Applications of Solder Alloys and Threats of Tin Whiskers

1

and Threats of Tin Whiskers

Dr. Ning-Cheng Lee

December 1st, 2015

Solder Alloys

• Low melting

– Lower thermal warpage

– Lower cost

• Medium melting

– Higher reliability

• High melting

• High Service Temperature & High Reliability

– Target 175C service temperature

– High thermal aging & TCT reliability

– High vibration reliability

– SMT solder paste, wire, preform process

Medium Melting Alloys

3

– Automotive, medical device, server, telecommunication applications

Alloy Designs - Indium Confidential

• SnAgCuSbBi (4)• SnAgCuSbBiNiCo• SACM (high Ag)

High Melting Solders

• Melting temperature > 260C, preferably > 280C

• Lead-free

• Applications

– Die attach

– High power devices, such as IGBT– High power devices, such as IGBT

– Automotive

4

Medium Melting

Prevailing Solder Alloys

5

Prevailing Solder Alloys

Prevailing Lead-Free Alloys

210

220

230 SnCu (+ dopants, e.g. Ni, Co, Ce)

SnAg (+ Cu, +Sb, + dopants, e.g. Mn, Ti,

Al, Ni, Zn, Co, Pt, P, Ce)

SnAg (+ Bi, + Cu, + In, + dopants)

Reflow Wave Hand

√

√√

√ √

√

√

√

6

170

180

190

200

Tem

p (

C)

SnZn (+ Bi) √

Most prevailing alloys: SnAgCu, with Ag 3-4%. Trend toward further reduced Ag.

BiSn(+Ag) (mp 140°C) on the rise.

High Thermal Fatigue ResistanceResistance

7

High Ag High TCT Life

• Changes in Ag content can have significant impact on thermal fatigue reliability

• Terashima et al. found that a decrease of Ag content from 4% to 1% decreases the thermal fatigue life (first failure) by a factor of about 2 about 2

– -40/125°C, 10 min dwell.– All alloys had 0.5% Cu

– Performance relative to eutectic Sn-Pb not reported

• Addition of other alloying elements which affect undercooling, formation of various IMCs, matrix properties & microstructure not well understood

S. Terashima, et al., J. Elec. Mater., Vol. 32, No. 12, p.1527 (2003). 8

Effect of Ag Content

High Ag result in long TCT life

All BGA assembled with SAC305 paste

9

Richard Parker, Richard Coyle, Gregory Henshall, Joe Smetana, Elizabeth Benedetto, “iNEMI Pb-FREE ALLOY

CHARACTERIZATION PROJECT REPORT: PART II - THERMAL FATIGUE RESULTS FOR TWO COMMON TEMPERATURE

CYCLES”, SMTAI, p.348-358, Orlando, FL, Oct. 14-18, 2012

Evolution of Interface with Increasing Cu Conc.

Cu (die side)

Ni (PCB side)

SAC

0, 0.5

0% Cu

0.5, 2

1

(Cu, Ni)6Sn5

Cu

(Cu, Ni)6Sn5 (2.7 um)

Cu

(Cu, Ni)6Sn5

Cu

(Cu, Ni)6Sn5

Cu

Crack

location

High Cu result in more

ductile failure (bulk solder) than brittle failure (IMC interface)

Ref: Henry Y. Lu, Haluk

Balkan, Joan Vrtis, and K.Y. Simon Ng, " Impact of Cu Content on the Sn-Ag-Cu Interconnects", 55th ECTC, P.113-119,

May 31-June 3, 2005

10Cu suppress dissolution of Ni. Hence, Ni3(P,Sn) disappear first, followed by NiPSn. But it also promotes more IMC formation on PCB (Ni) side & nucleation of Ag3Sn plates.

(Ni,Cu)3Sn4

NiPSn (500 nm)

Ni3(P,Sn) (300 nm)

Ni (3.84 um)

SnAg

(Cu, Ni)6Sn5

NiSnP + Ni3(P,Sn) (250 nm)

(Cu, Ni)6Sn5 (2.8 um)

(Cu, Ni)6Sn5 (2.7 um)

SAC305

Ni (4.68 um)

NiSnP (240 nm)

(Cu, Ni)6Sn5

(Cu, Ni)6Sn5

SAC3010

Ni

(Cu, Ni)6Sn5

(Cu, Ni)6Sn5

SAC3020

Ni

Cu conc.

No Ag3Sn plates in any locations of the solder joints for the 0.0Cu and 0.5Cu at time zero

High Cu lead to flourishing growth of Cu-Sn IMCs, which promotes the growth of Ag3Sn platelets.

High Shock Resistance

11

12Ref:Vijay Wakharkar& Ashay Dani, “Microelectronic Packaging Materials Microelectronic Packaging Materials Development & Integration Development & Integration Challenges for Lead Free Challenges for Lead Free”, Lead-free workshop, TMS, San Antonio, TX, March 12, 2006.

How Hard is SAC Alloys

13M. Date, T. Shoji, M. Fujiyoshi, and K. Sato (Hitachi), “Pb-free Solder Ball with Higher Impact Reliability”, Intel Pb-free

Technology Forum, 18th – 20th July 2005, Penang, Malaysia

14Ref:Vijay Wakharkar& Ashay Dani, “Microelectronic Packaging Materials Microelectronic Packaging Materials Development & Integration Development & Integration Challenges for Lead Free Challenges for Lead Free”, Lead-free workshop, TMS, San Antonio, TX, March 12, 2006.

Effect of Dopant Mn on IMC

GrowthGrowth

15

• High Shock Resistance, Good TCT Performance (Low Ag)

– Example (SACM0510 – Sn0.5Ag1Cu+Mn)-Sphere

Medium Melting Alloys

16“The Second Generation Shock Resistant And Thermally Reliable

Low Ag SAC Solder Doped With Mn”, SMTA International, Fort

Worth, TX, Oct. 13-17, 2013

Thermal Cycling Test Results

Bump Alloy

Characteristic Life 63.2% (ηηηη)

ValueRatio

(alloy/105)

SAC105 1468 1

SAC105M 2034 1.39

SAC305 1905 1.30

TCT (-40°°°°C/125°°°°C) performance of BGA assemblies

-55C/125C

Bump AlloyAssembly

Condition

Characteristic Life

63.2% (ηηηη)

Characteristic Life 50%

Weibull

Slope (ββββ)Value

Ratio

(Mn/105)Value

Ratio

(Mn/105)

SAC0510MFlux

24271.61

23171.62

7.26

SAC105 1510 1431 6.68

SAC0510MSAC305 paste

23611.37

22581.45

8.03

SAC105 1726 1559 4.3617

SAC305 1905 1.30

Liu etc, SMTAI, p.920-934, October 4-8, 2009, San Diego, CA.

“The Second Generation Shock Resistant And Thermally Reliable Low Ag SAC Solder Doped With Mn”, SMTA International, Fort Worth, TX, Oct. 13-17, 2013

TCT Results

Mn105

0510M

18

Significant

recrystallization

Minute

recrystallization

Earlier work Mn stabilize microstructureLiu etc, SMTAI, p.920-934, October 4-8, 2009, San Diego, CA.

“The Second Generation Shock Resistant And Thermally

Reliable Low Ag SAC Solder Doped With Mn”, SMTA

International, Fort Worth, TX, Oct. 13-17, 2013

High Melting

High Power Die Attach Solution Development

19

TLP Composite Solder TLP Composite Solder Preform

20

Composite Preform TLP Bonding Process

Sn -Ag So ld er

LayersAg L ayer

Su bs trate

Com p osite Preform

Assem bly

Sn -Ag So ld er

LayersAg L ayer

Su bs trate

Com p osite Preform

Assem bly(a)

S ubs trate

Co mp osite Preform

S ubstrate

S ubs trate

Join t Layer

After Reflow Solderin g

S ubs trate

Co mp osite Preform

S ubstrate

S ubs trate

Join t Layer

After Reflow Solderin g

(c)

(b)

21

Microstructures of TLP Bonds

Cu

Ag

Ag-Sn ζCu3Sn

(a) Optical and (b) SEM back-scattered electron micrographs of a solder joint

made with a composite preform between two Cu substrates (Peak reflow temperature 380ºC, TAL 9 min)

Cu(a)

(b)

Ag-Sn ζCu3Sn

22

BiAgX®

High-TemperatureHigh-TemperatureLead-free

Drop-in Replacement for

High-Pb Solder Paste

BiAgX® is a registered trademark of Indium Corporation23

Design of BiAgX ® - The Mixed Solder Paste

1. Using two metalpowders in the paste

2. 1st alloy dominates themelting behavior & themechanical properties.

3. 2nd alloy dominatesinterfacial reaction forwetting improvement.wetting improvement.

24

Thermal Storage

Tanimoto S., etc, JSAP, Sept. 2012

200oC

2mmx2mmx0.3mm Ti/Ni/Ag SiC/AMBC-SiN

25

Tin Whisker

26

27Swaminath Prasad, Flynn Carson, G.S. Kim, J.S. Lee, Y.C. Park, Y.S. Kim & K.S. Min, S.S. Lu, Liu Hui, & Xu Hai, S.H. Khor & C.L. Tan, “Plating Chemical Evaluations and Reliability of Pb-Free Leadframe Packages”,Pan Pacific: February 13, 2001.

28

Tension

Compression

29

4Tension

Tin Whisker Growth Mechanism

• Growth due to compressive stress

• Augmented by

– Cu-Sn IMC formation at grain boundary

– Small grain size

30

– Thermal stress due to mismatch in TCE

Chen Xu, Yun Zhang, Chonglun Fan and Joseph A. Abys, "Driving Force for Sn Whisker Formation:

Compressive Stress - Pathways for its generation and Remedies for its Elimination and

Minimization", QuickStart Conference, Libertyville, IL, Sept. 25, 2004

31

(microvoids & creep corrosion)

Tradeoff

Tradeoff

Reason

32

Reason

33

Density Ni=8.902,

34

Density Ni=8.902, Sn=7.31g/cc

Atomic wt Ni=58.69, Sn=118.71

Molar volume of 3 Ni + 4 Sn = 84.7 cc, larger than Ni3Sn4 molar volume (75.25cc). IMC formation cause net shrinkage.

Tin Whisker Suppression

• Alleviated by

– Ni plating on Cu

– Large grain size

– Thin solder layer

– Surface oxide

– Alloying with Bi

35

– Alloying with Bi

– Baking treatment (150°C/1 hr)

Chen Xu, Yun Zhang, Chonglun Fan and Joseph A. Abys, "Driving Force for Sn Whisker Formation: Compressive Stress - Pathways

for its generation and Remedies for its Elimination and Minimization", QuickStart Conference, Libertyville, IL, Sept. 25, 2004

36

IMC grow into Sn grain boundary

Flattened IMC won’t grow into grain boundary

37Coarsened Sn grain size

38

Direction of Alloy Development to Meet Evolving Applications to Meet Evolving Applications

39

Wetting Challenge

• Oxide thickness does not decrease with miniaturization.

• Reduced drilled hole size & increased aspect ratio (pin-to-hole) cause partial hole filling at wave soldering (due to decreasing laminar flow rate).

• Alloys with better wetting needed in order to avoid use of higher soldering temperature.

40

temperature.• Incorporation of low surface tension

elements, such as Bi, P, Sb beneficial.• Alloys with lower viscosity will also help.

(HP)

0

0.5

1

1.5

2

2.5

0.5 0.52 0.54 0.56 0.58

Surface Tension (N/M)W

ett

ing

Tim

e (

sec)

(Indium)0

0.5

1

1.5

2

Ag Al Au Bi Ce Co Cu Fe Ga Ge In La Mn Ni P Pb Pd Pt Si Sb Sn Ti Zn

Su

rface T

en

sio

n (

N/m

)

Wetting Speed Challenge

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

• Increase surface tension may reduce wetting speed, but cause more defect rate due to higher horizontal pulling force

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

Mass Fraction of Solid EstimationSymmetry assumed for eutectic

41

0%

1%

2%

3%

4%

5%

6%

7%

Sn2Ag0

.5C

uSn2

.5Ag0

.8Cu

Sn3Ag0

.5C

uSn3

.5Ag1

Cu

Sn3.8

Ag0.7

Cu63

Sn3

7Pb

To

mb

sto

nin

g R

ate

(%

) .

(Indium)

0.56

N/m

0.51

N/m

y = 0.082e-5.0766x

R2 = 0.8882

0%

2%

4%

6%

8%

0% 20% 40% 60% 80%

Mass Fraction of Solid (%)

To

mb

sto

nin

g R

ate

(%

). 3.5-1

3.8-0.7Sn63

3-0.5

2.5-0.82-0.5

(Indium)

Fragility Challenge

• Miniaturized devices more portable, thus more risk to be dropped.

• Small joints more vulnerable to shock.• Joints with low fragility desired

– Lower hardness (such as low Ag SAC)

– Dopant which reduce IMC thickness, scallop size, or fragility, such as Mn, Ti, Y, Bi, Ce, Ni, Co, Pt

Drop Test Performance (Mean value)

0

10

20

30

40

Sn

1.1

Ag

0.4

5C

u0

.1G

e

Sn

1.1

Ag

0.4

7C

u0

.06

Ni

Sn

1.0

7A

g0

.47

Cu

0.0

85

Mn

Sn

1.1

Ag

0.6

4C

u0

.13

Mn

Sn

1.1

3A

g0.6

Cu

0.1

6M

n

Sn

1.1

Ag

0.4

5C

u0

.25

Mn

Sn1

.07

Ag

0.5

8C

u0

.03

7C

e

Sn

1.0

9A

g0.4

7C

u0

.12

Ce

Sn

1.0

5A

g0

.56

Cu

0.3

Bi

Sn

1.1

6A

g0

.5C

u0

.08

Y

Sn

1.0

Ag

0.4

9C

u0

.17

Y

Sn

1.0

5A

g0

.73

Cu

0.0

67

Ti

Sn

1.0

Ag

0.4

6C

u0

.3B

i0.1

Mn

Sn

1.0

5A

g0

.46

Cu

0.6

Bi0

.06

7M

n

Sn

1.1

9A

g0.4

9C

u0

.4B

i0.0

6Y

Sn

1.1

5A

g0.4

6C

u0

.8B

i0.0

8Y

Sn

1.0

5A

g0

.64

Cu0

.2M

n0.0

2C

e

SA

C30

5

SA

c38

7

SA

C10

5

Sn6

3

No

. o

f D

rop

s t

o F

ail

ure

As-reflowed

After aging

42

scallop size, or fragility, such as Mn, Ti, Y, Bi, Ce, Ni, Co, Pt

– Form more ductile solder via high Cu

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

– Reduce spalling, such as high Cu– Fine grains to nullify effect of anisotropic

orientation of Sn crystal

(Kao)

(Indium)

(Lu et al)

Cu (die side)

Ni (PCB side)

SAC

0, 0.5

0% Cu

0.5, 2

1

Crack location

Sn

1.1

Ag

0.4

5C

u0

.1G

e

Sn

1.1

Ag

0.4

7C

u0

.06

Ni

Sn

1.0

7A

g0

.47

Cu

0.0

85

Mn

Sn

1.1

Ag

0.6

4C

u0

.13

Mn

Sn

1.1

3A

g0.6

Cu

0.1

6M

n

Sn

1.1

Ag

0.4

5C

u0

.25

Mn

Sn1

.07

Ag

0.5

8C

u0

.03

7C

e

Sn

1.0

9A

g0.4

7C

u0

.12

Ce

Sn

1.0

5A

g0

.56

Cu

0.3

Bi

Sn

1.1

6A

g0

.5C

u0

.08

Y

Sn

1.0

Ag

0.4

9C

u0

.17

Y

Sn

1.0

5A

g0

.73

Cu

0.0

67

Ti

Sn

1.0

Ag

0.4

6C

u0

.3B

i0.1

Mn

Sn

1.0

5A

g0

.46

Cu

0.6

Bi0

.06

7M

n

Sn

1.1

9A

g0.4

9C

u0

.4B

i0.0

6Y

Sn

1.1

5A

g0.4

6C

u0

.8B

i0.0

8Y

Sn

1.0

5A

g0

.64

Cu0

.2M

n0.0

2C

e

IMC Plate Challenge

• 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. (Lee etc.) (SAC387)

43

forming large IMC plate needed.

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

• 97Sn2.3Ag0.5Cu0.2Bi is favored by IBM

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

(Lee etc.) (SAC387)

(IBM)