Study on interfacial reaction between lead-free solders and alternative surface finishes

Abstract

This study investigates the interfacial reactions occurring during reflow soldering between Sn-Ag-Cu lead-free solder and two surface finishes: electroless nickel/ immersion gold (ENIG) and immersion silver (IAg). The study focuses on interfacial reactions evolution and growth kinetics of intermetallic compounds (IMC) formed during soldering and isothermal ageing at 150 degree Celsius for up to 2000 hours. Optical and scanning electron microscopy were used to measure IMC thickness and examine the morphology of IMC respectively, whereas the IMC phases were identified by energy dispersive X-ray analysis (EDX). The results showed that the IMC formed on ENIG finish is thinner compared to that formed on IAg finish. For IAg surface finish, Cu6Sn5 IMCs with scallop morphology are formed at the solder/ surface finish interface after reflow while a second IMC, Cu3Sn was formed between the copper and Cu6Sn5 IMC after the isothermal ageing treatment. For ENIG surface finish both (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 are formed after soldering. Isothermal aging of the solder joints formed on ENIG finish was found to have a significant effect on the morphology of the intermetallics by transforming to more spherical and denser morphology in addition to increase i their thickness with increased ageing time.