Old Solder

I've never had trouble "wetting" an old joint with a bit of new rosin-core solder.

The RS solder I tried would spit blobs of solder, due to an uneven flux core. I checked several rolls of it of a few years but cutting it open. There were places the wall was very thin, and others with no flux. I finally wadded it up and tossed it into a solder pot we used to tin wire.

What you may have been missing is the speed, ease, and quality of soldering with liquid flux vs. relying solely on the resin core. Here's just a couple of examples off the top of my head, without benefit of a full pot of coffee yet:

1. Try assembling a through-hole board with several hundred components by hand. Hell, try installing one component, say a 14-pin DIP. Then inspect the top side to see how much solder actually flowed all the way through to the top side. Without flux, many or most of the joints would be rejected. Without flux, you will spend at least five times as long doing the assembly.
2. Try installing a QFP100 without flux. Time your effort. Inspect the results under a microscope. Can you spell "slow and ugly?" Now try flux. Once the chip is positioned, you can easily make all 25 solder connections along each side with a single drop of solder on the tip of the iron. Just spread a drop of flux down the row, and skate the iron down it. Takes about 1-2 seconds to make 25 perfect solder connections, with no bridging thanks to the flux. Same technique used on any surface mount package of more than a few pins.
3. Try tinning stranded wires without a little additional flux. Chances are you'll take five times as long, and the solder won't penetrate to the center very well.
4. For cold or cracked solder joints on repairs, there's already plenty of solder on the joint. A single tiny drop of liquid flux will let you reflow what's there with far more elegance than gobbing on a big additional wad of solder just to release a speck of flux from it.
5. Maybe this one should have been first: Ever needed three hands to solder something? One to hold one of the parts, one for the soldering iron, one for the solder? Say a butt joint on a couple of wires, or adding a pigtail to something, or hanging a capacitor off the back side of a board as a temporary fix? Try this:

Put a bit of solder on each part. Hold parts next to each other and put a drop of flux on them both. Now reflow the solder with the iron. You absolutely, positively cannot do this well without liquid flux, yet the need to do it comes up almost daily for anyone that does any reasonable amount of soldering.

So don't take my word for it. Go to any manufacturing facility that does any hand soldering, and you'll find a little bottle of flux at every workstation. Try it yourself and you'll see why.

Oh yeah, I keep one on each active bench here.

Jeff

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Nice list.

I've been soldering for about 50 years and only recently have started to use bottled flux. I quickly tin both ends of whatever I'm soldering first. That leaves a little flux on each end. If done correctly, I haven't found the need for liquid flux for large components. Tinning and intentionally leaving flux on the components requires more skill than is commonly available and isn't suitable for tiny components.

I've recently been dealing with badly reflow soldered BGA chips, usually inside laptops. The trick is to reflow the solder bumps (usually with a hot air gun) without moving anything. Without a wetting agent (flux), the solder bump and PCB solder pad will not reflow together. Getting the flux to flow under the BGA is a major problem. My current method is to use a bent needle syringe, but that only works for small BGA's. Spraying with an atomizer and air hose works well, but also makes a huge mess.

For soldering big heavy lugs and cables, flux is also required. The small amount of flux (about 5%) found in common solder is insufficient. By the time the solder melts and flows into the wire, all the flux has been burned off. The only choice is to add more flux. To avoid the drippy mess, I use rosin paste flux for lugs and cables.

Another headache is mixing leaded and unleaded solder. I have separate irons and solder rolls for each. I can mix solder types and get a usable joint, but it takes more effort and often looks more like dross than a proper connection.

There's also such a thing as defective solder. I once bought a small roll of "electronic solder" at a hardware store that was awful. The flux would sputter and spray small balls of solder everywhere. I managed to do the soldering only to find that nothing less than acetone would remove the flux residue. Alcohol wouldn't touch it.

More on flux:

Make your own flux:

I've done this with violin rosin and pine tree pitch. It works. Unfortunately, it smells like a burning forest, causing everyone in the lab to panic.

As for the old solder, I haven't had much trouble with flux loss with rosin core solder. I have several rolls "liberated" from a previous employer that are easily 20 years old, and are still in use. That's not the case with acid core solder (not suitable for electronics). My roll of acid core plumbing solder looks corroded. The steel reel on which it's rolled is also corroded, indicating that the acid flux is somehow leaking and attacking everything nearby. The OP's solder collection is rosin, so flux loss should not be a problem.