Stencil Printing: Single pass vs. Double pass

"Any risks associated with double pass stencil printing?"

Mostly just too much paste, plus the associated bridging, and your stencil gets dirty quickly. Any gap between the stencil and the board gets filled with paste on the first pass, then migrates laterally during the second pass.

In my experience the only reason you should "need" to do it is that something else is wrong with your process, like a bum printer, old paste, worn out stencil, etc. You can probably get away with it for a .050" product, but not with fine pitch unless everything else is in your favor (your stencil is in really good shape, you have nice flat pads, good layout, no thick silkscreening, etc.).

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You printer is not solely designed for printing solder paste. This is why some of the options you would never use for paste appear in the menu. You may actually be printing overlays or yes, even t-shirts. This is why you may want to flood/print, print/print etc. In olden times we actually used silk screens which were basically the mesh stretched across the frame with block out or mask applied to the mesh to prevent printing in these ares. Often the mesh was attached to the frame via a lace and eyelets so the tension was never even and the image would move differently according to the print direction. You could be printing with paints or dyes, different adhesives, any sort of medium really. As the other members stated you would generally try to avoid this double print with paste or smd adhesive unless you are trying to overcome some other sort of defect or process problem.

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Search the fine SMTnet Archives for previous discussion, like: http://www.smtnet.com/forums/Index.cfm?CFApp=1&Message_ID=48507

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Many years ago, we used to double print. The way we did it was with a rubber squeegee set to do a flood stroke, followed by a return cleaning stroke with a metal squeegee.

Why?!

Well, this was in the days before Laser etch, and because the chem bath screens would have jagged apertures. Also, we used to do this when screening not only for pads, but putting paste into thru-hole holes as well. The double sweep put enough paste into the hole to allow us to put thru-hole components into the board, and reflow like normal.

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