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I have a really bent heatbed PCB, the middle is elevated about 3 mm with respect to all edges.

I have found this thread Warped PCBs, where a heating method is applied by baking a PCB in the oven, as described here: 3.2 Bow and Twist Repair.

Can this help straightening out a Prusa heatbed PCB? If so, can I apply the heat by the heatbed itself, or do I need to utilize an oven? Will the pressure from the strongly clamped glass plate be enough or will the glass break at these temperatures (given that the heatbed can reach them).

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Baking PCB in an oven is not a good idea, I would say. I know PCBs are resistive to the heat (especially heatbeds) but, still, it sounds odd. But the real question is how baking would help. Let's leave it.

If your heatbed is such bent you can do few things depending on your situation/environment.

You can:

  1. Add two glass plates (at the bottom and at the top) and clip them all together;
  2. Support your HB with flat aluminium or even wood;
  3. Add an aluminium frame, or;
  4. If you use glass plate, clip the HB to the glass using stronger (wider) clips.

Ad#1 the thermistor can even stay sandwiched between the PCB and the bottom glass plate.

Ad#2 If your HB is bent up in the middle, you can use the middle thermistor hole. Drill it a bit with fi8mm drill (but not too much, I would say halfway through) and use a cone head screw to screw it flat on to the aluminium/wood support. Of course you will have to install a thermistor into the new place.

The simplest, and less destructive, solution is #1 and I would recommend that.

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    $\begingroup$ Some feedback after some time, without using the heatbed too much. The bend didn't go away so far, but with the bulldog clips I use now (it was a weird temporary solution before), the bed straightens out enough under the glass sheet. This solution is good enough for me. I will report if the glass suffers a total loss at some point. $\endgroup$ – kamuro Jun 1 '16 at 9:19
  • $\begingroup$ While the suggestions in the answer are OK, the premise (that "Backing PCB in an oven is not a good idea") is factually wrong, given that is exactly as PCB are manufactured, and how they are fixed when surface mounted components become loose. In fact you can most definitively use heat to straighten up a PCB, see this other answer: 3dprinting.stackexchange.com/a/5682/9134 $\endgroup$ – mac Mar 24 '18 at 15:07
  • $\begingroup$ Agree with Mac on this. PCB heat beds are essentially PCBs and they can be put in ovens, baked at lower temps (say around 100 deg C) and can withstand temperatures up to 240 deg C (for less than 30s). That’s is how components are soldered to the PCB. This of course requires a reflow oven or a temperature controlled skillet. $\endgroup$ – electrophile Mar 25 '18 at 3:21
  • $\begingroup$ AFAIK the question was - "could HB be straightened out ?" I did answer how it can be straightened out and in the first sentence I did mention that PCBs are heat resistive. I also asked how baking could help to straighten it out if his HB did bend after making it hot so how it could go the opposite with the same action? I don't see ANY reason to downvote. $\endgroup$ – darth pixel Mar 25 '18 at 13:08
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Electrical engineer here. There is nothing odd about putting a PCB in an oven. Any surface mount PCB is assembled using a reflow oven which heats all the components, as well as the PCB itself, several degrees for quite some time up to the 'soak' temperature, which is 150 °C.

After that, the PCB (and components, still not soldered but held down with solder paste) are heated up to the reflow temperature, which for lead-free processes (i.e., all of them) is 245 °C. After this point, they are held at that temperature for 60-90 seconds to allow the solder to completely reflow. Note, this is not time spent with the oven temperature that hot - this is time the entire PCB, which edge to edge has reached 245 °C, spends at that temperature.

There is absolutely nothing wrong with, nor anything odd about putting a PCB in an oven or otherwise getting it hotter than it's glass transition temperature. Doing so is a key part of their commercial assembly into electronics and has already been done to every PCB you own.

Please ignore the 'accepted' answer - it's incorrect.

PCBs are usually made out of glass epoxy composite (FR4) with a glass transition temperature of 140°C. There are high heat varieties with a glass transition temperature of 170°C. I do not know which variety the Prusa MK2/MK2S uses specifically, but FR4 doesn't have a 'sharp' glass transition anyway, so you want to err on the side of hot regardless of the specific type. Get it nice and cozy at at 190-200 °C, and increase the temperature until it unwarps. The solder's liquidus is about 220°C, so you should try to keep 20 °C shy of this just because kitchen oven thermostats are not to be trusted.

I own a MK2S, and have personally heated my heat bed up to above the glass transition temperature of PEI (don't ask) which is 217°C, and this was not a problem (well, except for the PEI but again, don't ask). It didn't suffer any ill effects, nor did I expect it too. I'm printing on it right now, as I type this. It's perfectly fine to heat it up that hot. That was the entire reason FR laminate was even made - to be rigid and to tolerate being hot. Solder ain't gonna melt itself.

Note, however, you will need to remove the PEI/Ultem sheet as well as the adhesive before you do this. Prusa has instructions on how, look under replacing the PEI print surface.

Now, as for the actual procedure, the pcb isn't going to magically just become flat. You have to force it to flatness. Flatten it into submission. It will only be as flat as the hopefully flat surface it is resting on. And, given the gradual transition of the material, it isn't going to exactly become super pliable even when a good deal past its glass transition temperature.

What you ideally want is two plates of aluminum large enough to sandwich it between. Or steel, or any metal. They need to be thick enough to not flex so they are very flat. A pizza stone, if you can find one that is flat, or a slab of granite also works. I know none of these are exactly 'just laying around the house' type objects, but really any flat surface that will tolerate the heat that you can rest the pcb on is acceptable. (Kids - ask your parents first)

The only problem is it will likely not unwarp itself under its own weight. You'll need to put a large flat thing on top of it too, and either have it be heavy, or add heavy things on top of your large flat thing. You want to make a heat bed sandwich. Also, it needs to be shielded from the oven heating elements (be they electric or gas), as these will radiate heat a lot hotter than the oven temperature when they turn on. Don't worry - aluminum foil is enough to shield anything from the heat, but again, you should have something very flat and substantial on top.

Oh, and remove the screw hole sockets from the PCB. They just need a hex key and some pliers to unscrew. Don't lose the lock washers.

Anyway, the above procedure, while definitely a pain, will restore (or if it was always warped, imbue for the first time) flatness as good as the flat things it is sandwiched between.

The flatness of kings.

This isn't exactly unknown either. People do it. It works.

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  • $\begingroup$ YT shows that you can heat up composite material and bend it (in this case make it flat). But on the YT movie we can see a brand new HB. There is no info about it after using it for some time. If there is internal tension - it's possible the HB will bend itself again when heated up without pressure. $\endgroup$ – darth pixel Mar 27 '18 at 7:03
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Since aluminium has a melting point at 600°C, you're unlikely to get it to a state where it flexes much by heating. I would doubt you even manage to aneal it (release the internal stresses).

Your best option is to construct a jig which allows you to press the distortion out (or pull, using a screw in the existing holes), whilst also taking care not to apply pressure (at least with a sharp edge) to the tracks or coatings.

This will only be a partial fix, but might be worth trying with a glass sheet in addition.

Edit: This might be relevant for a Mk3 heatbed, but not the Mk2 as asked in the question.

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  • $\begingroup$ I am not completely sure why you bring up the melting point of aluminium, can you elaborate on that? $\endgroup$ – kamuro Nov 29 '16 at 13:27
  • $\begingroup$ You have a bent metal plate, which you're trying to make less bent. In the case of a warped PCB, heating will soften the resin. The same is not really true in your case, I think - or maybe I mis-understood the question, and it is just a bare PCB you have? $\endgroup$ – Sean Houlihane Nov 29 '16 at 13:36
  • $\begingroup$ Yes, - sorry, I could have a) understood what you meant (MK3 type PCB heatbed) or mentioned that I only use a MK2 type heatbed PCB as portrayed in reprap.org/wiki/PCB_Heatbed $\endgroup$ – kamuro Nov 29 '16 at 13:39
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Easiest way would be to place a glass sheet on top of the bed and run a calibration... Although, that way you may lose a few millimeters of print height.

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