Electronic – Layer stack up & thickness calculations for Metal-core PCB

The last metal-core PCB I designed, I found the Bergquist guidelines pdf (via Bergquist t-clad overview) and Bergquist thermal clad white paper helpful.

Other PCB manufacturers that make metal-core boards also have guidelines on their web sites that you may find useful.

• you'll probably bolt the metal-core PCB directly to an even larger heat sink. Consider where to put the mounting bolts; you'll probably want at least a donut of copper around those holes (for the bolt head to wear against) or more likely a big ground plane around those holes.
• The connectors you put on these PCBs -- you don't want through-hole connectors. You may or may not want the kind of "surface-mount connector" that requires holes in the board for "locator pins"; you'll probably want the kind of connectors that mount without any holes.
• The power transistors and the "FET drivers" directly connected to them should go on the metal-core board. In principle you could put all the other components on the metal-core board also, but in practice they typically end up on a FR4 board connected to it.
• You may want board-to-board connectors to transfer signals from a FR4 board (if any) to the metal-core board.
• Trace/space on metal-core boards generally must be several times wider than on typical FR4 boards, especially if you use thick copper.

Take a look at this publication from Texas Instruments (you can go directly to pages 12+ for a quick look) : http://www.ti.com/lit/ml/slup230/slup230.pdf

Someone made an online spreadsheet calculator about it : http://circuitcalculator.com/wordpress/2007/02/16/pcb-thermal-copper-area/

You can also read this Application Note from Fairchild about "Maximum Power Enhancement Techniques for SOT-223 Power MOSFETs", which seems to be exactly what you're looking for :) http://www.fairchildsemi.com/an/AN/AN-1028.pdf

As far I as know, there is no heatsink for SOT223 packages, the only way to dissipate the power is through copper via pin 4.

I doubt you will be able to use your mosfet at full power (1.5W) and at 100°C+ when the Tj max is 150°C, unless you have a quite big copper area and a cooling fan.