Typically these are heat sinks. Find them on alot of power devices, to sin= k 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 curren= t, then you want to have a good patch to ground rather than just out the on= e 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 wrote: On 18 Sep 2006 at 13:47, Har= old 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 2= 8 pin = QFN. There is no mention of it in anywhere I could find. FWIW the Altium/Pr= otel = PCB library connects this pad to GND with a whole bunch of vias. Big omissi= on 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 man= aged = to "overlook" the existence of the exposed metal pad until the chips arrive= d and I = had a look at them. My PCB design was ready to roll and I had placed severa= l vias = under the chip that would have been shorted by this pad. The 11th hour solu= tion = 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 anythi= ng. = 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 somethin= g I = would reccomend for production. It will be a real pain laying out this boar= d again = with no top side tracks/vias under the PIC. It's hard to imagine the PIC wo= uld 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 -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist = --------------------------------- How low will we go? Check out Yahoo! Messenger=92s low PC-to-Phone call ra= tes. -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist