Diffusion barrier property of electroless Ni-W-P coating in high temperature Zn-5Al/Cu solder interconnects

The operating temperature of high-temperature electronics can significantly promote the growth of intermetallic compounds (IMCs) at solder/substrate interfaces, particularly for low-cost Zn-based solders because of the rapid rate of reaction of Zn with Cu. Thus, a reliable and robust diffusion barrier is indispensable for suppressing the reactions between solder and substrate. In this work, a ternary Ni-W-P alloy was prepared via electroless plating. Its diffusion barrier property was evaluated by comparing the microstructures of IMC layers in Zn-5Al/Ni-W-P/Cu and Zn-5Al/Cu interconnects after liquid-solid reaction for prolonged durations. When the reaction lasted for 30 min, the thickness of the Al₃Ni₂ produced in the Zn-5Al/Ni-W-P/Cu solder interconnects was only 2.15 μm, whereas the thickness of the interfacial layer of Cu-Zn IMCs (CuZn₄, Cu₅Zn₈ and CuZn) at the Zn-5Al/Cu interface was 94 μm. Because of the unbalanced growth of the IMCs in the Zn-5Al/Cu interconnects, notable numbers of Kirkendall voids were identified at the CuZn₄/Cu₅Zn₈, Cu₅Zn₈/CuZn and CuZn/Cu interfaces after prolonged liquid-solid reaction. By contrast, the Al₃Ni₂ layer in the Zn-5Al/Ni-W-P/Cu solder joints remained intact, showing the potential to effectively enhance the mechanical reliability of electronic devices. - Highlights: • An electroless Ni-W-P alloy was employed in Zn-5Al/Cu as a diffusion barrier. • The growth rate of IMCs in Zn-5Al/Cu fell 97.75% with aid of a Ni-W-P interlayer. • Kirkendall voids in Zn-5Al/Cu can be eliminated by introducing a Ni-W-P layer. • Diffusional formation mechanisms are proposed of IMCs in Zn-5Al/Cu.