Kamal Karimanal ANSYS Inc. - · PDF fileANSYS Inc. SEMATECH Workshop, March, 16 2010 Reliability Modeling Techniques Applicable to 3D-Packaging Using TSV ... • Fracture and interface

1ANSYS Inc. SEMATECH Workshop, March, 16 2010

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Reliability Modeling Techniques Applicable to 3D-Packaging Using TSV

Reliability Modeling Techniques Applicable to 3D-Packaging Using TSV

Kamal KarimanalANSYS Inc.

Presented at

SEMATECH Workshop on Stress Management for 3D ICs Using

Through Silicon Vias March, 16 2010

Kamal KarimanalANSYS Inc.

Presented at

SEMATECH Workshop on Stress Management for 3D ICs Using

Through Silicon Vias March, 16 2010


IC Packaging Challenges addressable by FEA Simulation

Board Level Interconnect Reliability

Solder Bump ReliabilityAccounting for Underfill

Interface Delamination

Fracture Mechanics at Silicon Level


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Today’s Focus:

Techniques Applicable to TSV based 3D packaging


Causes for damage

• Externally imposed loads– Heat sink mounting forces– Shock, vibration, drop event

• Thermo-Mechanical stresses originating from within the IC– Due to CTE Mismatch


Simulation Techniques for Common Package Design Challenges

• Creep Failure of solder joints– Rate Dependent and Rate independent plasticity modeling– Coffin Manson type fatigue models

• Fracture of Silicon– Fracture Mechanics Models

• Interface Delamination– Cohesive Zone Modeling

• Multi-Scale Modeling Challenges– Sub-modeling

• Temperature Prediction Accuracy– Non uniform substrate conductivity mapping due to copper-FR4

non homogeneities• This is applicable to predicting 3D Package temperature distribution


Thermo-Mechanics 101

CTEVdTdV

−= αα ,

At Elevated Temperature

At Temperatures levels below un-deformed state

Low CTE

High CTE

Un-deformed state


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Simulation Techniques & Their Applicability to 3D-Packages

Simulation Techniques & Their Applicability to 3D-Packages


Solder Joint Reliability at PCB/substrate Interface

• This may be treated the same way as being done for the traditional packages– Based on 2 types of models

• Constitutive models for the solder material

• Coffin Manson type fatigue model

Board & substrate Level SJR

( ) ( ) ( ) ( )Tftfffcr 4321 εσε =&


Traditional Solder Joint Reliability

• Popular Constitutive models– Material properties

• Anand Rate Dependent Plasticity Models• Hyperbolic Sine Creep Law (Gerafalo Model)

• Generic form of Coffin Manson Fatigue Relationship• Crack Initiation:

• Crack growth:

• Characteristic life:

210

KKN ψ=

43

KKdNda ψ=

dNda

aNW += 0α

Ψ is the damage sensitive parameterExample, strain energy density

K1, K2, K3, K4 determined by curve fitting test vs. simulation


SJR at stacked die interface?

• Probably not an issue– Since same material is on either side of the

micro bumps– Differential thermal expansion comes from

temperature gradients

Micro bumps at stacked die interface


Silicon Fracture due to TSV CTE Mismatch with Silicon

• The use of yield point as a failure criteria is simplistic

• FEA Technology offers different Fracture Mechanics models

TSVs


Thermo-Mechanical Fracture Modes

Generic Classification

Thermo-mechanical Fracture Modes applicable to TSV (CTE (cu) ~ 17PPM, CTE (Si) ~ 2PPM; Ref Temp ~ 120 (electro plating) (Lu et al)

DT > 0, R- Crack due to tensile mode

DT < 0, C- Crack due to tensile mode

Interfacial, Shear mode @ any DT Tensile mode @ DT<0

Tensile modeShearmode

http://146.6.104.11/~ruihuang/talks/TSV_June2009.pdf


Fracture Mechanics Models

• J Integral approach

• In nonlinear materials, the energy release rate is calculated using the integral J, which is a more general measure of G.

• The J values are compared against a Critical J value at which fracture is expected to happen.

• The J integral approach is a general energy release rate approach applicable non-linear materials

∫Γ ⎟⎠⎞

⎜⎝⎛

∂∂

−= dsxuTwdyJ i

i


Fracture Mechanics Models

• Stress Intensity Factor approach– This approach is applicable to linear Materials

– KI, KII & KIII are the 3 crack modes– The engineering approach is to utilize ANSYS to calculate J-Integrals or

stress intensity factors (K) directly and then compare these to experimental determined critical values (Kc or Gc)


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DelaminationDelamination


Delamination in 3D Packages

Delamination could occur at any of these interfaces

Die Thinning, microbumps, newMaterials & associated processesMakes delamination a non-trivial challenge


Models in FEA for Delamination prediction

• Cohesive Zone Model

• Virtual Crack Closure Technique (VCCT)


Models in FEA for Delamination prediction

• Cohesive Zone Model

• Virtual Crack Closure Technique (VCCT)


Cohesive Zone Model Inputs

• The user inputs at least three constants :– C1: Maximum normal traction at the interface σmax

– C2: Normal separation across interface at σmax when there is no shear separation

– C3: Shear separation across interface at τmax when there is no normal separation

nδ

tt δδ 2=


Two types of models

• Exponential CZM Law not suited when there are unloading or reverse loading effects

• Bilinear Law accounts for unloading and reverse loading

• Can be specified using Traction-Separation or Critical Fracture energies


Summary of CZM Inputs in ANSYS

Separation-Based σmax unc Gn

c τmax utc Gt

c η βNormal Stress Mode C1 C2 C5 C6Shear Stress Mode C3 C4 C5 C6Mixed Mode C1 C2 C3 C4 C5 C6Energy SpecificationNormal Stress Mode C1 C2 C5 C6Shear Stress Mode C3 C4 C5 C6Mixed Mode C1 C2 C3 C4 C5 C6


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Tackling the Multi-Scale ChallengeTackling the Multi-Scale Challenge


Multiple Size Scales of Influence/Interest

25 mm

2 mm0.1 mm0.01 – 0.1mm


Submodeling - A Solution to the Size scale Challenge

• The Sub Modeling approacha) Full Package model with lumped properties

to identify critical risk areas• Hot spots and heat transfer pathways make this a

non-trivial taskb) Cut out Region with detailed geometry and

material models• Transfer Displacement profile from (a) to the cut

boundary– St. Venant’s Principle

• Transfer temperature distribution from the cut volume


Sub Modeling Example

PCBLumped Solder

Ball Array

SubstrateEncapsulation

Lumped Bump Array

Traditional 0.3 mm Silicon

Lumped Bump Array

Thinned SiliconWith TSV (Lumped)

Lumped Micro Bump Array


Sub Modeling Example (Continued)

2W Uniform & Steady

3 W on for 30 Sec & Off for 30 sec on 2 mm hot spot

300 Sec

330 Sec

360 Sec

Average power until Pseudo Steady state1.5 W

3 W

H = 25 W/m2/K on all external surfaces


Multi Scale Modeling Approach

Transient Temperature

Total Deformation

Total Deformation

Refined Model at Level II


Stress distribution in silicon with TSV


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Temperature Prediction Accuracy

Temperature Prediction Accuracy


Importance of Temperature Prediction• Once past the road mapping and technology feasibility analysis,

tools are needed for analysis of specific designs.

• Design variations could be– Bump array distribution– Non-uniform TSV distribution– UBM & RDL metallization

• The above Distribution of metal play a crucial role in temperature distribution

• Techniques exist in ANSYS Icepak Software to account for metal distribution


PCB & Substrate trace and plane modeling

• Trace & Planes can be read from Cadence, Gerber file formats

• Proprietary algorithm captures local variation of metal conductivity without needing to mesh traces– As result computationally practical

• Trace & Planes can be read from Cadence, Gerber file formats

• Proprietary algorithm captures local variation of metal conductivity without needing to mesh traces– As result computationally practical

ECAD Imported PCB


Workflow for 3D-Packages

• Icepak tracemodeling poduces accurate temperature distribution

• WB platform allows easy coupling to Temperature distribution in Static structural analaysis

• Icepak tracemodeling poduces accurate temperature distribution

• WB platform allows easy coupling to Temperature distribution in Static structural analaysis


Upcoming Opportunities for Learning ANSYS Techniques for Electronics Reliability

• Registration site for the 2 day workshopshttp://www.ansys.com/services/ts-courses-detail.asp?TRAINING_ID=197&Location=NA

http://www.ansys.com/services/ts-courses-detail.asp?TRAINING_ID=197&Location=NA


Summary

• 3D Packaging with TSV poses new challenges– Can be addressed using traditional package FEA

modeling techniques

• Thermo-Mechanical stresses due to hot spots are dominant challenges

• Fracture and interface delamination challenges can be evaluated using FEA

• Sub-modeling is a key technique for refining multiple scales of interest

Documents

Kamal Karimanal ANSYS Inc. - · PDF fileANSYS Inc. SEMATECH Workshop, March, 16 2010 Reliability Modeling Techniques Applicable to 3D-Packaging Using TSV ... • Fracture and interface