Hi Nila,
Your PCB sounds like a "challenge"—not to say nightmare in this brave new 
LF-world of ours.
I sure hope you are planning on using vapor phase for your solder assembly 
process, otherwise the thermal requirements are going to be "hellish".
When you say 'metal core', do you mean a real metal-core substrate, or a PCB 
with a heat sink   in the middle surrounded by 2 FR-4 PCBs?
What are you doing to make your PCB survive the LF-assembly process?
Did you read my White Paper on the subject?

Werner



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