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A QFN chip I'm working with (atmega32u4) has multiple pads connected to ground, see the picture. Is it really necessary to connect all these pads? Could I say omit one or is it really bad practice? Would it be a big deal? enter image description here

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    \$\begingroup\$ Short answer - absolutely :) \$\endgroup\$ – Ken Jun 12 '17 at 21:42
  • \$\begingroup\$ Generally, but this is determined by the specific chip, not the package. Doubly so when you talk about a possible ground pad on the underside of the QFN (there are actually examples there where the manufacturer has come to recommend leaving that floating) \$\endgroup\$ – Chris Stratton Jun 12 '17 at 21:47
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    \$\begingroup\$ @ChrisStratton I believe you, but... can you name even one, off hand? I'd like to see how that's written up. I haven't every encountered a case, yet. But my experience is admittedly limited. \$\endgroup\$ – jonk Jun 12 '17 at 21:49
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    \$\begingroup\$ You have an equal amount of +VCC pins. Think about that for a while. Then google "atmega inside" and see how the pins are connected to the die. \$\endgroup\$ – Dejvid_no1 Jun 12 '17 at 21:50
  • \$\begingroup\$ @jonk nRF24L01+ though now that I look again, it may have been that the data sheet said not to connect the center pad, and later they said it was okay to. Of course that's an entirely different part than this question about, and only useful to demonstrate that "QFN" from the original title doesn't imply much by itself. \$\endgroup\$ – Chris Stratton Jun 12 '17 at 21:57
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Having multiple GND pins serves several purposes:

  • Lower ground inductance. In this case, the pins will be internally connected, and not connecting one to GND will worsen performance, but it could still work. Maybe. I mean, probably. I mean, I could stop my bicycle when one brake cable snapped, so why do I need two brakes, eh?
  • Isolate AGND from DGND. In this case, they are not connected internally, and omitting the GND via from AGND means your ADC will not work at all, or just burn.

Are vias that expensive? Last time I checked they weren't, really...

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  • \$\begingroup\$ Thanks for the response, I have no use for the ADC in this case, though I'll still be connecting it to ground. I guess I'll just be safe and connect em' all together. \$\endgroup\$ – Jhonti Todd-Simpson Jun 12 '17 at 21:57
  • \$\begingroup\$ I recall recently seeing an indication that there are key parts of an ATmega other than what you would think of as the ADC, which derive their power from that supply pin. Sorry I can't remember where. \$\endgroup\$ – Chris Stratton Jun 12 '17 at 22:00
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    \$\begingroup\$ Yeah, and it's only 3 GND pins, not that hard to add 3 vias, right? \$\endgroup\$ – bobflux Jun 12 '17 at 22:37

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