CISPR 25 Conducted Emissions: Shield Can Design for Automotive DC-DC Converters

Executive Summary

Non-isolated step-down converters based on the LM2596 and its widespread clones are a default choice for automotive 12 V to 5 V/3.3 V rails, but their 150 kHz–500 kHz switching fundamentals and sub-nanosecond MOSFET edges produce broadband conducted emissions that routinely fail CISPR 25 Class 3–5 between 150 kHz and 108 MHz. The dominant failure mode is common-mode current injected into the vehicle harness through the converter's switching-node parasitic capacitance to chassis, radiated locally by the board and re-coupled into the LISN. This application note specifies how POCONS USA two-piece shield cans, SMD pan nuts, and precision spring contacts form a board-level Faraday enclosure that, in combination with a properly dimensioned CM/DM input filter, achieves compliant margin against CISPR 25, ISO 11452-4 BCI, and the conducted-emissions clauses of IEC 61000-6-3 for derivative consumer hardware.

Technical Specifications & Attenuation Data

Shielding effectiveness (SE) at the board level is governed by seam impedance, aperture length relative to wavelength, and the quality of the ground return at every bond point. A continuous can with zero apertures approaches the bulk SE of the wall material; in practice, lid seams, vent slots, and cable penetrations dominate the response above 500 MHz. The table below captures the envelope POCONS specifies for tin-plated cold-rolled steel (CRS) two-piece cans with spring-contact lid closure, measured per a modified IEEE 299-2006 methodology on a 50 mm × 50 mm cavity.

| Parameter | Specification | Standard | |-----------|--------------|----------| | SE, 30 MHz – 200 MHz | ≥40 dB | IEEE 299 (adapted) | | SE, 200 MHz – 1 GHz | ≥60 dB | IEEE 299 (adapted) | | SE, 1 GHz – 6 GHz | ≥55 dB | IEEE 299 (adapted) | | Wall material sheet resistance | ≤5 mΩ/sq (tin-plated CRS, 0.2 mm) | ASTM B193 | | Relative permeability (CRS) | µr ≈ 200 @ 1 kHz | — | | Spring contact resistance | 15–30 mΩ, ΔR ≤10 mΩ @ 100k cycles | MIL-STD-1344 Method 3004 | | Pan nut pull-off force | ≥25 N after reflow | IPC-9708 | | CISPR 25 Class 5 peak limit (LW, 150–300 kHz) | 70 dBµV | CISPR 25 Ed. 5 | | CISPR 25 Class 5 peak limit (FM, 76–108 MHz) | 30 dBµV | CISPR 25 Ed. 5 |

Material selection is not neutral. Tin-plated CRS gives meaningful magnetic absorption below 10 MHz where thin nonferrous cans degrade; nickel-silver or brass are preferred above 1 GHz where skin depth in steel (≈2 µm at 1 GHz) is adequate but solderability and seam reproducibility matter more. The 30 dB LISN insertion factor in the CISPR 25 voltage method must be accounted for when back-calculating required SE from spectrum analyzer readings; Biricha's and Baltic Lab's bench setups both demonstrate this offset directly.

Common Design Pitfalls

1. Undersized ground return under the shield frame. A shield can perimeter that lands on a hatched or thermally relieved ground pour creates an inductive return path; emissions at 100–300 MHz rise 10–15 dB as the cavity loses its low-impedance reference. Mitigation: solid ground pad under the entire frame footprint, stitching vias on ≤1.5 mm pitch connecting to the nearest internal ground plane, ≥0.3 mm annular ring around each via.

2. Cavity resonance at λ/2 of the longest internal dimension. A 40 mm × 25 mm × 5 mm cavity resonates near 3.75 GHz (TE101). If a switching harmonic, clock, or LDO noise source excites this mode, internal field strength peaks and leaks through any aperture >λ/20. Mitigation: partition large cans into sub-cavities with internal walls, or apply lossy absorber (µ″ > 1 from 1–6 GHz) to the lid interior.

3. Aperture length exceeding λ/20. Vent slots and lid gaps longer than 5 mm begin to leak noticeably at 3 GHz. The commonly repeated "λ/50" rule is overkill for 40 dB targets but correct for 60 dB. Mitigation: multiple small round vents (≤2 mm diameter) totaling the required open area instead of a single slot.

4. Single-point lid grounding. A lid grounded only at corners behaves as a patch antenna driven by the cavity. Mitigation: POCONS spring contacts distributed at ≤λ/20 of the highest suppressed frequency — 5 mm pitch for 3 GHz coverage, 2.5 mm for 6 GHz. Minimum three contacts per edge regardless of length.

5. Filter placed inside the shielded volume with unfiltered penetrations. If the CM choke and X/Y capacitors sit inside the can but the 12 V input trace exits through an unfiltered aperture, the trace re-radiates filtered noise back onto the harness. Mitigation: feed-through the shield wall with a Y-cap or bulkhead ferrite at the penetration, and route filtered conductors outside the can only after the filter's output node.

PCB Footprint & Soldering Profile Guidelines

The frame footprint for a POCONS two-piece can uses a continuous solder pad 0.8 mm wide matching the frame wall thickness plus 0.2 mm outboard tolerance. Courtyard clearance is 1.0 mm to nearest component body per IPC-7351B Level B. Stencil aperture is a 1:1 match to pad in the long dimension, with 85% area ratio on the short dimension to prevent solder bridging into the cavity; stencil thickness 0.12 mm (4.7 mil) laser-cut stainless with electropolish. Pan nut pads are 1.6 mm diameter with a 0.75 mm non-plated through-hole; paste aperture is an annular ring leaving the hole clear.

Reflow profile for SAC305 per J-STD-020 and IPC J-STD-001: preheat ramp 1.5–2.5 °C/s to 150 °C; soak 60–90 s between 150–190 °C; ramp to peak at ≤3 °C/s; peak 245 ± 5 °C; time above liquidus (TAL) 60–90 s; cool-down ≤4 °C/s. Larger thermal mass cans (>2 g) benefit from the upper end of TAL to ensure full frame-to-pad wetting without starving smaller adjacent 0402 parts. Post-reflow AOI should verify ≥75% wetted perimeter on the frame and pan-nut pull-off per IPC-9708 sampling.

For rework, remove the lid (snap-fit or pan-nut-secured) with a nylon spudger; never pry against the frame. IPC-7711/7721 procedures 5.2 and 5.3 apply for frame removal using convection rework stations at 240 °C nozzle, 45 s dwell. Replace the lid against fresh spring contacts — compressed contacts from prior assembly lose 3–5 mΩ of contact resistance guarantee.

Recommended POCONS Components

Two-Piece Shield Cans (Custom) — Tin-plated CRS or nickel-silver frames with removable lids, sized to application with internal partition walls when cavity resonance analysis demands sub-cavities. The removable lid enables rework and test-point access without reflow, and the spring-contact closure maintains ≥60 dB SE across thousands of lid cycles. Specify cavity dimensions, lid retention style (snap, pan nut, or hybrid), and any vent requirements. See /products/shield-cans/.

SMD Pan Nuts — Reflow-compatible threaded standoffs for mechanically captive lid retention where automotive shock and vibration per ISO 16750-3 would compromise snap-fit lids. Pan nuts withstand ≥25 N pull-off after SAC305 reflow and accept M1.6 or M2 screws for service-accessible lids. See /products/pan-nuts/.

Spring Contacts / Pogo Pins — Beryllium-copper spring contacts, gold-over-nickel plated, deliver 15–30 mΩ across 100,000 actuation cycles. Use as perimeter lid grounding at ≤5 mm pitch to maintain aperture discipline above 1 GHz, or as board-to-board shield interconnect for stacked modules. Specify working height, force range (typically 0.3–1.5 N per contact), and plating per the target corrosion environment. See /products/spring-contacts/.

A representative automotive DC-DC shielded module uses a custom two-piece can with internal partition, four M1.6 pan nuts for lid retention, and 24 perimeter spring contacts at 5 mm pitch — paired with a 10 µH CM choke and 4.7 nF Y-caps at the input penetration, this configuration consistently delivers 8–12 dB CISPR 25 Class 5 margin from 150 kHz to 1 GHz in bench measurements mirroring the Baltic Lab voltage-method setup.

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