Intermetallic reactions in a Sn-3.5Ag-1.5In solder ball-grid-array package with Au/Ni/Cu pads

In this paper, the interfacial reactions between Sn-3.5Ag solder and Sn-3.5Ag-1.5In solder and Au/Ni/Cu pads in ball-grid-array (BGA) packages during solid aging were investigated by microstructural observations and phase analysis. During the solid aging, the intermetallic compound (IMC) layer in Sn-3.5Ag/Au/Ni/Cu solder joints evolved from the (Ni, Au)Sn-4 phase to the Ni3Sn4 phase, but the rate of growth of the IMC layer did not change significantly. While, in Sn-3.5Ag-1.5In/Au/Ni/Cu solder joints, the phases evolved from the (Ni, Au)Sn-4 and Ni3Sn4 phases into Ni-3(Sn, In)(4) phase. The distribution of In atoms in the solder alloy weakened interatomic force in the Sn-3.5Ag-1.5In solder alloy and the involvement of In atoms in the interfacial reaction generated more energy of distortion of the Ni-3(Sn, In)(4) and (Ni, Au)(Sn, In)(4) lattices. These both accelerated the diffusion of Sn atoms and the rate of growth of the whole IMC layer, but this effect reduced gradually after prolonged aging.