Reflow Toaster Oven

For the WSU hardware hackathon, my team decided to build a reflow oven. We had 24 hours to complete our build and write some documentation to support our build. Please forgive my quality since I am currently writing this on about 1 hours of sleep in the last 24 hours.

Here we go!!!

Reflow ovens are incredibly useful building PCBs with surface mount components. The basic concept is that a toaster oven are cheap and with a small capacity and high temperature. The hope is that a microcontroller can detect the current temperature and modulate a relay to turn on the oven fast enough to cause solder paste to correctly bond to the PCB and components. This temperature curve is called a reflow profile where certain temperatures are supposed to met with a certain timing. This seems fairly relaxed but doing the best to adhere to these guidelines should cause the best result.

Parts needed:

- Old Infrared Toaster Oven (more on this later)

- 120V solid state relay

- Digilent MX3 microcontroller board

- Digilent Thermocouple Pmod with library (TC1)

- Digilent USB to UART Pmod (USBUART)

- Digilent Connector 1 Pmod (CON1)

- Raspberry Pi with Raspbian

Links at the bottom for examples of all parts

Sample code is currently in an embarrassing state but will be soon found at: https://github.com/mwingerson

The first time I tried to build a reflow oven, it did not work. It would not heat up nearly fast enough cooled down too slowly.

Things to avoid at the risk of a failed attempt:

1. Big toaster ovens - The bigger the oven then the slower that it will heat up and cool. BAD BAD BAD
2. Resistive heating elements - Just too slow to heat up and dissipate heat when turned off
3. Fancy electronics - Basic turn knobs are easy to work with

The pictures are good examples of toasters oven that should work. Remember, small and infrared.

The easiest way think about it is when all the knobs are in the on position then the toaster oven can be turned on using the relay.

For this step, cut the live wire between the power cord and the control knob. This will create a system that allows the microcontroller to turn on the toaster oven. Attach the wire coming from the power cord to one of the AC terminals and the wire from the control knobs to the other AC terminal. Now connect some spare wire to each of the ground and signal terminals of the solid state relay.

The k-type thermocouple that is included with the PmodTC1 is rated for -270°C to 1372°C, so it is perfect for a reflow oven. The thermocouple wire has a polarity so if the initial values do not make sense the flip the wires. You want to mount the TC1 as far away from heat sources as you can.

I Attached the PmodTC1 using a ziptie to the back vent of the enclosure. I also ran the Thermocouple wire through the crack in the heating chamber and run the 6-pin pmod header through the lower vent to attach to the microcontroller.

To protect the thermocouple and get consistent readings.

Use a paper clip to create a cradle for the thermocouple wire. Bend the wire so it rests in the cradle.

Now we just need to connect the microcontroller and run some basic calibration tests.

• Attach the PmodTC1 to the upper row of JE.
• Attach the PmodCON1 to the upper row of JD.
• Connect the ground and signal wires as indicated in the picture. A 470Ohm pullup resistor was added across the 5V to signal wire. This pulls up the logic level to bring the voltage into the saturation region of the solid state region.
• Attach the Pmod USBUART to the top row JC.
• Install the library for the PmodTC1 from here: http://digilentinc.com/Products/Detail.cfm?NavPath...
• Try to run the demo from the Github here: no link.com

The microcontroller will provide updates over UART1 and send data to the webserver over UART2 which is the Serial1 class.

The microcontroller should now be driving the relay in a controlled fashion.

The next step will be coming soon. It will add a Raspberry Pi that will display a limited amount of statistics but open up a lot of the work.

Please send me questions and reviews!

Cheers!

Marshall