>From: Kelly Kovalovsky, PCB Quality Engineering
>~     EMail:[log in to unmask]
>Subject: PCB vs. Paste Stencil Comp
>We have a fairly large circuit board that we are assembling. The card
>has fine pitch SMT at extreme ends. We have noticed a mismatch between
>the solder paste stencil and the printed circuit board. The circuit
>board features are actually closer together than the stencil.
>
>My question to any card assembly site is whether it is common practice
>to compensate a solder paste stencil for shrinkage of a PCB?

Some may do so (never heard of it, myself), but we never do, and have some
9U VMEbus boards as large as 16" x 16" (you metric folks, multiply by 2.54
to get cm) with which we have not seen such problems.

At the risk of demeaning someone from that tower of technology known as
International Business Machines - no sarcasm in my voice, I assure you; we
always (the last 2 yrs I've been here) send Gerber files directly to the
stencil fab houses, and I think that the use of even 7 mil mylar stabilized
film is inadvisable due to the possibility of image growth related to temp
or humidty changes.

We have had good success with laser cut stencils, and I think that one
advantage they offer is that there is no film generation required to create
diazos - they call them rubyliths, I think - as for chem etching. On larger
panel sizes, it would seem film stability could be a concern. The laser
burns metal according to the data provided, I have been told. They do seem
to release paste noticeably better, as well. Ours come from Alpha Sigma in
New Jersey so long as my budget permits. They run up to well over $500 for
0.5 mm pitch (often called 20 mil by us Texans) cuts mounted in 18 x 18
cast frames. If you contact your Alpha Metals rep, they might offer you a
"free trial" to replace that questionable stencil.

And then, there's the possiblity that board film had been
"grown/shrunk"...You could try laying a mylar photoplot (we keep 'em for
reference) of the layer 1 film on the stencil and checking it...

Again, I would guess you knew this.

cheers,

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