Best way to hand solder pads with castelations to wires

Question

I have ordered a cheap XS3868 A2DP Bluetooth model, and don't know what is the best way to solder its joints to a thin wire such as AWG 30 or similar. For reference, here is a picture of the joints:

I do have a hot air gun, a soldering station with regular soldering iron, flux and soldering paste, but not a rework station. Since it seems that this design is intended for stacking it on top of another PCB, I can't figure out what would be best practice in the case where I just want to robustly solder a wire to it.

Also, since the joints are so close to each other, I have managed to ruin couple of similar boards by shorting connections and by over-heating the joints.

As a side note, it's pretty frustrating that most of these cheap A2DP Bluetooth models I could find come with this kind of soldering pads. The ones that has the extra header male pins are significantly more expensive.

Answer 1

This type of PCB connection is relatively easy to solder to - but it takes practice.

1) Small tip diameter. I personally use a Metcal sttc-126 tip, which is only about 0.016" diameter at its' smallest point. This particular tip runs at 700F and I normally use 63/37 tin/lead solder.

2) Tiny amount of solder. Although I regularly use thick solder (0.050"), I would recommend that you use solder that is somewhere between 0.015" - 0.031" diameter. Rosin flux works well. Avoid using no-clean flux - I find that it just isn't active enough.

3) Thin wire. Someone else mentioned 30 AWG wire, which is what I would use. Strip about 1/8" insulation from the wire, then trim the bare portion so that it is only about 1/16" of exposed conductor. Now tin the exposed conductor with solder.

Prep all the wires ahead of time. It makes the whole process quicker and you will find that attaching the wires to the PCB just 'seems' to go better as you go along.

Start by ensuring that your solder tip is clean and well-tinned. Support the PCB so that it is vertical, with the little wells of the connection at the top. Quickly, very quickly, use the soldering iron and solder and fill the little well about half full of solder. Each solder joint should take no more than 1 or 2 seconds. Do all of the connections on the PCB along that top edge.

Now take one of your prepared wires and lay the exposed conductor into one of the wells. Touch the soldering iron on top of the wire, with the side of the iron also touching the side of the well. If your soldering iron is well tinned, the solder in the well will melt within a second or two. Let the wire drop into the molten solder and remove the iron while holding the wire still.

Now do all of the connections, one at a time.

This skill takes practice. You mention that you have already wrecked a couple of boards. That's fine - use those boards to practice on.

Answer 2

I would use fairly fine wire- AWG 30 wire-wrap Kynar or PTFE insulated works nicely for this sort of work (you do need a good wire stripper that does not nick the wire), but if you can't get ahold of that easily, AWG 30 solder-through magnet wire works okay too.

Here's what a wire-wrap wire dispenser from OK looks like:

I suggest you mount the board to whatever else you're using with it first before attaching any of the wires so they don't get bent back and forth and break off..