Fracture Life Evaluation of Cu-Cored Solder Joint in BGA Package

Abstract

A Cu-cored solder joint has an accurate height, a low thermal resistance, and a low electric resistance. However, the fracture mechanism of Cu-cored solder joints has yet to be clarified, and thus the fracture life cannot be predicted. We evaluated the fracture life of Cu-cored solder joints by using our molten-solder-shape analysis and crack-propagation analysis methods. Our molten-solder-shape analysis is based on the moving-particle semi-implicit (MPS) method. In the MPS method, a continuum is expressed as an assembly of particles. In contrast to finite element analysis (FEA), the MPS method can easily express a large deformation and any geometric topology changes, because the continuum does not need to be divided into elements. Using our molten-solder-shape analysis, we could calculate the shapes of Cu-cored solder after the reflow process. Our crack-propagation analysis has a feature where a crack initiation point and the crack propagation paths are automatically calculated and where the fracture life is quantitatively evaluated using FEA. Using our crack-propagation analysis, we could analyze the fracture mechanism of Cu-cored solder joints. By combining our molten-solder-shape and crack-propagation analyses, we could evaluate the fracture life of Cu-cored solder joints in a ball grid array package. As a result, we found that the fracture life of Cu-cored solder joints is longer than that of conventional joints. The height of a joint is one of the reasons for the improved fracture life. Since the height of a Cu-cored solder joint is controlled by the size of the core ball, the height is larger and more highly accurate than that in conventional joints. Accordingly, the solder strain and strain variation are decreased. Joint stiffness is the second reason for the improved fracture life. Cu is harder than solder, so the joint stiffness of a Cu-cored joint is greater than that of conventional joints. Accordingly, the displacement of a joint is decreased. The crack-propagation behavior is the third reason for the improved fracture life. In a conventional solder joint, a solder crack only propagates near the interface of the solder and the land. In a Cu-cored solder joint, a solder crack not only propagates near the interface of the solder and the land, but also at the interface of the solder and core ball. The crack-propagation life is longer than that in a conventional joint due to crack-path scattering. We found that the fracture life of Cu-cored solder joints is improved by using these mechanisms.