Geometry effect on mechanical performance and fracture behavior of micro-scale ball grid array structure Cu/Sn-3.0Ag-0.5Cu/Cu solder joints

Solder joint integrity has long been recognized as a key issue affecting the reliability of integrated circuit packages. In this study, both experimental and finite element simulation methods were used to characterize the mechanical performance and fracture behavior of micro-scale ball grid array (BGA) structure Cu/Sn-3.0Ag-0.5Cu/Cu solder joints with different standoff heights (h, varying from 500 to 100 mu m) and constant pad diameter (d, d= 480 mu m) and contact angle under shear loading. With decreasing h (or the ratio of hid), results show that the stiffness of BGA solder joints clearly increases with decreasing coefficient of stress state and torque. The stress triaxiality reflects the mechanical constraint effect on the mechanical strength of the solder joints and it is dependent on the loading mode and increases dramatically with decreasing h under tensile loading, while the change of h has very limited influence on the stress triaxiality under shear loading. Moreover, when h is decreased, the concentration of stress and plastic strain energy along the interface of solder and pad decreases, and the fracture location of BGA solder joints changes from near the interface to the middle of the solder. Both geometry and microstructure greatly affect the shear behavior of joints, the average shear strength shows a parabolic trend with decreasing standoff height. Furthermore, the brittle fracture of BGA solder joints after long-time isothermal aging was investigated. Results obtained show that, under the same shear force, the stress intensity factors, K-I and K-II, and the strain energy release rate, G(I), at the Sn-3.0Ag-0.5Cu/Cu6Sn5 interface and in the Cu6Sn5 layer obviously decrease with decreasing h, hence brittle fracture is more prone to occur in the joint with a large standoff height. (C) 2015 Elsevier Ltd. All rights reserved.