Well, I think the other responses have just about covered all the
variables.  I have seen this problem and wanted to add a little.  As they
said, check you aspect ratio, but if you had a stencil that was working
(why did you change to a new one?) and it is the same as this one, you
probably have the correct ratio.  I would not use nickel plating.  You
should electropolish laser cut.  I have tried it with and without
electropolish and results lean toward better release with electropolishing.
A rough sidewall will not release as well, there is more surface area then.
Think about paste release like a sand-pail on the beach when you are
building a sand castle.  It is trapezoid shaped and smooth on the inside.
If it worked better rough, sand-pail manufacturers would texture the inside
of the pails.  The smooth surface offers less surface area and easy
release.

Do check your paste as Steve G. mentioned.  I had a similar problem a few
years back and we looked at the stencil and process, then tested the paste
and found it was a bad batch.  It was way thick and upon further
investigation found it had been exposed to extreme cold (froze) during
shipment.  You may be in an area that you don't have to worry about this,
but it could have gotten too warm also.  Or it could just be bad from the
manufacturer.  But we had paste clogging like crazy with that bad paste.

Plugging a stencil manufacturer ehh Steve.  Well, I have tried several and
always end up back at IRI.  I have had outstanding success with IRI and
would not trade a stencil from them for anyone elses.  I have not tried
Beam-on since they are in CA and IRI has a facility closer.  But IRI has an
East and West coast facility to support the entire US.

Good luck solving the problem.

Brad Kendall
Hella Electronics Corp.




[log in to unmask] on 07/10/98 05:13:28 AM

Please respond to [log in to unmask]; Please respond to [log in to unmask]

To:   [log in to unmask]
cc:    (bcc: Brad Kendall/Hella North America Inc.)
Subject:  [TN] Electropolishing Laser-Cut Stencils




Hi all,

We are currently having problems with a laser-cut stencil that is
electropolished and nickel plated with 9-mil openings. The paste does
not release well from the apertures. We are having to wipe after every
print. The board is fixtured well and we have been using the same
paste for the last two years. We had a different laser-cut stencil in
production from a different manufacturer with the same 9-mil opening,
but this stencil was not electropolised or nickel-plated. With this
stencil, the paste release was excellent and we never had to worry
about wiping.

Does anyone have any input into whether or not laser-cut stencils
should be electropolished and nickel plated? I have heard
contradicting opinions on this subject. One source has said that you
do not want to electropolish laser-cut stencils because the
rough laser walls will give you a better paste release. Whereas,
another source has said that the smooth laser walls will give you a
better paste release.

Also, what benefit does nickel plating give you? Does the paste tend
to roll better with nickel plating? Do you tend to get more or less
paste deposit on the boards with nickel plating? Does it increase the
life of the stencil?

Has anyone using laser stencils seen major differences between
the quality of different stencil manufacturers?

Any input into this subject would be greatly appreciated.

Thanks,
Todd

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