Which board analysis is RIGHT for Y OUR board?

1. Whichboard analysisis RIGHTfor YOURboard?

2. Types of board flex analysis available How to determine which is best for your application Results and how to apply them Today’s Agenda

3. strain – a numerical value of the stretching that occurs in an object under stress. stress– relates to the flow of forces through an object. micro strain – equal to 1 millionth of a unit of strain (.000001). Von Mises– is one of the most widely accepted methods of calculating an effective strain value based on strain’s three directional components.. TermsYou be Hearing:

4. Probe Force 3D Finite Element Strain Gage Testing AvailableTypesof Analysis

5. PFA is a visual design tool not requiring the extensive modeling and computing time associated with a FEA. Components and other board features are not modeled, resulting in probe forces being overstated. High probe force areas are quickly identified. PFA is performed during design allowing the engineer to proactively place pushdown features and change probe spring forces to reduce possible board flex. Probe Force Analysis(PFA)

6. PFA example Areas of high probe force under sensitive components

7. Facts 0.032 thick board Panelized board with cut outs Slightly elevated probe force under BGAs With history of high field returns. Recommendations Engineer’s opinion and experience is to perform a Finite Element Analysis PFAResults/Recommendations

8. FEA is a method to mathematically analyze a model by dividing it into thousands of smaller pieces called elements. FEA is a refined tool used to pin point excessive strain on the UUT caused by probe force and hold down features. 3D Finite Element Analysis (FEA)

9. FEA is used as a design tool so it must be completed before initiating any fixture fabrication. 3D geometric model of circuit board and fixture is constructed 3D model is imported into FEA program The model is “meshed” – solids are broken into their finite element blocks Constraints are applied, usually edges of plates Probe forces are applied FEA program applies results to model. Results are studied and applied to fixture to reduce stress applied to the UUT. Upfront Design Tool

10. Vacuum Box push plate Vac Box Levelers Push Down Fingers UUT Top Plate Board Stops Dead Stops Probe Plate Spacer Plates Fixture Model Cross Section T-Board Walls of vacuum box removed for visual purposes

11. Fixture Model Top View Vacuum Box Push Down Plate Vac Box Levelers Push Down Fingers UUT Model shown without vacuum box cover

12. The analysis takes into account the flexing of the fixture itself. Which enables the fixture design to be easily modified reducing strain of the UUT. The PCB and fixture are modeled in 3D, since stresses and strains are not uniform on all surfaces as they are in a 2D model. The increased accuracy that 3D provides allows precise changes to thefixture leading to reduce strains applied to the UUT. Why Modelthe Whole Fixture?

13. UUT Modeled in the3D FEA

14. Board SupportsandProbe Info UUT Supports: 114 Pushdown fingers Contoured top plate Probes: 0 Top probes 640 Bottom probes Plates: 1 HDG pushdown plate Other: 1 UUT 48 UUT components (top side) 1 Hold Down Gate

15. Contact Types Frictionless – allows modeled parts to react as independent parts- UUT surface between: Pushdown features (push fingers, BGA stilts, push blocks) Dead stops Contour plates Tooling shrouds Bonded – modeled parts that are joined react as a singular unit Most primary fixture plates All contact pairs that are not modeled as frictionless Degree of Freedom (DoF) each node of the model has 3 degrees of freedom. This is especially important when modeling the fixture, it allows features to slide (nonlinear) on the UUT surface, or loose contact just like they could in an actual fixture. Z Y X 3D Model Simulation

16. 3D FEA Output Top of UUT– Strain Run A Elevated Strain around BGA For reference, 1.000e-004 is equal to 100µs, and 1.000e-005 is equal to 10µs.

17. 3D FEA Output Top of UUT– Strain Areas shaded red are higher than customer’s maximum limit of 500 µs. Run A Close-up

18. An initial FEA was run on a model based on the original fixture design (Run A). Because strains were higher than the customer’s specification that required them to be under 500 μs, the support system for the UUT was modified and the FEA was rerun (Run B). 3D FEA OutputResults Run A

19. Added 174 push down locations Changes in DesignResulting from Run A Run B

20. 3D FEA Output Top of UUT– Strain Run B Elevated Strain Reduced to acceptable levels For reference, 1.000e-004 is equal to 100µs, and 1.000e-005 is equal to 10µs.

21. 3D FEA Output Top of UUT– Strain All areas are within customer’s acceptable limits. Run B Close-up

22. Strains were significantly reduced across the entire board by more than doubling the number of pushdown fingers. Run B showed strains falling below the customer’s spec at all places on the board except an internal radius at the edge of the board: an effect that is typically ignored in FEA results analysis. 3D FEA OutputResults Run B

23. Dense populations of probes contacting the UUT under BGAs Difficulty providing enough UUT support UUT is <0.062” w/moderate probe population Similar boards have a history of field failures Strict warranty policy on components from suppliers When UUT is not available or cannot be sacrificed for Strain Gage Test When is3D FEA Advisable?

24. Remove, add or relocate pushdown fingers, stilts, push blocks, deadstops, levelers and rebars. Reduction of probe forces under BGAs Attempted cooperation with customer in making more real estate available for additional pushdown features Design Recommendations from FEA

25. Strain Gage Testing is the actual measurement of strain on discrete points on the UUT while actuating the test fixture. Measurement is taken from sensors called “rosettes” mounted to the UUT. A scanner is connected to the rosettes taking measurements at between 50 to 10,000 cycles per second. This is a destructive test Strain GageTesting

26. Strain GageOrdering Tip! BGA#1 BGA#2 BGA#3 BGA#4 BGA#1 BGA#2 BGA#3 BGA#4

27. Strain Gage UUT Set-up • Determine rosette locations • Components removed if necessary • UUT prepared • Rosettes mounted • Lead wires routed 7 8

28. Strain Gage Readings Run A

29. Testing Run A Modifications: Original fixture state Results: The customer’s specified diagonal strain limit was 600 ms. Strains were above this limit. Strain Gage Readings Run A

30. Spot faces were added in the contour plate near rosettes 4 and10 to give room to thin solder pads on the UUT. Though the solder pads were approximately 0.010” thick, they were close enough to the rosette to have a strong effect on the strain readings. A total of eight pushdown fingers were removed near rosettes 5, 6, 8, and 9. Fixture ModificationsResulting from Run A

31. Fixture ModificationsResulting from Run A 8 Push Down Fingers Removed

32. Strain Gage Readings Run B

33. Strain Gage Readings Run B

34. In its current state and with proper use, the fixture tested is not likely to cause damage to the circuit board or its components. The customer’s specified diagonal strain limit was 600 ms. Strains fell within the allowable range. Strain GageConclusions:

35. IPC/JEDEC-9704

36. Probe Force Analysis Pros: Quick & easy to perform Consistent information for evaluation Pro-Active design of fixture and feedback to customer No cost to customer Cons: Qualitative tool Does not provide actual strain values UUT components not modeled resulting in probe force overstatement Finite Element Analysis Pros: Advanced tool for predicting strain. Pro-Active design of fixture. UUT & fixture modeled Effectively reduces board damage from excessive strain Provides base line values for Strain Gage testing Strain reading for entire UUT Cons: Adds a few days to the fixture build process Additional $$ Does not guarantee UUT safety of fabricated fixture. ProsandConsofEach Analysis Strain Gage Testing Pros: • Validates safety of fabricated fixture. • No modeling required • Instant feedback results after each modification. • Ensures an extra measure of attention. Cons: • Re-Active approach • Requires UUT to be destroyed • Limited rosette locations • Finite number of locations measured • Additional time and cost

37. Explanation of types of board flex analysis available. Examples of each Results Recommendations Pros and Cons of each analysis Summary

38. Whichboard analysisis RIGHTfor YOURboard? Thank You! Neil Adams 763-694-4214 neil.adams@circuitcheck.com