Composite solders

Development of composite solders has been performed utilizing an intermetallic compound interspersed in a solder matrix. The principal work has been done on Cu₆Sn₅ intermetallic in 60/40 Sn/Pb. The composite solder was prepared by mixing powdered intermetallic in solder paste, with subsequent reflow. Formulations of 10-40% intermetallic (by weight) were investigated. Optical inspection of cross sections demonstrates that the differentiation of the three phases (Sn, Pb, Cu₆Sn₅) is not easy, owing to the similar appearance of Sn and Cu₆Sn₅; however, etching with ammonium fluoride/hydrogen peroxide preferentially removes the tin and darkens the lead, making characterization facile. Alternatively, scanning electron micrographs/energy dispersive X-ray (SEM/EDX) analysis of cross sections, using the backscattered detection mode, is an excellent tool for microcharacterization. The mechanical properties of composite solder were examined and were found to be superior to those of standard tin/lead solder. Solder joints were readily prepared utilizing the composite solder and were forced to failure by thermocyclic fatigue; the mechanism of failure was the same as for 60/40 Sn/Pb solder, i.e., heterogeneous coarsening