Effect of solder pads on the fatigue life of FBGA memory modules under harmonic excitation by using a global-local modeling technique

Abstract

This paper investigates the effect of solder pad size on the fatigue life of fine-pitch ball grid array (FBGA) solder joints in memory modules due to harmonic excitation by using a global-local modeling technique. Finite element analysis of a memory module requires enormous computer memory and computational time because some components such as solder balls are very small relative to the overall size of the memory module. The global-local modeling technique has been suggested as an alternative approach with reasonable accuracy. A finite element model of the memory module with simplified solder joints was developed as a global model, and the natural frequencies and modes were calculated and verified by experimental modal testing. Displacement variations were calculated from the global model due to vibration excitation using the mode superposition method. A finite element model of a part of the memory module, which is composed of a package, PCB, and detailed solder joints, was developed as a local model. Calculated displacements from the global model were then substituted along the boundary of the local model in order to detect vulnerable parts of solder joints under vibration. Utilizing the global-local modeling technique, the interface between the solder ball and pad near the PCB was found to be the most vulnerable part, and the effect of solder pad size on the fatigue life of the memory module was determined by using the Basquin equation and Miner's rule. It was experimentally verified that the solder pad size in solder joints affects fatigue life as well as the reliability of solder joints under harmonic excitation.