Investigation Of Interfacial Stress Singularity In Microelectronics Packages

There are various interfaces in the electronic packages, and the failure generally occurs at these locations. Due to the difference of material property between two sides of the interface, the stress singularity will be observed at the edge. Therefore, it is critical to analyze the interfacial stress singularity for deeply understanding the reliability of electronic packaging. In this paper, the stress singularity in solder joint was investigated based on the theory of interface mechanics.In this paper, the border angle 02 was used to describe the shape of a solder joint. Five shapes, includingθ2 equals to 600,750,900,1050nd1200, and three solder materials, Sn3.0Ag0.5Cu, Sn3.5Ag and Sn37Pb, were analyzed. The eigenvalues of interface in solder joints for different shapes and materials were obtained by solving the eigen equations. The shape of solder joint has significant influence on the eigenvalue of interfaces. Since the eigenvalues represents the stress singularity, it indicates that with the increasing of the angleθ2, the stress singularity increases. Additionally, the stress singularity in Sn37Pb solder joint is more obvious than those in Sn3.5Ag and Sn3.0Ag0.5Cu.Although the interface eigenvalues are able to reflect the stress singularity, it is difficult to obtain the analytic solution of stress field for complicated structure and loading condition. Therefore, the finite element method was used in this paper to calculate the interface stresses. A 2-D model of board level electronic package was established, and then the peeling stresses in solder joints under static loading were obtained. The result shows, in an identical material solder joints with five different shapes, the stress singularity exhibited by sharply change of the peeling stresses at the edge of interface with the increasing ofθ2. In an identical shape of solder joint with three different materials, the stress singularity in Sn37Pb solder is the greatest among the three materials.Additionally, the peeling stresses in solder joints with elastic, trilinear elastic-plastic material model for static loading and Johnson-Cook model for drop impact loading were analyzed. The stress singularity of solder joints with linear elasticity material model is greater than those of plastic model and the Johnson-Cook model.