How to solder something as tiny as this?

Question

I have an LED strip that is a little over 1 cm wide with 5 connectors on it, each about 1 mm wide I estimate. The wires on the left are the ones that were originally on this strip. It's supposed to be possible to cut this strip so I'm not doing anything out of spec.

My problem is that anytime I solder a wire to one of the copper traces on the strip, and then try the next one, the heat de-solders the existing wire. I sometimes manage 2 or 3, but then I invariably loosen one after which I have to desolder and clean all of them.

So far I have tried after the 'usual' technique (putting wire on the spot, apply heat to wire, push tin on wire until it starts to flow):

• Tinning wire, putting it on trace, apply heat
• Solder a solder copper piece first (the stranded wire has a tendency to 'fan out' to nearby wires), using all of the fore-mentioned techniques
• Drill a hole in the trace with a dremel, put tinned wired in there, apply heat to wire a bit away from the trace until tin starts to flow. Very hard to get hole right, plus once the tin starts flowing, it flows to the next trace.

I'm running out of ideas. I've tried the above with those tools with the two crocodile clamps to keep everything in place (not sure what they're called), and with magnifying glasses, and it's not just a matter of keeping steady or seeing everything correctly - it just seems that there is too much solder in too small an area (I've tried to keep it minimal, of course). Is this possible with only a soldering iron? Maybe there is a way to fab my own push-on connector so that I don't have to bother with soldering so close together anymore? I'm just a DIY amateur, I wouldn't know what sort of equipment this sort of thing is done with in a real lab (the connections that were pre-soldered look so near and precise).

Thanks.

Answer 1

In looking at your picture, I'd say it looks like to me that:

1. Your wires are too long (too much insulation stripped)
2. Your wires don't appear to be adequately tinned
3. You appear to have stripped the wires with a pocket knife or similar (shouldn't impact solderability, but looks bad and may contribute to #1)
4. It doesn't look like you twisted the wires together before you tinned them.

In general, I squeeze wire strippers on the insulated wire to cut through the insulation, then twist the cut end as I pull it off to get a tidy twisted end, like this (though that's a lot longer than I would generally ever expose)

Once that's done, I coat the neat wire end with a copious amount of flux, then get a gob of solder on the tip of the soldering iron, then lay the gob on the wire until the solder is wicked into the wire. No need to rub the iron on the wire, or press, or do anything else - it'll all get soaked up as soon as the wire gets hot enough.

Once that's done, keep adding solder until right at the point you can barely distinguish the original stranding. At this point, the wire's tinned. If you tin the pads with a **little* solder, then soldering the two parts together should just involve laying the soldering iron on the wire with the wire on the pad.

You shouldn't ever need to push or rub or poke with the iron. Be sure to clean the tip off with a damp sponge immediately before you do any work.

If you're looking to get a solder joint like the one below, check out this tutorial on tinning wires and this tutorial on soldering to LED strips.

Answer 2

Try some kapton tape, maybe the 1/4" variety. Tape the LED strip as close as possible to the pads, then place the wires on the pads and tape them to your work bench so that they are always in contact with the pads. Should prevent the wires from lifting off the pads when the adjacent pad heats up.

Answer 3

Starting from scratch I would feed each wire in from underneath, then bend the wire so that the whole thing will lay flat. Then I'd tape the whole thing down so it stays in place and solder across all the wires. I'd probably use a small tip and tack each one, although you can probably get away with a large one and solder them all in a row.

If your wire is too big for this use smaller wire. I'd also think, if you're going to do this a lot, that finding a small connector that's the same pitch as the end of those holes there and soldering it in there first will be a lot less trouble and probably more reliable.

Answer 4

Tin the wire. Heat the pads and melt a small amount of solder on to each pad and let it cool. Then place the wire on top of the cool solder and use the soldering iron to melt it all together. Done. I just did exactly this the other day - these LED strips are neat!

Answer 5

It appears that the combination of too much solder, too wide soldering iron, and little soldering experience, are preventing you from obtaining the results we see in Chuck's picture. Two of the three problems can be fixed right away. If you tin the wires and the holes, you will need very little, or none, additional solder. Use a pencil soldering iron, like the one in the same picture. After you put the wire in a hole, heat the junction of the wire and hole, until the solder on the wire and hole melt (should take no more than 2 seconds). If you do need to use a little additional solder, use the very thin kind (about 2 mm diam, rosin core). I recommend you first practice this in a PCB with similar spacing, then do the strip. Good luck!