A characterization study of direct imaging technique for stencil printing of thick boards (0.125") in the alternative assembly and reflow technology (AART) or pin-in-paste process

Abstract

Stencil printing poses significant challenges in the successful implementation of the alternative assembly and reflow technology (AART) or pin-in-paste process for thick (0.125) boards. When the AART process is used, it is extremely important to deposit adequate volumes of solder paste in the plated through holes (PTHs). In some cases, adequate amounts of solder paste cannot be deposited into the PTH with the conventional stencil printing method that uses a squeegee. Therefore, alternate techniques must be used to meet the solder paste volume requirements for thick boards. The direct imaging system is a unique method of stencil printing. It is a viable, if not better, alternative to the traditional squeegee-solder paste system. The ProFlow system consists of a pressure application mechanism that exerts a vertical force on an enclosed volume of solder paste. This research characterized the ProFlow system for the stencil printing of thick boards. Three PTH diameters were considered on the board: 35, 37 and 39 mils. The effect of the significant variables in the ProFlow system, such as transfer pressure and print speed, on hole fill and printability was systematically investigated. Also, the role of the ProFlow system's wiper pressure on the stencil printing process was examined. Thick boards were assembled with surface mount as well as through hole components to validate the applicability of the ProFlow system to the AART process.