Solder Printability of a Stencil with a Hydrophobic Organic Coating

This study investigated the effects of hydrophobic-film-coated stencils on bridging rate and solder print efficiency, defined as the volume ratio of printed solder to the stencil aperture size. Printed solder paste volume was measured by solder paste inspection (SPI). The test printed circuit board (PCB) in this study possessed 0603/0402, ball-grid-array (BGA) package, and bridging test pads printed using type 5 solder paste (Sn 96.5%, Ag 3.0%, Cu 0.5%, by weight). The coated stencil showed slightly higher printing efficiency than the uncoated stencil. The cross-sectional view of the aperture wall showed substantial roughness on the top and bottom sides of the aperture, which degrades the improvement in printing efficiency offered by hydrophobic thin films. The solder bridging rate of the coated stencil was higher than that of the uncoated stencil. The printed solder paste sent through the coated stencil had high surface tension, leading to a low bridging rate.