Interfacial reactions between Sn-3.8 Ag-0.7Cu solder and Ni-W alloy films

In this study, interfacial reactions between Ni-W alloy films and Sn-3.8Ag-0.7Cu solder have been investigated. Ni-W alloys films with tungsten content in the range of 5.0-18.0 at.% was prepared on copper substrate by electrodeposition in ammonia citrate bath. Solder joints were prepared on the Ni-W coated substrate at a reflow temperature of 250°C. The solder joint interface was investigated by cross-sectional scanning electron microscopy, energy dispersive X-ray spectroscopy and electron back scatter diffraction. It has been observed that a (Cu, Ni)6Sn5 layer formed on the Ni-W alloy film after reflow. The thickness of the (Cu, Ni)6Sn5 layer was found to decrease with the increase of tungsten content in the Ni-W film. An additional layer with a bright appearance was also found to form below the (Cu, Ni)6Sn5 layer. The bright layer was identified to be a ternary phase containing Sn, Cu, W and Ni. The bright layer is found to be amorphous and is suggested to have formed through solid state amorphization caused by anomalously fast diffusion of Sn into Ni-W film.