Atomistic Modeling to Predict and Improve the Strength of Doped Sn-Cu Solder Interfaces

In this work, we have used atomistic modeling based on density functional theory (DFT) and ab initio molecular dynamics (AIMD) to study the mechanical strength of the Sn-Cu interface under various conditions. We have investigated the cleavage energy (CE) of the Sn-Cu interface, and how it changes with various substitutional alloys (Ag, Au, Bi, Cu, Ni, and Zn) to determine the benefit (or detriment) to the strength of this simulated solder joint. Our simulations show that each of the dopants considered, except for Bi, increases the strength of the interface. In all our constructed Sn-Cu interfaces, a single atomic layer of Sn atoms deposits on the Cu and binds strongly to it. The weakest point of the interface is located between the deposited Sn layer and the remaining bulk Sn. As a complementary method for investigating this interface, we have used AIMD to simulate a mechanically controlled break junction. The process yields an energy versus distance curve, which provides information about the strength of the solder joint (similar to a stress–strain curve). The peak of this curve indicates the maximum amount of energy required to separate the joint, and it is in good agreement with our CE calculations. The first principles nature of the methods employed makes them highly transferable and applicable to any chemical composition.