Skillet Reflow Controller

Using an electric skillet to reflow surface mount circuit boards is a popular alternate use for those kitchen appliances. The real trick is monitoring and controlling the temperature. [Mechatronics Guy] built his own skillet temperature controller using a thermistor, a solid state relay, and an Arduino.

He was inspired by [Ladyada’s] work which used a servo to adjust the temperature dial on the skillet’s power supply. This started by attaching the thermistor to the bottom of the skillet using JB weld. since this area will be heating up he also attached a terminal block for connecting the feed wires as the heat would melt any solder joints. Those wires travel back to a control box housing the Arduino and solid state relay. To gain finer control over the heating element the relay is switched on and off, resulting in low-frequency Pulse Width Modulation, which should help maintain a consistent temperature better than just turning the temperature dial on the cord.

Pair this up with the vacuum tweezers hack and you’re on your way to a surface mount assembly line. If you want to see this process in action check out this post. It goes from stenciling, to populating, to reflowing in a toaster oven.

[Thanks Rob]

21 thoughts on “Skillet Reflow Controller

  1. @BiOzZ I want to build one. I think it is relatively easy to build a low res one using a thermopile sensor in a scanning mechanism. Problem is that fairly accurate thermopiles aren’t cheap either.

    Any suggestions?

  2. Hi everybody,

    Man, now I wish I’d finished the writeup. Basically I had a hard time tuning it so I ended up using manual control to solder the makerbot boards.

    The problem was that the sensor was located too far away from the heating coils, so there was about a 30sec lag in the control, which made it unstable. This lead to a lot of interesting discussions with control systems engineers about the best way of avoiding oscillation (reducing the feedback rate, using a virtual ‘observer’ to simulate the correct temp and respond off that, mapping the open-loop response and others), but in the end if you’re making your own controller I’d just recommended moving the sensor closer to the coils :)

    (The Mechatronics Guy)

  3. @Hirudinea No kidding, I touched the hot tube of my soldering iron once (I was grabbing it while starring to a PCB) and it was painful enough for me to cover the plastic part with glowing orange so it doesn’t happen again in the future. The burnt bacon toke some months to disappear and even now, after 4 years, I still have a scar in my hand and half of my fingerprint smoothed.

    * Attach a string between the soldering iron and your desk, to prevent falling accidents.
    * Always stare to the soldering iron when you grab and when you store it.
    * Work in a non inflammable zone (my electronics desk is covered by a hard layer of marble).
    * Always use grounded soldering irons, connected to mains switches that are easily accessible.

    Sorry for being such a safety freak, but starters usually don’t care until something bad happens.

  4. string is a good idea, but where I store the cold iron to the workspace is just about darn near the same distance tween my workspace and floor

    I also stare at the soldering iron while moving it( with 2 hands and a stand), but dont zone on it

    anyway good tips, MrX and if I may add one, get used to tiny burns and heat, it happens flux + solder = bacon grease, holding a wire a bit too close = owie, but the natural reaction is drop everything and freak out, which can make things worse

  5. I was building a reflow oven out of a toaster oven and considered this technique. I ended up using a thermocouple, microcontroller, and an SSR to get the job done.
    PS Most thermistors are inaccurate at the temps that it takes to reflow solder. Check your datasheets!

  6. @Bill D. Williams

    Well, I did a bunch of measurements and temp responses to various things. it was reasonably stable at certain temps, but the response time wasn’t fast enough to follow the same ISO standard temperature curves for soldering that I’d seen on the web. (they have ramp up rates, soak times and cool down periods)

    Eventually I ran the soldering with the controller in place, but in open-loop mode. So it’s a digitally controlled heating value that’s more accurate than the onboard themostat. This turned out to be more than adequate for surface mount reflow.

    The controller is made to be totally generic. It has GPO sockets so you can plug any device in, and the thermistor is replacable. So you can make other devices connect in a jiffy.

    I’ll probably make a sous-vide system at some point using the same equipment. But for now I’m satisfied with open-loop for SMD.


  7. I built a reflow oven that I’ve used a couple of times with success. I used the following parts:

    Black and Decker Infrawave oven (ebay 60) — open the unit and just bypass the circuit board so that when it is plugged in it powers the heating elements.

    Stainless steel thermocouple (ebay 5) — basic K type thermocuople in a stainless sleeve with a stiffener so it keeps its shape.

    Auber Instruments PID Controller (ebay 75?) — Basic PID controller that allows you to program a profile, basically an all in one unit that can control an SSR

    25A SSR (bgmicro 10) — This SSR has screw terminals so it is easy to attach to heavy gauge wire for the oven. The PID controller can drive it directly, so it is easy to attach.

    Cheap chassis from Mouser ($10) and it works like a charm. The oven is pretty big and can bake several boards at once. For cooling it down, I usually open the door, even with the ambient air it won’t exceed the profile ramps. I use Syntech LF paste.

  8. Many wire lead thermistor assemblies melt at solder reflow temperatures because they are made with … um, solder. Use a thermocouple. Attach one to your pcb to see what the temperature really is. Still good work. Have fun.

  9. Thermocouples are expensive, reasonably fragile, and very, very annoying. They need dedicated circuitry and calibration, where a thermistor can be dropped into a simple voltage divider and work fine. If you can find a thermistor that is specd for your temperature, use it instead.

  10. yawn, another reflow oven post… clearly, the most interesting part about this is the use of a thermal camera.

    seriously, where are the diy thermal cams? seems like the hacker community hasn’t tried anything beyond just mounting IR thermometers to servos. this is one of those instruments that have hundreds of uses in dozens of industries, but still cost thousands of dollars.

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