AVR-Chat

Chuck,

The TQFP (and any other TQFP as far as I know) version of that chip does not
have a bottom pad. That note is there because the same pinout is used for
both the TQFP and MLF/QFN version of the chip, and the latter does have and
require the bottom pad. That is why it is not mentioned at the bottom in the
package description, but only at the top pinout diagram.

One thing to note for future reference: If you use Eagle, many of the
standard MLF/QFN packages do not include the bottom pad. That may work fine
for hobbyist designs, but it will definitely be a potential trouble spot. I
usually avoid QFNs, but if you have to use one, be sure the package includes
the center pad. If not, make your own package.

On Sat, Jan 23, 2010 at 6:59 PM, Chuck Hackett <egroupscdh@up844.us> wrote:

> I am going to try a project using the SMT version of the Atmega16.  This
> will be my first SMT project.
>
> I must admit, I�m a bit nervous at the thought of not being able to pull
> the
> processor out of a socket to replace it � not that I�ve ever had to in the
> past, but at least I knew the option was there  :-)
>
> I note that, in the datasheet - near the top where it shows the pin
> assignments, it shows the TQFP (gull wing) package along with "NOTE: Bottom
> pad should be soldered to ground." with an arrow to a large pad on the
> bottom of the package.  This note does not appear near the end of the
> datasheet where they show the package dimensions.
>
> What is the reason for this pad to Gnd - thermal stability?
>
> In a home environment how does one solder this pad - provide a hole under
> the part and solder from the back side?  Would thermal/conducting grease
> work rather than soldering?
>
> My application does not use a lot of output drive so heat dissipation
> should
> not be a consideration ...
>
>
> Another issue:  In the past I have only used commercial prototyping boards
> with the processor already in place.  I had never noticed that, in the
> through-hole part, there are two "GND" pins.  The TQFP package has three
> VCC
> and three GND pins.
>
> I assume that good design requires that all of these pins be connected
> (i.e.: connecting one won't work)?  Do all the VCC pins require their own
> decoupling or will one do (and if so, which one?) ?
>
> I'm looking forward to designing a board with 603 size resistors that are
> WAY smaller than my usual 3/8" long beasts.  I'll be able to get a whole
> lot
> more in the board area provided by the free version of Eagle PCB  :-)
>
> Thanks in advance for the help ...
>
> Cheers,
>
> Chuck Hackett
> "Good judgment comes from experience, experience comes from bad judgment"
> 7.5" gauge Union Pacific Northern (4-8-4) 844
> http://www.whitetrout.net/Chuck
>
>
>
>
>
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