Basically, to perform reflow soldering, solder paste is placed on a printed circuit board, and the components to be soldered is placed on top of the solder paste. When the oven heats the solder paste past the melting temperature, the solder paste melts and solders the component to the circuit board.
To control the oven's temperature, I created my own reflow toaster oven controller circuit. This circuit uses an ATmega32U4 microcontroller to monitor the oven's temperature using a thermocouple and AD595AQ, and then control the oven's heating element using a solid state relay. The controller features USB logging/debugging, USB bootloading, a graphic LCD display, and 3 buttons. The firmware features tweaking for all settings, manual temperature control, manual heating element control, and automatic temperature profile control (with a nice temperature history graph display). This circuit will plug into a wall outlet, and the oven will plug into this circuit, while the solid state relay basically acts as a switch between the wall outlet and the oven's heating element. Safety is the main design objective (but some things were limited by cost), and ease of use is the second objective.
Here is a demonstration video:
Some more key features:
- The thermocouple is rated to over 500 degrees Celcius. Soldering using my solder paste requires only up to maybe 250 degrees Celcius.
- The solid state relay is used because they can provide better control than mechanical relays, and they are more reliable. Mechanical relays can wear out because of their mechanical nature, and from internal sparks that occur during switching. I plan on switching the relay at 1 Hz, so this is important.
- The relay is rated for 240V and 25A so it should work with any typical toaster oven in any part of the world. The relay is cooled with a custom made aluminum heat sink and a small cooling fan.
- Everything on this circuit is powered from the wall outlet because I'm using a tiny USB charger as a built-in AC-to-DC converter. This is good since one cable powers everything. A computer is not required to operate it.
- A plastic cover is created to protect the circuit from things touching it accidentally. I can safely handle the circuit without getting shocked.
Included here (see bottom of this step) are all of the project files. This package contains the CadSoft EAGLE 5.11 schematic and PCB files, the PCB gerber files, the source code for the microcontroller firmware (including the bootloader), and some mechanical drawings for the heat sink and plastic shielding.
Step 1: Before You Begin
Safety NotesThis is probably the most dangerous electronics project I've done to date, involving high voltages and fire hazards. I am not responsible for your safety, and I am not liable for any claims, damages, or other liability. Do this project at your own risk.
- Do not leave anything unattended, you need to know if sparks happen or a fire starts, and either power down everything quickly or evacuate.
- Make sure you have a fire extinguisher handy.
- Make sure that you are using a wall outlet that has a fuse or circuit breaker, just in case something short circuits.
- Please use a GFCI wall outlet if you have one, this is the type of outlet that will cut power if you drop a hair dryer into a bathtub.
- Remember the rule about wires: Any extension cords must be thicker than the cord that is already used on the toaster.
- Once you use the toaster oven for soldering, you shouldn't use it for food again. Once you've put lead in this oven, your food will not be safe in the oven. Remember that lead is a slow accumulating poision, and it can cause mental diseases later.
- Work in a well ventilated area.
- Make sure you are aware of all materials that are near the toaster oven.
- The circuit I've build does not have an ON/OFF switch for a reason. It forces you to completely unplug it when you are done.