Phase Stability Assessment and Microstructure Modification at Lead-free Solder Joint

The microstructure modification of the solder joint formed between the OSP Cu substrate and Sn3.5Ag lead-free solder was explored in this paper. The 0.2Co0.1Ni additives were added into Sn3.5Ag solder matrix. And the refined bulk microstructure, namely, the depressed Sn-halo growth, was found, which could be interpreted by the eutectic growth theory and interaction behavior between the component elements. After soldering on Cu substrate, a large solubility of Co and Ni was found at Cu₆Sn₅-based IMC, which could be labeled by (Cu, Co, Ni)₆Sn₅. The thermodynamic assessment was carried out and revealed that the solubility of Co and Ni at Cu₆ Sn₅-based IMC could reduce the system energy and thus stabilize the system. The calculated results also implied a higher affinity of Ni/Sn and Co/Sn couples than that of Cu/Sn, which supplied another evidence of the phase stability of (Cu, Co, Ni)₆Sn ₅. The Ag₃Sn + (Cu, Co, Ni)₆Sn₅ cluster structure was found in the solder matrix after soldering. After isothermal annealing, the Cu₃Sn phase was remarkably depressed partly due to the more stable (Cu, Co, Ni)₆Sn₅ phase