About the center pad, exposure pad
I'm usually make a via hole from the back
and pour a solder, that is perfect.
but, the thermal grease would be better
I think so.
According to the PSoC3 data sheet, the center pad should be connected to digital ground. You can leave it unconnected, but it might affect electrical performance.
You might want to look into AN61290 (PSoC3 and 5 hardware design considerations). There you also find the hint that unused pins should be left floating (page 7).
AN61290, unused pins left floating, I am going to file a case on this as I
think that statement might be in error, from a sleep current and EMC,
crosstalk point of view.
Ok. So I'll ground the center pad, but still don"t have a verdic for unused pins. Ihav mixed suggestions from here, documentaion, and other sources.
The response I got from Cypress -
We checked this internally. Actually you can leave the unused pins floating. By default, the GPIOs that are not used in your design will be in the High Impedance Analog state when the chip comes out of Reset.Here both the output driver and digital input buffer are turned off. This prevents any current from flowing in the I/O’s digital input buffer due to a floating voltage.
Even in the Low Power Application Note AN77900 we have mentioned that any unused GPIO should be configured as Analog Hi-Z unless there is a specific reason to use a different drive mode, so as to reduce the current consumption.
If a Pin is not configured for any particular drive mode in your design, by default it will be in the Analog Hi-Z Mode. So, if it is unused, you can leave that pin unconnected.