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Hari SubramaniamTechnical Services Engineer
ANSYS, Irvine, [email protected]
Solder Joint Reliability (SJR) underThermal Cycling
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1-2ANSYS, Inc. Proprietary
2009 ANSYS, Inc. All rights reserved.February 23, 2009Inventory #002593
2011 ANSYS, Inc. October 8, 20142
Solder Joint Failure
CTE mismatchAt Elevated Temperature
Low CTE
High CTE
Undeformed state
Stiffness mismatch
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2011 ANSYS, Inc. October 8, 20143
For most consumer electronics, thermal cycling is the main cause for fatigue Plasticity and Creep causes solder joints in electronics to undergo incremental
damage with every thermal cycle Each load cycle causes stress damage to solder Damage accumulates over time
Damage Accumulation
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2011 ANSYS, Inc. October 8, 20144
Fracture is typically not modeled explicitly for SJR since this would becomputationally expensive
Instead, crack size or life are related to other parameters such as plastic work orstrain
By having ANSYS calculate plastic strain or work, we can relate this to the life ofthe part
Life Prediction
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2011 ANSYS, Inc. October 8, 20145
Utilize constitutive material model Possible inclusion of inelastic, rate-dependent and
temperature-dependent effects ANAND, Creep Models
Volume-averaged approach Pick a region and determine a volume-averaged
quantity for Life calculation
Predict Life Darveaux, Syed
Solder Joint Reliability - Workflow
Possible failure regions. VolumeAveraging done within this toPredict TTF
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2011 ANSYS, Inc. October 8, 20146
Plasticity models describe inelastic behavior that develops when the stress levelgoes beyond the yield strength of the material
Usually, a type of kinematic hardening is suitable for cyclic loading behavior Can be rate-independent or rate-dependent or coupled with creep
Review of Plasticity
Yield Strength y
Elastic Plastic
Unloading
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2011 ANSYS, Inc. October 8, 20147
Creep strain is driven by stress and temperature without a distinct yield surface. Usually separated into three regions Primary, Secondary, Tertiary. Solder
joint reliability is typically interested in secondary creep effects to get stabilizedcycles
Review of Creep
Generalized Garofalo (Hyperbolic Sine)
Combined Time Hardening (Double Power Law)
( )[ ] 34
21 sinh C T C
cr C eC
=&
( ) /T C C
/T C C C
cr te C C
et C
76
432
5
3
1
1
1
+
++
=
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2011 ANSYS, Inc. October 8, 20148
Popularized by Darveaux and others forsolder joint simulations
ANAND Model can be thought of as aspecial form of hyperbolic sine law with an
evolution term s
Review of ANAND Model
R. Darveaux, Effect of Simulation Methodology on Solder Joint CrackGrowth Correlation, ECTC 2000
m RT
Q
ins
Ae
1
sinh
= &
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2011 ANSYS, Inc. October 8, 20149
Inelastic Strain (or sometimes referred to as creep strain or plastic strain) is theirrecoverable strain that develops
Accumulated inelastic strain is an always increasing quantity Plastic Work (or creep strain energy) is the energy associated with this inelastic
strain
Volume Averaging technique will be used to reduce the sensitivity to meshing Plastic Work
Plastic Work and Strain
Possible failure regions. VolumeAveraging done within this toPredict TTF
=
V
V W W avg
volumeunit per work PlasticW ....=
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2011 ANSYS, Inc. October 8, 201410
Model proposed by Syed et al
Model proposed by Darveaux et al
Crack Initiation
Crack Growth
Characteristic Life
Life Prediction Methods
( ) 1= acc f W N
( ) 1= acc f C N
2
10K
avgW K N =
4
3K
avgW K dN da =
dN da
a N W += 0
Nf Cycles to FailureC and W Material Parameterseacc Accumulated creep strain per cyclewacc Accumulated creep energy density per cycle
Here, K 1 through K 4 are material
parametersa is the joint diameter at the interface(final crack length)
W is the plastic work per cycle
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Geometry Split the solder region for Volumetric Averaging Use DesignModeler, Spaceclaim
Workflow - Geometry
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Input Material properties in the Engineering Data
Workflow Material Data
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Check Connections Contact Create Named Selection
Named Selection will be used during post-processingfor calculating the volume averaged plastic work
Apply Thermal condition up to 3 to 4 cycles
Workflow - Loading
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2011 ANSYS, Inc. October 8, 201414
Workflow - Postprocessing APDL Commands are used for volumetric
averaging and Life calculations
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2011 ANSYS, Inc. October 8, 201415
NL-PLWK can be plotted using User-Defined Results
Workflow - Postprocessing
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2011 ANSYS, Inc. October 8, 201416
ANSYS also provides other approaches for failure prediction Cohesive Zone Model (CZM) Fracture Parameter
J-Integral Calculations [J]
Stress Intensity Factor [K] Energy Release Rate [G]
Crack Growth VCCT based crack growth
APPENDIX Other Methodologies
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ANSYS nCode DesignLife
PSD Cycle Counters
NarrowBand Only suitable for signals with single frequency content Steinberg Electronics Dirlik First general purpose (wideband) technique LaLanne Military Standard
APPENDIX Other Methodolgies
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Q & A
Thank you
Hari [email protected]