Soldering 101

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A few things to remember before you start:

  • Most people use crimp-style bullet connectors on automotive applications. One reason is that they do better with high power applications. Also, mechanical vibration can cause solder to crystallize, leading to joint failure. That said, Wes, who rides in salted slush six months of the year, has never had a connection fail on his bike, either soldered or crimped.
You still have to solder certain things, like grounds onto your 1157 sockets when adding extra rear lights.
  • Get a ~30 watt pencil soldering iron. They are available at Radio Shack or, in Canada, The Source, for about $10. Try to pick up a soldering sponge or two at the same time.
  • Make sure you tin your tip before using it. That means: soak it in solder, knock the excess off, and wipe it clean with a wet soldering sponge. If you don't have a soldering sponge, a damp cotton or terrycloth rag will do the trick. This will not only extend the life of your tip, but by having a clean tip, you will produce better joints, more quickly, and with less heat damage to surrounding components.
  • You want to be using rosin-core electronics solder, about 16-22 awg, not that big thick stuff designed for plumbing. You want either a 63/37 or 60/40 tin/lead alloy. The 97/3 and the "lead free" stuff are both inferior. They make it harder to visually inspect the joint, because they have a naturally duller appearance; also, they require higher temperatures, which means that your iron or your wire's insulation may not be up to the task.
  • Flux is superfluous when soldering electronics (not copper pipe), as flux is included inside proper electronics solder ("rosin-core").
  • Always apply heat to the object being soldered, then apply solder to the object, not the iron, no matter how tempting it is. Solder flows toward heat (ie, the iron), so put the iron at the end-point of where you want solder to go.
  • A good solder joint will be shiny silver, not dull. You should be able to tug lightly on the joint (don't yank on it, just gently pull), and it shouldn't come apart.
  • In general, you want to start with a good mechanical connection (ie, wires twisted solidly together) before soldering, then use solder to "glue" the joint together. This isn't always possible, but it's a good goal. Laying one wire on top of another and soldering them together will make a joint which is more likely to fail than one in which the things being soldered are already solidly connected.

Soldering a wire to something

Explanation: "Tin the X" means to cover X in a super-thin layer of solder. One effective way to solder two pieces together is to tin each piece, touch them together, and apply heat to the larger one until the smaller one's solder melts and the pools join together. This will create a good electrical and mechanical joint.

Steps 1 and 2 are to tin the bulb socket and wire; the sub-steps describe how to achieve this with each piece.

Step One: Tin the wire you are about to solder

1a. Heat the wire with the soldering iron
1b. Touch the solder to the wire so that the (stranded) wire "wicks" it in. Don't leave the iron on so long that the insulation melts.
1c. Use as little solder as possible

Step Two: Tin the socket

2a. Sand a spot on the socket so that bare metal is showing
2b. Heat that area of the socket with the iron. Hold the iron at a 45 degree angle to the socket so that the tip of your iron makes maximal contact
2c. Touch the solder to the socket until it melts and pools
2d. Use as little solder as possible

Step Three: Make the connection

3a. Hold the iron as above, but also touch the tinned portion of the wire to the socket
3b. When the solder which was on both the socket and the wire has melted, remove the iron. Do NOT move the wire until things have cooled down again
3c. Your goal is for a nice, shiny connection
3d. Note that in this stage you are NOT adding solder

The finished product should look something like this:

Soldering 1.jpg

Next, it is advisable to protect the joint from the elements, but this is not really necessary. 3M rubber electrical tape works well here. It's like regular electrical tape, except it's made of rubber rather than vinyl, and is quite a lot thicker. Hot glue also works for this.

Soldering wires together

For repairing a wiring harness or other wire-to-wire soldering:

  • Perform a Western Union splice without tinning the wires. The Western Union (or Lineman's) splice is a general-purpose splice suitable for joining wires of similar diameter (4 awg difference max). It has excellent mechanical strength, small size, regular (inline) shape, and very low resistance. Before splicing, cut and strip the wires. If they are stranded, twist them loosely so that there are no frayed bits in the way.
Soldering 2.jpg
  • Put a length of heat shrink tubing about 2" long over one of the wires. Move it out of the way, so you can use it to seal your joint when you're finished.
  • Heat the bottom of the wires with your iron, and apply solder at the top. As soon as the solder melts, it will go all the way through the wires, just like water into a sponge. This will yield an excellent mechanical and electrical connection. If you have trouble with insulation melting, try clipping it at each end with alligator clips, vice grips, etc, to act as a heat sink.
  • Use as little solder as will do the job. You're not trying to make a big goopy ball.
  • Slide the heat shrink tubing over your new connection, and shrink it. Use a very hot hair dryer to do this. You can use a lighter or your iron if you're very careful.
  • Multiple splices in the same wiring harness should be offset in a step pattern (ie... don't put them all in the same place) to keep the diameter of the wiring harness down.


Consult this page for more information. Look for the Soldering Photo Gallery at the top of the article.

There is also another article called Soldering 101, with more photographs.