Liquid/solid and solid/solid reactions between SnAgCu lead-free solders and Ni surface finish

The SnAgCu series of solders are the most promising lead-free candidates to replace the Pb-Sn solders. The Ni metallization is one of the most common surface finishes for the electronic packages. During soldering (a liquid/solid reaction), the reaction between SnAgCu solders and Ni strongly depended on the Cu concentration. When the Cu concentration ≤ 0.2wt.%, only a continuous (Ni_1-xCu_x)₃Sn₄ layer formed at the interface. When the Cu concentration´s 0.6 wt.%, (Ni_1-xCu_x)₃Sn₄ disappeared, and only (Cu_1-yNi_y)₆Sn₅ was present. After soldering, samples were subjected to several different temperatures isothermal aging (a solid/solid reaction) for time as long as 1000 hours. When the reaction time was 200 hours at 180°C, it was surprising found that a discontinuous (Cu_1-yNi_y)₆Sn₅ intermetallic particles formed over the (Ni_1-xCu_x)₃Sn₄ layer in the solid/solid reaction between Sn3.9Ag0.2Cu and Ni. At higher Cu concentration (≥0.6 wt.%), (Ni_1-xCu_x)₃Sn₄ disappeared, and only (Cu_1-yNi_y)₆Sn₅ was present. It showed that the initial difference in the intermetallic compounds right after reflow can be aged out at high temperatures.