Enhancement of Sn-Bi-Ag Solder Joints with ENEPIG Surface Finish for Low-Temperature Interconnection

Low-temperature soldering constitutes a promising solution in interconnect technology with the increasing trend of heat-sensitive materials in integrated circuit packaging. Experimental work was carried out to investigate the effect of electroless Ni/electroless Pd/immersion gold (ENEPIG) layer thicknesses on Sn-Bi-Ag solder joint integrity during extended reflow at peak temperatures as low as 175°C. Optimizations are proposed to obtain reliable solder joints through analysis of interfacial microstructure with the resulting joint integrity under extended reflow time. A thin Ni(P) layer with thin Pd led to diffusion of Cu onto the interface resulting in Ni₃Sn₄ intermetallic compound (IMC) spalling with the formation of thin interfacial (Ni,Cu)₃Sn₄ IMCs which enhance the robustness of the solder after extended reflow, while thick Ni(P) with thin Pd resulted in weakened solder joints with reflow time due to thick interfacial Ni₃Sn₄ IMCs with the entrapped brittle Bi-phase. With a suitable thin Ni(P), the Pd thickness has to be optimized to prevent excessive Ni–P consumption and early Cu outward diffusion to enhance the solder joint during extended reflow. Based on these findings, suitable Ni(P) and Pd thicknesses of ENEPIG are recommended for the formation of robust low-temperature solder joints.