CBGA Solder Fillet Shape Prediction and Design Optimization

Abstract

In this paper, we first present a mathematical method that can be used to predict the eutectic solder fillet shape for ceramic ball grid array joints. An underlying assumption is that the solder fillets on both the module and the card sides can be represented as arcs. The fillets’ profiles are then calculated for the factors affecting the shape including solder volume, pad size, solder ball size, the wetting angle between eutectic solder and solder ball, and the gap between solder ball and pad. The second part of the paper focuses on design for reliability and investigates the effect of the interactions between the card-side and the module-side solder fillets on CBGA solder joint reliability. To this end, a central composite design of experiment is set up to systematically vary the pad size and the eutectic solder volume on both the module and the card sides. For each of the design settings, the proposed mathematical method is used to calculate the solder fillet shape. Using ABAQUS and the modified Coffin-Manson relationship, the mean fatigue life is predicted. The implications of the simulations are discussed. In addition, a response surface model is presented to find the optimum settings for maximum reliability. Finally, a comparison is made for the fatigue life predictions obtained using the proposed mathematical method and the linear solder fillet assumption.