Parts Library Strikes Again

Here's a little bit of a parts library or wrong package problem. I'm not exactly sure how it happened. The Qfn_worng_part_library_2 board may have been laid out for a small QFP footprint but then a QFN part was purchased. The footprint looks like that of a QFP, but QFNs are usually smaller, so it may be a custom made part library with some errors in measurement.

It's surprisingly easy to do that; especially if the person making the library part isn't the same person that's using it. Maybe the person drawing the footprint only had part of the dimension information for the QFN. I can easily see how someone could take the outside dimensions of the part and the pitch and then come up with something like this based of years of making QFP footprints.

The lesson is to always double check the component fit before sending a board out for fab. In this case, the lead pads are so long that they would short every connection to the ground pad in the middle. Oh, and there's also that giant open via in the middle of the center pad. That's not a good thing.

Duane Benson
Analysis, Mr Schlock?

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Comments

Hi Nick - that's a very good point to add: the dimensions of the part, need to be specified as well as the pin count. Thanks for the note.

QFN packages need pin count _and_ dimensions to specify a footprint in a library. For example, there are "5mm x 5mm 28-QFN" and "6mm x 6mm 28-QFN". Just "28-QFN" (or 16-QFN in case of this picture) isn't enough information.

Hi Jeffrey - I don't know who fabbed this particular board. Ourselves, we buy boards from Sunstone, Prototron, Merix and maybe a few others. When we get kitted jobs in, we often don't know where the customer purchased the boards.

Interestingly, some manufacturers will recommend not putting mask between the pads, as does FreeScale on page 3 of this appnote. http://www.freescale.com/files/rf_if/doc/app_note/AN3003.pdf

Can I ask who manufactures boards that well? Your partner, Sunstone, requires .005" web on the solder mask and that's not enough to put a web between pins on a .5mm pitch QFN. But the board in your picture has a perfectly good solder mask web.

Hi Reider - I bet you're right that the big hole in the middle was designed for hand soldering the pad. If the via is for thermal or grounding purposes, we usually do see a bunch of small ones (hopefully capped or plugged) instead of one big one.

Seems to be a footprint for a completely different component... even the pitch is wrong. Still could be a QFN or LLP footprint, but different pitch and size from the purchased one.
The huge via in the middle... hmm, just weird: a few small vias will provide much better thermal transfer, not to mention solder drainage... unless of course :) the original designer's idea was to try and hand-solder the thermal pad from the opposite side of the board. :)

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