One of the most critical workflow items in producing your own PCBs is the soldering method. There are a number of approaches that work for basic PCBs: you can assemble fully by hand with a soldering iron and wire solder, or you can use paste and a reflow oven or hotplate. I had a toaster oven set aside for conversion into a reflow oven, but it was rather bulky, and in my small shop this was weighing on my mind: how can I justify the amount of space this thing is going to take up for something I use relatively sparingly? The idea of a hotplate for basic SMD soldering sounded great, especially if it were tiny and easy to use, maybe even the ability to run soldering profiles (preheat, soak, reflow, cool, etc). There are a couple decent size mains hotplates on Aliexpress, but still somewhat expensive with shipping, plus they can't run profiles without a controller upgrade. There is the really awesome MHP30 from Miniware, but I felt this was maybe too small for what I need.

I saw a couple projects on twitter using newly-available 'PTC Heating Plate LED Remover' hotplates on Aliexpress. This seemed like the perfect upgrade for a cheap, compact, and reliable hotplate assembly to reflow boards at home. While these plates are capable of being plugged directly into mains, I wanted something slightly safer and more robust. I came up with this block diagram for a relatively safe system using a Solid State Relay (SSR) to control mains to the hotplate, a thermal fuse to prevent runaways, and a thermistor to read temperature. I could use a PID or Bang-Bang control loop on a microcontroller to regulate the hotplate temperature, and even run profiles. With this I started work on a breadboard prototype

Block diagram of PCB and mains wiring

Getting Pico and screen working with Arduino

SSRs and hotplates came in, along with finished breadboarded circuit

Adding a 3D printer thermistor to the heating plate

To be able to hold the hotplate while it was in use with this prototype, I threw together some leftover 20x20 aluminum extrusion and some printed parts. The FreeCAD source files are available in the GitHub if you'd like to use them, I will definitely be updating them as I work on the PCB.

FreeCAD model of prototype

For the prototype of the mains wiring, I grabbed an IEC connector out of an old projector, added an inline mains fuse, and grounded the hotplate with a grounding wire, the latter two mostly for safety reasons

Underside showing wiring to IEC connector, SSR, and fuse holder. Fuse is being held between the bright red wires, clipped to frame with another printed piece

Fully assembled with breadboard prototype controller

The code as it currently exists only reads temperature and looks for a button input. As I further refine the code I'd like to be able to set temperature, detect safety events, and run soldering profiles, but with that minimal amount of coding I was at least able to test functionality: I cobbled everything together and gave it a test to see if it worked

Reflowing a scrap board to test

Looks like flowed joints to me!!! Now I just need to get better at paste application and placement

So obviously I'd like to get some PCBs made up to get rid of the breadboard controller, and if possible I'd love to have the hotplate controller reflow its own PCB! I designed this board in KiCAD to replicate everything that's on the breadboard, with a couple extra useful features (SSR LED, optional thermocouple input for the future). The KiCAD source files are on Github and is fully open source, CC-BY-SA.

So am I entering this contest so I get my most expensive BOM item for free? Yes, definitely! Do I hope that others try to build this and create awesome tools to help them work better? Also yes! I hope that you enjoyed reading about this project and can use it as inspiration for more cool stuff to build!