The content below is from wiki, Allen just wants more PCB people to know the real MCPCB knowledge.
MCPCB (Metal Core PCB) is those boards which incorporate a base metal material as heat spreader as an integral part of the circuit board.
The metal core usually consists of aluminum alloy.
Furthermore MCPCB can take advantage of incorporating a dielectric polymer layer with high thermal conductivity for lower thermal resistance.
Source:http://4mcpcb.com/mcpcb-wiki-introducing-for-you.html