I am a lay person researching the application of non-woven material in stencil rolls either using branded material or non-branded alternatives for DEK, MPM Panasonic machines in SMT environments. I have read a number of articles referring to aqueous, ultrasound techniques as well as use of paper rather than non-woven fabric. Are these alternatives to the use of non-wovens when cleaning stencils in SMT or relating to other cleaning processes within PCB / SMT manufacturing? Any pointers would be appreciated Bryan
I would like to know if there is any information regarding how often to clean the stencil by the printer cleaning system. I am running a board with an ultra fine pithc QFP. Is there any guide available?
Bryan: * Most of our stencils are made from stainless steel that is held by a aluminum frame, attached with epoxy with a fiber glass intermediary between the frame and the stencil. * We use stencils to print either glue [thermoset epoxy] or solder paste * Plastic stencils are almost exclusively used for glue. * Paper is used as a stencil underwipe to clean the bottom of the stencil during production as a function of printer operation. * Dedicated ultrasonic or water wash cleaners clean both sides of the stencil off-line during a stoppage of production or [as mentioned earlier] at the end of a production run.
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.
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.
Thanks to all for clarification - I now appreciate the different methodology between "in-line" & "end of run" but am still unclear as to advantages / disadvantages of paper versus non-woven material ie stencil rolls for in-line cleaning. Any info would be gratefully received. Thanks again
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.
You can stop cleaning after a job when there is no more paste in the apertures. Under wiping should be done with a quality cloth and should leave no paste on the underside of the stencil (assures good contact on the next print and reduces chances of solderballs) and should leave no cleaning liquid (e.g. alcohol) on the stencil.