Optimization of MicroSpring Contact Design Parameters for Low Pressure Probing. Rod Martens, Larry Levy Southwest Test Workshop 2004

Transcription

1 Optimization of MicroSpring Contact Design Parameters for Low Pressure Probing Rod Martens, Larry Levy Southwest Test Workshop 2004

2 Contents Industry Driven Probing Requirements Probing Operating Space Cres Scrub Mark Metrics Case Study Predictive Probing at FormFactor Scrub Mark Cres Customer Validation Conclusion p. 2

3 Industry Trends New Bond Pad Stackups Copper metallization Low K dielectric Probing over active area Lower Power Devices, driven by mobile products, reduce test margins p. 3

4 Probing Operating Space Cres Vs. Pressure Cres Abritrary Scrub Mark Metric Pressure p. 4 Pressure is an independent variable that modulates both Cres and the scrub mark metric. Must balance two opposing requirements

5 Probing Operating Space Scrub Mark Metrics Length Scrub Envelope XErr YErr Pad Size Keepout + Scrub Length Scrub End Margin Width Depth Expose lower stack metals Package reliability issues Size/Location Hit passivation Non-Scrub Margin Tip Scrub Start Margin # of TD s Multiple sorts common Double touch recipes common p. 5

6 Case Study New Bond Pad Stack-up Metallization Requirements: No exposure of base metal after repeated touchdowns Thin Al over base metal Low, stable Cres Approach: Predictive Probing at FFI Scrub Characterization Cres Customer Site Validation Scrub Characterization Qualification p. 6

7 Experimental Setup Predictive Probing at FormFactor Force/Displacement Fixture Force Displacement Test Wafer/Coupon Data Acquisition Test Spring Resistance Micro-ohmmeter (4-wire) p. 7

8 Experimental Setup Predictive Probing at FormFactor Test Spring p. 8 Au plated Cres Test Coupon shown

9 Predictive Probing Evaluation of Pressure Parameters Pressure is a first order variable for both Scrub mark and Cres. Pressure=Applied Force/Contact Area p. 9 FFI design parameters and probing recipes can be optimized to control pressure for specific applications. Overtravel K Tip size Tip Size K Force Area OT Pressure

10 Predictive Probing Pressure Evaluation Effects of K and Overtravel OT High 15 touchdowns in same spot Standard tip size Blanket wafer with customer metallurgy/stack Low p. 10 Low High K

11 Predictive Probing Pressure Evaluation Effects of K and Tip Size Tip Size Large Standard 15 touchdowns in same spot The low and high g/mil springs were grown from standard to large by repeated touchdowns on a controlled abrasive surface High OT for all cases. Tip size is dominant variable. p. 11 Low High K

12 Pressure Surface Relationship For Constant Overtravel p. 12 Pressure K vs. Pressure 2 more shallow Increasing tip size is most effective in reducing pressure. K Tip Size Tip Size vs. Pressure steeper

13 Predictive Probing Cres Evaluation Au and Al Coated Substrates Aluminum sputtered Stainless Steel coupon R c ρ = 2 πh F σ f H + F Resistance (ohms) Gold plated Stainless Steel coupon Calcluated Bulk Resistance of T2 spring = 48.3mΩ. p R. Holm, Electric Contacts, Theory and Application, (4th ed.) Berlin/New York: Springer Sense Connection affixed here Force (g)

14 Predictive Probing Cres Evaluation Force vs. Resistance Holm Theory R c ρ = 2 πh F σ f H + F p. 14 Aluminum Material Properties (from MatWeb): H al = 15 Kg/mm 2 ρ al = 2.7E-6 Ω-cm σ f is unknown. Since all other variables are known, the film resistivity for the Aluminum case can be estimated from the experimental data. σ f ~ 2.5E-7 Ω-cm 2 Establishing this material parameter allows for more accurate estimates of Cres for future applications.

15 Predictive Probing Summary Customer Site Evaluation Predictive Probing Summary Evaluated significant parameters for scrub mark Design parameters selected for field evaluation Characterized Cres to high resolution Customer evaluation Several cards were evaluated at customer site Varied K, tip size and number of touchdowns Probed test wafers with production metal stack Cres monitored for all tests Objectives: Define design parameters for qualification Define probing recipe for qualification p. 15

16 Field Characterization 100 Scrub Mark Metric Touchdowns 12 Touchdowns Based on experimental data, set target specification here for qualification. 25 p Mean pressure Note: Cres met customer specification for all runs.

17 Cres/Scrub Mark Metric Vs. Pressure Touchdowns Touchdowns Al Theory 4 Scrub Mark Metric Al Cres curve transformed from force to pressure space. (Does not account for scrub) Operating point away from Knee Cres (ohms) p. 17 Pressure

18 Product Qualified All Qualification testing passed Lifetime Test ILD damage Scrub Cpk Multiple TD analysis Package reliability p. 18

19 Conclusion FormFactor s MicroSpring Contact probing solution allows for customization for specific applications Overtravel K Tip size Predictive Probing enables FFI to simulate customer environments Validated at multiple customers and applications p. 19

20 Acknowledgments Intel Trung Nguyen, Mike Dang, Reuben Gallegos FFI Fred Lane, Eric Watje p. 20