We frequently receive questions from customers on how to solder thick cables. Either two thick cables to each other or a thick cable to a connector. For us, a cable is considered thick if it has a cross-sectional area of 2.5 mm² (14 AWG) or more. Soldering these thick cables is troublesome because the solder doesn’t stick to the cable. Why does this happen and how can it be resolved?
Why is it so difficult to solder thick cables?
The cable consists of a significant amount of copper, which conducts heat very efficiently. When you apply the soldering tip to the end of the cable, the heat transfers easily away into the rest of the cable. The thicker the cable, the larger the cross-sectional area that can transfer heat, making it even easier for the heat to dissipate. This efficient heat transfer prevents the solder from properly melting and adhering to the cable.
You can compare the end of the cable you want to solder to a barrel. The temperature of the cable end is represented by the liquid level in the barrel. To be able to solder, the liquid level needs to be near the top. The barrel has one small input on top through which liquid is pumped in. This input is represented by the soldering tip and the pump at the soldering station. On the bottom of the barrel, there is a large hole through which the liquid escapes, symbolising the thick cable which causes the heat to leak away. Because of this hole, the barrel will never fill up, meaning the temperature will never be high enough for effective soldering.
At a certain point, the amount of heat you apply with the soldering iron to the cable equals the amount of heat that dissipates. If this equilibrium temperature is below the ideal soldering range of 320℃ - 350℃, the solder won’t melt properly. This results in a poor connection since the cable doesn't reach the necessary temperature for effective soldering.
But how are thick cables soldered then?
The first thing most people try is increasing the soldering temperature. While this can help by transferring more heat into the cable, it also comes with a few downsides. Higher temperatures burn through flux more quickly and cause the soldering tip to degrade quickly. In general, increasing the temperature will not lead to satisfactory results.
The root cause of the issue is that the surface area of the tip-to-cable contact is much smaller than the cross-sectional area of the cable. In the barrel analogy, the hole on the top of the barrel is much smaller than the hole at the bottom. The best way to address this is by increasing the size of the hole at the top of the barrel. This means increasing the contact area between the soldering tip and the cable to transfer more heat efficiently.
There are several ways to increase the surface area of the tip-to-cable contact:
- Use the biggest tip you can find with a large surface area to maximize contact with the cable.
- Add a bit of solder to the tip before making contact with the cable. The solder bridges all small gaps between the tip and the cable, significantly increasing the heat transfer. While this is normally not recommended, this technique is acceptable in this case.
- Use a hot-air station together with the soldering iron to insert additional heat. This helps to maintain the necessary temperature for effective soldering.
There are also some less refined methods that might lead to acceptable results for the occasional joint. Firstly using a gas-powered torch to heat the cable, gives little temperature control, but quickly adds a lot of heat. Secondly, using leaded solder with a lower melting point can help, but it introduces health, environmental and compliance issues.
What is the best solution?
The provided solutions above might work but are not ideal. The best solution is not to solder thick cables. The best practice is to use crimping connectors on thick, but even thin cables. Crimping a thick cable is quick, easy, reliable and consistent. A cheap €40 crimper and a €0,20 cable lug provide excellent results for even the thickest cables. Another advantage of crimping over soldering is that crimped connections are much more resilient to mechanical stress. This is particularly important for thick cables, where mechanical stress is often a major issue.
If you encounter a situation where you need to replace connectors on thick cables and the existing connectors are soldered, contact the manufacturer to find an alternative connector which can be soldered. Crimping is today’s industry standard, and soldering is only intended for joints on printed circuit boards.
