ATMEGA32U4 QFN Bottom Ground Pad Question

The datasheet for the ATMEGA32U4 (http://www.atmel.com/Images/Atmel-7766-8-bit-AVR-ATmega16U4-32U4_Datasheet.pdf) doesn't mention the bottom ground pad for the QFN package. Besides for thermal reasons, does the bottom pad need to be connected to ground (i.e., is it not already electrically connected to the other GND pins)?

On the Teensy 2.0 it appears that it is connected (with an array of vias on the pad area to dissipate the heat), but on other boards like the Arduino Micro and SparkFun's Pro Micro (https://www.sparkfun.com/products/12640) the bottom pad doesn't appear to be connected at all, at least not with an array of vias going to the back side of the board. I understand soldering the exposed bottom pad helps with structural stability, but I'm more concerned with the electrical connection.

So, the question is, do I need to connect the bottom ground pad?

Why are you considering not connecting the bottom ground pad? If you're hand soldering a custom board, you might just add a plated through hole large enough to solder from the back.

pictographer said:

Why are you considering not connecting the bottom ground pad? If you're hand soldering a custom board, you might just add a plated through hole large enough to solder from the back.

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I was posing the question for 2 main reasons:

1) I have a hot air rework station (not a reflow oven), so I'm not sure how well the solder on the exposed pad would do with this method.

2) It takes up a lot of real estate for routing (on both the top and bottom layers for a 2-layer board) since it needs to be masked off essentially.

joe_prince said:

1) I have a hot air rework station (not a reflow oven), so I'm not sure how well the solder on the exposed pad would do with this method.

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To hand solder that way, usually a bottom-side heater is needed, in addition to the hot air on the top of the chip. I'd done it many times with an Aoyue 853A, but honestly it's quite unpleasant. For prototyping, I usually just leave the bottom pad unsoldered, unless it's electrically or thermally required.

PaulStoffregen said:

To hand solder that way, usually a bottom-side heater is needed, in addition to the hot air on the top of the chip. I'd done it many times with an Aoyue 853A, but honestly it's quite unpleasant. For prototyping, I usually just leave the bottom pad unsoldered, unless it's electrically or thermally required.

Click to expand...

Thanks Paul. So is the exposed ground pad internally connected to the other grounds? What are the side-affects of letting it float besides thermally and structurally? If I leave it floating, is it perfectly find to route beneath the chip as if the exposed pad wasn't there?