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I'm always having problems soldering wires to connectors. I've read through the Solder wont stick question, but nothing there seems to have helped.

Here's my procedure:

  • If the connector is being difficult, I rub the surface of the connector with fine-grain sandpaper to clear any oxidisation.
  • Get my iron up to about 175 deg C (it's a temp-controlled iron)
  • Brush the tip across a damp sponge.
  • Add a small spot of solder to the tip.
  • Line up the wire to the connector, usually with a clamp.
  • Hold the tip of the iron on the connector for a short time.
  • Push some solder onto the wire at the edge of the tip, so the solder melts onto the wire.
  • Remove the iron.
  • Brush the tip across the sponge again to clean off the excess solder.

The problem I'm having is that the solder just doesn't stick the wire to the connector. It usually holds for a second, then unsticks from the connector as soon as the wire moves. I've tried more solder, less solder, different solder - same issue.

I've started to notice the same issue when soldering onto stripboard, too. The flow seems to be poor, the solder doesn't look shiny, and it's all rather viscous.

Am I doing something wrong? Is my iron / tip bad?

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put a picture of your wire up, if possible with the label on the thingy it came on. – jippie May 6 '12 at 17:21
I don't have any to hand, and it didn't come off a reel. I mainly tend to use the type that computer PSUs use - in fact some of it is actually scavenged from old PSUs. I've also tried with some stuff from one of those beginners kits that comes pre-tinned. – Polynomial May 6 '12 at 17:24
6  
The fact that you have a temperature-controlled iron doesn't mean you can solder at low temperatures. Even more so if you use lead-free solder. Come back if you have problems at 300°C. – Armandas May 6 '12 at 17:56
I think that you may be using the sponge a bit too much, especially since you're using the low temperature. You're constantly mentioning excess solder. If that's a problem, get a smaller diameter solder wire and practice precisely controlling how much solder you use. Also you're using sponge after finishing soldering. Don't do that as it leaves the tip unprotected and open to oxidation. – AndrejaKo May 6 '12 at 18:20
1  
175 C is only 347 F, which is way too low for soldering. I consider 600 F (316 C) the minimum for normal soldering with lead/tin solder. For certain tasks I use 650 or 700 F, although I turn down the temperature as soon as I'm done with that to avoid unnecessary tip degradation. – Olin Lathrop May 6 '12 at 19:21
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2 Answers

up vote 5 down vote accepted

175C is pretty low for soldering. Low temperature lead solder ("63/37") melts around 185C, RHoS solder even melts around 250C. You also do not seem to use flux - do it. The solder flows fast and evely if the pads/pins have the right temperature. If you have problems with soldering in general check solderingguide.com, the soldering is easy comic or the tons of videos on youtube.

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A further point to consider is that it's not enough to simply place molten solder next to copper. The temperature and time must be high enough to activate the flux and allow the two metals to diffuse and form a metallurgical bond. – Theran May 7 '12 at 19:50
Grabbed myself some Sn 95.5% / Cu 0.5% / Ag 4% solder and a flux pen, retried with the higher temperature and it works wonders. I've now got perfectly neat, smooth, strong solder joints. Thank you! – Polynomial May 8 '12 at 21:43
up vote 2 down vote
  1. Apply flux to both the connector and the wire.
  2. Stay away from lead-free solder; use 60/40 Tin/Lead solder.
  3. Use copper wire. (I've rarely seen solder not bond to copper wire; what are you using?)
  4. Simultaneously heat both parts that are being soldered. The solder should flow onto both of them. You will see that the flux assists this greatly.

Flux is very important. When it heats up, its acidity "eats away" the thin layer of corrosion that can prevent the solder from making contact with the metal.

Never use the abrasive to clean the tip of your soldering iron, by the way. (In case you're tempted.)

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