Surface Mount Reflow Oven

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Description

Ready to take the next step in electronics? Surface mount components are the way to go when considering any of the following:

While you can get away for a while with a heat gun, hot plate, or a hot air reflow tool, you may eventually want a reflow oven. This write-up documents what I did to make mine. Because this project involved some salvaged and used materials, you will not be able to replicate my exact setup. You will probably have to make some minor substitutions and design/layout changes when making yours.

ReflowOven.jpg

Materials

Part Cost Description
Toaster Oven Varies Get one with multiple heating elements. You want a relatively quick heat ramp-up so your parts and PCBs don't overbake. I got a used one from Goodwill for about $10.
Arduino Uno $12 Get any 5V Arduino Uno clone. You can get one shipped from China on Amazon.com for around $12.
RocketScream Reflow Oven Controller Shield $38 No skimping here. It's expensive, but you don't have to do any coding and it has an built-in LCD display.
Type-K Thermocouple $5 I ordered mine along with my Rocketscream shield
Solid State Relay $5 I got the 40 Amp SSR40-DA on Ebay.
110v to 5V USB Converter Varies I used an old cell phone charger for this. Try to get one that outputs around 1 amp. Mine output 800 mA.
USB Cable Varies If you use an old cell phone charger like I did, you will need a USB cable that fits the Arduino clone you use
5V Case Fan $7 I bought a 5V case fan off Ebay to ensure my Arduino stays cool. Getting a 5V fan ensures you don't have to deal with additional voltage conversions.
Momentary Push Button Varies To start the oven. Mine was salvaged.
Wire Nuts To join electrical connections together
Terminal Lug Connectors To join up wiring to the solid state relay
Heat Shrink Tubing Where wire nuts don't work, heat shrink tubing comes to the rescue.
High-Temp RTV Silicone $5 For sealing gaps and holes in cheap toaster ovens. Buy at auto parts store. Be aware that if you use this stuff, your completed oven may smoke the first couple of times you use it.
Nuts and bolts For mounting fan and Arduino. With the way the face of my toaster oven was, I needed some super long bolts to mount my Arduino. If you do too, Ace Hardware is your friend. The big box stores don't carry odd sizes.

Wiring Diagram

ReflowWiring.png

Step 1 - Test Toaster Oven

You will want to test your toaster oven to ensure it functions ok and that its heat output can come close enough to looking like a reflow soldering curve when you plug in all the captured data points into a graph.

ToasterTest.jpg
ToasterTest2.jpg

I had an outlet box wired with a cheap extension cord from another project. I took this outlet and cut one of the wires, stripped some sheathing off and tinned the ends with my soldering iron. This way I could hook the two ends into my solid state relay (SSR) and control electricity flow to the toaster without even cracking the case open. The reflow shield gets connected up to the other end of the SSR and the thermocouple. The thermocouple gets put into the toaster oven. Hook your Arduino up to your computer and load the Rocketscream reflow sketch. Make sure your toaster hardware is set to be on and full temperature. Fire up the Arduino serial monitor, press the start button on the reflow shield, and capture the data that is output into the serial monitor. Once the sketch is done, you can throw all the data into Excel and generate a line graph from it. It should look something like this:

ToasterTestGraph.jpg

Compare your results with the following "standard profile":

SolderStandard.png

My initial trial run showed that my toaster was less than ideal. It took too long to heat up, took too long to cool down, and because it took too long to heat up, the soak period didn't seem to be long enough. I was about to give up on my Goodwill toaster and then noticed that only one set of elements turned on at any given time and that element set depended on which toaster function you were using. So I figured what the heck, let's tear it open and find out the story on the elements. Worst case on the cooldown was that I manually open the door to help it cool down faster.

Step 2 - Tear Down

Here's what I found inside my toaster oven. It was an amazingly simple metal thermostat. This thermostat was engineered so that only the top set of elements or the bottom set of elements could be turned on at any given time.

ToasterCaseOpen.jpg

Tear out everything but the wires. The knobs, thermostat, wire crimps all had to go. The only thing I left was a 110v lamp which I kept as part of the project to indicate that the toaster was on. If you have one, you could do this or you could use a panel mount LED and attach it to the Rocketscream shield.

Step 3 - AC Wiring

I crimped some circular terminal lug connectors onto the wires that attach to the AC terminals on the SSR. You could do the same or you could just strip the wires, tin the ends with your soldering iron, and then form them into a loop around the SSR screw terminals. Be very careful with whichever method you use. Give the wires a good tug after they are put in place to ensure that the terminal crimps or wire loops aren't faulty. When dealing with high voltages you don't want to risk a wire slipping free.

ToasterSSRWired.jpg

Throw some wire nuts on the other AC points. Refer to the wiring diagram above.

Step 4 - 5V Power

I needed 5V power to run my Arduino clone. It didn't take much looking until I found what is probably the easiest solution to convert 110V AC to 5V DC, a cell phone charger. I had several old chargers laying in my parts box from previous cell phones and from having worked a bit with Raspberry Pi. I used the one that had the highest amperage rating. I honestly had no idea what my current requirements were, but I figured it'd be better to have excess capacity from the start rather than having to tear into my project again if I needed more current. My cell charger was rated at 800 mA. Rather than opening up the charger brick, I took the easy path and just soldered some wire leads to the wall outlet blades and then slipped some heat shrink tubing over the solder joint and blades to avoid having exposed 110V contacts inside my oven. I then took the wire leads and joined them up to the existing 110V wire nut connections. From there it was only a matter of plugging in a USB cable and I had the 5V I needed.

Toaster5VConnection.jpg

Step 5 - Test Assembly

Before investing time in drilling and cutting holes in the toaster chassis, I figured it was a good time to test the electrical assembly. Hook the Arduino with reflow shield up to the solid state relay, feed the thermocouple through the oven door like earlier, and power the Arduino via the embedded cell phone charger. Initiate a test with the button on the front of the reflow shield and sit back and watch. The toaster should run a full reflow cycle. Let it cool down before doing any more work on it.

Step 6 - Cut Out For LCD

If everything checks out from the test, it's time to mount the Arduino assembly. The Rocketscream shield has an LCD screen built in to display the reflow phase and temperature. This was the only part of the shield I wanted exposed. Thankfully, the Rocketscream shield is open source hardware. I was able to download their design files and print out the board layout to use as a cutting template. Because they designed in Diptrace, I had to download a new tool to do this. See links below for the Rocketscream GitHub and Diptrace.

Rocketscream GitHub

Diptrace

Once I had the cutting template, I found the optimal place to put my Arduino assembly, taped it in place, and used a permanent marker to mark my cutout and drill locations.

ReflowCutoutTemplate.jpg

Note that this picture shows the cutout already done. I forgot to take a picture pre-cutting.

After marking the cut and drill locations, I pulled the template off, drilled the holes, and cut the slot for the LCD with my rotary tool.

ReflowCutout.jpg

Step 7 - Cut Out Above Arduino

I wanted to maintain quick access to the USB port on my Arduino for data logging and in case I ever wanted to modify the firmware. Because I was using the USB port to power it, this mean that a bulkhead connector was out. I took the easy route here again and just cut out a spot above it. Turned out later that this also had the added benefit of being a heat vent. After doing some quick measurements and marking, I again used my rotary tool to cut out a spot in the toaster casing. I then found some tubing that I sliced and put over the sharp edges.

ReflowChimney.jpg

Step 8 - Mount Arduino + Shield

I ran across an issue trying to mount my Arduino assembly to face of the toaster oven chassis. The LCD pins were clearly going to short out when pressed against the chassis.

ReflowLCDShort.jpg

To remedy this, I cut some foam to keep the pins from contacting metal. I later thought better of this. The foam would likely melt being very near the oven compartment. My final solution was to buy a rubber washer and cut out a strip to use in place of the foam. Note that the picture below still shows the foam. I did not get a picture with the rubber.

ReflowLCDSolved.jpg

Mount the Arduino assembly in place using some long bolts and nuts. Be careful to not overtighten the nuts. This could shatter the Arduino or shield PCB.

ReflowArduinoMounted.jpg
ReflowArduinoMounted2.jpg

Feed the thermocouple through internal firewall into the oven compartment. You may have to drill a hole. You want the thermocouple to reach mid-way into the oven. Since the thermocouple doesn't really stay put, I used a scrap piece of PCB to hold mine down so heat measurements are taken of the area my project PCB sits in.

Step 9 - Seal Holes

I used High Temperature RTV Silicone to seal all of holes and gaps in the internal oven firewall. There were a lot of these gaps due to the cheap construction of the oven.

Step 10 - Add a Fan?

I thought I was done at this point. I had put my toaster back together and started doing a reflow cycle test. However, about three-quarter through the test, I started noticing smoke coming from my toaster. I wasn't sure exactly what was smoking. It could have been cable shielding, the RTV silicone, the rubber gasket I used, the Arduino... Rather than risk pushing through the test, I powered it off and cooled it down. I opted for cheap insurance and decided to put a fan in to protect the electronics. The problem I was faced with is that most fans are 12V because they are designed to work with computer power supplies. I didn't have a 12V supply in my toaster. I had 110V and 5V. Rather than cobbling together a way to get 12V, I special ordered a 5V fan. To install it, I cut a hole in the bottom of the chassis right under the Arduino. I mounted the fan so that it sucked cold air from under the toaster and blew the hot air out the top where I had cut the hole for USB port access. For powering the fan, I just spliced into the USB cable, tapping into the 5V and GND lines.

ReflowFan.jpg

Step 11 - Cure Silicone, Reassemble, and Test

Let the silicone dry & cure for a day, reassemble the toaster unit, and test it out. I am pretty sure that the smoke that I encountered during initial testing was the silicone heat curing. I haven't seen any additional smoke on my finished toaster.

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