I saw some questions come up on a model railroading forum about soldering, and I guess there’s a lot of confusing information out there about it. I’ve been soldering for 30 years, so hopefully I can clear up the common questions about soldering for model railroaders.
The main reason you find conflicting information about soldering is the application. If you’re soldering wires to the track, the advice for soldering sheet metal together to scratchbuild cars will be very different.
Rosin core vs acid core solder
The biggest point of confusion when it comes to soldering for model railroaders is the two solder types. You’ll find one book that says to use acid core and another book that says to use rosin core and put yourself in a bind.
Rosin core solder is for electrical work. Always always always use rosin core solder for soldering wires to track. A rosin core solder connection won’t degrade over time.
Acid core solder is for plumbing and sheet metal work. If you’re building rolling stock out of sheet metal, acid core solder is OK for that. It will probably be less frustrating to use. But acid eats metal over time, so that type of connection isn’t electrically sound. The joint will hold together just fine but it won’t conduct electricity for long.
Rosin core solder works fine on copper, brass, and tin. It’s not so great on steel. If you’re going to solder steel, you probably want acid core.
In short, here’s what you need to remember:
- Rosin core is for soldering wires to track and anything else electrical
- Rosin core also works for scratchbuilding out of tin, brass, and/or copper
- Acid core is for soldering steel
Flux in model railroad soldering
The other controversy is whether to use flux. Some people say no, since solder contains a bit of flux already. But I find using flux makes soldering far less frustrating. Flux means I don’t have to use as much heat and I don’t have to keep the iron on the joint as long, which means I’m less likely to do any damage.
The only caveat with flux is that it’s best if you clean it up after the solder joint cools. Dip a cotton swab into a bit of isopropyl alcohol and then scrub the joint, or spray a bit of contact cleaner onto it. This will keep the joint from corroding and discoloring. Some people may prefer to spend more time soldering so they don’t have to clean up the residue afterward. But I find I can solder a joint in less than 1/3 the time with flux than without, and cleaning the joint takes no longer than soldering it took.
Can you skip the cleanup? One of my other hobbies involves vintage electronics, and old circuit boards contain a lot of solder connections. I can tell when a board has been repaired and the technician didn’t clean up the flux afterward. Old repairs from the 80s and 90s look pretty bad today if the flux wasn’t cleaned up. Does the connection still work even if it looks bad? Sure, sometimes.
But when it comes to your track, rework is almost always difficult. You have to take up the track around it too, and you may have to re-ballast. If you want smooth, reliable running trains, take the extra few seconds to clean off the flux.
When not to solder
I always solder my wires to my track to get a good electrical connection on the track. But I don’t solder under the table. It’s a safety hazard. When I need to make an electrical connection between two wires under my table, I use spring lever terminal blocks. They are super quick and convenient, and eliminate the chance of severe injury. They’re more expensive than solder, but completely worth it. One visit to urgent care pays for a lot of terminal blocks.
What soldering iron to use
The other question you may have is what soldering iron to use. If you’re soldering to track with plastic ties or integrated plastic roadbed, use a small iron, in the 15-25 watt range, like in the photo above. A soldering iron for delicate electronics work is less likely to melt your plastic. Believe me, it can still do it, but not instantly.
For large scale railroading, especially soldering to S gauge or larger track that may even have metal ties, use a bigger soldering gun. I love my Weller for soldering my O gauge track.