Shape Prediction of Molten Micro-Solder Joints Using Particle Method and Application to Evaluation of Fracture Life

The reliability of a micro-solder joint in a semiconductor structure greatly depends on the solder shape. Therefore, many methods to predict the shape of the molten solder have been proposed [1–3]. However, some problems arise when conventional methods are applied to predict the solder shape of miniaturized and lead-free joints. The first problem is the difficulty in expressing the large deformation and topology change of the solder. In a miniaturized joint, the shape of molten solder changes significantly during the reflow process, and even topology changes (e.g., merging with other solder in a neighboring joint or splitting into several pieces) can occur. These phenomena need to be expressed if we are to predict the solder shape of the miniaturized joint. The second problem is the difficulty in expressing the effect of solder wettability. The solder shape is known to depend on the solder wettability, and the wettability of lead-free solders is different from conventional solders. To predict the lead-free solder shape, we need to express the effect of the wettability. Therefore, I developed a new shape prediction method that solves these problems using the moving-particle semi-implicit (MPS) method [4, 5]. MPS is suitable for calculating incompressible flow and can be used to easily express large deformation and topology changes. However, the original MPS method cannot sufficiently express the effect of solder wettability. Therefore, I enhanced the surface tension formulation of MPS, making it possible to express this effect. I applied this method to predict the solder shapes of various packages (e.g., a thin small outline package (TSOP) and a flip-chip package) and found that the method is effective in predicting the solder shapes of miniaturized joints. Moreover, I was able to evaluate the fracture life of a solder joint with the predicted solder shape by coupling the shape prediction method with our crack propagation analysis method, which was demonstrated at a previous InterPACK [6]. In this crack propagation analysis method, crack initiation points and propagation paths are automatically calculated, and the fracture life is evaluated quantitatively by finite element analysis. I applied these combined methods to evaluate the fracture life of solder joints that had different solder shapes due to different wettability conditions. As a result, I was able to find the differences in crack initiation points and to evaluate crack propagation paths and fracture lives in different wettability conditions.