Saturday, January 4, 2014

The Hacker Dojo reflow oven

The oven is complete and operational. In case folks want more information, I'll put up this page as a clearing house.

While I was there, I ran through a cycle while someone took a video. When that video gets posted somewhere, I'll link to it here.

The gist of it is that you need to supply the oven itself with 120vac (just like its makers intended), and the controller board (with the LCD) needs 7-12 VDC @ 200 mA, 2.1mm center positive.

The normal waiting state will show "Waiting" and the thermocouple temperature on the left side of the display.

The thermocouple is the brownish wire-looking thing snaking up from the bottom of the oven. You want to put your boards as close to the end of that wire as practicable, but you want the thermocouple in air, not touching your board or any other part of the oven.

There is a button on the back left (when viewed from the front) side of the controller. Press it to start a cycle. The display will show four pieces of information. Top left is the name of the current step in the cycle. The steps are "Pre-Heat," "Soak," "Reflow," and "Cool." The bottom left will show the current temperature and the bottom right will show the current set point (target temperature) of the oven. The top right will show the elapsed time from the start of the cycle.

The way the firmware works is best illustrated with the concept of a graph. The axes of the graph are time and temperature. The beginning and end of each step represent points on that graph. The end of one step is the same as the beginning of the next. The set point of the oven will gradually move along the graph described by line segments joining each of the points.

The points are:
  1. The beginning of pre-heat starts at 0 seconds, 25 ºC. Pre-heat lasts 90 seconds and ends at 150 ºC.
  2. Soak lasts 60 seconds, and ends at 180 ºC.
  3. At the end of Soak, the line has a discontinuity. It breaks and immediately snaps to a set point of 225 ºC.
  4. The Reflow step stays steady at 225 ºC for the duration, which is 75 seconds.
  5. The Cool step goes downwards from 225 ºC to 150 ºC for 60 seconds. In reality, the oven will cool slower than this left on its own. Impatient users may wish to open the door a crack to speed cooling, but there is a danger of thermal shock if the cooling proceeds too quickly. The cool stage will attempt to prevent this by heating the oven further if cooling is proceeding too quickly.
This profile is appropriate for Sn-Pb solder paste. I have not attempted RoHS soldering with this oven. I am skeptical that it can make it up to 260 ºC in a reasonable amount of time, but I'd be happy to be proven wrong.

The low voltage cable coming out of the oven to the controller is directly connected to a pair of optoisolators with a common cathode (the green wire). There are no series resistors - you must add them yourself. The target is 20 mA of current. Do not exceed 50 mA or you'll damage the optoisolators. If you want to experiment with alternate control systems, the optoisolators are MOC-3020s. The thermocouple is a K type. Note that the K type thermocouple lead color code is yellow for positive and (perversely enough) red for negative.

The controller board that's there is a prototype for the backpack controller design. There is a wiring error in the LCD wiring that was worked-around in software. You must modify the LiquidTWI2 library to reverse the order of the LCD data bus lines for this particular board. I have an order into OSH park for corrected boards and will bring one in when they arrive.

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