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The Effect of Electromigration on Eutectic SnPb and Pb-free Solders in Wafer Level-Chip Scale Packages

Introduction Reliability is an ongoing, crucial part of the microelectronics industry as companies continue to increase the population of I/O connections and shrink package dimensions. As outlined in the International Technology Roadmap for Semiconductors (ITRS), this trend causes increased current density in solder joints, making electromigration the limiting reliability factor in high density packages. Electromigration leads to serious reliability factors such as joule heating and solder/silicon interface voiding which ultimately lead to failure. In this experiment, both the electrical and thermal effects due to electromigration are studied with eutectic 63Sn37Pb (e-SnPb) and Pb-free 95.5Sn-4.0Ag-0.5Cu (SAC 405) solder bumps in Wafer Level-Chip Scale Packages. Wafer Level-Chip Scale Packaging (WLCSP) is fabricated efficiently, completed directly on the wafer, and has a true chip size due to the wafer level processing. The test chip layout used for this experiment consists of a 6-by-6 array of 300 μm diameter solder bumps with a pitch of 500 μm, tested at current densities ranging from 3.36x10 A/cm to 3.67x10 A/cm in an ambient temperature of 50°C. The large amount of joule heating and the package junction-to-air factor leads to actual die temperatures about 80% of the solder’s melting temperature.