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TE-Migration II

Idea/Definition Information
Background: 
•In the previous project, we studied mainly the failure due to thermo-electromigration. We developed an MTTF equation on the basis of two current densities and two temperatures.
 
•To have a better fit to Black’s equation, we need to test at the least one more temperature and one more current density.
 
•We have not studied thermomigration. While we have studied the combined effect of electromigration and thermomigration on failure. We need to decouple them.
 
•In electromigration tests of flip chip solder joints, there are two factors which affect the MTTF analysis; one is the effect of current crowding due to the line-to-bump configuration, and the other is joule heating induced thermomigration which accompanies electromigration. We have studied the former, but not the latter.
Problem: 
Owing to the ban of Pb-based solder in consumer electronic products and the trend of miniaturization in wireless and portable devices, the reliability of Pb-free solder interconnection is one of the most challenging problems in electronic manufacturing industry, especially the failure causes by thermo-electromigration. Joule heating due to the on-chip Al interconnect has generated thermomigration to accompany electromigration in solder joints. For a joint of 200 micron in size, a temperature difference of 20 °C across it will cause a temperature gradient of 1000 °C/cm, which can lead to thermomigration.   We define the combined effect as thermo-electromigration. 
Approach: 

Y1Q1
Conduct electromigration tests as a function of low current density of 5 x 103 A/cm2, time and temperature.   Cross-section of failed samples at higher current density prepared by FIB and SEM.

Y1Q2
Electromigration experiment tests continue as a function of current density, time and temperature. To confirm the mode of failure so that we can confirm the test parameters for MTTF. 
 Complete the failure analysis, complete project report, and prepare journal publications.
 Do additional experimental testing to quantify the thermomigration and stressmigration effects. If these effects are deemed to be significant, consider whether the project can be extended for another 6 months to run 3D FEM modeling quantifying the individual effects of electromigration, thermomigration and stressmigration. 

Click on this link to download: Thermo-electromigration in WL-CSP Pb-free solder joints Phase II Proposal 2009.