Phosphor bronze springs: conductivity, corrosion and fatigue

What phosphor bronze is

Phosphor bronze is a copper alloy with tin as the main addition and a small amount of phosphorus. The common spring grade contains about 5% tin and up to about 0.35% phosphorus, and is known in different standards as C51000, CuSn6 or PB102. The tin gives strength and the phosphorus helps deoxidation during casting and improves fluidity.

The alloy is supplied as strip, wire and rod in several tempers. Spring temper is produced by cold rolling or drawing, and the amount of cold work sets the strength and the springback. A phosphor bronze spring is therefore a wrought product with a direction of grain, and the bend direction relative to the rolling direction can affect formability.

It is not the same as beryllium copper. Phosphor bronze has lower conductivity and lower strength, but it costs less and does not carry the beryllium handling controls. The two are often compared for contacts, and the choice usually turns on conductivity, strength and cost.

Conductivity and contact resistance

Phosphor bronze typically offers around 15% of the conductivity of pure copper, depending on temper and grade. That is far below copper or brass, but it is still enough for signal circuits, light current paths and springs that carry a small current while applying contact force. The voltage drop across a short spring is usually small compared with the contact interface.

The contact interface matters more than the bulk alloy in most connector designs. Contact resistance depends on the force, the surface finish, the roughness and the film on the surface. A phosphor bronze contact with good force and a clean surface can outperform a higher conductivity alloy with poor force or a contaminated surface.

Where the spring carries a significant current, calculate resistive heating and voltage drop rather than assuming the alloy is adequate. A thin spring with a long current path has more resistance than a short, thick section, and the temperature rise can soften the material and reduce contact force.

Corrosion behaviour

Phosphor bronze performs well in fresh water, humid air and many industrial atmospheres. It forms a stable surface film that slows further attack, and it does not suffer from dezincification because it contains no zinc. That makes it a practical choice for springs that are exposed rather than plated.

It is not immune to every environment. Ammonia and amine compounds can attack copper alloys, and sulphur compounds can darken and eventually tarnish the surface. Strong chlorides can also cause trouble in a crevice. If the spring sits in a sealed housing with outgassing materials, the local chemistry may be more aggressive than the room it is installed in.

For many electrical contacts, the natural film is acceptable, but a tarnish film can raise contact resistance. A thin tin or nickel plating is sometimes used to keep the interface stable, and that choice becomes a contact resistance decision as much as a corrosion one.

Fatigue and stress relaxation

Phosphor bronze has good fatigue resistance for a copper alloy, which is why it is used for bellows, diaphragms and contact springs. The fatigue strength depends on the temper, the surface condition and the stress range, and it falls as the stress range rises. A spring that is designed close to its yield point will not enjoy a long cycle life even in a good alloy.

Stress relaxation is the other limit. At elevated temperature, copper alloys lose load over time even when the stress is below yield. A contact spring that holds 2 N at room temperature may hold noticeably less after hours at 100 C or above. If the application runs warm, the design should be tested at temperature rather than at room conditions.

Bend radii matter as they do in steel. A sharp bend on the inside of a contact spring concentrates strain and becomes the fatigue origin. A generous radius, a clean edge and stress relief after forming all help.

Phosphor bronze against the alternatives

The table compares phosphor bronze with the two materials it most often competes with for a current-carrying spring. The right answer depends on which property is limiting the design.

Where bronze beats steel

The clearest case is a part that must conduct and spring at the same time. A stainless steel contact spring needs a copper path or a plated surface to carry current well; a phosphor bronze spring does both jobs in one part. That can remove a joint, a wire or a secondary operation, and the saving often pays for the alloy.

The second case is a spring that must work without plating in a damp or mildly corrosive environment. Phosphor bronze can be used bare in many indoor and fresh water applications, while a steel spring would need a coating that adds process steps and hydrogen risk.

The third case is non-magnetic operation. Phosphor bronze is effectively non-magnetic, which matters near sensors, relays and instruments. A cold worked stainless spring may be slightly magnetic, and that can be enough to cause a problem.

Where bronze loses to steel

When the spring is only a spring, stainless or music wire usually wins on strength, fatigue and cost. Phosphor bronze has a lower modulus and lower strength, so a bronze spring of the same envelope carries less load and needs more material for the same force. In a tight space with a high load, that is decisive.

Temperature is the other limit. Above about 100 C, stress relaxation becomes significant and the allowable stress falls. A steel spring can run hotter, and a stainless spring can run hotter still. If the part is near a motor, a heater or a power device, check the actual surface temperature before choosing bronze.

Finally, bronze is not the cheap option. It costs more than brass and often more than a steel spring, and it forms differently. The justification has to come from function, not from a wish to avoid plating.

Specifying a phosphor bronze spring

State the alloy designation, the temper and the product form. Give the conductivity only if it matters, and give the contact force at the working deflection, because force is what makes the electrical joint. Add the environment and the maximum service temperature.

• Alloy and standard, for example C51000 or CuSn6
• Temper and product form, wire or strip
• Spring rate and force at working deflection
• Current, voltage drop limit and temperature rise
• Maximum service temperature and environment
• Plating, if any, and the contact resistance limit