Microstructural evolution of Au-Sn solder prepared by laminate rolling during annealing process

The microstructural evolution and interfacial reaction of the Au/Sn/Au/Sn/Au/Sn/Au couples were investigated during annealing at 453, 523, and 543 K for up to 240 h. The Au/Sn combination formed a rapid diffusion system. Even in rolled Au-Sn solder, three phases, such as AuSn, AuSn₂, and AuSn₄, were formed. After initial annealing at 453 K, the diffusion layers of AuSn, AuSn₂, and AuSn₄, which were formed after rolling, expanded gradually and then fully transformed into ζ phase (containing Sn from 10% to 18.5%, mole fraction) and δ (AuSn) phase. As a whole, the microstructure of the couple was stable during annealing at 453 K. The solid-state interfacial reaction was much faster at 523 K than at 453 K. After annealing at 523 K for 6 h, the AuSn, AuSn₂, and AuSn4 were fully transformed into the ζ phase and δ phase (AuSn). In spite of the prolonged annealing time for up to 240 h, no significant change of the interfacial microstructure occurred, and the microstructure of the couple was stable during annealing at 523 K. When annealing at 543 K, however, the interfacial of Au/Sn was transformed into solid-liquid state, and the whole couple formed a eutectic structure rapidly, causing the solder to be brittle. The study results clearly demonstrate that the service temperature of the Au-Sn solder should be lower than 543 K.