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1
Moisture Related ReliabilityMoisture Related Reliability
The 58th Electronic Components and Technology Conference (ECTC)May 27 – 30, 2008, Lake Buena Vista, Florida
LAMAR UNIVERSITYA Member of The Texas State University System
Moisture Related Reliability in Electronic Packaging
Moisture Related Reliability in Electronic Packaging
InstructorXuejun Fan
Department of Mechanical Engineering
EM
Lamar UniversityBeaumont, TX 77710
E-mail: xuejun.fan@lamar.edu
2IntroductionIntroduction
• Name
• Organization
• Responsibility
• Expectations
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
2
3ReminderReminder
• Request– Please keep pagers / phones turned off
– No email / web surfing– No email / web surfing
• Active participation is encouraged
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
4AcknowledgementAcknowledgement
Institute of Microelectronics
T.Y. TeeT.B. LimPhilips
G.Q. ZhangW.D. van Driel
IntelYi He
Steve ChoDaniel Shi
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Daniel ShiBin Xie
Hyunchul KimLay Foong Siah
Alan LuceroShubhada Sahasrabudhe
3
5Outline Outline • Introduction • Moisture absorption, desorption, and diffusion• Vapor pressure model
C t d I d fill l ti f FC BGA k• Case study I – underfill selection for FC BGA packages• Case study II – delamination/cracking in stacked-die
chip scale packages• Accelerated moisture sensitivity test• Effect of moisture on material properties• Hygroscopic swelling
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Hygroscopic swelling• Electrochemical metal migration • Summary • References
6
IntroductionIntroduction
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4
7Electronic Packaging EvolutionElectronic Packaging Evolution
Leadframe based
Substrate based ball grid array (BGA)
Flip chip BGAQuad Flat Leadless Stacked chip scale
Flip chip CSP
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higher performance smaller sizecheaper price
• Chip-scale and wafer-level packaging (CSP, WLP)• High-density, high-performance packaging• System in package (SIP)
Quad Flat Leadless Stacked chip scale
8Wafer, Package, and Board Levels Wafer, Package, and Board Levels
Electronic PackagingWafer Fabrication & Backend Process
Surface Mounting
Packaging level
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• Design of a package must consider the interactions among wafer, package, and board
Waferlevel
Board level
5
9Thermo-Mismatch Induced StressesThermo-Mismatch Induced Stresses
Coefficient of thermal expansion (CTE): ppm/˚CAl Heat Sink: 24Adhesive: 60
Silicon Chip: 3
U d fill 20 30
Solder: 23
Substrate: 20Underfill: 20-30
Printed Circuit Board: 20
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Reference temperature state Thermal mismatch-induced deformation
10Failures by Thermal-StressesFailures by Thermal-Stresses
crack inside die
die
10 μ substrate
Wire bond damage Thin film delamination Die crack
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Solder ball fatigue crack Interface delaminationSubstrate cracking
6
11Moisture Absorption of Electronic PackagesMoisture Absorption of Electronic Packages
Individual component Surface mount to board
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• Electronic packages absorb moisture in uncontrolled humid conditions prior to the surface mount on board.
storage shipment
12Popcorn Failures at Soldering ReflowPopcorn Failures at Soldering ReflowDevice exposed to reflow temperature, typically 260°C
Stage 1: Moisture absorption Stage 2: Initiation of delamination
St 3 D l i ti tiSt 4 P k ki d l
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– Vapor pressure and adhesion reduction due to moisture vaporization are key mechanisms
Stage 3: Delamination propagation Stage 4: Package cracking and vapor release
7
13
• Polymer expands upon absorbing moisture – hygroscopic swelling
• Differential expansion results in hygro-mechanical strain and
Hygroscopic Swelling under HASTHygroscopic Swelling under HAST
stress (similar to thermal strain and stress)
• Magnitude comparable to & may be larger than thermal stress
• Adhesion degradation
H lli i d
Typical Failure in flip-chip under Pressure cooker test (120°C, 100%RH)
Underfill
Solder cracksDie
Solder
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Hygro-swelling induces• solder cracking• underfill delamination• BLM, ILD delamination Substrate
• Hygroscopic swelling and adhesion reduction are main failure mechanisms under HAST.
14Electrochemical Migration under BiHASTElectrochemical Migration under BiHAST• Conditions
– Moisture absorption/condensation – Voltage – Contamination
3 Basic Steps f S C f #
dendrites
+ -–CAF
e–
SR
• 3 Basic Steps– Oxidation or dissolution of metal at anode– Transport of metal ions, across insulator towards cathode– Reduction and deposition at cathode
Ref: Katayanagi et al. ESPEC Japan Tech-info Field Report #5, 1996
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Moisture provides electromigration transport path
8
15
• IPC/JEDEC J-STD-020C
Moisture Sensitivity Test (Precon)
MOISTURE SATURATION LEVELS PER IPC/JEDEC J-STD-020A (4/99)
MSL 3
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16Classification Reflow ProfilesClassification Reflow Profiles• IPC/JEDEC J-STD-020C
– Soldering reflow traditionally @ 220°C degrees, now moving to 260°C (for no-lead solders).
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
9
17Moisture Sensitivity Test
G G
Stage 1: Moisture absorption (e.g.: 30°C/60%RH for 168 hours)
Stage 2: Soldering reflow (peak temp: 220°C → 260°C)
G
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• Moisture– generates high vapor pressure– can degrade adhesion strength– induces stresses due to hygroscopic swelling
storage & shipment surface mount
18Highly Accelerated Stress Test (HAST)Highly Accelerated Stress Test (HAST)• Biased HAST• Unbiased HAST
– HAST– Autoclave (Steam) – 121°C/100%RH
Environmental Test Temperature (°C) Relative Humidity
(%RH)
Static/DynamicBias (V)
THB 85 85 / 60 0.1 to 755 85 / 6030 85 / 60
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30 85 / 60HAST 156 85 / 60 / 50
130 85 /60 / 50120 85 /60 / 50110 85 /60
10
19Summary: Kinetic & Moisture Driven Failures Summary: Kinetic & Moisture Driven Failures MECHANISM DESCRIPTION DRIVING
FORCESRELIABILITY STRESS
M+ Migration Metal ion migration between contacts or traces that results in a short circuit
Temperature HumidityVoltage
Biased HAST
Interfacial Delamination between two materials that were Temperature HAST / Bi HASTInterfacial delamination
Delamination between two materials that were bonded together that result in cracks and open circuits or migration paths (bond breaking with added energy)
TemperatureHumidity
HAST / Bi HAST/Precon
Intermetallic IMC formation
Formation of intermetallic compound that is different in volume and with brittle properties that may result in open circuits or shorts
Temperature Bake
Kirkendall voiding
Occurs with IMCs as charge moves from higher to lower potential area in material
Temperature BakeManufacturing
Electromigration voiding
Void left as material is picked up with electron wind (current flow)
TemperatureCurrent
Electromigration
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voiding (current flow) CurrentMech. Stress
Thermal material degradation
Thermal resistance and mechanical degradation resulting from polymer degradation and micro-crack
TemperatureMech. Stress
Bake
Dielectric cracking
Cracking in polymers or glasses that results from moisture assisted crack growth propagation
HumidityTemperature
HASTSteam/TH/Precon
20SummarySummary
• 3 failure mechanisms due to moisture– ‘Popcorn’ at soldering reflow (vapor pressure, adhesion reduction)– Delamination under HAST (hygroscopic swelling, adhesion
reduction moisture aging)reduction, moisture aging)– Metal migration under BiHAST (e.g. dendritic growth. Moisture,
voltage, contamination)• 3 reliability tests
– Moisture sensitivity test (MSL 1, MSL 2, MSL 3 …)• MSL 3 - 30°C/60%RH for 192 hours
– HAST/TH
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HAST/TH– BiHAST/BiTH
Understanding moisture diffusion is a key
11
21
Moisture Absorption, Desorption and Diffusion
Moisture Absorption, Desorption and Diffusion
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22Polymer Materials in Electronic Packaging Polymer Materials in Electronic Packaging • Bulk-form
– encapsulation (e.g. mold compound)
– substrate …Adh i
A leadframe package
Mold compoundDie attach
Thermal adhesive
• Adhesives– die-attach, underfill– thermal adhesives …
• Thick- or thin- film– solder mask– passivation Solder mask
Passivation
DieSolder bump
A ball grid array (BGA) packageCarrier
Underfill
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Substrate
• Materials change over a range of temperatures• Susceptible to moisture absorption
12
23Moisture Absorption in Polymeric Materials
85°C/85RH
ρw: moisture density in polymeric materialρa: ambient moisture density under 85°C/85%RH
ρw= 81.2ρa
ρa
ρw
Moisture condensation in a typical underfill
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– Under 85°C/85%RH condition, ρw= Csat = 2.47e-2 g/cm3 = 81.2 ρa (Csat: saturated moisture concentration)
• Moisture is condensed into liquid state• Moisture exists in micro-pores or free volumes (in bulk or at interface)• Moisture vaporizes at reflow, possibly still at mixed liquid/vapor phases
24Key Messages in Moisture AbsorptionKey Messages in Moisture Absorption
• At equilibrium, moisture density inside material is a few orders higher than ambient moisture density;– Polymer material behaves like sponge – you can squeeze out
water after moisture absorptionwater after moisture absorption.
• Moisture in material will be in liquid/vapor mixed phases;– Liquid-form moisture will evaporate during heating up – to
generate high vapor pressure.
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
13
25Fundamentals of Moisture AbsorptionFundamentals of Moisture Absorption
• What is the Relative Humidity (RH)? – Defined as vapor pressure ratio associated with
temperature T
%100×=airtheofpressurevaporSaturated
airtheofpressurevaporActualRH
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26
• Fundamental variables and properties– Moisture concentration, C(x, t; T, RH)
• Mass of moisture per unit volume of substance.– Saturated moisture concentration, Csat(T, RH)
• Maximum mass of moisture absorbed per unit volume of substance at given
Fundamentals of Moisture Diffusion
temperature and humidity.– Diffusivity, D(T, RH)
• A measure of the rate of moisture mass diffusion• Defined as the amount of mass flux per unit concentration gradient
• Fick’s diffusion equation
⎟⎟⎠
⎞⎜⎜⎝
⎛++= 2
2
2
2
2
2
zC
yC
xCD
tC
∂∂
∂∂
∂∂
∂∂
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⎠⎝ zyxt ∂∂∂∂
14
27Comparing Moisture Diffusion to Heat Transfer
C1C2
C2’
Moisture
T1 T2
Thermal
Mat A Mat B T3Mat A Mat B
Discontinuity for moisture concentration Continuity for temperature
Normalize moisture concentration (ϕ =C/S or w = C/Csat )
Moisture concentration C
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C1C2
C2’Mat A Mat B
w1 w2
w3Mat A Mat B
28
• Thermal-moisture analogy
Properties Thermal Moisture Field variable Temperature, T ϕ = C/S
Moisture Diffusion Modeling
p , ϕDensity ρ 1
Conductivity k D S Specific capacity c S
Moisture concentration: C= ϕ S
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Moisture diffusivity: D = DS/(1xS) = D
15
29Moisture Diffusion ModelingMoisture Diffusion Modeling• Input
– Preconditioning conditions – e.g. 60°C/60RH– Diffusivities for each material (die-attach, mold compound, BT, and
solder mask)– Saturated moisture concentration for each material (die-attach,
mold compound BT and solder mask)mold compound, BT, and solder mask) – From material characterization or supplier’s data
MoldingCompound
BT Solder Mask
DieAttach
Cu
– Output
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Output• Moisture concentration at each location• Package total weight gain: ∑ C* Velement
– Compare• Package weight gain data
30
• 3-D PBGA
MoldingCompound
DieAttach
FEA Moisture distribution
Moisture Diffusion Modeling: ValidationMoisture Diffusion Modeling: Validation
FEA vs Experiment Weight Gain
at 0 9
1
850C/85%RHBT Solder MaskCu
42 hrs
84 hrs
atio
of m
oist
ure,
M(t
)/Msa
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Experiment
FEA
85 C/85%RH
300C/60%RH
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
168 hrs
Time (hour)
Mas
s r
0
0.1
0 20 40 60 80 100 120 140 160
FEA
16
31
• 35mmx35mm PBGA
Moisture Diffusion Modeling: ValidationMoisture Diffusion Modeling: Validation
FEA Moisture distribution (quarter model) after 168 hrs at 850C/85%RH FEA Vs Experiment Weight Gain
Mold compound
Solder Cu l
Substrate
0.004
0.008
0.012
0.016
stur
e W
eigh
t Gai
n (m
g)
ExperimentFEA
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resist
Thermal via
planeDie flag Die
00 200 400 600 800 1000
Time (hr)
Moi
s
32
• PBGA at Level 3 (30°C/60%RH)Moisture Absorption
24h
Finite Element Moisture Diffusion ModelingFinite Element Moisture Diffusion Modeling
Moisture Absorption 48h
• PBGA at Level 1 (85°C/85%RH)
48h
24h
168h
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192h
17
33Moisture Diffusion vs. Heat ConductionMoisture Diffusion vs. Heat Conduction
cm2.0=h
x
sat),0( CtC =
0),( =∂∂ =hxxtxC
14.0at 1.2
Thickness: 2mm
h/2
x=0
h/2
2 0
4.0
6.0
8.0
10.0
12.0
moi
stur
e co
ncen
trat
ion
ade
of x
=h (g
/cm
^3 *e
-3) 30C/60%RH
85C/60%RH85C/85%RH
x=h
0 2
0.4
0.6
0.8
1.0
erat
ure
at th
e si
de o
f x=h
cm20=h
x=h
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0.0
2.0
0 100 200 300 400 500
Time (hours)
Loca
l sid
cm2.0=h
0.0
0.2
0 10 20 30 40
Time (second)Te
mpe cm2.0=h
• In most cases, the thermal modeling can be decoupled with moisture diffusion modeling
34Moisture DesorptionMoisture Desorption
G G
• Moisture desorption takes place at reflow
Moisture absorption(e.g. 30°C/60%RH:168 hours)
(Factory environment after opening dry-bag)
Moisture desorption(reflow process)
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Moisture desorption takes place at reflow – Elevated temperature, varying as function of time– Short time period (3 – 5 minutes)– Diffusivities a few orders higher
18
35Moisture Desorption ModelingMoisture Desorption Modeling
tS
SDtDzyx ∂∂
⋅+=
∂∂
+∂∂
+∂∂ ϕ
∂ϕ∂ϕϕϕ 1
2
2
2
2
2
2
0≠∂∂
⋅ tS
SDϕ
• ϕ =C/S, S=S(T) = S[T(t)] = S(t)• Normalization approach doesn’t work!• Direct moisture concentration approach (DCA) can be
introduced (B. Xie et. al. ECTC 2007) • An exception
∂tSD
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p– Normalization approach with C/Csat is still applicable when Csat
is assumed independent of temperature over entire reflow period
36Moisture Desorption at ReflowMoisture Desorption at Reflow
• After 168 hours at 85°C/85%RH– The package is almost fully
saturated with moistureD i fl
WetnessDie MCDA C
QFN Half Model
WetnessDie MCDA C
QFN Half Model
Transient Moisture Distribution
• Desorption effect is considered (QFN Package)
• During reflow, – External package surface
loses a significant amount of moisture due to high moisture desorption rate
– Moisture concentration in the interior of the package remains relatively unchanged
– The local moisture concentration along critical
1hr
12hr
168h
DA CuModel
1hr
12hr
168h
DA CuModel
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concentration along critical interfaces determines the strength of interfacial adhesion and magnitude of internal vapor pressure induced
168hr
Reflow
168hr
Reflow
19
37
0 min85°C
Moisture distribution
Moisture Distribution at Reflow• Flip Chip Package without Mold
– Substrate thickness: 1mm
85 C
1 min183°C
1.5 min200°C
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2 min220°C
– Desorption during reflow affects the moisture distribution greatly,– Moisture distribution at critical interfaces inside may not change at all.
38
8 8
‘Over-Saturation’ at Reflow‘Over-Saturation’ at ReflowFlux=0
Mat10.2mm
0.03mm Mat2
C
Flux=0
Mat10.2mm
0.03mm Mat2
C
0
1
2
3
4
5
6
7
8
0 50000 100000 150000 200000 250000 300000 350000
Time (s)
Con
cent
ratio
n (k
g/m
^3) BT
Film
0
1
2
3
4
5
6
7
8
316780 316800 316820 316840 316860 316880 316900 316920 316940 316960
Time (s)
Con
cent
ratio
n (k
g/m
^3) BT
Film
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( ) ( )
– In the beginning of desorption, since the film tends to absorb more moisturedue to increasing the saturated moisture concentration, the moistureconcentration increases at the beginning, and then decreases
20
39Package Total Weight Gain vs. Local Moisture Concentration
Package Total Weight Gain vs. Local Moisture Concentration
• QFP package - moisture absorption – 40% saturated weight gain
• QFP package - moisture absorption until saturated weight. Then desorp until 40% saturated weight gain
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• The first case passed pre-con test, will the second case pass?
40
Mt/M
∝ o
f pac
kage
Mt/M∝ ≈ 75%Failure
No failure
FailureNo failure
Mt/M∝ ≈ 38%
Moisture Absorption versus Popcorn Cracking
M
Local moisture concentration C for the
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Package cracking is independent of the total mass of moisture in the package (Kinato et al, IRPS, 1988);
− But dependent on the local moisture concentration in the package
Local moisture concentration C for the first case is less than the second case
21
41Moisture Absorption vs. Water AbsorptionMoisture Absorption vs. Water Absorption
Water-resistant material
In air (30°C/60%RH) In water (30°C/100%RH)
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• Some materials are water-resistant, but absorb moisture in air.
Moisture gain in package No moisture gain in package
42
• Diffusion Coefficient / Diffusivity, D(T)– Measures the rate of mass diffusion– Defined as the amount of mass flux per unit concentration
gradient (m2/s) A function of material and temperature
Material Related PropertiesMaterial Related Properties
– A function of material and temperature
• Saturated Moisture Concentration, Csat(RH, T)– The maximum mass of moisture per unit volume of the
substance kg/m3.• Solubility, S(T)
– The ability of the substance to absorb moisture– Defined as the maximum mass of moisture per unit volume
f th b t it (k /( 3P ))
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of the substance per unit pressure (kg/(m3Pa)). – A function of material and temperature
PCS sat= Where P = ambient pressure in given RH
22
43Characterization Method• Procedures - documented in the following standards
– Semi G66-96 Test method for the measurement of moisture absorption characteristics for semiconductor plastic molding compound
– ASTM D570-95 Standard test method for moisture absorption of plastics– BS 2782: Part 4-1983, ISO 62 -1980BS 2782: Part 4 1983, ISO 62 1980– Method 430A Determination of water absorption at 230C– Method 430B Determination of water absorption at 230C with allowance for
water-soluble matter– Method 430C Determination of boiling water absorption– Method 430D Determination of boiling water absorption with allowance for
water-soluble matter
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Ø50mm 1 mm for diffusivity
3mm for solubility
44
• Source of Error – Specimen Preparation
• Minimize voids• Sufficient aspect ratio to
Solubility and Diffusivity - Experimental Characterization
– promote 1-D diffusion and minimize edge effect– Measurement
• Inaccurate measurement
– Total weight of specimen*1%*1% > scale resolution• Improper measurement frequency
– Too frequent : excessive disturbance to TH chamber– Too sparse : missing data
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– Too sparse : missing data
23
45Experimental Data AnalysisExperimental Data Analysis
• Sample requirement: t << length or width• 1D diffusion valid, and
2
2
xCD
tC
∂∂
=∂∂
40
60
wei
ght g
ain
(mg)
ExperimentalCalculated
– C: moisture concentration (mg/cm3) – D: diffusivity (cm2/sec)
• Diff. eq. can be solved with initial and boundary conditions
∞ ⎤⎡ + 22)12(18 DnM π Mt: weight gain at time t
0
20
0 50 100 150 200 250 300 350 400 450 500 550 600
Time (hours)
Moi
stur
e
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∑∞
=∞⎥⎦
⎤⎢⎣
⎡ +−
+−=
0222)12(exp
)12(181
n
t th
DnnM
M ππ
tM∞: saturated weight gainh: thickness
46Temperature Dependency of DiffusivityTemperature Dependency of Diffusivity• Diffusivity D(T) vs. temperature T – Arrhenius relation
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• D increases with temperature exponentially • Near or above Tg, the Arrhenius relation is described with a new set
of constants reflecting the change in the molecular structure of the material across the transient temperature.
24
47Temperature Dependency of Solubility Temperature Dependency of Solubility • Solubility vs. temperature – Arrhenius Relation
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• When temperature increases solubility decreases ( activation energy positive)
48Temperature and Humidity Dependency of CsatTemperature and Humidity Dependency of Csat• Csat = p*S• p = RH * psat (T) = RH * p0 exp (- Ep/ kT)• Csat = RH * p0 exp (- Ep/ kT)* S0 exp(Es/kT) = RH*C0 exp [(Es-Ep)/kT ]
10
9
In-Situ Moisture Measurement for BT-CoreIn-Situ Moisture Measurement for BT-Core
30 40 50 60 70 80
4
6
8
70 µm BT core
60% R.H. 80% R.H.
Csa
t (m
g/cm
3 )
0 10 20 30 40 50 60 70 80 900
1
2
3
4
5
6
7
8
9
Slope = 0.11
Experimental Data Linear Fit
Csa
t (m
g/cm
3 )
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Temperature (°C)
• Saturated moisture concentration under fixed humidity independent of temperature below Tg (e.g. BT-core)
• Saturated moisture concentration linear with humidity
0 10 20 30 40 50 60 70 80 90Relative Humidity (%)
25
49Saturated Moisture Concentration vs. Temperature
Saturated Moisture Concentration vs. Temperature
1.2
)
• A low-Tg die-attach film • 60%RH
0.4
0.6
0.8
1.0
Y = 0.34+0.0084T
Measured Linear Fit
Sat
urat
ed M
oist
ure
Upt
ake
(%)
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– Within the temperature range, it seems that the saturated moisture content increases linearly with increasing temperature
30 40 50 60 70 80Temperature (°C)
50Moisture Absorption and DesorptionMoisture Absorption and Desorption
100.2
100.360oC
%)
Weight
40
50
60
Referen
RH
Moisture absorption-desorption experiment for BT-core
0 200 400 600 800 1000 1200
99.9
100.0
100.1
Time (min)
Wei
ght (
%
0
10
20
30
nce Sensor RH
(%)
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• The subsequent absorption-desorption cycles were repeatable– The sample reaches approximately the same saturated moisture
level during sorption and it loses the same weight upon drying• This indicates that there is no chemical reaction between the water
molecules and the material
26
Fickian and Non Fickian Kinetics Fickian and Non Fickian Kinetics 51
10
15
20
2560oC / 60% RH
Gai
n (μ
g)
Saturation
( )CDtC 2∇=∂∂
⎟⎠
⎞⎜⎝
⎛−=kTE
DD do exp
0 1000 2000 3000 40000
5
10
Experimental Data Fit 1 Fit 2
Wei
ght
Time (sec)
BT resin weight gain curve
Linear plot initially
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Two characteristic features of Fickian behavior– Initially linear plot vs. t1/2
– Level off to a saturation level • An ideal case of moisture transport without interference of polymer chain
structural relaxation
Fickian and Non-Fickian KineticsFickian and Non-Fickian Kinetics 52
Substrate
0.02
0.04
0.06
0.08
0.1
0.12
0.14
Wei
ght G
ain
(%)
Sub SH1Sub S6FMean
D
mDDEB
θλ
=)(
Deborah Number (DEB)D
00 24 48 72 96 120 144 168 192 216 240
Time (hr)
“two-stage” sorption
∑ tk
BT core composite moisture weight gain
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Two processes – Moisture diffusion– Polymer relaxation
∑ −∞∞ −=
i
tki
ieMtM )1()( ,
27
Non-Fickian Moisture Absorption and DesorptionNon-Fickian Moisture Absorption and Desorption• Two types of mold compound (2mm thickness)
53
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Moisture absorption Moisture desorption
• After 12 weeks, saturation condition has not been reached• After 24 hours desorption at 110°C, a residual moisture content of
about 40% saturation level was observed Ack: H. Shirangi, J. Auersperg, EuroSimE 2008, 455-462
States of Moisture in Polymers States of Moisture in Polymers 54
85°C/85RHρw: the moisture density in
polymeric materialρa: the ambient moisture
• Where to stay?
ρa
ρw
ρa: density under 85°C/85%RH
ρw = (20 -200)ρa
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– Free volume or micro/nano pores • Moisture state
– Bound water (hydrogen bond)– Unbound water (liquid or gas)
28
55SummarySummary• Moisture in polymer materials will be in liquid/vapor
mixed phases• Moisture concentration is discontinuous along bi-
material interface –special treatment such asmaterial interface special treatment such as normalization must be applied
• Local moisture concentration, not total moisture weight gain, determines package moisture performance
• Moisture absorption is different from water absorption • Moisture absorption is a reversible process for Fickian
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
type of moisture diffusion• Diffusivity, solubility and saturated moisture
concentration are moisture related properties
56
Vapor Pressure Model
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
29
57Introduction - Popcorn FailureIntroduction - Popcorn Failure
• Moisture is vaporized at high temperature• Vapor pressure plays a critical role in popcorn failure
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
p p p y p p• Vapor pressure exists anywhere in package where moisture resides
58Example: Pressure Cooker Example: Pressure Cooker • Vapor phase
T1 →T2
Single vapor phase
p1 p2
– Moisture in single vapor phase;– ρ < ρ (T) ρ: moisture density over the total volume of
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– ρ < ρg(T), ρ: moisture density over the total volume of cooker; ρg(T): saturated moisture vapor density
– Ideal gas law can be used: p2=p1T2/T1
30
59Example: Pressure Cooker Example: Pressure Cooker
• Liquid/vapor phase
Given temperature T (e.g., 170°C)
– Moisture in two-phases – water/vapor mixed;– ρ > ρg(T) - liquid-vapor phase , ρ: moisture density over
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
g
the total volume of cooker; ρg(T): saturated moisture vapor density
– ρ = mm/volume – apparent moisture density– Saturated vapor pressure remains regardless of water level
60Multi-Scale AnalysisMulti-Scale Analysis• Representative Elementary Volume (REV) concept
– Microscopic level
• Moisture condensed into mixed liquid/vapor phases
• From macroscopic to microscopicFrom macroscopic to microscopic – f: interstitial space fraction (or free-volume fraction)
– C: moisture concentration (from moisture diffusion at macroscopic level)
– ρ: moisture density in pores
fCVV
Vm
Vm /
dd
dd
dd
===ρ
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– ρ < ρg (T): single vapor phase
– ρ ≥ ρg (T): mixed liquid/vapor phase
VVV ff ddd
A representative elementary volume
31
61Example: Moisture Density in Free Volumes Example: Moisture Density in Free Volumes
85°C/85RH85°C/85RH
ρw
fCVV
Vm
Vm
ff
/dd
dd
dd
===ρ
• Apparent moisture density over voids- ρ
ρaρa
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
ρw= Csat = 81.2 ρa
ρ =Csat/f = 1624 ρaf=5%
62Steam TableT(°C) 20 30 40 50 60 70 80
ρg(g/cm2×10-3) 0.017 0.03 0.05 0.08 0.13 0.2 0.29
pg(MPa) 0.002 0.004 0.007 0.01 0.02 0.03 0.05
T(°C) 90 100 110 120 130 140 150
ρg(g/cm2×10-3) 0.42 0.6 0.83 1.12 1.5 1.97 2.55
pg(MPa) 0.07 0.1 0.14 0.2 0.27 0.36 0.48
T(°C) 160 170 180 190 200 210 220
ρg(g/cm2×10-3) 3.26 4.12 5.16 6.4 7.86 9.59 11.62
pg(MPa) 0.62 0.79 1 1.26 1.55 1.91 2.32
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
T(°C) 230 240 250 260 270 280 290
ρg(g/cm2×10-3) 14 16.76 19.99 23.73 28.1 33.19 39.16
pg(MPa) 2.8 3.35 3.98 4.69 5.51 6.42 7.45
32
63Saturated Vapor Pressure Saturated Vapor Pressure
)(2731
)exp()( 33
2210
CTxwhere
xaxaxaakPaP
o
sat
+=
+++=
)(273 CT+
63
52
310 100972361.5 ,109085058.2,105151386.3 ,033225.16 ×=×−=×−== aaaa
• The saturated vapor pressure, Psat(T), is the pressure at which liquid water and water vapor can coexist at temperature T.
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• The amount of water vapor (Psat(T) ) the void can hold increases with temperature.
64Moisture State in VoidsMoisture State in Voids
• Single vapor phase when )
0(T
gρρ ≤
T0
• Mixed liquid/vapor phase when
– ρg - the saturated vapor density
• Phase transition temperature T1
)0
(Tg
ρρ >T0
Single vapor phase
Mixed liquid/vapor phase
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Phase transition temperature T1T1)()( 11 TT gρρ =
TSingle vapor phase
T0: moisture pre-conditioning temperature
33
65Vapor Pressure ModelVapor Pressure ModelCase 2
ρ (T) ≥ ρg(T)
ρ (T1) = ρg(T1)
T0, f0, C0 → p(T0), ρ (T0)
T, f
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Case 1ρ (T0) ≤ρg(T0)
Case 3ρ (T0) > ρg(T0)
andρ (T) < ρg(T)
66Vapor Pressure ModelVapor Pressure Model
)(3
000
0 0
11
)()(
)( TTg eff
TT
fTTpC
Tp −−
−−
= α
ρ
Case 1: )(/ 000 TfC gρ≤ Case 2: )(11 )(3
0
0 0 Teff
fC
gTT ρα ≥
−− −−
)()( TpTp g=
Case 3:and
)(/ 000 TfC gρ>
)(11 )(3
0
0 0 Teff
fC
gTT ρα <
−
− −−
)(3
1
1
11
1
)(11)()()( TT
g eTff
fTf
TTTpTp −−
−−
= α
000 1)(g fTfTρ 11 )(1 TffT
)(1
)(1)( 1
)(3
0
1
1
1 TefTf
TfC
gTT ρα =
−− −−
where
Vapor phase only Liquid/vapor phase Transition from liquid to vapor
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• The vapor pressure is related to the local moisture concentration C• Local moisture concentration C, changes with moisture desorption in reflow
34
67Vapor Pressure Modeling for PBGA (Level 3 (30°C/60%RH)) – No Desorption
Moisture absorption
24h
Vapor pressure at reflow (220°C)
48h
24hMPa
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Moisture and vapor pressure have different distributions
192h
68Vapor Pressure Modeling for PBGA (Level 1, 85°C/85%RH) – No Desorption
Vapor pressure at reflow (220°C)Moisture absorption
48h
168h
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
168h
• More moisture, no more vapor pressure
35
69
Interfacial adhesionMoistureabsorption
Failure Mechanism – Interfacial Delamination
Vapor pressure
Moisture absorptionMSL 3 MSL 2 MSL 1
Adhesion Vapor pressure
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Moisture affects the package reliability at reflow from two aspects: generating vapor pressure and degrading the interfacial adhesion, respectively.
Schematic, not scaled
70Vapor Pressure Induced Expansion
Swelling :β C
Thermal expansion :α T
Vapor pressure induced : γ p
Before preconditioning At reflow temperatureAfter preconditioning
Volume expansion:Thermal expansion-induced 3α ΔT ≈ 21 e-3 Vapor pressure induced dV/V = 3 (1 2 ) p/ E 9 3 e 3
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Vapor pressure-induced dV/V = 3 (1 - 2ν ) p/ E≈ 9.3 e-3
ΔT : 220-150 = 70°C,E: 300 MPa , p: 2.32MPa ,ν : 0.3, α: 100ppm
• Vapor pressure introduces additional mismatch.• Vapor pressure-induced expansion is directly related to the vapor pressure
distribution, rather than moisture distribution.
36
71
M o ld C o m p o u n d D ie A t ta c h
T o ta l S tr a in E q u iv a le n t m e a nC T E (p p m /°C ) T o ta l S tr a in E q u iv a le n t m e a n
C T E (p p m /°C )T h e r m o -
m e c h a n ic a l 1 .5 3 e -3 3 4 7 .6 5 e -3 1 7 0
H
Vapor Pressure Induced Expansion
H y g r o -m e c h a n ic a l 1 .5 7 e -3 3 4 .9 3 .2 2 e -3 7 1 .6
V a p o rP r e s s u r e 8 .1 4 e -4 1 8 .1 2 .1 6 e -2 4 7 9 .6
I n te g r a te d( to ta l) 3 .9 1 e -3 8 7 3 .2 5 e -2 7 2 1 .2
• When vapor-pressure induced expansion is included– Stress-free at T0 and cooling down (or heating up) to T1
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
TY Tee et al, ECTC 2002
Stress free at T0 and cooling down (or heating up) to T1– The total ‘thermal strain’ = α (T1-T0) + (1 - 2ν ) p/ E– Equivalent coefficient of thermal expansion
• α + (1 - 2ν ) p/ E /(T1-T0)
72SummarySummary
• The saturated vapor pressure, Psat(T), is the pressure at which liquid water and water vapor can coexist at temperature T
• Vapor pressure will remain saturated as long as the moisture is in liquid phase during reflowq p g
• Moisture affects the package reliability at reflow from two aspects: generating vapor pressure and degrading the interfacial adhesion, respectively.
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
37
73
Case Study I: Underfill Selection for Flip Chip BGA on Moisture Sensitivity
Performance
Case Study I: Underfill Selection for Flip Chip BGA on Moisture Sensitivity
Performance
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
74
• To perform the moisture sensitivity test to evaluate underfill performance
• Test vehicleFli hi PBGA 10 10 di 27 27 BT b t t (0 4
Underfill Selection in Flip Chip Packages
– Flip chip PBGA, 10x10mm die, 27x27cm BT substrate (0.4 mm core thickness), double layer
– Underfill materials: 6 kinds of underfills• Material properties
– Modulus (E)– Viscosity – CTE– Tg
underfill
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Tg– Moisture related properties (diffusivity, solubility …)– Adhesion (fracture toughness)
38
75
1.4
1.6
Underfill Material Properties - Moisture Absorption (85°C/85%RH)
0.2
0.4
0.6
0.8
1
1.2%
wei
ght g
ain
UF-A UF-B UF-CUF-D UF-E UF-F
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
00 60 121 182 244 305 366 425 486 547 609 670 731 790 851 912
No. of moisturized hours
• UF-A < UF-B < UF-C < UF-D < UF-E < UF-F
76Underfill Material Properties - Mechanical and Thermal Properties
CTE1 CTE2 Tgppm/°C ppm/°C °CUF
E1, E2GPa
UF-A 31 90 133 8 1.7
UF-E 68 197 155 2.2 1.05
UF-F 61 199 102 2.7 0.05
78 144 7 4.5
40 128 12 4UF-B 18
UF-C
UF-D
25 93 117 9.6 1.4
27
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– It seems the underfill A is an ideal candidate material.
UF F 61 199 102 2.7 0.05
39
77Adhesion at Room Temperature with Moisture
Adhesion at Room Temperature with Moisture
Shear Test Strength (PI/UF) At Room Temperature
30
35
40
G)
0
5
10
15
20
25
30
JM-A JM-B Sumito.
Moisture: 85°C/85RH
Shea
r Str
engt
h (K
G
0days
11days
17days
21days
UF-A UF-B UF-C
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• No significant difference in underfill adhesion strength at room temperature with moisture
• Adhesion at room temperature is not sensitive to moisture
78
• For controlled samples tested in Level 3 (30°C/60 %RH) (UF-A)
Summary of Moisture Sensitivity Test Results for Flip Chip Packages
Leg ID Configuration # unit with this failure mode
A1 ball layout 1, molded
A2 ball layout 1, not molded
A3 ball layout 2, molded
A4 ball layout 2, not molded 6/24
3/24
5/24
4/24
Underfill Total number of failure units
• For UF-C and UF-E
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
MSL 3 MSL 2
UF-C 0/24 0/18
UF-E 0/18 Not Available
Underfill Total number of failure units
40
79Failure MechanismFailure MechanismSolder spread
Before moisture sensitivity test
delamination
After moisture sensitivity testX-ray images for failed unit with interface delamination at PI/UF
El t i l h t d b ld t i diePI
Cross-section
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Electrical short caused by solder extrusion die
solder
NiCu
Extrudedsolder
UF
SM
Cross-section image
80Adhesion at High Temperature with MoistureAdhesion at High Temperature with Moisture
25
30with moisturewithout moisture
0
5
10
15
20
UF A UF B UF C UF D UF E UF F
shea
r st
reng
th (k
g)
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Adhesion strength at high temperature with moisture is a key.
UF-A UF-B UF-C UF-D UF-E UF-F
underfill
41
81
Interfacial adhesionMoistureabsorption
Failure Mechanism – Interfacial Delamination
Vapor pressure
Moisture absorptionMSL 3 MSL 2 MSL 1
Adhesion Vapor pressure
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Moisture affects the package reliability at reflow from two aspects: generating vapor pressure and degrading the interfacial adhesion, respectively.
Schematic, not scaled
82
Failure Analysis – Solder Ball Flow-OutFailure Analysis – Solder Ball Flow-Out
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
42
83Failure Mechanism• Initiation of delamination around thermal via between UF and
solder resist
• Vapor pressure propagates the delamination
• If the delam reaches edge of underfill (exit point) the pressure of i t ill k th lt ld t fl tmoisture will make the melt solder to flow out
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
84Key MessagesKey Messages
• For underfill material, since material’s modulus is relatively high, cohesive delamination is not a concern
• Underfill material properties of diffusivity and solubility are not critical properties for package performance at reflow;
• The adhesion at high temperature with moisture is a
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• The adhesion at high temperature with moisture is a key.
43
85SummarySummary• Delamination at UF/PI or UF/SR is common failure
mode for FC BGA. No cohesive failure observed since UF has relatively high modulus
• Diffusivity and saturated moisture absorption are not• Diffusivity and saturated moisture absorption are not critical parameters in selecting underfill for moisture performance
• Adhesion at room temperature with moisture may not reflect the material behavior at reflow temperature
• Adhesion at high temperature with moisture is key
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Adhesion at high temperature with moisture is key
86
C St d IICase Study II : Delamination/Cracking in Stacked-
Die Chip Scale Packages
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
44
87From Single-Die Chip Scale Package (CSP) to Stack-Die Chip Scale Package
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
88General Assembly ProcessGeneral Assembly Process• Assembly issues with traditional die-attach paste by
liquid dispensing – Bleed-out to contaminate bond-pad with thinner die down to 3mil
or below– Die cracking and handling
Liquid dispensing on substrate
substrate DA paste
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Bleed-out
die
45
89
Wafer
Wafer laminate UV irradiationDicing Picking-up
Process Flow Using Wafer-Level FilmProcess Flow Using Wafer-Level Film
Wafer
DB Film
Die Bonding Wire Bonding Molding
DC Tape
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
90Die-Attach Film Delamination/CrackingDie-Attach Film Delamination/Cracking
• Experimental observations– Failures at the bottom die-attach film after precon – Cohesive voiding/cracking dominant– Very sensitive to soldering reflow profile
100120140160180200220240260280
mpe
ratu
re (C
)
FMSH
y g p
G
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
20406080
100
0 60 120 180 240 300 360 420reflow time (second)
Tem
46
91Failure MechanismFailure Mechanism
G
Before reflow After reflow
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
92
Effect of moisture on DA material properties
800
900
No RH
Fundamentals: Material Characterization
0 3
0.4
n
Die attach material moisture uptake
0
100
200
300
400
500
600
700
0 20 40 60 80 100 120 140
Temperature (oC)
Mod
ulus
(MPa
)
No RH
85 % RH
Dramatic decrease in modulus of DA
material in moisture
DA fil T i l d d i h i
-0.1
0
0.1
0.2
0.3
0 1 2 3 4 5 6 7
time (minute)
% w
eigh
t gai
n
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– DA film Tg is low, and drops with moisture exposure– DA modulus is less than 3MPa at reflow temperature, even lower than the
saturated vapor pressure (4.7MPa@260°C)– Moisture uptake in DA is very fast
• Faster than observed in previous literature
47
93Hypothesis: Two Scenarios on Vapor Pressure Buildup During Reflow
Hypothesis: Two Scenarios on Vapor Pressure Buildup During Reflow
180
210
240
270re
(C)
0.48MPa
2.32MPa
4.69MPa > film strength
0
30
60
90
120
150
0 60 120 180 240 300
reflow time (second)
Tem
pera
tur
0.02MPaReflow profile
120
150
180
210
240
270
pera
ture
(C)
0.48MPa
2.32MPa
< film strength
Reflow profile
Scenario II
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
0
30
60
90
0 60 120 180 240 300
reflow time (second)
Tem
p0.02MPa
Reflow profileScenario I
– Vapor pressure is dominant driving force for cohesive failure
94Moisture Diffusion Modeling
Fully drySi 600C/60% 96 hr PreconSaturated
No appreciable difference in moisture saturation after precon with difference substrate thicknessthick- substrate
thi b
film 1
fil 1
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Comments:•Substrate thickness has no effect on final moisture saturation of packagematerials after moisture preconditioning.
thin- substratefilm 1
48
95
Moisture diffusion modeling Moisture diffusion modeling -- before reflowbefore reflow
Modeling: Effect of Substrate Core Thickness
20406080
100120140160180200220240260280
0 60 120 180 240 300 360 420
reflow time (second)
Tem
pera
ture
(C)
SH
SH (Model)
Moisture desorption Moisture desorption modeling modeling -- after reflowafter reflow
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
1.3xx
Comments:•Moisture at DA film interface is reduced ~40%
96
CSP with thinner substrate
Vapor Pressure ModelingVapor Pressure Modeling
CSP with thicker substrate
– About 50% reduction on vapor pressure at 250˚C in the bottom DA film
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
About 50% reduction on vapor pressure at 250 C in the bottom DA film between two thicknesses of substrate
49
97
Interface moisture content comparison between two reflow profiles from 60oC~200oCInterface moisture content comparison between two reflow profiles from 60oC~200oC
180200220240260280
ure
(o C)
180200220240260280
re (o C
)
Modeling: Effect of Reflow Profile
20406080
100120140160
0 60 120 180 240 300 360 420
Reflow Time (s)
Tem
pera
tu
SH
SH (Model)
20406080
100120140160180
0 60 120 180 240 300 360 420
Reflow Time (s)
Tem
pera
tur
FM
FM (Model)
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Comments:•moisture at DA film interface is reduced ~40%
98
Thickness (μm) Leg1 Leg 2 Leg 3 Leg 4 Leg 5
Solder Mask 1x 1.02x 1.04x 1.04x 1.37x
Inner Cu density 0% 50% 50% 50% 50%
Validation: Experimental Results
Inner Cu density 0% 50% 50% 50% 50%
BT-Core 1y 1.09y 1.43y 1.47y 1.44y
Total 1z 1.20z 1.47z 1.47z 1.53z
Delam Rate 0% 7% 32% 47% 100%
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Comments:• BT-core thickness is largest modulator
50
99Desorption ModelDesorption Model
)/)(cos(])1(2[/),(0
/
sat
22
hxheCtxCn
nn
htDn n
−−
=∑∞
=
−
λλ
λ
πλ )2
12( +=
nn
C moisture concentration in filmC t t d i t t ti2 Csat saturated moisture concentration
2htD D: substrate diffusivity
t: timeh: thickness
C/Csat ~ exp (- )
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
The local moisture concentration depends on the thickness and the diffusivity of substrate, and reflow time.
D ↑ t ↑ h↓ C/Csat ↓
100How Would Moisture Escape? How Would Moisture Escape?
Die-Attach Film (reservoir)
D t h C/Csat
(reservoir)
D: diffusivityt: timeh: thickness
G
G
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Substrate (moisture escape path)
Reflow time, substrate diffusivity, and substrate thickness are three key parameters to determine the moisture loss.
51
101Summary
• Cohesive voiding/cracking of die-attach film was observed
• Very sensitive to reflow profilesy p– Desorption plays a significant role
• Very sensitive to substrate thickness• Reflow time, substrate diffusivity, and substrate
thickness are three key parameters to determine the moisture loss
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
102
Accelerated Moisture Sensitivity Test
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
52
103Accelerated Equivalent Accelerated Equivalent • IPC/JEDEC J-STD-020C
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
104Accelerated Equivalent Accelerated Equivalent
• CAUTION - The ‘‘accelerated equivalent’’ soak requirements shall not be used until correlation of damage response, including electrical, after soak and reflow is established with the ‘‘standard’’ soak requirements or if theestablished with the standard soak requirements or if the known activation energy for diffusion is 0.4 - 0.48 eV. Accelerated soak times may vary due to material properties, e.g., mold compound, encapsulant, etc. JEDEC document JESD22-A120 provides a method for determining the diffusion coefficient
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
53
105
• Equivalent Concentration Methodology– Local Moisture Concentration as primary failure driver
• R.L. Shook, B.T. Vaccaro, D.L. Gerlach, “Method for Equivalent Acceleration of JEDEC/IPC Moisture Sensitivity Levels”, 36th IRPS, 1998
Acceleration Methodology
1998– Technique
– Use 1-D moisture diffusion analytical analysis– Established a consistent accelerated M.S. test which is
independent of package form (for leaded package only) and materials (assuming that MC has similar diffusivity)
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
106Acceleration MethodologyAcceleration Methodology
– Failure occurs in interface involving only one diffusive material; e.g. Cu pad - mold compound – leaded package
– Acceleration factor is independent of package form and thickness– FEA moisture diffusion modeling is not even necessaryFEA moisture diffusion modeling is not even necessary– Acceleration factor (AF) can be simply computed from the diffusivity
of mold compound as
std
acc
DD
AF =
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
54
Existing Reliability Testing Methods
BHAST ~ 4.2days
Method <= 7days
Acceleration Methodology - OverallAcceleration Methodology - Overall
Bake ~ 21days
~ 4.2days UHAST
Precon ~ 10days
TCB ~ 21days
TCC ~ 11days
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Bend
TCG
Time
> 84days*
* Including design & SMT time
New Methods Have Been/Are Being Developed
BHAST ~ 4 2days
Method <= 7daysAccelerated MS
Acceleration Methodology - OverallAcceleration Methodology - Overall
Bake ~ 21days
~ 4.2days UHAST
Precon ~ 10days
TCB ~ 21days
TCC ~ 11days
BHAST ~ 4.2days
~ 84days*
Fast TCC
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Bend
TCG
Time * Including design & SMT time
55
Testing Method
Bake 7hrs/125C ( i t )
Acceleration MethodologyAcceleration Methodology
Stage 1: Moisture absorption (e.g.: 60C/60%RH for 88 hours)
G G
Stage 2: Soldering reflow (peak temp: 220C → 260C)
(moisture)
TCB 5cycles (shipment)
TH 60RH/60C 88hrs (moisture)
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Storage & Shipment
G
surface mount
IRX3 260C (reflow)
Accelerated test should ensure same failure mode/mechanism;
similar failure rate.
Acceleration MethodologyAcceleration Methodology
similar failure rate.
TSAM image
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
56
σT + σM + PThermo-stress Hygro-stress Vapor Pressure
Adhesion Strength
Accelerated & Standard Precon. Correlation Methodology
Acceleration Methodology
Soaking for Standard
Stress
Adhesion Strength
Same Failure Rate
Reflow for Standard
Failure Start Point
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Time
Soaking for Fast
Vapor Pressure + Thermal Stress+ Hygro Stress
Delam OccurrenceReflow for Fast
*Time Not Scaled
4.55
g/m3 )
30C/60%RH ~ 100hrs; 60C/60%RH ~ 30hrs.
Local Moisture Concentration Correlation
11.5
22.5
33.5
4
stur
e C
once
ntra
tion
(kg
30C/60%RH
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
00.5
1
0 20 40 60 80 100Time (hrs)
Moi
s
60C/60%RH
57
216 hrs 30C/60%RH matches with 70hrs 60C/60%RH
Global Moisture Concentration Correlation
216hrs under 30oC/60%RH
70hrs under 60oC/60%RH
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
45hrs under 60oC/60%RH 88hrs under 60oC/60%RH
216hrs 30C/60%RH matches with 70hrs 60C/60%RH
Vapor Pressure Correlation
70hrs under 60oC/60%RH
216hrs under 30oC/60%RH
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
88hrs under 60oC/60%RH 45hrs under 60oC/60%RH
58
30C/60%RH, 216hrs
Failure Mode/MechanismValidation
60C/60%RH, 60hrs
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
60C/60%RH, 75hrs
60C/60%RH, 88hrs
Failure RateFR at 216hrs 30C/60%RH is similar with that at 68.5hrs 60C/60%RH
100
Validation
0.1
1
10
0 50 100 150 200 250
Failu
re R
ate
(%)
30C/60%RH
30 45 60 75 88 216
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
0.01
0.1
Time (hrs)
30C/60%RH
60C/60%RH
59
117SummarySummary• Equivalent concentration methodology
– Identifies local moisture concentration as the primary failure driver
• J-STD-020C• Simple to use• Caution necessary when applied to leaded packages of different
materials• Caution necessary when applied to organic substrate based
packages• New methodology
– Local moisture concentration equivalency
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– Global moisture distribution equivalency
118
Effect of Moisture on Material Properties
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
60
119General ObservationsGeneral Observations
• Moisture has little effect on Young’s modulus• Moisture has little effect on visco-elastic properties• Moisture has little effect on CTE• Moisture generally reduces the Tg in the range of 10°C–
20°C• Moisture has significant impact on interface adhesion
strength
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
120
• Effect of moisture on storage modulus
16
20
Pa)
Typical Example : Mold Compound
Example: Effect of Moisture on Modulus
0
4
8
12
16
0 2 4 6 8
Sto
rage
Mod
ulus
(GP
Modulus @ 30CModulus @ 100CModulus @ 160CModulus @ 220C
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
0 2 4 6 8Moisture Concentration (mg/cc)
61
121Example: Effect of Moisture on Viscoelastic Properties
Example: Effect of Moisture on Viscoelastic Properties
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– A slight drop in modulus with moisture was observed
122Example: Effect of Moisture on TgExample: Effect of Moisture on Tg
800
900
No RH
0
100
200
300
400
500
600
700
0 20 40 60 80 100 120 140
Temperature (oC)
Mod
ulus
(MPa
)
No RH
85 % RH
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Temperature ( C)
62
123Example: Effect of Moisture on CTE and TgExample: Effect of Moisture on CTE and Tg
60
70 143Typical Example : Molding Compound
0
10
20
30
40
50C
TE (p
pm/C
)
141
141.5
142
142.5
Tg (C
)
CTE 1CTE 2
Tg
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
0 2 4 6 8Moisture Concentration (mg/cc)
124Adhesion Test TechniquesAdhesion Test Techniques• Interfacial fracture toughness measurement
– 4-point bend test– Double cantilever beam test– Compact tension test
• Adhesion measurementShear Test
• Adhesion measurement – Shear test– Pull test– Peel test– Torque test
• Sample configurations– Pre-cracked
Pull Test
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– Not pre-cracked– Sandwiched – Button shape
Peel Test
63
125Adhesion Test Sample Configuration: Pull and Shear
Pull Test Samples
Shear Test Samples
• Test samples
subs.
substrateuf
chip
chipuf
substrateuf
substrate
chip
chipuf
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Test samples– Same materials on both sides
• To eliminate the thermal stress effect• To keep the interface same on both sides
126Test Sample Preparation
• Process for the test sample preparation
• The adhesion samples were done on the whole wafer
Wafer/Substratewith PI/SM coating
Chip attachment Curing
Dicing Test Samples
Stencil printing for underfill
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
p– Consistent underfill wetting area– Consistent underfill wetting location– Consistent underfill height– Failure mode can be controlled
64
127
• For underfill/polyimide interface (UF/PI)– Shear test at room temperature (25°C)
Adhesion Test DOE
Underf ill Dry 85°C/85RH 85°C/85RH 85°C/85RH 11 days 17 days 21 days
UF/PI, Shear Test, Room Temperature, Chip to Chip Sample
– Shear test at reflow temperature (220 °C)
UF-1 v v v vUF-2 v v v vUF-3 v v v v
Underf ill Dry 30°C/60RH 85°C/60RH 85°C/85RH 21 days 21 days 21 days
• Sample size: 16
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
UF-1 v v v vUF-2 v v v vUF-3 v v v v
• Sample size: 16
128Pull Test Set-Uppull tester
• New pull fixture for pull test– Can operate under high temperature.
• Adhesion between sample and fixture
chip
chip
adh. fixture
uf
pull fixture
adh. fixture
fixture– Require high Tg (>200°C) with low curing
temperature (<160°C) and short curing time.
• Since the underfill wetting area << interface area at sample/fixture – failure always occurs at UF/PI interface
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
temperature chamber
clamp
failure always occurs at UF/PI interface.
65
129Shear Test Set-Up• Micro-mechanical test system:
DAGE Series 4000 with hot plate (temperature range 25°C - 300°C)– Fixed (hot plate &) test table– Full automatic test processFull automatic test process – Adjustable test parameters
• Adjustable parameters– Shear speed– Shear height– Load range
Tester: DAGE Series 4000
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Shear height
• The failure mode and adhesion results are very sensitive to the parameters setting such as shear height and shear speed.
Shear test sample
130Effect of Shear Height• For underfill/solder mask interface (UF/SM)
– Shear test at room temperature under different set-up conditions
ShearShear Height 20um 50um 100um 150um 200umAverage 7 014125 8 796125 15 329 11 60275 20 013
Load Speed 200um/s Str.Unit:KG
• Sample size: 16
Shear height
Average SM/UF Adhesion Shear Strength (KG) (shear speed 200um/s, different shear height)
89
10111213141516171819202122
Shea
r Stre
ngth
(KG
)
Average 7.014125 8.796125 15.329 11.60275 20.013Stdev. 1.227535 1.851912 8.304412 4.213173 6.81709
Shear Load Speed 200um/s,Room Temp.Failure Mode Vs. Shear Height
40%
50%
60%
70%
80%
90%
100%
(alo
ng S
M/U
F In
terfa
ce)
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
012345678
20um 50um 100um 150um 200um
Shear Height
Aver
age
– The adhesion strength and failure mode strongly depend on the shear height
0%
10%
20%
30%
20um 50um 100um 150um 200um
Shear Height
Failu
re M
ode
66
131
• Adhesion test results– Comparison between pull and shear at room temperature for UF/PI
Room Temp. (UF/PI) Pull and Shear Strength
Adhesion Results: Pull vs. Shear
2468
101214161820222426283032
Stre
ngth
(MPa
)
Pull
Shear
shear
pull + shearpull
φ (loading mode)
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Pull strength is lower than shear strength, but not significant
02
JM-A JM-B SumitomoUF-1 UF-2 UF-3
132
• Summary for shear test at room temperature for UF/PI
Shear Test Strength (PI/UF) At Room Temperature
40
Adhesion Test Results at Room Temperature
0
5
10
15
20
25
30
35
JM-A JM-B Sumito.
Moisture: 85°C/85RH
Shea
r Stre
ngth
(KG
)
0days11days17days21days
UF-1 UF-2 UF-3
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– No significant difference for the adhesion strength among three underfills at room temperature
– For these three underfills, the adhesion strength at room temperature is not sensitive to moisture
Moisture: 85 C/85RH
67
133
• Summary of adhesion tests at reflow temperature 220°C• UF/PI interface
UF/PI 220°C Shear Speed 200um/s Shear Height 30um
7
Adhesion Test Results at Reflow Temperature
1
2
3
4
5
6
Ave
rage
She
ar S
treng
th (K
G)
SumitomoJM-AJM-B
Sample size: 16
Failure mode: 100%
Stdev.: < 15%
Moisture absorption time: 21 days
UF-1UF-3
UF-2
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– Interfacial adhesion at UF/PI decreases with moisture absorption
0Dry 30°C/60RH 85°C/60RH 85°C/85RH
134
• UF/PI interface: Failure mode
Spacer
chip
UF
chip
UF Spacer
chip
UF Spacer
pull
Adhesion Test Results: Failure Mode
chipchip
U/F surface PI surface
chip chip
U/F surfacePI surface
chip
chip
U/F surface U/F surface
PI surface
chipchip chip
chipchipchip
shear
Si substrate U/F surface
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– Almost 100% failures occur at UF/PI interface under both pull and shear tests
Mode 1 Mode 2 Mode 3U/F surface PI surface
68
135
• UF/SM interface: failure mode
Mode 1 Mode 2 Mode 3
Adhesion Test Results
Mode 4 Mode 5 Mode 6
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– Failure modes are more complicated than that of UF/PI adhesion test.
136
Hygroscopic Swelling
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
69
137Hygroscopic SwellingHygroscopic Swelling85°C/85%RH
0 29%
Expansion strain due to hygroscopic
swelling
Expansion strain due to temperature change of 100°C
0 25%
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
~0.29% ~0.25%
εhygro=β∗C εthermal=α∗ΔTβ– the coefficient of hygroscopic swellingC – moisture concentration
Hygroscopic mismatch is comparable to thermal mismatch in causing mechanical stresses
138Validation: Moiré Interferometry Measurement Validation: Moiré Interferometry Measurement
Before HAST
Time zero before HAST at 85°CAfter HAST
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Time 168h after HAST at 85°C/85RH
70
139Hygroscopic Swelling-Induced DeformationHygroscopic Swelling-Induced Deformation
Time zero at 85°C Time 20h at 85°C/85RH
Time 60h at 85°C/85RH Time 200h at 85°C/85RH
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
140Hygroscopic Swelling Characteristics• Characterization Technique – TMA/TGA Method
–Machine• Two standard thermal analysis instruments (TGA & TMA)
–Specimen• Identical shape and size so as to ensure identical moisture absorption and
desorption rate
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
TGA TMA
71
141Hygroscopic Swelling CharacterizationHygroscopic Swelling Characterization
εave =Δh/hh
TMA
time
h
• TMA/TGA MethodThermal Mechanical Analyzer
Moisture absorption
MTGA
time
ΔM Cave= ΔM/V
0.0012
ave. strain
time
Thermal Gravimetric Analyzer
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
εave = βaveCave
βave – coefficient of hygroscopic swelling0
0.0004
0.0008
0.0 1.0 2.0 3.0 4.0 5.0ave. moisture concentration (mg/cm3)
142Hygroswelling Characterization Hygroswelling Characterization
• Method #1
Graphs of TGA and TMA
36 06
36.08
36.1
36.12
TGA (mg)
1.0363
1.0365
1.0367
1.0369
TMA (mm)
TGA
TMA
Graph of Strain vs. Concentration
y = 0.2223xR2 = 0.9946
0.0006
0.0008
0.001
0.0012
Strain
Method #1
36
36.02
36.04
36.06
0 200 400 600 800 1000Time (min)
1.0355
1.0357
1.0359
1.0361
0
0.0002
0.0004
0 0.001 0.002 0.003 0.004 0.005
Concentration (mg/mm3)
• Method #2
)()( thhthI Δ− satII thh )(−=ε
• Method #1
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
0
0
0
0 )()(h
thh
hthIave
Δ==ε
0
moisture
0
0 )()(V
tMV
MtMC I
ave =−
=
satave h
=ε
0
)(V
tMMC satII
ave−
=
72
143IssuesIssues
0.0012
ave. strain Uniform moisture distribution
Non-uniform moisture
0
0.0004
0.0008
0.0 1.0 2.0 3.0 4.0 5.0
ave. moisture concentration (mg/cm3)
distribution
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• What’s impact of non-uniform moisture distribution across the test specimen during measurement?
• βave = εave/Cave– Can the averaged βave represent the true material property of hygroscopic swelling?
144Theoretical PredictionsTheoretical Predictions
Material constant
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• The error can be as big as 250% due to moisture non-uniformity.
•Iave
IIave βββ ≤≤
73
145Experimental VerificationExperimental Verification
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Good Correlation between theoretical prediction and experimental results is obtained.
146Regression AnalysisRegression Analysis
0.003
0.0035
0.004
0)
βave(t2) > βave(t1) > βave(t0) t0
aveβ
0.0005
0.001
0.0015
0.002
0.0025
Stra
in (d
elta
_L/L
_0
βave(t0) = β
βave(t1) > βave(t0)
t1
t2
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
00 0.003 0.006 0.009 0.012 0.015
Moisture concentration (C_ave, mg/mm^3)
• Linear regression further introduces the randomness in determining the coefficient of hygroscopic swelling.
74
147Guideline in Hygroswelling CharacterizationGuideline in Hygroswelling Characterization
• Specimen fully saturated before measurement • Dry the specimen completely • Measure moisture loss and dimension change under
TGA and TMA for only two points – fully saturatedTGA and TMA for only two points – fully saturated and fully dry conditions
• β = (Lsat – Ldry)*V/Ldry*(Msat- Mdry)
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Hygroscopic Swelling and Free VolumeHygroscopic Swelling and Free Volume• How hygroscopic swelling is induced?
148
%3.0=ΔVV
Hygroscopic swelling induced volume change
• The hygroscopic swelling is only a small fraction of the total free volume S lli i d b t l l b d t th l
V
Free volume fraction
%30 ==V
Vf volumefree
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Swelling is caused by water molecules bound to the polymer matrix and not by free water molecules
• Free volume fraction estimate without considering swelling effect is acceptable
Cβε =swellingwaterbound
swelling Cβε =
75
149What Are Gaps Now?What Are Gaps Now?
• The dependency of the coefficient of hygroscopic swelling on temperature unknown
• The dependency of the coefficient of hygroscopic swelling on RH unknownswelling on RH unknown
• β = β (T, RH) ??? Will follow Arrehnius relation?
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Hygroscopic Swelling Measurement at Different Temperatures
Hygroscopic Swelling Measurement at Different Temperatures
150
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Ack: H. Shirangi, J. Auersperg, EuroSimE 2008, 455-462
76
151
chip
BLM/ILD Failure Mechanism Hypothesis during HAST
underfill
substrate
bump
HAST
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Due to hygro-swelling, tensile stress/strain is generated in BLM/ILD layer to cause delamination/cracking.
152BLM Stresses after HASTBLM Stresses after HAST
ILD stresses (MPa) at 85C after HAST
50
60 with moisturewithout moisture
0
10
20
30
40
Normal stress Shear Stress
stre
sses
(MPa
)
ILD stresses (MPa) at 25C after HAST
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu 0
1020304050607080
Normal stress Shear Stress
stre
sses
(MPa
)
with moisturewithout moisture
77
153Equivalent Coefficient of Thermal Expansion (CTE) –Linear Analysis
Equivalent Coefficient of Thermal Expansion (CTE) –Linear Analysis
• The hygroswelling introduces additional mismatch – Hydroswelling strain can be treated as additional thermal strain in
addition to thermal strain• Linear thermal stress analysis y
– Stress-free at T0 and cooling down (or heating up) to T1– The total ‘expansion strain’ = α (T1-T0) + β*C– Equivalent coefficient of thermal expansion
• α + β*C/ (T1-T0) • When vapor-pressure induced expansion is included
– Stress-free at T0 and cooling down (or heating up) to T1– The total ‘thermal strain’ = α (T1-T0) + β*C + (1 - 2ν ) p/ E– Equivalent coefficient of thermal expansion
β*C/ (T T ) (1 2 ) / E /(T T )
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• α + β*C/ (T1-T0) + (1 - 2ν ) p/ E /(T1-T0)
– The above analysis assumes• Linear analysis• Vapor pressure and moisture is uniformly distributed (the worst case)
154Moisture and Hygroswelling PropertiesMoisture and Hygroswelling Properties
1.55e-6
9.02e-6
D (mm2/s)
0.0329
0.0152
Csat (mg/mm3)
0.00720.22Underfill B
0.0027
Total hygro strain(CME x Csat)
0.18Underfill A
CME (mm3/mg)
Materials
2.13e-6
4.83e-5
2.79e-6
1.14e-5
0.00170.00430.4Mold Compound
0.00290.01430.2Solder Mask
0.00300.00750.4BT Substrate
0.0112 0.00350.31Underfill C
Mold Compound Die Attach
Total Strain Equivalent meanC ( / C) Total Strain Equivalent mean
C ( / C)
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Total Strain CTE (ppm/°C) Total Strain CTE (ppm/°C)Thermo-
mechanical 1.53e-3 34 7.65e-3 170
Hygro-mechanical 1.57e-3 34.9 3.22e-3 71.6
78
155Integrated Package Stress and Warpage (QFN Package)
Integrated Package Stress and Warpage (QFN Package)
Package Warpage during Reflow
Thermo-mechanical
Warpage (mm)
Thermo-mechanical
Warpage (mm)
Hygro-mechanical
Vapor Pressure
Hygro-mechanical
Vapor Pressure
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Thermo-mechanical model has upward warpage, opposite in direction as compared to hygro-mechanical, and vapor pressure induced.
IntegratedIntegrated
156SummarySummary
• Polymer expands upon absorbing moisture –hygroscopic swelling
• Hygroscopic strain is comparable to & may be larger th th l t ithan thermal strain
• ε = β C: β - hygroswelling coefficient – can be measured by TGA/TMA
• Hygroscopic swelling and adhesion reduction are main failure mechanisms under HAST
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
79
157
Electrochemical Metal MigrationElectrochemical Metal Migration
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
158Electrochemical Migration under BiHASTElectrochemical Migration under BiHAST• Conditions
– Moisture absorption/condensation – Voltage – Contamination
• Two different failure mechanismsTwo different failure mechanisms– On surface: dendritic growth
• Electrolytic dissolution of metal at anode followed by the reduction and deposition of metal ion at cathode
– Below surface: conductive anodic filament (CAF)/ conductive filament formation (CFF)
• Growth initiates at anode and proceeds along separated fiber/epoxy interface
dendrites
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Ref: Katayanagi et al. ESPEC Japan Tech-info Field Report #5, 1996
+ -–
CAFe–
SR
• Moisture provides electromigration transport path
80
159
@ V ≥ 1.23V – electrolytic decomposition of water:H2O → H+ + OH–
• At the Anode: oxidation and metal lossproduction of hydronium ion:
↑
Fundamentals of Electrochemical MigrationFundamentals of Electrochemical Migration• Dendritic growth
H2O → ½O2↑ + 2H+ + 2e–
electrolytic dissolution of metal:M → Mn+ + ne–
Cu → Cun+ + ne– (n = 1,2)• At the Cathode: reduction and protective effects
production of hydroxyl ion:2H2O + 2e– → H2 ↑ + 2OH–
reduction of metal:Mn+ + ne– → MCun+ + ne– → Cu (n = 1 2)
Changes in pH in the vicinity of the electrodes
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Cun + ne → Cu (n = 1, 2)
The production of H+ at the anode and OH− ions at the cathode creates a pH gradient between these electrodes
The Electrochemical Cell
160
• From the Pourbaix diagram for copper- pH 7 to 11: copper is passivated; no corrosion will occur- < pH 7: copper corrosion occurs at potentials greater than 0.2V- > pH 5: solubility of copper ions declines rapidly, becoming nearly insoluble at ~ pH 8.6
• At the anode, H+ caused a drop in local pH → soluble Cun+
• At the cathode, the copper ions become insoluble and precipitate out
Fundamentals of Electrochemical MigrationFundamentals of Electrochemical Migration
• Electrical failure occurs when contact is made
• If bias voltage is removed prior to contact, growth will terminate due to the cessation of the electrochemical reaction
, pp p p
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Simplified Pourbaix Diagram for Copper
81
161
200X500X
Area 1AnodeAnode
200X
500XArea 2
CathodeCathode
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- Several Cu dendrites were observed - Numerous areas with extremely small Cu
migration particles were also observed (red arrows)
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500X
Anode
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- Several Cu dendrites were observed
200XCathode
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Ni Migration (bulk)
Examples: Metal Migration TypesExamples: Metal Migration Types
Surface and bulk metal migration
Sn Migration (surface)
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Cu Migration (bulk)
164HAST Metal Migration - ContinuedHAST Metal Migration - Continued
Surface Migration: in plane
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*Sn/Sn oxide stains on the periphery of pins with the solder bridging material.
Crystal particles ready as nuclei for further growth due to water condensation
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Anode Reactions
1. Metal oxidation creating metal ions that migrate towards the cathode. When the metal ions reach the cathode, dendritic growth can occur.
2. Halide ion breakdown of the passive film creating freshly exposed metal surface
Dendritic GrowthDendritic Growth
surface. 3. The oxidation can eventually lead to an open circuit failure.
Cathode Reactions
1. Chemically formed metal oxides or hydroxides followed by dissolution of these species. This will lead to erosion of the cathode causing an open circuit failure.
2 Reduction of dissolved metal ions leading to nucleation and growth of
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2. Reduction of dissolved metal ions leading to nucleation and growth of metal dendrites. This would lead to the formation of anode-cathode short failure.
John W Osenbach, Semicond.Sci. Technol. 11
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• Electro-dissolution (anode)• Ion transport
Metal migration process:
Summary: Metal Migration Mechanism
• Electro-deposition (cathode)
Dendrites growth:
• Initiation• Propagation
Dendrites shape: • flow of species involved in the
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John W Osenbach, Semicond.Sci. Technol. 11
flow of species involved in the electrolytic process
• path of current flow (electric field)
• Critical over-potential or critical current density
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167Conductive Anodic Filament (CAF) FailureConductive Anodic Filament (CAF) FailureAnode Cathode
Glass fiber
CAF
Epoxy resin
Thru-Via
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• Electrochemical corrosion process that grows from anode to cathode along delaminated fiber/epoxy interfaces
• Dropping pH value at anode results in soluble Cu corrosion product. This corrosion product proceeds through any weak interface/voids/opening from anode to cathode due to the pH gradient
168Conductive Anodic Filament (CAF) FailureConductive Anodic Filament (CAF) Failure
a: hole-to-hole b: hole-to-trace
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c: trace-to-hole d: trace-to-trace
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169
(-)
SEM on X-section
Example: Via to ViaExample: Via to Via
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
(+)CAF growth
Void
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x-section
Prepreg
(-) (+)
Example: Via to TraceExample: Via to Trace
SM
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu Intel package FA
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X-section
Example: CAF FailureExample: CAF Failure
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
172Dendritic Growth vs. CAF Growth Dendritic Growth vs. CAF Growth
• A dendrite can be observed on the surface of PCB & substrate. However, CAF can occur in sub-surface associated with glass fibers/epoxy resin interface.
• A CAF grows from an anode to a cathode. However, a dendrite grows from a cathode to an anode with needle like/tree like shape.
• A CAF is made from soluble copper salt at anode and built at anode by turning to insoluble salt due to pH effect.
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
Dendrite occurs as a result of solution at anode and plating at cathode.
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ENVIRONMENT MATERIALS PROCESS
Reaction Mechanism (schematic) Acceleration Factors
Water adsorption and diffusion
• Moisture content• Temperature• Material quality
Changes in pH due to • Voltage
Contributory Factors Contributory Factors
Contributory Factors:• En ironment Temperat re H midit Voltage Contaminants
the electrolysis of water (acidization)
• Moisture content• Temperature
Copper elution and copper ion diffusion (diffusion)
• Voltage• Moisture content• Material quality• pH, impurity ions• Dissolved oxygen content
Electron transfer and ion migration (reduction)
• Voltage• Material quality• pH, impurity ions
Ref: Katayanagi et al. ESPEC Japan Tech-info Field Report #5, 1996
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• Environment – Temperature, Humidity, Voltage, Contaminants• Materials – Package Materials Selection and Suppliers
substrate dielectrics, solder resist, flux and flux residue, Cu-plating chemistry, Cu metallization (line width/spacing and geometry) underfill chemistry (water adsorption isotherm, ionic content/contamination, CTE), etc.
• Process – Package Assemblybaking, fluxing, soldering, cleaning, etc.
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Foreign materials/defects/contaminants sources• materials-based sources – substrate manufacturing process• assembly processes – residual ionic contaminants• environmental sources – airborne dusts, etc.
Contributory Factors Contributory Factors
Foreign materials and defects contribution to ECM failure – extrinsic factors• conductive bridges• trace defects – extraneous material, mouse bites, breaks• moisture condensation sites
+d
+d +d
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−d
−d
−d
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175SummarySummary
• Moisture provides metal migration /electron transport path. In a dry environment, metal migration is not a concern at all
• If bias voltage is removed prior to contact growth willIf bias voltage is removed prior to contact, growth will terminate due to the cessation of the electrochemical reaction
• Contamination changes local pH environmental conditions and thus accelerate the metal migration. For example, Na+ residue from SR developing solution and flux
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
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SummarySummary
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
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177Summary Summary
• Moisture Related Reliability Tests • Moisture absorption, desorption, and diffusion• Vapor pressure modelp p• Case study I – underfill selection for FC BGA packages• Case study II – delamination/cracking in stacked-die
chip scale packages• Accelerated moisture sensitivity test• Effect of moisture on material properties
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
• Hygroscopic swelling• Electrochemical metal migration
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ReferencesReferences
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
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ReferencesReferences• General
– X.J. Fan, “Moisture related reliability in electronic packging”, 2005/2006/2007 ECTC Professional Development Course Note, 2005/2006/2007
– X.J. Fan, “Mechanics of moisture for polymers: fundamental concepts and model study”, 9th IEEE International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, (EuroSimE), April 20-23, 2008, pp159-172X J F J Zh d A Ch d “P k i t it l i ith th id ti f i t
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– X.J. Fan, J. Zhou, and A. Chandra “Package integrity analysis with the consideration of moisture effects”, 58th Electronic Components and Technology Conference (ECTC), 2008
– G.Q. Zhang, W.D. van Driel, and X.J. Fan, “Mechanics of Microelectronics”, Springer, 2006• Moisture diffusion modeling
– B. Xie, X.Q. Shi, X.J. Fan, and H. Ding, “Direct concentration approach of moisture diffusion and whole field vapor pressure modeling for reflow process”, submitted, 2008
– B. Xie, X. Q. Shi, and X.J. Fan, Sensitivity investigation of substrate thickness and reflow profile on wafer level film failures in 3D chip scale packages by finite element modeling, 57th Electronic Components and Technology Conference 2007, ECTC '07, 2007, p 242-248
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
p gy p– T.Y. Tee, X.J. Fan and T. B. Lim, “Modeling of whole field vapor pressure during reflow for flip chip
and wire-bond PGBA Packages”, 1st International Workshop on Electronic Materials & Packaging, 1999
– J.E. Galloway and B.M. Miles, “Moisture absorption and desorption predictions for plastic ball grid array packages”, IEEE Transactions on Components, Packaging and Manufacturing Technology, Part A, 20(3), pp. 274-279, 1997
ReferencesReferences• Moisture diffusion modeling (cont’d)
– E. H. Wong, Y. C. Teo, and T. B. Lim, “Moisture diffusion and vapor pressure modeling of IC packaging”, 48th Electronic Components and Technology Conference, pp.1372-1378, 1998.
– T. Y. Tee and Z. W. Zhong, “Integrated vapor pressure, hygroswelling and thermo-mechanical stress modeling of QFN package during reflow with interfacial fracture mechanics analysis”, Microelectronics Reliability, Vol. 44(1), pp. 105-114, 2004.
• Characterization, adhesionX J Fan J Zhou and A Chandra “Package integrity analysis with the consideration of moisture
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– X.J. Fan, J. Zhou, and A. Chandra Package integrity analysis with the consideration of moisture effects”, 58th Electronic Components and Technology Conference (ECTC), 2008
– Y. He, and X.J. Fan, “In-situ characterization of moisture absorption and desorption in a thin BT core substrate”, Electronic Components and Technology Conference, pp. 1375-1383, 2007
– X.Q. Shi, Y.L. Zhang, W. Zhou, and X.J. Fan, “Effect of hygrothermal aging on interfacial reliability of silicon/underfill/FR-4 assembly”, IEEE Transactions of Components and Packaging Technologies, 2008, 31(1), 94-103
– H. Shirangi, J. Auersperg et al, “Characterization of dual-stage moisture diffusion, residual moisture content and hygroscopic swelling of epoxy molding compound EuroSimE 2008 455-462
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
moisture content, and hygroscopic swelling of epoxy molding compound, EuroSimE 2008, 455 462– T. Ferguson and J. Qu, “Moisture absorption analysis of interfacial fracture test specimens
composed of no-flow underfill materials”, Journal of Electronic Packaging, Vol. 125, pp 24-30, 2003.
– S. Luo and C.P. Wong, “Moisture absorption in uncured underfill materials”, IEEE Transactions of Components and Packaging Technologies ,Vol. 27, No.2, 345-351, 2004
– S. Luo and C.P. Wong, “Influence of temperature and humidity on adhesion of underfills for flip chip packaging”, IEEE Transactions of Components and Packaging Technologies ,Vol. 28, No.1, 88-94, 2005
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ReferencesReferences• Vapor pressure, moisture sensitivity test correlation
– X.J. Fan, G.Q. Zhang, and L.J. Ernst, “Interfacial delamination mechanisms during reflow with moisture preconditioning”, IEEE Transactions of Components and Packaging Technologies, 2008 (in press).
– X.J. Fan, J. Zhou, G.Q. Zhang and L.J. Ernst, “A micromechanics based vapor pressure model in electronic packages”, ASME Journal of Electronic Packaging, 127 (3), pp. 262-267, 2005.
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– X.J. Fan; G. Q. Zhang; W. D. van Driel; L. J. Ernst, Analytical solution for moisture-induced interface delamination in electronic packaging, Electronic Components and Technology Conference, 2003, May 27-30, 2003 Page(s):733-738
– E. Prack and X.J. Fan, “Root cause mechanisms for delamination/cracking in stack-die chip scale packages”, International Symposium on Semiconductor Manufacturing (ISSM), 2006, September 25 - 27, Tokyo, Japan.
– X.Q. Shi and X.J. Fan, “Wafer-level film selection for stacked-die chip scale packages”, Electronic Components and Technology Conference, pp. 1731-1736, 2007
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– X.J. Fan and T. B. Lim, “Mechanism analysis for moisture-induced failures in IC packages”, ASME 1999 International Mechanical Engineering Congress, IMECE/EPE-14, 1999.
ReferencesReferences• Hygroscopic swelling
– X.J. Fan, J. Zhou, and A. Chandra “Package integrity analysis with the consideration of moisture effects”, 58th Electronic Components and Technology Conference (ECTC), 2008
– X.J. Fan, “Mechanics of moisture for polymers: fundamental concepts and model study”, 8th IEEE International Conference on Thermal and Mechanical Simulation and Experiments in Microelectronics and Microsystems, (EuroSimE), April 20-23, 2008
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– T.Y. Tee, C. Kho, D. Yap, C. Toh, X. Baraton, Z. Zhong, “Reliability assessment and hygroswelling modeling of FCBGA with no-flow underfill” Microelectronics Reliablity, 2003, pp. 741-749.
– H. Ardebili, E.H. Wong, and M. Pecht, “Hygroscopic swelling and sorption characteristics of epoxy molding compounds used in electronic packaging”, IEEE Trans. Comp. Packag. Technol., Vol. 26, No. 1 (2003) pp. 206-214.
– E.H. Wong, K.C. Chan, R. Rajoo, T.B. Lim, “The mechanics and impact of hygroscopic swelling of polymeric materials in electronic packaging,” Proc. 50th Electron. Comp. Technol. Conf., Las Vegas, NV, 2000, pp. 576–580.
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
– J. Zhou, “Investigation of non-uniform moisture distribution on determination of hygroscopic swelling coefficient and finite element modeling for a flip chip package, IEEE Transactions of Components and Packaging Technologies, 2008 (in press)
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ReferencesReferences• Accelerated moisture sensitivity test
– X.Q. Shi, X.J. Fan, B. Xie “A new method for equivalent acceleration of JEDEC moisture sensitivity levels”, 58th Electronic Components and Technology Conference (ECTC), 2008
– B. Xie and X. Shi, and X.J. Fan, “Accelerated moisture sensitivity test methodology for stacked-die molded matrix array package”, Proceedings of IEEE 9th Electronics Packaging Technology Conference, p.100-104, December 2007
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– R. Shook, T. Conrad, V. Sastry and D. Steele, “Diffusion Model to Derate Moisture Sensitive Surface Mount IC’s for Factory Use Conditions”, IEEE Transaction on Components, Packaging and Manufacturing Technology, Vol. 19, No. 2, pp. 110-118, 1996.
– R. Shook, R. Vaccaro and D. Gerlach, “Method for Equivalent Acceleration of JEDEC/IPC Moisture Sensitivity Levels”, Annual International Reliability Physics Symposium, pp. 214-219, 1998.
• Chemical-electromigration – J.W. Osenbach, “Corrosion-induced degradation of microelectronic devices”, Journal of Semicond.
Sci. Technol., 11, 155-162, 1996
Xuejun Fan Moisture-Related Reliability xuejun.fan@lamar.edu
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End of the Course
Questions?
End of the Course
Questions?
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