Closing the Loop on Surface Mount Soldering

Step 4 Surface Mount Soldering

Using the temperature control system described in this Instructable improves Extreme Surface Mount Soldering in two ways. First, temperature control is more accurate and significantly quicker. So instead of having a slow ramp from about 120C to 180C over 6 minutes or so, we can go quickly to 180C, hold for 2 ½ to 3 minutes, and go quickly to 220C to 240C for about a minute. We still have to watch for the point when the solder flows and turn off the power, or just quickly lower the target temperature. Since the temperature comes down very slowly, I usually slide my circuits off the hot plate as soon as the temperature has cooled below 210C. Put them on a piece of perf board or wood, not metal. The metal could cause them to cool too quickly. Note also that you might have to raise the target temperature over 250C (the maximum the sensor will read) in order to get the plate hot enough in certain areas. The plate will not reach a single temperature over the entire surface but will be cooler in certain areas than others. You’ll learn this by experimenting.

The second area of improvement is a reduction in the time between soldering cycles. With the open loop system, I had to wait for the hot plate to cool to room temperature (about 20C) to start a new soldering cycle. If I didn’t do this, then the temperature cycle would not be correct (change of initial conditions). Now I only need to wait for a stable temperature around 100C and I can start a new cycle.

The temperature cycle I now use is implied above, but here it is exactly. Start at 100C. Put your boards on the hot plate for two to three minutes to warm up - longer with large components. Set the target temperature to 180C. This temperature is reached in less than one minute. Hold here for 2 ½ minutes. Set your target to 250C. As soon as all the solder flows, decrease the target temperature to about 100C. The temperature of your plate will stay high. As soon as it decreases to 210C, or time of 1 minute elapses, slide your boards off the hot plate onto a cooling platform of perfboard or wood. Soldering is done.

If you wish to use a different temperature profile, you should have no trouble achieving it with this control system.

You may want to experiment with the position of the temperature sensor above your hot plate. I found that not all areas of the hot plate reach the same temperature at the same time. So depending on where you position your sensor, the actual time and temperature required to make the solder flow can vary. Once you work out a recipe, use the same positioning of the sensor for repeatable results.

Happy Soldering!

Comments

[buddhafragt]

hello,
everything is working fine, but I dont understand if it is possible to save a soldering time/temp table.
I can adjust the temp manually, but can I save a profile??

best regards
Michael

[doctek]

Great idea, and you could certainly modify the software to do this. The reason I haven't tried to do that is that the environment (mostly the ambient temperature) changes enough that I would need to correct for it and controlling the process by eye is pretty easy. The type of parts on the board and presence or absence of ground plane makes a difference also. Some sort of sensor of board temperature or solder flow would be needed.