Microstructure study of lead free solders in electroless nickel gold under bump metallurgy solder bump

Abstract

In electroless nickel immersion gold (ENIG) under bump metallurgy (UBM) solder bump system, the nickel UBM and solder deposition aspects, are two of the most important factors affecting the stability of solder joints. In this study solder bump shearing strength, materials interaction and inter-metallic compound (IMC) growth of Al/electroless Ni immersion gold/solder bumps were investigated with respect to multiple-reflow conditions. Three different chemical composition of lead free solder balls were used for investigate with the multiple reflow effect of 1, 2, 3, 4, 5, 7 and 10 times. Solder materials were selected for this study, were Sn3.8Ag0.7Cu, Sn0.07Cu and Sn3.5Ag, along with Sn37Pb as a reference. Results show that, shearing strength of Pb-free solders in stable condition with multiple reflow effect and indicated that Sn-Ag-Cu has the highest shearing strength and Sn-Cu is the lowest. Sn-Ag system has a compatible shearing value with the Sn-Pb. During reflow process, the formations of Ni-Sn IMC are detected at the nickel and solder material interfaces and the IMC thickness increased with multiple reflow.