Complete Circuit Board Lab & POV Business Card

Step 4 Build a Reflow Oven & Controller

You may have noticed a trend here toward automation, and I don't deny it. This whole project was an attempt at small scale mass production. So here we go: instead of hand soldering every board, hey, guess what- I can just stencil on solder paste, glob it on real good sorta, and stuff the whole thing in a toaster for a minute or two? 

Yup, that's the general idea.

Surface Mount Technology
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The part that makes this possible is choosing Surface Mount components over conventional through-hole components. They are significantly smaller and less expensive, there are no holes to drill, and board size is reduced. Any disadvantages they have are just side effects of their advantages; they can easily be too small to manipulate without the proper tools.

Typical SMT fabrication works like this: solder paste is spread through a stencil unto the solder pads of the board. Then the components are stuck into the paste, and the entire thing is ramped at a set rate to several set temperature points, depending on the solder and heat resistance of the components. In our case, every component except two can be baked; the battery and the switch will need to be hand soldered later.

Regardless, the higher entry cost is outweighed by the long term advantages, so it's time to build a temperature controlled reflow oven so we don't need to get carpal tunnel soldering hundreds of little grains of sand.
 

The Controller
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 The cheapest commercially available reflow oven is certainly several thousand dollars. On the other end of the spectrum, the cheapest stand alone controllers are in the $100 range. Since the expensive ones are too expensive, and the cheap ones are too crippled, I had to build my own.

All the controller has to do is monitor the temperature of the oven and change it when it need to be changed. The ideal way to do this is with a Ramp/Soak temperature controller, used in factory automation for just such a purpose. I was lucky enough to get a Watlow SD6R-LCJE-AARG ramp./soak controller for less than 50 dollars, and I built the whole thing around it. You should be able to find something similar in the same price range without too much trouble. Just make sure it can ramp the temperature to multiple setpoints, as opposed to simply maintaining one stable temperature.

The Walow is pretty forgiving; it will take 24 AC/DC as power input it can display temperature in Fahrenheit or Celsius, and it accepts almost any type of thermocouple. More importantly, it has low voltage DC as a control output that we will use to drive a solid state relay, which will in turn switch on and off the heaters of a standard toaster oven.
 

The Toaster Oven
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Any toaster oven will work, though the smaller the better, as the smaller volume of air will mean it can heat up much faster. Try and look for a newer oven with quartz heaters, but ceramic heaters would also be suitable. If your oven has a setting for always on operation, such as bake, you'll be fine. Otherwise, you will need to open it up and wire the heaters to always be on - a dangerous affair that I would not recommend.

You'll also need a thermocouple with a heat-resistant insulation, as our oven will reach temperatures of 260º C. This presents yet another fire hazard, so don't leave it unattended, keep it away from the walls, and don't set anything on it (unless it has heat resistant plastic feet!). The last thing to do is simply to drill a hole in the back of the over large enough for the thermocouple to fit through. There is no need to worry too much about heat loss with our oven, but the hole shouldn't be too much larger than the thermocouple. On the other hand, too small a hole and the fiberglass insulation on the wire will begin to fray from repositioning it. The hole should be placed slightly above where the center rack in the oven would be, so that the thermocouple can just touch the surface of the circuit board.

References

1. Solder Reflow Guide for Surface Mount Devices, Paul N. Houston, et. all
2. Design and Construction of a Reflow Soldering Oven, Dept of EE, University of Washington
3. Conversion of a toaster oven into a SMT reflow oven
4. Building A Reflow Controller For Griddle/Hotplate/Toaster Oven

Parts

• Toaster oven, preferably small volume, with four quartz heating elements
• Metal project enclosure
• Ramp/Soak Temperature Controller, here's one option, but you should try eBay first
• 10A or greater Solid State Relay, Crydom d2410f
• High temp thermocouple K-Type, 270 to +1372º C (-454 to +2501º F)
• heat resistant rubber or plastic feet & hardware
• indicator lamp, panel mount 120V
• electrical socket, panel mount
• two wire screw down terminals, panel mount

Tools & Supplies

• Power Drill
• Dremel or other rotary tool, with cutoff wheel
• Crimping tool
• Soldering iron, solder
•  mounting hardware
• various crimp on connectors sized for your parts
• several feet of 14-16 gauge wire, various colors
• Thermal heatsink paste

Comments

[dustinandrews]

http://www.instructables.com/id/Make-your-own-programmable-thermostat-with-Arduino/ You could also make your own PID controller. :)

[Taoiseach]

I live in Ireland, and, for the most part, we don't have Toaster Ovens (Actually, when I saw the picture, I thought it was a Microwave, which could have ended badly). Could you recommend an equivalent appliance I could use? Cheers.

[hughperman]

Hey, Argos do actually sell toaster ovens (search on their website), although they only go up to 230C (allegedly - the picture actually shows 400F as the max, so may not even be 230C). I am quite interested in picking up a cheap toaster oven for some SMT work, if you have found anything workable in Ireland please let me know!

[wagnerlip]

Question:

How come you guys bake potatoes? make pizza, toast bread cheese and tomatoes, bake frozen chiken, pre-heat hotdog buns, broil a bunch of half garlic and olive oil, bake tomatoes, hot sandwich, etc?

[Taoiseach]

You mean how do we? The magic of the oven grill. Same principle as the above, only attached to an oven and a hob, which would make the whole thing terribly awkward and expensive to use as a reflow solder oven. Basically, the Toaster Oven above is to the Double Oven, what the Hotplate is to the hob.

[wagnerlip]

Interesting. Here in USA we can buy one oven toaster like that in hundreds of department stores, like WalMart, Target, etc. They ALL are manufactured in China and you can buy then from 10 to 40 US Dollars. If you have electric stove, probably you may have available to sell the heating elements. Here we use a coiled resistance, electrically isolated, that heat to a point (if you want) to get really red and heat the pan over it. The pan really sits over the element. There 3 or 4 sizes of those heating elements, the second size, the most common around, is for 900W, if I remember well. They feed on 220Vac. In the past I build a SMD reflow solder oven using a toaster, but I was thinking to build another from scratch, stainless steel box, thermal insulation another box inside, fans, a sliding tray for the SMD boards, may be even a metalic belt for automation. Using those stove elements is easy, just one on top, another on bottom, the PCB in middle, 20 to 30mm distant from the heaters. The oven I build 3 to 4 years ago, you may find it at http://www.ustr.net/smt/oven.htm and http://www.ustr.net/smt/ . My process is much simpler than the posted here, since I use a microcontroller already programmed with the temperature profiles and the temperature is captures from a real small circuit board inside the oven, so it is not "air temperature", is an electronic circuit board temperature. Cost much less, since all the electronic control is made with less than $10. One day I need to publish that circuit and software.

[sponges]

That's a weird question! I have no idea what the equivalent is over there. Lots of things would work; it just needs a metal enclosure and some kind of heating element. The risk is always how safe you could make something that heats to up to 300 C. What we call a toaster oven is capable of that just as a matter of course, and so is very safe.

You might be able to take a regular vertical toaster with side heating elements and place it on it's side. I don't know how hot those can get, but it may be close. It would need to be sealed with a door somehow. And you'd need to find out what the regular operating temperature and wattage was so you didn't exceed that and start a fire.

Also, if you don't need your design in massive quantities, you can just go over the board with a hot air soldering gun and get similar results. Even a regular pen type soldering iron works, with practice.

Just don't do anything I wouldn't do!

[rthrockmorton]

that sounds like a "oops - i kinda set the kitchen on fire making a business card," incident waiting to happen.

:)

[sponges]

I agree! This is kind of dangerous. :D

[Taoiseach]

Thanks, you've been very helpful. I'll see what I can do with a regular toaster, and if not, like you said, just solder manually. Thanks!