What shielding effectiveness do I need between an LNA and a synthesizer to keep reciprocal mixing noise below −110 dBc/Hz at 1 MHz offset?

Budget ≥55 dB of isolation across the LO fundamental band. A stamped nickel-silver two-piece can with ≤2 mm perimeter via pitch and a fence of ground vias around the LNA cavity typically delivers 55–70 dB from 700 MHz to 6 GHz, sufficient to keep injected LO leakage below the mixer's reciprocal-mixing floor.

At what seam gap does a 5.8 GHz shield can start leaking significantly?

Slot radiation becomes material when the longest unbroken seam exceeds λ/20 — about 2.6 mm at 5.8 GHz. Maintain contact-finger or via-stitch pitch ≤2 mm for Class 5 targets, and ≤1.5 mm where the cavity resonates near the operating band.

Can I use a one-piece soldered can instead of a two-piece frame-and-lid for a production radio?

Only when post-assembly rework and tuning access are not required. One-piece cans offer slightly better low-frequency attenuation but force board-level scrap on any VCO, PA, or filter trim. For volume OEM builds with field-replaceable modules or factory tuning, specify a two-piece can with a removable lid and ≥0.35 N per finger contact force.

RF Transceiver Front-End EMI Shielding: CISPR 25 Compliance for Sub-6 GHz Radios

Executive Summary

RF and microwave transceiver front-ends — the LNA, mixer, synthesizer, and PA subsystems described in canonical transmitter/receiver architectures — concentrate high-gain analog nodes, high-power output stages, and sensitive local-oscillator paths within centimeters of each other on a single PCB. The dominant failure mode at compliance is radiated emissions leakage from the PA harmonics and LO feedthrough coupling into adjacent victim circuits, producing CISPR 25 Class 5 exceedances in the 150 kHz–2.5 GHz sweep and self-desense that destroys receiver sensitivity before the product ever reaches an EMC chamber. This application note specifies the shield-can geometry, via-fence design, grounding contact force, and reflow constraints required to hit CISPR 25 Class 5, 3GPP TS 36.101 spurious emission masks, and ISO 11452-2 radiated immunity simultaneously, using POCONS two-piece stamped shield cans, SMD pan nuts, and spring-loaded pogo-pin grounding contacts.

Technical Specifications & Attenuation Data

Shield-can performance is dominated by three mechanisms: absorption loss through the wall (negligible for thin sheet metal above 100 MHz), reflection loss at the air-metal boundary (dominant at low frequencies), and aperture leakage through seams, vents, and tuning holes (dominant above 1 GHz). Target specifications below assume a two-piece nickel-silver (C7521) frame-and-lid construction with tin-plated finger contacts and a continuous ground-via fence on the PCB.

| Parameter | Specification | Standard | |-----------|--------------|----------| | Shielding effectiveness, 30 MHz–1 GHz | ≥70 dB | IEEE 299 / CISPR 25 | | Shielding effectiveness, 1 GHz–6 GHz | ≥55 dB | IEEE 299 / CISPR 25 Class 5 | | Shielding effectiveness, 6–18 GHz | ≥40 dB | MIL-STD-461 RE102 | | Base material — nickel-silver C7521 | σ ≈ 4.1 × 10⁶ S/m, μᵣ ≈ 1 | ASTM B122 | | Base material — tin-plated SPCC (alt.) | σ ≈ 1.0 × 10⁷ S/m, μᵣ ≈ 200–500 | JIS G 3141 | | Sheet thickness, frame | 0.20 mm ±0.02 mm | IPC-7525 | | Sheet resistance, plated surface | ≤5 mΩ/sq | ASTM B193 | | Finger contact force (per finger) | 0.35–0.60 N | POCONS TDS | | Finger pitch (perimeter) | ≤2.0 mm @ CISPR 25 Class 5 | Derived λ/20 @ 7.5 GHz | | Ground via pitch (PCB fence) | ≤2.0 mm; ≤1.5 mm near PA/LO | IPC-2221B | | Spring contact (pogo) resistance | ≤30 mΩ initial, ≤50 mΩ EoL | POCONS TDS | | Spring contact current rating | 2.0 A continuous per pin | UL 310 | | Cavity resonance target | f₁₀₁ > 1.5 × f_op,max | Cavity resonator theory | | Reflow peak, Pb-free | 245 °C ±5 °C | J-STD-020E |

Attenuation values assume the cavity is not operating at a resonant mode in the band of interest. The TEₘₙₚ resonant frequency of a rectangular cavity with dimensions a × b × d is f = (c/2)·√((m/a)² + (n/b)² + (p/d)²); for a 20 × 20 × 3 mm cavity, the lowest TE₁₀₁ mode lands near 10.6 GHz, safely above a 5.8 GHz Wi-Fi/LTE-U front-end. Stretch a can to 40 × 25 × 3 mm and the same mode falls to ~6.8 GHz — now inside the operating band, and measured SE collapses by 20–30 dB at resonance regardless of seam quality.

Common Design Pitfalls

Insufficient ground-pad copper under the can perimeter. Root cause: a narrow, thermal-relieved ground ring presents series inductance (typically 0.5–2 nH per segment), which at 2.4 GHz is 7.5–30 Ω of impedance back to the reference plane. Observable consequence: the shield can itself radiates as a driven slot antenna at the PA harmonic, producing CISPR 25 peaks 15–25 dB above Class 5 limits at 2.4, 3.6, and 4.8 GHz. Mitigation: ≥1.0 mm wide solid copper perimeter ring tied to the reference plane with via pitch ≤2.0 mm (≤1.5 mm adjacent to PA/LO cavities), no thermal reliefs on shield ground pads.
Cavity resonance coincident with operating band. Root cause: designers size the can to whatever empty board area is available rather than to the half-wavelength constraint. Observable consequence: a sharp 20–30 dB SE notch at f_res, visible as a narrow spike in the radiated-emissions sweep and as a sensitivity dip at a specific channel. Mitigation: keep the longest internal dimension below c/(2·f_op,max·√εᵣ_eff); when unavoidable, install a ferrite-loaded absorber patch (≥3 dB/cm at f_res) on the underside of the lid at the field maximum.
Seam gap from lid-to-frame fit tolerance stacking. Root cause: nominal 0.05 mm fit gap plus ±0.05 mm frame flatness plus ±0.03 mm lid flatness produces worst-case 0.13 mm slots distributed around the perimeter. Observable consequence: slot radiation at frequencies where seam length exceeds λ/20; typically appears above 3 GHz. Mitigation: specify finger-contact lids with ≥0.35 N per finger at 0.10 mm deflection, and limit continuous seam length between contacts to ≤2.0 mm.
Single-point grounding of multi-cavity cans. Root cause: partitioning walls are tied to the PCB only through the outer perimeter via fence, leaving the internal wall floating at RF. Observable consequence: LO-to-LNA isolation degrades from the ≥55 dB design target to 30–35 dB, causing reciprocal mixing noise rise and failed 3GPP spurious emission masks. Mitigation: dedicate a via row (≤1.5 mm pitch) directly under every internal partition wall, landed on the same reference plane as the outer fence.
Reflow-induced frame warp. Root cause: asymmetric solder-pad layout or uneven paste deposition creates thermal gradients during reflow, warping a 0.20 mm frame by 0.10–0.20 mm. Observable consequence: lid no longer makes uniform finger contact; SE drops 10–15 dB in a non-repeatable, unit-to-unit pattern. Mitigation: symmetric pad layout, uniform 0.12 mm stencil, paste aperture ratio 0.66–0.75, and a soak-zone dwell of 60–90 s at 150–180 °C to equalize frame temperature before the liquidus excursion.

PCB Footprint & Soldering Profile Guidelines

Footprint: solid copper perimeter pad 1.0 mm wide minimum, 1.2 mm preferred, with no thermal reliefs and no solder mask over the contact land. Mask pullback 0.1 mm outside the pad edge. Courtyard clearance to adjacent SMD components: 0.5 mm minimum; 1.0 mm where rework access to can-side components is required. Ground vias filled or tented on the opposite side to prevent solder wicking; via drill 0.25 mm, annular ring 0.15 mm, pitch ≤2.0 mm around the perimeter and ≤1.5 mm under any internal partition wall or adjacent to PA and synthesizer cavities.

Paste stencil: 0.12 mm laser-cut stainless steel, electropolished, with aperture ratio 0.66–0.75 calculated per IPC-7525B. For a 1.0 × 6.0 mm perimeter segment, aperture 0.9 × 5.8 mm yields ~72% paste coverage, sufficient for a continuous solder fillet without bridging to adjacent SMD pads. Paste: SAC305, Type 4 (20–38 μm) for fine-pitch adjacent components.

Reflow profile (J-STD-020E, Pb-free): preheat ramp 1.0–2.5 °C/s from 25 °C to 150 °C; soak 60–90 s between 150–200 °C; ramp 1.0–3.0 °C/s to peak; peak 245 °C ±5 °C; time above liquidus (217 °C) 60–90 s; cooling ramp ≤4 °C/s. Exceeding 250 °C peak or 90 s TAL degrades the tin plating on finger contacts and raises contact resistance by 20–50 mΩ after three reflow cycles. Reference IPC/JEDEC J-STD-020E for moisture sensitivity and IPC-A-610 Class 3 for acceptance.

For field-serviceable assemblies using POCONS SMD pan nuts, torque shield-can lid screws to 0.08–0.12 N·m; over-torque strips the nut's solder joint from the PCB and produces intermittent grounding failures that only manifest after thermal cycling per IEC 60068-2-14.

Recommended POCONS Components

Custom Two-Piece Shield Cans — Frame-and-lid construction in nickel-silver C7521 or tin-plated SPCC, 0.20 mm wall, stamped finger contacts at ≤2.0 mm pitch. Solves the rework-access, tuning-access, and cavity-isolation requirements of multi-stage transceiver front-ends without sacrificing the ≥55 dB SE target across the sub-6 GHz band. Specify cavity dimensions to keep TEₘₙₚ resonances outside the operating band. /products/shield-cans/two-piece/
SMD Pan Nuts — Reflow-compatible fastening points for removable lids and chassis bonding, 0.08–0.12 N·m torque rating. Enables field service, EMC-chamber tuning, and repeated rework without compromising the RF ground return path. Use where lid retention force exceeds what finger contacts alone can provide. /products/smd-hardware/pan-nuts/
Spring Contacts / Pogo Pins — ≤30 mΩ initial contact resistance, 2.0 A continuous, 0.35–0.60 N working force. Deploy as board-to-board, board-to-chassis, or antenna-to-PCB ground bonds where solder is not mechanically viable or where service disassembly is required. Critical for maintaining the low-impedance reference plane continuity that every shield-can SE figure in this note assumes. /products/spring-contacts/

Application note produced by POCONS USA engineering team. Contact applications@poconsusa.com for design review.