Stencil printing of solder paste for fine-pitch surface mount assembly

A generic study involving solder paste characterization, stencil configuration, printed wiring board layout, and printing parameters was performed. The authors summarize findings in the following areas: solder paste-powder particle size distribution, rheology, slump, and solder ball formation; stencils-single versus dual thickness, brass or other materials, opening sizes, vendor capabilities; printed wiring boards-pad geometry, dimensional tolerance, pad surface finish; printing process-setup and alignment, squeegee materials, printing speed downward pressure; and, component placement-handling, placement accuracy, force limitations. Some practical concerns, including the current process operating window and future fineness of print limitations, are discussed