Let’s look at how to solder wires to terminals. Most people prefer crimping, but soldering is also effective if done correctly.
The key to creating a solid solder joint is to heat the wire evenly to the core. If not, you’ll have to deal with a cold solder joint (a molten metal shell on the exterior).
So here’s how to create solid solder/wire joints in vehicles.
Table of Contents
- Why Solder Wires to Terminals?
- How to Solder Wires to Terminals
- Comparison Between Different Soldering/Wire Termination Techniques
- Wrap Up
Why Solder Wires to Terminals?
Soldering is not as effective as crimping, meaning soldered terminals corrode faster.
Crimping offers better protection from moisture, debris, and dirt, so you might have to redo solder joints between wires and terminals more often than crimps.
When that time comes, follow these steps to create an effective strong connection.
How to Solder Wires to Terminals
Gather the following tools for this process.
- Soldering iron/soldering gun
- Solder wire
- Wire cutter
- Wire stripper
- Heat shrink tubing
A car audio technician soldering electrical wires
Step 1: Remove the Old Connector
Before beginning the soldering process, remove the old, damaged, or corroded connector.
It is easier to cut it off using the wire stripper tool, but you can desolder the wire. If cutting, use a sharp wire cutter to create a straight face at the end.
It is vital to note that if the connector handles two or more wires, don’t cut them simultaneously.
You can short the electrical connection. Always cut the wires separately, even if you unplug the battery.
And after cutting, separate them or cover one with an insulator, such as a heat shrink tube, for safety.
A technician repairing a car door’s wiring by soldering using a soldering iron
Step 2: Prepare the Electrical Wires
Use the wire stripper to remove the wire insulation to about the length of the barrel depth in the connector.
This exposed wire will dip into the barrel and should not be too long to avoid exposing the copper.
Also, it should not be too short because the surface area for soldering won’t be sufficient to create a solid joint.
For instance, with an EC3 connector, strip about 0.01 inches (3-4mm) from the end.
Step 3: Tin the Wire
Tinning involves applying molten solder around the wire before soldering to hold the wire strands together.
This step is necessary because it is easier to weld the strands of wire as one piece.
Apply flux to the cable first to help the molten adhere to the wire strands perfectly. And don’t use too much of it to avoid damaging the wire.
Melting tin on a soldering iron tip (tinning)
Flux residue is acidic, meaning it can corrode the wires. But it is necessary to eliminate oxide layers and impurities above the wire surface for tight adhesion.
The best practice is to wipe the tinned wire when cool using alcohol to remove the flux residue.
You can forfeit flux application if soldering using a rosin core solder or rosin flux paste.
And if using these latter options, preheat the wire to ensure the solder melts into the core. Otherwise, you’ll have a cold joint.
Flux core (rosin core solder)
It is vital to note that tinning is unnecessary when using a solid wire. But vehicle wiring only uses stranded cables because they are more flexible and durable.
So remember to tin the car wire harnesses because none has solid core wires.
Step 4: Solder the Connector
Repeat the same tinning process in the connector to fill the lining with the solder material.
This process will ensure the internal sections bond with the tinned wire to form a solid core when heated.
Next, insert a piece of heat shrink tube into the wire before plunging it into the terminal’s barrel.
Push the wire into the hole, then apply heat and solder around the short exposed outer section to create a solid base.
After that, use the soldering gun or iron to heat the surface around the barrel to activate the solder tinned inside.
This heat transfer creates a solid bond throughout the wire and terminal.
A technician soldering a terminal to a wire
If using a soldering cup, insert rosin-core solder inside, melt it using the soldering iron, then quickly insert the wire inside to touch the bottom.
Hold the wire in place until the solder cools and solidifies.
Once the solder joint is solid, swab it with alcohol to eliminate flux residue and other corrosion-causing contaminants.
Finish off by pushing the heat shrink tube above the joint, then heat it using the gun to wrap it firmly around the exposed metal.
Repeat this process for the other wires, and that’s it.
Comparison Between Different Soldering/Wire Termination Techniques
As stated earlier, crimping gives the most solid wire termination.
So let’s compare different termination techniques and soldering materials against barrel crimping to rank them and determine the best one.
Factors to Consider
We’ll consider these factors in the comparison.
- Joint strength: Sturdy joints are more resistant to vibrations and mechanical stress. So the sturdier the joint, the more reliable it is.
- Thermal conductivity: Although wires generally produce little to no heat, higher thermal conductivity is better for dissipating heat quickly if some resistance develops in the joint.
- Electrical conductivity: High conductivity reduces resistance along the joint, resulting in faster electric current flow to the load.
- Ease of use: This factor shows how easy or challenging it is to create the termination. The latter requires more expertise, making the technique unsuitable for DIYers or beginners.
- Cost: The cost factor is self-explanatory.
| Material or Technique | Joint Strength | Thermal Conductivity | Electrical Conductivity | Ease of Use | Cost |
| Lead-free soldering | Good | Fair | Good | Fairly easy | Low |
| Lead-based soldering | Superb | Superb | Superb | Fairly easy | Low |
| Silver solder | Superb | Superb | Superb | Challenging | High |
| Twist-on connectors | Fair | Low | Fair | Easy | Low |
| Brazing | Superb | Superb | Superb | Challenging | High |
| Crimp connections | Good | Good | Good | Easy | Low |
Although Lead-based solder is better than its Lead-free counterpart, the material contains a toxic element (Lead). So the technique is no longer in use, except for specific high-performance applications. But cars don’t need such high-performance joints.
Lead-based solder wire with a rosin core
Even OEM automotive manufacturers ditched the toxic material for Lead-free solder years ago. So avoid using it in creating solder terminations in vehicles.
Also, avoid twist-on connectors (wire nuts) and electrical tape because their joint strength and thermal/electrical conductivity properties are not the best.
Wrap Up
Similar to when making crimped connections, you need high-quality tools to create reliable solder joints between wires and terminals.
So get high-quality solder wire and flux, wire cutters, strippers, and heat guns/irons for the job. And avoid cold solder joints at all costs.
These brittle connections experience physical failure quickly, increasing resistance along the wire.
Once you sort out these two issues, you will easily create reliable connections between the wire strands and terminals.
Follow the steps above, and the soldered terminals will be ready in no time.
