SQUEEGEE BLADE DESIGN FOR STEP STENCILS

摘要

There are several printing applications that require multilevel stencils to achieve the correct paste height/volume. Overprinting of the PCB pad or through-hole is one method to achieve more paste volume. However to achieve additional paste height the thickness of the stencil must be increased. This will normally require a step stencil since not all components need additional paste height. Examples where additional paste height is required are components with coplanarity issues like CBGA's, Surface Mount connectors, and RF shields. The difference in height for these applications is typically 25 microns (1 mil) to 75 microns (3 mils). The rule of thumb for the distance between an aperture and the step wall is 900 micron (35 mils) per 25 micron (1 mil) of step difference. As long as the step is 75 microns (3 mils) or less a normal metal squeegee blade is usually effective for printing. However there are other step stencil applications that require steps much larger then 75 micron (3 mils). These step stencils require special blades. A slit squeegee blade for printing step differences up to 750 microns (30 mils) and a notched squeegee blade for printing step differences up to 3 mm (.120 mils) are described in this paper.