Author: William E. Coleman Ph.D. Company: Photo Stencil Date Published: 4/1/2010
Abstract: As smaller components are introduced into SMT assembly the paste printing process faces new challenges. Small components have small stencil apertures. The normal accepted Area Ratio guideline is .66 although some types of stencils claim that .5 is an acceptable. A .3 mm CSP device with circular stencil apertures of 175 micron would require a stencil thickness of 67 micron to achieve an area ratio of .66. SMT assembly process engineers normally would prefer a stencil thickness of at least 100 microns to print solder paste for boards having .3 mm CSP and other SMT devices. The area ratio for a stencil with this thickness is .44, far below the recommended .66 and even below .5. This paper will explore coatings applied to the aperture walls of stencils as an enabling process to lower the area ratio below .5. Laser-cut and Electroform stencils have apertures coated with two different nanocoat materials. These stencils have apertures yielding area ratio’s ranging from .75 on the high end to .31 on the low end. Paste volume and paste volume variation will be measured and reported over the range of area ratios to determine the lowest area ratio acceptable for consistent paste printing.
Key words: Area Ratio, Stencil, Electroform, Laser, Coatings, Step Stencil, Paste Transfer