To simulate the formation of TSSOP solder joint with SAC solder and assess on the effects of the stencil design and the misalignment on the joint shape

Abstract

Robustness of solder interconnection highly depends on solder joint geometry which is largely governed by the surface tension of solder paste, the wettability of the solder paste on the component finish and on the solder land, and the geometry/dimension of the solder land on the substrate and the component lead. With the mandatory trends to launch Pb-free in most electronics application and the increasing interconnect miniaturisation in electronics packaging, the effect of the surface tension of liquid solder alloy, the design parameters, e.g. solder volume, and the process parameters, e.g. the misalignment, on the solder joint geometry, and the criteria for acceptable design/process parameters for a reliable interconnection are more and more attractive in actual industry manufacturing. In this study, with a typical Philips component/package TSSOP (thin shrink small outline package) as a example, a developed numerical model to predict the equilibrium shape of a liquid was applied in Surface Evolver, and the formation of the solder joint between the TSSOP package to the Cu pad on the PCB during reflow is simulated. With the model, the geometries of the TSSOP solder joint with eutectic Sn-Ag-Cu (SAC) solder and traditional eutectic SnPb solder were predicted and compared, and the effect of the different stencil design and the misalignment of the lead to the pad on the solder shapes is predicted, and the predicted result showed a good match with the result from the lab test. Finally, a criteria/margin is suggested to define these design/process parameters for the TSSOP package.