Hey Ya'll,

Well, judging from the responses I've read on a number of issues about dry
film mask issues, it almost seems that there are more and more boards being
specified with dry film mask on them.

This seems to be driven by how well dry film tents vias. I recieved many
responses about via fill, and most of them said that it was better to have no
via fill at all, or if you did, it had better be at least 75%...something
that cannot be guaranteed with LPI masks.

We all know that LPI mask won't tent vias, but then sometimes because of the
mask characteristics, a via may just try to fill during wave and fill less
than 75%. This is the reason (I feel) that hi-rel companies still continue to
specify dry film. They'd rather make sure that none of the vias fill, and not
even worry about what a pain in the butt it is to try and place and solder a
300-pin plus QFP without solder bridges with a "windowed" dry film.

Dry film mask is usually at least 1-2 mils higher than the pad surface, so
when I print, I'm getting a extra mil or two over and above the stencil
thickness because the dry film is holding my stencil up that high above the
pads. From now on, when I'm going to specify a stencil thickness for 20-mil
pitch with dry film masks, I'm going to ask for a 4-mil stencil from now on,
and have my stencil vendor "mickey-mouse" or "dogbone" my 50-mil apertures to
give me the additional volume I need for my J-leaded devices and whatnot.

The stencil I've been using with the most recent assembly has been 6-mil with
a industry standard aperture reduction and I've been getting mucho bridges
(with this stupid dry film).

In the past, 6-mil stencil thickness with LPI masks have NEVER been a problem
with bridging from my experience...

Just ranting is all...

-Steve Gregory-

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