Novel Bi-based High Temperature Solder for Power Semiconductor Joining

A novel high temperature solder for power semiconductor joining of hybrid and fuel-cell vehicle's inverter has been studied. The melting temperatures of well-known lead-free solders such as Sn-based alloys are around 220 deg C, that are not sufficient for high temperature operation of compound power semiconductor device such as GaN and SiC. New Bi-based solders; Bi with CuAlMn particles whose melting temperature is 270 deg C are developed to improve the brittleness of Bi. The CuAlMn particles were prepared by gas-atomizing method, then they were mixed with Bi melt. According to mechanical property measurements, it was found that the tensile strength of the fabricated solder was almost 2 times higher than that of pure Bi. Consequently, joining samples were fabricated using metal plates and their reliabilities were examined by thermal cycling test. After almost 2000 cycles of -40/200 deg C test, almost no intermetallic compounds and cracks were observed for Cu interface of CTE (Coefficient of Thermal Expansion) matched sample, on the contrary, certain amount of brittle Bi3Ni was found for Ni interface sample. In addition, joining samples using Sn-Cu solder were peeled off during -40/250 deg C test, however no degradations were found on the samples using the fabricated solders. In conclusion, the developed solder seems to have a capability for high temperature operation of power semiconductor device.