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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