Toaster Oven Reflow Controller

Introduction

• This Project Is Inspired By Henrik Forsten Nowadays many of the most “exciting” chips come only in leadless packages, such as BGA and QFN which are hard or impossible to solder just by soldering iron, because leads are under the chip where they can’t be reached. These kinds of chips are usually soldered using reflow soldering.

Temperature profile in reflow soldering. Source: Wikipedia

For the solder paste to work correctly, the temperature of the oven must follow the correct temperature profile accurately. Real reflow ovens are usually expensive, but ordinary toaster ovens can be used with an external controller board.

Reflow Oven Controller

I wanted to make a simple and cheap controller that wouldn’t use outdated parts. Instead of buttons and a dedicated display, it uses a USB port for setting the temperature profile and monitoring.

The controller works by switching the power on and off rapidly with a solid state relay (SSR) connected to an extension cord. Temperature is measured using a thermocouple inside the oven.

Hardware Selection

• Microcontroller: ATmega8U2 (cheapest AVR with hardware USB support).
• Temperature Conversion: MAX31855 thermocouple-to-digital converter.
• Switching: 25A Solid State Relay.

Schematic for the reflow controller.

Firmware

The program reads the oven temperature via SPI from the MAX31855. A state machine manages the reflow states and target temperatures.

A PID controller is used to adjust the relay PWM signal. Because the oven responds slowly, the derivative component isn’t very useful, making it effectively a PI controller. To save space on the 8kB program memory, all arithmetic is done without floating point numbers.

Green: Target temperature | Blue: Current temperature.

Testing

Reflow soldering works well, and the quality is significantly better than hand-soldering. Surface tension helps pull components into alignment during the reflow process.

Bill of Materials