Interface reaction between an electroless Ni–Co–P metallization and Sn–3.5Ag lead-free solder with improved joint reliability

To address the reliability challenges brought about by the accelerated reaction with the implementation of lead-free solders, an electrolessly plated Ni–Co–P alloy (3–4wt.% P and 9–12wt.% Co) was developed as the solder metallization in this study. Three compounds layers, (Ni,Co)3Sn4, (Ni,Co)3P and (Ni,Co)12P5, are formed at the reaction interface. Nano-sized voids are visible in the (Ni,Co)3P layer under transmission electron microscopy, but no large voids are found under scanning electron microscopy. This is an indication of effective diffusion barrier performance by the Ni–Co–P metallization compared with the binary Ni–P metallization. The influence of interfacial reaction on the solder joint reliability was reported through the evaluation of the tensile strength of solder micro-joints. Upon aging at 180°C for 600h, the tensile strength of Ni–Co–P/Sn–3.5Ag solder joint remains high, and the failure is caused by the bulk solder necking and collapse. As a comparison, the tensile strength of the Ni–P/Sn–3.5Ag solder joint drops significantly after aging for 400h at 180°C, and the fracture mode has shifted from ductile failure in the bulk solder to brittle failure at the solder joint interface. The Ni–Co–P metallization, having a much slower consumption rate and improved resistance to joint strength degradation during long-term aging treatment, is a potential candidate for future microelectronic solder metallization materials.