Electroless Ni-Cu-P barrier between Si/Ti/Al pad and Sn-Pb flip-chip solder bumps

This work investigates the barrier effect of the electroless Ni-Cu-P deposit between Al conductor and Sn-Pb solder bump, as well as the interfacial reaction with the Sn-Pb solder. For the Ni-Cu-P/63Sn-37Pb system, a (Ni, Cu)₃Sn₄ compound with three different morphologies: fine-grain, whisker, and polygon are formed at the Ni-Cu-P/63Sn-37Pb interface after reflow at 220 °C for 15 s. These (Ni, Cu)₃Sn₄ crystals transform into polygon shape with smooth appearance during 150 °C aging. For the Ni-Cu-P/95Pb-5Sn system, equiaxial (Ni, Cu)₃ Sn₄ crystals are formed at the Ni-Cu-P/95Pb-5Sn interface after reflow at 350 °C for 15 s, and they also transform into polygon shape during 150 °C aging. In addition, the Ni-Cu-P deposit will crystallize to form Ni₅ P₂ during 350 °C reflow. The growth of the (Ni, Cu)₃ Sn₄ compound by solid state reaction is a diffusion controlled process for both Ni-Cu-P/63Sn-37Pb and Ni-Cu-P/95Pb-5Sn systems. A 4 μm Ni-Cu-P deposit can provide adequate barrier function between an Al conductor and two Sn-Pb solders under 150 °C aging for 1000 h.