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Thread: Current on a PCB
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face BY : Dan Michaels email (remove spam text)



Andrew Seddon wrote:
>> Hard to imagine anyone would do this. Even though the nomograph
>> indicates you can run 750 mA thru such a trace, and only have a 10degC
>> temp rise, there are several "other" things to worry about. Trace series
>> R producing IR drops. Digital noise in this narrow channel. Inductive
>> effects of narrow trace width at hi speeds. Impedance issues. On and on.
>
>I always thought that it was best to use the least ammount of copper you can
>with digital logic, come to think of it I think I made this rule up myself
>so any input appreciated. BTW I just got a board back from the shop, it has
>8 mil ground and power traces. Seems to be working OK. I dind`t mean to do
>it but it was a rush job and I fogot to manually route the power traces.
>

Andy,

Everything I have read indicates you have this exactly backwards. The
problems become more critcal as frequencies been used increase, and when
analog and digital are mixed on the same pcb.

The ideal case would be to have a multilayer board with gnd and power
planes. This gives you minimum possible IR drops in Vcc/Vss lines,
less possibility for heat buildup since planes act like heat sinks,
better overall bypassing since the planes act like distributed caps
[the smaller the vertical spacing between planes the better], smaller
inductive current loops so there is less brodcast EMI [high speed
signals follow the path of least "impedance"], on and on. Tiny traces
are going to be problematical in all of these areas.

I do mostly 2-layer boards, and typically use 70-100 mil traces for
gnd and 30-50 for Vcc. I run them on the upper surface of the pcb in
parallel with each other, directly under the ICs, with feeding from
teh same end of the pcb. The majority of chip interconnections go on
the lower surface. Also, with wide gnd traces, it is easier to run
hi-speed lines, such as clock signals, directly beneath the gnd trace
on the lower pcb surface. This helps to limit EMI generated by the
clock edges/etc.

On and on - this just scratches the surface. Howard Johnson's book
"High-Speed Digital Design A handbook of Black Magic" has lots more.
Expensive, but well worth the price.

best regards,
- Dan Michaels
Oricom Technologies
===================

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