Interfacial microstructures and mechanical properties of molten Sn reacting with Ni-xP films

Deposition of Ni via autocatalytic (electroless) plating inevitably contains a specific amount of P (usually 6-10 wt.%) due to the use of hypophosphite reducing agent in the autocatalytic reaction. The focus of this study is to investigate the effect of P content on the solderability between pure Sn and a Au/Ni-xP/Cu pad, where x was 6 wt.% (10.8 at.%), 8 wt.% (14.1 at.%), and 10 wt.% (17.4 at.%). We found that the intermetallic compound (IMC) species formed at the interface between solder and Ni-xP and the Ni-xP pad's depletion both strongly depended on x. The P-dependent Ni-xP depletion was ascribed to various IMC formations. The dependence of the IMC species on the P content can be rationalized using a Ni-P-Sn isotherm. Additionally, high-speed ball shear test showed that the difference in the interfacial microstructures arising from various x values might significantly affect the mechanical properties of solder joints. These findings revealed that the P content in the Ni-P platings is a very important factor of solderability.