Comparison of Pb-free Solder on UIC Test Boards and Components

6/3/2002 Universal Instruments Consortium – NokiaAll Rights Reserved

Comparison of Pb-free Solder on UIC Test Boards and Components

Steve DunfordNokia Mobile Phones


Agenda• UIC Pb-free Test Nokia

– Configurations– Test Conditions

• X-ray Inspection• First Failures• Weibull Plots• Intermetallic Formations in Pb-free solder

– Definition and examples– Reliability Improvements and Concerns– Solder Joint Voiding– Failure Mechanisms– Sn Whiskers

• Discussion• Conclusions


Configurations Tested At Nokia

• Configuration Sn62 Sn/Ag3.8/Cu 0.7Sn/Ag3.5 Castin OSP 8 32 32 8 Ni/Au 8 32 32 8• Stencil

– .005 inch– 1/1 aperture to pad

• Reflow Profile– Peak 244°C (SnAg)– Peak 241°C (Castin, SAC)– 3 Zone Oven– Ambient Conditions


Test Conditions

• Temp Cycle– 14 min. dwell+4 min. transition– -40°C to 125°C in single chamber– Components at temperature

• 11 minutes cold• 9.5 minutes hot


Assembly Results• Castin

– All Solder Joints Good– Popcorned During Reflow - Improperly dried – Significant Voiding in Solder Joints – Results

Invalid– Temp Cycle Failure Data Not Used - Invalid

• SAC – 1 Open

• Polyimide tape on package• Bridged daisy chain around open

• SnAg– 1 Open

• Bridged daisy chain around open• Sn62

– All Solder Joints Good


Assembly Results

SnAg Open SAC Open – Caused by Polyimide Tape on Component


Early Failures – Temp Cycle 0 to 12

SAC – Ni/Au board (461) Package side failure at cycle 12


Pb-Free Ni/Au Failures Up to 2700 Cycles

•SnAg – Ni/Au PWB, Failure on PWB at 2700 cycles Shown above.

•SAC – Ni/Au PWB, Board Side Failures 180 to 450 cycles


SnAg – Ni3Sn layer formed above Phosphorous rich Ni layer. Adhered well on most components

SAC Ni/Au Failures 2700 cycle


Weibull Plots – SAC NiAu

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Generated by: ReliaSoft's Weibull++ 5.0 - www.Weibull.com - 888-886-0410

Probability Plot

Time, (t)

Unre

liability,

F(t

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6:44:58 PM6/3/2002nokiasd

WeibullSAC-NiAu

P=2, A=RRX-R F=12 | S=0

“black pad” on PWB or Component contributed to SAC on Ni/Au failures in less than 200 cycles. Other failures not fully investigated at this time.

Solder fatigue failures


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Probability Plot

Time, (t)

Unre

liability,

F(t

)

12:25:19 AM6/4/2002nokiasd

WeibullSnAg-NiAu

P=2, A=RRX-R F=13 | S=0

Weibull Plots – SnAg on NiAu

Two Failure Mechanisms – not fully investigated at this time

Solder fatigue failures


Package Damage – 2000 cycles

Underfill separation, substrate cracks – Likely contributor to different failure mechanism. Sn62 – Ni/Au board, Failure at 2700 cycles


Package Damage – 2700 cycles

Underfill separation, substrate cracks – Likely contributor to different failure mechanism. Sn62 – Ni/Au board, Failure at 2700 cycles


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Probability Plot

Time, (t)

Unre

liability,

F(t

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6:38:32 PM6/3/2002nokiasd

WeibullSAC-OSP

P=2, A=RRX-R F=10 | S=0

Sn62-NiAu

P=2, A=RRX-R F=12 | S=0

Sn62-OSP

P=2, A=RRX-R F=14 | S=0

Weibull Plots – Best Performers

SAC on OSP and Sn62 on OSP


Intermetallic Formations in Pb-free Solder

• Necessary for durable interface between solder and solder pads

• Can Restrict Evolution of gases– Increased voiding

• Increase solder joint strength• Continue to form throughout lifetime

– Needles, dendrites, plates and tubes– Cu Pad dissolution

• Can re-direct fractures– Not Necessarily the preferred crack propagation

path• Potential for Spalling


X-Ray Inspection of Cycled Boards

SnAg on OSP 6500 cycles



SAC on Ni/Au 6500 cycles



CASTIN on OSP 6500 cycles



SAC on OSP 6500 cycles



SAC on OSP 6500 Cycles



SAC OSP 6500 cycles



SAC on Ni/Au 6500 cycles


Whisker growth similar to that seen in Previous X-ray

Whisker Growth – Not Typical Sn Whisker

SnAg on OSP 6500 Cycles


Whisker Growth - Inside Voids

SnAg Solder after 6500 cycles, -40 to 125°C


Whisker Growth - Inside Voids

Ag/Sn forms on the end of the Sn Whisker



Sn/Ag/Cu on OSP – Intermetallic formations and Void attached to Sn/Ag intermetallic plate



Sn/Ag Plate Redirects Crack, 2700 cycles (-40 to 125°C)



Ag/Sn intermetallic redirects crack propagation, SAC/OSP (2700 cycles, -40 to 125°C)



SPALLING – Portions of Solder Joint Separating (6500 cycles, -40 to 125°C)




Crack Propagation Arrested by Ag/Sn Flake


After etch



Sink Holes

Au/Sn Intermetallics

May be associated with continued formation of intermetallic compounds

Ag/Sn Spheres

Grain Boundary



Unique Failure Mechanisms & Continued Intermetallic Growth (6500 cycles, -40 to 125°C)



SPALLING – Portions of Solder Joint Separating along grain boundaries. (SAC, 6500 cycles, -40 to 125°C)



SAC solder joints 6500 cycles and extended etch


Intermetallic Formations SAC

SAC solder joint 6500 cycles and extended etch

SnAg3SnPlate



Ag3Sn spheres and dendrites form in subgrain boundaries

Sn

Ag3SnPlate



Fractures Form at Grain Boundaries


Discussions• SAC on OSP shows equivalent or

better performance than eutectic Sn/Pb solder

– Solder Paste Print– Temperature cycling tests from –40 to 125°C

• Intermetallics in Solder Joint – Ag/Sn Plates, Spheres and dendrites in

subgrain boundaries. – Can Arrest or Redirect Cracks

• Increase Joint life– More Extensive in SAC solder


Discussions

• Grain Boundaries Can be Preferred crack Propagation Path

– Cracks Any Location in Solder Joint• Vertical or horizontal• Potential for Spalling

• Some Concerns Remain– Increased Voiding is Likely to Occur

• Increased Intermetallic formation– Spalling Potential– Potential for Sn Whisker Growth


Conclusions

• Pb-free processing is Viable for many applications

– Can Provide Equivalent or Better Performance over Sn/Pb solders

• Our work is not done

Comparison of Pb-free Solder on UIC Test Boards and Components

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