1. Pb-free issues for the Semiconductor Industry
Dr. Ir. Pascal Oberndorff
NXP Semiconductors
IC Manufacturing Operations Back End Innovation
Nijmegen, The Netherlands
COST531, Vienna, 18th of May 2007
Pascal Oberndorff
NXP Semiconductors
IC Manufacturing Operations Back End Innovation
Nijmegen, The Netherlands

2. Introduction
Pb could be found on the leads in electrolytic finish of semiconductor components
Pb also present in some die – attach, but exemption: alternatives for high Pb solder??
In case of Ball Grid Arrays (BGA) Pb was present in the solder balls
Semiconductor Industry is also impacted by higher temperature of the soldering process in Pb-free (Moisture Sensitivity)

3. Introduction Leaded Devices: BGA Change Electroplating
NiPdAu Preplated Leadframes (costly)
Pure Sn (Whiskers)
BGA
Change Solder balls to Pb-free balls
Sometimes balls go missing?!

4. Leaded Components Electroplating Pre Plated Leadframes NiPdAu

5. NiPdAu Structure Au or Au Alloy Pd Ni Base Material Thicknesses:
Prevent Pd Oxidation
Prevent Outgas
absorbed by Pd
Prevent Ni Surface Oxidation
Prevent Cu Defusing into Pd Layer
Source : Sumitomo
Typical thicknesses of NiPdAu is about 2-3 nm Au, ~ 20 nm Pd, 1.5 micron Ni.
Thicknesses:
Ni m; Pd nm; Au 3-10 nm

6. Soldering NiPdAu Molten Solder Molten Solder Au or Au Alloy Pd Ni
Base Material
Molten Solder Pd Au
Au or Au Alloy Ni
Base Material
Ni has to be protected before soldering because if it is exposed to air it will oxidize and can’t be soldered.
During soldering this protective layer of PdAu will dissolve in liquid solder.
Source : Sumitomo
Note that when come into contact with molten solder, the Pd and Au/Au Alloy layer dissolves into the matrix structure of the solder, leaving only the Ni Layer.

7. Conclusion for PPF
Only applicable for Small Components due to moisture absorption

8. Moisture Sensitivity
During storage the component willl absorb moisture from the ambient humidity
During board assembly the absorbed moisture will vaporize and escape from the package
More critical for Pb-free reflow, due to higher temperatures of Pb-free solder
Source Infineon
Adhesion of moulding compound on PPF is worse than on Cu alloys

9. Moisture Sensitivity
IC damage risk due to:
Popcorn effect

10. Remarks on PPF Good solution for small packages but costly!
How to improve adhesion to plastic;
Different plastic
Rougher surface of leadframe
Chemicals?

11. Leaded Components Electroplating Sn based Finishes

12. Comparison of Various Plating Options!
+/-
Matte Sn was selected, mass production within NXP since 2003

13. Problem: Whisker Growth
Most literature agrees whiskers grow because of internal compressive stress in plating layer
Important: whiskers not only grow on pure Sn but also on Sn alloys; even SnPb!

14. Whisker growth not only with pure Sn!
Whisker growth on SnBi after 8 months storage at ambient storage
Whisker growth on SnCu1 after one year storage at ambient temperature

15. Reasons for Stress in Plating
Irregular intermetallic formation
Mismatch of CTE with leadframe
Incorporation of foreign particles in plating
Mismatch of crystal orientation
Mechanical operations
The growth of whiskers is relieving the present stress

16. Whisker Growth @ Ambient
Whiskers grow because of compressive stress in the plating which is caused by irregular growth of intermetallics
Tin Whisker
is forced out
Cu6Sn5
Sn Deposit
Whisker
Cu L/F
Cu6Sn5
Cu Substrate

17. Countermeasure: Postbake (1h, 150 oC) (Within 24 hours of plating)
Because of higher temperature diffusion will shift from grain boundary to bulk diffusion and thus regular intermetallics
Grain Growth of Sn
Diffusion barrier for further intermetallic growth
Possible annealing of stress
Postbake does NOT change CTE mismatch!
No whisker!
Sn deposit
Cu6Sn5/Cu3Sn
Cu based LF

18. Experimental: Selective Etching
Useful for looking at morphologies of Cu6Sn5 and by differential measuring able to calculate the amount of intermetallic.
Cu6Sn5
Sn Deposit
Cu based LF

19. Intermetallic Growth Directly after Plating
1 hour after plating 1 week after plating
Intermetallic (Cu6Sn5) grows after plating by grain boundary diffusion of Cu into Sn. After one week the height is more than 3 m.

20. Growth of intermetallic Cu6Sn5 compared @ room temperature and @ 55 oC
Growth of Cu6Sn5
Growth of intermetallic Cu6Sn5 compared
@ room temperature 55 oC

21. Growth Curves at Elevated Temperature
Growth rate gives idea about amount of intermetallics
However, morphology of IMC is different

22. Morphology of the Intermetallics
Bulk Diffusion
Recrystallization
Grain Boundary Diffusion
Less Recrystallization
1 h 150 oC
4 h 125 oC
42 days 55 oC
1 year RT
Same amount of intermetallics!

23. Calculation of Volume Change
6 Cu + 5 Sn => Cu6Sn5
Calculation of the volume change with help of X-ray data of the unit cell results in:
 V = / = = 3.2%
Not taking into account the contribution of the reacting Cu:
V = /81.4= = 57.3 %
Thus: Cu6Sn5 formation can result in compressive stress.

24. Grain Growth Sn Cu6Sn5 C19400 Before postbake: 3-10 m
After postbake: 5-20 m

25. Diffusion barrier
Intermetallic Growth 1 2 1 2
This graph shows that @ ambient there is no additional intermetallic growth up to 2 years after postbake
No whisker observed after postbake @ ambient for > 4 years, irrespective of Sn thickness

26. CTE Mismatch Difference in CTE Sn= ~ 23 ppm/o FeNi42= ~4 ppm/o
Whiskers will grow during temperature cycling Sn
~23 ppm/o Cu
~17 ppm/o
FeNi42
~4 ppm/o
This situation is only relevant for assembled components
4/16/2017
CPD AIT

27. CTE Mismatches
No whisker growth on NiFe leadframes in isothermal storage,but after temperature cycling whiskers grow, contrary to Cu-based leadframes.
5 m Sn on NiFe after 500 cycles
-55/85oC
5 m Sn on CuFe after 500 cycles
-55/85oC

28. Temperature Cycling on Assembly
Source: Infineon
Whisker length after –40 oC/ 85 oC, 5K/min, 30 minutes dwell time on unsoldered components and modules
(SnPbAg and SnAgCu soldered)

29. Whisker in high humidity
Corrosion may happen due to water condensation in association with exposed base material forming a galvanic couple (exposed base material)
Also local defects within the tin may cause localised different nobility forming such a galvanic couple
Presence of water w/o condensation may support the corrosive effect
Cathode
Cu2+ / Cu = 0.34 V
Anode
Sn / Sn2+ = V
O2 in H2O
Sn Sn2+ + 2e-
Cu2+ + 2e-  Cu
Sn + 2OH-  Sn(OH)2 + 2e-
O2 + 2H2O + 4e- 4OH-
2Sn + 2H2O + O2 2Sn(OH)2

30. Whisker Growth Associated with Corrosion
Whiskers grow because of compressive stress in the plating which is caused by irregular growth of intermetallics or oxidation/corrosion products in the Sn layer.
Formation of oxide results in volume increase (29/47% SnO and 32% SnO2)
Tin Whisker
is forced out
Cu6Sn5
SnOx
Sn Deposit
Cu based LF

31. Relation to Field Life
After 4000 hrs even SnPb shows whisker growth (max length ~ 100 m)
Board assembled units do not show this corrosion or extreme whisker growth during the test time

32. Remarks on Pure Sn finish
Three mechanisms for whisker growth related to field conditions of SMT components have been idenified:
Irregular intermetallic growth at ambient temperatures
CTE mismatch during temperature cycling
Corrosion induced at elevated temperatures & high humidities
Open Questions:
Were does Sn from a whisker come from?
What is mechanism on micro scale?
Why SnPb helps mitigation of whisker growth?
What does board soldering/reflow do to whisker growth?
What can be used as accelerated test with a relation to field life?

33. Leadfree solder ball adhesion improvement in BGA-packages
Philips Applied Technologies
Update of May 2007
Jo Caers, Zhao Xiujuan, Jan Kloosterman

34. BGA Soldering

35. Root Cause Description of Missing Ball Issue
Present used Pb-free solder (SAC405) is very sensitive for impact loading because of:
Very high strength of the bulk solder
Formation of brittle intermetallic layer between solder ball and package substrate.
Formation of large Ag3Sn needles
Image of 0hr SAC-ball overview Ni
(CuNi)6Sn5
(CuNi)3Sn4

36. Cross sections of products showing missing SAC405 solder balls on NiAu pad
SEM/BSE images of PSK sample (missing bump), showing almost no intermetallic residues Ni

37. Solution Composition of SAC101 SAC405 Ag : 1% 4% Cu : 0.1% 0.5%
Controlling growth and structure of the IMC layer
Reducing the strength, while improving the ductility of the Pb-free solder used
￫ modification of the SAC solder ball composition from SAC405 to SAC101
Composition of
SAC101 SAC405
Ag : 1% 4%
Cu : 0.1% 0.5%
Ni : dopant no
X : dopant no
Sn : balance balance

38. Effects of Ag and Cu concentration
Effect of Cu-content (Supplier Presentation)
Effect of Ag-content (Supplier presentation)

39. Effect of Ag on liquidus temperature

40. High speed shear test Test equipment : Instron micro tester
Test location : Instron Singapore
Instruction and test set up by IME-Instron-Apptech Singapore
Test speed : 0.45m/s
Shear height : 50m
Ball diameter : 600m
High speed ball shear tester

41. High speed shear test-Failure modes
Three typical failure modes were found:
Fracture in IMC
Fracture in Bulk solder
Pad peel
Fracture in IMC
Fracture in Bulk solder
Fracture of Pad peel

42. High Speed Shear testing
Alloy
Composition
Supplier
SnPb (ref1)
eutectic A
SAC405 (ref2)
Sn-4Ag-0.5Cu
SAC305 + NiGe
Sn-3Ag-0.5Cu-xNiGe B
Castin 258
Sn-2.5Ag-0.8Cu-0.5Sb C
(SAC101 + doping)
Sn-1Ag-0.1Cu-0.02Ni-x
SACX
Sn-0.3Ag-0.7Cu-0.1Bi – X D

43. High speed shear test Failure modes on Supplier A finish
Bulk failure happens in SnPb and in SAC101 solder joints
The SAC0535 (SAC-X) alloy, SAC305 and SAC405 mainly fail in the intermetallic compounds
In general more intermetallic failure than for Supplier B finish

44. High speed shear test – Failure modes on Supplier B finish
Bulk failure only happen in SnPb solder joints
SAC101 only failed with Pad peel
Assume the IMC strength > the strength in Pad interface if failure in pad peel, SAC101 is better than other solder alloys to resist the high speed impact.

45. BLR tests: JEDEC Drop testing
Test board : standard JEDEC test board according to JESD22-B111

46. BLR tests: JEDEC Drop testing
Test packages:
Component
UBM finish
Solder ball
Supplier
LFBGA240
NiAu
SAC 101 (0.3mm) A B
OSP
SAC101 (0.3mm)
SAC405 (0.3 mm)
PbSn

47. BLR tests: JEDEC Drop testing Method
Drop test set-up
To event detector (AnaTech: Model STD)

48. BLR tests: JEDEC Drop testing Method
Test conditions
Acceleration: 1500g, 0.5ms half-sine pulse
Test duration: 30 drops
Failure criteria: daisy chain resistance larger than 500 and the first event of intermittent discontinuity followed by 3 additional such events during 5 subsequent drops
Sample size: 6 boards per type of package
Acceleration measured from base plate

49. BLR tests: JEDEC Drop testing results. SAC 101 versus SAC405
Supplier B-NiAu SAC101
Supplier A-NiAu SAC101
Supplier A-OSP SAC101
Supplier A-NiAu SAC405
SAC 101 clearly shows improved drop test performance compared to SAC405.
Failure distribution of supplier B is more narrow than that of supplier A
Following table shows the characteristic lifetime at 63.2% failure with 95% 2-sided confidence level, for SAC 101.

50. BLR tests: JEDEC Drop testing results. PbSn versus SAC 101
PbSn SAC 101 1.
100.
10. 5.
50.
90.
99.
ReliaSoft's Weibull
Probability
N-drops
Cumulative failures (%)
:17
Philips CFT
Philips
Weibull
NiAu
W2 RRX - SRM MED
F=9 / S=36
2-Sided-B [T1]
b=2.9551, h= , r=0.9898
Drop test performance of SAC 101 on NiAu is comparable to that of PbSn on NiAu!

51. Failure analysis – NiAu-SAC 405
3rd
2nd
1st
(a) U5 right corner 2nd bump, 7 drops. Crack through Ni/Ni3Sn interface at the component side.
(b) U5, right corner 1st bump. Crack in PCB.
(c) U13, down corner. Complete fracture through Ni/Ni3Sn interface at the component side.
(d) Details of (c).

52. Failure analysis (1) – NiAu-SAC 101
(b) Details of (a). Fracture through first Cu6Sn5 IMC layer then metal line at the substrate side.
(a) U3 up corner, 16 drops.
(c) U3 down corner, 16 drops.
(d) Details of (c).

53. Failure analysis – summary
SAC 101 solder ball, supplier A and B
From the cross sectioning analysis, it becomes clear that all failures are caused by the Cu track fracture in the daisy chain on PCB
Drop test performance of SAC 101 /NiAu is comparable to that of PbSn / NiAu. Drop test performance of SAC 101/OSP is a factor 2 better than PbSn / NiAu
SAC405 solder ball, supplier A
Complete fracture through Ni/Ni3Sn interface at the component side is observed in many cases. Fracture of the Cu traces in the PCB, and of the PCB does occur, but is not the main failure mode.

54. Reliability Test Results of new solder ball
High Temperature Storage Tests
No missing balls after 1500 hrs testing
Single uniform IMC layer with decreased thickness compared to SAC 405 (see picture)
Image of 0hr SAC101-ball overview
Image of 0hr SAC405-ball overview

55. Summary BGA Open Questions: What is influence of dopants?????
JEDEC drop test improvement
All SAC101and SAC105 products showed failures at the PCB side.
SAC101on NiAu shows equivalent performance as PbSn on NiAu
SAC101 on OSP shows a factor 2 improvement in JEDEC drop testing compared to PbSn on NiAu
Temperature Cycling Data
SAC101 outperforms SAC405
SAC101 ‘outperforms’ SnPb
Results from high speed shear test:
SAC101 closest to SnPb with respect to failure mode; no intermetallic failure observed for this alloy
Open Questions:
What is influence of dopants?????
What does the Ag and Cu concentration do exactly??

56. But remember:
It’s not easy
being green