Machined parts for RF and microwave: finish, conductivity and geometry

Why the surface does the work

At microwave frequency, current does not flow through the whole conductor. It concentrates in a thin layer at the surface, and the depth of that layer shrinks as frequency rises. That is why an RF part is judged by its surface: the inside of a cavity, the face of a flange and the bore of a connector carry the current that the signal depends on.

A rough surface makes the current travel a longer path, which raises the loss. A scratched or pitted surface does the same. A plating that is thin, porous or poorly bonded adds resistance at the point where the current crosses from one part to the next.

The mechanical drawing rarely says this. It shows a pocket depth and a wall thickness. The RF requirement is a separate set of calls on finish, plating and joint flatness, and those calls have to reach the machine shop and the plater, not just the RF engineer.

The features that carry the RF path

A machined RF housing or component has a small number of features that decide performance. Reading the part in terms of them makes the drawing review faster.

Surface finish inside the cavity

The finish call inside a cavity is usually tighter than the outside of the part. A common target for a machined aluminium or copper cavity is a roughness in the region of 0.8 micrometres Ra or better on the walls that carry current. The value should come from the loss budget at the operating frequency, not from a habit.

Achieving that finish in a deep pocket is harder than on an open face. A long, slender tool deflects, and the wall can chatter. The result is a finish that varies with depth. If the drawing calls a single value, agree where it is measured and whether the floor, the side walls and the corners are all included.

Bead blasting and tumbling can improve the appearance but they can also round an edge or leave media in a blind hole. For an RF part, a controlled machining finish is usually better than a cosmetic blast, because the geometry stays as designed.

Plating and conductivity

Aluminium is light and machines well but it oxidises quickly and its oxide is a poor conductor. An RF aluminium housing is usually plated to provide a stable, conductive surface. Silver is common for low loss, with a thin nickel or copper strike underneath for adhesion. The plating thickness and the adhesion are both part of the specification.

Copper and brass are easier to plate and are often used for smaller components. Copper has high conductivity but tarnishes, so a protective or a silver finish is usual. Brass machines freely and is common for connectors and smaller bodies.

Plating thickness needs a number and a method. A thin flash may cover the surface but not survive handling or a connector mate. A thick deposit can round a thread or close a tight bore. State the minimum on the current-carrying faces, the maximum where a fit matters, and the areas that must be masked.

Silver tarnishes in sulphur-bearing air. A tarnish film raises contact resistance at a joint. Storage in a sealed bag with a suitable protector, and a defined shelf life before assembly, are part of the finish specification, not an afterthought.

Geometry that machines well and works well

RF geometry and machining geometry pull in different directions. A sharp internal corner is good for a defined cavity mode and bad for a cutter, which leaves a radius anyway. Design the corner with the cutter radius in mind so the drawing matches what the machine can produce.

Deep, narrow pockets need long tools and light cuts. If the pocket is deeper than about three times its width, expect the supplier to raise the cost or to propose a split body. A split body introduces a joint, and a joint is an RF loss point, so the trade-off needs a decision rather than a default.

Thin walls distort. A cavity wall that is a few tenths of a millimetre thick will move under clamping and may spring after machining. A slightly thicker wall, or a rib, keeps the geometry and reduces scrap. The RF effect of a small change in wall thickness is usually far less than the effect of a distorted wall.

Inspection and first article

Inspection of an RF part mixes dimensional checks with surface and plating checks. A first article should record the cavity dimensions, the flange flatness, the finish inside the cavity and the plating thickness on a sample. If the part has a connector interface, add the concentricity and the bore size after plating.

A visual check under magnification is useful for plating defects, but it does not prove thickness. An X-ray fluorescence gauge gives a fast thickness reading on a flat area. A cross-section gives a direct reading and shows the bond between the layers, at the cost of a sample.

For a critical part, a network analyser measurement on a test coupon or a first assembly is the only check that proves the electrical result. Agree the test fixture, the reference and the acceptance limit before production, because a measurement without a reference cannot be repeated.

What to send with the RFQ

An RF machining quote needs more than a model. The supplier needs to know which surfaces carry current, which are cosmetic and which are only structural.

• Drawing with the cavity, flange and connector dimensions
• Finish call for the current-carrying faces and the cosmetic faces
• Plating type, thickness, adhesion and masked areas
• Material grade and certificate requirement
• Assembly and joint details, including gaskets or conductive seals
• Test method and the acceptance limit
• Volume and the number of setups expected