Novel Sn–Cu Based Composite Solder Preforms Capable of Low Temperature Reflow for Die Attachment of High Temperature Power Electronics and the Transient Liquid Phase Bonding Process

Novel Sn–Cu based composite solder preforms capable of low temperature reflow for die attachment in high temperature power electronics are fabricated by an ingenious composition design and accumulative roll bonding (ARB) process, which consist of Sn–6Cu alloy and reinforcement phases (including Cu microparticles and flexible copper woven wire mesh). After five passes of ARB, in the Sn–Cu composite solder preform the reinforcement phases are tightly embedded in the solder matrix with Cu particles distributing uniformly in the preform. Although it is difficult for β-Sn to completely consume Cu particles in the preform under a mild reflow condition (e.g., 255 °C for 5min), a dense skeleton of high temperature phases consisting of Cu–Sn intermetallic compounds and Cu particles forms in joints, which is beneficial to mechanical properties of the joints. Some voids can be formed in reflowed joints due to volume shrinkage of Cu–Sn TLP reaction, whereas the joints still exhibit excellent mechanical performance with shear strength close to 30 MPa. In addition, shear fracture morphologies of the joints indicate a mixed mode of ductile fracture and brittle fracture, and the proportion of brittle fracture increases with the increase of reinforcement phase content in the composite solder preform.