Electroless Ni-W-P alloy as a barrier layer between Zn-based high temperature solders and Cu substrates

A need for a drop-in lead-free high temperature solder to replace high lead solders has been stringently desired for the die attachment in SiC power devices. Currently, the gold-based solders (Au-20Sn, Au-12Ge, and Au-3Si eutectic solders) and nano-silver paste are widely applied for their excellent high-temperature properties. But due to high price and high-demand of a processing facility, their applications are hindered. Zn-based solders, which exhibit numerous advantages such as low cost, high melting point, high thermal and electrical conductivity, show their potential uses to replace high lead solders. However, the oxidation and significantly reactions of Zn-based solders on copper substrates lead to excessive Cu-Zn intermetallic compounds (IMCs) at elevated temperature and further makes the joints brittle and easy to fracture. In this work, in order to effectively prohibit the growth speed of Cu-Zn IMCs under harsh environment, a ternary Ni-W-P coating (6-7 wt% of P and 18-19 wt% of W) on copper substrate was developed for its good thermal stability. The interfacial reactions at Cu/Ni-W-P/Zn-5Al interface and Cu/Zn-5Al interface were studied and compared after soldering at 450°C for different time. It was found that Cu-Zn IMCs grew rapidly after soldering for 30 minutes and cracks were observed in IMC layer of Cu/Zn-5Al interface. However, no voids and cracks were found at the Ni-W-P/Zn-5Al interface with the same thermal treatment. Moreo ver, the thickne ss of γ-Ni₅Zn₂₁ a nd Al₃Ni₂ formed in Ni-W-P/Zn-5Al solder joints were quite thin and stable during soldering for different time, which proves the barrier-effect of Ni-W-P layer. These results indicate that Ni-W-P metallization is a promising under bump metallization (UBM) to improve reliability in long term aging and multiple reflow.