BGA Simulation in Workbench: Challenges and Successes Presented by Kaan Divringi Ozen Engineering Inc

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1 BGA Simulation in Workbench: Challenges and Successes Presented by Kaan Divringi Ozen Engineering Inc 1

2 Ozen Engineering Inc We are the local ANSYS channel partner 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. 2

3 BGAs Analysis Background 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 3

4 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 4

5 Material Nonlinearities ANANDs material model is a material model well suited for modeling solderballs under 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! 5 Darveux2002

6 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 6

7 Meshing Large number of identical bodies will need to be meshed efficiently Without careful mesh controls, mesh may not be acceptable 7

8 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 8

9 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 9

10 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 10

11 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) Single solder ball mesh High order element (midside nodes) With midside nodes: 29,531 nodes Without midside nodes: 7,551 nodes 11

12 How To Mesh a Solderball: Low Order Method Multizone mesh method Cross section Revolve slice Sweep mesh 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 12

13 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 13

14 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 solderball geometry 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 14

15 Solderball mesh order No sizing: 2738 elements But with sizing it doesn t mesh automatically Separate mesh out in stages: 698 Elements 15

16 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 16

17 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 Normal X Y Z Frictionless Support: DSYM Command: 3D Constraints UX, ROTZ, ROTY UY, ROTZ, ROTX UZ, ROTX, ROTY 17

18 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 18

19 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 19

20 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 20

21 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 21

22 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 22

23 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 23

24 QUESTIONS? Thank you for your attention Please join us for Happy Hour following the conference. Ozen Engineering Inc: 1210 E. Arques Ave #207 Sunnyvale, CA (408) Directions: Right on Tasman Left on Lawrence Left on Arques Ave. Second Right into OEI parking lot. 24