Soldering Wires and Connectors

Solder used in automotive electrical applications is an alloy typically made up of 60% tin and 40% lead. Solder needs to change from a solid state into liquid easily and return to its solid state quickly. Solder is available as solid or flux cored. Solid solder requires an external flux to be applied in the soldering process. Flux is needed to prevent the metals from being joined due to oxidization when they are heated. Flux-cored solder has a bead of flux within the center of the solder. Flux in flux-cored solder can have either an acid base or a rosin base. Acid flux is designed to be used on nonelectrical metal joints such as radiators and must be removed after the soldering process so that the joint does not corrode. Rosin flux solder is used on electrical connections because it is much less likely to corrode the metals than acid flux. Acid flux and rosin flux also come in paste form that can be brushed onto the joint if using solid-core solder.

Solder is applied with a hot soldering iron. The soldering iron is heated electrically or by an external source such as a butane or oxyacetylene torch. The soldering iron tip absorbs heat that is then applied to the materials to be joined. Once they are hot enough, solder can be melted between the components. It solidifies as it cools, “gluing” the metal pieces together.

For a connection to be successful, the soldering iron needs to be clean and “tinned.” Cleaning may be as simple as heating the tip and wiping it on a damp cloth. Or, with the soldering iron cold, you may need to use a file to remove oxidized metal and reshape it so it can effectively transfer heat to the wires. The tinning process assists in transferring heat to the wire, by leaving a small amount of liquid solder on the tip which increases the surface area where the tip contacts the wires. To tin the soldering iron, the tip is heated and a small amount of solder is applied to the tip. Excess solder is removed with a cloth rag. The soldering iron tip is heated and then applied to the wire so heat is transferred to the wire. The solder is then applied to the wire opposite the soldering iron. Once the wire is up to soldering temperature, it will melt the solder and pull the solder into the strands of wire producing a strong, effective joint. Do not apply too much heat to the wire or two things will happen. First, the solder will be drawn too far up the strands of wire making a very long, nonflexible joint that is subject to breaking. The second problem is that the insulation may overheat and melt.

To solder wires and connectors, follow the steps in Skill Drill 36-3:

1
When using a soldering iron, you must be careful not to burn yourself or any part of the vehicle you are working on. The tip of the soldering iron has to be hot enough to melt metal solder, so make sure it is in a safe position and not touching anything while it is heating up.
2
While the soldering iron is heating, remove an appropriate amount of the protective insulation from the wires. Always use a proper stripping tool that is in good condition. If you intend to seal the joint with a heat-shrink sleeve, cut a section of this tubular material long enough to overlap the wire insulation on both sides of the joint and slide it over the end of one of the wires before joining them. (Photo 1)
3
Twist the wires together to make a good mechanical connection between them. If there are impurities in the solder, and the wires are not directly touching each other, then although there may be a strong physical connection, there may not be a good electrical connection. It is also very important that the surfaces be very clean before soldering or there will be a poor connection. (Photo 2)
4
Tin the soldering iron tip, and use the soldering iron to gently heat up the wires while placing the solder opposite of the soldering iron. Allow the solder to be drawn into the joint, ensuring that just enough solder runs smoothly into the wires. (Photo 3)
5
Be careful not to use too much solder. Also, if you apply too much heat, you will melt the wire insulation. When you have finished soldering, clean any excess flux from the joint with a damp rag. (Photo 4)
6
Once the electrical connection has been made and it has cooled enough for you to be able to handle it, slide the insulator sleeve over the joint. There are different types of sleeves. The most popular type is heat-shrink tubing, which shrinks when heat is applied to it with a heat gun. Another type contains a glue, which when heated with the heat gun melts into and seals the joint. If there is no heat-shrink tubing available, then it is possible to seal and protect the splice with electrical insulating tape. (Photo 5)
7
To solder a wire to a terminal connector, it is best to crimp it in place as before and just use the solder to “glue” the joint together.
8
Place the heated iron onto the terminal to get it hot enough to melt the solder applied to the end of the crimped wire tabs. Some solder will be pulled between the terminal and the wire. Be careful not to use too much solder. If you get the terminal too hot, the wire insulation will start to melt.
9
Once the electrical connection has been made and it has cooled down enough for you to be able to handle it, you can place the heat-shrink tubing over the terminal, heat it up with a heat gun, and place the connection into service. (Photo 6)
SKILL DRILL
36-3
Soldering Wires and Connectors
No Image
1
Safely position the soldering iron while it is heating up. While the soldering iron is heating, remove an appropriate amount of the protective insulation from the wires with wire strippers.
No Image
2
Twist the wires together to make a good mechanical connection between them.
No Image
3
Tin the soldering iron tip and gently heat up the wires while placing the solder opposite of the soldering iron. Allow the solder to be drawn into the joint.
No Image
4
A good solder joint where the solder has been drawn in.
No Image
5
Once the electrical connection has been made and it has cooled enough for you to handle it, slide the insulator sleeve cover over the joint and use a heat gun to shrink the tubing around the joint.
No Image
6
To solder a wire to a terminal connector, it is best to crimp it in place as before and use the solder to “glue” the joint together. Place the heated iron onto the terminal to get it hot enough to melt the solder applied to the end of the crimped wire tabs. Some solder will be pulled between the terminal and the wire. Cover the terminal with heat-shrink tubing.
 
SAFETY
While soldering is generally thought of as a simple process, it can be very dangerous. The solder, soldering iron, and wires are very hot and can cause severe burns. Be careful what you grab or where you set hot items. Molten solder can be flicked by a springy wire up into your eyes, so always wear safety glasses or goggles.
 
Technician Tip
One mistake students make is trying to apply the solder directly to the tip of the soldering iron while the iron is heating up the wires. This does melt the solder, but it is likely that the wire is not hot enough for the solder to stick to it; instead the solder just globs on top of the wires, leading to what is called a cold joint. A cold joint has high resistance and the wires are likely to break loose from the solder. One sign that the solder joint is good is that you can clearly see the outline of the wires on the surface of the solder, all the way around the joint.