Copper Surface Finishes for Precision Parts

Silver plating (electroplated silver, ASTM B700, 0.0003–0.001 in thick) is the usual choice for copper electrical contacts. Silver has the highest electrical conductivity of any metal (106% IACS) and forms a conductive oxide (Ag₂O) that reduces under contact pressure, maintaining low contact resistance. For contacts operating below 212°F (100°C), tin plating is an acceptable lower-cost alternative. For contacts requiring long-term reliability above 212°F (100°C), use silver or gold flash over nickel.

Copper reacts with atmospheric oxygen to form Cu₂O (cuprous oxide, red-brown) and CuO (cupric oxide, black). The reaction begins immediately on freshly machined copper exposed to ambient air. Cu₂O is semiconducting and increases contact resistance; CuO is an insulator. Oxidation is accelerated by humidity, heat, and sulfur compounds. Prevention options: electroless nickel (protects indefinitely), silver plating (conductive oxide, reduces under pressure), tin plating (RoHS-compliant, good shelf life), or OSP (organic solderability preservative, 6–12 month shelf life).

No. Anodizing is an electrochemical process specific to aluminum (and titanium). It converts the surface aluminum to aluminum oxide (Al₂O₃) through an anodic oxidation reaction. Copper does not form a stable, adherent oxide layer under anodizing conditions — the copper simply dissolves or forms loose cupric oxide. Copper parts are surface-treated by plating (electroless nickel, silver, tin, gold), organic coatings (OSP), or chemical conversion (benzotriazole treatment for short-term protection).

OSP is an azole-based organic compound (typically benzimidazole or imidazole) applied to bare copper by immersion in an acidic bath. It forms a thin (0.2–0.5 µm), transparent, oxidation-resistant film on the copper surface. OSP preserves solderability for 6–12 months under controlled storage conditions (sealed packaging, low humidity). During reflow soldering, OSP decomposes cleanly above ~428°F (~220°C), leaving the copper surface exposed for solder wetting. OSP is not suitable for contact applications — it does not improve contact resistance and is removed by handling.

Electroless nickel (ENi) can be applied to C360, but the 3% lead content requires additional surface preparation. Lead is poorly wetted by ENi chemistry and creates adhesion voids at lead-phase grain boundaries. The standard mitigation is a bright dip (dilute nitric/sulfuric acid) to remove surface lead, followed by a copper strike (electrolytic copper at 0.0001–0.0002 in) before ENi application. Without this pre-treatment, ENi on C360 fails adhesion testing. Specify the pre-treatment requirement on the drawing or fabrication note.

Use the ASTM B700 standard for electrodeposited silver: "Silver plate per ASTM B700, Grade A (99.9% Ag minimum), Class N (matte) or Class S (semi-bright), 0.0005 in (12.7 µm) minimum thickness." If the contact surface requires a specific thickness, call it out as a local note: "0.0005 in silver on contact surfaces shown — 0.0002 in minimum on non-contact surfaces." For high-frequency or high-reliability contacts, add: "Underplate with electroless nickel 0.0002 in per ASTM B733 before silver."