Effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu and SnPb solders with Au/Ni/Cu substrate during aging

The effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu (SAC) and SnPb solders with Au/Ni/Cu substrate during aging was investigated in this study. AuSn4 needle-like intermetallic compound (IMC) formed at the interfaces of both SAC and SnPb solder joints after laser reflow. In SnPb solder joints, a layer-type ternary IMC, (Au,Ni)Sn4, formed over a thin layer quaternary IMC (Au,Ni,Cu)3Sn4 after aging at 150 °C for 4 days, in which Cu came from the Cu pad below the Ni layer through diffusion. This (Au,Ni,Cu)3Sn4 IMC evolved into a new kind of quaternary IMC (Au,Ni,Cu)6Sn5 with further aging at 150 °C for 8 days because more Cu diffused through Ni and was involved into the IMC formation. For SAC solder joints, needle-like binary AuSn4 IMC in as-soldered state changed into a layer-type quaternary IMC (Au,Ni,Cu)Sn4. The quaternary IMC (Au,Ni,Cu)6Sn5, which has the same composition as those found in SnPb solder joints, was developed below the (Au,Ni,Cu)Sn4 layer after aging at 150 °C for 8 days. It was found that Cu diffusion played a key role in the interfacial reaction and compound formation.