Hi Steve Gregory & Jorge,
         Thousand thanks on the advises, it is very much valuable to me.

Thks,
Alvin

At 07:41 PM 11-08-1999 -0400, Stephen R. Gregory wrote:
>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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