----------
>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.
>
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Ben Davis
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