PCB Hot Plate: Reflow Soldering with PID Control

A PCB hot plate with PID temperature control enables precise reflow soldering at home — essential for SMD components that cannot be hand soldered. From QFN packages to 0402 passives, a controlled hot plate gives you professional-quality solder joints. This guide covers building a temperature-controlled reflow hot plate for under ₹2,000.

What Is a PCB Hot Plate?

A PCB hot plate is a flat, electrically heated surface with precise temperature control. You place the PCB (with solder paste and components) on the plate, and it follows a reflow temperature profile: preheat, soak, reflow, and cooling. The PID controller ensures the plate reaches target temperatures accurately without overshooting.

Advantages over a hot air station: uniform heating (no component shifting from air pressure), hands-free operation, and consistent results across the entire board.

How Reflow Soldering Works

Reflow soldering has four phases:

1. Preheat (25→150°C, 60-90s): Gradual heating activates flux and drives off solvents. Rate: 1-2°C/second.
2. Soak (150-200°C, 60-90s): Uniform temperature across the board. Flux fully activates.
3. Reflow (200-250°C, 30-60s): Solder paste melts and wets the pads. For leaded solder: 183°C liquidus. For lead-free (SAC305): 217°C liquidus.
4. Cooling (250→25°C): Controlled cooling. Too fast causes thermal shock; too slow causes large grain structures. Target: 2-4°C/second.

Components for the Build

• Heating element: Electric stove top plate (₹300-500), or old clothes iron element, or custom nichrome wire on aluminium plate
• K-type thermocouple + MAX6675 module (₹229) for high-temperature sensing
• Arduino Uno or Nano for PID control
• SSR (Solid State Relay) 25A for switching AC heating element
• 16×2 LCD for temperature display
• Pushbuttons for profile selection and manual override
• Aluminium plate (6mm thick, 150×150mm) as the hot plate surface

PID Temperature Controller Design

The Arduino PID library controls the SSR to match the desired temperature profile. The thermocouple reads actual temperature, PID calculates the required heater duty cycle, and the SSR switches the heating element accordingly.

#include <PID_v1.h>
#include <max6675.h>

// Thermocouple pins
MAX6675 thermocouple(13, 12, 11); // CLK, CS, DO

// PID variables
double setpoint = 0, input = 0, output = 0;
PID myPID(&input, &output, &setpoint, 20, 1, 5, DIRECT);

// SSR on pin 9
#define SSR_PIN 9
#define WINDOW_SIZE 2000  // 2 second PWM window
unsigned long windowStartTime;

void setup() {
  myPID.SetOutputLimits(0, WINDOW_SIZE);
  myPID.SetMode(AUTOMATIC);
  pinMode(SSR_PIN, OUTPUT);
  windowStartTime = millis();
}

void loop() {
  input = thermocouple.readCelsius();
  myPID.Compute();
  
  unsigned long now = millis();
  if (now - windowStartTime > WINDOW_SIZE) {
    windowStartTime += WINDOW_SIZE;
  }
  digitalWrite(SSR_PIN, (output > now - windowStartTime) ? HIGH : LOW);
}

Building the Hot Plate

The simplest hot plate uses a cheap electric stove (₹300-500) with its built-in thermostat bypassed and replaced with the Arduino PID controller. The flat cooking surface becomes the reflow surface.

For better performance, mount a 6mm aluminium plate on top of the heating element with thermal paste between them. The aluminium plate provides more uniform temperature distribution.

Temperature Profile Programming

Program temperature profiles as arrays in the Arduino code. A simple leaded solder profile:

• Preheat: ramp from 25°C to 150°C over 90 seconds
• Soak: hold 150-180°C for 60 seconds
• Reflow: ramp to 220°C, hold for 30 seconds
• Cool: turn off heater, allow natural cooling

Recommended Components

Safety and Usage Tips

• The hot plate reaches 250°C+ — use heat-resistant gloves and tweezers
• Never leave unattended while powered
• Ensure SSR has a heat sink — it switches several amps of AC power
• Use in a ventilated area — solder paste flux produces fumes
• Start with leaded solder paste (lower temperatures, easier) before attempting lead-free