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Hi all,
I've been playing with a sensor design where I want to use a small PIC package. The QFN is just perfect since it is pretty small. I have done some research to find out if the center pad should be grounded or not. I couldn't find any information in the datasheet. And in the electronic forums, nobody seems to have a clear idea of this. Does anybody have any experience designing boards with a QFN PIC part?
Thank you for your help,
Robert
Well, after doing some more research I found a thread in the Microchip forum where somebody else was having the same question. This person got the next reply from the Microchip Support Team,
Quote:
<From our Knowledge Base, available on http://support.microchip.com>
The center pad is primarily for heat transfer, and is integral to the QFN package design.
Most of our products do not generate enough heat to be concerned with, but the exposed pad is unavoidable. The exposed pad is the underside of the die attach paddle.
The die is typically attached to the paddle using a conductive epoxy. That means the exposed pad is electrically conductive to the substrate of the die, which is usually ground in our products. We do not recommend putting PCB traces under the exposed paddle, except for thermal relief.
The center pad should not be connected electrically to ground or power.
If you have device, just measure resistance from central pad to Vss. If is it short, then is safe to put ground. If isn't, then leave it unconnected.
i had the same question (why don't they just make this simple and put the info in the data sheet?)
their answer was similar but slightly different, copied MC's response at bottom here
i think the most important things is to reserve the space so you don't forget and run traces,
*** AUTOMATED MESSAGE - SEE RESPONSE METHODS BELOW ****
Below is a proposed resolution from Microchip Engineering Support Team for your Ticket xxx
Area : PIC® Microcontrollers
Product Group: S16FHP
Product : PIC16F883
ds wrote >>>
Problem Description:
The data sheet for this part does not explicitly say what to do with the exposed pad on the QFN packages
The exposed pad is not shown in the pin list
I know that by convention these are generally grounded for thermal and / or electrical reasons but the data sheet really should say something
My questions are these:
1] May i leave the exposed pad unconnected ?
2] Is the exposed pad internally connected ?
3] Are there any mandatory mechanical or thermal reasons to connect it when there is no expected power dissipation much above quiescent operating ?
thank you
ds
<<< end ds
Problem Resolution:
Hello Dennis,
Unless otherwise specified in the datasheet, you may connect the exposed pad to ground, or leave it floating, but it may not be connected to any other signals or potentials.
See
http://support2.microchip.com/KBSear...=SQ6UJ9A00I7WF
Regards,
Nicu