CISPR 25 Conducted Emissions Compliance: Shield Can Design and Filtering Integration for Automotive DC-DC Converters

Executive Summary

Automotive DC-DC converters—particularly buck regulators operating between 100 kHz and 2.5 MHz—are the dominant source of conducted emissions failures during CISPR 25 qualification. The voltage-method test procedure specified in CISPR 25:2021 Clause 6.4 captures both differential-mode and common-mode noise currents on power harness conductors from 150 kHz to 108 MHz, with Class 5 peak limits as low as 18 dBμV in the FM broadcast band. When input and output LC filters alone cannot achieve sufficient margin, board-level shield cans isolating the switching stage eliminate residual radiated coupling between the inductor, switch node, and harness-side traces. POCONS USA two-piece shield cans and low-impedance spring contacts provide the mechanical and electrical foundation for this containment strategy, delivering repeatable shielding effectiveness exceeding 55 dB from 200 MHz through 6 GHz while surviving the thermal cycling and vibration profiles mandated by ISO 16750-3.

Technical Specifications & Attenuation Data

The conducted emissions limits in CISPR 25:2021 are organized into five severity classes. Class 5 is the most stringent and is the de facto requirement for Tier 1 automotive OEMs. For the voltage method (artificial network / LISN), the limit line descends from approximately 40 dBμV at 150 kHz to 18 dBμV at 1.8 MHz, holding flat through 30 MHz for narrowband peak measurements. Above 30 MHz, the standard transitions to radiated emissions under Clause 6.5, but common-mode noise conducted onto cable harnesses remains a dominant radiation mechanism up to 108 MHz.

A buck converter switching at 500 kHz produces harmonic content extending to several hundred megahertz. The fundamental and first five to eight harmonics fall within the conducted emissions band and must be attenuated by input differential-mode LC filters. However, the switch-node voltage slew rate—typically 5–20 V/ns for modern automotive MOSFETs—couples capacitively to adjacent copper, creating common-mode currents that bypass the differential filter entirely. This is where board-level shielding becomes essential: a grounded metallic enclosure over the switching stage attenuates both electric-field coupling to nearby traces and magnetic-field coupling from the power inductor's fringing flux.

| Parameter | Specification | Standard / Reference | |---|---|---| | Shielding effectiveness, 200 MHz–1 GHz | ≥55 dB (plane wave, per IEEE 299 adapted) | IEEE 299, measured with POCONS two-piece can on 1.6 mm FR-4 | | Shielding effectiveness, 1 GHz–6 GHz | ≥50 dB | IEEE 299 | | Spring contact resistance, initial | ≤2 mΩ per contact point | POCONS datasheet, 4-wire Kelvin measurement | | Spring contact resistance, after 10,000 cycles | ≤5 mΩ per contact point | Per EIA-364-06 contact resistance method | | Contact normal force per spring finger | 0.3–0.8 N | POCONS spring contact design spec | | Shield can material | C5210 phosphor bronze or C7701 nickel silver, 0.20 mm nominal | — | | Plating | Tin (3–8 μm) or tin-lead (5–10 μm) over nickel (1.5–3 μm) barrier | IPC J-STD-001 Class 3 | | Sheet resistance of can wall | ≤1.5 mΩ/sq at 0.20 mm thickness | Calculated from bulk resistivity | | Operating temperature range | −40 °C to +125 °C continuous | ISO 16750-3 | | Reflow compatibility | 260 °C peak, Pb-free process | IPC/JEDEC J-STD-020 | | CISPR 25 Class 5 conducted limit (peak, 1.8–30 MHz) | ≤18 dBμV | CISPR 25:2021 Table 1 | | CISPR 25 Class 5 conducted limit (average, 1.8–30 MHz) | ≤6 dBμV | CISPR 25:2021 Table 1 |

The skin depth of tin-plated phosphor bronze at 200 MHz is approximately 6.5 μm, meaning the 0.20 mm wall thickness represents over 30 skin depths at this frequency. Absorption loss alone exceeds 250 dB; the practical limit on shielding effectiveness is therefore determined entirely by aperture leakage at seams, cable penetrations, and contact impedance at the can-to-PCB interface.

Common Design Pitfalls

1. Insufficient ground pad copper area under shield can perimeter. The PCB footprint for the shield can fence must present a continuous, low-inductance ground plane connection. When the ground pad is narrowed to route signal traces underneath the shield wall, the resulting inductive gap in the return path creates a slot antenna. At frequencies where the gap length approaches λ/20, radiated leakage increases sharply. A 5 mm gap in the ground pad produces measurable degradation above 300 MHz. Mitigation: Maintain a minimum 1.0 mm continuous copper pad width under the entire can perimeter. Route all signals through designated apertures with ground-stitching vias on both sides, spaced at ≤λ/20 of the highest frequency of concern (e.g., ≤2.5 mm spacing for 6 GHz containment).

2. Cavity resonance excited by switch-node or inductor placement. A shield can forms a resonant cavity. The lowest resonant mode (TE₁₀) occurs when the longest internal dimension equals λ/2. For a 30 mm × 20 mm × 5 mm can, the TE₁₀ mode falls at approximately 5 GHz, but the first higher-order mode with vertical electric field (TM₁₁₀) can be excited at lower frequencies if the switch-node trace runs along the longest axis. When harmonics of the switching frequency coincide with a cavity mode, the shield can amplifies rather than attenuates. Mitigation: Place absorptive material (e.g., ferrite-loaded polymer sheet) on the inside surface of the lid if cavity modes fall within the emissions band. Alternatively, offset the switching node from the geometric center and longest axis to reduce coupling to the dominant mode.

3. Thermal relief spokes on shield can ground pads preventing solder wetting. Standard thermal relief patterns—four spokes connecting the pad to the ground plane—are appropriate for through-hole soldering but catastrophic for shield can reflow. The spokes create discontinuities in the solder fillet at the can perimeter, producing intermittent contact points with resistance values 10–50× higher than a solid joint. Mitigation: Use solid, non-thermally-relieved connections on all shield can perimeter pads. If thermal management during reflow is a concern, increase preheat soak duration rather than adding spokes. The thermal mass of the shield can itself conducts heat to the joint adequately if soak temperatures reach 180–200 °C.

4. Neglecting the impedance contribution of via transitions under the shield. Signal and power traces entering the shielded volume transition through vias at the can boundary. Each via introduces 0.5–1.2 nH of parasitic inductance depending on PCB thickness and pad geometry. At 500 MHz, 1 nH represents 3.1 Ω of impedance—sufficient to create a voltage divider that allows noise to escape the shielded region along the trace. Mitigation: Use two ground-stitching vias flanking every signal via at the shield boundary, spaced within 0.5 mm. For power traces, use multiple parallel vias to reduce the net inductance below 0.3 nH.

5. Using the shield can fence as a structural solder joint without mechanical reinforcement. Two-piece shield cans (fence soldered to PCB, lid clipped onto fence) rely on the fence solder joint for both electrical grounding and mechanical retention during vibration. Under the random vibration profiles of ISO 16750-3 (e.g., 10–2000 Hz at 30 m/s² RMS for body-mounted electronics), fatigue cracking at the solder fillet initiates after as few as 500 thermal cycles if the can mass exceeds 3 g without additional anchoring. Mitigation: Specify POCONS SMD pan nuts at two or more corners of the shield fence footprint. The threaded fastener provides a secondary mechanical load path, reducing stress on the solder joint by over 60%. For cans larger than 25 mm × 25 mm, a minimum of four corner anchors is recommended.

PCB Footprint & Soldering Profile Guidelines

Pad Geometry

The shield can fence footprint is a continuous perimeter pad on the top copper layer, connected to the ground plane on all layers through stitching vias. Key dimensions:

• Pad width: 1.0 mm minimum, 1.5 mm recommended for cans exceeding 20 mm in any dimension
• Courtyard clearance: 0.25 mm from outer pad edge to nearest non-ground copper feature
• Via stitching pitch: 2.0 mm maximum center-to-center along the pad perimeter; reduce to 1.0 mm for designs requiring ≥50 dB SE above 3 GHz
• Via diameter: 0.3 mm finished hole, 0.6 mm pad, connecting all ground layers
• Solder paste aperture ratio: 80% of pad area for 0.20 mm wall thickness fence; increase to 90% for 0.15 mm wall. Use a 1:1 aspect ratio aperture with 0.12 mm stencil thickness. Excessive paste causes solder bridging between the fence and adjacent components; insufficient paste produces voids that increase contact resistance above 10 mΩ.

For POCONS spring contacts (used on the lid-to-fence interface), the PCB must include a dedicated contact pad at each spring location. Each pad should be 1.2 mm diameter minimum, with a direct via to the ground plane within 0.5 mm of the pad center. Surface finish must be ENIG (electroless nickel immersion gold) or hard gold (0.75 μm minimum) to ensure stable contact resistance over the product lifecycle. HASL (hot-air solder leveling) is acceptable but introduces surface planarity variation of ±15 μm that can increase initial contact resistance by 1–2 mΩ.

Reflow Soldering Profile

The following profile applies to Pb-free (SAC305) assembly of POCONS shield can fences onto FR-4 or high-Tg laminate substrates:

• Preheat ramp rate: 1.0–2.0 °C/s from 25 °C to 150 °C
• Soak zone: 150–200 °C for 60–90 seconds. The shield can's thermal mass (typically 1.5–4.0 g for automotive module-scale cans) requires the upper end of this soak to achieve thermal equilibrium across the full perimeter.
• Ramp to peak: 2.0–3.0 °C/s from soak to peak
• Peak reflow temperature: 245–250 °C (measured at the solder joint, not the can surface). The can surface will lag the PCB pad temperature by 5–10 °C; profile accordingly.
• Time above liquidus (TAL): 40–70 seconds. Below 40 s, wetting on the nickel barrier layer is incomplete. Above 70 s, intermetallic growth at the Sn-Ni interface increases brittleness.
• Cooling rate: 2.0–4.0 °C/s from peak to 200 °C. Avoid exceeding 6 °C/s to prevent thermal shock cracking of solder joints on large-footprint cans.

These parameters align with IPC/JEDEC J-STD-020 and IPC J-STD-001 Class 3 requirements for automotive electronics workmanship. Post-reflow inspection should verify 100% fillet formation around the can perimeter using automated optical inspection (AOI) configured for shield can geometries. X-ray inspection is recommended for voiding assessment on first-article builds, with a void area target of ≤25% per pad segment per IPC-7095D criteria.

Rework Considerations

Shield can rework per IPC-7711/7721 requires localized preheating of the PCB to 125–150 °C before applying focused hot air to the can perimeter at 350–380 °C nozzle temperature. The can should lift cleanly once all perimeter joints reach liquidus. Residual solder on pads must be dressed with solder wick before reattachment. Spring contacts on the lid interface are not affected by fence rework and do not require requalification unless the lid is also removed.

Recommended POCONS Components

Custom Two-Piece Shield Cans

POCONS two-piece shield cans are the primary containment solution for automotive DC-DC converter stages. The fence is reflow-soldered to the PCB, providing a permanent, low-impedance ground connection. The removable lid clips onto the fence via integrated spring tabs, enabling debug access and post-reflow component rework without desoldering the fence. Available in C5210 phosphor bronze (higher spring retention force) or C7701 nickel silver (superior corrosion resistance), with tin or tin-lead plating over nickel barrier. Custom geometries are tooled to match any rectangular, L-shaped, or irregular PCB keep-out zone.

Why this product solves the CISPR 25 problem: The two-piece design allows engineers to characterize conducted emissions with and without the lid installed during pre-compliance bench testing—isolating the shielding contribution from the filter contribution. This is critical for root-cause analysis when margin is insufficient. The soldered fence eliminates the intermittent contact issues that plague clip-on single-piece designs under vibration.

View Custom Two-Piece Shield Cans →

Spring Contacts / Pogo Pins

POCONS spring contacts provide the electrical interface between the shield can lid and the PCB ground plane or the lid and the fence rail. With initial contact resistance ≤2 mΩ and cycle life exceeding 10,000 insertions, they maintain shielding effectiveness across the full product lifecycle. Available in through-hole and SMD configurations with travel ranges from 0.3 mm to 2.0 mm, accommodating PCB warpage and assembly tolerance stack-up.

Why this product solves the CISPR 25 problem: The dominant leakage path in a two-piece shield can is the lid-to-fence or lid-to-PCB interface. Each spring contact replaces a mechanical clip point with a controlled-force, controlled-impedance ground connection. Distributing spring contacts at ≤10 mm spacing around the lid perimeter closes the aperture leakage paths that otherwise limit shielding effectiveness above 1 GHz.

View Spring Contacts & Pogo Pins →

SMD Pan Nuts

POCONS SMD pan nuts provide threaded mechanical anchoring for shield can fences on PCBs subject to automotive vibration and thermal cycling profiles. Reflow-soldered to dedicated pads at the can perimeter corners, they accept M1.6 or M2 machine screws that clamp the fence to the PCB with controlled torque. This secondary load path prevents fatigue cracking of perimeter solder joints under ISO 16750-3 vibration profiles, extending the mechanical life of the shield can assembly beyond 2,000 thermal cycles (−40 °C to +125 °C).

Why this product solves the CISPR 25 problem: A cracked solder joint on the shield can fence is electrically equivalent to a slot aperture. A single 3 mm crack at a corner raises contact impedance to hundreds of milliohms and degrades shielding effectiveness by 15–20 dB in the 1–3 GHz range. SMD pan nuts eliminate this failure mode entirely by providing a compression-based ground path that is independent of solder joint integrity.

View SMD Pan Nuts →

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