We build 0.5mm pitch (20 thou) QFPs also with 12 thou wide pads.
This is the right combination. The stencil should be reduced by 2 thou in
the width, so 10 thou apertures with the length left alone. The length of
the pad is an issue though. There are two reasons, 1) to provide sufficient
solder paste to form a good joint (the idea being to provide sufficient
paste by printing a long thin line thereby reducing the risk of bridging due
to the proximity of neighbouring strips of solder), the solder then wicks
back onto the lead/termination. 2) due to the aspect ratio of the width to
the height of the inner wall, the solder paste can remain in the aperture
after the stencil seperates from the board, unless there is sufficient
length of pad to adhere to and pull it from the inner wall of the aperture.
Pads of only 55-65 thou will be a problem for 10 thou apertures.
My prefferred minimum figure is 70 thou but 80 or 90 thou lengths are much
nicer. 
So 70 X 12 thou pads should have 70 X 10 thou apertures on a 6 thou stencil.
We get excellent results even with 7 thou stencils which I prefer (full
frame mounted lazer of course). Our particle size is 20 to 45 microns.
Thanks

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]>   *
* with <subject: subscribe/unsubscribe> and no text in the body.          *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************