Screen Printing

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I know from previous experiences that off contact printing h... - May 01, 2006 by DHolt

....am I reading this correctly? "Screen Printing" or "St... - May 01, 2006 by

We agree with Ken's points about off contact / screen versus... - May 02, 2006 by davef

We are using a Metal Screen. We used a metal screen in the ... - May 02, 2006 by DHolt

DHolt

#41278

Screen Printing | 1 May, 2006

I know from previous experiences that off contact printing have helped us reduce bridges and smears under the stencil. I am looking for ANY documentation regarding this topic in order to try to implement a change at our current factory. If anyone has a place to go and anyone to talk to would be greatly appreciated.

Thanks

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KEN

#41284

Screen Printing | 1 May, 2006

....am I reading this correctly?

"Screen Printing" or "Stencil Printing"

Screen = agree = off-contact.....it's fuzzy going back into the memory banks to the early 90's. Screen mesh and emulsion will stretch to produce the gasket affect while snapping off to release the solder for a crisp print.

Stencil print. Disagree. On-contact = preferred method. Need gasket function of stencil; foil is not as elastic as mesh and do not need to snap-off because there is no mesh interfering with the solder paste.

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davef

#41291

Screen Printing | 2 May, 2006

We agree with Ken's points about off contact / screen versus stencil, but this DHolt is talking stencil [regardless of what is typed about screens].

Comments are: * On "smears under the stencil": smearing happens. It makes us wonder about the frequency that you underwipe your stencil. For more on underwipe frequency search the fine SMTnet Archives. Though how badly the smearing occur may largely depend on rheology of solder paste [viscosity too low], some printer parameter adjustment [eg, reduce speed, reduce pressure] may help to improve the situation. * On bridging: search the fine SMTnet Archives. * Printing off contact with a stencil will reduce stencil life. * Consider asking your printer equipment supplier for advice on machine setup.

For more on printing paste, look here: * http://www.smtinfocus.com/processguide_printing.html

* http://www.smtinfo.net/Db/_Stencil%20Printing.html

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DHolt

#41306

Screen Printing | 2 May, 2006

We are using a Metal Screen. We used a metal screen in the past and found that stencil life wasn't a problem. We are a Low Volume, High mix spot where the average lot size is no more than 100 pcbs and as low as 5 pcbs. I am concerned with wiping in genral. The philosophy is sound but wouldn't that in some ways create more problems than solve. Once we went to off contact in previous applications the need to wipe went away due the crisp prints we recieved. Thanks for the info. I will do a more extensive search.

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