This presentation was given to a meeting of the ELFNET project (European Lead-Free Soldering Network) in Zurich in March 2006. http://tinyurl.com/ybcumh3

1. Philips Leadfree solder ball adhesion
improvement in BGA-packages
Philips Applied Technologies
March 2006
Jo Caers, Zhao Xiujuan, Jan Kloosterman

2. Problem Description
• Missing SAC405 solder balls are observed on some BGA
packages after shipment to the customer
• Drop test performance for on board / mobile applications of
Pb-free (L)FBGA packages shows decreased performance
compared to Pb-containing packages
Jo Caers c.s. – Elfnet workshop, March 2006 2

3. Cross sections of products showing missing
SAC405 solder balls on NiAu pad
Ni
Ni
Ni Ni
SEM/BSE images of PSK sample (missing bump), showing almost no intermetallic residues
Jo Caers c.s. – Elfnet workshop, March 2006 3

4. 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
(CuNi)6Sn5
(CuNi)3Sn4
Ni
Image of 0hr SAC-ball overview
Jo Caers c.s. – Elfnet workshop, March 2006 4

5. Solution to the problem
1. Controlling growth and structure of the IMC
layer
2. Reducing the strength, while improving the
ductility of the Pb-free solder used
Above items resulted in proposal to modify
the composition of the SAC solder ball
(SAC101).
Jo Caers c.s. – Elfnet workshop, March 2006 5

6. New Solderball
• Composition of
• SAC101 SAC405
– Ag : 1% 4%
– Cu : 0.1% 0.5%
– Ni : dopant no
– X : dopant no
– Sn : balance balance
– Remarks:
• Ag : lowering the melting point; improving the wet-ability; increasing the stiffness of the
bulk solder
• Cu : lowering the melting point; suppressing the diffusion of Ni into the solder; reduces
the elongation. However, lowering the Cu-content results in improved drop test
performance.
• Ni : suppressing mutual diffusion between Sn of the solder bulk and Cu or Ni of the land;
reduces the elongation. Improves TMCL behavior, and IMC composition
• X : restores wet-ability decrease due to lowering the Ag-content, restores elongation
Jo Caers c.s. – Elfnet workshop, March 2006 6

7. Effects of Ag and Cu concentration
Effect of Cu in SAC alloys on brittle failure
60
50
Brittle failures (%)
40
30
20
10
0
0 0.2 0.4 0.6 0.8 1 1.2
Cu level (%)
Effect of Cu-content (Supplier Presentation)
Effect of Ag-content (Supplier presentation)
Jo Caers c.s. – Elfnet workshop, March 2006 7

8. Effect of Ag on liquidus temperature
Jo Caers c.s. – Elfnet workshop, March 2006 8

9. Evaluation methods for new solder ball
– Component level high speed shear testing : BGA 23 x 23
– Board level JEDEC drop test : LFBGA240
– Board level TMCL : BGA256, TFBGA228
– HTSL : 1500 hours @150 °C
– Wetting test of solder balls
Jo Caers c.s. – Elfnet workshop, March 2006 9

10. High Speed Shear testing
Alloy Composition Supplier
SnPb (ref1) eutectic A
SAC405 (ref2) Sn-4Ag-0.5Cu A
SAC305 + NiGe Sn-3Ag-0.5Cu- B
xNiGe
Castin 258 Sn-2.5Ag-0.8Cu- C
0.5Sb
(SAC101 + doping) Sn-1Ag-0.1Cu- A
0.02Ni-x
SACX Sn-0.3Ag-0.7Cu- D
0.1Bi – X
Jo Caers c.s. – Elfnet workshop, March 2006 10

11. 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 : 50μm
• Ball diameter : 600μm
High speed ball shear tester
Jo Caers c.s. – Elfnet workshop, March 2006 11

12. 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
Jo Caers c.s. – Elfnet workshop, March 2006 12

13. High speed shear test - Failure modes on Supplier A finish
Failure m odes of SAC 0535 on Failure m odes of SAC101 on supplier Failure m odes of SAC305 + NiGe on
supplier A substrate A substrate supplier A substrate
0%
2%
3%
Pad peel
38%
Pad peel IMC Pad peel
48% IMC
50% Bulk IMC
Bulk 62% Bulk
0%
97%
Failure m odes of Castin 258 on Failure m odes of SAC 405 on supplier Failure m odes of SnPb on supplier
supplier A substrate A substrate A substrate
0% 9%
0%
16%
0%
38%
Pad peel Pad peel Pad peel
IMC IMC IMC
Bulk 62% Bulk Bulk
84%
91%
• 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
Jo Caers c.s. – Elfnet workshop, March 2006 13

14. High speed shear test - Failure modes on Supplier B finish
Failure m odes of SAC 0535 on Failure m odes of SAC101 on Supplier Failure m odes of SAC305 + NiGe on
Supplier B substrate B substrate Supplier B substrate
0%
15%
29%
Pad peel Pad peel
Pad peel
IMC IMC
IMC
71%
85%
100%
Failure m odes of Castin 258 on Failure m odes of SAC 405 on Supplier Failure m odes of SnPb on Supplier
Supplier B substrate B substrate B substrate
14%
17%
0%
Pad peel
44% Pad peel Pad peel IMC
IMC IMC Bulk
56%
86%
83%
• 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.
Jo Caers c.s. – Elfnet workshop, March 2006 14

15. Summary of high speed shear test
• 6 solder alloys selected
• Evaluation method selected and made available
• Results from high speed shear test:
– SAC101 closest to SnPb with respect to failure mode; no intermetallic failure
observed for this alloy
– Except from SAC101, SAC-X shows most pad peel failures and least
intermetallic failures of the Pb-free alloys
– No physical explanation for the differences in failure mode yet:
• effect from intrinsic strength of the intermetallic layer?
• effect of inhomogeneities causing a tail in the strength distribution at the lower
strength values?
• effect of ductility of the bulk solder?
– Resolution of max. force, total deflection and total energy still being evaluated
• Results from wetting:
– From on-line monitoring: uniform proceeding of the wetting for all alloys
– From the Pb-free alloys, SAC101 performs best, based on wetted area
Jo Caers c.s. – Elfnet workshop, March 2006 15

16. BLR tests: JEDEC Drop testing
• Test board : standard JEDEC test board
according to JESD22-B111
Jo Caers c.s. – Elfnet workshop, March 2006 16

17. BLR tests: JEDEC Drop testing
• Test packages:
Component UBM finish Solder ball Supplier
LFBGA240 NiAu SAC 101 (0.3mm) A
LFBGA240 NiAu SAC 101 (0.3mm) B
LFBGA240 OSP SAC101 (0.3mm) A
LFBGA240 NiAu SAC405 (0.3 mm) A
LFBGA240 NiAu PbSn A
Jo Caers c.s. – Elfnet workshop, March 2006 17

18. BLR tests: JEDEC Drop testing. Method.
• Board assembly
– SAC 305 solder paste
– Standard lead-free reflow profile with peak reflow
temperature 245°C
Jo Caers c.s. – Elfnet workshop, March 2006 18

19. BLR tests: JEDEC Drop testing. Method.
• Drop test set-up
To event detector (AnaTech: Model
128-256 STD)
Jo Caers c.s. – Elfnet workshop, March 2006 19

20. 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 500Ω 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
1800
1600
1400
1200
1000
exp
a (g)
800
calc
600
400
200
0
-200
398.0 398.5 399.0 399.5 400.0 400.5 401.0
t (ms)
Acceleration measured from base plate
Jo Caers c.s. – Elfnet workshop, March 2006 20

21. BLR tests: JEDEC Drop testing results.
SAC 101 versus SAC405
Probability
99.
90. Supplier B-NiAu SAC101
Supplier A-NiAu SAC101
50.
Cumulative failures (%)
Supplier A-OSP SAC101
Supplier A-NiAu SAC405
10.
Supplier A-NiAu SAC405
5.
1.
1. 10. 100.
N-drops
• 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.
A-NiAu B-NiAu
Upper Limit 84 68
Drops 56 45
Lower Limit 37 30
Jo Caers c.s. – Elfnet workshop, March 2006 21

22. BLR tests: JEDEC Drop testing results.
PbSn versus SAC 101
PbSn SAC 101
ReliaSoft's Weibull++ 6.0 - www.Weibull.com
Probability
99.
Weibull
90. NiAu
W2 RRX - SRM MED
F=9 / S=36
50. CB[FM]@95.00%
Cumulative failures (%)
2-Sided-B [T1]
10.
5.
Philips
Philips CFT
2005-11-02 13:17
1.
10. 1. 100.
N-drops
β=2.9551, η=51.6013, ρ=0.9898
• Drop test performance of SAC 101 on NiAu is comparable to
that of PbSn on NiAu!
Jo Caers c.s. – Elfnet workshop, March 2006 22

23. Failure analysis – NiAu-SAC 405
3rd 2nd 1st
(a) U5 right corner 2nd bump, fail @ 7 (b) U5, right corner 1st bump. Crack
drops. Crack through Ni/Ni3Sn interface at in PCB.
the component side.
(c) U13, down corner. Complete (d) Details of (c).
fracture through Ni/Ni3Sn interface at
the component side.
Jo Caers c.s. – Elfnet workshop, March 2006 23

24. Failure analysis (1) – NiAu-SAC 101
(a) U3 up corner, fail @ 16 drops. (b) Details of (a). Fracture through first
Cu6Sn5 IMC layer then metal line at the
substrate side.
(c) U3 down corner, fail @ 16 drops. (d) Details of (c).
Jo Caers c.s. – Elfnet workshop, March 2006 24

25. 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.
Jo Caers c.s. – Elfnet workshop, March 2006 25

26. Reliability Test Results of new solder ball
– HTSL
• No missing balls after 1500 hrs testing
• Single uniform IMC layer with decreased
thickness compared to SAC 405 (see picture)
Jo Caers c.s. – Elfnet workshop, March 2006 26

27. BLR TMCL data
• BGA256, NiAu finish (SAC101 vs SnPb)
– Test condition : -55 125 °C (20/20 minutes)
– Preliminary result : 0 failures after 3000 cycles with SAC 101
– First failures : 2 after 3016 and 3053 cycles resp.
– SAC101 1 % failures : ~ 3000 cycles
– PbSn 1 % failures : 2464 cycles
– PbSn first failures : 668, 1638, 2051, 2514 etc.
On-line monitoring with event detector
• TFBGA228 (SAC101 vs SAC405)
– Test condition : -40 125 °C (30/30 minutes)
– Preliminary result : 0 failures after 820 cycles with SAC 101
– SAC405 1 % failures : 317 cycles
– SAC405 50% failures : 594 cycles
Off-line evaluation
Jo Caers c.s. – Elfnet workshop, March 2006 27

28. BLR TMCL data
• Conclusion:
– BLR TMCL behavior of SAC 101 is much better than that of
SAC405
– BLR TMCL behavior of SAC 101 is comparable or better
than PbSn
Jo Caers c.s. – Elfnet workshop, March 2006 28

29. Summary
• Pro’s of SAC101
– Excellent solution to missing ball issue, for all substrate suppliers!!
– Excellent BLR drop test performance, for all suppliers!! So, excellent for mobile
applications.
– Excellent BLR TMCL and wetting behavior
• Con’s of SAC101
– Liquidus* is higher compared to SAC405 (228 v. 220°C); however, reflowing the
SAC101 30 to 60 seconds above liquidus, results in a stable and reliable joint.
– Material properties?
• Philips is recommending the introduction of the SAC 101 because of
it’s excellent reliability, both wrt. JEDEC drop testing and BLR TMCL
testing.
• High speed shear tester excellent evaluation tool
Jo Caers c.s. – Elfnet workshop, March 2006 29

30. Acknowledgements
• Dr. J. de Vries (Apptech E’hven)
• Wong Ee Hua, Ranjan Ranjoo (IME, Sgp)
• Dr. Tsai (Instron, Sgp)
Jo Caers c.s. – Elfnet workshop, March 2006 30