Fabrication and characteristics of Cu@Ag composite solder preform by electromagnetic compaction for power electronics

Abstract Cu@Ag core-shell material not only has advantages of Cu and Ag elements but also inhibits Ag migration and Cu oxidation issues, showing potential in die attachments for power electronics. Currently, micro/nano sintering has been widely used for die attachments and inevitably forms residuals and voids due to solvents, leading to poor bonding strength and weak reliability. In this work, a high-temperature Cu@Ag solder preform was firstly fabricated by electromagnetic compaction (EMC) on Cu@Ag core-shell powder. Unlike conventional solder preforms, a void-free joint can be directly achieved with this Cu@Ag solder preform by simple thermal compression at low temperature and pressure without melting. Moreover, with the benefits of high-energy forming process, other vital properties of solder preforms, including electrical property, thermal stability, corrosion resistance and joint integrity, were also improved compared to conventional die attach materials. For instance, the corrosion potential of this Cu@Ag solder preform was −0.14 V, which indicates excellent corrosion resistance similar to pure Ag (−0.12 V). And no oxidation can be detected on this solder preform after storage in air for at least one year. These performance improvements were a result of enormous pressure generated instantaneously on Cu@Ag powder during EMC. The results showed that a flaw-free Cu@Ag solder preform with a high compactness of over 99 % can be successfully fabricated by appropriate compaction pressure of 13.4 MPa.