Bill,
Not sure which specific Ablefilm you are using, but most are glass supported [550K, 5020K etc.] having thermal conductivities in the 0.2-0.8 W/m °K range.
You also don't specify the voltage differential, or the application ... and I am not really fond of the idea, but the fiberglass mat is nominally 0.001" in dia. Therefore at each knuckle in the mat you have a mounting surface spacing of 0.002". If there were nice smooth surfaces with no protuberances, and low voltages do you absolutely require glass beads? What are the dia of the beads?
Most of the Ablefilms are B-staged products and can to be pre-applied to one of the substrates at 70-80°C followed by a full cure of the [clamped] assembly at ~125-150°C. During cure, they will squeeze out, or through holes. Are you using the standard materials or something of lower cure temp?
It would be an extra step, but you could pre-fill & cure any vias with a lower-temp cure non-electrically conductive material like the 2035SC
Most of the standard materials have a high modulus when fully cured. You may opt for a modified, lower modulus version.
By comparison there was one unsupported electrically conductive film that I recall was something like 3.5 W/m °K [the 5025E]
I really like the Ablestik products, but you may also wish to consider the 'other' thermal film suppliers. Just be careful since they don't all spec things the same way. Remember that Henkel, Ablestik, Emerson & Cuming, Hysol, Loctite, and a few others are now one big 'happy' family. There is Bergquist, and a few other film suppliers to work with
Finally, I would seek to maximize the copper area to transfer heat from. The more spreading you can get, the lower the wattage/unit area.
Regards,
Steve C
-----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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