Every so often a question arises about the value of time spent on TechNet.
I forwarded Steve's post to my lead PC designer asking her to check some
designs we'd recently done. She discovered we'd specified the convex parts,
so she contacted the engineer at our subcontract manufacturer who then
admitted they'd had significant bridging problems on the last few builds
and, yes, the concave parts would have been better. Ouch! We even allow
them to review and comment our designs before releasing and bend over
backwards to correct things like this, but for some unknown reason, they
never notified us. The worst is that the footprints (feetprints?) on the
board don't even change. So, thanks to the exchange of information, we're
processing ECO's to easily correct a problem which would have continued had
it not been for this post.
Thanks to Steve and others who commented.
Bob Croslin
Nielsen Media Research
-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask]]
Sent: Tuesday, April 06, 2004 10:03 AM
To: [log in to unmask]
Subject: Re: [TN] Chip Resistor Arrays
Hi Siegfried!
From experience at a past job, concave terminations are best for reflow
soldering. I worked for a company named SMART Modular Technologies where we
built
memory modules. The one I remember well that used chip resistor networks was
a
Pipeline Burst Cache Module that had 21 chip resistor networks on the
backside...this was a double-sided reflow assembly. When we first started
building it,
we had many bridges and mis-alignments. The footprint was what was
recommended by the manufacturer of the resistor network that we were using
at the time,
and it was a convex terminated network. I think I must have bought 2-3
different stencils all with different aperture openings to try and improve
yields.
Things got a little better, but the problems never really went away.
Then we had this Panasonic Rep in, and I started talking with him about the
problems we were having. He immediately pulls this paper out of a study that
was done by Panasonic on concave vs. convex chip resistor networks reflow
soldering performance, and it showed that concave network terminations had
better
self-centering characteristics, and less bridging. I tried to find that
paper up
on the web, but no joy. Here is a link to a page in Digikeys catalog to a
Panasonic resistor network that says that concave terminations solder
better:
http://dkc3.digikey.com/pdf/EU041/0601.pdf
In my case at SMART Modular, switching to networks with concave terminations
almost completely eliminated the solder defects we were experiencing with
chip
resistor networks.
I've requested the paper I remember from Panasonic, it's pretty good with
some nice images. If they still have it, and I get it, I'll share with
everybody.
-Steve Gregory-
> Dear TechNetters,
>
> we are using more and more chip resistor arrays. They are available in
> concave and two convex (scalloped edge, square edge) types. The concave
one is
> some expensive and less disposable, but I heard that it is easier to
solder
> giving better quality.
>
> Any opinions and informations comparing the different types (concave,
> convex) are welcome.
>
> Thank you in advance for your help.
>
> Siegfried Meier
> Dipl.-Ing.
>
> Entwicklung selbstfahrende Erntemaschinen
> Müller-Elektronik GmbH u. Co.
> Franz-Kleine-Str. 18
> D-33154 Salzkotten
>
> Tel: +49 5258 9834-70
> Fax: +49 5258 9834-94
> email: [log in to unmask]
> Internet: http://www.mueller-elektronik.de
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