It's probably a heat sink, and most likely connected to the leadframe 
 of the device (ie Vss)



> 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?
> 
> Thanks!
> 
> Harold
> 
> 
> 
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