AVR-Chat

Dave VanHorn wrote:

> The data sheet says that the large pad under the device "should" be 
> soldered to ground.. 
> 
> Does anyone have direct knowlege of wether is HAS to be?
> Assume all perimiter ground pins are in fact grounded.

Not direct knowledge, no.

Speaking generally, these type pads have three roles--thermal
conduction, EMI reduction, and lowered ground inductance.

The first is hard to get away from, unless you can guarantee the device
will be operated at a reduced ambient temperature or at a reduced power
consumption, or with a beefier heat sink.  Some parts specify different
limits with or without the pad soldered.

Having a ground shield under the die can improve noise immunity,
especially in sensitive circuits like ADCs or other mixed analog-digital
chips.  You can get something like this with a circuit board ground
plane underneath, but it's not as close to the die as the pad is in the
package.

Finally, the large pad can serve as a low-inductance ground return that
helps avoid ground differentials in the chip.

Personally (not with AVRs, though), for home brew stuff, I've put in a
ground plane on both sides and an array of plated vias inside that,
filling the vias with solder and hoping at least some of it gets wicked
up onto the pad surface.

Your mileage may vary, of course!

-Johnathan, AE6HO