Milos Rasic's Raspberry Pi Pico Hot Plate Allows for Precise SMD Soldering

The process of SMD soldering

The vast majority of printed circuit boards (PCBs) are designed with surface-mount components in mind, as their small footprints and ease of placement makes them superior to through-hole components in almost every way. But unlike commercial assembly where robots precisely apply solder paste and place components, doing it by-hand for low volume runs typically leads to messy soldering and requires a lot of cleanup. Further compounding this issue is how solder is reflowed, as hobbyists will normally use hot air or a small oven instead of a more accurate reflow setup.

Element14 Presents host Milos Rasic's alternative is a DIY hot plate that he was able to build from low-cost components. It not only provides a stable, accessible surface for boards but also includes digitally adjustable thermal controls for setting the exact temperatures required by the solder paste.

Designing a custom hot plate

For the hot plate itself, Rasic sourced an inexpensive, 400W heating element that accepts 220V AC power and can be externally grounded as a safety measure. A Raspberry Pi Pico acts as the controller by generating a PWM signal for the plate's attached solid state relay wherein a larger duty cycle corresponds to a higher power output. Lastly, a thermocouple provides the Pico with fast temperature feedback while an OLED and dial make up the user interface.

Temperature feedback

Just like any other device that relies on a feedback loop to modulate its output, such as an air conditioner, refrigerator, or oven, the hot plate needed a reliable sensor that could withstand the high heat and provide accurate values. Rasic's choice of a negative temperature coefficient (NTC) thermistor could do exactly this, as it supports a wide range of conditions while being easy to read. In principle, the resistance drops along a curve when the temperature increases, and by using the manufacturer-supplied approximated curve formula, Rasic could quickly determine the temperature with the Pico.

Controlling power output

With the sensing portion of the feedback loop created, Rasic now had to get the Pico to output a PWM signal that gently ramps to match the temperature setpoint while simultaneously accounting for safety. The code is running a PID loop where the proportional value is set by the difference between the setpoint and measured temperatures whereas the integral is the accumulated error. The output is clamped between a minimum and maximum duty cycle for overshoot prevention along with a safety check that ensures thermal runaway cannot occur if the thermistor becomes disconnected.

Fabrication and assembly

Most of the hot plate's enclosure was fabricated from plywood due to its great thermal insulating properties and light weight. At the top are four standoffs that further separate the hot plate from the 3D-printed enclosure body/electronics. Once everything had been fully assembled, Rasic tested the thermistor safety feature and then promptly showcased his design by soldering several types of ICs onto a PCB with great success.

To see more about this project, you can watch Rasic's build log video on the element14 Presents YouTube channel.