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June 2002

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Subject:
From:
Ron Olinyk <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 22 Jun 2002 14:07:18 -0500
Content-Type:
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Steve,
I use Orcad Layout quite regularly as well and the situation you're
descibing is a 'regular feature' in Orcad. A good majority of the SMD
footprints in their libraries have this alternating oblong/rectangular pad
pattern. Mostly, as the others have mentioned, for troubleshooting but also
for artwork inspection/net checking. Many of their QFPs and SOICs use this
pattern. On most fine-pitched devices you'd never be able to tell the
difference after soldering (unless you magnify). I generally change all the
pad shapes to oblong -and generally before loading them into the design.
It's not that much work to accomplish after the fact but is definitely a
pain -you'd have to scan thru the padstack spreadsheet for that decal and
replace each rect. pad reference with an oblong ref. for both horizontal
and vertical pads - not as easy as modifying a gerber dcode (which you
should always avoid anyway).
There is a chance though that similar devices may use different decals in
which case each one would have to be examined and modified -this may be
what the designer was referring to as 'too much trouble'.  The only problem
I can see with the rect type pads is it may make routing a little more
dense.  From what I've read on the use of rounded corner or oval/oblong
pads, is that they help in centering (to a small degree) the IC onto the
pattern during reflow. If it's a production run it might be worth the
trouble to change them but if prototype could be left as is.
Ron O.
Mindlink Technologies

At 02:53 PM 6/21/02 -0400, you wrote:
>Hi all,
>
>We're having a board lay-out done outside, and as with new designs, I will
>get asked to look at the preliminary gerbers to see if I can see anything
>out of place that might need changing.
>
>I looked the the gerbers that were given to me and the first think I
>noticed was that on some IC footprints, there was 1 rectangular pad, then
>9 pads where the corners were rounded, then a rectangular pad, and on and
>on...
>
>So I fed that back to our engineer here who is over this project, then he
>fed that back to the individual who is doing the lay-out. Our engineer
>here then came back to me and asked if that would be a problem, and I told
>him it could be, but why not make all the pads uniform? Go to:
>http://www.stevezeva.homestead.com and look at "OrCad" to see what I mean.
>
>His reply was that the individual doing the layout told him he would have
>to go in and manually edit each one of the pads that were rectangular, and
>that it would be too much trouble. He said this is the way OrCad put the
>footprint down, and if it was such a problem, why would OrCad have this
>footprint in the library? Then he added that the IPC-SM-782 says that you
>can either use rectangular pads or optionally round the corners, so that
>tells him you can use either one or both, and it doesn't make any
>difference. So he told the layout person to leave things as they are.
>
>I don't know anything at all about OrCad, but something tells me there's
>an operator problem here. I've never seen a footprint like this before,
>and no matter what I say, I can't convince our engineer here that the
>footprint needs to be fixed.
>
>HELP!!! PLEASE!!!
>
>Thanks,
>
>-Steve Gregory-

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