Terminal material: copper, brass and phosphor bronze

Three jobs, three properties

A terminal has to conduct current, clamp a conductor or a mating tab, and in many designs provide a spring. Copper conducts well but is soft. Brass is stronger and easier to machine but conducts much less. Phosphor bronze has good spring properties and reasonable conductivity, which is why it appears in contact and terminal springs.

The material choice is rarely a single property decision. A high current lug needs copper because the section would be impractical in brass. A small earth terminal can use brass because the current is low and the strength and cost matter more. A spring contact needs a material that holds its force after many cycles.

The alloy and the temper matter as much as the family. Two brasses with different copper and zinc ratios can behave differently in conductivity, machinability and stress relaxation. A drawing that says brass without a grade is an invitation to substitute.

There is also a manufacturing dimension. A material that machines well may form poorly, and a material that forms well may need a stress relief step after bending. The choice has to fit the process route as well as the electrical duty, which is why the alloy, the temper and the forming sequence are usually decided together.

Copper: the conductivity benchmark

Electrolytic copper is the reference for electrical conductivity. It is soft, ductile and easy to crimp, and it forms a reliable joint with a correctly sized barrel and die. For power lugs and current carrying terminals, it is usually the default.

The weakness is strength. A copper terminal can deform under a high clamping load or creep at a bolted joint if the contact area is small. The usual answer is to increase the section, use a hardened copper alloy, or change the design so the copper carries current and a separate part carries the spring load.

Copper also work hardens. A formed or crimped area is stronger than the annealed starting stock, but it is less ductile. If the part is bent sharply after annealing, the bend can crack. The temper and the forming sequence belong on the process sheet.

Brass: strength and machinability

Brass is a copper and zinc alloy. It is stronger than pure copper, machines easily and takes a good finish, which makes it popular for small terminals, pins, screwed connectors and earth parts. It is also cheaper than copper on a volume basis for many shapes.

The trade-off is conductivity. A typical brass has a conductivity in the region of a quarter to a third of copper, so a brass conductor carrying the same current needs a much larger section or it runs hotter. That is why brass is common in low current and mechanical parts and uncommon in a high current power lug.

Brass is also susceptible to dezincification in some environments, and to stress cracking if the part is heavily cold formed and carries residual stress. A brass part for a wet or coastal application needs a grade and a finish that suit the exposure.

Phosphor bronze: the spring choice

Phosphor bronze is a copper alloy with tin and phosphorus. It has good spring properties, reasonable conductivity and better fatigue resistance than plain copper. It is used for contact springs, terminal clips and parts that must hold a force over many cycles.

The conductivity is lower than copper but usually higher than brass, depending on the grade. It is not a high current conductor in a small section, but it is a good compromise where the part has to conduct and spring at the same time.

Phosphor bronze is harder to form than copper and may need a larger bend radius. It also work hardens quickly, so a multi-stage form can need an anneal between operations. Those process steps add cost that is not visible in the material price.

Conductivity and section: a worked comparison

Resistance is proportional to resistivity and length, and inversely proportional to cross-section. If a brass has about 28 percent of the conductivity of copper, its resistivity is about 3.6 times higher. To match the resistance of a copper part at the same length, the brass part needs about 3.6 times the cross-section.

That is an illustrative figure, not a design value, because the actual grades differ. It shows why a direct material swap at the same geometry is rarely acceptable. A brass part that looks like a copper part will run hotter or drop more voltage.

The practical answer is to size the conductor for the current and the allowable temperature rise, then choose the material that can provide that section at an acceptable cost and size. A small brass terminal can be perfectly adequate for a signal or earth connection and quite wrong for a 200 A feeder.

Temper, spring and stress relaxation

A spring property is not a material name. It comes from the alloy, the temper and the forming history. A phosphor bronze in a soft temper will not hold the same force as the same alloy in a hard temper, and a brass spring can relax at a lower temperature than a bronze one.

Stress relaxation is the loss of force over time under a sustained deflection. It is faster at higher temperature. A terminal that holds its force at 20 degrees Celsius can lose a significant part of it at 80 degrees, which is why the operating temperature belongs in the material choice.

Fatigue is the other limit. A contact that flexes on every mating cycle needs a material and a stress level that survive the expected cycles. A material with good conductivity but poor fatigue life is a false economy in a spring contact.

Machinability, forming and cost

Brass is the easiest of the three to machine. It produces short chips, takes a good surface finish and holds tolerances well, which is why turned brass terminals and pins are common. Copper is gummier and can need sharper tools and more care to avoid smearing. Phosphor bronze sits between the two and can be abrasive.

For a stamped part, copper and brass both form well, but the springback and the bend radius differ. Phosphor bronze needs more attention to bend radius and may need stress relief after forming. The tool design should follow the material, not the other way round.

The total cost is not the material price per kilogram. It includes the scrap rate, the tool life, the plating, the forming steps and the inspection. A cheaper alloy that doubles the scrap rate is not cheaper.

A selection table

Use the table as a starting point, then confirm the grade and the temper with the supplier. The values are comparative and illustrative, not a substitute for the data sheet.

Write the alloy designation and the temper on the drawing, not just the family name. Add the conductivity or the maximum resistance if the part is current carrying, and add the spring force or the stress relaxation limit if the part is a contact. Those calls give the supplier a reason to choose a grade rather than the cheapest stock that fits the shape.

If a substitute is proposed, compare it on the properties that the joint uses. A change of alloy can be acceptable for a bracket and unacceptable for a current carrying spring, even when the dimensions and the finish are unchanged.