Soldering is one of those skills that every maker and DIY'er should have under their belt. It's actually easy to do, but it's easy to do wrong too.
What is soldering? Basically, it's like gluing two wires together*, but instead of a glue, you're using another metal. That metal is called "solder". In the old days, solder was an alloy of lead and tin, typically around a 40/60 mix. Because of health concerns about lead in the environment, most solders today are mostly tin, with small amounts of silver, zinc, copper and/or other metals added to deliver desirable properties. You can still find lead solder for use in roofing and plumbing applications**, where it's sold in bar and rod form as a 50/50 mix of tin and lead, with small amounts of zinc or copper thrown in.
This Instructible will focus on soldering wires together, and I'll discuss two very useful joints - the splice (aka end splice, aka Western Union splice) and the t-tap.
* Crazy as this sounds, it's actually technically correct. Both gluing and soldering use inter-molecular forces to achieve a bond between surfaces.
** Fun fact - the first pipes used for indoor water lines were made from lead, and used widely in Roman bath houses. The Latin name for lead is "Plumbum", which is why indoor water piping is called "plumbing" today.
Step 1: Strip and Join Mechanically
The end splice is used to tie the ends of two wires together. Since the wire is broken, we can use heat shrink tubing to insulate the connection after it's made.
First we use wire strippers to remove about 3/4" of insulation from the end of the wire. If you don't have a set of wire strippers, then you can use a sharp knife to carefully trim the insulation as though you were sharpening a pencil. Take care to avoid cutting the wire. If you you're working with stranded wire, and you cut too many strands off, it will reduce the current carrying capacity of the wire. If you are using solid wire, a nick on the wire could lead to wire breakage if the wire is stressed.
When working with stranded wire, as soon you strip the insulation, the strands will want to fray. Tightly twist the end of the wire in order to keep all the strands together. These twists also add additional strength by binding the strands together.
At this point, you'll want to cut a section of heat shrink tubing. The size of the tubing should be about twice the diameter of the wire so that you can easily fit the tubing over the soldered joint we're about to make. The length of the tubing should be at least 3/4" longer than length of the joint so that it provides some overlap on either wide. This is one of those places where you'll want to be generous. It can never hurt to have more heat shrink tubing on either end of the joint. Slide this section of heat shrink tubing over one of the wire ends, and push it up the wire far from the joint so that when we solder, the heat from the soldering iron doesn't shrink the tubing prematurely.
Now we're ready to mechanically join our splice. Cross the stripped ends of the wire at 90 degrees to each other. Pinching the free ends between the thumb and forefinger, twist each end in opposite directions so that they coil around each other. Keep twisting until every bit of the free ends are wrapped around the opposite wire. Use your fingers to mold the ends down so that there are no stray strands sticking out. The end result should look similar to the last picture above.
Trivia: Old Timer's call this joint the Western Union splice. Back in the early days of the telegraph, it was common for telegraph wires to either break under stress or be deliberately cut. This is the joint that lineman developed for rejoining wires. The twist gave it good mechanical strength, and could often be achieved without the need for additional wire just by stretching the wire that was in place. The mechanical connection would be sufficient to restore service on the wire by itself, which was good during bad weather when communications needed to be restored quickly. The lineman could then come back later and solder the lines. I learned to solder from an Old Timer, and this is one of those things you pick up from those guys.
Step 2: Apply Heat and Solder
In this Instructible, I'm using my mini-torch with the soldering attachment. You can also use a soldering iron of at least 35 watts for this size wire, but you'll get better results with a more powerful iron. You also need to wait for your soldering iron to reach temperature. An iron can take up to five minutes to reach working temperature. My gas torch takes about 20 seconds. A decent soldering gun takes about five seconds, but I don't have one of those right now.
When your iron is at temperature, apply it to the wire with firm pressure and hold in there for a few seconds to allow heat to transfer from the tip of the iron to the wire. If your iron is not clean you will have a hard time getting heat to transfer into the wire. A best practice is to use a scraper, some coarse steel wool or a file to knock the oxidation and dirt off your soldering tip. Then apply a small amount of solder to the newly cleaned surface to get a shiny layer of solder on it. This process is known as tinning. It improves heat transfer to the work, and also prevents contamination of the solder joint. In my pictures, you'll see that my solder tip is not tinned. There's a patina of metal oxide. In this case, it won't contaminate the joint, but it does affect heat transfer. Because my torch gets very hot, it doesn't impede my ability to get heat into the joint.
In the next step, we apply our solder to the wire itself. The goal is to heat the wire up to the point that the solder melts to it. If the wire isn't hot enough to melt the solder, then keep applying heat. It should only take a few seconds to get the joint to temperature, so if the solder refuses to melt, go back, clean your tip and tin it. At this point, people who have soldered a few times are tempted to cheat by putting the solder on the iron, letting it melt on the tip then flow to the joint. If you do this, you're at great risk for a cold solder joint. That means that the wire wasn't hot enough to encourage the solder to flow, and so the solder forms lumps on the outside of the wire. These kinds of joints are doomed to failure. Some folks with a little more experience might also cheat - they touch the solder to the iron, and after it starts to flow, the touch the solder to the wire. The solder from the iron will aid in heat transfer to the wire, which allows you to then flow the solder directly onto the wire. Here's the problem with this: If you're not getting good heat transfer, its likely from dirt and debris on your iron as well as some of those oxides. The solder that flows from the iron into the joint will carry the dirt and debris onto the wire. The technically correct thing to do is clean your tip and tin it to maximize heat transfer and minimize contamination.
You're done applying solder when every part of the joint has a shiny coating of solder on it. However, do not use too much solder - it should not drip off the joint or form bubbles. It should actually look like the copper wires turned silver. If you get a little too much on the wire, you can rub your iron across the joint to draw some solder off. But if you got too much solder on it, you'll need to use a copper de-soldering braid to draw off the excess.
There is one final step here that most people skip. In fact, I did not do it here, but it is part of "best practices". There is a substance that is included in electrical solder called "flux". It's the cruddy brown stuff you see in the photo. It flows into the joint ahead of the solder (due to its lower melting point) and helps the molten solder to flow better. Unlike acid flux, rosin isn't considered corrosive. However, rosin does decompose over time releasing small amounts of abietic acid. This will promote surface corrosion of the joint in moist environments. If you're going to do everything else the 'correct' way, you might as well use a small brush to wipe on 70% Isopropyl Alcohol to clean the rosin residue off. Some people say this step is un-necessary since it would take decades for the corrosion to make a difference. I would advise that if you want your work to last decades, it's a small price to pay in terms of extra effort. (Just wait for the alcohol to dry before the next step.)
Note: While the torch I'm using can also solder by direct flame, that's entirely too much heat for this kind of work. You could burn the thin strands of copper wire, and you will probably burn the insulation. I know it's tempting to use the more powerful direct flame method... just don't.
Step 3: Insulate the Joint
Now that our joint is well and soldered (and dry), slide the heat shrink tubing over the joint so that it's centered.
Heat shrink tubing - as it's name implies - will shrink when heat is applied. You can use a heat gun, a hair drier on the high setting, a cigarette lighter or a torch as I did to shrink the tubing.
No matter what method you choose, you need to rapidly move the heat source back and forth along the work. If you keep the heat in any one place too long, the tubing will shrink unevenly. You could also potentially burn the heat shrink or the wire insulation, particularly with the open flame methods. If you do this, you'll have to tape the joint.
Pro-Tip: If you're using this method to join electrical wires in a home, know that this IS an NEC code approved method. Believe it or not, even your local code inspector might not know this. Most people in the electrical trades were taught that the only place you can locate a splice is in a junction box that is accessible from outside the wall. If you've ever seen a wall plate on the wall for no particular reason... that's why.
So if you are doing a remodel and you need to tap into a wire or extend a wire run, you can use this method. Use separate heat shrink tubing for each conductor, and then use one big piece of heat shrink tubing over all the conductors. And if your code inspector questions it, refer them to NEC 110.14(B) which states "Conductors shall be spliced or joined with splicing devices identified for the use or by brazing, welding, or soldering with a fusible metal or alloy. Soldered splices shall first be spliced or joined so as to be mechanically and electrically secure without solder and then be soldered. All splices and joints and the free ends of conductors shall be covered with an insulation equivalent to that of the conductors or with an insulating device identified for the purpose."
Step 4: Preparing the T-Tap
A t-tap is for when you need to splice into a wire, but you don't want to cut it. A common example for this is in automotive electrical applications where you may want to get electrical power from the ignition harness instead of going directly to the battery.
With a t-tap, you're going to strip insulation from your tap wire the same way you did with the end splice. For the wire to tap into (we'll call this the feed wire), you'll use your wire cutters to cut the insulation in two places about 1 1/2" apart. Use a knife to carefully shave off the insulation without damaging the feed wire.
Cross the tap wire over the feed wire at a 90 degree angle, leaving about 1/4" of length from the feed line. Wrap the tap wire around the feed wire, making sure to keep it tight to the wire and to mold out any stray strands if using stranded wire. If you're using solid wire, I recommend heating it up with a torch to make it more malleable, but don't make it so hot that you burn your fingers.
Once the tap is wrapped, solder the joint using the same steps described previously.
Step 5: Finishing the Tap
Even though this is called a t-tap, we're not going to leave it in a T shape. With your fingers, fold the tap wire down alongside the feed wire.
In the next step, you're going to use a generous strip of electrical tape. Wrap the electrical tape in a helical fashion, keeping the tape under tension as it's applied. You'll start about 1/2" before the bare wire, and wrap until about 1/2" past the bare wire on the other end of the joint. Each successive wrapping of the tape should overlap the last by half the width of the tape.
The tension is key to getting the tape joint done well. You should also make sure you have clean hands when you do this - any dirt, oil or sweat on your hands will transfer to the adhesive side of the tape and interfere with it. I'm also going to be pretty explicit - only use a high quality vinyl electrical tape, not some no-name knockoff. I'm a big fan of 3M Super 33+ for indoor use. For automotive and outdoor use, I step it up to 3M Super 88. I hate to plug any particular brand, but after years as an automotive electrician in the car audio field, I've seen poor quality tapes come apart after only a few months of hot weather, but I've seen 3M products perform for years under demanding conditions.
(Incidentally, you can use tape on the end-splice solder joint we just did as well. I prefer heat shrink overall because it's more attractive and more durable. However, you have to be able to cut the wire to apply heat shrink, and if you could cut the wire, you wouldn't be using a t-tap.)
There's one final step with a t-tap that I strongly recommend. If someone or something were to apply pressure to the tap wire, it could pull back the tape to the point of exposing bare wire from the tap or feed lines. I recommend taking a zip tie and applying it around both the tap and the feed wire approximately 1" down from the joint. This will act as an additional strain relief.
And that's all there is to it. If you follow these steps, you can wire up your projects like a pro.