Soldering a splice has both advantages and disadvantages when compared to crimping cable. Advantages include a corrosion resistant connection as the tinned wires and solder present little if any corrosion potential, as well, a good solder joint will not generally come apart easily. However, disadvantages include increased failure potential as the soldered area presents a hard spot, as it is essentially solid wire, and vibration in the boat can cause a stress related failure of the connection. Also, some degree of skill is required to properly solder the wires, which probably limits its use to technicans familiar with the practice.
Again, we start out with the same type of connection as we did in the crimp example. However, in the soldering example, notice that the wires are stripped a bit longer. In practice, 1/2" or more is needed. The longer leads are required to provide some mechanical rigidity to the solder joint. As well, in this example, we are going to use Heat Shrink (although we could have also used Heat Shrink for the crimp connection as well). Consequently, the cable's jacket may have to be stripped back further to allow for the Heat Shrink.
Notice that we placed Heat Shrink on each individual wire as well as the jacket. Obvously the heat shrink must be placed on the wire before soldering them together. Notice that the reason we had to cut back the jacket in the last step is to have sufficient room to put the Heat Shrink on the Red and Yellow wires, without interfering with the bare wire to be soldered. Heat Shrink is available in plain and adhesive versions. The adhesive version will provide more waterproofing capability, so where the cable is going to end up will dictate what Heat Shrink you will use. You can buy Heat Shrink in rolls, strips, or in a kit, with pre-cut lengths of a couple inches. The kit usually has different sizes, and if you plan on a lot of Heat Shrinking, it would be worth the investment.
The best practice in soldering is to create a mechanical bond; that is, with the solder not the sole means of connection. While the mechanical options are limited in this situation, twisting the wire is sufficient. The longer the bare leads, and the more twists you can put into the connection the better. You may also find that in practice, you may only be able to twist and solder one wire at a time, as the other wires may un-twist when you are handling the cable during the soldering process. This should actually provide some indication that the mechanical bond isn't quite as good as it should be. If this happens, try to put more twists into the connections.
Now the hard part. I have to emphasize again that this does require some skill to properly solder the wires. The soldering tool of choice is going to depend somewhat on the size of the wires you will be soldering. I have a 25Watt soldering pencil, a 40Watt soldering iron, and a 125Watt soldering "Gun" (Weller) that I use, depending on how large the wires are. Most boat wiring will have PVC insulation, which will melt if too much heat is applied during the soldering process. Often is it better to use a hotter soldering instrument which will quickly heat the solder joint without melting the insulation. This is because the longer you leave the iron on the wires, the more time the heat has to heat up the insulation. Again, its all skill and technique.
Solder choice is also important. You must never use ACID CORE solder, the type often used for plumbing and sheet-metal work. Only use solder intended for electrical work, which will be ROSIN CORE (or have no flux at all). The cored solder has a FLUX, which is a pasty substance that helps clean the connection as the solder flows. Acid core will corrode and damage the wiring over time, where Rosin core will not. However, those with accomplished soldering skills can often solder the wires without using any flux at all.
The proper technique for soldering is to touch both wires simultaneously with the iron to heat them, then apply the solder to the joint - not the soldering iron's tip. This ensures the proper temperature has been achieved necessary for a good solder joint. A good tight twist in the wires also helps in this regard. If insufficient heat has been put into the wires, a "cold" solder joint will result. Such a connection is prone to failure. When the wires are soldered, the solder on the joint should have a shiny metallic look to it. A dull metal look is a sure indicator of a cold solder joint.
Once the solder job is completed, slide the Heat Shrink into the solder joint and heat it up with a heat gun. A flame, such as a lighter can be used, but there is an increased chance of burning the wire or Heat Shrink. So buy a heat gun if it is in your budget.
As in the crimping example, to prepare the cable for final dressing, cut the unused wires off at different lengths, Optionally, after cutting the unused wires, you may cap them off with Heat Shrink.
Like the Crimp example, dress the wires on the cable so that they all lay down as best as you can. Then slide the Heat Shrink you placed over the jacket into place, and again with a heat gun, shrink it into place. While this example showed the use of Heat Shrink, electrical tape could have been used to close the jacket. However, it is highly recommended to use Heat Shrink and not electrical tape at the solder joint for insulating each individual wire.
The chief disadvantage to soldering wires may be vibration. The sole purpose of using stranded wire in a boat is to prevent wire breakage, which could occur with solid wire. As the boat vibrates, solid wire cannot flex nearly enough, and especially at connectors where one surface is relatively fixed, constant flexing can easily break a wire. Therefore, the best practice dictates that soldered joints should only be done in an area where the wires leading to the solder joint can be prevented from moving. For a spliced wire, this typically means securing the joint with cable clamps or similar methods as shown. Soldering connectors is permissible if the connector has a method to secure the wires from moving. Circular connectors usually have a wire-gland that does this, so they would be suitable for soldering. White molex-type connectors do not have this additional support, and are not appropriate for soldering.
Next we'll present different (and allowable) terminal types, as well as a few other methods of splicing wires.