Liquid-state and solid-state interfacial reactions of fluxless-bonded Au–20Sn/ENIG solder joint

The microstructural evolution and interfacial reactions of fluxless-bonded, Au–20 wt.%Sn/electroless nickel-immersion gold (ENIG) solder joint were investigated during reflow and aging. After the reflow process, only (Au,Ni)Sn intermetallic compound (IMC) was formed at the interface. After prolonged reflow reaction, the (Au,Ni)Sn IMCs that had been formed in the initial reflow stage grew into the solder and a (Ni,Au)3Sn2 IMC was formed at the interface. In the aging treatment, the solid-state interfacial reaction was much faster at 250 °C than at 150 °C. After aging at 250 °C for 100 h, a thick (Au,Ni)Sn IMC layer was formed at the interface. With increasing aging temperature and time, the δ(AuSn)-phase adjacent to the interfacial layer was gradually replaced by the ζ(Au5Sn)-phase. The interfacial (Au,Ni)Sn IMC grew with the preferential consumption of the available δ-phase in the solder. The Ni3P layer formed from the Ni(P) layer in the initial 500 h of aging was subsequently further transformed into a Ni5P4 layer during further aging treatment between 500 and 1000 h. The original electroless Ni(P) plating layer lost its initial role of a diffusion barrier due to its crystallization. After aging for 1000 h, the Ni5P4 layer was partially destroyed, and the Au–Cu IMCs were formed below the P-rich Ni layer.