snni and snnicu intermetallic compounds found when …...• the behavior of lead-based solders and...

SnNi and SnNiCu IntermetallicCompounds Found When UsingSnAgCu Solders

Tom Gregorich and Pat Holmes, Qualcomm Inc., USAJeffrey Lee, ASE Group, TaiwanChin Lee, University of California at Irvine, USA

IPC/Soldertec Global 2nd International Conference on Lead Free Electronics June 23, 2004

Introduction• The behavior of lead-based solders and their intermetallics well understood

- Industry is only beginning to understand the behavior of lead-free solders

• Many believe that lead-free solders perform better than lead-based solders- Usually they do perform better. But not always…

Topics• Product design and discovery of issue• Analysis of failures• Theory of IMC formation• DOE and results• Conclusion and future plans


Product Design

• Family of lead-free 0.5mm CSPs• Body size from 12mm to 15mm• Solder ball rows selectively depopulated• Standard construction except for solder ball alloy:

- 2-layer BT resin substrate- Mold+saw singulation- 300µm solder ball on 275µm pad- Electrolytic NiAu plating on Cu- Sn4.0Ag0.5Cu solder ball


Pb-free CSP Development

• Qualified using same tests as components which use SnPb solder ball• Meets all performance requirements• Equal-to or better-than SnPb performance except for drop shock test


Results of Initial Production• Assembly of lead-free CSP was as-expected

• However, after electrical test a smallnumber

of packages were missing one or moresolder balls

• Missing balls were:- Usually found in the outer 4 rows of balls- Not always consecutive solder balls;

adjacent balls showed no evidence ofdamage

- Exposed pads had uniform matte-greyfinish with little evidence of solder

- Pads usually had a visible “halo” mark


SEM Analysis

• Little evidence of solder residue on the pads• Appearance similar to phosphorus-rich electroless NiAu• Fracture was brittle in nature and left “halo” mark


EDX Analysis

SnNiIMC

SnNiCuIMC

• Upper “sandy” region is NiSn and located adjacent to substrate pad• Lower “rocky” region is CuNiSn and located between solder ball and SnNi


Dual IMC Formation

Cu

Ni3Sn4formed

Cu6Sn5formed

SnAgCu

NiAu

Cu

SnAgCu

NiSnNi

Cu

SnAgCu

Ni

SnCuSnNi

Beforeassembly

+

(Cu1-xNix)6Sn5formed

Cu

SnAgCu

Ni

SnNiCuSnNi

Potentialfailure

Cu

SnAgCu

Ni

SnNiCu

SnNiForce

Solder Ball and SMT Reflow Assembly


Countermeasure DOE

• 2x2 DOE with solder flux and reflow profileLeg #1 - Original solder flux / original reflow profileLeg #2 - New solder flux / original reflow profileLeg #3 - Original solder flux / new reflow profileLeg #4 - New solder flux / new reflow profile

• Data collected from each leg- Cross Section SEM/EDX Analysis- IMC thickness measurements- Ball Shear and Pull- PCB Peel Test


Reflow Profile

Profile Time above 150°C Time above 217°C Peak tempOriginal over 200 seconds 85 seconds 250°C New under 100 seconds 37 seconds 243°C

Original Profile

New Profile


Leg #1: Original Flux/Original Profile

Ball-side IMC: ~2.0µm layer ofSnNiCu

Substrate-side IMC: ~0.4µm layer ofSnNi

Analysis of cross-section is similar to analysis of fracture interface: 2 IMCSnNiCu IMC is ~2.0µm thick and SnNi IMC is ~0.4µm thick


Leg #2 : New Flux/Original Profile



Similar grain structure to original processSimilar IMC thickness to original process


Leg #3: Original Flux/New Profile



Similar grain structure to original processIMC thickness reduced by 50~60% compared to original process


Leg #4: New Flux/New Profile



IMC thickness reduced by 50~60% compared to original process


Solder Ball Shear/Pull Test Results

• Specification for both tests is > 0.30 kg• No statistical difference found between the four DOE legs• Minimum spec was far exceeded in both tests

DOE Leg (Flux/Profile) Result (kg)Leg #1 - Original / Original 0.51Leg #2 - New / Original 0.53Leg #3 - Original / New 0.50Leg #4 - New / New 0.51

Solder BallShear Test

DOE Leg (Flux/Profile) Result (kg)Leg #1 - Original / Original 0.58Leg #2 - New / Original 0.56Leg #3 - Original / New 0.57Leg #4 - New / New 0.55

Solder BallPull Test


Solder Ball Peel Test Results

Package

PCB

Mode A

Mode B

Mode C

Profile Flux Mode CMode BMode AOriginal Original 63%29%8%

New Original 86%14%0%Original New 87%13%0%

New New 84%16%0%

IMC Thick Structure2.0/0.4µm fine

0.7/0.2µm fine1.8/0.4µm fine

0.9/0.2µm coarse

Leg#1

#3#2

#4


Conclusion• Formation of dual-compound IMC

- (Cu1-xNix)6Sn5 grown on top of Ni3Sn4- High dislocation density from differences in crystal structure & lattice- Reaction time increases IMC thickness and probability of failure

• Traditional tests cannot detect this failure mechanism- Solder ball shear and pull tests were not able to induce brittle failures- Peel test, drop test or other mechanical stresses required

• Countermeasures- Reduction in reflow temperature and reflow time can limit IMC thickness- Flux chemistry can affect IMC grain structure- Tighter control of substrate plating may have an effect (not studied)


Thank You!

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