How to define solder paste printing 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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