Reflow Toaster Oven Conversion

If you spend enough time on electronics as a hobby, you'll eventually outgrow the design and construction of circuit boards comprised solely of through-hole parts, and will want to graduate to surface-mount components.

Relative to surface-mount parts, through-hole components are large and take up a lot of space on a circuit board; they're often more expensive; and they have to be soldered onto the board by hand (unless you have a very large capital equipment budget). Surface-mount parts, on the other hand, are tiny, inexpensive, and, with the proper equipment, easy to solder. That "proper equipment" requirement is the catch. Surface-mount components are small enough to be a challenge to solder by hand; for the smallest of SMT devices, it's nearly impossible. With a solder paste and reflow oven, however, it becomes straightforward.

Photo of through-hole components next to a photo of surface-mount components.

Through-hole components (top) versus surface-mount components (bottom). Photos from Wikipedia (through-hole; surface-mount).

When soldering components by hand, the tip of a hot soldering iron is held against the parts, and a solid strand of solder is fed onto the joint. With the reflow approach, solder paste is deposited on the circuit board, components are placed on top of the paste, and the entire board is baked in an oven, whereupon the paste is heated up to the point that the solder balls contained within liquefy. The Evil Mad Scientist weblog's "Basics: Up Close and Personal with Solder Paste" is an excellent overview of how this is accomplished at the hobbyist level. DigiKey has a short video of surface-mount components in the midst of a reflow cycle.

Close-up photo of solder paste, revealing its composition of myriad tiny solder balls.

Solder paste, up close and personal. Photo by Windell Oskay.

Until recently, the only reflow ovens available were for designed for commercial purposes (very large and very expensive). Around ten years ago, though, enterprising hardware hackers discovered that a common toaster oven could be repurposed for service as a reliable, desktop reflow oven. Fast-forward several years, and there are multiple firms offering controllers and conversion kits.

After much research and deliberation, I purchased a conversion kit from Whizoo. It seemed to be the most well-engineered, fully documented, and complete of all the kits I considered. The kit includes an Arduino-based controller and all of the other parts required to convert a toaster oven to reflow service. Including the donor oven (sold separately) and shipping, the total cost was approximately US$230.

Photo of completed Whizoo reflow toaster oven conversion.

A $30 toaster oven converted for reflow solder service, using parts and a controller from Whizoo.

Unlike other kits that insert a controller between the oven and a wall outlet (AC mains), the Whizoo kit involves the gutting of the oven's original equipment, replacing them with three solid state relays that individually activate the heating elements at the controller's bidding. The conversion is not a trivial process; there's a fair amount of drilling and wiring to be done. It's well within the reach of anyone with a modicum of mechanical experience, though. I spent about 12 hours on it, but I'm deliberately slow.

And the verdict? The converted oven performs as advertised, which is to say, quite well. I've only baked a handful of boards so far, but there have been no failures.

Kit Contents

The Whizoo kit includes nearly everything you'll need to convert your toaster oven, with one or two caveats. The parts it does include are precisely enough to get the job done without any screw-ups. Even if you're experienced, this can be difficult to pull off. It's best to have spare crimp connectors and fasteners on hand.

Photo of partially-wired DC power supply and solid state relays.

DC power supply and solid state relays, partially wired up.

The kit does not include the wiring necessary to connect the relays and the DC power supply. Whatever you have on hand will probably suffice for the DC side (low voltage, low current). Don't skimp on the AC side (high voltage, potentially high current). Each relay will be drawing 5+ amps. I used 14 ga. stranded wire for the DC circuits, and 16 ga. for the low side and DC power supply.

The kit included more than enough insulating tape. I'd read that the earlier versions did not include enough to cover the entire interior of the oven, but I had no problem with that.

Photo of fully insulated interior, with "boost" heating element and thermocouple installed.

The fully insulated interior, with the "boost" heating element and thermocouple installed.

Donor Oven

I used a Black & Decker TO1303SB oven, the model featured in the build guide on the Whizoo web site. It is non-convection (no fan to circulate the air inside the oven), and is, as Dave Jones might say, "really built to a price." Taking it apart and putting it back together again was a bit of a chore, mostly because the sheet metal is so flexible.

Photo of disassembled toaster oven with parts from the Whizoo conversion kit.

Partially disassembled toaster oven next to the pile of parts included in the Whizoo conversion kit.

Once I had the oven completely reassembled, the door did not close properly. The frame of the oven seemed to have gotten tweaked slightly from all of the handling involved in the conversion process. Luckily, a couple of learning cycles of the ControLeo2 heated the oven up enough that whatever part was out of whack was able to relax back into place. The door opens and closes just fine now. If you have the time and energy to browse the ovens available at local retailers, you may end up finding a sturdier model for not much more money.

ControLeo2

The oven controller, ControLeo2, is the heart of the conversion. A thermocouple mounted inside the oven measures the temperature, and the controller turns the individual heating elements on and off accordingly in an attempt to maintain a predetermined thermal profile. LEDs on the front panel indicate which heating elements are active at any given moment. The interface is comprised of a character-based LCD display and a pair of push buttons. The only setting to be adjusted during regular use is the maximum temperature.

Close-up photo of the ControLeo2's front panel.

ControLeo2, in action.

Assembling the ControLeo2 was the most difficult part of the conversion. In order to drive the servo that opens and closes the oven door, a pin header and a couple of components need to be soldered to the controller's circuit board. It's a bit of a bodge, but in practice works well enough. My difficulty was primarily a result of my mediocre skills with a soldering iron. I screwed up the soldering of the capacitor, destroying the 220 μF electrolytic included in the kit. I replaced it with a spare 1000 μF cap I had on hand. Since it's only there to smooth out any voltage dips when the servo draws power, it works fine.

Close-up photos of ControLeo2's circuit board with extra servo drive components soldered in place.

Pin headers, capacitor, and resistor that had to be soldered to the ControLeo2 before a servo can be connected. The charred residue is over-heated solder flux, a result of having to rework the components a few times before getting it just right.

Finally, I ran into a small speedbump while updating the firmware on the ControLeo2. I followed the instructions on the Whizoo web site, and the Arduino IDE kept complaining, "can't open device "/dev/ttyACM0": No such file or directory". The device node was certainly present, both in /dev and the Arduino IDE's Tools->Port menu. The answer was in the official Arduino troubleshooting FAQ:

If, however, you're using the RXTX package from your distribution, you may need to symlink from /dev/ttyACM0 to /dev/ttyUSB0 (for example) so that the serial port appears in the Arduino software.

This appears to be the case with Debian. The prescribed symlink made the problem go away, and I was able to update my ControLeo2's firmware without further issue.

Parting Thoughts

Remember to always think thrice, measure twice, and drill or cut once. I decided I would mount the ControLeo2 to the oven in a different fashion than was outlined in the build instructions. Unfortunately, I made this decision after I'd drilled a few holes in the enclosure suitable for the aluminum mounting tab the instructions discuss. It's hardly the end of the world, but had I spent more time thinking it through before drilling, the enclosure would look a bit tidier than it does today.

Photo of empty ControLeo2 enclosure mounted to disassembled toaster oven.

The ControLeo2's enclosure and door servo mounted on the disassembled toaster oven.