In a message dated 11/4/98 1:29:04 PM Pacific Standard Time,
[log in to unmask] writes:

<< Hi folks,

 I'm looking for sources of information, recommendations, etc. for laying out
 epoxy stencils for mounting bottom side surface mount components. Are there
 any standards for this? Anyone care to share their ideas, preferences, etc?

 Thanks.


                 Rick Thompson
                 Ventura Electronics Assembly
                 2665A Park Center Dr.
                 Simi Valley, CA 93065

             [log in to unmask]

                 (805) 584-9858 voice
                 (805) 584-1529 fax
  >>

Hi Rick!

That's one thing I like about my stencil vendor, I just give them the gerber
and tell em' I want an epoxy stencil for the bottomside and they take care of
the rest...and every one I've had from them works fine. They charge an extra
$60.00 to design and generate an additional layer of gerber data based on the
PCB's gerbers that's needed to plot the photowork for etching the stencil. I
use a company called Beam On Technology (408) 982-0161.

I just ran a couple of assemblies that I stenciled the epoxy. One had 0805
passives on the backside, the other had 0603's. On the 0805 stencil, I used a
6-mil thick stencil and Beam On made me .050" X .020" rectangles, the long
dimension was perpendicular to the orientation of the part...the rectangles
had rounded corners too, seems to keep things cleaner on the bottom of the
stencil with rounded corners. The 0603 stencil also was 6-mils thick, but the
openings were .040" X .015"...both stencils worked great.

One thing to not forget when spec'ing your stencil is to have them etch two
fiducials, or two pads out in opposing corners of the board so you'll have
some way of aligning the stencil when you're setting up. Then just before you
run, don't forget to take small pieces of tape and cover the openings, or your
pick and place machine will remind you when it gets a board and tries to read
the fiducials.

I'm sure you've heard about making sure you use the right kind of epoxy,
formulas that are non-hydroscopic and made specifically for stenciling (I use
Loctite 4011). The stuff is expensive...close to $300 a 300ml tube. You'll
leave at least $50-60 worth on the stencil and squeegees no matter how
fastidious you are about not wasting any. But there isn't a dispenser I know
of made that can come close to the throughput of stenciling epoxy when you
have a fair amount of components on the bottomside.

-Steve Gregory-

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