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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:

solder pads with castelations

zoom on the solder joint

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.

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  • \$\begingroup\$ I wouldn't use smaller than 24 for this, even if it ends up making the other side harder to work with. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 1 '15 at 22:39
  • \$\begingroup\$ Works for me, I'll use a thicker one then. \$\endgroup\$ – Omer Jan 1 '15 at 22:41
  • \$\begingroup\$ If you want to solder wires directly, stick with the thin stuff so that the wires (which you can always replace) break under stress rather than the pads. For something durable, whip up a PC board, either in-house toner transfer or cheap 10-day turn from one of the panelization services. An iron is fine - get yourself some thin desolder braid for clearing bridges. \$\endgroup\$ – Chris Stratton Jan 2 '15 at 0:03
  • \$\begingroup\$ Thank you. I am not experienced with etching nor drawing eagle files.. hence the wires :) actually there is a very cheap breakout board out there for this module: ebay.com/itm/371171955942 but I was hoping to avoid it since it doesn't suite my needs (both size-wise and feature-wise) \$\endgroup\$ – Omer Jan 2 '15 at 0:16
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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.

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  • \$\begingroup\$ Thank you very much for the elaborated answer. This is exactly what I was looking for. I hope it will help others as well. \$\endgroup\$ – Omer Feb 1 '15 at 9:46
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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:

http://i5.walmartimages.com/dfw/dce07b8c-8063/k2-_e4f7c989-361e-4e79-a7d3-293d07413741.v1.jpg

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..

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  • \$\begingroup\$ Thank you for the wire advise. Any tip regarding the soldering process itself? Usually with SMT I use the reflow technique, but here there's too little space on the pad to fill with enough copper for reflow. \$\endgroup\$ – Omer Jan 2 '15 at 0:40
  • \$\begingroup\$ With the board immobilized and wire pre-tinned, I use the three-hand method. First hand holds the iron, second hand holds the solder, third hand holds the wire. Kind of a learned thing. If you can use chopsticks, a bit like that. \$\endgroup\$ – Spehro Pefhany Jan 2 '15 at 1:49
  • \$\begingroup\$ ..which leaves me one more free hand :) still, I can't seem to see how I can overcome the tiny soldering pad issue. \$\endgroup\$ – Omer Jan 2 '15 at 8:30
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    \$\begingroup\$ Sounds like you need more soldering practice- I'm pretty good at soldering after 34 years in the industry, but there are people who can solder the most amazingly tiny things that you would never have believed possible. A microscope can really help too. \$\endgroup\$ – John D Jan 2 '15 at 20:47
  • \$\begingroup\$ I could definitely benefit from improving my soldering skills. Unfortunately there are not enough good teachers around.. in the meantime I managed to destroy another PCB's pads :( \$\endgroup\$ – Omer Jan 28 '15 at 10:08

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