Pb-Free High Temperature Solder Joints for Power Semiconductor Devices

Three types of inexpensive Pb-free solder joints, namely Zn-based and Bi-based solders, and a CuSn alloy were studied for application to the high-temperature operation of wide band-gap power semiconductor devices using GaN or SiC. Zn– Al solder sheets, whose melting point is 380°C were prepared, and then surface oxides were removed by RF plasma etching. Subsequently, Cu thin films were deposited by the DC sputtering method on the solder sheet. After that, joint samples were fabricated using a conventional electrical furnace with H2/N2 gas flow. That the joints were sufficiently dense was confirmed by scanning acoustic microscope. The wettability of the solder was improved due to contact between the pure Zn–Al solder without oxide and the Ni surface of the substrate, and the chemical reaction between the Zn–Al solder and the deposited Cu thin film. Bi with CuAlMn particle solders, whose melting point is 270°C, were fabricated and it was found that the solder has almost double the tensile strength of pure Bi. Excellent wettability was observed with the Ni or Pd thin film deposited on the Cu substrate. It was found that deposited Ni or Pd was not observed in the interface between the substrates after the joint samples were fabricated. CuSn alloy joints were fabricated using Sn thin films deposited and annealed on Cu substrates. The temperature during the annealing was kept at approximately 350°C; therefore, only the Sn film was melted and some chemical reaction occurred between the liquid Sn and solid Cu. Then, Cu3Sn alloy, whose melting point is 640°C, appeared in the joint and the composition was confirmed by X-ray diffraction analysis. Furthermore, it was found that the CuSn alloy is harder than Cu as measured by nano indentation analysis. In addition, the joints showed excellent thermal cycle reliability. Finally, the properties of these joints are summarized.