Application of multicomponent phase diagrams in understandting phase evolution of solders/UBM system

Abstract

There are two thermodynamic methods to predict the intermetallic compound (IMC) phase that forms first at the joint interface of solder/UBM (under bump metallurgy) during the soldering process. One is based on a key idea that the thermodynamic state at the interface would be under metastable local equilibrium between the initial phases before formation of any interfacial reaction product. Then, the phase with a highest driving force of formation under this metastable equilibrium at the interface would form first. The other is based on a general assumption of local equilibrium at the interface. Therefore, the tie lines in multicomponent equilibrium phase diagrams can be used in predicting the sequence of phases that form and react. In this presentation, the difference between two views is described and discussed mostly with an emphasis on Pb-bearing and Pb-free solders with various types of substrates and UBM.