Heat dissipation Aluminum PCB

I'm not sure how to calculate the following. I want to SMD solder either 3 LEDs or 6 LEDs to an aluminium core PCB. The LEDs in question are PK2N (datasheet) and, according to the datasheet, the Thermal resistance of Junction to Slug is 8 deg C / W.

So, if I want to run these at around 2.2W (600mA * 3.6V typical), does that mean that they generate approx 18 deg C each? So, if I place 3 of them, do they raise the temperature by 54 deg C? For 6 of them, 108 deg C? Also, the resistors will dissipate approx 2W or 4W (if I use 3 or 6 leds).

The total area of the AL PCB will be approx 9.1 cm2 (17 mm radius, 34 mm diameter) and 1mm thick. It is 35um Copper (1oz) but I don't have the datasheet for thermal conductivity figures.

How would you go about trying to work this problem out? Do I need extra fins on the PCB? Or should I limit myself to just 3 LEDs?

No, it means that each junction runs 18C hotter than the heatsink(PCB).

You need to know the heatsink's thermal resistance to air, and the total power you're dumping into it, and the air temperature, to calculate the heatsink temperature. Then add 18C to that, to get the chip temperature.

If the heatsink is just the PCB, its resistance will be high and its temperature will be high. It's difficult to calculate (as in, read a thermodynamics book first), maybe possible to simulate with professional design tools, but easiest to measure (bolt a power resistor and some thermocouples to it). But there's no point, you won't like the answers.

Practically this PCB isn't the heatsink, it just transfers the heat to the real heatsink (big lump of metal).

Now you need to know the real heatsink's thermal resistance to air, and the total power you're dumping into it, and the air temperature, to calculate the heatsink temperature. This thermal resistance will be in the heatsink's data, if you buy a commercial one. Otherwise, see above re: calculation, simulation, measurement.

But the PCB will run hotter than the heatsink : you also need to know its thermal resistance in transferring heat from chip through PCB to heatsink. Again, probably easiest to measure.

This is where you need to know the total power from all LEDs - if it's 1C/Watt, it will add 6.6C or 13.2C for 3 or 6 LEDs at 2.2W each.

Now, calculate the heatsink's temperature. Add the temperature due to the PCB. Then add 18C, and that's the chip temperature.