Electrical conductivity testing: methods and interpretation

What conductivity means

Electrical conductivity describes how easily a material carries current. It is the inverse of resistivity. A high conductivity means a low resistance for a given length and cross-section. The unit is siemens per metre, often written as S/m, or megasiemens per metre for metals.

The other common scale is percentage IACS, the international annealed copper standard. Pure annealed copper is defined as 100 per cent IACS, which corresponds to 58 megasiemens per metre at 20 degrees Celsius. A material at 50 per cent IACS has half that conductivity.

The two scales are easy to convert once the reference is fixed, but they are not interchangeable in a specification. State the scale, the reference temperature and the units. A drawing that says conductivity 58 without a unit is not a requirement.

The four-wire method

The four-wire method, also called the Kelvin method, passes a known current through the sample and measures the voltage drop across a defined length with a separate pair of contacts. Because the voltage contacts carry almost no current, the resistance of the leads and contacts is removed from the result.

The sample geometry must be known. For a bar or a wire, the cross-section and the distance between the voltage contacts give the resistivity. For a plated part, the measurement may need a coupon of the same thickness and process, because the substrate and the layer act in parallel.

The method is accurate but slow. It needs a stable current source, a sensitive voltmeter, clean contacts and a controlled temperature. It is the method to use when the result has to be defended, such as a material certificate or a dispute.

The current should be high enough to produce a measurable voltage but low enough to avoid heating the sample. A small current through a large bar can give a voltage that is hard to read, while a large current through a thin wire can raise its temperature during the test. Choose the current for the geometry, and let the sample settle before reading.

Eddy current and portable meters

An eddy current conductivity meter induces a current in the surface and measures the response. It is fast, non-destructive and works on a finished part. That makes it useful for goods-in checks and for sorting alloys.

The reading depends on the depth of the induced current, which changes with frequency and with the material. A thin plating, a coating or a surface layer can dominate the result. The meter needs a reference standard and a flat, clean surface of sufficient thickness.

Portable meters are often used to check whether a copper bar is the expected grade or whether an aluminium part has the right temper. They are less suitable for a thin foil, a small wire or a part with a complex geometry, where the four-wire method or a coupon test is more reliable.

The meter needs to know the thickness of the material, because the eddy current field extends only so far. If the part is thinner than the field depth, the reading is influenced by what is behind it. A curved surface or a small radius can also change the coupling, so a flat reference block is not always a fair comparison.

Methods compared

The table sets out the trade-offs. The right method depends on the part, the accuracy needed and whether the test is destructive.

Correct for temperature

Metals become more resistive as they warm. Copper changes by roughly 0.4 per cent per degree Celsius near room temperature. A reading taken on a warm shop floor can be several per cent away from the 20 degree value before any material difference is considered.

The test record should include the sample temperature and the correction applied. Many meters correct automatically to 20 degrees Celsius, but the correction assumes a temperature coefficient. If the alloy is unusual, confirm the coefficient rather than accepting the default.

For a comparison between two suppliers, measure both at the same temperature with the same method. A difference of a few per cent may be temperature, not material. A difference of 10 per cent or more is worth investigating.

Let the sample stabilise before measuring. A part that has just come off a press or out of a drying oven may still be cooling, and the reading will drift while the meter settles. A surface probe and a bulk reading can also differ, so state where the temperature was taken.

Specimen preparation and contact

Clean the surface before measuring. Oxide, oil and plating can add contact resistance or change an eddy current reading. Use a contact area large enough for the current, and keep the voltage contacts away from the current contacts.

For a four-wire test, measure the distance between the voltage contacts accurately. A small error in the length becomes a small error in resistivity, but a small error in the cross-section of a thin strip can be larger. Measure the thickness at several points.

For a plated part, decide whether the test is for the substrate, the plating or the combination. Each needs a different specimen. A test that mixes them produces a number that cannot be compared with a material specification.

Interpreting the result

A conductivity result is only meaningful against a specification. For copper busbar, the requirement may be a minimum percentage IACS. For an aluminium heat sink, it may be a grade check rather than an electrical requirement. For a contact, conductivity may matter less than hardness and arc resistance.

Direction matters. A rolled strip can have different conductivity along and across the rolling direction. A drawn wire can differ from a cast bar. State the direction and the location of the test, especially for a part that carries current in a specific path.

Heat treatment changes the result. Annealing raises conductivity in some alloys and lowers strength. Welding, brazing and bending create local changes. A test on the parent material does not describe the joint, so a joint may need its own resistance check.

Compare like with like. A reading from a portable meter on a finished busbar is not directly comparable with a four-wire result from a laboratory coupon. The method, the specimen and the temperature all differ. Put the method on the record so the next person does not compare two numbers that were never the same measurement.

When to ask for a test

Ask for a conductivity test when the material grade is critical, when the part carries a high current, or when a supplier changes the source or the process. Ask for it at goods-in if the risk is alloy substitution, and on a finished part if the process can change the property.

For a busbar or a lug, combine the conductivity check with a dimensional and plating check. A bar with the right conductivity and the wrong cross-section still fails. A joint with the right conductivity and a poor crimp still heats.

Keep the record with the lot. Note the method, the instrument, the temperature, the specimen and the result. A number on its own is not traceable, and a traceable number is what a customer or an auditor will ask for.