Conducted Emissions Containment with PCB-Level Shield Cans: A CISPR 25 Design Guide

Executive Summary

Automotive electronic control units routinely fail CISPR 25 conducted emissions testing in the 30–108 MHz band due to inadequate containment of switching regulator harmonics at the PCB level. The root failure mode is straightforward: high-frequency current from DC-DC converters couples through shared ground planes and unshielded traces into harness-connected pins, where it appears as conducted emissions on the voltage method test bench. Board-level shield cans, when properly designed with low-impedance perimeter grounding and correct cavity dimensioning, suppress this coupling path by 40–80 dB across the CISPR 25 frequency range of 150 kHz to 2.5 GHz. POCONS USA manufactures precision two-piece shield cans and BeCu spring contacts engineered specifically for this application, providing the mechanical compliance, solderability, and electrical performance required to close the gap between prototype and Class 5 certification.

Technical Specifications & Attenuation Data

Shield can effectiveness is not a single number. It is frequency-dependent, geometry-dependent, and dominated at high frequencies by the quality of the perimeter ground bond rather than the bulk shielding material. The following specifications establish the engineering envelope for POCONS shield cans deployed in CISPR 25 conducted emissions applications.

The shield material is tin-plated cold-rolled steel (CRS) at 0.20 mm nominal thickness for standard cans, with mumetal options available for applications requiring magnetic-field attenuation below 1 MHz. Tin plating thickness is 2–5 µm per IPC-4552, providing both solderability and corrosion resistance across the –40°C to +125°C automotive operating range.

| Parameter | Specification | Applicable Standard | |---|---|---| | Shielding effectiveness (SE), 200 MHz–1 GHz | ≥ 60 dB (plane wave, IEEE 299 method) | IEEE 299-2006 | | Shielding effectiveness (SE), 1 GHz–6 GHz | ≥ 50 dB | IEEE 299-2006 | | Shielding effectiveness (SE), 30 MHz–200 MHz | ≥ 40 dB (near-field magnetic) | MIL-STD-461G RE101 | | Material thickness (CRS) | 0.20 ± 0.02 mm | — | | Material sheet resistance | ≤ 1.2 mΩ/sq (tin-plated CRS, 0.20 mm) | — | | Relative permeability (CRS) | µ_r ≈ 200 (DC), decreasing above 1 MHz | — | | Spring contact resistance (BeCu) | 5–15 mΩ per contact, initial | EIA-364-06 | | Spring contact resistance after 1,000 cycles | ≤ 30 mΩ per contact | EIA-364-06 | | Contact normal force (per spring finger) | 0.3–0.8 N | — | | Reflow compatibility | Pb-free, peak 260°C, MSL 1 | J-STD-020E | | Operating temperature range | –40°C to +125°C | AEC-Q200 |

For conducted emissions per CISPR 25 (voltage method, 150 kHz–108 MHz), the shield can does not directly filter the conducted path on the harness. Its role is to prevent the radiated near-field from the switching converter's inductor and hot-loop from coupling into adjacent traces routed to connector pins. In bench testing using the CISPR 25 voltage method with a 5 µH/50 Ω LISN, a properly grounded POCONS two-piece shield can over a 2 MHz buck converter reduced emissions measured at the LISN by 25–35 dB in the 30–108 MHz FM broadcast band. This margin is the difference between a Class 3 marginal pass and a clean Class 5 result.

Above 108 MHz, the shield's radiated SE becomes the dominant factor. At 1 GHz, the skin depth in tin-plated CRS is approximately 1.3 µm — meaning the 0.20 mm wall provides roughly 150 skin depths of attenuation. The material is not the weak link. The perimeter bond is.

Common Design Pitfalls

1. Insufficient ground pad width creates inductive perimeter impedance. The single most common shield can failure is a PCB footprint with ground pads narrower than 0.8 mm. Each millimeter of pad width corresponds to roughly 0.5 nH of inductance per pad segment at the shield wall-to-board interface. When pad width drops below 0.8 mm, the total perimeter inductance can exceed 5 nH, creating a resonant slot antenna at frequencies where the perimeter length approaches λ/2. For a 30 mm × 30 mm shield can (120 mm perimeter), this resonance falls near 1.25 GHz — squarely in the LTE and GPS bands. Mitigation: maintain ground pad width ≥ 1.0 mm continuously around the full perimeter with stitching vias at ≤ 2.5 mm pitch connecting the pad to the internal ground plane.

2. Cavity resonance from oversized shield cans. Engineers frequently specify shield cans larger than necessary to allow layout flexibility, but the cavity's first resonant mode occurs at f = c / (2L√ε_eff), where L is the longest internal dimension. A 50 mm shield can with FR-4 substrate (ε_eff ≈ 3.2) resonates at approximately 1.67 GHz. At resonance, internal fields amplify rather than attenuate, and the shield can becomes a radiator. Mitigation: keep the longest internal dimension below 40 mm for applications requiring SE above 1.5 GHz, or partition with internal fence walls using POCONS clip-in divider strips.

3. Shared ground plane with noisy switching node. If the shield can's perimeter ground pads connect to the same copper pour carrying the return current of the switching converter, the converter's di/dt directly modulates the shield's ground reference. This is equivalent to driving the shield as an antenna with the switching noise. Mitigation: dedicate a separate ground pour island for the shield perimeter, connected to the main ground plane at a single star point located at the quietest corner of the shielded region, or use a full internal ground plane layer with via stitching.

4. Missing or misplaced apertures for thermal relief. Shield cans require ventilation apertures when covering components dissipating more than 0.5 W in automotive ambient conditions. However, apertures larger than λ/20 at the highest frequency of concern degrade SE by 10–20 dB. A 3 mm round hole becomes λ/20 at 5 GHz. Mitigation: use arrays of sub-1.5 mm holes rather than single large openings; orient slot apertures perpendicular to the dominant internal E-field polarization; keep total aperture area below 1% of the shield can surface area.

5. Neglecting the lid-to-frame contact interface in two-piece designs. Two-piece shield cans (soldered frame + removable lid) are preferred for rework access, but the lid-to-frame contact determines high-frequency SE. Without spring contacts, the lid relies on friction fit, producing contact gaps of 50–200 µm that behave as slot radiators above 500 MHz. Mitigation: specify POCONS BeCu spring contacts integrated into the frame at ≤ 5 mm spacing around the perimeter. Each spring finger provides 0.3–0.8 N normal force, maintaining contact resistance below 20 mΩ through thermal cycling and mechanical shock per AEC-Q200 requirements.

PCB Footprint & Soldering Profile Guidelines

Pad Geometry

The shield can frame is soldered to a continuous ground pad ring on the PCB surface. POCONS provides component-specific footprint recommendations with each shield can drawing, but the following general rules apply:

• Pad width: 1.0–1.5 mm (wider is better for SE; 1.2 mm is the standard recommendation)
• Pad-to-pad gap at corners: 0.0 mm — pads must form a continuous ring with no interruption at corners
• Courtyard clearance: 0.25 mm from shield can outer wall to nearest non-ground copper or component
• Ground via stitching: 0.3 mm finished hole diameter, 0.6 mm pad diameter, placed at ≤ 2.5 mm pitch along the full perimeter, centered on the ground pad ring
• Solder paste aperture ratio: 70–80% of pad area, using a home-plate or segmented pattern to control solder volume and prevent bridging to internal traces
• Stencil thickness: 0.12 mm (120 µm) for 0.20 mm wall thickness cans; increase to 0.15 mm for 0.30 mm wall thickness to ensure adequate solder fillet

For two-piece designs, the removable lid has no solder footprint. The frame footprint carries the full mechanical and electrical interface. Spring contact locations on the frame's inner ledge are defined by the POCONS drawing; no additional PCB features are required for the lid interface.

Reflow Soldering Profile

POCONS shield can frames are compatible with standard Pb-free reflow processes per J-STD-020E. The following profile targets SAC305 solder paste:

| Phase | Parameter | Value | |---|---|---| | Preheat ramp rate | ΔT/Δt | 1.0–2.5 °C/s | | Soak zone temperature | T_soak | 150–200 °C | | Soak zone duration | t_soak | 60–120 s | | Ramp to peak | ΔT/Δt | 1.0–2.5 °C/s | | Peak reflow temperature | T_peak | 245–255 °C (260 °C absolute max) | | Time above liquidus (TAL) | t > 217 °C | 40–70 s | | Cooling rate | ΔT/Δt | –2.0 to –4.0 °C/s |

Critical notes: The large thermal mass of the shield can frame (typically 0.5–3.0 g depending on size) acts as a heat sink, delaying the frame solder joints from reaching liquidus relative to surrounding SMD components. Ensure that the reflow profile is validated with a thermocouple attached directly to the shield can frame, not just to adjacent component pads. Insufficient TAL on the frame results in cold joints with contact resistance exceeding 100 mΩ — a direct SE degradation mechanism. Per IPC J-STD-001, Class 3 (automotive), solder fillet height must be ≥ 75% of the frame wall height on the soldered side.

Post-Reflow Inspection

Per IPC-A-610 Class 3 criteria, inspect for continuous solder fillets around the full perimeter. Any gap exceeding 1.0 mm in the fillet is a reject condition, as it creates a slot aperture in the ground bond. X-ray inspection is recommended for production validation of the first article to verify solder wetting beneath the frame wall, where visual inspection is impossible.

Recommended POCONS Components

Custom Two-Piece Shield Cans

The POCONS two-piece shield can system is the primary solution for CISPR 25 conducted emissions applications. The soldered base frame establishes the low-impedance perimeter ground bond, while the snap-fit lid with integrated spring contacts provides field-removable access for debug, rework, and programming.

• Material: Tin-plated CRS, 0.20 mm standard (0.15 mm and 0.30 mm available)
• Customization: Any rectangular footprint from 8 mm × 8 mm to 80 mm × 60 mm; internal fence walls for multi-cavity partitioning; custom height from 1.5 mm to 8.0 mm
• Why it solves this problem: The two-piece design allows full compliance testing with the lid installed while preserving rework access — critical during the iterative EMC debug cycle that CISPR 25 Class 5 typically demands

Explore the full range at /products/emi-shielding-components/.

BeCu Spring Contacts

POCONS beryllium copper spring contacts are the enabling component for reliable lid-to-frame conductivity in two-piece shield can systems.

• Contact resistance: 5–15 mΩ initial, ≤ 30 mΩ after 1,000 insertion cycles
• Normal force: 0.3–0.8 N per finger, tunable via geometry
• Finish: Gold over nickel (standard), tin over nickel (cost-optimized)
• Why it solves this problem: Without spring contacts, a two-piece shield can loses 15–25 dB of SE above 500 MHz due to the lid-to-frame air gap. Spring contacts eliminate this gap with controlled, repeatable force, maintaining SE through thermal cycling, vibration, and mechanical shock per automotive qualification standards

See spring contact configurations at /products/spring-contacts/.

SMD Pan Nuts

For shield cans requiring screw-down attachment in high-vibration environments (engine bay modules, ADAS sensor housings), POCONS SMD pan nuts provide a surface-mount threaded fastening point that is reflow-soldered to the PCB ground pad.

• Thread sizes: M2, M2.5, M3
• Material: Nickel-plated brass
• Solder interface: Four-pad footprint with ≥ 2.0 mm² total ground contact area per nut
• Why it solves this problem: In environments exceeding 20 G_rms vibration (ISO 16750-3), snap-fit lids with spring contacts alone may not maintain the required contact force. SMD pan nuts provide a deterministic clamping force via torque specification, eliminating contact degradation under sustained vibration

Browse SMD fastener options at /products/smd-pan-nuts/.

Design Review Service

POCONS USA offers complimentary DFM and shielding effectiveness reviews for OEM design teams working toward CISPR 25 Class 5 certification. Submit your PCB layout (ODB++ or Gerber) along with your target frequency plan and conducted emissions test data to applications@poconsusa.com. Our engineering team will return a shield can placement recommendation, footprint DRC report, and cavity resonance analysis within 5 business days.

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