The influence of phosphorus concentration of electroless plated Ni-P film on interfacial structures in the joints between Sn-Ag solder and Ni-P alloy film

The interface structure between Sn-3Ag solder and electroless plated Ni film and the structure near that interface were examined. Plated electroless Ni films contained 3.7 mass% Phosphorus or 8.5 mass% phosphorus. A P-enriched layer is formed at the joining interface between plated electroless Ni film and Sn-3Ag solder, in each sample with 3.7 mass%P and 8.5 mass%P. P-enriched layers of both P concentration samples contained double the P concentration than the original plated Ni films. Also, the P-enriched layer of the Ni-8.5 mass%P sample was much thicker than that of the Ni-3.7 mass%P sample. Both P-enriched layers have been composed of Ni-Sn-P layer and P enriched Ni-P layer, Kirkendall voids were formed between the 1st Ni-Sn-P layer and 2nd Ni-P layer. The number of voids observed in Ni-8.5 mass%P sample was much greater than those in the Ni-3.7 mass%P sample. Intermetallic compounds, mixtures of Ni3Sn2 and Ni3Sn4, were formed by the interfacial reaction. In the case of the Ni-3.7 mass%P sample, Ni-Sn inter-metallic compounds continuously crystallized on the P-enriched layer, while in the case of the Ni-8.5 mass%P sample, Ni-Sn intermetallic compound crystallized dispersively in the solder.