Steve,

How big you make your apertures is dependent on the following (some of which
you mentioned):

1)      Barrel fill requirements
2)      Solids content solder paste  (need larger aperture to make up for
flux volume)
3)      Diameter of leads
4)      Diameter of holes
5)      Board thickness
6)      Stencil Thickness.
7)      lead stick through  (increases the size of the meniscus)

Note that your aperture doesn't have to be round as long as it gets you the
required volume of paste with a portion of it over the PTH, oval apertures
work well.

A better way to think about it is how much surface area does the aperture
require to give you the required barrel fill for your given stencil
thickness

Also don't forget about the volume consumed by the solder meniscus (best
estimated)this may be why you were short on a couple of pins in your
experiment.

Hope this helps.
Scott

-----Original Message-----
From: Stephen R. Gregory [mailto:[log in to unmask]]
Sent: Thursday, November 11, 1999 1:52 PM
To: [log in to unmask]
Subject: [TN] Intrusive Reflow


Hi ya'll!

I'm getting ready to do some intrusive reflow on this board we got
here...(this is my first time! I'm s-o-o-o nervous...not really...well,
maybe
a little...hehehe) Anyways,
the parts are going to be 2, 20-pin SIP sockets. The board is .063" thick,
the holes
are spaced .100" apart, and the hole diameter is .040". I'm trying to decide
how big I should make my apertures.

I've looked at all the archive postings (there are a 168 of them on
intrusive
reflow) to see if anybody posted any recommended stencil thicknesses, and/or
aperture sizes,
but there was none. I looked on the web, but couldn't find any real
guidelines either...
(hmmmm, something I couldn't find...now THAT's a change!)

I need to keep my stencil thickness at 6-mils, 'cause I've got some 20-mil
pitch stuff on the same side. I was thinking of doing .060-.070" diameter
circles over the holes...
does that sound okie-dokie?

I did a little test with the SIP socket and a syringe full of solder paste.
I
drew a bead of paste (about .030" dia.) down the row of holes and dropped
the
connector in, then reflowed it. Then I got my dremel tool out and did me a
rough cross section to check the barrel fill. It actually wasn't too
bad...there were about 2-3 holes that were a little shy volume-wise but that
was because (I think) the solder wicked over to the holes next to it because
of the continuous bead of paste I had laid down. That shoudn't happen when I
have individual deposits of paste for each hole.

So how big should I make my openings? Did I guess right at .060-.070"?

-Steve Gregory-

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