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PICList Thread
'[EE]: Current on a PCB'
2000\05\31@195146 by Donald L Burdette

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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


'[EE]: Current on a PCB'
2000\06\01@115144 by Dan Michaels
flavicon
face
Don Burdette wrote:
>Dan Michaels wrote:
.....
>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.
>

Don, hopefully my designs are a little more precise than my
explanations!

Yes, I always calculate everything I can - general and "worst
case" - during the design phase, add in a substantial safety
margin on things like power dissipation, always use the widest
possible traces for power/gnd as a matter of course, cook it up,
and then test/measure the resulting hardware. There are always a
few things that take a 2nd cut to get right, but most things are
pretty good on the 1st go.

After I sent that last memo, I realized it sounded a lot like I
just totally wing everything. Not quite. <:-)).
===============

>
>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.
>

As I pointed out last time, I was especially struck by the fact
that the nomographs show that it takes "only" a 3x increase in current
to go from the 5degC to the 100degC temp rise curve. That's an
enormous effect for not so enourmous a change in current. Essentially
no effect at all to blistering your finger/whatever.

I would not want my pcbs to be designed anywhere close to the point
where this could occur. I would calculate the worst case currents as
best as possible, find the trace width that produces say a conservative
10degC rise at that current, and then try to allow a substantial
safety margin over **that**.

10degC rise for a 100mil 1oz cooper trace is ~4A, so I would think
that operation at currents <= 1A will "never" be a problem here,
since it would take ~12A to get 100degC rise. At currents much beyond
1A, I think I would go with heavier copper, 2-3oz, from the start.

Looking at the nomographs, and thinking about the physical size of
relays and switches required to handle large currents, it seems you
pretty much have enough room to widen/thicken the traces so heating
should rarely be an issue. Eg, for a 10A design, a 300mil 1oz trace
gives about 10degC rise - there should be plenty of room for parallel
150mil traces top and bottom - and it would handle 30A before you get
to 100deg rise. Plus, there is probably room here to widen the traces
further. 3oz copper would be much better.
===============

>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.

I could not find this in Johnson's book, "High-Speed Digital Design",
but as indicated above, I think one can generally design so that you
shouldn't have to worry about it.

best regards,
- Dan Michaels
==============

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