Optimal thickness of nanocrystalline cobalt-phosphorus coating with good diffusion resistance and small space requirement for solder joints

In advanced chip packaging, the miniaturization of solder joints has increased the requirements for diffusion barrier layers. Cobalt-phosphorus (Co–P) coatings, with superior solderability and diffusion resistance, emerge as an ideal alternative to under-bump metallization (UBM). However, the absence of systematic research on the optimal thickness of Co–P coatings hinders further solder joint miniaturization. This study investigated the diffusion resistance of various UBMs, focusing on how the thickness of nanocrystalline Co–4P coating affects their diffusion resistance and mechanical properties. SEM, EPMA, and shear test were used to investigate the morphology, composition of intermetallic compounds (IMCs), and mechanical properties in solder joints. First-principles calculations were used to study the interface bonding of Co–P/Sn–Ag and the effect of Cu diffusion concentration on it. After aging at 180 °C for 15 days, nanocrystalline Co–4P with a thickness of 2.6 μm exhibited enough diffusion resistance, which inhibited the excessive generation of brittle Cu–Sn IMCs and enhanced the mechanical properties in joints, which is a UBM alternative with high-performance, economy, and resource-saving. The simulation results showed that the bonding properties at the Co–P/Sn–Ag interface were better than that of Ni/Sn–Ag and Cu/Sn–Ag. When Cu diffusion was below 9.375 at.%, the limited Cu diffusion due to Co–4P coating thinning had minimal impact on interface bonding, which was according to EPMA and shear test results. This work provides a scientific basis for the application of ultra-thin nanocrystalline Co–P coatings and offers prospective guidance for coating design in package solder joints.