Modified Multipoint Constraints of Finite Element Model for SJR Prediction Accuracy

Finite element (FE) method has been widely used for package level and board level reliability predictions. Recently, there are more components packaged in single unit and complicated geometry combining with finer mesh should be built in FE model. To simplify model building procedure and reduce modeling time, FE model is often separated into two or more parts with discontinuous mesh, which are package and solder joint with a printed circuit board (PCB). By using Multipoint constraints (MPC) method, the degree of freedom (DOF) will be constrained and transmitted between fine and coarse mesh. This simplified FE method usually induces some numerical errors. To balance efficiency and accuracy, two geometric parameters, constraint location and mesh size ratio between package and solder joint, will be discussed in this paper. The results show that setting the constraint layer farther from the solder will get lower error percentage. Furthermore, constraint location, at interface of two substrate layers or at inner of single material, isn't a significant factor. In mesh size discussion, a finer mesh usually gets a more accurate solution until it converged. From this study, an appropriate meshed size ratio, P/S ratio, is proposed for getting the converged result in reasonable time. In addition, two Pb-free alloys, LF35 and SACQ, are compared as well since the discrepancies of material properties lead to varying degrees of numerical error.