Is there a problem that the tinned wire is loose in the screw terminal?

Many companies and manufacturers adopt the practice of dipping the end of the wire, but this can lead to loose connections.

In addition to ensuring that all screws are tightened to the correct torque specification, this document also explains the reason for the loosening of the tinned wire.

Why use tinned wire?

Multi-strand copper wires are usually connected to PCB or other equipment through screw terminals.

“tinning” is a practice of coating a thin layer of tin compounds on stripped wires.

However, when used with screw terminals, there will be problems related to loose wires.

A common reason for tinning is to prevent wire harness from wearing when connected to the termination point.

For example, pushing a stranded wire into a screw junction box.

In this case, the wiring harness may not be fully pushed into the cage, resulting in a short circuit.

If wires are frequently installed and removed from their termination points, the wires will become more and more difficult to use.

Tinning seems to be a good solution.

Why is the tinned wire loose in the screw terminal?

The root of the problem is that when the wire is tinned, the tin material flows into the narrow space between the copper brackets of the wire.

This turns copper wire and tin into solid materials.

Once the wire is inserted into the junction box, the cage is closed and sandwiched to the tinned wire, and the tin & wire block is compressed and shaped.

In doing so, the tin may break and cause the strands to pull apart from each other, creating gaps (figure 1).

In essence, when the wire is tightened, it separates the tin and begins to loosen the wire.

Although the wire may be fixed at first, normal operation may cause the wire to become loose.

Operation and thermal expansion

During operation, as the environment heats up and / or current flows through the wire, the wire heats up and cools.

As the temperature of the wire rises, thermal expansion may be the main culprit for loosening the termination.

The higher temperatures in wires and tin materials cause the metals in each of them to expand naturally and occupy a larger volume.

However, material properties (coefficient of thermal expansion) lead to different amounts of expansion of different metals.

In this case, the tin material expands more than the copper (figure 2).

As the metal cools, they contract and occupy a smaller volume.

Expansion and shrinkage will cause a slight change in the shape of the tinned wire, which can no longer be properly clamped between the cage and the pressing plate (figure 3).

Other common misunderstandings

Tinned wire will increase corrosion resistance.

Yes, but when the tinning wire is compressed in the terminal, the tin will separate, so the connection may no longer be airtight.

Bare strands will form a better airtight connection and can be a more effective way to reduce corrosion.

Tinned wire is still suitable for terminals.

In some cases, the added material will prevent the wire from being installed correctly in the block.

The addition of tin will make the wire larger than the rated size of the terminal.

This will prevent the terminal from being improperly closed / clamped on the wire.

The last thought

All in all, the tinned wire may be the reason for the loosening of the screw terminal connection.

While tinning does have benefits, there are better alternatives that can help you avoid headaches caused by loose terminations.

A good alternative is to use rings and make sure the screws are tightened to the correct specifications.