Hi guys I'll attach 2 examples one FR-4 PCB and one Aluminum PCB both 25mm with 4 x 3535 leds in a 6V configuration.

This example is intended to be an FR-4 PCB with big vias of 3mm and they will be filled up with leaded solder,
the vias are part of the pour that it's connected to the thermal pad of the leds.


Or this one this is just a big polygon connected to the thermal pads of the leds on an Aluminum PCB


Which one do you think will work better at 18W?

There will be a heatsink below the PCB. I know a DTP Copper will work way better, but they're just too pricey.

 

I use small VIAs, filled, to transfer heat to the back side of the board.
It looks like most of the copper is not connected to the LEDs. Not good.
I do not know the schematic so probably wrong.
The idea is to have the LED pad connected to the maximum copper. Red= cut in copper.

 

I would go with the metal core PCB for the best performance and longest life of the LEDs.

 

Exactly what Dick suggested.
One of the most important applications for aluminum substrate boards is precisely high power LEDs.

 

Thank you schmitt trigger and DickCappels for reply!

The Aluminium PCB that I can afford has a dieletric layer of 1wm/k, why is this better than big vias filled with leaded solder (50wm/k)?