5 Replies Latest reply on Oct 31, 2013 3:02 PM by user_14586677

    Unused Pins and Center Pad (QFN-64)

    john.ramshur

       What should be done with pins that are not used in the design? Is it ok to connect them to ground? On the QFN 64 footprint the pads are really small and it would help stabalize the pin footprint if I connected them to ground. The pads easily come off when I try to apply solder by hand in preperation for reflow. 

         

       

         

      What should I do with the center pad of the QFN-64? I don't think I need it for thermal purposes in my design. Currently I"m not soldering it to the pcb and it's not connected to ground.

        • 1. Re: Unused Pins and Center Pad (QFN-64)
          junichi.hiraoka
                  Hi jram   
          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.   
          • 2. Re: Unused Pins and Center Pad (QFN-64)
            user_78878863

            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).

            • 3. Re: Unused Pins and Center Pad (QFN-64)
              user_14586677

              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.

                 

               

                 

              Regards, Dana.

              • 4. Re: Unused Pins and Center Pad (QFN-64)
                john.ramshur

                 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.

                • 5. Re: Unused Pins and Center Pad (QFN-64)
                  user_14586677

                  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.

                     

                   

                     

                  Regards, Dana.