Technical Paper

Applying FEA Simulation for Test Interface Unit

Prepared by Jason Koh Shiann Chern.

Presented at BiTS Workshop on March 5 – 8, 2017

 

ATE & TIU Stack Up

Automatic Test Equipment (ATE)

Test Interface Unit Stack Up (Manual Test)

TIU Challenges

TIU Challenges

Challenges

  • Narrow Pitch (<0.20mm)
  • High Pin Count (4000++)
  • High bandwidth, short pin requirement (Pin test height < 2.0mm and below)
  • High insertion force (TIU need to overcome 100kgf)
  • Time to market, reliability and cost

Impacts

  • Spring probe diameter become more challenging.
  • High warpage and high stress concentration on test socket.
  • Thinner socket design potential to cause high deformation on socket body during preload stage.
  • Hand socket lid, package, test socket, PCB and stiffener have structure stiffness concern.
  • Product development cycle, factor of safety, material selection.

How to Approach New Challenges?

  • Design something that worked in the past and made it bigger/smaller.
  • Use spreadsheets or hand calculations
  • Build and test prototypes
  • Trial and error method
OR
Utilize Finite Element Analysis method to solve your engineering problems

TIU products that involved for FEA Simulation

Benefits and Challenges in FEA Simulation

Benefits of FEA Simulation

  • Solve a wide variety of engineering problems
  • Can handle very complex geometry
  • Useful for problem with complicated restrains and loading
  • Analyze the impact of different material properties

Challenge in FEA Simulation

  • Simulation building requires experience and judgment in order to construct a good finite element model
  • Simulation results may be difficult to interpret
  • The simulation results provide "approximate" solutions
  • The simulation has "inherent" errors
  • Mistakes by users can be fatal

FEA Simulation Process Flow

Typical FEA Process Flow Chart

Example Test Socket Warpage

Problem Statement:

i. Pin counts increase, stress and displacement on the test socket stiffness becomes a major concern.

ii. Stable electrical performance of spring probe, bottom plunger of spring probe is always compressed when test socket is mounted on test board.

Inputs for Analysis

Target

  • Less than 0.15mm for Top Plate coplanarity
  • Material stress for factor of safety (FOS) at least or more than 2.0

Input

  • 3piece design, consist of Guide Plate (GP), Top Plate (TP) and Bottom Plate (BP)
  • Spring Probe Preload 13gf
  • Total 4352 pins counts
  • Total preload acting on TP = 4352 * 0.013 kgf = 56.576kgf

Type of Analysis

Linear Static FEA

CAD Modelling for FEA

Guideline for 3D CAD Simplification

i. Remove outside corner chamfer and fillet.
ii. Remove small holes, slot and step cut outside the load path.
iii. Remove decorative and indication features
iv. Use of quarter of CAD model, if load and support are symmetry.

Build Simulation Model

i. Assign Materials

Linear static FEA material property required are,

  • Elastic Modulus, (E)
  • Poisson’s Ratio, (v)
  • Shear Modulus, (G), G = E / (2(1+v))
  • Yield Strength (or Ultimate Strength)

ii. Apply Boundary Conditions & Loads

  • The choices of boundary conditions & external loadings have a direct impact on the overall accuracy of the model.
  • Over-constrained model will cause stiff model due to apply incorrect boundary conditions.

Build Finite Element Mesh

Generate & Review the Mesh Modelling

  • Accuracy of the solution is primarily dependent on the quality of the mesh.
  • Check the mesh quality (Aspect Ratio, Distortion Element)
  • Apply mesh control on critical area

FEA Solver

Run Solver

Factors when choose the proper solver

Test Socket Analysis & Example

Analyzing the Simulation Results

Physical & Virtual Correlation

Meet Requirement?

  • Target 0.150mm for TP coplanarity
  • Maximum displacement from simulation 0.328mm
  • Unstable electrical spring probe performance cause by high warpage
  • TP material stiffness is not enough
  • Thin TP thickness cannot withstand high pin force
  • Improvement study is required

Optimization Study

Simulation model failed to meet target

Conclusions

Conclusion

  • FEA simulation help engineers filter out potential risk and gain a much broader picture for better decisions making.
  • Training is required and the opportunity to practice extensively.
  • Comparing simulation results with physical data if possible.
  • Finite Element Analysis make a good engineer great, and make a bad engineer dangerous.
  • “Garbage in = Garbage Out” - magic box dilemma