Question about the QFN package.

MarkS

I guess that this is more general than Prop specific, but I've never used a QFN chip before. Is the center pad to hold the chip in place? If it has a more specific use, should it be left floating or routed to ground/Vcc? If it does not serve a purpose, can it be left off of the board? It takes up quite a bit of valuable routing space under the chip.

big_mark

At a guess I'd say its there to act as a heatsink

Leon

I've always grounded it with a large pad on similar chips. It would normally be soldered as part of the reflow soldering process with solder paste. For hand-soldered prototypes, I put a large via in the centre and feed solder in with the soldering iron tip. The pad might need to be divided into sections for reflow soldering.

Leon

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Paul Baker

The pad must be grounded, and as Leon indicated the stencil needs to be checkerboarded to prevent floating in a reflow oven. For hand assembly, we recommend using the QFP version of the chip for your surface mount needs.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

MarkS

Paul Baker (Parallax) said...
The pad must be grounded, and as Leon indicated the stencil needs to be checkerboarded to prevent floating in a reflow oven. For hand assembly, we recommend using the QFP version of the chip for your surface mount needs.

Yeah, the QFP would probably be better. I'm not saving much more board space with the QFN and its definitely more complicated.

Thanks for the info everyone.