Reinforced solder joint performance by incorporation of ZrO₂ nanoparticles in electroless Ni-P composite layer

To reinforce the reliability issue brought by excessive interfacial reaction with the dimensional scale-down of electronic device, an electroless Ni-P-ZrO₂ (17.5 at.% of P) composite coating was developed as the under bump metallization (UBM) for lead-free solder interconnect. ZrO₂ nanoparticles were proved to be homogeneously distributed and helped improve wetting ability of the layer. Both Sn-3.5Ag/Ni-P-ZrO₂ and Sn-3.5Ag/Ni-P solder joints were prepared and aged at various conditions to study the interfacial reaction. Growth of intermetallic compounds (IMCs) without serious spalling in solder/Ni-P-ZrO₂ joint was slowed down because of the barrier property of incorporation of ZrO₂ nanoparticles, which blocked the diffusion of Ni and Cu atoms. Based on the IMC growth, the activation energy of solder/Ni-P-ZrO₂ was estimated to be higher than that of plain solder joint. The top-view of IMCs demonstrated a much finer grain size compared with that of solder/Ni-P joint. A reactive diffusion-induced compound formation mechanism was proposed to address the microstructural evolution in detail. Moreover, solder/Ni-P-ZrO₂ joint demonstrated higher shear strength than did solder/Ni-P joint for different aging durations. The fracture surface of solder/Ni-P joint after shear test showed ductile transition failure, with big dimples and plastic deformation.