| With the development of SMT and BGA technology, especially the fine pitch technology, the size and pitch of I/O of IC have been decreasing constantly. Solder bridge between adjacent pads is one of the most common and critical defect in the high density assembly process. The arrangement and size of solder pads on PCB becomes more and more important for the reliability of IC packaging, and optimized geometry of solder pad and solder liquid is the guarantee of manufacturing process. Solder mask plays an increasingly crucial role in controlling solder defects during fabrication of high density PCB, which can block the distribution of solder liquid and avoid solder bridge between adjacent pads during reflow process. So it is of great importance to conduct study of morphology control for the wetting of the solder liquid.In this paper, numerical simulation technology by means of using finite element analysis software, Surface Evolver, is applied to study the wetting behavior of SnAgCu solder liquid on structured surface consisting of solder pad and solder mask. There are three major aspects explored:1. The dewetting process on substrate with nonwetting line and equilibrium morphology of SnAgCu solder liquid is simulated. Based on the simulation, the mechanism of the solder bridge is proposed. What's more, the relationship between critical height of the solder liquid HC and the width of nonwetting line W is proposed based on the simulation result, which is comparable with the experiment data and previously proposed theoretical model. All in all, these results gave a clear hint on the origin of Solder Bridge.2. The dewetting process on substrate with nonwetting round region and equilibrium morphology of SnAgCu solder liquid is simulated as well. Our investigation shows that the dewetting of solder liquid on such region depends on whether the height of solder liquid is smaller than the critical height of solder. At the same time, the relationship between the radium of round region and the critical height to dewetting is derived based on simulation results, which can also be verified by previous experimental results.3. The dewetting of the SnAgCu solder liquid on structured surface with 4×4 pad array is studied by simulation. A new method to fabricate solder bump by wetting and dewetting on structured inhomogeneous surface is proposed and simulated. These researches on morphology control and bridging of solder liquid on patterned substrate is not only helpful to reveal the limitation of the highest connection density, but also valuable for the development of new high-density assembly techniques. It is of great significance in the miniaturization of electronic products and devices.
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