EV High Voltage DC Contact Plate: C110 Copper Machining Case Study

A flat contact plate machined from C110 copper that carries high current between the moving armature and fixed terminals in an EV DC relay or contactor. The part geometry is straightforward — a flat plate with mounting holes and a contact surface. The manufacturing challenge is not complexity but material behavior. C110 copper is among the most electrically conductive metals available, but it is soft, gummy during machining, and prone to oxidation. Getting consistent dimensional accuracy, clean machined edges, and a reliable silver-plated surface requires specific tooling choices and process discipline.

1. Material Selection: Conductivity vs Machinability vs Cost

Contact plates in EV high voltage relays serve as the conductive bridge between the relay's moving armature and the fixed busbar or terminal connection. The primary requirement is maximum current-carrying capacity with minimum voltage drop. This points directly to high-purity copper. However, the choice between copper alloys involves trade-offs in strength, machinability, and cost that are worth examining.

2. Why C110 Copper for This Application

C110 (UNS C11000), also known as ETP (Electrolytic Tough Pitch) copper, is 99.9% pure copper with a small amount of oxygen (0.04%) that improves workability. It has the highest electrical conductivity of any commercially available copper alloy, which is the primary reason it is specified for contact plates in high-current switching applications.

The Trade-off: Soft Material, Handling Challenges

C110's main disadvantage is its low hardness. At ~40 HRB in the annealed condition, the material dents and scratches easily. During machining, thin plates can deform under clamping pressure. During handling between operations, the contact surface can be scratched by fixtures, conveyors, or operator gloves. Any surface damage that survives into the plating stage becomes a permanent defect — silver plating follows the substrate contour, so a scratch in the copper appears as a scratch in the finished part.

Silver Plating Necessity

Bare C110 copper oxidizes quickly in air, forming a dark copper oxide layer within hours. This oxide layer has significantly higher contact resistance than clean copper, which would cause excessive voltage drop and localized heating at the contact interface. Silver plating (5–10 μm on the contact face) provides two benefits:

1. Oxidation resistance: Silver oxide is conductive, unlike copper oxide. The contact face maintains low and stable contact resistance over the relay's service life.
2. Enhanced conductivity: Silver has ~106% IACS conductivity — marginally better than copper. On a 5–10 μm layer, this contribution is small in absolute terms, but it ensures the contact interface is the best possible conductor.

3. Machining Strategy

3.1 CNC Milling: The Primary Process

Contact plates are flat parts with relatively simple geometry — the main profile, mounting holes, and sometimes locating features or alignment tabs. CNC milling is the appropriate process. The part is machined from a copper blank (saw-cut plate or saw-cut bar stock, depending on geometry).

3.2 The Core Challenge: Copper Is Gummy

Unlike aluminum or steel, C110 copper does not produce clean, well-broken chips during milling. Instead, it generates long, stringy chips that can wrap around the tool, drag across the machined surface, and leave embedded debris. This is the single biggest machining challenge for copper contact plates. The solutions are specific tool geometry and cutting parameters:

• Tool geometry: Use sharp carbide end mills with high positive rake angle (15–25 degrees). Two-flute cutters are preferred over three or four flute because the larger flute valley provides better chip clearance. For finishing passes on the contact face, PCD (polycrystalline diamond) end mills produce a superior surface finish and generate less built-up edge.
• Cutting parameters: High spindle speed (3,000–6,000 RPM for 6–12 mm end mills), moderate feed rate, and shallow depth of cut for finishing passes (0.05–0.1 mm). Flood coolant is essential — it flushes chips away from the cutting zone and prevents chip re-cutting.
• Chip evacuation: Use coolant-through tools where possible. For deep pockets or slots, peck milling or ramp milling helps clear chips. Air blast alone is insufficient for copper.

3.3 Surface Preparation for Silver Plating

The quality of the silver-plated surface depends on the condition of the copper substrate. Before plating, the contact face must meet specific requirements:

• Surface roughness: Ra ≤ 1.6 μm before plating. Rougher surfaces require thicker silver plating to achieve uniform coverage, increasing cost without improving performance.
• Cleanliness: No cutting fluid residue, no copper fines, no oxide film. Parts go through a multi-stage cleaning process (alkaline degreaser, acid dip, DI water rinse) before entering the silver plating bath.
• No scratches or dents: Any surface damage will be visible through the plated layer. Handle parts with ESD-safe gloves and place them on soft trays between operations.

3.4 Handling Thin Plates

Contact plates are typically 2–6 mm thick. Thin copper plates deflect under clamping pressure, which causes dimensional variation in flatness and thickness. The approach is to machine the parts from oversized blanks and use soft-jaw fixtures with even clamping distribution. Vacuum fixtures are another option for very thin plates (< 2 mm). After machining, parts are inspected for flatness before proceeding to plating.

4. Quality Testing

5. Cost Drivers: Where the Money Goes

6. Common Mistakes That Reduce First-Article Yield

7. Production Timeline