Trend of Solder Alloys Development

Dr. Ning-Cheng Lee

Indium Corporation

1

Solder Is The Choice of Most Electronic Bonding For Years To Come

Property Soldering Wire Bonding Conductive Adhesive

Reliability Good Good Medium

Electrical resistivity(10-6 ohm-m) 0.1-0.2 0.016-0.027 0.1-10

Thermal conductivity(W/m.K) 35-70 200-400 1.3-2.2

Bonding temperature Medium Medium to high Low

Pitch constraint None Bonding tool size Metal heterogeneity

Footprint Area array Linear Area array

Cost Low Low to high Medium

Bonding speed Mass reflow, fast Slow Medium

Reworkability Good Poor Poor2

(Toleno)(Aprova)(Indium)

Popular Solder Alloys

3

300C

250C

200C

150C

92.5Pb5Sn2.5Ag

10Sn88Pb2Ag

97.5Pb2.5Ag

10Sb85Pb5Sn

92.5Pb5In2.5Ag

97Pb3Sn; 95Pb5Sn

90Pb10Sn80Au20Sn

99.3Sn0.7Cu96.5Sn3.5Ag

63Sn37Pb; 62Sn36Pb2Ag

95Sn5Sb

89Sn8Zn3Bi 91.8Sn3.4Ag4.8Bi 95.5Sn4Ag0.5Cu; 95.5Sn3Ag0.5Cu

58Bi42Sn

High Pb

Pb-free Pb

1st Level Interconnect

2nd Level Interconnect

Solder bump Cu pillar

(2012)

Electronic Solder Joint Feature Consideration

• Solder joint feature requirement– Corrosion resistance (PCB level reliability)– Shock resistance (PCB level reliability)– Thermal fatigue resistance (PCB & package level reliability)– Heterogeneity stability (PCB & package level stability)– Electromigration resistance (> 103 amp/cm2 application, PCB & package level reliability)– Composition stability (TLPB, Au, Cu, package level reliability)

1µ10µ100µ1000µ

PCB level Package level

Joint dimension

ShockThermal fatigue

Composition stabilityElectromigration

Corrosion

Heterogeneity

4

Business Drivers

• Smaller & smarter– Smaller feature size

(smaller, thinner, lighter)– Higher I/O density– Faster

• More reliable– Non-fragile– Fatigue resistant– Stable & uniform

microstructure– Corrosion resistant– Electromigration resistant

• Lower cost– Lower processing cost (air,

lower temperature, shorter cycle time, higher yield)

– Cheaper material (solder, components, boards)

• Green– Less toxic (Pb-free)– Low carbon

• Higher power density– Higher temperature

tolerance, higher current resistance

5

More Oxidation Resistant, Lower Surface Tension, Higher Fluidity

• Smaller & smarter (driver 1)– Smaller feature size

(smaller, thinner, lighter)

6

Although solder size can shrink with feature size, solder oxide thickness does not shrink, unless solder is more oxidation resistant.

Anti-oxidation dopants such as P, Ge, desired for alloying. Ingredients prone to oxidize should be avoided.

Since oxide thickness on parts also does not shrink, solder need to wet better, with lower surface tension or higher fluidity. Elements such as Bi, Ni, Co beneficial.

solder

0

0.5

1

1.5

2

2.5

0.5 0.52 0.54 0.56 0.58

Surface Tension (N/M)

Wet

ting

Tim

e (s

ec)

Lower Melting Point, Lower Process Temperature For Miniaturized Applications

• Smaller & smarter (driver 2)– Higher I/O density

• Lower processing cost (lower temperature)

7

(Intel)

(Intel)

• Reduced via-via spacing increase risk of CAF (conductive anodic filament) formation due to thermal damage.

• A lower melting temperature solder desired, possibly with addition of Bi, In, Zn, etc.– e.g. 57Bi42Sn1Ag (138-140C)

Lower Hardness For Portable Devices. Dopants Reduce And Stabilize IMC

• More reliable (driver 3)– Non-fragile

8

• Small joints more vulnerable to shock.

• Joints with low fragility desired– Lower hardness (such as

low Ag SAC)– Dopant which reduce IMC

thickness or fragility, such as Mn, Ti, Ni, Co, Pt

– Dopants which reduce Kirdendall void formation or spalling, such as Ni, In, and high Cu

Drop Test Results of As-Reflowed Samples (Min, Max, 2X-StDev)

0

20

40

60

80

Sn1.

1Ag0

.45C

u0.1

Ge

Sn1.

1Ag0

.47C

u0.0

6Ni

Sn1

.07A

g0.4

7Cu0

.085

Mn

Sn1.

1Ag0

.64C

u0.1

3Mn

Sn1.

13Ag0

.6Cu0

.16M

n

Sn1.

1Ag0

.45C

u0.2

5Mn

Sn1.

07Ag0

.58C

u0.0

37Ce

Sn1.

09Ag0

.47C

u0.1

2Ce

Sn1.

05Ag0

.56C

u0.3

Bi

Sn1

.16A

g0.5

Cu0

.08Y

Sn1

.0Ag0

.49C

u0.1

7Y

Sn1

.05A

g0.7

3Cu0

.067

Ti

Sn1

.0Ag0

.46C

u0.3

Bi0

.1M

n

Sn1.

05Ag0

.46C

u0.6

Bi0

.067

Mn

Sn1

.19A

g0.4

9Cu0

.4Bi

0.06

Y

Sn1

.15A

g0.4

6Cu0

.8Bi

0.08

Y

Sn1

.05A

g0.6

4Cu0

.2M

n0.0

2Ce

SAC

305

SAC

387

SAC

105

Sn6

3

No.

of D

rops

to F

ailu

re

Mn CeBi

Y

Ti

(Kao)

(Indium)

High Ag & Cu For Fatigue Resistance. Dopants Refine Grain And Stabilize Microstructure

• More reliable (driver 4)– Fatigue resistant

(Kao)

9

• High Ag for slow creep. High Cu for stable IMC on Ni.

• Dopants Mn, Ce stabilize microstructure.

• Dopants which refine grain size may ease anisotropic Sn crystal issue.

Nano-filler May Also Render Fatigue Resistance By Pinning Down Grain Boundary

• More reliable (driver 4)– Fatigue resistant

10

• The fatigue resistance may also be introduced by adding nano-filler into the solder

BiSn solder doped with nano SAC305 can be reflowed at low temperature but exhibit high strength and high thermal fatigue resistance.

SnBi doped

SAC

SnBiNano SAC particles reinforced BiSn matrix, and pinned down the grain boundary.

Low Ag, Dopants (Zn, etc.) Which Suppress Undercooling To Suppress Ag3Sn IMC

• More reliable (driver 5)– Stable & uniform

microstructure

11

(Lee etc.) (SAC387)

(IBM)

• Small joint more vulnerable to IMC plate (such as Ag3Sn) formation, which can cause early failure.

• Alloys with low tendency of forming large IMC plate needed.

• Reduced Ag content or rapid cooling process through whole manufacturing process critical.

• Addition of dopants such as Znwill suppress formation of plate through reducing undercooling.

Avoid Composition With Galvanic Corrosion Potential

• More reliable (driver 6)– Corrosion resistant

12(Song et al)

corroded

Non-corroded

• Small joint more sensitive to damage induced by corrosion, such as cyclic bending load.

• Presence of Ag appear to aggravate galvanic corrosion.

• New alloys with composition with low propensity toward galvanic corrosion desired.

Ag & IMC Joint Resist EM

• More reliable (driver 7)– Electromigration resistant

13

• Ag > 1% resist EM.• The diffusivity of Ag in Sn is ~ 3000X slower

than Cu at 150°C.• IMC joint more resistant toward EM. • The current density needed to cause EM

damage for Cu6Sn5 IMC joint is at least one order of magnitude larger than Sn based solder.

Cu/SnAg w EM

Cu dissolved

No Cu dissolved

Cu/SnAg w aging + EM

Ag helpsIMC helps

Wide Pasty Range Alloy Critical For Slow Wetting Hence Low Reflow Defect Rate

• Lower cost (driver 8)– Lower processing cost

(higher yield)

14

y = 0.082e-5.0766x

R2 = 0.8882

0%

2%

4%

6%

8%

0% 20% 40% 60% 80%

Mass Fraction of Solid (%)

Tom

bsto

ning

Rat

e (%

). 3.5-1

3.8-0.7Sn63

3-0.52.5-0.8

2-0.5

0%

1%

2%

3%

4%

5%

6%

7%

Sn2Ag0

.5Cu

Sn2.5A

g0.8C

uSn3

Ag0.5C

uSn3

.5Ag1

CuSn3

.8Ag0

.7Cu

63Sn3

7Pb

Tom

bsto

ning

Rat

e (%

) .

(Indium)

(Indium)

• Reduced chip size cause greater vulnerability toward chip disturbance at reflow soldering (Tombstoning, swimming, billboarding, wicking)

• Need alloy with a slower wetting speed at melting temp, such as a pasty alloy with high mass fraction of solid at melting temp.

Low Ag, and Lower• Lower cost (driver 9)

– Cheaper material (solder)

15

• Reduced Ag content.• Elements introduce hardening,

slow in diffusion, and dopants stabilize microstructure and IMC desired.

Metal Price (USD/Kg)

Ag 1,043Au 53,376

Bi 29

Co 37.1

Cu 7.20In 760

Ni 18.9

Pb 1.95

Pd 19,292

Sb 15.6

Sn 22.3Ti 27.8

Zn 1.88

Ag

Sb Is Next ?

• Environmental friendly (driver 10)– Less toxic (Pb-free)– Low carbon

16

• Pb & Cd free• Sb and many of its compounds

are toxic, and the effects of antimony poisoning are similar to As poisoning.

Region Limits for Sb in tap water (µg/L)

EU 5Germany 5US (EPA) 6Japan 15World Health Org 20

Ag Or Cu Desired For Future High Power Semiconductor Devices

(Baliga)

For die-attachment, the joint of future high power devices is expected to see a higher service temperature which may cause early solder fatigue failure. Ag or Cu bonding will be promising alternative in this regard, if processable at soldering temperature.

(Ning et al.)

• Higher power density (driver 11)– Higher temperature tolerance,

higher current resistant

Nano-Ag May Serve As Ag-Solder For High Temp High Power Applications

18

Material Thermal Conductivity (W/m*C)

Melting Points ©

SAC305 57.8 217SnCu 64.0 227Nano-Ag paste 240 960

• Virginia Tech study showed the transient thermal impedance of the device attached by nano Ag paste (sintered at 275C/30min, < 5MPa) is 12.1% lower than those by the solder alloys. (IMAPS 4/2010).

• Fraunhofer Institute reported high bonding strength > 40 Mpa can be achieved with sintering at 275C for 60 sec and pressure at 2 Mpa (290 psi). (CIPS’2010).

• Volkswagen, Danfoss, Bosch, GM, TI, BAE pursue or explore this.• Low or zero pressure sintering desired.

Nano-Ag may seal the gap between solder & high melting metal, and serve as a Ag “solder”. Cu may have similar potential.

19

Summary• Smaller & Smarter

– More oxidation resistant, lower surface tension, higher fluidity– Lower melting temperature for miniaturized applications

• More Reliable– Lower hardness for portable. Dopants for stabilize microstructure– High Ag & Cu for fatigue resistance. Dopants refine grain and stabilize

microstructure – Nano-filler may also render fatigue resistance by pinning down grain

boundary– Low Ag, plus dopants (Zn, etc.) which suppress undercooling to stabilize

Ag3Sn IMC.– Avoid composition with galvanic corrosion potential– Employ high Ag and IMC for EM resistance

• Lower Cost– Wide pasty range alloy critical for slow wetting hence low reflow defect rate– Low Ag for low cost

• Green– More environmentally friendly. Sb next to ban?

• Higher Power Density– Nano-Ag/Cu bonding desired for future high power semiconductor devices

20

Trends of Solder Alloys Development

• Trends of getting better diversify according to applications, mainly into portable devices, high thermal reliability, and high temperature high power applications.

• Regulating Ag and Cu content plus doping or adding nano-filler will be the primary means for improvement.

• Quasi-solder joint formation such as forming IMC joint and sintering of nano-high-melting metal are new twist of achieving improvement.