How do I design a shielded compartment on a PCB to isolate sensitive RF stages?
Board-Level RF Shielding
Board-level shielding is essential in modern wireless devices (smartphones, IoT modules, base stations) where RF receivers, transmitters, and digital processors share the same PCB.
Shield Can Design
(1) Material: tin-plated steel: SE > 50 dB, magnetic, solderable. Most common. Nickel-silver alloy: SE = 40-50 dB, good mechanical properties, corrosion-resistant. Tin-plated copper: highest conductivity, SE > 60 dB, but more expensive. (2) Construction: one-piece can: simple, low cost. The can is soldered to the PCB ground pads during SMT reflow. Rework requires desoldering the entire can. Two-piece can (fence + lid): the fence (perimeter wall) is soldered to the PCB. The lid clips onto the fence. The lid can be removed for rework and debugging without disturbing the fence solder joints. More expensive but much easier for manufacturing and repair. (3) Can dimensions: height is limited by the tallest component inside + 0.5-1 mm clearance. Width and length should minimize the enclosed area (smaller = fewer cavity resonances). A resonance occurs when the can dimensions equal half-wavelength at a specific frequency: f_res = c/(2×sqrt(epsilon_r)) × sqrt((m/a)^2 + (n/b)^2), where a and b are the can dimensions and m, n are mode indices. For a 20 × 15 mm can: lowest resonance (TE10) at approximately 7.5 GHz (in air). If the circuit operates near this frequency: the cavity resonance amplifies coupling between components inside the can (the opposite of the intended shielding). Mitigation: add absorber material to the inside of the lid, or reduce the can dimensions, or add internal partitions to break the cavity into smaller sub-cavities with higher resonant frequencies.
Via Fence Design
(1) Via pitch rule: the via fence acts as a slot array; each gap between vias is an aperture. The SE of the fence = SE of a single slot - 10×log10(N_slots). For a gap of 1 mm at 10 GHz (lambda = 30 mm): SE per slot = 20×log10(30/(2×1)) = 23.5 dB. For 40 slots (40 mm perimeter): total SE = 23.5 - 10×log10(40) = 23.5 - 16 = 7.5 dB. Insufficient at 10 GHz. To achieve SE > 30 dB at 10 GHz: gap must be < 0.3 mm. This requires a via pitch of < 0.5 mm (tight but achievable with modern PCB processes). (2) Via size: 0.2-0.3 mm drill, 0.4-0.5 mm pad. Smaller vias allow tighter pitch. (3) Layer transitions: the via fence must connect ALL ground planes in the stack-up. If the PCB has 4 ground layers: each via must pass through all 4 layers. This creates a continuous grounded wall from top to bottom. (4) Stitching vias vs fence vias: stitching vias are scattered throughout the ground plane area (inside the shield) to prevent the ground plane from resonating as a patch antenna. Fence vias are specifically at the perimeter to create the shielding wall. Both are needed for good SE.
RF Feedthrough Design
Signals entering and leaving the shielded compartment must pass through the shield wall without degrading the SE: (1) RF signals: route on stripline layers (between ground planes) that pass under the via fence. The ground planes above and below the trace provide continuous shielding. The trace passes through the fence without any gap in the shielding. (2) DC and low-frequency control signals: use feedthrough capacitors at the shield wall. The capacitor passes DC (or low-frequency signals) while shorting RF to ground. For a shielded LNA compartment: the DC bias enters through a feedthrough cap (100 pF-100 nF depending on the RF frequency). (3) Digital signals: route on inner stripline layers and add a ferrite bead or filter at the shield wall crossing to prevent digital noise from entering the shielded compartment. (4) Power supply: add a pi-filter (C-ferrite-C) at the shield wall to prevent power supply noise from contaminating the RF section.
f_res = c/(2√ε_r) × √((m/a)² + (n/b)²)
SE_fence = SE_slot - 10log₁₀(N_slots)
SE_slot = 20log₁₀(λ/(2×gap))
Shield can SE: 40-60 dB typical
Frequently Asked Questions
Do I need a shield can for every RF stage?
Not necessarily. Selective shielding: shield only the stages that are most sensitive (LNA, VCO) or most noisy (PA, synthesizer, digital clock). If two stages are on the same PCB and separated by 20+ mm with a good ground plane: the natural isolation may be sufficient (30-50 dB from ground plane isolation alone). Use a shield can when: (1) The sensitivity of the receiver requires > 50 dB isolation from noise sources. (2) The VCO or synthesizer is susceptible to injection locking from nearby digital clocks. (3) The PA harmonics or spurious emissions must be contained. (4) FCC/CE compliance requires additional isolation beyond the PCB layout and enclosure.
How do I attach the shield can to the PCB?
Three common methods: (1) Solder reflow: the can has a solder-tinned rim that is reflowed to solder paste on the PCB ground pads during the standard SMT reflow process. The can is placed by the pick-and-place machine (for small cans) or manually (for large cans). Best SE (continuous solder bond). (2) Snap-fit (clip-on): the can has spring tabs that snap into slots or holes in the PCB. No soldering required. SE is slightly lower (the spring contact is not as continuous as solder). Easy to remove for rework. (3) Press-fit: the fence posts press into plated through-holes on the PCB. No soldering. Moderate SE. For production: two-piece (soldered fence + clip-on lid) is the most common approach. The fence is permanently soldered for best SE, and the lid is removable for rework and testing.
What about EMI between shield cans?
If two adjacent shield cans share a common ground pad wall: the coupling between them is limited by the via fence quality and the shared ground impedance. If the shared wall has insufficient vias: signals can leak from one compartment to the other through the common ground plane. To maximize inter-compartment isolation: use separate via fences with a gap (no shared wall). Add extra vias in the shared wall region. Ensure the ground planes in the shared wall area are solid (no signal traces crossing the wall). Add an absorber material to the insides of both lids to suppress cavity resonances. Isolation between adjacent well-designed shield cans: 40-70 dB (depending on frequency and via fence quality).