Dwayne,

Not a dumb question at all.

Not only can the lead pitch change, so can the chip body size. I found this=
=20
out when I had to go to surface mount and a lot of footprints seemed to be=
=20
wrong.

I found this standard IPC-7351 at=20
http://landpatterns.ipc.org/IPC-7351BNamingConvention.pdf
Names end up being quite long though. Maybe a local variation might have to=
=20
be adopted.

Mentor Grpahics also have a viewer and wizard for calculating land patterns=
..=20
In the wizard, you can enter the dimensions and tolerances of a given part,=
=20
and it will give you a pattern and dimensions, including courtyards, masks,=
=20
overlays etc.  It also allows you to choose between several different=20
density spacings (ie how densley packed your board has to be).

Tom Hausherr seems to be rather knowledgeable in this area. A google search=
=20
should find him.

Cheers,

David

----- Original Message -----=20
From: "Dwayne Reid" <dwayner@planet.eon.net>
To: "pic microcontroller discussion list" <piclist@mit.edu>
Sent: Friday, July 15, 2011 10:30 AM
Subject: [EE] TQFP lead pitch variations


> Good day to all.
>
> Got a dumb question here: we just got back some proto boards done
> with the PIC 18F67J60.  Went to populate the boards - whoops! Got a
> problem here.
>
> The TQFP-64 package that was in the CAD library doesn't match the PIC
> received from Digikey.
>
> Turns out that the TQFP package in the CAD library has a 0.80mm lead
> pitch whereas the PIC has a 0.50mm lead pitch.
>
> Like I said: Whoops!
>
> So - my co-worker is busy making a new footprint to match the actual
> chip.  But that has me wondering: how do people define these footprints?
>
> I did some minor searching via Google and find that we aren't the
> only people perplexed by this - Microchip apparently offers TQFP
> packages with 4 different lead pitches: 0.40mm, 0.50mm, 0.65mm, 0.80mm.
>
> I'm looking for suggestions as to how to name these footprints - I'd
> like to follow whatever convention has been developed rather than
> come up with something totally different from what everyone else uses.
>
> This particular incident wasn't my mistake but it easily could have
> been - I'm sorta used to a package name (eg SOIC) having a specified
> lead pitch and I might have decided to skip my usual "check twice"
> policy.  As it was, I did have a good look at the board layout before
> it was sent out and it *NEVER* occurred to me that the PIC footprint
> might be wrong.
>
> dwayne
>
> --=20
> Dwayne Reid   <dwayner@planet.eon.net>
> Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
> (780) 489-3199 voice          (780) 487-6397 fax
> www.trinity-electronics.com
> Custom Electronics Design and Manufacturing
>
> --=20
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>=20




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