Thin film joining for high-temperature performance of power semi-conductor devices

Pb-based solders are used as high-temperature solders for power semi-conductor devices. Although the use of Pb has been globally restricted, there are no alternative materials and technologies available to replace the Pb-based solders. This study investigated the replacement of Pb-based solder with intermetallic compounds. For a joint between Si chip and Cu substrate, Sn/Cu/Sn/Ag/Sn films were deposited as joining material on backside of the Si chip by electron-beam evaporation. A joint specimen was produced by joining the Si chip with the evaporated films to Cu substrate on a hot plate in N2 atmosphere. Heating condition was set at 573 K for 30 s. After joining, the evaporated films were completely transformed into intermetallic compounds consisting of Cu6Sn5/(Ag,Cu)3Sn/Cu3Sn. The result of die shear test proved that the joining strength at 543 K was higher than that of Pb-based solder joint. Thermal stress generated in the joint part was also simulated by finite element analysis. The simulation result revealed that the thermal stress was less than yield stress of each IMC. Therefore, the stress would not induce a mechanical fracture of the intermetallic compounds including the Si chip because their deformations occurred only in an elastic range.