用现成品改装的烤箱回流焊

liyf · 发表于 2012-10-6 11:32:25

http://www.instructables.com/id/ ... ard/?ALLSTEPS#step4

Step 4Build 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

Solder Reflow Guide for Surface Mount Devices, Paul N. Houston, et. all
Design and Construction of a Reflow Soldering Oven, Dept of EE, University of Washington
Conversion of a toaster oven into a SMT reflow oven
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