Evolution of AuSnx Intermetallic Components in Laser Reflowed Right-angled Solder Joints during Isothermal Aging Process

Au surface finish is common used as a protection layer on pads. In the traditional reflow process, Au will be dissolved into solder and form AuSn₄ IMCs (intermetallic components). However, as for the solder joints fabricated by laser soldering, the morphology and distribution of the AuSn_x IMCs are quite different from that in solder joints fabricated by traditional reflow methods. The formation, distribution and evolution of IMCs will affect the properly and reliability of solder joints directly. This paper presents a detailed analysis of AuSn_x IMCs evolution in laser reflowed solder joints during isothermal aging process. To investigate the effect of Au thickness on the evolution of AuSn_x IMCs, pads with 0.1, 0.5, 0.9 or 4.0 mum Au surface finish were introduced into our experiment. The solder joints were aged in the 125 degC isothermal oven. Experiment results showed that few IMCs formed at the interface of the solder and the pad with 0.1mum Au surface finish. Even in the condition of 31 days aging, the thickness of IMCs didn´t increase a lot; as for the laser reflowed solder joints with 0.5 or 0.9mum Au surface finish on the pad, most of the AuSn₄ IMCs stayed at the interface and formed in needle-like or dendritic morphology. With the increase of aging time, the AuSn₄ IMCs became more flat and changed to a continuous layer; In the laser reflowed solder joints with 4.0mum Au surface finish on the pad, AuSn, AuSn₂, AuSn₄ IMCs and Au₂Sn phase formed at the interface. With the increase of aging time, more Sn rich IMCs formed at the interface, and evolved to AuSn₄ IMCs in the condition of long time aging. The thickness of AuSn₄ IMCs can reach about 30mum