1

Achieving High Reliability Low Cost Lead-Free SAC Solder Joints Via Mn Or Ce Doping

Dr. Weiping Liu1, Dr. Ning-Cheng Lee1, Adriana Porras2, Dr. Min Ding2, Anthony Gallagher3, Austin Huang4, Scott Chen4, and

Jeffrey ChangBing Lee5

1 Indium Corporation; 2 Freescale Semiconductor3 Motorola Inc; 4 Advanced Semiconductor Engineering Group;

5 IST-Integrated Service Technology Inc

2

Introduction

• SAC with high Ag good in thermal fatigue performance, but poor in drop test performance

• SAC with low Ag OK in drop test, but poor in thermal fatigue performance

• A Pb-free alloy with improved drop test performance, and good in thermal fatigue performance badly needed.

• SAC105+Mn or Ce studied here

3

Experimental Design

• New Alloys– SAC105+0.05Mn (SACM)– SAC105+0.02Ce (SACC)

• DOE for JEDEC drop test & TCT

High Tg FR4/8 layer/NVIP/NSMD/OSP (ENIG, ImAg)PCB

245C245C245C245C245C220CReflow profile

SAC305SAC305SAC305SAC305 SAC305SnPbSolder paste

NiAuOSPNiAuNiAuNiAuNiAuSurface finish of substrate

SACCSACMSACMSAC305 SAC105 SnPbSolder ball

0.3/0.50.3/0.50.3/0.50.3/0.50.3/0.50.3/0.5Ball/pitch ( mm)

Daisy TFBGA244 12X12Package

4

Test Design

• JEDEC Drop Test (JESD22-B111)– Fail when > 1000 ohms

• Dynamic Bending Test– Board strain at 1st fail– Dye & pry

• Thermal Cycling Test– - 40C/125C– 42 min/cycle, ramp 11 min, dwell 10 min– Fail when > 20% Resistance increase

• Cyclic Bending Test– 1 Hz/2mm– Paste SAC387– Fail when > 1000 ohms

1500G

5

JEDEC Drop Test ResultsSACM, SACC ≥ SnPb, 105 > 305

1.00 500.0010.00 100.001.00

5.00

10.00

50.00

90.00

99.00 0.5

0.6

0.7

0.8

0.9

1.0

1.2

1.4

1.6

2.0

3.0

6.0

β

η

ReliaSoft's Weibull++ 6.0 - www.Weibull.com

Probabil i ty - Weibul l

Time, (t)

Unr

elia

bilit

y, F

(t)

2009/2/23 09:4CompanyCK

WeibullSAC105

W2 RRX - SRMF=10 / S=5SAC305

W2 RRX - SRMF=10 / S=5SACCe

W2 RRX - SRMF=10 / S=5SACMn

W2 RRX - SRMF=11 / S=4SnPb

W2 RRX - SRMF=10 / S=5

Ce

Mn

305105

SnPb

0

100

200

300

SnPb SAC105 SAC305 SACM SACC

C-L

ife in

Dro

p Te

st

150C/0 hr

150C/100 hrs

150C/250 hrs

TCT 250 cycles

6

Dynamic Bending Test ResultSACM ≥ SACC > 105 > 305

0.3/0.5 0.3/0.5 0.3/0.5 0.3/0.5NiAu NiAu NiAu NiAu

SAC105 SAC305 SAC105Mn SAC105Ce

80 80 80 80Thermal aging 150C/0hr Failure Criteria( Strain ) 0.5049% 0.2406% 0.7490% 0.6963%

Thermal aging 150C/250hr Failure Criteria( Strain ) 0.0999% 0.1493% 0.3550% 0.2968%

Package

Linear ramp profile, peak 235CSAC387Solder paste

Failure Criteria: 0.5% strain levelTFBGA ( Bravo)

PCB (FR4/8 layer/NVIP/SMD)

solder ball

ball/pitch ( mm)surface finish (substrate)

reflow profile

*SACM is SAC105 + Mn dopantSACC is SAC105 + Ce dopant

0.0%

0.2%

0.4%

0.6%

0.8%

SAC105 SAC305 SACM SACC

Stra

in to

failu

re

150C/0 hr150C/250 hrs

7

TCT (-40C/125C) Test Results TFBGA on PCB (OSP)

100.00 5000.001000.001.00

5.00

10.00

50.00

90.00

99.00 0.5

0.6

0.7

0.8

0.9

1.0

1.2

1.4

1.6

2.0

3.0

6.0

β

η

ReliaSoft's Weibull++ 6.0 - www.Weibull.com

Probabil i ty - Weibul l

Time, (t)

Unr

elia

bilit

y, F

(t)

2009/2/16 17:54CompanyCK

WeibullSAC105

W2 RRX - SRM MF=13 / S=2SAC305

W2 RRX - SRM MF=15 / S=0SACCe

W2 RRX - SRM MF=15 / S=0SACMn

W2 RRX - SRM MF=14 / S=0SnPb

W2 RRX - SRM MF=14 / S=1

Ce

Mn

305

105

SnPb

SACC, SACM, 305 > SAC105 > SnPb

0

500

1000

1500

2000

2500

SnPb SAC105 SAC305 SACM SACC

C-L

ife in

TC

T

150C/0 hr

150C/100 hrs

150C/250 hrs

150C/250 hr Aging

8

CBT Test Results TFBGA on PCB (OSP)

Ce

Mn

305

105

SnPb

305 > 105, SACM, SACC > SnPb

SAC305 exhibited the highest value in TS, YS, Young's modulus, and elongation (%).

All LF are better than SnPb.

as reflowed

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Effect of Surface Finish

0

100

200

300

NiAu BGA OSP BGA

C-L

ife o

f Dro

p Te

st

150C/0 hr

150C/100 hrs

150C/250 hrs

TCT 250 cycles

0

100

200

300

OSPPCB

ENIGPCB

ImAgPCB

OSPPCB

ENIGPCB

ImAgPCB

C-L

ife o

f Dro

p Te

st

150C/0 hr

150C/100 hrs

150C/250 hrs

TCT 250 cycles

SACM SACC

PCB:

No obvious trend.

0

2500

5000

7500

10000

NiAu BGA OSP BGA

C-L

ife in

Ben

ding

Tes

t

150C/0 hr

150C/100 hrs

150C/250 hrs

TCT 250 cycles

Package: NiAu ≥ OSP (in general)

JDT TCT CBT

SACM SACM SACM

0

4000

8000

12000

16000

OSPPCB

ENIGPCB

ImAgPCB

OSPPCB

ENIGPCB

ImAgPCB

C-L

ife o

f Ben

ding

Tes

t (cy

cles

)

150C/0 hr

150C/100 hrs

150C/250 hrs

TCT 250 cycles

SACM SACC

JDT TCT CBT

PCB: Weak trend ImAg > OSP > ENIG (in general)

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ResultsSACM and SACC

displayed

(a) thinner and smoother interfacial IMC layers

(b) finer IMC particleswithin bulk solder

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Mn & Ce Suppressed IMC Growth

0

2

4

6

8

10

12

0 400 800 1200

150C Aging Hrs

IMC

Thi

ckne

ss (m

icro

ns

SnPb/NiAu

SAC105/NiAu

SAC305/NiAu

SACM/NiAu

SACM/OSP

SACC/NiAu

0

2

4

6

8

0 400 800 1200

150C Time (hrs)

IMC

Thi

ckne

ss (m

icro

ns

SnPb/NiAu BGA SAC105/NiAu BGA

SAC305/NiAu BGA SACM /NiAu BGA

SACM /OSP BGA SACC/NiAu BGA

On package side On PCB side

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Mn & Ce suppressed coarsening of IMC rods at interface

150C

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Microstructure of solder joints of TFBGA (NiAu) on PCB (OSP) after TCT

Mn suppressed coarsening of IMC particles, thus maintained hardness of joint

0

5

10

15

0 100 200 300

150C Aging Time (hrs)

Vick

er H

ardn

ess

Num

ber

SAC105

SACM

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Mn & Ce suppressed IMC coarsening upon thermal aging, hence stabilized microstructure.

IMC particle coarsened

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Mn & Ce stabilized grain size upon thermal aging, presumably through stabilizing IMC particles.

Grain boundary fading away Grain size stabilized Grain size stabilized

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Discussion

• Drop Test– Both SACM and SACC exhibit finer and thinner IMC

structure at interface. – Inclusion of dopants in IMC may also alter the

crystallinity, hence reduce the brittleness of IMC layer.

• TCT Test– A stable and fine IMC structure may be the primary

contributing factor, and the stabilized grain structure resulted may be the secondary cause for SACM and SACC to exhibit a high TCT reliability.

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Conclusion

• The Mn or Ce doped low cost SAC105 alloys – Achieved a higher drop test and dynamic bending test reliability than

SAC105 and SAC305, and exceeded SnPb for some test conditions.– Matched high Ag SAC in thermal cycling performance

• The mechanism for high drop performance and high thermal cycling reliability can be attributed to – A stabilized microstructure, with uniform distribution of fine IMC

particles, presumably through the inclusion of Mn or Ce in the IMC.– A thinner IMC layer

• The cyclic bending results showed SAC305 being the best, and all lead-free alloys are equal or superior to SnPb.

• NiAu is preferred over OSP for BGA packages if assembled on PCB (OSP).

• Weak trend on preference of PCB finishes: ImAg > OSP > ENIG