Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Stencil Printing: Single pass vs. Double pass

Views: 2390

#52975

Stencil Printing: Single pass vs. Double pass | 19 December, 2007

Hi,

How do single pass and double pass stencil printing compare? Is one preferred to the other? Any risks associated with double pass stencil printing?

thanks in advance -Arun

reply »

#52976

Stencil Printing: Single pass vs. Double pass | 19 December, 2007

"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.).

reply »

#52979

Stencil Printing: Single pass vs. Double pass | 19 December, 2007

Double pass printing for solder paste is not needed and will cause defects. Redesign your stencil thickness or aperture sizes, change your paste mesh size or type, go to 45 degree blades.

reply »

#52980

Stencil Printing: Single pass vs. Double pass | 19 December, 2007

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.

reply »

#52984

Stencil Printing: Single pass vs. Double pass | 19 December, 2007

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

reply »

#52989

Stencil Printing: Single pass vs. Double pass | 20 December, 2007

Good to know there are a few smart people left in this industry like Darby. I love how everyone assumes the original poster was only talking solder paste. We use screen printers for five different materials. Some of which we triple print by design.

And even when you do use your printer just for paste, there a several very real good reason why one may want to use the double pass. I love some of the desk jockey replies... LOL!

reply »

#52990

Stencil Printing: Single pass vs. Double pass | 20 December, 2007

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.

reply »

#52993

Stencil Printing: Single pass vs. Double pass | 20 December, 2007

Wow, Chunks, condescension becomes you. Not.

I think that in this forum when someone mentions *stencil* printing and doesn't specify what material, we tend to gravitate towards a scenario that involves printing solder paste first, then adhesive. If I'm wrong, fine, sue me. Or slap me around on the internet. Hit me, beat me, make me feel cheap. Please.

I would be interested in knowing why, if all the usual factors are optimized, as stated in my original post, you would need/want to double print solder paste.

Desk jockey my ass.

reply »

#52995

Stencil Printing: Single pass vs. Double pass | 20 December, 2007

Not you Steven. Blanket comments like "Double pass printing for solder paste is not needed and will cause defects. Redesign your stencil thickness or aperture sizes, change your paste mesh size or type, go to 45 degree blades." are so way off base. Making such comments without knowing the process, or the reason why the original posted asked, as uncalled for. Double pass printing with solder paste may not cause defects, depending on a multitude of variables. There way too many know-it-alls in our field and only a handful that have actual technical know-how.

reply »

#52996

Stencil Printing: Single pass vs. Double pass | 20 December, 2007

Thanks for all your comments....my question is answered!

reply »

#52997

Stencil Printing: Single pass vs. Double pass | 20 December, 2007

Ahhhh.

Sorry. It's been a crap week and I've got a touch of hair trigger fever.

Somebody pour me a Scotch. It's noon somewhere.

reply »

SMT in-printer dispensing

PCB Soldering Tools