Effect of P content on diffusion resistance and interfacial mechanical properties of crystalline Co–P coatings in solder joints

Electroplating crystalline cobalt–phosphorus (Co–P, with 1 ~ 8 at.% P) is a potential interfacial layer for electronic packaging solder joints, however, the optimal content of P has been lack of systematic research. In this work, crystalline Co–P coatings with P contents of 2, 4 and 6 at.% (Co–2P, Co–4P, Co–6P) were prepared and then bonded with Sn–Ag solder to form Cu/Co– x P/Sn–Ag solder joints. Within aging at 180 °C for 120 h, the diffusion resistance and intermetallic ductility of crystalline Co–P coatings with different P contents were systematically studied. The microstructure, phase composition and mechanical properties of Co–Sn intermetallic compounds (IMCs) were comprehensively characterized using XRD, SEM, EBSD and nano-indentation techniques. The XRD results showed that the Co–P coatings were crystalline structure, with obvious sharp diffraction peaks of β-Co. The Co–4P system exhibited the slowest IMCs growth rate and the best diffusion resistance was obtained in this system, and the kinetic analysis showed that the Co–Sn IMCs growth at the interface was controlled by interface reaction and volume diffusion. It was found that CoSn 3 was generated in the three systems during the aging process, while CoSn 4 and Co–Sn–P were generated only in Co–6P system. Compared with Ni–Sn, Co–Sn exhibited significantly lower IMCs hardness, so the brittle fracture tendency of Co–P solder joints was lower. With the increase of P content, the hardness of Co–Sn IMCs increased first and then decreased, ranging from 3.20 to 4.94 GPa, and the minimum value was noticed in the Co–6P system. Co–4P coating would be a promising barrier layer owing to its good diffusion resistance, Co–6P coating may be used in solder joints with high mechanical load owing to its lowest IMCs hardness, and the Co–2P coating can maintain a balance in anti-diffusion and mechanical properties.