Steve Quinn asked:

>We have just started using a dry film mask(Vacrel), which is about .003"
>thick.  We are using a .006" stencil and printing .020" pitch parts, and are
>measuring paste thickness of about .009".

If you have 9 mil paste height with a 6 mil stencil, seems logical your
stencil is off contact on the pads, and it doubtlessly means you are not
gasketing. I would expect to see terrible paste definition.

>We are using a metal squeegee,
>but seeing some "dog ear" looks to the paste.  We seem to be getting a poor
>separation of paste from the stencil.

Or possibly you are simply extruding the paste down below the stencil...are
you getting more contamination on the bottom of the stencil during
printing?

>The mask is about .0005" higher than
>the pads.

Since the pads are not perfectly flat (assuming you use HASL finish), the
opening would likely be larger at the margin of the aperture/pad, where the
stencil opening "wall" must define the printed paste.

My hypothesis is that the mask _cannot_ be higher than the pad surface at
its _edge_. We have notes to that effect on our PWB fab drawings. It is
necessary for the stencil metal to gasket on the pad surface so that the
aperture will fill and not slobber (to use a technical term) paste through.

>Our variation in paste height measurements on a board has
>increased by .002" from Photo Imageable to dry film.

Yikes. The variation must be bad in X-Y, too.

>   Are there any ideas on how to process a board with Vacrel to improve our
>printing process?

Ask DuPont.

>Thanks for any ideas/suggestions.

My suggestion would be change to an LPI mask. Maybe things have changed,
but I thought dry film mask had gone the way of the dodo bird. Tried it in
about 1982, hated it (for reasons totally apart from yours).

These might be worth experimenting with:

Get some metallographic sections made to see just how the profile of the
mask and pads measure - more accurate than a laser section scope.

We believe we get better paste release using laser cut stencils (from
Alpha-Sigma) - although they are much more expensive, we use them on most
0.5 mm pitch product.

Could try reducing your stencil aperture sizes by 10% or so, but I would
give it slim chances.

Or try going to a softer urethane squeegee (say 80 durmometer) to
effectively displace some paste in the apertures. If my guess is right, you
could see even more paste on the bottom of the stencil if you are not
gasketing, however.

regards,

Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com