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Date: | Thu, 18 Apr 1996 07:44:00 EDT |
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>From: Walker, Don
>To: DesignerCouncil
>Subject: Re[2]: heavy copper...
>Date: Wednesday, April 17, 1996 9:55AM
>
>> ...you also have to know at what temperature (over ambient) that the
board
>> will be running at...
>
>Remember, what the chart is saying is that if you apply a specified current
>to a specified trace size, the temperature of the trace will rise by a
>specified amount.
>
Don, I've not heard this interpretation of the chart. In IPC-D-275 Figure
3-4 there seems to be a conflicting definition of the charts. Above the
first chart it says "For use in determining current carrying capacity....for
various temperature rises above ambient". We have always interpreted this
to mean take the expected temp. rise above ambient of the board
(environment) and select trace size. However the notes for the Figure (Note
1) indicates "The design chart has been prepared as an aid in estimating
temperature rises (above ambient) vs. current for various cross-sectional
areas of etched copper conductors." This seems to support your
interpretation, Don. Admittedly I haven't been using the IPC chart, rather
one that has been posted on the wall (a copy from the old Bishop Graphics
Design Aid). The old BG chart doesn't have the Note 1, but does have the
first label above.
So now I'm confused. Does the chart indicate the rise of the trace
temperature or does it indicate the rise above ambient of the environment?
Perhaps someone from the IPC committee that worked on the 275 (Gary are you
there?) could comment and/or elaborate on the charts.
>> ...As an example, say I need 5amps and 20 degrees over ambient is a safe
>> assumption. The chart gives ABOUT 100 square mils, which corresponds to
>> ABOUT a 75mil trace width on the 1oz copper curve.
>
<<<<<< cut >>>>>>>
>
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>
Ben Davis
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