Steve, Paul & Poh

We generally use 6 mil laser or chemical etched stencils with a 10%
reduction across the whole aperture. However, with 16 - 12 mil pitch
components we have been experimenting with a 10 % reduction in the length of
the aperture to improve the consistency of the paste volume. We were finding
that with an overall reduction on the aperture size the width of the
aperture was becoming to small to get a decent print.

Full Size
*****
*****
*****
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10% length reduction

*****
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*****

Something like this!!

On the subject of plastic stencils. 5mil thick?? Aren't they a bit fragile
and don't they suffer from stretch and general deformation. I have come
across the DEK 'Pump Print' plastic stencils but they are about 80mil thick,
with the idea that you pump the material through the stencil rather than
print it. (You always get material left in the stencil). I believe this
method was developed to improve the printing of glue and because of the
thickness of the screen you can carve pockets to give you clearance over pre
inserted conventional parts.

Can anyone let me know of any UK manf. of these polyimide screens so that I
can see for myself.

===== Original Message from [log in to unmask] (TechNet E-Mail Forum.) at
24/05/99 01:27
>While we're on the 20 mil subject ;
>the same apply on the polyimide ? ; as they have better release ratio than
>SS ?
>Any comments on plastic stencils in general ?
>
>Thanks        paul
>
>-----Original Message-----
>From: Stephen R. Gregory [mailto:[log in to unmask]]
>Sent: Saturday, 15 May 1999 2:04
>To: [log in to unmask]
>Subject: Re: [TN] Stencil Thickness
>
>
>In a message dated 5/14/99 8:59:39 AM Pacific Daylight Time,
>[log in to unmask] writes:
>
><< Hi Technetters,
>
> What kind of stencil thickness you would recommend for 20 mil pitch QFP?
>
> 4, 5 or 6 mil ?
>
> Thanks in advance.
> Poh >>
>
> Hi Poh!
>
>I always get a 6-mil with the industry standard aperture reduction (20%
>smaller)... always  works for me without a problem.
>
>-Steve Gregory-
>
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