William,
it seems to me as a question of viscosity. Ask for a film with somewhat
higher value or lower the processing  temperature. The later will result in
longer processing time, which you may not accept if it's a high volume
line. We have been bonding boards on aluminum heatsinks for years with
Staystik 811 without flowing issues. Just as an example.

Inge



On 24 January 2012 16:18, William Noel <[log in to unmask]> wrote:

> 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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