William,

  I have played with bonding PCBs to Al using low cost 10mil VHB and higher cost ceramic filled tapes. I also played with custom copper on ceramic materials. SO much depends on power level and allowed temp rise. Yes the ceramic carriers have fantastic performance but at greater complexity and cost. It all comes down to assembly costs and repeatability. 

 I recall numbers like 15C / W on .0625 FR4 on Al with 10 mil VHB tapes. I only had 3 watts on that device so it was OK. A thinner board made with higher conductivity material was my option. Also, heavy copper on internal layers helps spread the heat giving a lower temp rise. At 50 watts on a DPAK the ceramic DPAK carriers I had made were the "Cat's Meow", though at a cost. Soldering wires to those was difficult. It is like trying to solder to a copper plate. It is hard to head up just the location where you want to solder.

 Heat sink compounds with Mica Insulators on TO-220 are low cost materials but assembly is a pain. 

 Pick your poison,

Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of William Noel
Sent: Tuesday, January 24, 2012 10:19 AM
To: [log in to unmask]
Subject: [TN] Heatsink Bonding

Hello all - We have a project for which every PCB assembly needs to be bonded to a heatsink for thermal transfer from the devices on the board to the heatsink and out through the Heatsink mounting hardware to the box that houses the assembled board/heatsink "modules".  One of the requirements we have is that when the board assembly is mounted to the heatsink it needs to be electrically isolated from the ground potential associated with the heatsink and box mounting hardware.  As a result, we have used a combination of ablefilm and glass microspheres at the bonding interface between the board assembly and heatsink.  The ablefilm serves as the bond and the glass microspheres provide electrical isolation.  The problem we have consistently had with this process is squeeze out of the ablefilm through the holes in the vias of the board.  We have attempted to use soldermask to tent the vias and thereby essentially seal the top surface of the board.  This has worked somewhat but we still seem to have problems with this approach.  Does anyone else out there have experience with this and could share some advice with us.  Thanks in advance for any help you can share.

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