Pb - Sn Solder for Die Bonding of Silicon Chips

The thermal fatigue life of silicon devices die bonded with lead-tin (Pb-Sn) alloys of 14 different compositions, from Pb-3%Sn to Pb-95% Su was investigated. A silver-plated silicon chip and a nickelplated copper substrate were soldered together using a piece of thin foil (100µm
^t
) solder. Soldered silicon devices were exposed to thermal cycling tests of - 55 to 150°C (1 cycle/h). The thermal fatigue life of a device was defined as the number of thermal cycles when the heat resistance of the device reached 1.5 times its original value. The maximum thermal fatigue life was observed for Pb-50%Sn and was about 9 times that for Pb5%Sn. Solder grain growth during the thermal cycling was also observed, and scanning electron microscope (SEM) measurements showed that cracks in the solder propagated selectively in the at (Pb-rich) region.