Board Level Solder Joint Thermal Fatigue Reliability Improvement by Optimizing PCB Pad Size and Solder Paste Stencil Mask Designs

In the application of consumer electronics, thermal fatigue reliability of solder joints have been received considerable attention nowadays due to the increasing requirement of long-term usage. Ball grid array (BGA) package as one of high-density packages has many advantages including excellent electrical performance, heat dissipation and improved re-workability, and is therefore widely used. The BGA packaging solders in the field are normally under the circumstance of frequent temperature variations because of ambient temperature change and various usage scenarios. The coefficient of thermal expansion (CTE) mismatch between BGA package and printed circuit board (PCB) will induce fatigue stress in solder joints when the change of temperature occurs. Consequently, the fatigue cracks inside the solder joints may generate and propagate, leading to the severe facture of solder joints. Moreover, the underfill often apply in the interval of solder joints to enhance the mechanical behavior of bonding, which accelerates the thermal fatigue failure due to the larger difference of CTE introduced by underfilll. In order to improve the thermal fatigue reliability of BGA solder joints, the geometry appearance of joint was optimized by altering the size of stencil and PCB pad in this study. The BGA components were surface mounted on the PCB using different size of stencil and PCB pad, followed by the application of underfill. Thermal shock tests were carried out to measure the thermal fatigue life of BGAs. The cracks in solder joints were observed via X-ray computed tomography (CT) and cross-section cutting. Finite element method (FEM) using ABAQUS software was applied to simulate the stress and strain of solder joints under thermal shock, indicating the tendency of crack generation. The geometry appearance of solder joint used in FEM models was obtained by the numerical simulation of Surface Evolver.