Hi Alvin!

Jorge makes some good points, but I'd like to add to his input:

> Alvin;
>
>     There is no formula since the stencil opening depends on pad size at
PCB,
>  as rule of thumb they recommend reduce 10% from pad width to give a good
>  gasketing.

10% is not a bad percentage to reduce the apeture opening size, but most of
the stencil houses I use will reduce the openings 20% if I ask for a standard
industry reduction...and there is somewhat of a formula to use to keep you
outta trouble when dealing with fine pitch apetures. You want to stay at
least 1.5 times the thickness of the stencil in the width of the opening to
keep from having your paste hang-up in the apetures when you're printing.

>  Stencil thickness generally is 6 mils, laser cut, trapezoidal
>  shape with eletropolish finish.You can use eletroformed stencil as well,
but
>  NEVER use chemical etching type for fine pitch.

Well, that's almost true. But there's a stencil company that I've used in the
past that etches stencils in a different manner than most. The company is
named BeamOn Technology. Their process is called "Band-width Etch" or "Band
Etch". They actually etch only the outline of each apeture, not the whole
opening...much easier to control that way. They don't get the typical
"hourglass" cross section like what normally happens when etching from
double-sided photowork...very straight and smooth sidewalls. I've had
stencils made that contain 15.7-mil pitch devices and the stencils print
every bit as good as laser cut stencils at less cost. You can visit their web
site at: http://www.beamon.com. When I worked for Zevatech, we used BeamOn
stencils for that little keychain we built and passed out at the shows like
NEPCON, SMI, etc.
For those of you that have one of them you know that the QFP is 15.7-mil,
also got a microBGA, and 0402's...stencil worked great!

Just thought I'd add my two-cents...

-Steve Gregory-

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