Monte Carlo study of solder paste microstructure and ultra-fine-pitch stencil printing

Abstract

In this paper, we apply a Monte Carlo simulation technique to study the microstructure of solder pastes and investigate the influence of solder particle-size distribution on the ultra-fine-pitch stencil printing. First, the microstructures of bulk solder pastes with different particle-size distributions were generated using a random-packing model. Then a statistic model was applied to simulate the packing of solder paste inside the apertures. The numbers of solder particles and solid volume fraction embodied in the apertures were counted. Five particle-size distributions and two aperture shapes (circular and square) were investigated. Simulation results showed that the mean solid volume fraction of the solder particles inside the apertures is lower than that in the bulk solder paste. For the same aperture size and shape, as the particle size increases the mean solid volume fraction decreases and the standard deviation increases. This implies that to obtain consistent paste deposits in ultra-fine-pitch printing, the particle size must be proportionally reduced with the aperture size. The reasonable size ratio of the aperture to the solder particle was found to be around five. Excessive reduction in particle size could not improve the printing quality further, in contrast, it may lead to poor printability due to the increase in the paste viscosity and poor solder joints due to the generation of solder balls in the reflow soldering process.