Interfacial Reaction and Microstructural Evolution between Au-Ge Solder and Electroless Ni-W-P Metallization in High Temperature Electronics Interconnects

The elevated working temperature of high temperature electronics can inevitably cause potential excessive growth of interfacial intermetallic compounds (IMCs), which can significantly deteriorate the mechanical integrity of electronic devices. Therefore, a robust diffusion barrier that can operate reliably under elevated temperature is highly demanded to retard the interfacial reaction between solder and substrate. In this work, a ternary Ni-W-P alloy was deposited through electroless plating and applied as an Under Bump Metalisation (UBM) to Au-Ge solder joints. The interfacial reaction in Au-Ge/Ni-W-P solder joints after reflow and prolonged ageing durations was investigated. We found NiGe and Ni5Ge3 layers formed after reflow, however only NiGe was observed after 1000h aging at 300°C. The thickness of NiGe increases linearly with the square root of ageing time up to 1500h, indicating that the growth mechanism of NiGe is diffusion-control process when Ge atoms are sufficient. After ageing for 2000h, although Ge atoms from Au-Ge solder was fully consumed, the Ni-W-P coating remained stable and exhibited excellent diffusion barrier property. During various ageing durations, the top-view morphology of NiGe IMC grains changed from pyramid-like and polygon-like shape at as-built stage to granulate-like (up to 1500h), and finally a polygon-like shape (after 2000h).