Function of Shielding
Metal shields attenuate field strength through reflection and absorption, enclosing highly nonlinear circuits such as PA, VCO, and mixers within local cavities to reduce external radiation and internal crosstalk. Gaps, openings, and connector penetrations are the main leakage paths, and shorter wavelengths (higher frequency bands) are more sensitive to gaps. If an IoT device places the module inside a plastic case, sometimes metal coating or copper foil is added as a simple shield, but antenna coupling and SAR need to be evaluated.
Single-Point and Multi-Point Grounding
RF grounds usually require a low-impedance reference plane. When digital switching noise couples to the RF area through return currents, it can create spurious signals. In practice, partitioned layouts and a 'bridging' strategy are often used: digital and RF grounds ultimately converge near the power entry or battery, avoiding the formation of large loop antennas. Multi-point grounding can reduce high-frequency impedance, but if there is a potential difference, it may create ground loop noise. The specific topology should be designed comprehensively, taking into account the number of board layers, module positions, and cable exits.
Cables and Common-Mode Chokes
USB, power, and Ethernet cables can become radiating antennas, carrying common-mode currents during cellular transmissions. Ferrite cores and common-mode chokes can suppress high-frequency common-mode components on the cables. The outer layer of shielded cables should be grounded properly with single-ended or dual-ended strategies; incorrect grounding may be counterproductive. Outdoor device lightning protection and grounding systems relate to safety standards and should be designed separately, to avoid arbitrarily mixing RF ground with lightning protection ground.
Debugging Methods
Near-field probes and current clamps can help locate leakage points. Comparing spectra with the shield on and removed can verify shielding effectiveness. Note that the resonant frequency of the shield enclosure may be near the operating band, so if necessary, add absorbing material inside the enclosure or adjust its height.
Typical Lessons from IoT Complete Machines
Common mistakes include: removing the shielding in a module reference design to save costs, which causes failures in conducted spurious and radiated emissions during full-device testing; placing the USB and cellular antennas side by side on the same side, causing common-mode radiation as the return current flows through the cable outer shield; the metal case connected to the PCB ground only through a single screw, resulting in high-frequency impedance being too large. During remediation, it usually requires adding metal spring contacts to the mold, adding common-mode chokes at the interfaces, or adjusting the opening positions, which costs much more than leaving provisions during the design stage. It is recommended that customers review EMC risks and antenna layout simultaneously during schematic review and perform a pre-scan at the EVT stage to avoid concentrated failures right before certification.
Cavity Slots and Resonant Frequency
A shield gives the power amplifier, the oscillator, and the mixer a local metal boundary, so the electric field decays quickly at a slot. It is not a hope that the plastic housing itself absorbs RF. The longer a slot is relative to wavelength, the more it leaks, so the same assembly seam is more sensitive on a high band than on a low band. The can also has its own resonance. At the right height and side length, the cavity amplifies at some frequency instead of suppressing it. Grounding decides which layer the return current stays against. The RF reference ground needs to be low impedance and continuous. If digital switching current crosses that reference, spurious enters the receive band. A single-point join can avoid a low-frequency ground loop. A multi-point seam can lower high-frequency impedance. The two practices do not solve the same band.
In use, look at the cables at the same time. Power, USB, and Ethernet carry common-mode current out of the chassis and become antennas. A ferrite suppresses common mode on the cable. It does not replace the can. Which end of a shielded cable's outer conductor lands on the chassis depends on the direction of the interference. Bonding both ends, or bonding neither, can be worse. Outdoor lightning ground is a path that dumps a large current. RF ground is a small-signal reference. If the join is in the wrong place, lightning current or mains current flows across the RF reference plane.
Mixing Lightning Ground with the RF Reference
The most common rework is to omit the can from the reference design. Conducted results still look acceptable, and whole-device radiation exceeds the limit at slots and cables. Next is a can soldered at two diagonal points with the other edges open, so the slot length lands near the operating band. Third is a metal housing and the circuit ground joined by a single screw whose paint was never scraped. High-frequency impedance is large. A near-field probe reads a peak at the screw, and the investigation goes to change the match as if it were an antenna problem. When USB sits beside the antenna, return current leaves the chassis along the cable shield, and adding a can cannot hold it.
If the reference ground under the trace is slotted, soldering a can on cannot restore the return; see PCB RF routing. Nonlinearity in connectors and fasteners raises a separate noise floor under strong transmit. That is intermodulation, not leakage from the can; see passive intermodulation. Both effects change with transmit power. Compare before and after adding the can, and before and after changing the connector, as separate tests.
During rework, change one variable at a time: first close the can with a continuous solder joint, then add a cable ferrite, then move the ground point, and save the three spectra apart. Changing all three at once can lower the radiation without showing which change did it, and the next mechanical lot will omit the one that worked. If the can resonates near an operating channel, before raising the can or adding absorber inside it, confirm the slot length is not the main cause. Otherwise the absorber only hides an assembly seam. Measure the contact between the metal housing and circuit ground after assembly. Paint and anodizing turn a multi-point ground on the drawing into an open circuit in practice. The lightning discharge path should enter ground near the chassis entrance. It should not cross the RF reference plane and then return to system ground. Record the near-field probe readings at the screw, the opening, and the cable exit separately. A pre-scan before certification then shows whether to change the structure or the filtering.