I'm no PCB expert, but I can give you a few reasons why one might
consider larger traces than the current capacity calls for.

1) Traces are antennas
2) Traces are inductors
3) Traces are capacitors
4) Traces are resistors

The smaller a trace is, the less able it can handle surge current
demands.  Electricity, despite our digital desires, is not infinite in
speed, and a current demand at one end of a long wire does not
instantaneously mean a current demand at the other end of the wire.

Depending on the application, a thin power trace might be just fine.
Remember, however, that on power up a thin trace may take a longer
amount of time to energize capacitors you'd have to place around the
board to make up for the current limits of the trace, and your processor
might not be correctly reset.

As a general rule, oversizing your power and ground traces will limit
the problems, lower the noise floor, etc.  Specifically, however, only
you can tell whether it's enough.

-Adam

Larry Bradley wrote:

> I'm laying out a PCB for a PIC 16F877 project, and I'm wondering about
> the
> minimum trace widths to use for the power and ground traces. The standard
> width used for all the traces (so far) is 10 mils. And life is easier,
> layoutwise, if I can use 10 mil for the power and ground traces as well.
> The current consumption as around 35 mA, and the board is 5" by 2.5 ",
> double sided. I'm using some analog circuitry, but it has it's own ground
> which is tied to the power ground at one point close to the regulator.
> The
> analog 5V is decoupled from the digital 5V.
>
> I'm new to this layout stuff. Can any of you experts out there tell me
> if I
> can use a 10 mil trace, or should I use something larger?
>
> Thanks
>
> Larry Bradley
> Orleans (Ottawa), Ontario, CANADA
>
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