Board-Level Shield Cans for Power Supply EMI Filtering Beyond Ferrite Beads

Executive Summary

Switch-mode power supplies are the dominant source of conducted and radiated EMI on mixed-signal PCBs, and the instinct to suppress this noise with ferrite beads is so deeply embedded in design culture that engineers often exhaust bead selections before considering spatial containment. The failure mode is specific: ferrite beads attenuate conducted noise on the trace they sit on, but they cannot suppress radiated emissions from switching node copper, exposed inductor windings, or high-dV/dt planes underneath the converter. When a design passes CISPR 32 Class B conducted limits at the connector but fails radiated emission scans between 200 MHz and 1 GHz, the root cause is almost always uncontained near-field radiation from the power section—a problem no amount of series impedance on the rail will solve. POCONS USA two-piece shield cans and precision spring contacts provide 40–80 dB of radiated attenuation across this critical band, delivering the spatial containment that board-level filtering cannot, and enabling compliance with CISPR 32, IEC 61000-4-3, and MIL-STD-461 RE102.

Technical Specifications & Attenuation Data

The shielding effectiveness of a board-level can depends on three factors: the bulk conductivity of the can material, the electrical continuity of the can-to-ground interface, and the absence of apertures that act as slot antennas at frequencies of interest. POCONS shield cans are stamped from C5210 phosphor bronze (conductivity 1.1 × 10⁷ S/m) with tin plating at 1–3 µm thickness, providing a sheet resistance of approximately 2.5 mΩ/sq at 0.2 mm wall thickness. For applications requiring enhanced magnetic-field attenuation below 30 MHz, nickel-silver (C7521) variants offer relative permeability of 1.02 with improved corrosion resistance in high-humidity environments per IEC 60068-2-67.

The ground interface is where most designs fail. A soldered perimeter fence provides the lowest impedance path but prevents rework access. POCONS two-piece designs solve this with a soldered fence (base frame) and a snap-fit lid, maintaining shielding continuity through spring-loaded contact points at intervals no greater than λ/20 at the highest frequency of concern. For a 6 GHz upper bound, this means contact spacing of ≤2.5 mm along the lid perimeter.

Spring contacts in the POCONS PSC series deliver the following measured performance:

| Parameter | Specification | Test Standard | |-----------|--------------|---------------| | Shielding effectiveness, 200 MHz–1 GHz | ≥ 60 dB | IEEE 299 (modified for board-level) | | Shielding effectiveness, 1 GHz–6 GHz | ≥ 40 dB | IEEE 299 (modified for board-level) | | Contact resistance per spring point | 5–8 mΩ initial, < 10 mΩ after 10k cycles | EIA-364-06 | | Spring contact force | 0.3–0.8 N per contact | EIA-364-04 | | Operating temperature range | −40 °C to +125 °C | IEC 60068-2-14 | | Can material conductivity (C5210) | 1.1 × 10⁷ S/m | ASTM E1004 | | Wall thickness | 0.15–0.30 mm | — | | Tin plating thickness | 1–3 µm | ASTM B545 | | Contact spacing (lid-to-fence) | ≤ 2.5 mm for 6 GHz compliance | Derived from λ/20 rule |

These values are measured with the shield can soldered to a continuous ground ring on a 4-layer FR-4 test board (1.6 mm thickness, 1 oz copper) using a near-field probe per the methodology described in IEC 61967-6. Shielding effectiveness degrades by approximately 6 dB per doubling of the largest aperture dimension—this relationship is the single most important design rule for shield can specification.

Ferrite beads, by contrast, offer impedance insertion that is highly frequency-dependent and fundamentally limited by their rated current. A typical 0402 ferrite bead rated at 600 Ω at 100 MHz provides meaningful attenuation only between approximately 50 MHz and 500 MHz, with impedance falling sharply above the ferrite's resonant frequency as the parasitic capacitance (typically 0.2–0.5 pF) creates a low-impedance bypass. At 1 GHz, many common ferrite beads present less than 100 Ω of impedance—often less than the trace impedance they are inserted into, rendering them electrically invisible. Above 2 GHz, most ferrite bead materials have fully transitioned from inductive to capacitive behavior and provide zero EMI suppression. This is the regime where board-level shielding becomes not merely helpful but mandatory.

Common Design Pitfalls

1. Relying on ferrite beads for radiated emission failures above 500 MHz. Ferrite beads are conducted-mode devices. When the switching node of a buck converter radiates from the copper polygon it occupies, inserting a ferrite bead on the output rail has no effect on the radiated field. The observable consequence is a design that passes conducted emissions at the power connector but fails RE102 scans at 3-meter distance between 500 MHz and 2 GHz. The mitigation is to enclose the entire converter section—input capacitors, switching FETs, inductor, and output capacitors—inside a shield can with continuous ground ring connectivity. Do not shield only the inductor; the switching node trace and FET drain pad are equally significant radiators.

2. Insufficient ground ring copper width beneath the shield can fence. The shield can fence pads must provide a continuous, low-inductance return path. A ground ring narrower than 0.5 mm on inner layers, or with thermal relief spokes on the fence solder pads, introduces inductive discontinuities that degrade shielding effectiveness by 10–20 dB above 1 GHz. The design rule: ground ring width ≥ 1.0 mm on all copper layers, fence pads connected with solid (non-thermal-relief) connections to the ground plane, and at minimum one ground via per 2.0 mm of fence perimeter length. Vias should be 0.3 mm finished diameter, placed within 0.5 mm of the fence pad outer edge.

3. Shield can cavity resonance due to internal dimension selection. Every shield can is a resonant cavity. The lowest resonant mode (TE₁₀) occurs at f = c / (2L√εᵣ), where L is the longest internal dimension and εᵣ accounts for the effective dielectric of the PCB substrate and any potting material. For a 20 mm long shield can on FR-4 (εᵣ ≈ 4.3), the first resonance occurs at approximately 3.6 GHz. If the converter's harmonic content includes energy at this frequency, the shield can amplifies rather than attenuates the emission. Mitigation: size the can so that no internal dimension produces a resonance coinciding with known harmonic content. Alternatively, apply RF-absorbing material (µ″ > 5 at the resonant frequency) to the interior lid surface to damp the Q of the cavity below 10.

4. Using the shield can as a heatsink without thermal modeling. A shield can enclosing a 2 W buck converter in still air creates a localized thermal environment. Without ventilation apertures (which degrade shielding) or thermal pad connections to internal ground copper, junction temperatures of enclosed ICs can exceed ratings by 15–25 °C. The solution is to design thermal vias underneath the converter IC, connecting to internal ground planes that extend beyond the shield can perimeter, allowing heat to conduct laterally through the PCB stack-up rather than relying on convection within the enclosed volume.

5. Ignoring the ferrite bead's DC bias derating when selecting it as a pre-filter before the shield can. Ferrite beads lose impedance under DC bias. A bead rated at 1000 Ω at 100 MHz may present only 200 Ω when carrying 80% of its rated current, because the ferrite core partially saturates. If a ferrite bead is used as a conducted-mode pre-filter at the power entry to the shielded zone, its impedance under actual operating current—not its zero-bias datasheet value—must be used in the filter model. Failing to account for this results in a 10–15 dB gap between simulated and measured conducted attenuation at the power entry point.

PCB Footprint & Soldering Profile Guidelines

POCONS shield cans require a specific PCB footprint geometry to achieve rated shielding effectiveness. The fence footprint consists of a continuous perimeter pad with the following specifications:

• Pad width: 1.0 mm minimum for 0.2 mm wall thickness cans; 1.2 mm for 0.3 mm wall thickness
• Courtyard clearance: 0.25 mm from the outer edge of the fence pad to the nearest non-ground copper feature
• Solder paste aperture ratio: 80% of pad area, using a home plate or modified rectangle aperture pattern to prevent bridging to adjacent component pads
• Stencil thickness: 0.12 mm (5 mil) for standard assembly; 0.10 mm (4 mil) if fine-pitch components (≤ 0.4 mm pitch) share the same stencil
• Ground via pattern: 0.3 mm finished hole diameter, 0.6 mm pad diameter, placed on 2.0 mm centers along the fence perimeter centerline, with solid copper connections (no thermal relief) on all ground layers

For the two-piece design, the base fence is soldered during the primary SMT reflow pass alongside all other components. The lid is installed after programming, testing, and any rework of enclosed components, snapping onto the fence with spring contact engagement. This sequence is critical: the base fence must withstand reflow without the lid present, and the spring contacts must not be exposed to reflow temperatures.

Reflow Soldering Profile (SAC305, per IPC/JEDEC J-STD-020)

| Phase | Parameter | Value | |-------|-----------|-------| | Preheat ramp rate | Temperature rise | 1.0–2.5 °C/s | | Soak zone | Temperature range | 150–200 °C | | Soak zone | Duration | 60–120 s | | Ramp to peak | Temperature rise | 1.0–2.5 °C/s | | Peak reflow | Temperature | 245 ± 5 °C | | Time above liquidus (TAL) | Duration at > 217 °C | 40–70 s | | Cooling rate | Temperature decline | ≤ 3.0 °C/s (−6 °C/s max) |

The phosphor bronze fence material tolerates peak reflow temperatures up to 260 °C without annealing or spring temper degradation. Tin plating on the fence provides solderability with SAC305, SnPb, and SnBi low-temperature alloys. For rework of the base fence, follow IPC-7711/7721 procedures for through-hole and SMT shield removal: apply uniform heat across the entire fence perimeter using a dedicated nozzle sized to the can outline, and lift vertically once all solder joints reach liquidus simultaneously. Uneven heating causes fence distortion and pad lifting.

POCONS provides reference footprint files in KiCad, Altium Designer, and OrCAD/Allegro formats for all standard shield can sizes. Custom footprints are generated as part of the DFM review for custom shield can orders.

Recommended POCONS Components

Custom Two-Piece Shield Cans

The POCONS TPS series is engineered specifically for the problem described in this application note: enclosing power supply sections that have exhausted conducted-mode filtering options and require spatial containment for radiated compliance. The two-piece architecture allows the base fence to be soldered during standard SMT reflow, while the snap-fit lid enables post-assembly access for IC programming, boundary scan testing, and rework. Available in C5210 phosphor bronze with tin plating as standard, or nickel-silver for applications requiring enhanced corrosion resistance. Custom dimensions from 5 × 5 mm to 60 × 60 mm, heights from 1.5 mm to 8.0 mm. Internal divider walls available for multi-cavity configurations separating input and output stages of multi-rail power management ICs. View custom shield cans →

Spring Contacts / Pogo Pins

The POCONS PSC series spring contacts provide the electrical interface between the base fence and the snap-fit lid. Each contact point delivers 5–8 mΩ resistance with 0.3–0.8 N contact force, maintaining shielding continuity through mechanical vibration profiles per IEC 60068-2-6 (10–2000 Hz, 20 g). The gold-over-nickel plated spring tips resist oxidation through 10,000+ mating cycles, ensuring field-replaceable lid access does not degrade shielding effectiveness over the product lifetime. SMD-mountable variants are available for integration directly into the shield can fence during primary reflow. View spring contacts →

SMD Pan Nuts

For shield cans requiring screw-down attachment—common in high-vibration environments such as automotive ADAS modules and industrial motor drives—POCONS SMD pan nuts provide a surface-mount threaded fastener that reflows with the base fence. The pan nut design distributes clamping force across the lid perimeter, compressing gasket or spring contact interfaces to achieve consistent contact pressure across temperature cycling from −40 °C to +125 °C. M2 and M2.5 thread sizes in stock; custom threads available. View SMD pan nuts →

Integrating the Solution

For power supply sections failing radiated emissions between 200 MHz and 6 GHz after ferrite bead optimization, the recommended integration path is:

1. Define the shielded zone boundary to include all switching components, gate drive traces, and the first stage of output filtering
2. Specify a POCONS TPS-series two-piece shield can sized to the zone with ≥ 2.0 mm clearance to the tallest enclosed component
3. Design the PCB ground ring per the footprint guidelines above, ensuring continuous copper on all ground layers beneath the fence perimeter
4. Place PSC-series spring contacts at ≤ 2.5 mm intervals along the lid engagement perimeter
5. Validate shielding effectiveness with a near-field probe scan before and after lid installation per IEC 61967-6

Contact applications@poconsusa.com with your PCB layout files and radiated emission scan data for a complimentary design review and shield can recommendation.

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