Further insight into interfacial interactions in nickel/liquid Sn–Ag solder system at 230–350 °C

Abstract

Interfacial reactions are investigated between electrochemical deposited Sn-2 wt%Ag alloy and Ni for isothermal heating at various temperature (230–350 °C) and for various time to study initial stages (1–4 min) and latter stages of reaction (15 min–4 h). During the isothermal heating a continuous compound layer of Ni3Sn4 is formed at the interface between liquid Sn–Ag and solid Ni. In this study scallop like morphology with round and smooth surfaces of Ni3Sn4 intermetallic (IMC) layer is observed for shorter time of isothermal holding, which is in fact contradictory to the observations reported by recent studies which describe the morphology of IMC as elongated and faceted needles. For longer reaction times (>1 h) the scallop-like morphology is transformed gradually to a facetted abnormal growth morphology but not elongated structure. The average thickness of the reaction layer is proportional to a power function of the annealing time with an exponent n varying from 0.35 to 0.40 and the apparent activation energy for liquid–solid Ni3Sn4 formation was evaluated to be of about 21 kJ mol−1. The role of deposition method of Ni and Sn layers on the morphology and the growth kinetics of the reaction layer is discussed. A theoretical analysis of the initial formation and growth of Ni3Sn4 phase at the Ni/Sn interface is also presented.