Transient Liquid Phase Sintered Joints for Power Electronic Modules

Abstract

Pastes consisting of micron-sized particles of a low melting point metal (i.e. Sn) and a high melting point metal (e.g. Ag, Cu) embedded in organic binder have been developed to attach silicon or wideband gap semiconductor devices to metallic or ceramic substrates for power electronic applications requiring operation at high temperatures. The attachment is made by a pressure-less, low temperature transient liquid phase sintering (LT-TLPS) process in air. Process time and temperature, along with binder type and amount are adjusted to minimize the formation of voids in the joints. Test samples consisting of copper dice on copper substrates joined by these LT-TLPS sinter pastes have been manufactured for shear testing. A shear fixture for high-temperature testing has been designed, and shear tests have been performed at temperatures of 25°C, 125°C, 250°C, 400°C, and 600°C. The influence of process time, process temperature, and the ratio of low-melting point metal (Sn) to high-melting point metal (Ag, Cu) on the shear strength at each temperature has been assessed. It has been shown that the shear strength of TLPS sinter joints remains high up to the melting point of the dominant intermetallic phase of the joint. The joints show no softening below the melting point of these phases. AgSn sinter joints show only limited change in shear strength up to 400°C. CuSn joints exhibit high shear strength up to 600°C for high copper ratios. While process times of 5–15 minutes are sufficient to drive the sintering reaction to near completion, extended curing improves the strength of the sinter joints even more. Failure analyses for joints of different compositions have been conducted along with cross-sectioning of sintered but non-sheared specimens to correlate reliability to microstructure.