Reaction of solder with Ni/Au metallization for electronic packages during reflow soldering

Gold over Ni is one of the most common surface finishes for Cu soldering pads in ball-grid-array (BGA) and other electronic packages. The Au layer is for oxidation protection, and the Ni layer serves as a solderable diffusion barrier. In this study, eutectic Pb-Sn solder-balls were reflowed on the Au/Ni/Cu pads, and the chemical interactions between the solder and the surface finish were studied. Quenched-in microstructures at different stages of the reflow were carefully examined using the scanning electron microscopy. It was found that the solder melted locally along the solder/pad interface at the very early stages of the reflow before the whole solder ball had reached the Pb-Sn eutectic temperature. This was because a ternary eutectic reaction L=(Pb)+(Sn)+AuSn₄ occurred at 177°C, six degrees below the Pb-Sn eutectic temperature. Four distinct stages were identified for the reflow process. The four stages are: (1) partial melting of solder balls and the initial reaction of Au with Sn; (2) complete reaction of An with Sn; (3) separation of (Au_xNi_1-x)Sn₄ from the pad; (4) complete melting of solder balls and the reaction of Ni with Sn. After a typical reflow, with a 225°C peak reflow temperature and 115 s reflow time, all the An and Au-bearing intermetallic compounds left the interface and the only intermetallic compound at the interface was Ni₃Sn₄ with a thickness of about 2 μm