You may be interested in the
web via calculator. With the calculator you can play around with board thickness, size of hole, plating thickness of the via and get Amp capacity and thermal resistance.
I've been doing a lot of R&D on this subject lately and I prefer to order PCBs as thin as possible. Many PCB fab shops will make the PCBs as thin as 0.2mm. The thermal resistance using multiple vias is as good as an MCPCB or better.
Where does FR4 PCB wattage run out and MCPCB become more attractive. It's when your PCB can not move sufficient heat to a large heat sink and the device you are trying to keep cool is no longer kept within the MFG specifications.
A general observation. Since thermal resistance is rated in C/W (degrees C/Watt) it makes sense that at 1W your transition across the PCB will be the straight thermal resistance. If for example that number is 40C/W then at 1W your device will be 40C above the heat sink it is mounted to.
It gets worse as wattage goes up. At 2W the temperature rise is now 2X 40C. At 3W the temperature rise is now 3X. You can see that as power increases so does the multiplier on the thermal resistance. Once you get to 10W and higher you need thermal resistances that are less than 5C/W and preferred numbers that are less than 1.
With a Cree XR-E the thermal pad is large enough to put down some 20+ vias. With some tiny LEDs you are lucky if you can get 5. The two factors that will get you are power (watts) and thermal pad size if you are talking LEDs. The smaller LEDs are going to be tough as you won't be able to get 10+ vias on the power pad.
One 18 mil via on 0.8mm thick FR4 is according to the calculator 49.3C/W. 10 of these vias would give you 4.93C/W. An XM-L LED adds another 1.5-2C/W. If we combine the PCB and LED that makes it approximately 6.93C/W or so. At 3A the thermal resistance drop to the backside of the PCB will be (10W * 6.93C/W) or approximately 69.3C rise assuming you can put 10 18mil vias under the thermal pad. If the LED die is rated for 150C then with an infinite heat sink attached the maximum ambient you can stay in specifications is 150C/69.3 = 80C ambient.
It may be worth the small additional cost and go thinner. The same setup with 0.2mm PCB is a temperate drop from the die to backside of 33.1C at 10W of power. (3.3V @ 3A ~ 10W). In this case the XM-L LED is part of the limiting factor if I remember correctly the thermal resistance is 2C/W. Actually looking it up it's 2.5C/W for the LED. The board will be 1.31C/W with 10 vias on a 0.2mm board and the LED adds another 2.5C/W.
Wayne