Why It Is Important to Keep the SMT Stencil Clean SMT stencils to can be subject to a variety of process defects due to the removal of solder paste and uncured/unreacted SMT adhesives from stencils, misprinted printed circuit boards. The cleaning of stencils and misprinted PCBs has taken an increasingly important role in surface mount technology as pitches become finer and parts spacing narrows. Paste volume is a critical issue for fine and ultra-fine pitch components, chip-scale, BGA and other miniature components. Insufficient solder due to clogging of stencil apertures is a consistent cause of defects. Clogged stencil apertures results in poor paste printing. Therefore clean apertures in SMT stencils are important to delivering the proper volume of solder paste. Several studies and fishbone diagrams report that approximately 70% of surface mount technology defects are due to solder paste printing problems. Cleaning agents for SMT stencils must be effective, safe for workers and safe for the environment. Isopropyl alcohol (IPA), commonly used for cleaning SMT stencils, poses both environmental (VOC) and safety (i.e., fire hazard) concerns.
The residue encountered on stencils is generally non-reflowed solder paste. After hours of use, the solder paste may partially dry thereby increasing the difficulty of cleaning. Proper stencil cleaning needs to remove solder particles and organic flux media from stencil apertures without damaging the stencil, bonding adhesives, and the glue of the frame of the stencil. Solder paste residue on a stencil can result in transferring paste from the bottom of the stencil the next board being printed. These contaminants can interfere with the intimate contact or ‘‘gasketing’’ of the stencil to the board.