matte tin whisker evaluation

Matte Tin Whisker Evaluation

Lucy Hu

Integrated Device Technology

Santa Clara, CA

USA Introduction

?The use of lead(Pb) in electronic products has

environmental concerns:

–European Union —RoHS: restricts use of Pb, mercury,

cadmium, hexavalent chromium and certain

halogenated flame retardants (PBB and PBDE) by July

2006.

–United States —EPA reduced reporting threshold

for Pb containing items

–Japan —Recycling Law increased fees for recycling

Pb-containing items

?Pure tin plating is the finish chosen by many companies as a non-disruptive and economical replacement for

solder plate on leadframe products

Whisker Concern

?The main concern of pure tin plating is the known possibility for whisker growth under long term conditions

?Tin whiskers are electrically conductive, single crystal structures, spontaneously growing from the pure tin coating ?Tin whisker may cause long term reliability concern, because they have the potential to short adjacent leads or break off and short.

?Tin whiskers are believed to grow in order to relieve stress that exist within the plating. However the exact mechanism for

whisker growth is unknown as of this study

?As of this date, there are no universally accepted tests to accelerate whisker growth, although NEMI has recently

published a test proposal Evaluation matrix

Whisker Test Conditions

Tin Whisker Tests Condition Pre-condition Bias Inspection Points SS Aging60°C / 90% RH None No0,1000, 2000,3000hr 3 units , 3 leads per unit

Reflow @215°C No0,1000, 2000,3000hr 3 units , 3 leads per unit

Assembly @215°C Yes0,1000, 2000,3000hr 3 units , 3 leads per unit Storage25°C/30% - 80% RH None No0,1000, 2000,3000hr 3 units , 3 leads per unit

Reflow @215°C No0,1000, 2000,3000hr 3 units , 3 leads per unit

Assembly @215°C Yes0,1000, 2000,3000hr 3 units , 3 leads per unit T/C-55°C to +85°C None No0, 500 cycle 3 units , 3 leads per unit

Reflow @215°C No0, 500 cycle 3 units , 3 leads per unit

Assembly @215°C No0, 500 cycle 3 units , 3 leads per unit Qual Vehicle

Assy Facility Plating Chemistry Leg L/F Alloy Pkg Type

Subcon X Chemistry A1C7025TVSOP 80L

2Olin 194TVSOP48L

3EFTEC 64T TQFP 120L

Subcon Y Chemistry B4EFTEC 64T TQFP100L

5Olin 194LQFP 100L

6C7025PQFP 208L

How The Evaluation Matrix Was Defined

?Test methods are similar to NEMI recommendation

?2 pure tin plating chemistries were selected, chemistry A from Subcon X, and chemistry B from Subcon Y

?3 leadframe alloys(Olin 7025, Olin 194, Eftec64T) were selected for each chemistry

?Packages with the narrowest leadframe pitch (0.4mm lead pitch) were selected from each leadframe alloy family ?215oC reflow pre-conditioning was added to simulate the SnPb reflow process which customers may use Pb-free parts for SnPb process ?5V bias was added into evaluation matrix

?All evaluation lots went through 150oC 1 hour bake to mitigate whisker growth

?Each Leg has 3 evaluation lots(plated at different time) and a control (SnPb plated) lot, 3 units of each lot were inspected for whiskers

?3 randomly chosen leads of each component were inspected

?Whiskers were inspected at 300X, and verified by SEM How The Evaluation Matrix Was Defined (cont’)

The Effect of 215oC Pre-con

10

20

30405060B E e B E e R e lo w

B O n 194B O n 194R e o w

B

O n 7025B O n 7025R e lo w

M a x W h s k e L e n g h u m 0

102030405060

A E A E R w

A O A O R w

A O A O R lo w

M a x W h s k e L e n g h u m Chemistry A After 3000hr Aging

Chemistry B After 3000hr Aging

Chemistry A After 500 T/C

Chemistry B After 500 T/C

2468101214B E e c B E e c

R e o w

B O n 194B O n 194R e o w

B O n 7025B O n 7025R

e o w

M a x W h s k e r L e n g t h (u m )0

51015202530

A E A E R w

A O A O

R w

A O A O R w

M a x W h s k e L e n g h u m 3000hr 60oC90% Aging Test Result

10203040506070

80

1000

2000

3000

Time (hr)

M a x . W h i s k e r L e n g t h (u m )

A-Eftec

A-Eftec-Bias A-Olin194

A-Olin194-Bias A-Olin7025

A-Olin7025-bias 0

10203040506070800

1000

2000

3000

Time (hr)

M a x .W h i s k e r l e n g t h (u m )

B-Eftec B-Eftec-Bias B-Olin194B-Olin194-Bias B-Olin7025

B-Olin7025-bias

Chemistry A Chemistry B

Room Temp Storage 3000hr Result

0510152025303540B E e c

B E e c B a s

B O n 194B O n 194B a s

B O n 7025B O n 7025b a s

M a x . W h i s k e r L e n g t h (u m )

0510152025303540A -E e c

A -E e c -

B a s

A -O n 194A -O n 194-

B a s

A -O n 7025A -O n 7025-b a s

M a x . W h i s k e r L e n g t h (u m )

Chemistry A Chemistry B

Temp Cycle (500 cycles) Result

Chemistry A Chemistry B

5101520253035B-Eftec B-Eftec-ASM

B-Olin194B-Olin194-ASM

B-Olin7025B Olin7025-ASM

M a x .W h i s k e r L e n g t h (u m )

5101520253035

A-Eftec

A-Eftec-ASM

A-Olin194A-Olin194-ASM

A Olin7025A Olin7025-A S M

M a x .W h i s k e r L e n g t h (u m )

Summary

?Pre-conditioning at 215 oC reflow makes no significant difference, although 215 oC reflow is slightly worse for chemistry A at 500 T/C, but no obvious impact for 3000hr 60 oC90%RH Aging of chemistry A

?Chemistry A performs better than chemistry B at 60 oC90% RH Aging condition

?Chemistry B is slightly better than chemistry A after 500 T/C, but whiskers from both chemistries were less than 30um

?SnPb plating also produced whiskers under both T/C and Aging conditions ?60oC90% RH Aging is the most effective accelerated whisker test, most whiskers were found at this condition, including a 41um whisker on control(SnPb) lot

Summary (cont’)

?Room temperature shows the lowest propensity for whisker growth, only 2 tiny whiskers found on chemistry A after 3000hr, and one whisker found on chemistry B after 3000hr ?L/F alloys performed differently for different plating chemistries under Aging condition, Eftec 64T is more whisker prone for chemistry A, but Olin 194 had longest whisker for chemistry B

?No difference was observed for different leadframe alloys on chemistry A after 500 T/C, but Eftec 64T is the best on chemistry B after 500 T/C, with no whiskers observed ?5V Bias does not make significant difference for whisker growth

Tin Whisker Examples

A/Olin 194 after 3000hr Aging B/Olin 194 after 3000hr Aging B/Olin 7025 after 3000hr Aging B/Eftec after 3000hr Aging SnPb Whisker Examples

SnPb/Olin 194 after 500 T/C SnPb/Olin194 after 2000hr Aging, no

change after 3000hr