Low Cost DIY Automatic Reflow Hot Plate

Do you believe that I made a fully automatic reflow hotplate for just 8USD? (MCU excluded, of course :D)

I was working on my new DIY NAS project and I would need a custom SATA to USB adapter being made out of SMT parts. However, just before I start working on the design, I notice I have no tools that can be used to do reflowing. As I am just a poor university student and can't afford anything good enough for my purpose, I decided to make myself one.

I saw a few Youtubers actually uses 110 / 220V AC heat plate for the job (by modifying a clothes iron). The decision of making it fully DC is because I do not have the required knowledge to work on main voltages.

And there are also designs that uses PCB as reflow heat elements. The reason I am not going for PCB reflow hotplate is that the lifespan of the PCB is gonna be low as it pass through so many thermal cycles. As I need something relatively reliable for my development and I dont need to replace it every few uses, I decided to stick with a ceramic heater element instead.

• Mechanical Thermal Switch x 2 (150°C + 230°C, depending on the solder paste reflow curve)
• Ceramic Heater (24V 5A)
• LEDs
• Power Switch
• 24V 6A power supply
• Buck converter (I am using the cheapest one I can get from the market)
• Thermal conductive glue ( Max 280 °C)
• Silicon sealing glue (Max 250 °C)
• Seeed XIAO BLE (LINK: https://www.seeedstudio.com/Seeed-XIAO-BLE-nRF52840-p-5201.html, or any other MCU that can handle 85°C or above for > 3 minutes)
• Container
• 5.5 x 2.1 power input jack
• Cardboard (Max 420 °C)
• Kapton Tape

The total cost of materials are under 8 USD (excluding the MCU) which is perfect for poor Makers like me :)

The most important thing for this project is the mechanical thermal switch. These two switch are designed to follow the reflow curve of the solder paste you choose and allow switching off of the heat plate when it is too hot.

As I am using Sn63Pb37 as my solder paste of choice, according to the reflow profile, it should be around 150 °C when reached the soaking zone (or what I call it the pre-heating zone) and 230 °C when reaching the reflow zone. So for this reason, I picked the two thermal switch for150°C and 230°C.

These mechanical switches usually cost around 1 - 2 USD each and they are not that accurate. I bought a few of them and test all of them with the help with a digital thermometers so that I can use the one that switches closest to the aiming temperature.

This was the original circuit design of the hot plate. I modify a bit during my build to fit the real world situation. Notice there is a 280 °C thermal switch on the right side that suppose to act as a thermal fuse. However, eventually I cannot find a suitable place to fit it in so I didnt bought it. The Arduino (later I switched to Seeed Studio XIAO BLE) is for controlling the two relay so the thermal switch can transit from soaking zone to reflow zone.

Assemble the reflow hot plate following the circuit diagram above. The two LED are connected to the GPIO of the XIAO BLE D3 and D4 for signaling the current stage (soaking or reflowing)

You might notice that I switched out the variable resistor for the buck converter, as it was due to it accidentally got molten during earlier tests. You can replace it or simply snap in a LM7805 LDO for the same purpose.

Assembling the heater element is a bit tricky. The side wall in brown are cardboards wrapped in Kapton tape, a tape that is specially designed to withstand high temperature. The Kapton tape wrapped cardboard formed a square with a corner cutted out and rotated 45 degree. The heater element with two mechanical thermal switch is then fitted in the center with all the wires coming out from the cut out of the cardboard support structure.

The four corner where the heating element touches the cardboard is secured with silicon sealing glue that can withstand 250 °C. Just below what we need for reflowing.

In order to make the hot plate follow the reflow curve automatically, we need to control the timing of when the relay is switched on and off. I need something small that can fit inside the case as well as thermal resistance so it wont die or slow down deal to increasing temperature after a few reflow cycles. This is where I tried a few MCU from different manufacturers.

• Seeed Studio XIAO BLE
• ESP8266 (NodeMCU)
• CH552G

The CH552G and ESP8266 got some minor issues when I try to reflow the 3rd or 4th cycles including slow down or unstable. However, XIAO BLE works under the extreme working environment and survived multiple reflow cycles. That is why I am keeping it for this project.

For more information on how to setup the XIAO BLE board, take a look at https://wiki.seeedstudio.com/XIAO_BLE/.

Here attached with the code for controlling the reflow hot plate. Note that depending on your mechanical switch and solder paste speficiation, you might need to adjust the timing in the code.

The wiring of the MCU goes as follows.

Soaking Relay Toggle Pin --> D1

Reflow Relay Toogle Pin --> D2

Waiting LED (Yellow in my case) --> D3

Heating LED (Red in my case) --> D4

The program will go through the following cycle

1. Countdown -- Yellow light blink for 3 seconds after power on
2. Soaking Heating -- Heating up to the soaking temperature, Yellow LED constantly on
3. Soaking Wait -- Temperature reached soaking temperature, Red LED start blinking
4. Reflow Heating -- Heating up to reflow temperature, Red LED constantly on
5. Reflow finish -- All relay is now off, Red LED blinking slowly

After finish with the programming, you can close up the case by screwing in the screw at the bottom of the case. I also added a C shaped 3D printed fence to prevent myself accidentally touching the cardboard. As I am expecting the cardboard become fragile after a few more thermal cycles and touching it might breaks it.

On the front side, we got a power switch and two LED indicating the heating status. Also a 2.1 x 5.5mm power jack for powering the reflow hotplate via an external power brick.

Next, you can test out the reflow curve and record the time VS temperature curve by recording a dry run reflowing. This can help fine tune your heating time and make the temperature much more accurate. From my test, I am getting a temperature curve in blue (See the photo 3). It is not perfect but it is close enough for my purpose.

Lastly, you can put some solder paste on your PCB then place your favorite IC onto it.

After a few practices, I finally got the amount of solder paste right and the perfect reflowed results shows up like magic. Now, you have your own ultra low cost, fully automatic reflow hotplate ready for your next DIY electronic project!