© 2011 ANSYS, Inc. August 26, 20111

BGA Simulation in Workbench: Challenges and Successes

Presented by Kaan DivringiOzen Engineering Inc

© 2011 ANSYS, Inc. August 26, 20112

Ozen Engineering Inc

With over 25 years of experience in Finite Elements Simulations and Engineering Consulting, we collaborate with customers to provide the best in class expertise and solutions to their problems, enabling them to succeed.

We are the local ANSYS channel partner

www.ozeninc.com

© 2011 ANSYS, Inc. August 26, 20113

• BGA (Ball Grid Array) packages is a popular type of surface mount package

– The solder connections allow high density, robust connection to the PCB

– With packages becoming ever more complex and miniaturized, BGAs are an attractive form factor

• Unfortunately these solder ball connections are a likely point of failure

• Failure can occur from:

– Thermal stress from process conditions

– Cyclic fatigue

– Viscoelastic/plastic damage accumulation

BGAs Analysis Background

© 2011 ANSYS, Inc. August 26, 20114

Traditional Challenges of BGA Analysis: Geometric

• Lots of bodies due to the numerous solder joint connections alone

• As packages get more information dense and complex, this number will only increase!

• Slicing bodies for meshing increases this number even more

• Preprocessors will struggle with large numbers of geometric entities

• Angular features & length scales

• Makes stress results difficult, especially at material interfaces

• Long & thin elements susceptible to distortion, causing convergence failures

© 2011 ANSYS, Inc. August 26, 20115

Material Nonlinearities

• ANANDs material model is a material model well suited for modeling solderballsunder thermal cycling

• Many types of solder material characterized in available literature

• Material model is highly nonlinear:

– Rate dependent

– History dependent viscoplasticity

– Continuous creep under nonzero loading

• Not compatible with analysis restart!

Darveux2002

© 2011 ANSYS, Inc. August 26, 20116

Load Steps

• Thermal cycling is a multistep process that must be represented with many load steps

• This many loadsteps can result in prohibitive run times even in medium sized models!

• Can also involve manual setup work in GUIs• Thermal condition object (tabular load data)• Loadstep setup• Autotimestepping settings

© 2011 ANSYS, Inc. August 26, 20117

Meshing

Large number of identical bodies will need to be meshed efficiently

Without careful mesh controls, mesh may not be acceptable

© 2011 ANSYS, Inc. August 26, 20118

Workbench for BGA: Benefits

• Conventional wisdom is that you should use MAPDL (Ansys Classic) for these large & complex analysis types

• With the most recent versions, Workbench can better manage the complexity and handle these types of analysis, allowing you to take advantage of:

• Ease of use• Powerful geometry tools & integration• Intuitive pre & postprocessing• File management• Contact…that Workbench offers

© 2011 ANSYS, Inc. August 26, 20119

Workbench for BGA: Benefits – Meshing

• Workbench meshing is a separate technology from MAPDL and is constantly improving:

• Hex dominant mesh method

• Multizone mesh method (technology from ICEM-CFD acquisition)

• More advanced mapped meshing, especially within circular areas

• Key feature: body by body meshing within multibody parts

• This R13 feature allows the creation of detailed, conformal meshes within Workbench

Mapped meshing of circular body

MAPDL Workbench

© 2011 ANSYS, Inc. August 26, 201110

Tips & Tricks

• To make the most out of a BGA simulation in Workbench, there are certain features that will make life easier

• Some of these tips you may be aware of, others you might not have any idea about

• It’s possible to regularly use Workbench and not know about features recently added!

• The following slides contain practical advice to help your BGA or other analysis

© 2011 ANSYS, Inc. August 26, 201111

How to mesh a solderball

• In planning a BGA mesh, there is an important early tradeoff:• High order mesh or low order mesh?

• High order mesh (save on engineering time)• + You can use any mesh algorithm• + Don’t need to slice as much, will keep

the body count down• - Will end up with a relatively large

problem• Low order mesh (save on computation time)

• - Everything must be hex meshed (swept or multizone)

• - More time spent working on the mesh• - Necessary slicing will increase the

number of bodies• + Will save significantly on problem size• + If problem can fit in-core, computation

time will go down even more

Low order element(no midside nodes)

High order element(midside nodes)

Single solder ball mesh

With midside nodes: 29,531 nodesWithout midside nodes: 7,551 nodes

© 2011 ANSYS, Inc. August 26, 201112

How To Mesh a Solderball: Low Order Method

Will give a hex mesh without any sizing necessary

Will want to use sizing (hard edge sizing) to minimize the number of nodes, however…

Cross section

Revolve slice

Multizone mesh method

Sweep mesh method

© 2011 ANSYS, Inc. August 26, 201113

Mesh Considerations - Solver Arithmetic

• With a low order mesh, it may be possible to run your solution in-core

• This is special case of the direct solver where the entire model can be fit into the system RAM, minimizing use of the hard drive

• Since RAM is much faster than the hard drive, a significant performance gain will be realized from removing the hard drive bottleneck

• You can estimate a target mesh size for your model based off this:

# of nodes in model x 3 (degrees of freedom at each node)x 10 GB/Million DOFs

• You will want to compare this number to the Physical Memory Free/Available field in the Task Manager• Note that Workbench will create more nodes as it adds contact elements to the model during the solve

process• The Ansys solver will also need memory for the solver and prefers some extra available free memory for

‘breathing room’• See the Ansys Performance Guide in the MAPDL help for more information

© 2011 ANSYS, Inc. August 26, 201114

How to mesh many solderballs

• It’s easy enough to manually set mesh controls on a single solder ball, but what if you have 50 or more that need precise mesh controls in order to meet size requirements?

• The new named selection logic feature in R13 can save a lot of manual work

• Since the patterned solderballgeometry will be identical, you can make use of the “select items with the same size” criteria

• Use on bodies to assign uniform mesh method

• Use on edges to assign uniform sizing

• Note: you must have a geometrical entity selected and right click in the graphical view to get the dialog on the right

© 2011 ANSYS, Inc. August 26, 201115

Solderball mesh order

No sizing:2738 elements

But with sizing it doesn’t mesh automatically

Separate mesh out in stages:698 Elements

© 2011 ANSYS, Inc. August 26, 201116

Mesh order macro explanation

• Mesh order by body name macro included in Ansys installation

• <Ansys install location> \aisol\DesignSpace\DSPages\macros\MeshBodiesSequentially.js

• Modified macro works off of named selections named DM_##, where ## is the mesh order

• First part of code (not pictured) loops through named selections

• Second part sorts them by ## and sequentially selects and meshes the bodies

© 2011 ANSYS, Inc. August 26, 201117

Boundary conditions: Symmetry

• Symmetry will be an invaluable tool in saving on analysis time

• Without going into at least ½ symmetry, the model may be too big to be feasible

• Problem: Symmetry boundary condition conflicts with mesh order

• Workbench will mesh every part on the symmetry boundary condition at once

• Body by body mesh won’t be possible in these parts

• Workaround: Frictionless support gives you the same effect without affecting meshing

Frictionless Support:

Normal 3D Constraints

X UX, ROTZ, ROTY

Y UY, ROTZ, ROTX

Z UZ, ROTX, ROTY

DSYM Command:

© 2011 ANSYS, Inc. August 26, 201118

Boundary Conditions - Tabular

• In a BGA analysis with many loadsteps, it can be tedious to enter thermal loadings row by row

• The load history is in tabular format, just like Excel

• Make use of copy and paste

• Select rows in Excel -> Copy

• Select rows in Workbench – Paste

• Using Excel, you can build your load data with logic and functions and bring it into Ansys in one step

Copy

Paste

© 2011 ANSYS, Inc. August 26, 201119

Boundary Conditions – Loadsteps times

• The copy and paste method from Excel works with setting up your loadstep times as well but with a different procedure

• For every cell, Workbench makes sure the end time is less than the next step

• This can cause it to ignore all of your copy paste rows except the last

• An efficient procedure to copy and past your loadstep time data into Workbench is as follows:

• With a single loadstep, set the end time to equal the target end time of the entire analysis

• Change the number of loadsteps to the actual number in your analysis

• Copy and paste from Excel as normal

© 2011 ANSYS, Inc. August 26, 201120

Autotimestepping settings

• Autostepping is useful for an analysis but for BGA, the default initial/min/max timesteps may not be aggressive enough

• No way in the GUI to set them all at the same time

• You can see all your autotimestepping settings in the worksheet view but not edit them

• For now the only workaround would need to be a JScript macro like on the right

© 2011 ANSYS, Inc. August 26, 201121

Reference Temperatures by Body

• Start your simulation at the stress free temperature of solder and still account for the residual thermal stresses

• Use the reference temperature by body option to achieve this

• Assuming linear material properties for everything besides the solderballs, not necessary to simulate previous steps and EKILL/EALIVE the solder balls

Stress free temperature of ‘SolderBall’ equal to environment temperature

Stress free temperature of ‘TopFR4’ set individually to appropriate temperature

© 2011 ANSYS, Inc. August 26, 201122

BGA in Workbench – Wishlist for Mechanical

• Along with the successes, I also encountered challenges in using Workbench for this type of analysis

• Some of the things I encountered are actually fixed in R14, others are more on the horizon

• Here would be my wishlist for making Workbench more comfortable for BGA analysis:

• Better GUI performance with large number of bodies (1000+) [R14]

• Ability to turn off graphical indicator/preview

• Ability to preserve mesh order [R14]

• Ability to pattern an existing mesh across identical geometry

• Ability to set default timestepping options or modify in a batch

• Restart ability for ANANDs material model

© 2011 ANSYS, Inc. August 26, 201123

BGA in Workbench - Conclusions

• Workbench continues to evolve into an ever more powerful and easy to use GUI for finite element analysis

• There is constant improvement & enhancement ANSYS meshing and Mechanical within the Workbench platform

• If you are not using these tools, you are missing out on some great ANSYS technology advancements

• Workbench is well suited to undertake challenging analysis types, like BGA reliability

• Just as in any software program, it pays to know of and take full advantage of available capabilities

© 2011 ANSYS, Inc. August 26, 201124

QUESTIONS ?Thank you for your attention

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