Peltier cooler failure modes - what are they? - Page 1
Forum section: electronics / repair

author: solar
author_rank: Newbie
timestamp: January 27, 2025, 12:06:53 pm
content: I have two old cool boxes that use standard 12 volt 40mm square Peltier coolers. One is marked TEC1 12705, which is a very common 30 watt unit.One does not conduct until around 25V is applied briefly, which kicks it into life and then it works fine at 12V. Is it possible that a small crack has developed that, once the higher voltage is applied to jump the crack, the ensuing heat closes the gap to start full operation?On the other unit it worked fine initially but I think I misused it by trying it briefly without a heatsink. It then behaved as above but now has completely failed. These links sort of imply that the above may be true but they don’t mention kickstarting it with high voltage:https://www.sameskydevices.com/blog/reliability-considerations-for-peltier-modules#:~:text=The%20most%20common%20failure%20mechanism,series%20resistance%20of%20the%20device.https://thermoelectric-coolers.com/technical/trouble/#:~:text=Along%20with%20cracks%20developing%2C%20the,cause%20the%20breaking%20of%20wire.Have ordered new Peltiers. I guess I need to be more careful to not run them without a heatsink and to use even clamping forces.Is there anything special about the heatsink compound to use as I just have Servisol’s white stuff, those conductivity may not be the lowest, at 0.77W/mK. But from RS you can’t get anything much better. 0.4W/mK is their best and is pricey. The original compound seems to have a high grey mineral content that is hard to wash off ones hands; some sort of ceramic perhaps.

author: Gyro
author_rank: Super Contributor
timestamp: January 27, 2025, 02:22:33 pm
content: Welcome to the forum,Well in terms of failure modes, you seem to have found most of them, both in your links and first hand! The individual Peltier elements are soldered to the copper traces on the two ceramic outer plates using BiSn solder. This has a melting point of a mere 138'C so running the module without heatsink or clamping force will very quickly result in joints opening. The thermal cycling failures are a very real phenomenon as is the danger of corrosion on the inside of the cold surface, you can get elements that have a white silicone compound around the edges to reduce moisture ingress, together with modules that use a higher melting point solder.In terms of mounting, the ceramic plates are very smooth and hopefully flat, so the main issue is the quality of the mating surfaces in your cooler. They should be flat, without excessive milling marks. Clamping screws should ideally have some form of spring washer and should certainly be tightened in stages, in a diagonal pattern. They should be tightened to the point where they hold the module in firm thermal contact but not enough to risk crushing the module. Vibration is an issue too - if you have the cooler in the back of a car for instance with the heavy heatsink and fan sitting on the back of the cooler just held by the tension of 4 screws and the compression of the module. Higher quality coolers probably do a better job than cheap ones in terms of additional support. As far as heatsink compound, it really doesn't matter that much. It is just there to fill microscopic air voids in the surfaces, so should be applied very thinly and evenly. Most people overdo the quantity, which actually increases the thermal resistance and doesn't spread evenly under the limited clamping pressure of the clamping screws and large surface area. These are things that get little care during low cost manufacture. The heat density of these things is much lower than, say, a PC CPU, so you really don't need anything exotic.At the end of the day, Peltier modules will fail in this sort of use. They're not exactly pampered with things like controlled current and close loop monitoring, get bashed about and experience mechanical shock etc. So you need to be prepared to replace them occasionally.

author: solar
author_rank: Newbie
timestamp: January 29, 2025, 10:54:00 am
content: Many thanks for the reply. Great to finally find a forum that delivers some good responseThe coolbox that initially failed had been left in a very hot car in the sun. It was not very new, so maybe the extra heat finally accelerated some incipient degradation. These Peltiers are cheap so I don’t expect they are the finest quality, despite the edging of silicone sealer.The heatsink surfaces are quite smooth, being milled extrusions, but there are some milling marks, which don’t show up well in this photo due to heatsink compound residue. They only use 2 screws to clamp it and I guess that’s not really enough to give even clamping. Difficult to add more screws.

author: Gyro
author_rank: Super Contributor
timestamp: January 29, 2025, 07:17:53 pm
content: You're welcome.Two screws is probably pretty common, but not ideal for vibration resistance, I can't see how you could add any more either though. Running it a very hot car could certainly have been the failure trigger, Peltier elements are really quite inefficient and generate a lot of heat compared to what they are pumping. Blowing hot air over an already hot heatsink could well have pushed it over the (138'C+) edge. The thermal switch on the block wouldn't have been any help, irrc, that is only used when the box is in heating mode.One thing that might help is to use spring washers under the screw heads and don't bottom them out. There will be a difference in thermal expansion coefficient between the thick Aluminium block and the steel screws that might possibly cause quite a variation in force across the thickness of the element if there is no compliance in the system. You might need to combine it with a small amount of threadlock to stop the screws loosening though. It will also help with getting the clamping more even - at leastthatis easier with just 2 screws!P.S. It looks as if the manufacturer was a bit generous with the heatsink compound (as expected). It only needs to fill the milling marks plus a tiny fraction more.