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phase change thermal interface

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mark0310

mark0310

Mechanical
Sep 7, 2006
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Hi,
I have an application that requires an electrically insulating (breakdown voltage better than 2000 vac)material with excellent thermal conduction (better than 0.5 C-in2/W)to be used between a long pcb 150mm x 25mm and it's heat sink (has same mating area).
I have looked at the usual options of thin (circa 0.15mm) adhesive backed materials (Polyimide, Silicon) from suppliers such as Kunze, Bergquist, Chomerics and Aavid but the one big problem I have is that I cannot maintain a pressure between the pcb and the heat sink, so with the adhesives I am reliant upon the glue strength of the adhesive. The performance figures from the suppliers all show that the thermal conductivity is much lower when there is little pressure applied.
I have no knowledge of phase change materials, if I can get this insulation to behave as for the polymer (polyethylene??)from a glue gun then after application (heating up and cooling down in a low temperature oven)the bond strength between the pcb and heat sink should be good. I've enquired of 2 of the suppliers and they do market phase change materials, but they're soft at room temperature....so no good...any suggestions?
 
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Unfortunately we don't have the controls for ensuring an even distribution of a paste or spray nor the health and safety measures required for management of them. Also because the PCB will take European main voltage, both sides are exposed to this so there must be no risk or bridging and the above would be a concern.

So the best solution may appear to use adhesive backed materials such as Sil-Pad....but we have the problem that I cannot slip the parts together, so there's no clamping pressure which reduces the thermal conductivity significantly. This is why I am looking at Phase-change materials....at assembly we can clamp the pcb to the heat sink and put the assembly in an oven.....so we could get the material to "soften/melt" and form an excellent thermally conductive path....but from those I've looked at (eg: Chomerics PC07DM-7) they are also fairly soft at room temperature.....so if there was a grade of this that was rigid to say 50C, then I could bond the parts together in an oven at a higher temperature beyond it's phase change and I have a solution....problem is I don't know if it's available!!!!!!
Thanks for your suggestion, hope the prior ramble explains the situation?
Mark
 
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I understand. My suggestion was not to use just thermal epoxy to bridge the gap because you said you can't apply it evenly. That's why I suggested a thermally conductive pad with a blob of thermal epoxy on each side. When clamped it will spread and then set. I've used it to attach big metal heatsinks and it's great stuff.

Chris DeArmitt PhD FRSC CChem

Consultant to the plastics industry
 
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Right I see, with some practice and the right epoxy and conductive plastic this would work...however the annual volume is going to be up to 5000, so I'd really prefer a solution that can be assembled and work without any fitting skills.Although this pcb is 150mm long, we're also looking at one that is 375mm x 25mm.

In service the PCB (on 150mm length)generates 3W loss, and we're looking at 50C max in service temperature, so small amounts of heat to lose.

I'm hoping I can locate an electrically insulating, thermally conductive phase change material that is "rigid" to say 60C and "melts" at about 100C.

If I can find such then if I place it between heatsink and pcb, temporarily heat the assembly up so it "melts"(and temporarily clamp the parts together)then on cooling we have the bonded items.

But can I find a source of such material.....not yet!
 
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What you described is exactly the same fitting as I mentioned with the epoxy. Two blobs, clamp, heat so it sets, done.

Phase change sounds nifty but they are not good for thermal conductivity, the high performance CPU heatsinks don't use them. They are too thick for one thing.

The epoxy spreads out to a nice thin layer and sets stronger than an adhesive pad of phase change.

Chris DeArmitt PhD FRSC CChem

Consultant to the plastics industry
 
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Phase change is pointless unless you intend to go past the critical temperature, it which case it'll ooze out of the joint.

The whole point of using a phase change material is to actually make it change phase, since the phase change itself absorbs a chunk of heat without changing temperature.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize
 
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Chris; Yes your solution with epoxy/carrier/epoxy is practically the same as what I'm looking for with a phase change material. Where I have a concern is that for such a long and thin PCB in a production environment the consistent application of the epoxy would need to be very well managed (and all the H&S controls associated with it's use).
The Thermal Interface Material (TIM) suppliers can provide a phase change material either side of a carrier (eg Chomerics PC070DM-7 from Hagelec.com). So I can get this on a roll to the right width (25mm) and cut lengths (to 375mm) to suit. In a production environment for 5000+ repeats I know I'd get the same performance each time. I agree it's thermal conductivity is inferior but it's good enough...what isn't good enough, and where I'm looking for a solution is that the "stiffness" of the phase change material up to my maximum service temperature (to 50C)is very poor, all the suppliers have focused on providing tacky materials, not one that can double as heat activated adhesive.
IRStuff and Patprimer: The TIM suppliers provide phase change material because it wets out above the critical temperature and enables easy handling at room temperature, any benefit from thermal transfer as a consequence of passing through the phase change is secondary. The thermal resistance of the assembly is established once wetted out, so if the temperature subsequently drops below critical then the physical enhancement is unchanged.
Unclesyd: many thanks for the list, I'll work through them.
 
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Some epoxies are tacky paste or even solid at room temperature. Not all are the liquid kind from the hardware store. You can even get epoxy as prepreg, that is strips of epoxy tape with carbon fiber in it. Perhaps you could use something like that. If you could get epoxy tape with glass fiber instead of carbon fiber then you're set as it doesn't conduct electricity. I imagine Hexel or Cytec would be able to advise.

Chris DeArmitt PhD FRSC CChem

Consultant to the plastics industry
 
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Chris, Many thanks...I'll see if Hexel or Cytec happen to have a very thin epoxy pre-preg with an insulative (sufficient di-electric circa 2000+Vac) core and reasonable thermal conductivity....maybe a field they've looked into?

IRstuff..sure... or a thermo-set....for my application all I need is for the transition (the knee in the temperature/plasticity characteristic)to be fairly sharp and it's performance as a TIM to be adequate so I can lose the in service thermal energy. The TIM suppliers term a variety of composites as Phase Change when in application the energy used at transition isn't the issue, it's the wetting of the working surfaces and ease of use at assembly in the factory. Hence the terminology is now adopted irrespective of absolute accuracy.
 
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You would be looking for a film adhesive, not prepreg. These require clamping and heating to 250F to 350F to cure.

Phase change materials change phase, i.e. melt, during normal operation. This is why they cannot do what you imagine.
 
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