PTC Heaters For Reflow Soldering

Reflow soldering – setting components on a PCB in blobs of solder paste and heating the whole assembly at once to melt all joints simultaneously – has been the subject of many ingenious hacks. Once it was the sole preserve of industrial users with specialist microprocessor-controlled ovens, now there are a myriad Arduino-controlled toaster ovens, hot air blowers, and hotplates that allow hackers and makers to get in on the reflow act too.

This morning a fresh idea in the reflow soldering arena has come our way. It’s not the most earth-shattering, but it does have some advantages so is worth a second look. [Analog Two] has successfully used a PTC heating element as a reflow soldering hotplate.

PTC heating elements are thermistors with a positive temperature coefficient. As their temperature rises, so does their electrical resistance. By careful selection of materials they can be manufactured with a sharp increase in resistance at a particular temperature. Thus when an electrical current is passed through them they heat up until they reach that temperature, then the current decreases as the resistance goes up, and they do not heat beyond that point. Thus as heaters they are intrinsically self-regulating. From our point of view they have another advantage, they are also cheap. Fitted as they are to thousands of domestic heating products they are readily available, indeed [Analog Two] found his on Amazon.

The heater chosen was a 200W 110V model with a temperature of 230 Celcius to match the solder he was using. They are also available for other mains voltages, and even at 12 and 24V for automotive applications. He reports that the time to reflow was about 90 seconds.

We’ve mentioned the advantages of this heater as its price and regulated temperature. Looking at the pictures though a disadvantage is its size. This is a reflow plate for small boards. There are larger PTC heater elements available though, it would be interesting to hear people’s experiences reflowing with them.

Hotplates for reflow soldering have featured before a few times here at Hackaday. We recently had this tiny plate, but we’ve also had a PID-controlled plate, and an Arduino-controlled domestic hotplate. We’re sure this is an avenue with further to go.

12 thoughts on “PTC Heaters For Reflow Soldering

    1. Material used in PTC thermistor is its own control loop. When it reaches operating temperature its resistance rises so much, that current flow is limited and it starts to cool down. When that happens, resistance drops, current flow is increased and element heats up again…

  1. But this is just bang! up to the reflow temperature directy – no phases where moisture is driven out and where the fluxes can do their thing. I’m sure it kinda works but so does throwing the pcbs on the stove and turn it on :-)

    1. You could buy a couple PTCs of different temperature, then run the boards along them like a conveyer belt (somewhat emulating thermal zones of a real reflow oven). I’m not sure that this would work properly since the author said he had to push the boards to make contact with the PTC heater (i assume for better conduction).

      If you attached the PTCs to a metal plate at different spots, you could get a proper temperature curve (maybe)? Then push the PCB across the plate very slowly (and scratch up the bottom surface, or maybe lift and move).

  2. I think a more interesting use of these heaters would be as a pre-heater – you bring up the board to 100-150C and then finish the job using e.g. hot air. The heaters come in various temperatures, from about 50C all the way up to 270C which is good enough to melt even lead-free solder.

    The only pity is that these heaters are quite small – the largest I have seen was 100x30mm, so to make it more useful, two or three side-by-side would be needed. If there was one e.g. 200x200mm, that would be excellent. Even to make a heated bed for a 3D printer …

    1. What bugs me is when people say “a myriad”. Myriad literally means ten thousand, it’s not a collective noun. Colloquially it means “many” or “lots”.

  3. One might consider placing a $0.50 ceramic tile between the heater and the board. Enough thermal mass to slow down the heating process, better fitting the reflow curve; and also spreads out the heat through the whole tile more evenly so you don’t end up with a hotspot that melts the solder while the rest is still cool enough to touch.

    Might be a good idea to put on some kind of aluminum foil tray so it’s easy to pick up and move off though.

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