Heat Dissipation Aluminum Core PCB & LEDs

[made2hack]

Hello all,

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?