Stencil cleaning

Stencils should be cleaned frequently enough to ensure total removal of any bottom side residues, but not so infrequently as to allow the same residues to dry or cake on, making their removal much more difficult.

Look here: http://www.smtnet.com//forums/index.cfm?fuseaction=view_thread&CFApp=1&Thread_ID=9320&#Message37129

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Just to add to what dave said, the only limitation to how frequently you clean is the cost of the cleaning medium. You can wipe every board if you want to, but you'll go through material accordingly.

If you're printing 16mil pitch or smaller, cleaning every other cycle isn't out of the question but it depends on how well your stencils gasket the board (surface finish, stencil alignment), your paste rheology, etc.

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First, isn't paper a nonwoven? Maybe we start splitting hairs on whether paper is a 'fabric' or not, but what's the difference?

Second, paper versus nonwoven fabric boils down to performance and cost. We know 'paper', but it could be 'fabric' and we wouldn't know it.

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The most common rates of in-process cleaning (under wipe) I have encountered are once per 5 to 10 prints. But this depends a on a lot of variables. More often is typically better, but more costly in under-wipe materials. (For some more info see http://www.finelinestencil.com/Products/MicroCare/index.html) Stencil cleaning after the run is finished (or temporarily interrupted) can be done many different ways and the main controlling variable is the type of flux that is used in the solder paste. Methods range from hand cleaning with alcohol, brushes and wipes to cleaning in an ultrasonic bath using water, temperature and a cleaning agent. There are almost as many ways to be successful with this as there are ways to do it wrong.
Good luck.

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