How to solder electrical connections
Learning to solder is a skill well worth the effort to master. It doesn't require a lot of equipment, supplies or technique, but there are a few things to be aware of. The following text will give some tips and info on soldering electrical wire and components.
 Some things you'll need (and good additions to your Electrical Repair tool box)
 Soldering guns, irons, and torches
For work where there is no electricity to use an electric soldering gun or iron, you can use a butane torch. There are models available that heat ta tip with the flame rather than applying an open flame to the solder joint.
A supply of 63/37 (tin/lead) or 60/40 solder. 63/37 is a eutectic mixture having a low melting point (183° C or 361.4° F). Solder diameter varies, 24 gauge (0.0201") is sometimes used for harness making, and it works well for circuit board assembly also, a good mid-sized gauge. Generally staying at or below about 1/16" diameter is fine for automotive electrical work. Rosin core solder is OK but doesn't mean added flux won't be needed.
In recent years, lead solder has been replaced by "lead free" solder. This is made from a combination of different elements such as Sn, Ag, and Cu (tin, silver, and copper). Generally the lead free solders have a higher melting point than tin/lead solder. Lead free solder uses a flux made for it, as well.
Rosin flux is used exclusively for electrical work when using tin/lead soft solder. Rosin flux isn't water soluble, so it has to be removed w/hydrocarbons like acetone, lacquer thinner, xylol, etc. Avoid breathing or contacting these compounds. With the advent of lead free solders, water soluble fluxes have been introduced.
Acid core solder or flux should never be used when doing any type of electrical soldering, the acid will cause corrosion and failure of the connection over time.
 Other tools and supplies
- Small brass flux brush (about the size of a toothbrush)
- Dental pick
- a small flashlight or drop cord for working under a dashboard for instance
- De-soldering tools (some are spring loaded syringe-like devices, some are a simple suction bulb) that will suck the solder from a joint when the joint is sufficiently heated. Desoldering braid is another way to remove solder. It works by wicking the solder into the braid, sometimes made from tinned copper.
- A quality set of automatic strippers with a depth gauge.
- A quality tie wrap gun, various tie wraps.
- A selection of various terminals and connectors in the 3 popular sizes.
 Heat shrink tubing
- Shrink tubing of various sizes and colors (cheaper in bulk), makes neat, tidy, and professional transitions from joined wire section to section.
- A heat gun is required or a hair dryer (takes more time, but it works) for the shrink tubing.
- Do not use matches or a lighter for heat shrink tubing, this can melt the tubing, make it brittle, and it may not shrink to the proper size. It will slide right off the joint in some cases.
- While not advised, in a pinch shrink tubing can be applied by using your soldering iron. Start the shrinking from the ends just beyond the soldered joint. Shrinking from the ends first will make the tubing shrink to the wire size leaving the soldered joint encapsulated in a cocoon, it will prevent the tubing from slipping, then do the middle. It is not necessary to touch the tubing with the soldering iron just get it close.
 Some helpful tips
Typically you want to use as little solder as possible, because with large quantities of solder it ends up being a heat sink.
For automotive electrical work, you want a HOT iron. A good iron will have a capacity of at least 35 watts. If you can afford a temperature controlled iron buy it, you will be glad you did and you will never regret it.
It's always good practice to tin the wire or connector before soldering. This is nothing more than applying a small amount of flux to the wire to be soldered, then heating it and applying a small amount of solder. This will make the joint stronger and easier to make. Be aware that solder will wick up a wire and make it stiff. Avoid this, the tip of the wire is all that needs to be tinned.
Keep the soldering iron tip clean and tinned. Put it away tinned so it's ready for use the next time it's needed. A damp sponge can be used to occasionally wipe the tip during work, this removes any build up of solder or burned on flux. Another tip is to use a sal ammoniac block if the tip is especially bad. Rub each face of the well heated soldering iron(not red hot) on the sal ammoniac tinning block along with a little solder applied to the tip of the iron.
 The procedure
- Strip the wire to fit the terminal sleeve, with about 1/4 inch showing.
- Without twisting the stripped end, tin it until a good silver coat appears.
- Next, get two pieces of shrink tubing -- black for the first insulator, and a selected color for the top to slide over the black, about a 1/4 inch shorter than the bottom; cut and slide them on the wire.
- Next, crimp the terminal. A good mechanical connection is critical before it is soldered. Solder should not be used to support a connection, just to bond it.
- Next, solder the wire to the ring terminal, "wicking" the solder up into the terminal sleeve until it is filled, smooth, and shiny silver.
- Finally, slide the black (longer) tubing up the wire and flush with the end of the terminal sleeve on the ring end, and shrink it. Keep the wire straight until it cools, unless a pre-bent angle is desired.
- When cool, slide the colored tubing (red for example) over the first, and flush with the ring end, and shrink it down. Net result is you have a terminal with a red marker and about a 1/4" trailer behind it. Looks professional, and lasts forever.
- To extend a wire, strip a good length from both wires, slide the shrink tubing over the wire and out of the way, then make a good mechanical connection by placing the stripped ends parallel to each other, wrap one clockwise and the other counterclockwise. Solder until you have a good flow and coverage and silver (not gray) appearance, then slide the shrink tubing over the soldered joint and heat to shrink.
- Even on crimp terminals, solder AND crimp.
It is important to always heat whatever is being soldered (wire, terminal, etc.) to the point that the solder will melt when it contacts the wire or terminal. That assures good contact. Do not just apply the solder to the tip of the gun or soldering iron. This will result in a cold joint. Always melt the solder on the material being soldered.
Even if using rosin flux-core solder, adding flux to the joint is recommended.
Only use rosin flux when doing electrical work. They even make a rosin core solder especially for electrical work. Other flux can lead to corrosion. Acid fluxes are best used on non-electrical work like radiators, etc.
- Electrical abbreviations from archtoolbox
- Crankshaft Coalition wiki articles
- Troubleshooting electrical problems
- Understanding the math of electrical loads
- Wiring diagrams
- Remote Ford solenoid for GM- no hot start
- Automotive wiring 101
- Relay application guide- Bosch relays
- Cleaning soldering iron tip using a sal ammoniac block