Philips ELFNET 2006 SAC101

Philips Leadfree solder ball adhesion improvement in BGA-packages

Philips Applied TechnologiesMarch 2006Jo Caers, Zhao Xiujuan, Jan Kloosterman

2Jo Caers c.s. – Elfnet workshop, March 2006

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


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

NiNi

Ni


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


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).


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


Effects of Ag and Cu concentrationEffect of Cu in SAC alloys on brittle failure

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2

Cu level (%)

Brit

tle fa

ilure

s (%

)

Effect of Cu-content (Supplier Presentation)

Effect of Ag-content (Supplier presentation)


Effect of Ag on liquidus temperature


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


Alloy Composition SupplierSnPb (ref1) eutectic A

SAC405 (ref2) Sn-4Ag-0.5Cu A

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

A

SACX Sn-0.3Ag-0.7Cu-0.1Bi – X

D

High Speed Shear testing


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


High speed shear test-Failure modesThree 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


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 intermetalliccompounds

• In general more intermetallic failure than for Supplier B finish

Failure modes of SAC 0535 on supplier A substrate

48%50%

2%

Pad peel

IMC

Bulk

Failure modes of SAC101 on supplier A substrate

62%

0%

38%Pad peel

IMC

Bulk

Failure modes of SAC305 + NiGe on supplier A substrate

3%

97%

0%

Pad peel

IMC

Bulk

Failure modes of Castin 258 on supplier A substrate

16%

84%

0%

Pad peel

IMC

Bulk

Failure modes of SAC 405 on supplier A substrate

38%

62%

0%

Pad peel

IMC

Bulk

Failure modes of SnPb on supplier A substrate

9%

0%

91%

Pad peel

IMC

Bulk


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.

Failure modes of SAC 0535 on Supplier B substrate

85%

15%

Pad peel

IMC

Failure modes of SAC101 on Supplier B substrate

100%

0%

Pad peel

IMC

Failure modes of SAC305 + NiGe on Supplier B substrate

71%

29%

Pad peel

IMC

Failure modes of Castin 258 on Supplier B substrate

44%

56%

Pad peel

IMC

Failure modes of SAC 405 on Supplier B substrate

83%

17%

Pad peel

IMC

Failure modes of SnPb on Supplier B substrate

14%

0%

86%

Pad peel

IMC

Bulk


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


• Test board : standard JEDEC test board according to JESD22-B111

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

BLR tests: JEDEC Drop testing


• Board assembly– SAC 305 solder paste– Standard lead-free reflow profile with peak reflow

temperature 245°C

BLR tests: JEDEC Drop testing. Method.


• Drop test set-up


To event detector (AnaTech: Model 128-256 STD)


-200

0

200

400

600

800

1000

1200

1400

1600

1800

398.0 398.5 399.0 399.5 400.0 400.5 401.0t (ms)

a (g

) expcalc

Acceleration measured from base plate

• 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



A-NiAu B-NiAuUpper Limit 84 68Drops 56 45Lower Limit 37 30

BLR tests: JEDEC Drop testing results.SAC 101 versus 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.

Supplier A-NiAu SAC405

Supplier A-NiAu SAC101Supplier B-NiAu SAC101

Supplier A-NiAu SAC405

1. 100.10.1.

5.

10.

50.

90.

99.

Probability

N-drops

Cum

ulat

ive

failu

res

(%)

Supplier A-OSP SAC101


BLR tests: JEDEC Drop testing results.PbSn versus SAC 101

• Drop test performance of SAC 101 on NiAu is comparable to that of PbSn on NiAu!

1. 100.10.1.

5.

10.

50.

90. 99.

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

Probability

N-drops

Cum

ulat

ive

failu

res

(%)

2005-11-02 13:17Philips CFTPhilips

WeibullNiAuW2 RRX - SRM MED

F=9 / S=36CB[FM]@95.00%2-Sided-B [T1]

β=2.9551, η=51.6013, ρ=0.9898

PbSn SAC 101


(c) U13, down corner. Complete fracture through Ni/Ni3Sn interface at the component side.

1st2nd3rd

(d) Details of (c).

(a) U5 right corner 2nd bump, fail @ 7 drops. Crack through Ni/Ni3Sn interface at the component side.

(b) U5, right corner 1st bump. Crack in PCB.

Failure analysis – NiAu-SAC 405


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).


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.


– HTSL• No missing balls after 1500 hrs testing• Single uniform IMC layer with decreased

thickness compared to SAC 405 (see picture)

Reliability Test Results of new solder ball


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 cyclesOff-line evaluation


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


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


Acknowledgements

• Dr. J. de Vries (Apptech E’hven)• Wong Ee Hua, Ranjan Ranjoo (IME, Sgp)• Dr. Tsai (Instron, Sgp)

Technology

Philips ELFNET 2006 SAC101