Hack a Toaster Oven for Reflow Soldering

As I get more serious into my electronics hobby, I need to work with more SMD components. Some component packages are very difficult or impossible to solder with a traditional soldering iron. To solve this problem, I decided to hack a toaster oven to become a reflow soldering oven.

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

There are several videos in this Instructable. Please watch them in full screen and 720p resolution, because there will be some text that you might want to read. The text content of this Instructable will also include additional notes and links. Most images from the videos will also be uploaded into the gallery. Most images (there are over 150 images in this Instructable) are annoted and sized in such a way that they do not suffer from image compression. The pictures, text, images, and files may not match exactly due to having revisions and different versions, but I'll guarantee that the text and files are in their final versions. I usually learn something new with every one of my projects and I put the important stuff in a final thoughts page and appendix at the end.

Safety Notes

This 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.

Comments

[Ugifer]

sitopway.com are a Chinese firm making PCBs and stencils. Their SS stencils start at 20USD plus shipping. I have not used their stencils but their PCBs are very good.

[alex10]

here is a good stencil supplier u.k based [e-mail]

[ok1cdj]

If anyone need PCB and plastic cover (laser made) you can get it: http://hamshop.cz/pcb-c7/pcb-reflow-controler-i199/

[hillaryryan]

Howdy,
I stumbled upon this post because I was using the AD595 for another application and was surprised by the degenerate square wave output. It turns out that the square wave disappears when you ground the positive terminal of the thermocouple. This may sound strange, but you can see the connection if you look closely at figure 1 in the AD595 Datasheet.
Best,
Hill

[frank26080115]

huh, I did not see that before, or did not realize it meant a connection is required because the line is dotted without the dot that indicates a junction

[kd1s]

One thing that could change is you could take the 125VAC into a 1:1 isolation transformer, then through a step-down transformer into a rectifier and filter capacitor to get the 5V using just one cord. Granted the bulk of the transformers would be barrier to making it light and easy to use.

The only reason I know about this trick is because I'm building a controller for a Western Electric 1D2 pay phone and it requires 5VDC for the logic and 130VDC for the coin relay.

[Kinnishian]

I'm not sure why you would do that instead of using the 5v switching supply. You could always integrate the charger into the one cord.

[Kinnishian]

I'm not sure why you would do that instead of using the 5v switching supply. You could always integrate the charger into the one cord.

[skrubol]

iteadstudio has framed stainless stencils starting at $45. Probably overkill for most, but if you're making enough boards to justify having a jig to hold the frame, or durability comes into play, probably unbeatable.

[ohararp]

Great instructable and thanks for the great presentation on how to use our stencils! I love seeing ingenuity like this! Ryan O'Hara www.ohararp.com/Stencils

[Zenock]

Do you think you could get the same vibrating action from an electric toothbrush?

[wespow1]

this may sound like a dumb question but did you have the size of this board 5cm by 5cm or 10 cm by 10 cm

[wespow1]

I mean 5cm by 10 cm lol i don't think speed studio has that size.

[wespow1]

could i just make the pcb for a cheaper price?

[frank26080115]

My design has hidden vias, so if you can plate through vias yourself, then yes, if not, then no

[wespow1]

okay thanks.

[wespow1]

on the button switch would something like this work? http://www.ebay.com/itm/120770287927?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649#ht_2766wt_1163

[frank26080115]

yes it will work

it'll look kind of ugly though

[charlieb000]

if i build one its going to be with ole fashioned bits and peices, rather than bits and bytes!! step 15: you do realise the brick acts as a heatsink? if its at the bottom then i guess its ok. but the aluminium is a nice thought, though it may work better if the whole inside were coated. you have a "minute" typo here

[frank26080115]

the brick is acting like a heat capacitor, it will absorb heat while the air is hotter and release heat if the air is cooler, but it also reduces the amount of air inside the oven. Air also acts as a heat capacitor, and also a heat resistor.

it doesn't help heat dissipation to the outside of the oven that much, if at all, so it's a net benefit.

In the end, it does help the oven heat faster, but the oven also cools slower. opening the oven door slightly will help it cool faster so that's not a problem

[canucksgirl]

I think you did a great job, and so what if you have typos? - Celsius and aluminum ( I still knew what you meant )... :)

BTW, I am trying to repurpose 2 small appliances. I have a working coffee maker and a working toaster oven (both replaced with newer gifts). I like your project, but It's really not that useful for my needs. Just out of curiosity, do you have any other suggestions for reusing these appliances (or just taking out useful parts)?

If I can't come up with anything, I will donate them to either the Salvation Army or someone who wants them (and is willing to pay for shipping) - I posted this in the Q & A section the other day.

Would love to hear your ideas/feedback if you have any to share.

[frank26080115]

Thanks

I can't really come up with any other projects with those things, if they are working, personally I would just keep using them, or donate them. Just don't trash them.

[charlieb000]

if it were a simple mispellling i would have left it, it ought to have said "second". btw, the spelling/pronunciation of aluminium/aluminum depends on where you are from.

[frank26080115]

oh I was looking for the mistake but spell check didn't pick anything up, now I fixed it since you pointed out it was supposed to be seconds, thanks.

[lolito2011]

hi i like your proyect am in mexico idon't now nathing abaut electronic and i like to ask if you have one for sell i am rely interested .
and sorry i try to speck ingles thank's
[e-mail]

[garyacrowellsr]

First off, I like your project very much. I strongly suspect I'll borrow several elements of it when I build my own oven.

In your schematic I see the signals SCL and SDA on the cable connector and the processor. Do they go anywhere else? I'm asking because I can't find the pull-up resistors that are specified in the LCD and ATMEGA data sheets. If they aren't hiding on the page somewhere (this is why I really dislike 'disjoint' schematics), their absence may be why your I2C runs slowly.

[frank26080115]

Hi Gary

My code activates the internal pull-up resistors of the ATmega, but maybe that is not enough? It's worked for other projects flawlessly before though.

[garyacrowellsr]

I wondered about internal pull-ups, but quit looking when I saw that the data sheet explicitly recommended them.

Googling around suggests that internal pull-ups are generally too weak for high speed operation: http://wiki.nycresistor.com/wiki/Using_the_TWI/I2C_interface

[garyacrowellsr]

That should say: ... the data sheet explicitly recommended external pull-ups.

[praveenchowdary]

good work and good presentation in instructables .....:->

[elmesito]

Grounding all metal parts is safer than not grounding them. By this I mean that the ground should be earth ground, not the low voltage circuit ground.

The relay heatsink surface is always insulated from any of its contacts (just imagine how it would end if it was not).

Again, please, please, for your own safety and of anyone attempting the project, connect all touchable metal parts, to the earth ground.

by doing this, if there was any connection or insulation failure, the leak would be detected by the RCD (I hope you have one) and save your life.

[keithisit]

Was there any particular reason that you left the USB socket and AC prong panel attached?

[frank26080115]

I didn't leave the prong panel attached

The USB socket wasn't bothering anything, it was wrapped up in tape in the end anyway. There are much taller things on that assembly so the USB socket didn't matter.

[kmpres]

Fabulous! Things have changed since I went to an electrical engineering college about 35 years ago. Much more reliance on microcontrollers now - my old textbooks only talk about transistors and TTL circuits. I was building stereo amps and 8-bit computers back then. You've made a very complete instructible. Thanks for the high detail and good luck with your studies.

[udawatabhimanyu4]

It was like a complete journey through the syllabus of Electronics

[praveenchowdary]

its very good data:->

[dreadengineer]

This is clearly the right way, but for occasional surface mount soldering I've had success in just turning a toaster oven on, then turning it off when the temperature reaches 210C as measured by a temperature probe I put inside the oven. It only roughly follows the profile, so it won't be quite as reliable, but I wanted to mention it for people who only do occasional SMT work.

[cyberdove]

You can buy the thermocouple connectors at Newark.

[frank26080115]

I'm a cheapskate, Digikey has FedEx 1 day shipping for $8, I'd rather just do one order

[cyberdove]

Where the thermocouple wires connect to your board you should include cold-junction compensation to provide better accuracy. A great source book is "Intelligent Sensor Design" by Creed Huddleston.