Good ranting. Now get rid of the DF for ever and find another way to deal
with vias.

Earl Moon
----- Original Message -----
From: Stephen R. Gregory <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, September 14, 1999 5:20 PM
Subject: [TN] Dry Film Mask Issues...


> 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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