Ryan,
I agree with some of the points Ed listed, especially gasketing. But I would
like to add a concern to the size of yer (yes, yer is a word in the mid-west
dictionary) stencil apertures and pad sizes for this part. If you are below an
effective aspect ratio, you will have very inconsistent results and poor
release. The good rule of thumb is an aspect ratio of stencil to aperture width
is to be at or above 1.5...i.e., if yer stencil is .006", the smallest aperture
width you should use is .009" (.006 x 1.5). But you also have to insure good
gasketing, which means yer pad has to be larger then "the aperture opening plus
the alignment accuracy" of yer stencil printer, so if yer pad is the same size
as the aperture, and yer alignment accuracy is .0005" you will already lose
gasketing due to alignment tolerances. i.e., make your pad wider the aperture
without violating the aspect ratio rule.
Also, if these are HASL coated boards, insure the solder coating is not more
then .002" high, as this will cause poor gasketing.
Also, insure you have good quality laser-cut and electro-polished apertures in
yer stencil (alright all of you electro-form advocates, I've left myself wide
open for abuse here). Ahhhhhh, nice, firm, well round smooth stencil apertures,
every solder paste engineers dream (I've obviously been in the lab too long!!!).
There are a few other considerations, but I'll leave those for the other
responders.
Hope this helps. Regards.
Jeff Hempton
United Technologies Electronic Controls
______________________________ Forward Header __________________________________
Subject: [TN] Squeegee Speed
Author: Ryan Jennens <[log in to unmask]> at Internet
Date: 2/18/99 5:12 PM
Evenin' all!
My question is this: My stencil printer operator has said that he has
to
wipe our 6-mil stencil each pass to get the paste in the 20-mill pitch
aperatures for a QFP to release well. We are using Amtech NC-559 type 3
paste with 89.5% metal. The paste is not old. I considered using type 4
paste, which has a much finer powder in it. I was told by Amtech that this
finer powder paste will pack more densly into the aperatures and I could get
some bridging, unless I order a new stencil with aperature reductions. Our
squeegee speed is 0.6 inches/s, or 15mm/s. Is this too slow? Amtech
suggested that at this slow speed, the paste could be rising out of the
aperature, as if someone pressed on a wet sponge and it rose back to its
original shape. What is a good squeegee speed for 20 mil pitch parts? Thanks
all!
Ryan Jennens
TelGen Corp.
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