I found this diode I want to use for rectification in a full bridge configuration. It is the SK520BTR has significantly lower thermal resistance than other diodes. The only thing is that it says the condition for this is "DC operation" does that mean the thermal resistance will likely be higher for my rectification purpose or lower?
I have emailed the manufacturer but I have not gotten a response over a week later.

 

Seems ok for a SMC part

 

All I can think of this that noting that it is measured under DC is that it is not a not pulsed condition. If pulsed the thermal resistance would seem lower because of the transient thermal response being lower.

 

Yes, it's too good to be true.
There's no way that small part can have such a low thermal resistance to free air.
For example below is the thermal resistance of a similar part ( MBRS360).
So the 10 °C/W should be stated as from junction-to-lead, not junction-to-air.
It needs to be soldered to a thermal pad for any dissipation more than about 0.6W (for the maximum operating junction temperature of 125°C).
Even when the leads are each soldered to a 1" x ½" pad on a PCB, the thermal resistance is still over 70 °C/W.

And there is no significant difference whether it is AC (50Hz or above but not low duty-cycle pulses) or DC.
It's simply the average power generated by the junction.
Note that since diode dissipation is mostly not ohmic (proportional to the current and not the square of the current) the peak current of a sinewave has only a small effect on increasing the dissipation as compared to DC.