Power Supply EMI Filter Shielding: Cavity Design for Conducted and Radiated Compliance

Executive Summary

Switch-mode power supplies remain the dominant source of both conducted and radiated emissions failures in automotive, industrial, and medical hardware. Even a well-designed pi or LC input filter leaves the switching node, the flyback transformer leakage field, and the output rectifier loop radiating as broadband near-field sources from 30 MHz through 1 GHz. Passing CISPR 25 Class 5 (peak limits as low as 24 dBµV/m at 75–400 MHz) or IEC 61000-4-3 immunity at 30 V/m requires mechanical shielding integrated with the filter topology. This note specifies shield can geometry, footprint rules, and attenuation targets for engineers using POCONS two-piece shield cans, SMD pan nuts, and spring contacts to cage SMPS and filter stages on a single PCB.

Technical Specifications & Attenuation Data

The shielding effectiveness (SE) of a soldered shield can is governed by three loss mechanisms: absorption loss in the wall material, reflection loss at the air-metal boundary, and aperture leakage through seams, vents, and ground discontinuities. For a 0.2 mm nickel-silver (CuNi18Zn20) wall, absorption loss alone exceeds 80 dB above 100 MHz. Real-world SE is almost always dominated by aperture leakage at the PCB-to-skirt seam, which is why ground pad geometry is more critical than wall thickness.

| Parameter | Specification | Standard / Reference | |-----------|--------------|----------| | Wall material | Nickel silver C7521, 0.15–0.20 mm | ASTM B122 | | Shielding effectiveness (E-field) | ≥80 dB, 30 MHz–1 GHz | IEEE 299 (scaled) | | Shielding effectiveness (H-field) | ≥45 dB at 100 MHz, ≥65 dB at 1 GHz | MIL-STD-285 (modified) | | Sheet resistance | ≤5 mΩ/sq | ASTM B193 | | Skirt-to-PCB contact resistance | ≤10 mΩ DC | IPC-TM-650 2.5.14 | | Spring contact resistance (pogo) | 30–50 mΩ initial, ≤80 mΩ after 10k cycles | MIL-STD-1344 3004 | | Seam gap (two-piece lid) | ≤0.1 mm, continuous | POCONS internal | | Temperature range | −55 °C to +125 °C | IEC 60068-2-14 | | Applicable compliance | CISPR 25 Class 5, ISO 11452-2/-4, IEC 61000-4-3/-6, MIL-STD-461 RE102/CE102 | — |

For ferrite bead filter stages, the bead's impedance peak (typically 600–1000 Ω at 100 MHz for an 0805 bead) only attenuates differential-mode current flowing through it. Common-mode noise bypasses the bead via stray capacitance to chassis. A shield can over the filter converts the stray path into a defined return, restoring 20–30 dB of CM attenuation that the bead cannot deliver alone.

Common Design Pitfalls

1. Discrete ground pads under the skirt. Root cause: designers place individual 1.0 × 1.0 mm pads at 4–5 mm pitch to ease manual soldering. Consequence: each gap forms a slot antenna resonant at c/(2L). A 5 mm gap resonates at 30 GHz, but radiates meaningfully from 3 GHz upward; more importantly, the inductive return path degrades H-field shielding by 15 dB across the FM band. Mitigation: use a continuous ground rail 0.8 mm wide under the entire skirt, stitched to the ground plane with vias at ≤2 mm pitch.

2. Shield can placed after the filter stage instead of over it. Root cause: mechanical layout convenience. Consequence: the filter inductor couples magnetically to the switching node before the shield wall intercepts the field. Mitigation: the shield cavity must enclose the switching FET, transformer, and the first two filter stages. Only the bulk input capacitor and fuse remain outside.

3. No internal partition between primary and secondary. Root cause: treating the can as a single Faraday cage. Consequence: the transformer's leakage inductance radiates onto the secondary rectifier loop inside the same cavity, and the cavity itself resonates at λ/2 of its longest dimension (a 40 mm cavity resonates at 3.75 GHz, 20 mm at 7.5 GHz). Mitigation: specify a two-piece can with an internal wall separating primary and secondary, or keep the longest internal dimension below 25 mm for sub-6 GHz designs.

4. Ventilation holes sized for thermal convenience. Root cause: thermal engineers request large apertures for airflow. Consequence: a circular aperture of diameter d leaks at frequencies where λ ≤ 2d. A 10 mm hole leaks above 15 GHz, but a 20 mm slot leaks above 7.5 GHz. Mitigation: use arrays of holes ≤3 mm diameter with center-to-center spacing ≥2× diameter; total open area can equal a single large hole while maintaining SE.

5. Relying on lid friction fit for RF seal on two-piece cans. Root cause: mechanical tolerance stack allows 0.2–0.3 mm lid-to-frame gap. Consequence: gap acts as a slot antenna at the cavity resonance. Mitigation: specify spring-finger contact along the lid perimeter (POCONS part family PCS-L series) with ≥4 contact points per centimeter and ≤10 mΩ contact resistance.

PCB Footprint & Soldering Profile Guidelines

Shield can frame footprint shall use a continuous ground trace 0.8 mm wide matching the skirt outline, with solder mask opening 0.1 mm larger than the trace on each side. Paste aperture ratio: 85–90% of pad area, stencil thickness 0.12 mm (5 mil) for SnAgCu paste. Courtyard clearance around the skirt outline: 0.5 mm minimum to adjacent components, 1.0 mm to tall components or connectors that may interfere with pick-and-place placement nozzles.

For POCONS SMD pan nuts used to retain removable lids, the footprint requires a 2.4 mm circular pad with 0.15 mm solder mask relief and four thermal relief spokes to the ground plane. Paste aperture: cross-hatched at 80% coverage to prevent flotation during reflow.

Reflow profile per J-STD-020 for SnAgCu (SAC305):

• Preheat ramp: 1.0–3.0 °C/s, 25 °C to 150 °C
• Soak: 150–200 °C for 60–120 s
• Peak reflow: 245–250 °C
• Time above liquidus (217 °C): 60–90 s
• Cooling ramp: ≤6 °C/s

Nickel-silver shield cans have high thermal mass; specify a 10 s longer TAL than used for 0402 passives and verify fillet formation along 100% of the skirt using X-ray inspection per IPC-A-610 Class 3. Rework per IPC-7711/7721 procedure 5.3.5, using hot-air with bottom-side preheat at 130 °C.

Recommended POCONS Components

Two-Piece Custom Shield Cans (TPS Series) — Frame-and-lid design with internal partition walls for primary/secondary isolation on SMPS boards. Custom cavity sizes from 8 × 8 mm to 80 × 80 mm, wall heights 3–12 mm. Solves the rework constraint and internal cavity resonance in a single part. Specify via /products/shield-cans/ with cavity dimensions, partition layout, and target SE.

SMD Pan Nuts (PN Series) — M1.6 and M2.0 reflow-compatible nuts for retaining removable lids or chassis grounding bosses. Tin-plated brass body with nickel-silver base flange. Part numbering PN-[thread]-[height]. Essential for field-serviceable SMPS modules where lid must be removable without desoldering. Available at /products/smd-pan-nuts/.

Spring Contacts / Pogo Pins (SC Series) — Gold-over-nickel plated, 30–50 mΩ contact resistance, 10,000+ cycle life. Used for lid-to-frame RF bonding on two-piece cans and for board-to-chassis grounding where solder joints are not permitted. Specify travel, force, and plating via /products/spring-contacts/.

For hybrid filter/shield assemblies, combine a TPS-series frame with PCS-L lid spring fingers and PN-series nuts at each corner. This configuration has been qualified to CISPR 25 Class 5 and ISO 11452-4 BCI at 200 mA on automotive 48 V SMPS designs.

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