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September 1998

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Subject:
From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 9 Sep 1998 22:02:39 EDT
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Hi All!!

First, I wanted to thank everyone who responded to my post last week about
suggestions for an experiment to conduct to show the importance of cleaning, I
owe a few of you some beers! Thanks again!

Now I gotta' nuther one...it's to do with µBGA layout. We just got a prototype
kit in here that has a 144-pin µBGA with a cavity. I recieved the stencil
gerber and looked at the size of the apertures in order to know what I should
specify for my stencil. The apertures in the gerber were 15-mils in diameter,
which is not out of the ordinary.

The µBGA shares the board with a bunch of 50-mil stuff, so I ordered a 6-mil
thick stencil and had my stencil vendor do a rounded square for all the µBGA
apertures (per a suggestion by Vern Solberg on Tessera's WEB page AND it
works!). But when we got the fabs and I got a chance to see them, there's
something I noticed that concerns me...only because I've never seen it before
on a µBGA footprint.

What the designer did, was on pins that share a common signal, instead of
laying out a 15-mil diameter circle for one pad and running a 5-mil wide trace
to the next 15-mil circle, he laid out a big fat 15-mil wide trace between the
two points and defined each pad point with a circular opening in the solder
mask...does that sound okie-dokie to ya'll? To me, it seems like shades of
building RF boards where parts share the same pads and stuff...and what a pain
in the butt that is! There's about half a dozen places like that in the
footprint...one or two of those fat trace points have 3 or more pads for the
device that will solder to them. I have never seen this used before and I
don't know if it's gonna cause me problems assembling it...have any of you
ever seen this done before? Does it cause problems?

Thanks in Advance!

-Steve Gregory-

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