AVR-Chat

On Sat, Jan 23, 2010 at 9:59 PM, Chuck Hackett <egroupscdh@up844.us> wrote:

>
>   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?
>

The TQFP has no pad.  The MLF/QFN has pad, and it is for mechanical
stability in this case.
The solder on the pins alone will not hold the QFN to the board if the board
is dropped or vibrated.



>
> 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?
>

For a prototype or one-off project a hole under the part is fine for hand
soldering.
In a production environment you do not want to have that as it can theave
solder
into the hole away from the chip.

No grease, but you do want a *lot* of good flux.  A flux pen is a good first
step.



>
> 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?) ?
>

Unless the pin is marked "do not connect" or "not connected" always connect
the pin to something appropriate.
Just because you don't use the ADC does not mean you can not leave AVcc
unconnected.
 Unused I/O pins do not need to be connected, but they do need to have
pull-ups enabled, or set them to outputs.



>
> I'm looking forward to designing a board with 603 size resistors that are
> WAY smaller than my usual 3/8" long beasts.
>

The consensus on the gEDA/PCB list is that 0402's can are the smallest that
can be done by hand with practice, and good magnification.
01005 parts are hard to even see with the naked-eye at all. :-)


> I'll be able to get a whole lot
> more in the board area provided by the free version of Eagle PCB :-)
>
If you are not stuck on Windows give gEDA/PCB a shot, it has no such
arbitrary limits.
http://www.gpleda.org/  The Windows version is still a work in process...


-- 
http://www.wearablesmartsensors.com/
http://www.softwaresafety.net/
http://www.designer-iii.com/
http://www.unusualresearch.com/


[Non-text portions of this message have been removed]