Dave,
 Couple of questions,
1. What is the surface finish?
2. What is the plating of the leads?
3. What type of process, NC or Clean?
 Suggestions;
1. Have you tried a "Dog bone" or "Bow tie" aperture for your stencil?
Normally the aperture has a standard reduction of around 10% and then
depending on the lead width and  workmanship specs I have taken as much
as a 30% tapered reduction to the center of the lead. You can contact
your stencil vender and they can make some recommendations. Basically
this will allow for adequate paste to solder the heel and toe and
prevent excess paste escapement from the under the center of the lead
(influencing shorts).
2. Re calibrate your placement equipment to achieve more repeatability
so placement is more consistent. Another leading cause of shorts for
devices less than 20 mils is placement. We set up a process where we
calibrated our IP machines to have a 2.63 cpk. We reduced the normal
placement window and achieve greater repeatability.

With these modification we were able to run a 196 piece proto without a
single defect on the 16mil device. We are currently running this product
in production volumes (2800 per month) and this device is less than .01%
of defects.

Hope some of this helps.

>>> [log in to unmask] 9/5/2003 6:41:03 AM >>>
Well, we have our first 16 mil fine pitch device, 128 pin QFP. I am
worried
about printing shorts on a 6 mil stencil. Everything else on the board
is 20
mil or better so 5 mil is not good idea. we normally do a 10% reduction
and
round the rectangles for better release. The pad width is .7 mil, I
would be
pushing 6 mil with the reductions, when our normal rule is on 20 mil
pitch
do not reduce overall width to less than 9. If I don't reduce the width
I'm
sure we will get shorts.
Am I going to have problems if we leave the width alone and only take
the
reduction in volume garnered by rounding the  rectangles?
Dave C.

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