Study On Stress-Strain And Thermal Failure Of Ball Grid Array Package

The thermal stress failure of packaging has a significant impact on the reliability of a device, especially in Ball Grid Array (BGA) package. The stress is the direct cause to lead to solder balls failure, which causes microelectronic devices damage. The finite element analysis can conveniently calculate the total or part strain and stress of the packaging system and assist the packaging structure design. Thereby, the cost of design and the cycle period of analysis are cut down.In this paper, the mechanical behavior of BGA package devices has been analyzed under the thermal cycling. The morphology of solder balls has been observed by using a high power microscope. The failure mechanism of the devices has been studied. The manners of germinating and expending of the crack have been analyzed. The experiment results show that the coarsening rate of solder balls gradually becomes marked, and the outmost solder balls of device get failure firstly with the increasing of thermal cycling times. Because thermal expansion coefficients of different material mismatch, the crack is firstly germinated near the borderline between around the outmost balls and chip. The cracks expand from the borderline of outmost solder balls to the ball center along the interfaces. It is found that the germination and expanding of the cracks are caused by highly concentrated stress and strain as well as interaction between thermal cycling and creep deformation.In this paper,the finite element analysis software ANSYS has been used to simulate the stress distribution and displacement of the two-dimensional model of Ball Grid Array package under thermal cycling conditions in the range of -40℃~125℃. The model of two-dimensional BGA is simplified as chip, solder ball and PCB. The shape of solder ball is drum which is more effective than columnar for simulating the stress distribution. The simulation results show that maximum values of stress and displacement are at the joints between the peripheral solder ball and chip under thermal cycling conditions. The dangerous point that could be found while universal postprocessor. With the increase of thermal cycling times, the largest values of stress and strain increase and tend to a stable value, which is presented by time after course treatment.The optimization design of solder ball is important content of the reliability research on it. Because of symmetry of the model structure and load conditions, Von Mises stress and strain of a single solder ball were studied by the model of three-dimensional BGA for optimization design. The material of solder ball is 63Pb37Sn and the unified viscoplastic constitutive Anand model was applied. The simulation results show that with the increase of radius of solder ball,the maximum Von Mises stress and strain increase firstly, and then decrease when the height of solder ball is constant. When the radius of solder ball is a constant, with the increase of the height of solder ball, the maximum Von Mises stress and strain of solder ball decrease accordingly.What this paper has done has some important practical reference to improve the reliability of devices, and supplies the theory of improving the quality of solder balls during the practical jointing. Otherwise, the APDL language of ANSYS is used to write program in the process of modeling. So the ANSYS software can be used to do the simulation analysis of the packaging expediently.