Machined copper surface finish: Ra, contact resistance and plating

Finish is a functional call on copper

On a steel bracket, surface finish is often cosmetic. On a copper conductor, it is electrical. The real contact area between two metal surfaces is a small fraction of the apparent area, because the surfaces touch only at the highest asperities. A rougher surface has fewer, sharper contact points, so the current is forced through a smaller area and the resistance rises.

The same surface also controls plating adhesion, solder wetting and the way a gasket or a thermal pad seats. A finish that looks acceptable under a bright light can be wrong for all three. That is why a copper drawing should state the finish on the functional faces, not as a general note in the title block.

The distinction matters most on busbars, terminals and contact pads, where a few tens of micro-ohms at a joint become watts of heat at operating current. It matters less on a spacer or a cover, where the finish only has to look consistent.

What Ra does and does not tell you

Ra is the arithmetic mean of the profile height deviations over the evaluation length, measured about the mean line. It is a single number, and it throws away the shape of the profile. A surface with fine, even scratches and a surface with deep, widely spaced grooves can produce the same Ra.

Rz, the maximum height of the profile within a sampling length, captures the peaks better. For a contact face, the peaks are what touch first, so Rz is often the more useful number. Rmax, the largest single peak-to-valley height, catches the outlier that will hold two surfaces apart.

The evaluation length and the cutoff also matter. A 0.8 mm cutoff filters out waviness, which is the long-wavelength error that a surface plate or a straightedge would show. A contact face can pass an Ra check and still be bowed. If flatness matters, call it separately as a geometric tolerance.

Contact resistance and the real contact area

When two copper surfaces are bolted together, the contact resistance depends on the contact force, the hardness of the material, the finish and the cleanliness of the surfaces. The finish sets the number and size of the contact spots. The force deforms those spots until the real area is enough to carry the load and the current.

A finer finish increases the number of contact spots for the same force, which lowers the resistance. But the gain is not unlimited. Once the spots are dense enough, the resistance is dominated by the oxide film and by the bulk resistance of the parts, not by the roughness. Polishing a face from Ra 1.6 to Ra 0.4 µm helps a joint. Polishing from Ra 0.4 to Ra 0.1 µm usually does not, unless the joint is a low-force contact.

For low-force contacts, such as a spring contact or a plug, the force is small and the oxide is the dominant term. A finish that breaks through the oxide, or a plating that does not oxidise, matters more than the Ra. That is one reason silver plating is used on current-carrying contacts: it stays conductive at the surface.

Plating follows the surface it is given

Electroplated copper, nickel and silver all deposit conformally. A scratch, a tear or a burr is not filled in, it is copied. A thin silver layer over a torn surface will show the tear and may not cover the peak, leaving a bare copper spot that oxidises.

Adhesion depends on cleanliness as much as roughness. A moderate, uniform roughness gives the deposit a mechanical key and improves adhesion. An over-polished surface can be too smooth for a good mechanical bond, and a surface contaminated with oil, oxide or machining residue will fail an adhesion test regardless of the Ra.

The practical sequence is to machine to a defined pre-plate finish, clean and activate, then plate to a defined thickness. Specify both. If only the plated finish is specified, the supplier can meet it with a thick deposit over a rough surface, which uses more silver and can round the edges.

Which process gives which finish

Turning produces a helical lay with a repeatable peak-to-valley that depends on the feed per revolution and the tool nose radius. A 0.8 mm nose radius at 0.1 mm per revolution gives a theoretical Ra of about 0.4 µm, and in practice a little more. Reducing the feed is the direct lever, but it costs cycle time.

Milling leaves a scalloped path set by the cutter diameter and the stepover. A ball nose cutter at a small stepover gives a fine finish but a long cycle. A face mill with a wiper insert can produce a good finish at a high feed, which is often the better choice for a flat contact pad.

Lapping and fine grinding give the lowest Ra and the best flatness, at a cost. They are used on precision contact faces, on sealing faces and on surfaces that must be flat to within a few microns. Bead blasting gives a uniform matte finish and a good key for plating, but it raises the contact resistance and can embed media if the process is not controlled.

How to specify it on the drawing

Put the finish symbol on the face that matters, with the parameter, the value, the cutoff and the standard. Add a note for the faces that are cosmetic only. If the part is plated, give the pre-plate finish and the plated thickness, and say which one is checked at goods-in.

• Parameter and value, for example Ra 0.8 µm
• Standard and cutoff, for example ISO 4287, 0.8 mm
• The specific faces, not the whole part
• Flatness or waviness where the joint needs it
• Pre-plate finish and plated thickness
• Any masked or non-plated areas
• Handling and packing requirement for bare copper

Measuring a copper surface

A stylus instrument is the standard method. The stylus is dragged across the surface at a constant speed, and the profile is filtered and evaluated. On soft copper, the stylus can scratch the surface, so use a low-force pickup and check the instrument against a calibrated specimen.

The measurement direction should match the lay. A turned surface measured along the helix reads differently from one measured across it. State the direction, or measure both and report the worst case. For a contact face, measure across the lay, because that is the direction the current and the mating part see.

For very fine finishes or soft surfaces, an optical profilometer avoids contact damage. It gives areal parameters rather than a single profile, which is more informative for a contact face. The trade-off is cost and the need for a trained operator. A comparator plate is not a measurement, and it should not be the acceptance method for a critical face.

Finish does not survive bad packing

Bare copper oxidises. A part that leaves the machine at Ra 0.8 µm can arrive at the customer with a visible tarnish and a higher contact resistance, even though nothing touched it. Humidity, sulphur in the air and paper wrapping all accelerate the film.

Bag parts in a sealed barrier bag with a volatile corrosion inhibitor, or plate them before shipping. Avoid cardboard and uncoated paper in direct contact with a critical face. Handle with gloves, because skin oil leaves a residue that shows up after plating and can cause a adhesion failure at that spot.

If the part is stored for more than a few weeks, ask the supplier for the packing method and the shelf life. A finish call on the drawing is only half the requirement. The other half is the condition of the surface when it reaches the assembly line.