I feel silly asking this question, but I am mystified. I can't seem to tin the 24 gauge stranded wire used in Christmas tree lights.
I've tried every kind of solder/soldering iron combination I have. I even tried various paste fluxes, all to no avail. The solder won't wet the copper. Am I looking at an oxidation problem?
I have been running into a lot of that funny wire lately. It looks just like any other wire I have used for years - but it won't wet right. Upon closer inspection, I see some of it was copper coated steel wire ( magnet attracts it ). Some of it was copper plated aluminum ( wonder how they did that ), and other was plain old stranded aluminum wire ( it was a dollar-store USB cable I bought to hack into for a source of five volts ).
It looked great until I tried to solder it. At first it would seem to wet, then solder just rolled off it. Not even GOOT flux worked. What happened is the solder dissolved the thin layer of copper off. Must have been electroplated on. When I took it to the lab and saw what was going on under the microscope, it became obvious what the problem was.
I guess with today's technology where most everything is crimped, aluminum and steel wire is a lot cheaper than copper wire and works good enough to avoid a customer return.
Google sent me here while I was seeing if this situation existed. The evidence left here shows that is indeed what is going on. I leave my experience for others also trying to figure out they were losing their mind because they can't do something they have done for years.
It may be coated magnet wire or litz wire or plastic coated to prevent oxidation such as purelane. Is the copper surface clean? There are solvent solutions to dip the wire and then there are flame solutions to burn off the insulation. It must look like shiny white copper colour that turns reddish with oxide or orange with coating.. Careful scraping with an razor edge knife may work.
2022 calling.
Cars almost drive themselves, but video calls are still terrible.
Same story, had some apparently copper stranded speaker wire which I just couldn't tin. While looking into buying some more, I heard of copper-clad aluminum stranded, which led me here.
I scraped a section of the unsolderable wire, and sure enough, shiny underneath. It is not attracted to a strong magnet so I'm assuming it is aluminum.
I posted as an answer instead of a comment so I could include a photo. Oh, phone cameras have a ridiculous number of pixels now.
Copper-clad aluminum stranded speaker wire:
Aluminum oxidizes very quickly and forms a brittle insulating sheath (sapphire) around the wire. Once oxidized further oxidation stops. Increasing the gauge or number of strands allows aluminum wires to work as well as copper, except for flexing. When aluminum wire is bent the coating cracks exposing fresh aluminum underneath which further oxidizes and so on.
Plating copper onto aluminum is difficult, requiring several phases, but can be done. A coating may look like copper but may be something else. The coating flexes without exposing the aluminum underneath. This allows the full diameter of the aluminum to remain conducting while being flexible, which is the reason for stranding the wire.
If it is copper, a thin coating may not be enough to permit soldering. As mentioned by Steve, the solder can dissolve.
While the copper coating is thick enough to protect the aluminum from oxidation, it may not be thick enough to prevent the aluminum from interacting with the solder (a speculation).
