Gas station without pumps

2014 February 23

Soldering headers on a Freedom board

Filed under: Circuits course — gasstationwithoutpumps @ 16:14
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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.

There are 4 female headers to assemble—here they are shown next to the holes they need to go through.  The headers themselves should go on the component side of the board.

There are 4 female headers to assemble—here they are shown next to the holes they need to go through. The headers themselves should go on the component side of the board.

Place the headers in the holes and flip the board over onto the bench.  Sometimes it is easier to put the headers pins-up on the bench and put the board down over them.  Make sure that all the headers are flat on the bench and that the board is level on the headers.  Using 2×n headers (rather than 1×n as the Arduino boards use) makes aligning the headers easy. Make sure that the board has the feet up, so that the headers are soldered on the correct side of the board.

Place the headers in the holes and flip the board over onto the bench. Sometimes it is easier to put the headers pins-up on the bench and put the board down over them.
Make sure that all the headers are flat on the bench and that the board is level on the headers. Using 2×n headers (rather than 1×n as the Arduino boards use) makes aligning the headers easy.
Make sure that the board has the feet up, so that the headers are soldered on the correct side of the board.

To solder a pin to the board, touch both the pin and the copper ring around the hole with the soldering iron for about 3 seconds to heat both.

To solder a pin to the board, touch both the pin and the copper ring around the hole with the soldering iron for about 3 seconds to heat both.

The touch the solder to the pin and the soldering iron and hold it there until the solder melts and flows onto the pin. Remove the solder but leave the iron in contact with the pin for another second.

The touch the solder to the pin and the soldering iron and hold it there until the solder melts and flows onto the pin. Remove the solder but leave the iron in contact with the pin for another second.

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.

One common problem is a "solder bridge" where too much solder was applied and two adjacent pins are shorted.  Solder bridges can be fixed in two ways: 1) sometimes just reheating the solder with the iron will cause it to reflow onto the two pads separately. 2) If there is too much solder for reflow to fix the problem, the solder can be melted with the soldering iron and sucked away with a "solder sucker", or wicked away through capillary action with "solder wick".

One common problem is a “solder bridge” where too much solder was applied and two adjacent pins are shorted.
Solder bridges can be fixed in two ways:
1) sometimes just reheating the solder with the iron will cause it to reflow onto the two pads separately.
2) If there is too much solder for reflow to fix the problem, the solder can be melted with the soldering iron and sucked away with a “solder sucker”, or wicked away through capillary action with “solder wick”.

Sometimes there is not enough solder, and bare copper or voids can be seen.  In that case, reheat the pin, and add a tiny bit more solder.

Sometimes there is not enough solder, and bare copper or voids can be seen. In that case, reheat the pin, and add a tiny bit more solder.

Pin 1 (the rightmost pin in the top row) shows another soldering flaw—there is enough solder, but it hasn't flowed onto the pad completely.  Such "cold-solder" joints can be fixed by reheating the pin and pad, without adding more solder. Some of the other pins in this picture also show cold-solder joints, though less severely.  Whenever bare copper appears on the pad, the joint needs to be reheated.  (See pin 14 to the right of pin 16, for example.)

Pin 1 (the rightmost pin in the top row) shows another soldering flaw—there is enough solder, but it hasn’t flowed onto the pad completely. Such “cold-solder” joints can be fixed by reheating the pin and pad, without adding more solder.
Some of the other pins in this picture also show cold-solder joints, though less severely. Whenever bare copper appears on the pad, the joint needs to be reheated. (See pin 14 to the right of pin 16, for example.)

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.

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