Hi Mike,
The charts in question show a temperature rise above ambient, not an
absolute temperature.  You should take your maximum current (300mA) and
determine the proper trace width for the location of the trace (inner Vs
outer layer) using the appropriate temperature rise curve.  There are other
things you should take into consideration, such as the proximity of the
trace to a component which is dissipating significant power, because that
component's heating effect will cause the "ambient" temperature near the
trace to be higher.
Here is a simple example:
Max ambient temperature for the PWB = 70 C.
Safe operating temperature of PWB = Tg - safety margin = 90 C (remember,
this is only an example).
Your allowable temp rise would be 90-70 = 20 C.
Assuming your trace is on an internal layer, with 1 oz/ft^2 copper, using
the 20 C temp rise curve from Figures 3-4c and the 1 oz curve from 3-4b
gives you a minimum trace width of .005".  In this case, your .012" trace is
quite sufficient.
This is probably over-simplifying things a bit, but I hope it points you in
the correct direction.
Regards,
Mike

Michael E. Boldt
Electronic Packaging Engineer
Moog Inc., Motion Systems Division
East Aurora, New York 14052

My name is Mike Cubon and I work for ITW Chronomatic. We manufacture Polymer
Thick Film circuits and I have a technical question about figure 3-4 in
IPC-D-275. There are three graphs that show current rating for different
conductor traces at different temperatures. Are there any charts or perhaps
a formula for calculating current capacity below the temperatures shown. The
application is automotive and will be tested to -40C. The max. current will
be 300mA and trace width will be .012 inches.

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