piclist 2000\05\31\195146a >
Thread: Current on a PCB
www.piclist.com/techref/pcbs.htm?key=pcb
picon face BY : Donald L Burdette email (remove spam text)



Dan Michaels wrote:

>The general bent of this thread seems a little strange to me.

>As a general rule of thumb in electronics, it seems reasonable to
>allow 2-3x safety margin on most critical design decisions - especially
>those concerning power dissipation levels. It would seem that even so
>much as thinking about driving traces to the point where they are
>heating up or glowing or discoloring the pcb might be like flying
>too close to the sun on waxed wings.

How can you figure where 50% is unless you know what 100% is?  You have
to know where the failure occurs before you can estimate a safety margin.

>My general approach [which may not be the
>best of all possible worlds] is iterative. I figure it takes a couple
>of cuts to get it right [others may disagree here].
<snip>
>Then, build it and see how it goes. Next cut may need adjusting in some
>cases. [Is this approach too imprecise?? - I'm sure there are different
>tribes of opinion here - things like "safety margin" are opinion
>oriented].

This approach is not imprecise if you use your prototype to actually
measure how close you are to the limits.  You could easily build a
prototype that works, but is significantly beyond the safe limits, or
even the limits of what will work at all over the long term or under
extreme conditions.  Limited or short-term testing may not discover these
flaws.

If you guess at the right values and don't measure how close you got,
(which I suspect is rarely true in your case), then you are being
horribly imprecise.  In fact, you could be building a safety and/or
reliability nightmare.

I believe the most reasonable solution (at least for most commercial
applications) is to KNOW the right values.  Then there's no guesswork,
and you can build in "comfortable" safety margins.

Of course, in high-reliability applications (military, medical, space),
you should KNOW and then VERIFY through extensive testing.

I felt that the general bent of this thread was about KNOWing.



NOW - MY CONTRIBUTION

The topic of 'fusing' the traces is actually very different from the
topic of 'reasonable' currents.  The fusing (melting or vaporizing)
currents are at least an order of magnitude higher than what you want to
use.  A trace that will melt at 10 amps will discolor the solder mask and
the substrate, even melt the glue and peel off the board at currents of 1
amp or less.  Fusing normally occurs only if there's a major fault
somewhere.

I think the heart of the question is twofold:

1. What is the max allowable temperature of the trace.  If you know that
and the ambient temperature, you can calculate the allowable temperature
rise (the difference), from which you can calculate the current carrying
capacity of a trace, or conversely, the trace needed to carry a given
current.

2. What is the max allowable voltage drop across the trace.  This is
highly application dependant.

After answering these two questions, you can use the tools mentioned by
previous contributors to make the necessary calculations.

The vast majority of PCB's are made on either phenolic or FR4 fiberglass,
with one of several mask materials.  Does anyone know the max temp of
this stuff?  If nobody knows offhand, I'll call my PCB vendor on Friday
to ask.

Don

<20000531.195138.-1037989.1.dlburdette@juno.com> 7bit

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