The comparative study on interfacial IMCs growth of three Cu/SnAgCu/Cu solder joints with Bi and Cr additions during thermal aging

In this paper, the interfacial microstructure evolution of three Cu/solder/Cu joints with thermal aging at 150°C for 0, 168, and 500 hours respectively were studied using SEM+EDS. The object of study was the self-prepared Sn3.0Ag0.3Cu0.05Cr (SACC) and Sn3.0Ag0.5Cu3.0Bi0.05Cr (SACBC) and the common industrial Sn3.0Ag0.5Cu (SAC) solder was selected as the benchmark for comparison. The IMCs (intermetallic compounds) with different morphologies at the joint interfaces were enumerated and described. Results showed that most of Cu₆Sn₅ was rod-like, while Ag₃Sn was needle-like in three solder matrix of joints. The interfacial Cu₆Sn₅ growth of Cu/SAC/Cu joint during soldering was not in a typical scallop-shaped model, but in a so-called “bamboo shoots”. For three joints after soldering, the Cu₆Sn₅ in Cu/SACC/Cu joint had the smallest size and most fine strip-like Ag₃Sn was observed near the interfacial IMC layer, which could be attributed to the Cr addition and the redistribution of elements controlled by atomic diffusion along phase interfaces. It was also found that, differing from the coarsened granular Ag₃Sn at the Cu₆Sn₅ grain boundaries in Cu/SAC/Cu joints after aging, the tiny filamentous Ag₃Sn at the interface of Cu/SACBC/Cu joint had a tendency to break into nano or ultrafine particles evenly distributed on the surface of Cu₆Sn₅. Bi particles with nano size precipitated from solder matrix due to Sn consumption forming IMCs.