Microstructural evolution and shear strength of AuSn20/Ni single lap solder joints.

AuSn20 (mass fraction) solder was prepared by laminate rolling-annealing method in an argon atmosphere. We investigated the interfacial reactions between the AuSn20 solder and the metalized Ni layer, and evaluated the effect of the interfacial reactions on the mechanical reliability of AuSn20/Ni joints with a long term solid-state aging by the single lap shear test. After the reflow process, fine eutectic structure and some hexagon (Ni, Au)3Sn 2 phases were observed in the solder matrix. During the aging treatment, the microstructure of the joint was coarsening significantly. The effects of aging treatments on the structure of the joints were obtained: aging at 150 °C for different aging time, the microstructure of AuSn20/Ni joints did not change much; after aging at 200 °C for 300 h, a thin (Au, Ni)Sn + (Ni, Au)3Sn2 multi-layer was formed at the interface; by prolonging the aging time to 500 h, the (Au, Ni)Sn layer became thick with a distinct line above the (Ni, Au)3Sn2 layer; the total thickness of the intermetallic compounds (IMCs) layer grew with the increasing aging time. The as-reflowed joint had a higher single lap shear strength than the aged joints. All specimens fractured at the interface between the solder and IMC layer. The fracture surfaces demonstrate that the reduction in shear strength of the AuSn20/Ni joint was caused primarily by the coarsening of the microstructure. © 2013 Elsevier Ltd. All rights reserved.