Optimal design of VFBGA mixed solder joints under random vibration

This article provides a method to perform finite element analysis (FEA) and theoretical evaluation on stress and strain characteristics of very-thin fine-pitch ball grid array (VFBGA) assembly under the random vibration environment. Solder ball diameter, ball pitch, ball joint height, PCB pad diameter, Intermetallic compounds (IMCS) between solder alloy and PCB pad, and IMCS thickness were selected as critical elements. By using an L₁₈(3⁷) orthogonal array, which combined of 18 configuration parameters, the mixed solder joints of VFBGA components were designed. FEA of the VFBGA packages under random vibration load were performed, then the distributions of stress and strain of VFBGA mixed solder joints was investigated. Based on the FEA results (stress value), range analysis and variance analysis was researched. In this study, the stress of mixed solder joints is affected by solder ball pitch, ball joints height, ball diameter, PCB pad diameter, IMC type, IMC thickness in a descending order and the best level combination of configuration parameters that results in the minimum stress and strain is the solder ball pitch of 0.7mm, the solder ball joint height of 0.25mm, the solder ball diameter of 0.56mm,the PCB pad diameter of 0.335mm, the IMC is Cu6Sn5, the IMC height is 1.4μm compared to other selected level in paper. With 90% confidence, the ball pitch and the ball joint height have significant effect on the stress of VFBGA solder joints whereas the other critical parameters have little effect on the strain of the mixed solder joints of VFBGA under random vibration load.