How to define solder paste printing height tolerance

Start with targets based on the stencil thickness (for height) and stencil thickness times the aperture size (for volume). * Set reasonable upper and lower control limits (+ 25% and –25 % of the targets, for example). * Next, check the printer set-up by printing and measuring a test print on a flat sample surface. A piece of white poster board will work. ** Take sample measurements at critical locations (the corners of fine pitch sites, BGA and ?BGA sites, etc.) ** Gather readings at the corners and the middle of the complete print area, checking for parallelism. * Once you are confident in the printer set-up, record measurements on every PCB in the pilot lot. * If a print failure is found (i.e. exceeding your selected control limits) wash the sample, adjust the printer and reprint. * Process the boards through reflow. * After reflow, inspect and calculate the yield. * Adjust your process limits based on the defect analysis. If post reflow inspection identifies: ** Shorts, lower the upper control limit. ** Opens, consider increasing your targets.

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