Novel Bi-based High-temperature Solder for Mounting Power Semiconductor Devices

Ni Bi3Ni Cu -40/250 Sn-Cu We have studied a novel high-temperature solder for mounting the power semiconductor devices used in the inverters of hybrid and fuelcell vehicles. The melting point of well-known lead-free solders such as Sn-based alloys is around 220 C, which is too low to allow their use with high-temperature compound power semiconductor devices such as those based on GaN and SiC. To overcome the brittleness of Bi, we have developed a new Bi-based solder that consists of Bi with CuAlMn particles, the melting point of which is 270 C. The CuAlMn particles are prepared by a gas-atomizing method, and are then mixed with molten Bi. Mechanical property measurements revealed that the tensile strength of the fabricated solder was almost two times greater than that of pure Bi. Consequently, joint samples were fabricated using metal plates and their reliabilities were determined by subjecting them to a thermal cycling test. After almost 2000 cycles of -40/200 C test, neither intermetallic compounds nor cracks were observed at the Cu interface of a sample in which the CTE (Coefficient of Thermal Expansion) was matched. On the other hand, brittle Bi3Ni was observed in the Ni interface sample. In addition, although those joint samples using Sn-Cu solder peeled off during the -40/250 C test, no degradation was observed for those samples using the developed solders. In conclusion, the developed solder seems to offer the qualities needed for use with high-temperature power semiconductor devices.