Because I decided to switch to the Freedom KL25Z boards this year for the circuits class, rather than Arduino boards, I have a bit of a scheduling problem. The KL25Z boards come without headers, but the students can’t do much with them until headers have been soldered on. That means that I need to teach the students to solder on the first day of lab, rather than halfway through the course.
Because students will need instruction, I’ve tried to put together a tutorial on soldering specifically for this purpose. Because I don’t have a KL25Z board without headers, but do have a KL26Z board (which uses identical headers), I’ve taken pictures of soldering on the headers.
I find it best to tack down each of the headers by soldering one or two pins first, so that I don’t have to worry about the headers shifting around as I solder.
After every few solder points, it is a good idea to wipe off the tip of the iron on a wet sponge, to keep the tip clean. Otherwise burnt residue of the rosin flux builds up on the iron and makes good thermal contact difficult.
I should also warn the students not to touch the rubber feet with the soldering iron. Arranging the board so that the long edge is closest to the hand holding the soldering iron (rather than the short edge as shown here) probably makes this easier. Rotating the board to solder the other side, rather than reaching across, is also a good idea.
We will be using tin-lead solder (since it is easier to work with than lead-free solders), but the lead is not a serious hazard. A bigger problem is the smoke from heating the rosin-core flux, as the electronics lab is not as well ventilated as it should be. I can warn the students not to breathe the smoke, and to take a 5-minute break in another room after soldering. If we were doing a lot of soldering, we’d have to get some fans with filter units to remove the smoke from the work area.
I’m a little worried about students’ first soldering projects being with a $13 KL25Z board, rather than the 50¢ hysteresis-oscillator board, and needing 64 solder points, rather than 42, but I think that they can handle it. The ease of putting the board face down on the headers and tacking them all down (compared to inserting varying size and shape headers, capacitors, DIP, and resistor on the hysteresis-oscillator board) may make the KL25Z a better choice as a first solder project.