Toaster Reflow Oven (Early 2021)

I woke up one day with a realization: I need a solder reflow oven as I'm moving more towards surface mount components. The only issue was that a commercial one would've set me back 250-1000 George Washingtons. I began designing one with one constraint: make it for less than $100. I got a toaster oven from Target for a ridiculously cheap price of $20. I basically needed a heating element inside of a metal box with no other fancy controls.

The plan was to control the heating elements with a solid state relay (cheaper and easier to at the time use than a mechanical relay) and have feedback from a K-type thermocouple fed into a microcontroller which could follow solder paste temperature curves.

You can see the solid state relay on the right attached to an old CPU fan I had lying around. In addition to the electrical components, I added some ceramic insulation in order to mainly avoid burning myself, but also to allow the oven to heat up more quickly and retain its heat.

Here's a front view. I put in an OLED screen to display relevant information. The tinfoil on the door is for added IR insulation, which surprisingly made a huge difference in how fast the oven got up to temperature.

Here's a test run of the oven on an extra PCB I had. You can see the exact moment the reflow occurs, which is pretty cool.

The code is straightforward; upon booting it will wait for a button to be pressed, after which it will follow a predefined temperature curve. The curve is defined by a series of temp vs time 2D lines. The microcontroller will calculate what temperature it needs to be at for the current time and adjust the heating element accordingly. I didn't know what a PID loop was at the time, but I essentially implemented one.