phase change thermal interface
phase change thermal interface
(OP)
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?
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?





RE: phase change thermal interface
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Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: phase change thermal interface
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
RE: phase change thermal interface
RE: phase change thermal interface
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: phase change thermal interface
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!
RE: phase change thermal interface
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
www.phantomplastics.com
Consultant to the plastics industry
RE: phase change thermal interface
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http://www.nusil.com/index.aspx
RE: phase change thermal interface
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
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RE: phase change thermal interface
Regards
Pat
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RE: phase change thermal interface
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.
RE: phase change thermal interface
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: phase change thermal interface
They have different requirements and different behaviors.
TTFN
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RE: phase change thermal interface
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.
RE: phase change thermal interface
Phase change materials change phase, i.e. melt, during normal operation. This is why they cannot do what you imagine.