Another Toaster Reflow Oven Part One

As Friedcircuits grows, the need to produce more at once increases. Before making the OLED backpacks I was hand soldering each product. During the development and fundraiser stages, I figured it would be a good idea to look into a better way to manufacture them. After researching and talking to a few people, I decided to go with a custom-built reflow oven. Well, really, it’s a converted toaster oven, just modified to suit my needs. I used resources from across the Web that I’d like to bring together in one post as well as my experience.

Reflow Oven

Parts

Here is a list of my parts for my build:

Arduino Shield PID Controller from RocketScream

Arduino Uno R3 from Sparkfun

9V power supply or USB cable connected to a 5V USB power supply like from a smartphone. You can also power it from your laptop.

Type K Thermocoupler from Sparkfun (or Adafruit)

D2425 Crydom Solid State Relays w/heatsink - I got a good deal on eBay but DigiKey sells them as well. I am only using one at the moment.

Black & Decker 1500W Toaster Oven from Target (Can't find on their site but it is in store)

3 Prong extension cable from Target

Aluminum Foil

Project Box from RadioShack - For low voltage parts

Cardboard box or another project box  - For high voltage relays

STSP Monmentary Push buttom from radioshack (I had this one already, you can use what you have)

LED Holder from RadioShack (This is for 5mm but you could hot glue a 3mm in there too)

3-5mm RED LED.

Some people just manually control the temp on the toaster oven. I find that it is better to match  the temp profile required by the solder and achieve a more consistent result. 

You could purchase a reflow oven from eBay but I find that people were having mixed results and you wouldn’t be able to customize the software. Plus I’d rather support open source when possible. 

I haven't had the opportunity to connect it to my laptop yet but you would have the ability to log the temp data and adjust the curve as needed.

The Build

There are a few different ways of controlling the toaster oven but I decided to go for the least invasive method so that I wouldn't have to use high temp crimp connectors, reducing the chance for problems. Before connecting the relay I did some tests with the thermocouple to get a better idea of the performance capabilities of the toaster oven’s heating elements. You can do these tests by using the shield, Uno, and thermocouple. Just upload the code provided by RocketStream. Using the temp readout on the display you can time how fast it heats up.  In order to get a more accurate reading, I put the thermocouple on a scrap PCB. This is the most important test since the PCB temperature is what really matters. Also, during reflow, you need to be able to heat up quickly and therefore spend the least amount of time as possible at the peak temperature. 

Scrap PCB for temperature monitoring

My findings:

• The fan seems to slow down the heating speed.
• Broil shuts off the fan and only uses the top elements. 
• A lot of heat is lost through the front glass, sides and bottom.
• The first few heat cycles are smelly and smoky, probably due to packing oil burn-off.
• The top elements are higher wattage than bottom
• The PCB heats up faster with the rack closer to the top element

Putting it all together:

Since the fan tends to slow down the heating process, the first thing I did was disconnect it. The wiring of the toaster oven is pretty simple to figure out. It takes more time to disassemble the toaster oven than it does figuring out how it works. I didn't get pictures of the inside but you can see some here: http://danstrother.com/2011/01/15/reflow-oven-controller/. He opted to control the top element, bottom element and fan separately. As he mentioned it probably doesn't really help.

Trace the fan wires back to the control panel and disconnect it from the panel.

Note: The best settings I have found for reflow is to set the temperature knob to the max heat setting just before broil and set the timer to stay on. Once set, you don't have to adjust them again. The SSR and microcontroller will do all the work for you.

Speed up heating

Before finalizing the design I wanted to speed up the heating process as much as possible since this will give me the most flexibility, speed and energy efficiency. I noticed that a lot of the heat loss was coming from the glass door, so I wrapped it in foil and made sure that the gaps around the glass were sealed tight when the door is closed. Since the foil blocks the view of the PCB I thought about cutting a view hole but I am sure it will increase the heat loss. Finally I added foil beneath the lower elements since they don't produce as much heat as the upper elements. I may try to add high temperature insulation inside the toaster oven in the empty space between the oven chamber and the outer covering.

Toaster reflow oven with foil

Toaster reflow oven with foil

Enclosure - Low Voltage

I started with the enclosure for the low voltage electronics. The hardest part of this was cutting the hole for the LCD. My wife helped me by tracing out the LCD on a paper template. I tried a few different methods but ended up using a rotary tool to cut it out. It didn't come out quite how I envisioned it, but it works.

PID LCD display

Using a regular drill I cut holes for the LED, button and one hole on one side for low voltage power and control lines for the SSR and the other side for power and USB.

PID hole for thermocouple and SSR control

PID Power\USB hole

Once everything was inside and connected I could test the low voltage electronics. Turned it on and saw the temperature reading on the LCD. Finally we need to test the SSR. I spliced into the middle of the extension cord but realized later I should of spliced closer to the toaster oven side. Oh well, easily fixed by another extension cord.

PID Controller open on top of high voltage box with SSR

The high voltage goes in one side of the box (seen below) and the low voltage on the other (seen above). Once connected we can hit the button and see if the toaster oven turns on. If so we are ready to close it up. Since the height of the enclosure is more than the controller I used some folded cardboard to act like a spring to fill the gap. I only used two screws to close the case, in case I need to open it again.

SSR in high voltage box, only one is in use

Now it's time to test it out through an entire reflow process. Once it gets to the cooling stage and it starts to drop a few degrees, I usually open the door just a tad to speed up cooling. If it cools off too fast the controller will apply some heat. You can use the door to control the cool down speed. The max should be 6C/per second but double check your datasheet for the solder paste you are using. I find even with the door open it is hard to reach that cool down speed.

In part two we will look at framing, stencils and applying solder paste to the PCB. Then we can reflow it in the toaster reflow oven and make professional looking PCBs at home.