I did get an email back from Microchip indicating the bottom pad on the
chip indeed connected to Vss. I've used other chips (analog) with a pad on
the bottom that did NOT go to ground. Since the datasheet did not seem to
indiciate what it was connected to, I did not want to assume it was
ground. Microchip, however, says it IS ground on this particular chip.

Harold


> Typically these are heat sinks.  Find them on alot of power devices, to
> sink the heat into the copper of the board.  Sorta unusual for a micro to
> have that however since the package is rather small, and if your sinking
> current, then you want to have a good patch to ground rather than just out
> the one pin so its perhaps just as easy to put a layer of metal tied to
> the Vss (if its a flip chip arrangement as well) to sink the current.
>
> Brent Brown <brent.brown@clear.net.nz> wrote:  On 18 Sep 2006 at 13:47,
> Harold Hallikainen wrote:
>> The datasheet for the 16F687ML in the 20 pin QFN package shows an
>> exposed metal pad on the bottom of the chip, but does not, as far as I
>> can tell, show what that pad connects to. Is it Vss? Something else?
>
> Yeah, I would like to know too. I came across this with the PIC16F88ML in
> 28 pin
> QFN. There is no mention of it in anywhere I could find. FWIW the
> Altium/Protel
> PCB library connects this pad to GND with a whole bunch of vias. Big
> omission on
> the part of Microchips to not tell us about this pad.
>
> In actual fact in my prototype board design I made my own footprint and
> managed
> to "overlook" the existence of the exposed metal pad until the chips
> arrived and I
> had a look at them. My PCB design was ready to roll and I had placed
> several vias
> under the chip that would have been shorted by this pad. The 11th hour
> solution
> was to "tent" all the vias (cover with solder mask) and add a solid square
> on the top
> overlay layer to further prevent the exposed metal pad from touching
> anything.
> During assembly I put the chips down with double sided tape (another layer
> of
> insulation, just in case) and soldered by hand. Worked ok, but not
> something I
> would reccomend for production. It will be a real pain laying out this
> board again
> with no top side tracks/vias under the PIC. It's hard to imagine the PIC
> would need
> this pad for heat sinking.
>
> --
> Brent Brown, Electronic Design Solutions
> 16 English Street, St Andrews,
> Hamilton 3200, New Zealand
> Ph: +64 7 849 0069
> Fax: +64 7 849 0071
> Cell: 027 433 4069
> eMail: brent.brown@clear.net.nz
>
>
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