5G Aluminum Shell CNC Process for RM500U and FM650 Modules

• 1
  Key takeaways
• 2
  What the 5G aluminum shell CNC process has to solve
• 3
  Alloy and stock choice for a 5G aluminum shell
• 4
  Wall thickness, pockets and the thermal path
• 5
  RF grounding, masking and screw torque
• 6
  Anodizing, masking and post-machining
• 7
  Inspection and the mistakes that cost the most
• 8
  Step by step: running the 5G aluminum shell CNC process
• 9
  Alloy selection by shell function

A 5G module carrier is not a decorative box. The RM500U and FM650 sit on a PCB that has to stay flat, stay cool and stay grounded. The shell does three jobs at once: it stiffens the board, it moves heat into the air, and it gives the radio a reference plane. If any one of those jobs is weak, the product fails in the field rather than on the bench.

Heat is the first constraint. A 5G module running at full transmit can push several watts through a small footprint. Aluminum spreads that heat laterally, but only if the thermal path is continuous from the module shield can, through the pad, into the pocket floor, and out to the fins or the chassis. A gap of 0.05 mm in that chain is a real thermal resistance.

Grounding is the second constraint. Anodizing is an insulator. Every screw boss, every shield fence and every RF pad that must carry current has to be masked or machined after coating. We treat grounding as a machining feature, not a finishing afterthought.

The third constraint is mechanical. These shells go into gateways, CPE units and vehicle telematics boxes that see vibration and temperature cycling. Wall thickness, screw depth and corner radii decide whether the shell survives that or cracks at a boss.

Alloy choice changes the whole process. 6061-T6 is the default for module carriers: it machines cleanly, holds ±0.005 mm on critical bores, and has good thermal conductivity at roughly 167 W/m·K. It also anodizes to a consistent finish, which matters when the shell is a visible product surface.

5052 is the pick when the shell acts as a cover that flexes or clips into place. It bends without cracking and resists salt spray well, but it is gummy on the mill and harder to hold to tight flatness. Use it for covers and brackets, not for the main carrier with a precision module pocket.

6063 gives the best anodized appearance, so it suits visible gateway housings. Its yield strength is lower than 6061-T6, so keep screw bosses thicker and avoid thin unsupported webs. 7075 is worth considering only when weight is critical and the part is small; it is stronger but costs more and is less friendly to anodizing.

Stock matters as much as alloy. Extruded plate often carries internal stress that releases when you remove material, so a 100 mm carrier can bow 0.15 mm after roughing. We rough, stress-relieve where the geometry allows, then finish. Cast ADC12 is fine for a die-cast housing but not for a pocket that must hold ±0.05 mm.

Wall thickness is a trade between stiffness, weight and heat. For a carrier that holds an RM500U or FM650, we usually run 1.5–2.0 mm on the main walls and 4–6 mm at screw bosses. Below 1.2 mm the wall deflects under screw torque and the module pocket loses flatness. Above 2.5 mm you add weight without much thermal gain.

The module pocket is the critical feature. Its floor flatness should be 0.05 mm over the module footprint, and its depth must match the pad thickness plus the compression you want. A typical thermal pad compresses 10–30 percent, so a 1.0 mm pad wants a pocket 0.10–0.15 mm shallower than the nominal gap. Get this wrong and the pad either does not touch or pushes the PCB off its mounts.

Pocket floor finish matters for contact area. Ra 0.8–1.6 μm is a good working range: smooth enough for pad contact, rough enough to hold a thin thermal paste film. A mirror-polished floor can actually trap paste unevenly. We keep the floor as-machined at Ra 1.6–3.2 μm only when a thick gap filler is used.

Fins and ribs go on the outside where air moves. Internal ribs stiffen the shell but block airflow, so put stiffness where it does not fight convection. A 3 mm rib on a 1.5 mm wall can raise stiffness several times over for very little mass.

The radio needs a low-impedance path to the shell. That path is a bare aluminum pad or boss, not an anodized surface. On the drawing we mark every grounding feature as mask-on-anodize or machine-after-coat, and we keep those pads at Ra 1.6 μm or better with a flatness of 0.05 mm.

Screw bosses carry both the mechanical load and the ground return. For M2.5 screws into aluminum we target a tapped depth of at least 2× diameter, so 5 mm minimum, and we keep a 0.3 mm flat washer face around the hole. A boss that is too shallow strips during assembly, and a stripped boss is a scrapped housing.

Torque is a process parameter, not a suggestion. M2.5 into 6061-T6 typically lands around 0.4–0.6 N·m; M3 around 0.8–1.2 N·m. Over-torque crushes the pad and bows the PCB, which then breaks the very ground contact you were trying to make.

Shield fence grooves need a press fit, usually 0.05–0.10 mm interference, cut with a sharp small-diameter end mill. A worn tool leaves a burr that lifts the fence and opens a slot antenna in your shield. We deburr those grooves by hand before anodizing.

Anodizing grows the surface, typically 10–25 μm per side for a standard sulfuric bath. That growth moves the surface outward, so a pocket cut to final size before coating will close up by twice the coating thickness. On tight features we leave 0.02–0.05 mm of stock and machine after coating, or we mask the feature entirely.

Masking is selective and labor-heavy, so plan it early. Grounding pads, threaded holes and press-fit grooves are the usual mask points. Threads are a special case: anodizing a tapped hole makes the pitch diameter grow and the screw binds. Either mask the hole or plan a post-coat tap chase.

Hardcoat anodizing gives a 25–50 μm layer that is genuinely wear resistant but brittle at sharp corners. If the shell has thin ribs or a chamfer under 0.3 mm, hardcoat can chip there. We round those edges to 0.5 mm minimum radius before coating.

Color anodizing changes the cosmetic result but not the dimensional story. If the shell must match a color standard, send a physical sample. Color shifts with alloy and with bath age, and 6063 and 6061 will not match each other in the same dye lot.

Inspection should follow the function. Measure pocket depth and floor flatness first, because those two decide thermal performance. Check ground pad flatness and surface condition second, because those decide RF performance. Screw boss depth and thread condition come third. Cosmetic checks come last.

The most expensive mistake we see is finishing the pocket before anodizing and then losing the depth to coating growth. The part passes incoming inspection and fails thermal test. Leave stock or mask the pocket, and write the coating thickness on the drawing so the shop can plan for it.

The second is a datum that moves. If datum A is machined in setup one and then the part is flipped and re-clamped on a different face, flatness drifts. One datum, referenced in every operation, keeps the carrier flat.

The third is over-torqued screws at assembly. The shell is correct and the build still fails because the pad is crushed and the PCB bows. Publish a torque spec with the drawing and hand a torque driver to the line.

We inspect 100 percent of parts before shipment, with raw material checks, in-process monitoring and a final dimensional and visual inspection. Measurement reports are available on request.