Selecting gauge and material for stamped current-carrying parts

One stamping, three jobs

A stamped current-carrying part usually does three things at once. It carries current, it provides a spring force against a mating surface, and it survives the temperature rise that the current creates. A busbar tab mostly carries current. A contact spring mostly provides force. A terminal does both, and that is where the selection gets difficult.

The three jobs are linked through the same geometry. A thicker strip lowers the resistance and the heat, but it also raises the force for a given deflection and can make the spring too stiff for the housing. A wider strip does the same. A longer beam softens the spring but adds resistance and takes space. There is no single correct gauge, only a set of trade-offs that the function decides.

The materials and what each one is good at

Copper C11000 is the conductivity benchmark at around 101 per cent IACS in the annealed condition. It is soft, forms easily and is the natural choice for a current path that does not need to act as a spring. Its weakness is strength: a contact made from soft copper will relax and lose force, especially at temperature.

Brass C26000 is cheap, machines and forms well, and has moderate strength, but its conductivity is roughly a quarter of copper. It suits a terminal body or a mechanical part where the current path is short and the cross-section is generous. It is a poor choice for a long current path with a tight voltage drop budget.

Copper alloy C19400 sits between the two. It offers roughly 60 to 65 per cent IACS with much better strength than pure copper, which is why it appears in terminals and lead frames. Phosphor bronze C51000 has low conductivity, around 15 per cent IACS, but good spring properties and fatigue resistance, so it suits a contact spring that carries only a small current. Aluminium 5052 is light and moderately conductive at around 35 per cent IACS, but it needs a larger section for the same resistance and a careful joint design.

Gauge, cross-section and current density

Gauge is shorthand for thickness, but the electrical property is the cross-section. A 0.8 mm strip that is 6 mm wide has 4.8 square millimetres of copper. The same 0.8 mm strip at 3 mm wide has half the area and twice the resistance per unit length. When a drawing changes the width to fit a housing, it changes the electrical design, even if the gauge stays the same.

A common way to start is current density, expressed in amperes per square millimetre. For a small stamped conductor in free air, an illustrative working range might be 3 to 6 A per square millimetre, with the exact value set by the allowable temperature rise, the ambient temperature and how the part is mounted. A part bundled with others or run inside a closed housing will be closer to the low end.

Current density is not a rating and it is not a substitute for a heat rise test. It is a sizing check that tells you whether the section is in the right region. If the calculation lands near the limit, the next step is a temperature rise test on a representative assembly.

Heat rise and the thermal path

Resistance converts electrical energy into heat, and the temperature rise depends on how that heat leaves the part. A short, wide conductor with a large surface area runs cooler than a long, narrow one with the same cross-section, because the thermal path to the surrounding air and to the terminals is better. Plating, contact resistance at the joints and the housing material all change the result.

The contact interface is often the hottest point. A bolted or crimped joint with a small real contact area has a higher resistance than the conductor, and that resistance rises if the joint relaxes or corrodes. A stamped contact spring adds a second interface, because the force at the contact tip decides the contact resistance. Lower force means higher resistance and more heat, which softens the material and lowers the force further.

That loop is the reason a current-carrying spring cannot be selected on conductivity alone. The mechanical and electrical designs have to be checked together at the expected ambient and load.

Spring force and stress relaxation

A contact spring has to deliver a force at a defined deflection. The force depends on the material modulus, the beam geometry and the deflection. Thickness is the strongest lever, because the stiffness of a cantilever rises with the cube of the thickness. A small increase in gauge can double the force, which is why a gauge change is never only an electrical change.

Stress relaxation is the slow loss of force at temperature. A material loaded close to its yield point will lose force faster, and a brass or soft copper spring will lose more than a phosphor bronze or a high-strength copper alloy. If the part runs warm, the design should keep the working stress well below the yield strength at temperature, not just at room temperature.

Fatigue is the other limit. A contact that flexes on every mating cycle needs a stress range that the material can survive for the required number of cycles.

Plating and the contact interface

Tin plating is the default for a stamped terminal. It is cheap, solderable and gives a stable contact resistance at moderate current, but it has a higher contact resistance than silver and it can form a brittle intermetallic if the thickness is wrong. Silver plating lowers the contact resistance and suits higher current, at a higher cost and with a risk of tarnish in a sulphur-bearing environment.

Nickel is often used as a barrier under gold or silver. It stops diffusion between the base metal and the top coat, but it has its own resistance and it is hard, so the contact force has to be enough to break through the surface film. Gold is reserved for low-level signals or a very high cycle count, not for a power terminal.

The plating also changes the forming route. A pre-plated coil avoids a post-plating step, but the coating has to survive the bend without cracking. If the part is post-plated, the racking and masking add cost and the plating thickness has to be checked on the formed part.

A selection sequence you can defend

Start with the current, the allowable temperature rise and the ambient. Size the cross-section from an illustrative current density, then check the voltage drop over the length. Choose a material family that meets the conductivity and the strength, then set the beam geometry for the required force and check the stress at temperature.

Next, check the contact interface: force, plating, contact resistance and the expected environment. Then check the manufacturing route: can the material be stamped at the required radius, does the plating survive the bend, and does the gauge come in a standard coil. Finally, verify with a heat rise test and a force measurement on a production-representative sample.

• Current, ambient and allowable temperature rise
• Cross-section from width and thickness, not gauge alone
• Voltage drop over the full current path
• Force at the working deflection and stress at temperature
• Contact plating and the expected environment
• Heat rise and force verification on a production sample