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James E. MorrisDepartment of Electrical & Computer Engineering
Portland State University
P.O.Box 751, Portland OR 97207-0751, USA
IsotropicElectrically Conductive Adhesives
5/7/2012
ICA: Isotropic Conductive Adhesives
Current pathPowder
Flakes (Ag)Polymer
ICA: Flip Chip Interconnect
ICA: SMT Interconnect
Contents•Introduction
•Applications•Processing
•Resistance & measurement•Nanotechnology•Layering effect•Adhesion•Drop test•Galvanic corrosion•Cure models• Miscellaneous
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SMT joint showing Ag flakes
Flip-chip joint showing Ag flakes: Top: Si chip bump Bottom: PWB pad
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ICA applications:Solder replacement in SMT &
flip chip
Bosch Diesel Fuel Injection Pump
ICA Smart Cards & RF ID (IZM-Berlin)
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ICA Microvias: Ag, Cu, & LMP(Low Melting Point) Alloy
[Das et al (Endicott Interconnect), ECTC’06]
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Failure modes →James E. Morris OSU Seminar5
(Sintered NanoParticle) ICA Die Attach [Bai et al (VaTech), HDP’05]
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ACA: Anisotropic Conductive Adhesive (& ACF)IC
BumpAdhesive
Glass substrateElectrode
Conductive particles
Ni particleCoated polymer particleJames E. Morris OSU Seminar7
Technology Drivers
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Environment Environmental:
- No-flux, no-Pb
No-Pb solders -Melting temperatures
-Thermomechanical stress
Silver effects
Economics Manufacturing
Fewer process steps
Available technologies (stencil/screen/dispense)
Technology drivers(c.f. solder)
Thermomechanical stress Low process temperature High compliance
Fine-pitch (area array) interconnect Smaller particles than solder No slumping Inter-metallic diffusion in solders Electromigration
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Processinge.g. Flip-Chip Assembly (IZM, Berlin)
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Cure Process
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Log Time
Tem
per
atu
re
Tg0
gelTg
Tg∞
Liquid
Sol/Gel Rubber Gel Rubber
Gel Glass
Char
Sol/Gel GlassGelation
Vitrification
Complete CureVitrification
Devitrification
Tc>T∞
Tc<T∞
Sol Glass
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Thermal Cure: Optimization
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20
40
60
80
100
120
140
160
0 50 100
Time in min
Te
mp
era
ture
in º
C
Typical specification:
Temperature T and time t
Probably a range of T & t combinations
Possibly ramp rates dT/dt
Pre-cure thermal soak
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Cure Effects
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Adhesive &Type
Viscosity(Pa.s)
Potlife Glasstransition
VolumeResistivity
cm
Shear strength(MPa)
Curingtime
(min.)
CuringTemp.
(C)
ICA 1(1 comp.)
65 3 days 80 2.10-4 11 10-15 130-150
ICA 2(2 comp.)
25-35 3-4 days 85 1-4.10-4 10 30 130-140
ICA 3(2 comp.)
50 2 days 50 2-4.10-5 8 90 130-140
ICA 4(1 comp.)
160-200 4 days 80 5.10-3 5 20-30 120-130
ICA 5(1 comp.)
310-350 4 days 80 1.10-3 4 20-30 120-130
ICA 6(2 comp.)
150-200 8-12 hr 75 2-5.10-4 14 10-15 170-180
Compare intrinsically conducting polymers, e.g. Polyaniline: ρ = 105 (intrinsic) to 10-2 .cm (doped)
Compare Ag: ρAg = 1.6 x 10-6 .cm; ρICA = 12.5 to 312.5 x ρAg
Properties Commonly used isotropic conductive adhesives(Jagt, Chapter 11, Conductive adhesives for Electronics Packaging)
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Glass Transition
Temperature
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Compare CTEs:Si = 4-5 ppm/°CAg = 20 ppm/°CEpoxy = 54 ppm/°CFR4 = 36 ppm/°C
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Low Resistance Measurement
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Wheatstone bridgeJames E. Morris OSU Seminar15
Electrical Test: Four-terminal measurement
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Kelvin probe
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Three terminal measurement for pad contact resistance
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(a) Current Crowding(b) EquallyDistributed Current
Current crowding & contact shorting
(a)
(b)
(c)
(d)
4-point measurement using ICA track across proto-board current lines (a) Sense lines short ICA (b) thinned contacts (c) trimmed contacts (d) Surface point contacts
(a)
(b)
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ICA Resistance: Percolation
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Polymer Metal
Percolation threshold
Insulating
conducting
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Experimental Percolation
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Flakes: surface/vol ratio
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Percolation Threshold Comparison (Sancaktar )
Sphere/Powder Flakes
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Bi-modal: Flakes + powder
e.g.
10μm diameter flakes
(<1μm thick)
plus
1-3μm diameterpowder
(spheres)
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ICA Resistance
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Percolation Particle μm particles 10’s nm e- mfp Negligible size effects
Inter-particle And pad contact
Experimental (Modeling)
ECA ResistanceProtective
of Corrosion
Bulk Resistance
Contact
Percolation PathPossible Penetration of Oxide
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Monte Carlo Percolation Models
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Size effect in percolation
←p=0.5↓p=0.7
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Particle Contact Resistance
Constriction resistance: R = /2a Current crowding Circular contact Diameter 2a
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TEM of contact between nano-particles c. 7nm
diameter
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Polymer/Insulating Oxide
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Thermionic emission (with Schottky effect)
Electron tunneling (with image effect)
Poole-Frenkel emission
Fowler-Nordheim tunneling
TCR 0 & field effects for various mechanisms None of these observed
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AgO/Ag2O Degenerate Semiconductor
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Intrinsic Extrinsic Degenerate
Silver oxide (degenerate semiconductor) Also 1/a2
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Carbon on flake surface appears after cure:
Changes in peak intensities with temperature during cure
Miragliotta, Benson, Phillips, & Emerson: MRS ’01, 2002 Materials
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RHS: 744 cm-1 carboxylate peakLHS: 1600cm-1 C peak
(a) uncured (b) cured
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TCR Results
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TCRICATCRAg
RICA RAg
Contact R negligible
Resistivities for various temperatures.
(a) Brass stenciled CT-5047-02 thermoset sample (TCR-0.0039/ºC).
(b) (b) CSM-933-65-1 screen printed thermoplastic sample {TCR=0.0038/ºC}
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AC Effects: Sub-Threshold
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R vs f
L vs 1/f
Frequency Effects: Skin Effect Tunnel Gaps
Low frequency: Solder resistance < ICA resistanceHigh frequency: Solder resistance > ICA resistance
(because areas converge and ρAg << ρsolder
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0.0000001
0.000001
0.00001
0.0001
0.001
0.01
0.1
1
10
100
1000
0 1 2 3 4 5 6
lg(f^0.5)
Imp
ed
an
ce/m
Oh
m
RICA=RDC+R/80 RSolder=RDC+R/80 wL_ICA - ICA I path wL_Solder wL_ICA full area
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Skin Effect
SolderICA
ICA
Solder
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Nanotechnologies, e.g. Add CNTs
Electrically conductive fillers with CNT
(Courtesy Prof. Zhaonian Cheng, SMIT Center, Shanghai University)
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Negligible effect
Nanoparticles in Isotropic Conductive Adhesives (ICAs) (Stefan Kotthaus)
Percolation limits of different ICA filler particles5/7/2012James E. Morris OSU Seminar34
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Nanoparticle Sintering
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[Wong et al, ECTC’06]
Raut, Bhagat, Fichthorn, Nanostruct. Mater. 10(5) 1998, 837-851
Layering Effect
Constable et al (Adhesives’98)
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Z-axis Size Effect Expt
Resistivity as a function of the thickness.
Resistivity
0.00E+00
1.00E-03
2.00E-03
3.00E-03
4.00E-03
5.00E-03
6.00E-03
7.00E-03
8.00E-03
9.00E-03
0 0.5 1 1.5 2
h/mm
cm
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Adhesion
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(Detert, Herzog, Wolter, Zerna (TU-Dresden))
Danfoss
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Shear test result with plasma treatment
0
20
40
60
80
100
120
140
Shear strength (%)
M. Tr. Ox. - Ni. Tr. Ox. Tr. Ox. + Hy.Tr.
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Upper two columns show the standard deviation
M. Tr. - Mechanical treatment
Ox. - Ni. Tr. - Oxygen Nitrogen treatment
Ox. Tr. - Oxygen treatment
Ox. + Hy. Tr. - First oxygen treatment then hydrogen treatment
(1230psi)
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Adhesion: Surface Roughness(Chow, Li, Yuen: 2002 Adv Pkg Materials Sympos)
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Adhesive before and after vacuum process
0
50
100
150
200
250Shear strength
(%)
M. Tr. Ox. Tr. + 2hrvac. afterprinting
2hr. vac.beforeprinting
2hr. vac.after printing
ICA before vacuum process
ICA after vacuum process
Change of surface by:•removal of air and water•contraction of adhesive
100 = (1230 psi)
No Vacuum Vacuum
Average Resistivity cm1022.4 3 cm1092.2 3
Standard Deviation
cm1029.3 3 cm1053.1 3
No
Vacuum Vacuum
Average Pull Test strength
160 N 235 N
Standard Deviation
108 N 56 N
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Thermal Cure: Optimization
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20
40
60
80
100
120
140
160
0 50 100
Time in min
Te
mp
era
ture
in º
C
Typical specification:
Temperature T and time t
Probably a range of T & t combinations
Possibly ramp rates dT/dt
Pre-cure thermal soak
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Impact Strength/Drop Test (Tong)
Adhesion tests passed
Devices fall off PWB if dropped Large devices fail; small OK
No correlation between drop test results and adhesion strength testing
Complex shear modulus Storage and loss moduli
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2
tt
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Variation with inertial mass
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Blocks Drops till failure1 More than 120 (no failure)
2 30, 433 17, 214 7, 4,115 4, 46 3
y = 124.7e-0.6556x
R2 = 0 .9712
0
10
20
30
40
50
60
70
0 1 2 3 4 5 6 7
n u mb e r o f we ig h t b lo c ks
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Multiple Drop Testing:1. Cumulative damage
2. Pre-heat effects
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Drop test results for PLCC68 Samples cured at 150°C
0
10
20
30
1 2 3 4 5 6
Heig ht in f eet
Variable number oftest survived.
Def inate number oftests survived.
Drop test results for PLCC68 cured at 150°C w ith pre-heating and post-
cooling.
0102030
1 2 3 4 5 5.6
Heig ht in f eet
Variable number oftests survived
Definate number oftests survived
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Contact R 85°C/85% RH: Au & Cu contactsGalvanic Corrosion: Dissimilar metals + H2O
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A on Au
A on Cu
C on Au
C on Cu
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ICA: Lee, Cho, & Morris, Proc. EMAP 2007Ag-ICA 85/85 performance comparison on Cu and immersion-Ag PWB contacts
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A1Cu2
B1Cu1
BiCu2A1Cu1/All Ag
A3Cu2
B3Cu1
B3Cu2A3Cu1/All Ag
185oC cure 185oC cure
170oC cure 170oC cure
B1Cu1
B1Cu2A1Cu2A1Cu1/All Ag
A3Cu1
A4Ag1 B3Cu1
A3Cu2/B3Cu23x Ag
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•Contact Resistance Variation
• Ratio Variation of Resistances
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Creep & Coffin-Manson Lifetimes: Comparisons with
Solder (Rusanen)
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1
10
100
1 000
10 000
0.01 0.10
Non-recoverable strain range
Nf(
50) i
n nu
mbe
r of c
ycle
s
ICA (Rusanen &Lenkkeri 1999)
ICA (Rusanen 2000)
Sn63Pb37 (Doi et al.1996)
Sn63Pb37 (Dudeket al. 1997)
Sn5Pb95 (Darveaux& Banerji 1991)
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0.4 0.6 0.8 1
0
20
40
60
80
[mm]
For
ce [N
]
Multiple plateaus
Primary plateau ~15N (all samples)
Parallel slopes (epoxy shear modulus)
Cure Models
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Reaction rate d/dt = K f(), where K = A exp-(E/RT) K = chemical rate constant, f () reactant concentration. = degree of cure R=Gas constant=8.31J/K.mole=Nk=6.023x1023/mole x 1.38x10-23J/K
n-th order model: f() = (1 - )n
Calculate degree of cure (for constant T, i.e. isothermal cure): 1st order: d/dt = K(1- ), = 1-exp(-Kt) 2nd order: d/dt = K(1- )2, = 1 – [1 + Kt]-1
nth order: d/dt = K(1- )n, = 1 – [1 + (n-1)Kt]-1/(n-1)
Rate proportional to reagent mass available n=1/2 Phase boundary reaction(area) n=1/3 Phase boundary reaction(volume) n=2/3 Nucleation sphere (volume growth)
Auto-catalyzed model: Single-step: d/dt=K f()=K m (1- )n …….. but note d/dt=0 for α=0 Double step (linear combination): d/dt=(K1+K2 m)(1- )n
Note: ONLY model with more than a single activation energy E Modified double step: d/dt=K f()=K(y1+y2 m)(1- )n where y1+y2=1
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DSC: Directly determine degree of cure αand rate of cure dα/dt
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Cure Modeling:3 Adhesives
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A
B
C
Simple 1st order cure models work well
Manufacturer profiles → incomplete cure
Degree of Cure vs. Time
Resistance vs. degree of cure
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Particle Swarm Optimisation
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Fitness
Results – Optimal Coefficient Sets
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Model E1 A1 n m E2 A2 y1
1st order 37882 1021.64 1 ‐ ‐ ‐ ‐
2nd order 35917 630.83 2 ‐ ‐ ‐ ‐
3rd order 38364 841.82 3 ‐ ‐ ‐ ‐
Single step auto 31017 711.40 1.536 1.118 ‐ ‐ ‐
Double step auto 37039 666.04 0.767 3.958 309930 854.81 ‐
Modified double 33983 1074.40 1.326 1.115 ‐ ‐ 0.0393
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Results – Cure variation
55 Cure rate– Emerson and Cuming CE3103 conductive adhesive material
Basic (BADGE) curing reactions
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Etherification significant only at high T Model by 2-step auto-catalytic? For multiple reactions, e.g. 3 above
gcureondaryprimary
diffusiontionetherificaondaryprimaryeffective
TKK
KKKKK
T lowat 11
....11111
sec
sec
I
II
III
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Current (to 10A) and surface temperature
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50
100
150
200
250
0 50 100 150 200 250 300
temp/centigrade
resi
sta
nce
/mO
hm
HighCurrent Predicted with TCR
0
50
100
150
200
250
0 20 40 60 80 100 120
current ^2 / I^2
Surface T vs I2
R predicted by TCR
Measured RR vs Surface T
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Ablebond 84 - 1 LM1
0
50
100
150
200
250
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15I [A]
R [m ]sample #1
sample #2
Electric Field for Fake Alignment
Adhesive “F”
Adhes
ive F
Adhes
ive C
Adh
esive
Bwithout field
during cure
before cure0.0%
50.0%
100.0%
R/R
ma
x
Configuration C
0.8
0.85
0.9
0.95
mO
hm
Electric Field
Adhesive FConfiguration A
before
during
without
Only adhesive F was used for this test.
All measured resistances were in a range of 0.7mOhm to 1.2mOhm
There was no significant drop in volume resistance of the cured samples, whether supplied with an electric field before or during cure, or not.
Previous experiments (different ICA) found significantly lower R.
Adhesive F No significant difference with and without applied field.
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Imprint Boards
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Summary: ICA Applications; processing
Cure modeling
Electrical conduction Sources of resistance; measurement; layering effects
TCR/frequency data
(Conduction modeling)
Nanoparticles (sintering) & CNTs
Reliability issues Impact resistance
Adhesion
Continuing research Cure modeling
Mechanical cycling
Drop test & crack propagation modeling
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