ANSYS solutions to Lead Free Package Design Challenges
ANSYS solutions to Lead Free Package Design Challenges
ANSYS solutions to Lead Free Package Design Challenges
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<strong>ANSYS</strong> <strong>solutions</strong> <strong>to</strong><br />
<strong>Lead</strong> <strong>Free</strong> <strong>Package</strong><br />
<strong>Design</strong> <strong>Challenges</strong><br />
Kamal Karimanal<br />
Sheldon Imaoka<br />
Vamsi Krishna<br />
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Today’s Topic<br />
<strong>ANSYS</strong> Solutions <strong>to</strong><br />
Electronics System <strong>Design</strong><br />
<strong>Challenges</strong><br />
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Drivers for <strong>Design</strong> <strong>Challenges</strong><br />
• Shipping and handling exposes electronics <strong>to</strong> shock<br />
and drop events<br />
– Can result in catastrophic failure even from a single<br />
event<br />
• Even normal, recommended end use conditions<br />
subject electronics <strong>to</strong> temperature fluctuations due <strong>to</strong><br />
power on and off<br />
– Thermo mechanical stresses incrementally damage<br />
components with every cycle<br />
– Failure is not about “if”, its about “When”<br />
• Need for life prediction capabilities<br />
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<strong>Design</strong> Implication of the <strong>Lead</strong> <strong>Free</strong><br />
Change<br />
PCB Mounting <strong>Design</strong><br />
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<strong>Design</strong> Implication of the <strong>Lead</strong> <strong>Free</strong><br />
Change at the <strong>Package</strong> Level<br />
Underfill -<br />
Material & thickness are<br />
coupled with process as<br />
well as product designs<br />
Silicon, IHS, Attach Material selection &<br />
<strong>Design</strong> are completely coupled. Larger<br />
Die = more functionality as well as<br />
reliability issues<br />
Similar <strong>to</strong> balls but entirely<br />
Different size scale issues.<br />
Coupled with underfill delam.<br />
Solder Balls – size, pitch, material<br />
PCB & Substrate pad config, mask<br />
design criteria<br />
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<strong>ANSYS</strong> Technologies for <strong>Lead</strong> <strong>Free</strong><br />
<strong>Challenges</strong> at the <strong>Package</strong> Level<br />
Underfill -<br />
Visco elasticity and Cohesive Zone Models<br />
in <strong>ANSYS</strong> Mechanical for product design.<br />
+<br />
VOF modeling capabilities in <strong>ANSYS</strong> CFD <strong>to</strong> model<br />
Capillary driven underfill flow for process design<br />
CZM in <strong>ANSYS</strong> Mechanical<br />
+<br />
Implicit as well as explicit solver<br />
Technologies for shock & drop reliability<br />
Viscoplasticity, creep, physics<br />
coupling needs & scripting<br />
+<br />
Contacts & HPC technology for<br />
Potentially large models in<br />
<strong>ANSYS</strong> Mechanical<br />
Solder Balls – non-linear Visco Plasticity<br />
& Creep models, thermo-mechanical coupling,<br />
If necessary for power cycling analysis. Scripting<br />
Capabilities <strong>to</strong> aid au<strong>to</strong>matic estimation of Fatigue<br />
Life in <strong>ANSYS</strong> Mechanical<br />
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<strong>Design</strong> Implication of the <strong>Lead</strong> <strong>Free</strong><br />
Change at the System <strong>Design</strong> Stage<br />
PCB Mounting<br />
<strong>Design</strong><br />
Stiffener plate<br />
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<strong>ANSYS</strong> Technologies for the <strong>Lead</strong> <strong>Free</strong><br />
<strong>Challenges</strong> at the System <strong>Design</strong> Stage<br />
PCB Mounting<br />
<strong>Design</strong><br />
Linear Static Structural, Modal and Random<br />
Vibration solver technologies in <strong>ANSYS</strong><br />
Mechanical<br />
Linear/non-linear flexible dynamics using implicit<br />
as well as explicit solver technologies in the<br />
WB environment for shock & drop reliability<br />
Stiffener plate<br />
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Today’s Focus:<br />
Solder Joint Reliability<br />
Under Thermal Cycling<br />
Fatigue<br />
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Thermo-Mechanics 101<br />
dV<br />
dT<br />
= α V , α − CTE<br />
Un-deformed state<br />
At Elevated Temperature & Moisture Levels<br />
At Temperatures & Moisture levels below un-deformed state<br />
Low CTE<br />
High CTE<br />
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Review of Stress Vs. Strain Curve<br />
• Plasticity models describe inelastic behavior<br />
that develops when the stress level goes<br />
beyond the yield strength of the material<br />
– Can be rate-independent or rate-dependent or<br />
coupled with creep<br />
σ<br />
Unloading<br />
Yield Point σ y<br />
Elastic Plastic<br />
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ε
Review of Creep<br />
• Materials tend <strong>to</strong> undergo creep at temperature higher then their<br />
Homologous temperature<br />
– Homologous temperature = 0.5 X Melting point (in Kelvin)<br />
• Secondary creep has been known <strong>to</strong> cause Failure of Solder<br />
Joints<br />
ε<br />
Primary<br />
Secondary<br />
Rupture<br />
Tertiary<br />
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t<br />
&<br />
ε =<br />
f<br />
( σ ) f ( ε ) f ( t)<br />
f ( T )<br />
cr 1 2 3 4
Thermal Expansion Implications <strong>to</strong><br />
Electronics<br />
• No Permanent damage for perfectly elastic<br />
materials<br />
• Among the IC package parts that undergo plastic<br />
deformation and/or creep, solder joints are the<br />
most susceptible <strong>to</strong> failure<br />
• For Most Consumer electronics, thermal cycling<br />
is the main cause for fatigue failure<br />
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Time To Failure (TTF) Prediction<br />
• For solder joint reliability, prediction of life in<br />
<strong>ANSYS</strong> is based on two Choices:<br />
– Choice of fatigue model<br />
– Choice of constitutive model<br />
• This is a material model that <strong>to</strong> be used in FEA that<br />
captures<br />
&<br />
ε =<br />
f<br />
( σ ) f ( ε ) f ( t)<br />
f ( T )<br />
cr 1 2 3 4<br />
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Fatigue model chosen for this work<br />
• Following taken from R. Darveaux, “Effect of Simulation Methodology<br />
on Solder Joint Crack Growth Correlation”, ECTC 2000<br />
• Crack Initiation:<br />
• Crack growth:<br />
• Characteristic life:<br />
N = K ΔW<br />
0<br />
da<br />
dN<br />
α<br />
W<br />
=<br />
K<br />
1<br />
3<br />
ΔW<br />
• The characteristic life α W is then related <strong>to</strong> the failure free life and<br />
cycles <strong>to</strong> first failure for failure prediction<br />
K2<br />
avg<br />
K4<br />
avg<br />
a<br />
= N0<br />
+<br />
da<br />
dN<br />
Here, K 1 through K 4 are<br />
parameters estimated by<br />
correlating ΔW avg from<br />
simulation predictions <strong>to</strong> test<br />
data of N 0 and crack growth<br />
rate<br />
a is the joint diameter at the<br />
interface (‘final crack length’)<br />
ΔW is the plastic work per<br />
cycle<br />
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Choice of Constitutive Model<br />
• Choice of constitutive model is tied <strong>to</strong> the<br />
model used <strong>to</strong> determine K 1 through K 4<br />
• In this work we will use K 1 through K 4<br />
determined by Darveaux using Anand<br />
Constitutive Model<br />
• Hence Anand Constitutive model is used in<br />
this work<br />
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Characterization Methodology<br />
(Darveaux, 2000)<br />
Cycles <strong>to</strong> initiation<br />
Pro<strong>to</strong>type<br />
Geometry<br />
+<br />
Material Models<br />
Determine K 1 and K 2<br />
Strain Energy Density (from simulation)<br />
Thermal Cycling<br />
Test<br />
<strong>ANSYS</strong> Simulation<br />
of Thermal Cycling<br />
Test<br />
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Rate of Crack Growth<br />
N o, , Cycles <strong>to</strong> initiation<br />
Rate of Crack Growth<br />
Strain Energy Density<br />
Or<br />
Plastic work<br />
Determine K 3 and K 4<br />
Strain Energy Density (from simulation)
Solder Joint Fatigue<br />
Simulation in <strong>ANSYS</strong><br />
Workbench Environment<br />
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<strong>ANSYS</strong> Workbench Workflow for<br />
Fatigue Simulation<br />
Detailed BGA<br />
from Macro<br />
<strong>ANSYS</strong> Workbench<br />
<strong>ANSYS</strong><br />
<strong>Design</strong>Modeler<br />
<strong>ANSYS</strong> Unified<br />
Meshing Platform<br />
Material props, Solver Settings<br />
Ansys Mechanical<br />
for<br />
Thermal Cycling Stress<br />
Simulation<br />
Post processing APDL<br />
for Cycles <strong>to</strong> failure<br />
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BGA Geometry Creation Macro<br />
• Ball Grid Arrays (BGA)<br />
type <strong>Package</strong>s are well<br />
suited for cus<strong>to</strong>mization<br />
because of their<br />
standardized geometry<br />
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Geometric Details and Simulation<br />
Intent<br />
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Geometric Details and Simulation<br />
Intent<br />
Substrate<br />
PAD<br />
SMD Mask<br />
NSMD Pad<br />
PCB mask Opening<br />
(NSMD)<br />
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Geometric Details and Simulation<br />
Intent<br />
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Geometry - Possible Simplifications<br />
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Simulation Intent, WB Mesh Control and<br />
Contacts Technology<br />
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Material Models and Material<br />
Properties Used for Sample Case<br />
All materials are linear elastic except the solder ball, which uses Anand rate<br />
Dependent plasticity model<br />
Silicon<br />
Underfill<br />
Solder, Anand model<br />
Mask<br />
PCB and BT resin<br />
orthotropic Young’s<br />
Modulus and CTE<br />
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Property Values and Anand Model<br />
Constants<br />
• PCB, BT, mask, Silicon, and Solder property<br />
values were used from:<br />
– “Impact of ball via configurations on solder<br />
joint reliability in tape-based, chip-scale<br />
packages” Zahn Bret A. , 52nd electronic<br />
components & technology conference : ( San<br />
Diego CA, 28-31 May 2002 )<br />
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Commands Object for Orthotropic CTE<br />
and Plasticity Models<br />
Parameters for Commands objects can be input from Details window in the GUI<br />
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Commands Object for Anand Model<br />
• Variable names ARG1,<br />
ARG2 etc in the APDL<br />
instruct WB <strong>to</strong> take<br />
inputs from the Details<br />
window for the<br />
corresponding<br />
commands object.<br />
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Thermal Cycle Input<br />
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User Choice for Performing Fatigue<br />
Calculations<br />
Named selections for sections of the solder joint<br />
expected <strong>to</strong> experience highest plastic strain<br />
fluctuations<br />
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Fatigue Post-Processing Calculations<br />
APDL inserted as command<br />
Object in Solution branch<br />
Results are calculated after<br />
Solution is completed<br />
“My_...” variables in the script<br />
make results appear in the<br />
Details window<br />
• Sample macros will be available for download at<br />
<strong>ANSYS</strong> Cus<strong>to</strong>mer portal<br />
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Results and<br />
Discussion<br />
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Cycles <strong>to</strong> failure calculation<br />
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Plastic strain on all solders<br />
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Stresses on solder pad<br />
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<strong>ANSYS</strong> Software Products Needed<br />
• <strong>ANSYS</strong> <strong>Design</strong>Modeler<br />
• <strong>ANSYS</strong> Mechanical<br />
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How Can <strong>ANSYS</strong> Help You?<br />
• BGA geometry creation macro<br />
• Workflow instructions<br />
– <strong>ANSYS</strong> Workbench<br />
Capabilities for geometry<br />
processing and meshing<br />
– Material models in <strong>ANSYS</strong><br />
Mechanical<br />
– Sample APDL scripts for GUI<br />
based user inputs for Material<br />
models and Fatigue Model<br />
• Sample <strong>ANSYS</strong> Workbench<br />
Project<br />
• Jump start assistance through<br />
service projects<br />
• Focused Training?<br />
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Take Home Material<br />
• The following materials have already been<br />
uploaded <strong>to</strong> <strong>ANSYS</strong> Cus<strong>to</strong>mer Portal<br />
– A workflow Tu<strong>to</strong>rial for performing Fatigue<br />
Calculation using Anand constitutive model<br />
and Plasticity based TTF prediction<br />
– A sample model containing more details and<br />
finer mesh than the tu<strong>to</strong>rial problem will also<br />
be available from <strong>ANSYS</strong> Cus<strong>to</strong>mer Portal.<br />
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Conclusions<br />
• We presented an overview of design challenges in<br />
electronics due <strong>to</strong> lead free transition<br />
– Focused on package specific challenges<br />
• Highlighted why <strong>ANSYS</strong> technology is best suited <strong>to</strong><br />
address these challenges<br />
• Used Solder Joint Fatigue simulation of a BGA<br />
package <strong>to</strong> illustrate workbench based workflow<br />
using <strong>ANSYS</strong> Mechanical software<br />
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Conclusions<br />
• Example workflow illustrated the following unique strengths<br />
– Geometry creation time savings due <strong>to</strong> <strong>Design</strong>Modeler Macro for BGA<br />
– <strong>ANSYS</strong> Meshing platform for high quality refined mesh where needed<br />
– Contacts technology for computational cost savings<br />
– Productivity enhancements through Au<strong>to</strong>matic Contact detection<br />
– Time tested <strong>ANSYS</strong> Mechanical solver with advanced non-linear models<br />
• Viscoplasticty and creep modeling capabilities<br />
– Workbench platform bridges the power of solver lever manipulation using<br />
APDL with user friendliness<br />
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Thank You!<br />
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