Electronics > Metrology
PCR versus TCR
zlymex:
I'm wondering how your ‘naked’ R-T curve is measured.
Zeranin:
--- Quote from: zlymex on April 23, 2016, 02:10:40 am ---I'm wondering how your ‘naked’ R-T curve is measured.
--- End quote ---
I have not personally measured the 'naked' R-T curve for the zeranin, firstly because it came to me already bonded to an aluminium substrate, and secondly because the only R-T curve that matters in this case is the curve measured with the zeranin already bonded to the substrate, as that is how it will be used. When I speak of my measured R-T curve, I mean the curve with the zeranin already bonded to the substrate, and mounted in the current driver, clamped against the constant-temperature heatsinking aluminium plate. I explained how my R-T curve was measured, in my posting #13, the relevant part copied below :-
Let me explain how my R-T curve was measured. As explained, the zeranin sheet is pre-bonded to a 1.6mm thick aluminium substrate and clamped/bolted down to a Peltier-temperature-controlled 12mm thick heatsinking aluminium plate. This zeranin shunt is the current measuring element in a precision current driver circuit, that for measuring the R-T curve is set to produce a steady 16A. In series with the same current is my massive (100A rated) Leeds&Northrup (L+N) naked master-reference-shunt, temperature controlled to 0.02K. If the resistance of the zeranin shunt in the current driver should change, then the controlled current changes, and this is detected via the voltage across the L+N master shunt. The zeranin shunt is clamped down onto the temperature-controlled aluminium plate with a 12mm thick plate of copper, and a thermistor is embedded deep within this copper plate, essentially touching the zeranin sheet, so the thermistor therefore accurately measures the zeranin temperature, say within 0.1K. The heat flux flows from the zeranin to the temperature controlled aluminium plate, with a thermal resistance of around 0.01K/W, so at full dissipation of 25.6W, the zeranin temperature rises by ~0.25K with respect to the 12mm thick aluminium plate to which it is clamped, not much at all, and completely negligible on the R-T curve. Measuring the R-T curve is real easy. The heatsink temperature (and therefore the Zeranin temperature) is ‘dialled up’ on the Peltier temperature controller setpoint to values between 20 and 50 DegC. At each zeranin temperature, the voltage across the L+N master shunt is recorded, being an accurate measure of the zeranin resistance.
The 'naked' R-T curve can be found on the website of the zeranin manufacturer, Isabellenhutte. In this case, the resistance of a naked sample of zeranin is accurately measured as a function of the zeranin temperature, probably with the zeranin sample in a variable-temperature controlled oven.
Zeranin:
All of the discussions here predict that my PCR problems scale with the rise in temperature of the zeranin foil, and that therefore I should be able to reduce the problem by reducing the thermal resistance from foil to constant-temperature heatsinking plate.
To this end, I have ordered an 18” x 18” sheet of 0.003” thick Tpcm 580 Series Phase Change Material, part number Tcpm583, manufactured by Laird Technologies, priced at US $49.81 from Digikey, the highest performance thermal interface material that I have been able to find.
I have started a thread about on the Technical Stuff forum for a general discussion about the best thermal interface materials available. That thread has the calculation showing that the temperature rise of the zeranin should be reduced from ~0.25K down to 0.037K, a x6 improvement.
I will report back on this thread as to what improvement this gives to my 5ppm drift in shunt resistance, after the current is switched from zero to 16A. I’m not expecting a x6 improvement because there are other imperfections such as the constant temperature plate is not at exactly constant temperature under transient conditions, and there is still a thermal ‘contact resistance’ on each side of the thermal interface material, but I do expect a measurable improvement in my PCR induced drift of 5ppm. The result will be interesting.
sarepairman2:
:scared:
Zeranin:
There is a wise old saying, that if something appears too good to be true, then it probably is. As it turns out, the Laird Tcpm-583 thermal interface product that I ordered is utterly useless, being merely an expensive and inferior alternative to thermal grease.
Despite being an electrical insulator, this material (as it turns out) is apparently not intended to provide electrical isolation, though nowhere do Laird actually say this. The ‘film’ is supplied between 2 transparent protective plastic sheets. To use the material, you peel off the top plastic sheet, then press the sticky surface of the grey ‘thermal interface material’ onto one surface. Then you peel off the other protective plastic sheet, and finally, press your second surface down onto the tacky grey material. The interface ‘film’ has no mechanical strength or integrity whatsoever. I tried a 25mm square test sample of the material, clamped between 2 metal surfaces and heated to 65 DegC, and just as predicted, a few 10’s of volts was enough to break down the film, with the 2 metal surfaces thereafter shorted together. Oh, and once assembled, you can’t get the surfaces apart again, another great ‘feature’ of the product.
Apparently, the purpose of this material is simply to provide good thermal contact between two metal surfaces, but provide no electrical isolation. That being so, it is difficult to comprehend why they make the material electrically insulating in the first place, as better thermal conductivity can be obtained by using thermal grease loaded with microfine copper or silver particles. Furthermore, a metal-loaded grease will provide a thinner interface, as the excess grease is squeezed out under pressure, while with the Laird product you are lumbered with the thickness of the thermal interface material, 0.003” in this case, and much thicker again for most of their Tcpm-580 products. No thanks.
In summary, what we have here is an expensive and inferior alternative to a high-quality, metal-loaded thermal grease. I am extremely unimpressed with both the product and the company. As described in detail in a previous posting, the published thermal resistance specifications are nonsense. I sent two emails to Laird, to 2 different enquiry email addresses, seeking clarification, and still have received no reply to either. Their documentation, product, and customer service are all complete rubbish as far as I am concerned, and I won’t be buying anything from Laird again. I am seriously unimpressed. >:(
My conclusion from all of this, is that the only advantage of the modern thermal interface materials is ease of use. Sure, some of the modern insulating interface materials have quite good thermal conductivity (W/mK), but in order to get useable mechanical integrity and electrical isolation, the thickness is so great that the resultant thermal resistance is at best as good, and generally inferior, to the old-fashioned approach of using a thin film of Kapton or similar, with thermal grease. My original plan was to use the thermally conductive grade of Kapton, known as ‘Kapton MT’, and it looks like that would still be the best solution. The only reason that I didn’t do it that way in the first place was because I could not obtain a sample sheet of Kapton MT bigger than A4 size, which is too small.
Maybe I need to spend $500 or so on to get the minium order quantity of 37 mictron Kapton MT. At least I would have plenty left over for the next job.
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