AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Steve Hodge" <steve@...> wrote:
>
> I'm a newbie to SMT devices and I'm
> doing my first pcb layout with them.
> With DIP packages I usually arrange
> them in a column straddling a Vcc
> power bus.  Is it OK to do the same
> with SOIC packages?

VCC routing matters very little if you follow these two important rules.  One, connect all grounds with something as close as possible to a continuous plane.  Two, decouple each VCC pin as close to the pin as possible with a single, low-inductance ceramic capacitor connected to the ground plane.

For SMT designs, most of the ground plane will be on the bottom.  Try to route on the top as much as possible (that's where the component pads are) and cross to the bottom as necessary, but don't make long slits in the ground plane.

SMT ceramic capacitors will perform better than leaded capacitors for decoupling.  Use the smallest size you can handle.  0805 shouldn't be too hard, but I've moved on to 0603 and am starting to use 0402 in dense layouts (but I wouldn't recommend you start there).  A good value is 0.1uF.  Use the same value everywhere (using a high value and a low value in parallel was once popular but actually makes things worse).

Graham.