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July 2002

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Subject:
From:
Graham Collins <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 16 Jul 2002 06:58:46 -0400
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text/plain (148 lines)
Sorry Peter, no such luck.  The part is only available one way.

regards

Graham Collins
Process Engineer, 
Northrop Grumman
Atlantic Facility of Litton Systems Canada
(902) 873-2000 ext 6215

>>> [log in to unmask] 07/15/02 09:24PM >>>
Graham,

One other thought occurs. Some QFP's are available with different
footprints (2.6, 3.2, 3.9 mm). Are you by any chance trying to fit a 3.2mm
footprint component to a 3.9mm land pattern? The heel overhang dimension
you mention indicates this might be so? To check, take the distance  the
distance toe to toe across the leads and component body (full component
width) and subtract the distance across the component body - you can get
the figures from the data sheet. The answer gives you the footprint size.

You may simply have the wrong component size. I hope for that sake of all
that this is all the problem is.

Let me know - I'm keen to feel smug today.

Peter



Graham Collins <[log in to unmask]>   15/07/2002 10:28 PM
Sent by: TechNet <[log in to unmask]>

Please respond to "TechNet E-Mail Forum."; Please respond to Graham Collins

              To:  [log in to unmask] 
              cc:  (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
              Subject: Re: [TN] part/pad mismatch








A thank-you to Kathy and the other Technet folks who replied!

To clarify the issue:
gull wing lead, the foot is about .1" long and the heel overhangs the pad
by .015 or so.  Thus absolutely no heel fillet.  Lots of side fillet, but
as has been mentioned there isn't much strength there.

Bonding the part would help with vibration, but I would still be worried
about thermal expansion - this board will be seeing some temperature
extremes.  I wouldn't buy off a waiver for something like that, so I'm
doubting the customer would either.  On the other hand, it's their design
and their schedule, so...

Seriously though, a couple of adapter vendors have contacted me off line,
we shall see if something can be agreed to by all involved.   At this point
the 'ol dead bug option looks best to me but we shall see...

Thanks again all!

regards

Graham Collins
Process Engineer,
Northrop Grumman
Atlantic Facility of Litton Systems Canada
(902) 873-2000 ext 6215

>>> [log in to unmask] 07/12/02 05:58PM >>>
you might be able to find an after market adapter.  I forwarded your
original post to a company qfp-adapter.com and asked them to help out.

When you mentioned the heel hangs off of the end of the pad, did you mean
that the heel overhangs the front or back of the pad?  If the pad overhangs
the front, well, you are in a pickle.  But if the heel is the only part of
the solder joint that can not be achieved then why not determine the amount
of joint that could be maintained to section 12 and then go for a deviation
from any heel joint requirements due to an incorrect pad layout.

Kathy

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