Isothermal solid-state aging of Pb–5Sn solder bump on Ni/Cu/Ti under bump metallization

Isothermal solid-state aging of the Pb–5Sn solder bump on the Ni/Cu/Ti under bump metallization was investigated. Only a layered (Ni,Cu)3Sn4 phase was formed at the solder/Ni interface aged at 160 °C. The (Ni,Cu)3Sn4 layer attached closely to the Ni layer after 4320 h of aging. Another (Ni,Cu)3Sn2 phase was formed when the aging temperature was raised to 300 °C. The (Ni,Cu)3Sn2 phase did not form as a continuous layer along the interface but grew discontinuously in the Ni layer. After aging at 300 °C for 360 h, the majority of the (Ni,Cu)3Sn4 phase detached from the interface. Pb was found to penetrate the detached (Ni,Cu)3Sn4 phase and filled the gap between the detached (Ni,Cu)3Sn4 phase and the attached (Ni,Cu)3Sn2 phase. Based on microstructural analyses, the causes for the detachment of the (Ni,Cu)3Sn4 phase and the Pb penetration were discussed.