AVR-Chat

Chuck Hackett wrote:
> I am going to try a project using the SMT version of the Atmega16.  This
> will be my first SMT project.

Welcome to the world of SMT.  I think I'm a little older than you and 
enjoy using surface mount parts -- although I would hesitate using 0603 
size as my current limit is around 0805.

> 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  :-)

If you suspect a bad part with this type of mounting, it is often that 
it is best to sacrifice the mounted part by cutting the leads at the 
package edge and desolder the remnants in order to avoid ruining the 
PCB.  I've never had to do it.

> 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?

Yes.

> 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?

Yes to hole, no to grease.

> 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.

Correct.

> 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?) ?

YES! Definitely connect ALL Vcc and Gnd pins!!!  And yes, each Vcc pin 
should have its own decoupling.  Side note: I recall a customer's 
engineer who ran Vcc to one of two package pins and then connected a lot 
of the other board Vcc circuitry to the other package pin.  Not only is 
this horrendous practice, but depending on bonding wire size and current 
draw, it could act as a very expensive fuse.

> I'm looking forward to designing a board with 603 size resistors that are

As I said, that size is uncomfortable for me although I could probably 
do it if necessary -- it would not not be enjoyable.  One more thing 
I've found is in laying out boards and etching them myself (double 
sided), I can etch them fine, but when it comes to drilling the small 
holes with precision, it is beyond my equipment and age/eye/steadiness 
capability, so I've gone to services like SparkFun and their PCB for 
single boards.

Regards,
Don Jackson
http://ae5k.us http://hamavr.ae5k.us http://4smicrowave.ae5k.us