DIY Smart Hot Plate for Reflow Soldering

Soldering tiny SMD components by hand is painful. Reflow soldering is the answer, but commercial hot plates can be expensive or lack customizability. In this project, I built HEAT - a smart, microcontroller-based hot plate designed for reflow soldering.

It features a custom PCB, an OLED interface, a dedicated cooling fan, and a unique "Smart Pulse & Coast" algorithm that outperforms standard PID for this specific application.

⚠️ WARNING: MAINS VOLTAGE This project operates directly from 110–240V AC. Building this requires experience with high voltage safety. The heater and heatsink are electrically live. Do not attempt this if you are uncomfortable working with mains electricity.

To build this, you will need the following components.

Electronics:

1. Heater: PTC or Resistive Heating Element LINK
2. Temperature Sensor: 100kΩ NTC Thermistor (3950 Beta) LINK
3. Display: 0.96" SSD1306 OLED (I2C) LINK
4. Cooling: 5V DC Fan (40mm or similar) LINK
5. PCB LINK
6. 3.15 A, standard 6.3 × 32 mm (3AG) cartridge FUSE
7. 3 x Buttons - 5.5mm x 6mm

Hardware:

1. Heater Mounting:
2. 4x M4 × 60 mm bolts
3. 4x M4 lock nuts
4. Case Reinforcement (Embedded):
5. 4x M4 standard hex nuts (inserted into the 3D print) (not required but recommended)
6. Fan Mounting:
7. 4x M3 × 30 mm bolts
8. 4x M3 lock nuts
9. Power Inlet Mounting:
10. 2x M3 × 16 mm screws
11. 2x M3 lock nuts
12. Triac Mounting:
13. 1x M3 × 10 mm bolt (secures Triac to heatsink)
14. Safety Grounding:
15. 2x M4 lock nuts (additional to heater nuts)
16. 2x M4 washers

Before building, it is crucial to understand how this differs from a simple thermostat.

1. Zero-Cross Switching: We aren't just turning the power on and off randomly. We use a Zero-Cross SSR circuit. This switches the AC mains only when the voltage wave crosses zero volts. This prevents electrical noise (EMI) and reduces stress on the components.

2. The Control Algorithm (Why not PID?): I initially tried standard PID control, but the thermal mass of the small plate (30g) combined with the powerful heater (400W) caused massive temperature overshoots. The solution is a "Smart Pulse & Coast" algorithm. It fires heat in calculated windows (pulses) and then waits (coasts) to observe the thermal rise before firing again. This mimics how a human operator might manually pulse a powerful heater.

3. Safety Features:

1. Fused Input: Protects against shorts.
2. Grounded Metal: The heater block is bonded to Earth Ground.
3. Watchdog Logic: If the sensor fails or temperature freezes, the system shuts down.

Material: You MUST use ASA or ABS. PLA will deform under the heat. Recommended Settings: 0.2 mm layer height, 3 walls, 20% infill.

Reinforcing the Case (The "Pause" Trick): For a solid build, you should embed nuts directly inside the plastic walls.

1. Slice the Enclosure_Bottom file.
2. Set a Pause Command at Layer 155.
3. Start printing.
4. When the printer pauses, insert 4x M4 standard nuts into the hexagonal cavities in the print.
5. Resume the print. The printer will seal the nuts inside.

The circuit board is a custom design that separates High Voltage mains from Low Voltage logic. For this project, I used PCBWay's services to fabricate the boards and assemble the components on it. The Gerber files are available in the repository LINK .

1. Recommended Specs:
2. Base material: FR-4 S1000H (TG150)
3. Copper thickness: 1 oz
4. Surface finish: HASL (lead-free)
5. Board thickness: 1.6 mm
6. Pro Tip: If you want extra durability for high heat, order with 2oz copper and TG170 material and Surface finish: ENIG. (not necessary)

Also, when fabricating the board, it is important to note that the triac must be soldered at the heatsink level.

Cut your wires to the exact lengths listed below to ensure a tidy fit inside the enclosure.

They reflect the actual physical implementation of the current build:

Thermistor → PCB(~160 cm)

OLED (I²C) → PCB(~51 mm)

Buttons → PCB(~130 mm)

Fan → PCB(~120 mm)

AC inlet → PCB(~75 mm)

AC inlet Earth(~115 mm)

Also, the grounding wire needs to be stripped and bent into a hook shape so that it can later be inserted under one of the heater’s legs (the M4 bolt).

1. Solder Peripherals Before mounting the PCB, solder the wires for the thermistor, fan, OLED display, and buttons to their respective pads on the board.

2. Place PCB in Enclosure Insert the PCB into the bottom of the case.

1. Pro Tip: If you soldered the button wires to the bottom side of the PCB (highly recommended), the tension created by the wires will allow the PCB to "snap" perfectly into position and hold itself securely without needing hot glue.

3. Connect AC Inlet Solder the wires from the AC Power Socket to the AC IN pads on the PCB.

1. ⚠️ IMPORTANT: Perform this soldering with the PCB in a vertical position (lifted out of the case). If you try to solder these pads from above while the PCB is sitting flat inside, the heat will melt and destroy the 3D-printed enclosure walls.

4. Secure UI Components

1. OLED: Use a small amount of glue to secure the display to the front panel.
2. Buttons: Push the tactile buttons into their housing. If you used the specific 5.5mm x 6mm buttons listed, they should fit snugly without needing glue.

5. Protective Grounding Connect the Earth/Ground wire from the AC inlet directly to one of the heater's m4 bolts. Secure it tightly with washers and lock nuts to ensure the heater plate is grounded.

6. Secure the Fan Mount the fan into its dedicated slot in the enclosure and secure it with M3 screws.

7. Mount the Heater Insert the four M4 × 60mm bolts into the heater block and lower it onto the enclosure.

1. If you used the embedded nuts method during printing, simply screw the bolts into the case.

8. Install Thermistor Tuck the NTC thermistor head into the small hole on the heater block.

Download the firmware binary from the firmware/ folder in the repo.

Connect an ST-Link V2 programmer to the SWD header on the PCB (3.3V, GND, DIO, CLK).

Flash the firmware using STM32CubeProgrammer.

Important Note: The default values pre-loaded onto the device are intended strictly for hardware verification (to ensure the heating element activates). They are not suitable for actual production use or reflow profiles.

Once the device is fully assembled, follow these steps to configure it for real-world use:

1. Access Settings via UI: Power on the device. Do not attempt to change parameters in the source code. Instead, navigate to the Settings menu directly on the device's display interface.
2. Input Profile Parameters: Adjust the necessary variables to match your specific solder paste manufacturer's recommendations or your project requirements.
3. Verification Test: Before inserting any PCBs or components, run a "dry" test cycle with your new settings. Monitor the display to confirm the heater reaches the target temperature and holds it for the correct duration.
4. Operation: Once you have verified that the thermal profile is executing correctly, the device is ready for standard operation.