Reliability Issues in Lead free Electronic Assemblies COST MP 0602 Meeting IPM, Brno, CZ 27 th...

Reliability Issues in Lead free Electronic Assemblies

COST MP 0602 MeetingIPM, Brno, CZ

27th August 2007Paresh Limaye

imec/restricted 2007 2Paresh Limaye IPSI/REMO

Introduction

• Reliability• Types of Reliability issues• Component reliability• PCB reliability• Solder joint reliability• Others

– Sn whiskers

– Brittle Solder failures

• Summary


Reliability

• From the point of view of the end user Product has to perform as is promised to the

customer AND has to last for as long as possible (or until the user gets tired of using the product)

Probability that the device performs as expected for an expected duration.

• From the point of view of product seller Product has to perform as is promised to the

customer AND has to last for a certain period (Warranty period/time till an newer version of the product is introduced)


Reliability Issues

Broad categories• Component reliability• PCB reliability• Solder joint reliability• Electro-migration• Sn whiskers• Others…….. Many more


Reliability – Specific to Lead Free soldering

• Dealing with products, systems, specifications designed for tin lead backed with 50 years of field data

• The increased soldering temperature has a significant impact on product reliability.

• Degradation rate doubles with every 10oC.• Lead-free solder has different mechanical

properties compared to SnPb.


Components

• Components can be degraded/damaged during the reflow soldering process

• Thermal load: temperature-time– Damage to internal temperature sensitive structure of component:

electrolytes, insulating materials,…

– Shift in electrical performance, reduced life span,…


Components - Moisture

Absorbed moisture: rapidly expands during reflow soldering may lead to cracking of the component package: pop-corning.

•Absorbed moisture may lead to excessive component warpage

– Opens/Shorts

– Poor quality solder joints


Printed Circuit Board: laminate

• Lead-free reflow soldering / Hot Air Solder Leveling puts more thermal load on the board.

• Risks:• Via barrel cracking due to CTE mismatch

between Cu barrel and epoxy laminate in Z-direction.

• Delamination• Board sagging• Discoloration

• Important parameters:• Glass transition temperature Tg• Time-to-delamination: T260, T288• Decomposition temperature• Z-expansion: 50-250oC


Printed Circuit Board: Board finish

• Lead-free HASL (Sn100C, SAC)– High thermal load on PCB: not suitable for thick multilayers

• Immersion Sn– Sn whiskering in unsoldered areas

– Solderability: shelf life, multiple reflow

• Immersion Ag– Solderability: sensitive to sulphur

• NiAu– Au Embrittlement: Immersion OK, electroless (?), electroplated NOT OK.

– Black pad (PCB manufacturer)

– Skip plating (PCB manufacturer)

– Soldering to Ni instead of Cu: slower

• All cases: risk of harmful chemistry residues in via holes.• OSP (Organic Solderability Preservative)

– Solderability

– Invisible quality issues


Possible solder joint failures

Poor quality solder joint• Insufficient temperature-time: cold joint (assembly)• Excessive temperature-time: brittle joint due to excessive

intermetallics (assembly)• Solderability issue

– Component leads (component manufacturer, storage)

– PCB surface (PCB manufacturer, storage)

• Incompatible metallurgy of lead-finish (design)• Contaminated solder joint (design, assembly)

Good quality solder joint: Fracture failures • Shock• Fatigue

– Vibration

– Thermal cycling


Key aspects of SAC solders

SnAg3-4Cu leads to increased stress levels!• Stiffer material than SnPb: significantly higher

E-modulus. The same deformation leads to a higher stress level.

• Stronger than SnPb: can bear higher stress levels

• Lower plasticity than SnPb.• Higher solidification temperature leads to

increased stress levels in the component/joint after solidification when thermal mismatch is present.

• Creep rate (deformation under constant load) is 10 to 100 times slower than for SnPb.


Solder joint: SAC

• SA3-4C may lead to failures elsewhere than in the solder!

• Intermetallic layer• PCB pad lifting• Component pads and body

(ceramic chip)

• SA3-4C solder joints are more susceptible to shock.• SA3-4C solder joints are less resistant to strong

vibrations• Increasing trend to move towards lower Ag content

solders SAC 105 etc.• Metallurgical Mess!!!


Thermo-mechanical Fatigue

Package

Board

Package

Board

Board

Package

Board

Package has lower CTE – 7-12 ppm/C

Board has higher CTE – 16-18 ppm/C

Thermo-mechanical load is taken up by the solder


Thermal Cycling

• Solder joints experience creep-fatigue

• Fracture is initiated and the crack grows until the joint is mechanically separated


Recrystallize/Refined

Grain Zone

Cracking of the joints accompanied /preceded by recrystallization in the region of high strain accumulation

Microstructure – Damage Zone


Solder joint: Thermo-mechanical fatigue

How is reliability ensured?• Identify the loading mechanism during operation• Identify failure mode and failure distribution• Accelerate failure mode in testing• Compare to qualification standards (themselves

based on acceleration models and past experience/field data)



• Accelerated thermal cycling test: e.g. 0C-100C, 1 cycle/hour

• Determination of failure distribution: e.g. Weibull distribution

• Determination of acceleration factor with respect to field condition based on failure model: e.g. Coffin-Manson, Norris-Landzberg

• Lifetime estimation under field conditions


Is SAC Reliable enough?Is it as reliable as SnPb?

• NO SINGLE ANSWER

Stress leveldependency(J.-P. Clech)


Thermal cycle experiments

• 10 mm x 10 mm x 0.68 mm WLCSP device daisy chained– 64 I/O

• Assembled on 2.5 mm thick board (High Tg)• Two pad sizes: 250 m and 450 m • Sn 4%Ag 0.5% Cu – 300 m and 450 m

preformed BGA Spheres


Thermal Cycling


Acceleration Factors:SAC

• Norris-Landzberg equation:

• N.Pan et al., HP, 2005; Salmela et. al., Nokia, 2006 have published models for SAC

• Acceleration factor more sensitive to maximum test temperature as well as to the temperature amplitude. Increased acceleration factor compared to SnPb at constant dwell time.

max, max,

1 11.9 0.33

. field test

Ea

k T Ttest field

field test

T fA F e

T f


Acceleration Factors – Experimental vs Modeled

AFN63 = 1.068(AFe)1.239

R2 = 0.836

AFN63 = 1.037(AFW)1.385

R2 = 0.904

0.01

0.1

1

10

100

0.1 1 10FEM Based AF (N63)

Ex

pe

rim

en

tal A

F(N

63

)

Strain energy based AFStrain based AFCorrelation line+2XX/2Power (Strain based AF)Power (Strain energy based AF)

. field

test

NA F

NNf=C(e)n


Acceleration Factors – Comparison

0.1

1

10

0.1 1 10

Experimental AF's

Oth

er A

Fs Strain Based AF vs

Experimental AF

Salmela Model vsExperimental AF

Strain Energy based AFvs Experimental AF

Correlation line

Strain Energy basedfatigue life AF vsExperimental AFPower (Strain Energybased AF vs ExperimentalAF)Power (Strain Based AFvs Experimental AF)

Power (Salmela Model vsExperimental AF)

Power (Strain Energybased fatigue life AF vsExperimental AF)

max, max,

11 11.662 0.33

1.

fieldfield test

test

Eac

k T Tfieldtest field

field test ctest

corr TT fA F e

T fcorr T

Salmela’s Model


Acceleration Factors

• Salmela’s model accounts for the solder material and the component type used

• Tends to over predict the AF at higher range and under predicts at lower ranges

• AF’s based on strain energy density show better correlation with the experimental observations

• We are far from understanding the real accelerated behaviour of SAC solders

• Creep mechanisms and their activation • Creep behavior of the various new alloys being

introduced through the range of temperature of accelerated testing



• Reliability statements are based on accelerated thermal cycling tests.

• These tests have been designed for SnPb solders.

• Accelerated tests give different results depending on test conditions, joint configuration, failure criterion, stress level,..

• 10-100 times lower creep rate of lead-free solders reduces the acceleration factor of the accelerated test.

• Risk: accelerated tests may overestimate fatigue resistance of lead-free solders!


Solder joint: Contamination

• Solder joint contamination may have a negative impact on the solder joint reliability

• Pb in lead-free solder joint– Source: SnPb solderable finish, contaminated solder bath

– Effect: tendency to form low melting phase SnPbAg (179oC). Weakened solder joint in last solidified region.

• Bi in SnPb solder joint– Source: SnBi solderable finish

– Effect: tendency to form low melting phase PbBi (96oC). Severely weakened solder joint.

• Au in lead-free or SnPb solder joint– Source: NiAu solderable finish

– Effect: formation of highly brittle SnAu intermetallics. Au embrittled solder joint.


Solder joint: Contamination

• Reports from July 2007 suggest that leaded solder components are becoming scarce

• Assemblies relying on SnPb solders (high rel. applications) may end up being forced to use lead free components

• Long term reliability ????? • Need to understand

– Creep Behaviour of mixed/contaminated alloys

– Accelerated and field cycling behaviour


Intermetallic related issues

• Other causes of potential solder joint failure• Kirkendall voiding related to differences in diffusion

between elements at solder/base metal interface.– Effect: weakened interface, cracking

along interface

– More severe with lead-free soldering because of high Sn content.

– AgPd not compatible with SAC(?).

– Source of concern, not clear yet.

Electroless Ni/Immersion Au Ni3P precipitation ENIG requires typically 8% P in Ni. Intermetallics growth leads to Ni3P precipitation along interface Brittle interface: cracking Concern for both SnPb as well as lead-free soldering


Sn whiskering

• SnPb3-10% has been widely used as a solderable lead finish for components. Ban of Pb leads component manufacturers to go for pure Sn because of its low cost, availability and good solderability properties.

• Pure Sn whiskers!• Tin whisker (inspection definition):

A spontaneous columnar or cylindrical filament, which rarely branches, of tin emanating from the surface of a plating finish. (NEMI)

Kinked BranchedOdd-Shaped

Eruptions (OSE)


What do we know about whiskers?

•It may create shorts under field operation conditions. It is NOT a production issue!•Satellites/Cruise missiles even Nuclear plants affected by this• It is not only an issue of pure Sn.• Compressive stress in the Sn layer drives whisker growth.• No quantitative view yet on impacting parameters.• Several mitigation techniques: no clear solution.


Brittleness Testing of Leadfree solders –Charpy Test

0

10

20

30

40

50

60

70

-200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100

Temperature, oC

Fra

ctu

re t

ou

gh

ne

ss

, J/c

m2

Sn-5%AgSn-4%Ag-0.5%CuSn-3%Ag-0.5%CuSn-37%PbSn-0.7%Cu(Ni)99.99%SnSn-0.7%Cu

Clear ductile to brittle transition for Pb-free solders!


Brittleness Testing of Leadfree solders – Mini Charpy Test

12

E1=mgH1

(initial energy)

E2=mgH2

(final energy)

L

90°-1

H1=L+Lcos(90-1)=L(1-sin 1)

LL

E=mg(H1- H2)= mgL(sin 2 - sin 1)

hit point

Hammer

Cooling block (down to –150°C)

Sample location


0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

0.011

-120 -100 -80 -60 -40 -20 0 20 40

Temperature (°C)

En

erg

y A

bso

rbe

d (

J/jo

int)

SAC305

SAC405

SnAg

SnPb

Brittle Solder FailuresEnergy for breaking joints

To Be published in the proceedings of EPTC 2007


Brittleness Testing of Leadfree solders – Mini Charpy Test

Test at -88ºC

Test at 23ºC

K. Lambrinou

Potential concern for assemblies that operate in high shock/ extreme temperature environment


Summary

• Reliability issues in leadfree electronic assemblies – various sources– Component reliability, PCB reliability, Solder joint reliability, Sn

whiskers, Flip Chip related ……….. MANY MORE!!!

– Higher melting temperature

– Higher solder stiffness

– Higher propensity to form intermetallics

• Various alloys and surface finishes being used: Metallurgical Mess!!!

• Long term solder joint related effects – Will lead free solders perform as well as tin lead? NO clear answer


Summary

• Depends on the loading condition is which the solder joint is expected to fail.

• Need for identifying creep mechanisms active in field conditions AND accelerating those in testing

• Need good creep and acceleration models• Mixed leadfree-SnPb solders – very little

information• Sn Whiskers is an issue for high reliability.

applications for which we have no solution• Low temperature brittle behaviour of LF solder

can be an issue.


Special Thanks to

• Bart Vandevelde• Ingrid De Wolf• Geert Willems (www.rohsservice.be)• IPSI/REMO Group• Dr. Jean Paul Clech, Dr. Robert Darveaux• ALSHIRA Partners –Connectronics,TBP (Geel),

Alcatel-Lucent (Antwepren) Multiboard, IMEC Gent, Interflux, Electronic Apparatus: (http://www.imec.be/ALSHIRA)

Reliability Issues in Lead free Electronic Assemblies COST MP 0602 Meeting IPM, Brno, CZ 27 th...

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