AVR-Chat

Atmel produce a demo board for their ATR2406 radio IC which includes an 
ATmega88 in this package and they did not put a large pad under the 
mega88. They do say they will change it when they up issue the board but 
the board works fine and they are not too worried since the processor has multiple GND pins. The GND pad does make that package more difficult to 
track on a two layer board particularly because the package is so small. 

Dave Pallant | Hardware Design Engineer

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AVR-Chat@yahoogroups.com wrote on 03/11/2005 00:29:43:

> My T26 data sheet (12/03) does not seem to say anything
> about it, or even that it IS solderable. TI usually notes
> things about a thermal-pad quite carefully; nothing seen
> here. In fact, this data sheet only has the word "solder"
> once, and that refers to the tip region of SOIC pins.
> 
> Jim
> 
> 
> On Wed, 02 Nov 2005 15:47:04 -0800
>  Johnathan Corgan <jcorgan@aeinet.com> wrote:
> > 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
> > 
> > 
> > 
> 
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