Joe - Sorry to take so long. I wrote this up at about 04:30, but the 'cat wouldn't answer to let me post. Started a defrag - and it ended up at about 18 hours.

A brittle joint is usually due to too much heat, or a cold joint due to motion while the solder is cooling. It doesn't take much to get a copper wire hot enough to oxidize, and if you have a good flux, the oxide breaks up and mixes with the solder. Two bad things - the wire gets smaller, and the oxide makes the solder kind of like a wad of snot.

If you don't have the right "heat," you don't get the solder to "flow" into the joint on its own, and the tendency is to try to "push some more solder in," which makes it harder to get a good flow - and can also cause you to overheat the wire, insulation, plug, and sometimes fingers.

A soldering iron that's "just right" for a circuit board repair may be a little small to bring a cable wire up to temperature - because the wire "sucks" a lot of heat out of the joint area. On the other hand, too large an iron may get the joint so hot that it "burns" faster than the flux can work. Overheating the joint, while you're trying to "work it in" can also burn the tin out of the solder - making your good 60/40 low melting point stuff that you started with into something more like an 80/20 - that's like trying to melt cast iron. (a little exaggeration is permissible?).

If the contacts and wire are clean, and they're wrapped or crimped together so that everything comes up to temperature together, and if your iron (or gun) tip is tinned clean so that it makes good contact to transfer the heat, you should be able to press the iron against one spot, hold it there for two or three seconds, and then touch the solder to the joint and let it "flow" itself into the joint.

You only need enough solder to "wet" the joint, if you get it to flow in. Solder holds only by surface tension. More solder doesn't help - and usually will actually weaken the joint.

The right size soldering iron depends a lot on your technique. I've known some guys (experienced lab techs) who could assemble a brass auto radiator with a 15 Watt pencil. For most people of ordinary technique, a 35W or 47W would probably work on reasonably sized stranded wire, although if you've got one of those fancy "electronically controlled" irons a little smaller one may work.

A "clean" tip on the iron is important, and it's normal practice to "tin" the tip. The "liquid" tinned surface helps the heat transfer. Once you've scraped the tip clean (a fine-toothed file works well if it's rough - a wire brush if it's not too bad to start) you need to bring the tip up to temp and touch enough solder on it to coat it. (I'm assuming a flux-core solder. Flux before tin if not.) Excess solder here doesn't hurt, because just before you touch the tip to the joint, you wipe it across a damp (not soggy) sponge to take the excess off - so that the first contact with the joint is "fresh solder."

If the solder doesn't "suck in" within a couple of seconds (5 or 10 at the max) you probably need to make a better twist (better contact between parts of the joint), get a little bit bigger iron, and/or or use a smaller diameter solder wire. Once it "sucks," hold the iron on for just a couple of seconds, and then pull it off - but don't allow the joint to move until the whole surface "frosts" to indicate that the solder isn't liquid any longer. And resist the temptation to "push in a little more solder" once the joint "wets." More is not better.

One of the problems you'll run into is that a lot of "modern" cable isn't really "made to solder." The wire drawing lube frequently is left on, and additional or other lube may be added to make the cable more flexible. At the factory, they can "etch" clean, and probably use crimp (or spot welded) joints exclusively, before they vulcanize the whole thing.

"Tinning" both sides of the joint before you connect them will let you see if there's a problem, and will help once you're ready to make the joint. If the wire acts like it doesn't want to tin, then a little alcohol rinse (or ether if you're real brave and foolish?) may take the lubes off - let it drip off, to take the grease away, and air dry thoroughly, since wiping may just move more grease back. I've seen cases where there was so much lube left in a cable that sandpaper just pushed the grease around - you have to analyze each joint, I guess.

Clean - Clean - Clean.
Tin first, with a very thin application of solder
Just the right amount of heat.
Just enough solder (with a good flux).
Just the right (minimum) amount of heat to sweat it in.
And absolutely NO motion until its hard.

And of course - advice is easier to give than to use. That's why I mostly try to use crimp terminals when I can. (The bare ones can be "tinned" after they're crimped - if you're a belt and suspenders type.)

John