Crimping vs Soldering

Does soldering really anneal the wire and weaken it? Can a crimp join really be as strong as a chemical bond? Today we dig into the fundamentals of both methods and find out once and for all which is best.

Battlelines drawn

You have probably heard people arguing about the merits of soldering your wiring connections vs crimping. The “solder is best” camp swears that soldering is more reliable because it forms a chemical bond and that soldering is less bulky and thus better. On the other side of the fence, the crimping advocates warn that the heat generated by soldering is annealing the wire and weakening it while bringing up countless examples of a cracked solder joint.

So who is right? Does soldering really anneal the wire and weaken it? Can a crimp join really be as strong as a chemical bond? Let’s dig into the fundamentals of both methods and find out once and for all which is best.

The case for solder

Soldered connections are formed by melting a small amount of alloy into and around the two wires being joined. This requires the use of a soldering iron and of course the solder material itself. As the solder material is melted, it wicks its way up and into the two wires, and when left to cool reforms as a hard interpenetrating connection that mechanically bonds the strands of two wires together. Looking at a nicely formed solder joint on the surface it’s easy to see why many people believe this to be the ultimate in joining techniques.

Its strong, its not bulky and there is excellent electrical conductivity between the two wires being joined. It’s important to note, once the join has been made, it needs to be covered by an insulating material, either heatshrink or worst case a tape join.

The crimping option

Let’s contrast that now with a typical crimp connection. In this method of joining two wires there is no heat required and no chemical bonding happening. A crimp join just uses old fashion compression to hold the join together. For joining two wires together we typically use something like these open barrel crimps, we overlap the wire and crimp down using a correctly sized crimping tool.

A crimped join is also strong join, it doesn’t need to be bulky and also offers excellent electrical conductivity between the two wires being joined. Again, after the join is made, it should be covered by an insulating material.

Good Join vs Bad Join

We need to be clear here; when done poorly – both soldered and crimped connections are a bad! A good solder join is better than a bad crimp, and vice versa a good crimp is better than a bad solder join.

The Twist-and-Tape method

Okay, let’s quickly address this well-known and frequently used technique. The twist and tape method may have its place – it’s just not in your car, and certainly not in engine management wiring.

The Verdict

So what IS the best way to join a couple of wires in an automotive wire harness? In 90% of cases, the crimp is the superior connection method in a wiring harness and that’s the method we recommend for ECU installations.

But why?

The two biggest problems with solder connections in a wiring harness is corrosion and cracking due to mechanical stress or vibration. Both of these issues are generally preventable with adequate strain relief and joint protection such as heatshrink, or better yet, heatshrink with a glue lining. Cracking of solder joins in wiring harnesses is not an uncommon occurrence, especially after many years of service. There are many examples of OEM vehicle manufacturers’ solder joints cracking or corroding causing any number of issues.

Best applications of solder

That’s not to say there is no place for solder joins in an automotive application. Wherever there is a PCB (printed circuit board) involved, we recommend using a solder join. Another instance where a solder is preferred over a crimp is where we don’t have a mating connector for a sensor and we have to make a direct wire connection between a pin and a wire. In that case we would also solder the wire directly to the pin rather than use a pin to pin connection. It’s highly recommended to also backpot the enclosure to prevent any movement of the wires away from the pin.

Is more better?

If crimping is good, and soldering is not as good but still OK – then is soldering a crimp join the best option available, being a combination of both methods? The simple answer to that, even if it’s somewhat counter intuitive, – is no. Adding solder to a crimp join does not improve the crimp. In fact, it’s detrimental because the additional heat that is added to the wire and the solder inevitably wicks up the wire and can actually weaken the wire itself making the join more susceptible to cracking.  

Before you start crimping or soldering

First get a quality set of crimping tools, they’ll make the whole process a lot easier. Find a reliable supplier of open barrel crimps in a multitude of sizes for joining one, two or ten wires together. Get some heatshrink, again in a variety of lengths and diameters. We recommend using a quality glue lined heathrink where the space allows. Don’t forget to add the heatshrink to the the join BEFORE you make the join.

And finally – Don’t be afraid to pull out the soldering iron to make those one-off random connections, but only if you have a way of mechanically retaining the entire join.