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Luca , 01-29-2017, 10:47 AM

I would like to use ATMEGA32U4RC-MU microcontroller in QFN package.
Reading the pins configuration, at page 3, and the pins description from page 5 to 7 i can't find any information about exposed pad under the QFN package.
Anyone can help me about this topic?
In detail for my project the board is very tiny and it's crucial for me understand if that exposed pad it can be omitted or not, and if not where it can be electrically connected.
Sorry for not perfect english.

robertferanec , 01-30-2017, 10:14 AM

Nice one It really looks like the information is missing there. I would try to search for a reference schematic or ask Atmel support.

Luca , 01-30-2017, 01:17 PM

Thanks for your time and help Robert!
This is the link where i've downloaded the datasheet: http://www.atmel.com/Images/Atmel-77..._Datasheet.pdf
Searching on Atmel official website i really can't find any documents or app-note about this information and, to be honest, it don't have understand how to send an email in order to directly ask to
Atmel... the web site is not really friendly for that section in my opinion.
Tomorrow morning i'll receive 5 microcontroller from Mouser Electronics, i work in an SMT mounting factory and i can treat very easy SMD components without ESD risk or similar.
If i simply measure the connectionc, if it's, from EP from GND pins can be a significative sign in order to understand about that?

robertferanec , 01-31-2017, 11:03 AM

Yep, if you have the component, try to measure if there is short circuit between EP (Exposed Pad) and one of the GND pins. If yes, you can simply connect EP to GND, that should be fine - that's what I would do.

Luca , 02-01-2017, 12:22 PM

Hi Robert, i've check if EP are in short circuit with pin number 15, GND and... they don't are in short circuit!!
Do you think that i've to try with other pins.. ?
Now my question is, the EP can be electrically connected, or must be left floating?

robertferanec , 02-02-2017, 09:22 AM

Sometimes EP is not connected inside chip, but they still recommend to connect it to GND. In case there is no short circuit between EPAD and GND, you really may want to find reference schematic and check how they connect it there.

Luca , 02-03-2017, 11:53 AM

Yes, surfing on internet i've founded this document

where the EP is connected to GND.
The microcontroller it's not exactly the same because i don't have space for crystal placement, infact the version that i use run the main clock by internal RC oscillator instead crystal.
This difference can affect somenthing if i connect the EP of this version of microcontroller to GND?

robertferanec , 02-04-2017, 10:42 AM

You may need reference schematic for the exact chip you use. Possibly, if you are still not sure, just add 0R resistor between EPAD and GND - but this would be a weird solution, you should be able to figure out how other people connect the EPAD.

Luca , 02-06-2017, 03:43 AM

Thanks for reply, yesterday i've search about reference schematics and i've found somenthing with that particular microcontroller and seems that can be connected to GND.
But the schematics that i've found aren't from Atmel, i mean not official information, but i think is still ok...
I can't search that document now because i'm at work in SMT line and i don't have much time, tonight when i return at home i'll try to post here.

Luca , 02-08-2017, 02:23 PM

I've talked with Atmel application engineer, that says the EP can be connected to GND or left floating without problem for this particular microcontroller.

Wesperos , 02-09-2017, 03:22 AM

Hey I've designed multiple boards with QFN ATmega32u4 and I've always connected it to the GND. I'd put couple of thermal vias to the bottom layer on the bigger GND surface, while on the top layer I'd only have a couple of traces to the GND pins.

Luca , 02-09-2017, 08:51 AM

Thanks for your experience, can i ask if you have also used ATMEGA32U4RC?