Using Kapton with phase change layer - Is it as great as advertised in papers ? - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 02, 2020, 08:56:41 am
content: Hi folks,I have a question, have you experience with Kapton film pads with phase change layer ?Like this DiaPhase60They are advertising about 0.13 K-in2/W for thinnest versionThis should give me about 0.2 K/W at TO247 packageIs that realistic or too optimistic ?

author: Siwastaja
author_rank: Super Contributor
timestamp: April 02, 2020, 09:30:44 am
content: Note that people here have reported bad experience using very thin layers of Kapton for thermal insulation, because any surface defect on the heatsink or component punctures through the thin Kapton layer, given some time and vibration. I wouldn't use the 0.025mm version at least unless I could guarantee perfect rounding and deburring of the device edges, smooth surfaces, balanced clamp force, and the cleanliness standard of the assembly. A tiny little metal dust particle is everything that's needed to cause a nasty, latent damage.Thin kapton would offer more safety, but the thermal conductivity starts sucking. At 0.46W/(mK), the Kapton kind of sucks if you want to have the physical distance (and not rely on the perfect insulation properties of the thin, undamaged material). Note that classical enforced, tough thermal pad materials exist up to about 3 W/(mK), so they could be 6 times thicker for the same thermal resistance.

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 02, 2020, 09:40:20 am
content: For mass production it can be a problemThis I want for single piece prototype where I can polish devices and heatsink surfacesMain problem is I want to cool 150W in TO247 case IGBTs what is on extreme side, and should last about 1000 hours on bench

author: H713
author_rank: Regular Contributor
timestamp: April 05, 2020, 08:35:45 am
content: I used .001" Kapton film for my HV bench supply. It works great, but it's quite tricky to use. The film is incredibly fragile, so careful prep is necessary. I ended up polishing the heatsink surface and the transistors to make absolutely sure there were no sharp edges. Be incredibly careful applying the compound- it MUST be clean. A little spec of dirt in the compound from being careless is enough to puncture the Kapton.When I blew up the supply a few weeks ago, I used the white ceramic insulators. I don't remember if these particular ones were Beryllium or Aluminum Oxide, but they have great thermal properties. Significantly better than Mica.Keep in mind that Kapton is a rather lousy thermal conductor, we just use it for insulators because we can make it so thin.

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 05, 2020, 09:59:24 am
content: Aluminum Oxide ceramic pad must be 0.5mm thin to reach desired conductivityThey are not commonly available

author: Siwastaja
author_rank: Super Contributor
timestamp: April 05, 2020, 10:16:25 am
content: Other options:1) Spread the heat over larger area by soldering (or other ways of non-isolating mounting with very low thermal resistance) the device to a copper heatspreader (like a 50x50x5mm copper block), this directly reduces the thermal requirements for the isolating material by increasing contact area, so very typical 2-3 W/mK 0.5mm thick or so silpads could be used.2) Throw more silicon at the problem. If the IGBT is of easily parallelable positive Vcesat thermal coeff type, this would be obvious. Yes, it increases the silicon cost, but by cutting the per device dissipation, you might even be able to reduce heatsink size (and cost!) and run the heatsink hotter while maintaining acceptable Tj. Not saying this is necessarily the case here, but I do see the trend that today, silicon is often cheaper than aluminium, so to speak!My go-to strategy nowadays seems to be, optimize for highest efficiency, use more silicon, and/or better silicon; power not dissipated at all is easiest to heatsink! Energy savings are a bonus.I'm mentioning the other options because having to think about the isolation layer this hard may be a sign that you are running pretty close to the limits.

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 05, 2020, 10:36:32 am
content: Copper heat heatspreader sounds like interesting alternativeParalleling at device level dont look like easy way as this is non insulated boost and inverting buck-boost for rather high current and speed switched at 60kHz or even higher at lighter loadAnd switching losses arent shared evenly without some additional skew controlI already have it as 2 or 3 phases of buck boost followed by 1 or 2 phases of boost converter (with output to 3kW 400V load resistor)If I can get to 0.2 K/W for TO247 package then I can use just 2 and 1 phasesWith about 0.3-0.4 K/W I will need to use 3 and 2 phases

author: Siwastaja
author_rank: Super Contributor
timestamp: April 05, 2020, 11:51:21 am
content: Adding phases is usually a good idea, I did exactly that for a 22V->75V 750W boost converter. After doing the full math with 2 phases, I realized the need for special cooling efforts, but with 4 phases, I could handle it with standard 4-layer board and standard SMD assembly, with a cheap thermal pad and standard heatsink on the bottom of the board. Efficiency went up only very slightly, but the heat is now over a larger number of semiconductors (similarly, inductor losses were spread out to multiple inductors, as well!). Reduced ripple on the capacitors is an added bonus. With full phases, no issues arise from the paralleling, but yes, the semiconductor cost was slightly higher because two 2-phase DC/DC controller ICs were needed instead of 1.From the design time perspective, duplicating a properly designed phase is very quick work.