FLOW PROCESSES IN SOLDER PASTE DURING STENCIL PRINTING FOR SMT ASSEMBLY

Solder paste is used for reflow soldering of Surface Mount Devices (SM Ds). In this paper we discuss how the various stages of the stencil pr inting cycle affect the rheological properties of the solder paste. Fi rst the heat generated in the paste roll is examined to see what effec t it has on solder paste rheology, then we analyse in detail the proce ss of paste withdrawal from a metal mask stencil and discuss those pro perties of solder paste that lead to a good print in terms of the size and shape of the solder paste particles, and their packing. In order to do this, we review some of the experiments and phenomena that have been shown to occur in dense suspensions, and see what aspects of that work are applicable to solder paste printing.