Copper stampings: conductivity, temper and tooling wear

Why copper earns its place in a stamping

Copper carries current with less loss than any common engineering metal apart from silver. It also conducts heat well, solders readily and forms at room temperature without cracking. That combination makes it the default for current carrying stampings: terminals, contact carriers, busbar links, fuse clips, battery tabs and connector shells.

The same properties that make copper useful make it awkward in a press. It is soft, it work hardens quickly, it galls against tool steel and it oxidises within days of being cleaned. A copper part that is dimensionally correct but has a torn edge, a smeared surface or a patch of oxide can fail at the next process even though it passed the dimensional check.

The alloys that cover most stamped parts

Pure copper is the best conductor and the weakest structural choice. Alloying adds strength, spring and resistance to softening, at the cost of conductivity. The table gives the usual families and where each one earns its cost.

Temper decides the bend and the spring

Temper is the amount of cold work left in the strip. Annealed material bends almost on itself and carries current well, but it has little spring and yields under a contact load. Half hard material is the usual compromise for terminals that must hold a force without cracking at a tight bend. Hard temper gives the highest strength and the smallest springback window, and it will crack if the bend radius is too tight.

Temper designations differ between standards and mills. ASTM B601 is the common language in North America, while European mills often use EN 1172 or EN 1652 designations. A drawing that says half hard without a standard is ambiguous. Quote the temper designation, the standard and the tensile range or hardness range that goes with it.

Stress relaxation matters as much as yield strength. A terminal that is bent and then held under load at 80 C will lose force over time, and a high conductivity alloy with poor relaxation resistance can lose more than half its initial contact force. If the part must hold a force for years, ask for stress relaxation data at the service temperature, not just a room temperature tensile result.

Why copper dies need more attention

Copper is soft enough to weld to a tool steel surface under pressure. The symptom is a gradual build-up of metal on the punch face and the die land, which tears the part surface, raises friction and changes the clearance. Once the pick-up starts it accelerates, because the transferred copper is softer than the tool and holds abrasive particles.

The usual countermeasures are a polished punch face, a coating such as titanium nitride, chromium or a diamond-like carbon, a slightly larger clearance than for steel, and a lubricant that suits copper rather than a general purpose drawing oil.

Copper work hardens at the sheared edge and at tight bends. That is useful in a terminal because the bend becomes stronger than the parent strip, but it also means a part that is straightened and re-bent in the same station can crack. Toolroom trials should include the actual bend sequence, not a single bend on a flat coupon.

• Polish and coat punches and die inserts that see copper
• Set clearance from the alloy and temper, not from a steel rule of thumb
• Use a copper compatible lubricant and control the film thickness
• Keep strippers in contact until the punch clears the strip
• Clean slugs and dust from the die at every shift change
• Inspect for pick-up at the start of each run, not only at the end

Bend radius and grain direction

Copper bends well, but not equally in every direction. A bend line across the rolling direction is more tolerant than one along it, because the elongated grain structure gives less ductility through the thickness. For a tight bend on a hard temper strip, orient the bend across the rolling direction and state the grain direction on the drawing.

The minimum bend radius depends on temper. Annealed copper can be bent to a very small inside radius, while a hard temper strip may need one to two times the thickness to avoid cracking on the outside of the bend. Those values are starting points, not a guarantee. The real limit depends on the alloy, the edge condition and whether the bend is in a sheared or a rolled direction.

Bend relief and a generous radius at the corner reduce the strain concentration. A sharp internal corner in a formed tab is a crack starter, especially after the edge has been work hardened by blanking. If the part must have a sharp outer corner, add a relief notch or move the bend away from the blanked edge.

Surface, oxide and plating

Copper oxidises in air. A freshly stamped part can show a visible tint within days in a humid warehouse, and the oxide layer raises contact resistance and interferes with soldering. For a part that will be stored or shipped, specify the surface condition and the packaging: interleaved paper, sealed bags with a desiccant, or a protective coating that can be removed before assembly.

Plating is common for terminals that must solder or make a low resistance contact. Nickel is often used as a barrier under tin or gold, because copper diffuses into the finish at elevated temperature. Silver gives the lowest contact resistance but tarnishes, and the tarnish film is not always acceptable on a low current signal contact.

Cleanliness before plating is part of the stamping process, not a separate concern. Lubricant residue, metal dust and oxide all cause adhesion failures that appear as blisters or peeling after the part has been in service. If the part is plated after stamping, agree the cleaning route and the maximum time between stamping and plating.

What a copper stamping quote needs

Copper prices move, so a quote without a metal basis is a snapshot that expires quickly. Send the alloy and temper, the strip thickness with tolerance, the finish, the annual volume, the scrap credit arrangement and the delivery basis. If the part is a current carrying terminal, add the current, the mating material and the contact force target.

Ask how the supplier will control pick-up and oxide, and what the first article will include. A copper first article should show the dimensions, the material certificate, the temper, the bend radii and the surface condition. If the part is plated, add thickness, adhesion and a solderability check where relevant.

For a programme that runs for years, agree a metal index and a review cadence. Copper is a traded commodity and a fixed price over a long programme is either expensive or unsustainable.