Testing the Mettle of STENCIL FOILS

摘要

In the quest to elevate the stencil printing process from its original black magic art form to the (mostly) science it is today, we've developed a number of guidelines to steer us toward success. One of my favorites is the area ratio (AR) calculation. It assumes solder paste sticks to everything equally, and to get a good quality deposit, the force of the paste sticking to the PCB pad must overcome the force of the paste sticking to the stencil when the two separate. We calcu-late it by dividing the area of the aperture walls by the area of the pad. Fast forward through the algebra, and that boils down to the size of the aperture (diameter if it's a circle;edge length if it's a square) divided by four times the foil thickness. The higher the ratio, the more paste is released.