The old MIL-STD-275E spec page 34 and 36 has charts in it that give you a guide that is de-rated based on the Military standards. http://www.dscc.dla.mil/Downloads/MilSpec/Docs/MIL-STD-275/std275.pdf
The IPC has the same charts in IPC2221 page 38.
According to what I have heard, these charts are approximately 50% de-rated... and based upon phenolic material. There is some impressive work being done by
Mike Jouppi, "Thermalman", that gives more realistic numbers to a thermal design. http://www.thermalman.com/
Also he gives an explanation of the history of the
Mil-Std-275 Current Carrying charts on his website at
If you want to use a free online calculator there's one
developed by Brad Suppanz at
Also, a good book on Thermal is "Cooling Techniques for
Electronic Equipment, Second Edition" by Dave S. Steinburg pub
1991 by John Wiley and Sons, Inc. ISBN 0-471-52451-4
Also there is a publication called "Electronics Cooling"
Of course the heat sink manufacturers are very helpful to
get more info on thermal designs -
Thermal Interface materials