A study of solder paste release from small stencil apertures of different geometries with constant volumes

Stencil printing is a critical first step in surface mount assembly. It is often cited that about 50% or more of the defects found in the assembly of PCBs are attributed to stencil printing. Manufacturing techniques for the assembly of certain flip chips, chip scale packages and fine pitch ball grid arrays are testing the limits of current stencil printing capabilities. A thorough understanding of basic stencil printing principles would facilitate the design of printers, stencils and pastes, and would ultimately permit the extension of reliable print techniques to the very fine print arena. For small apertures, solder paste volume and consistency are critical to solder joint reliability. The work described in this paper examines the release performance of various solder pastes from a variety of aperture sizes and geometries. The focus of this study is a comparison of square versus circular apertures when the nominal volume of paste to be deposited is kept constant. This method of study is contrasted with published work wherein squares versus circles have been studied, but, in those, the dimensions (not volumes) were the same (e.g., 12 mil diameter circle as compared to a 12 mil (on a side) square aperture).