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

Executive Summary

Automotive switch-mode DC-DC converters are the dominant source of conducted emissions failures against CISPR 25 Class 5 limits. The failure mode is predictable: fast switching edges (di/dt > 1 A/ns, dv/dt > 10 V/ns) on the input power bus inject broadband noise from 150 kHz through 108 MHz, with harmonic content extending well into the 30 MHz–1 GHz range where radiated coupling from PCB traces becomes the primary propagation path. Board-level LC filtering alone rarely closes the last 10–15 dB of margin above 30 MHz without an unacceptable increase in filter component count and cost. A properly designed shield can over the converter stage provides 40–60 dB of additional attenuation in this range, converting a marginal design into a compliant one. POCONS USA's two-piece SMD shield cans and precision spring contacts are engineered specifically for this application, delivering repeatable shielding effectiveness with solderless rework access and contact resistance below 3 mΩ per spring finger.

Technical Specifications & Attenuation Data

Shielding effectiveness (SE) of a stamped metal shield can is governed by three factors: the bulk conductivity of the wall material, the electrical integrity of the perimeter ground contact, and the aperture leakage through any slots, seams, or ventilation openings. For CISPR 25 conducted emissions compliance on automotive power converters, the relevant frequency range spans 150 kHz to 108 MHz (broadcast bands AM/FM), with extended requirements to 2.5 GHz for modules near antenna feeds.

POCONS USA shield cans are manufactured from tin-plated cold-rolled steel (CRS) and nickel-silver alloys. CRS provides superior magnetic-field attenuation below 30 MHz due to its ferromagnetic permeability (μr ≈ 200–300), while nickel-silver offers better corrosion resistance in high-humidity environments and excellent solderability.

| Parameter | CRS Tin-Plated | Nickel-Silver | Test Standard | |---|---|---|---| | Wall thickness | 0.20 mm / 0.30 mm | 0.20 mm | — | | Sheet resistance | 0.72 mΩ/sq (0.20 mm) | 1.85 mΩ/sq (0.20 mm) | ASTM B63 | | Relative permeability (μr) | 200–300 | 1.0 | — | | SE, 1–30 MHz (H-field) | ≥45 dB | ≥20 dB | IEEE 299 (adapted) | | SE, 30–200 MHz (plane wave) | ≥55 dB | ≥50 dB | IEEE 299 | | SE, 200 MHz–1 GHz | ≥60 dB | ≥58 dB | IEEE 299 | | SE, 1–6 GHz | ≥55 dB | ≥55 dB | IEEE 299 | | Contact resistance per pad | ≤5 mΩ (soldered) | ≤5 mΩ (soldered) | MIL-STD-1344 | | Spring contact resistance | ≤3 mΩ per finger | ≤3 mΩ per finger | EIA-364-06 | | Operating temperature | −40 °C to +125 °C | −40 °C to +125 °C | AEC-Q200 | | RoHS / REACH | Compliant | Compliant | 2011/65/EU |

The SE values above assume a continuous ground perimeter with pad pitch ≤5.0 mm. Every discontinuity in the ground contact degrades SE by approximately 6 dB per slot length equal to λ/10 at the frequency of interest. At 1 GHz (λ = 300 mm), a 30 mm gap in the perimeter ground acts as an efficient slot antenna and can reduce SE to below 20 dB locally.

For conducted emissions specifically, the shield can acts as a secondary containment. The primary emissions path is through the power bus wiring harness, and the shield prevents re-radiation from internal PCB traces from coupling into that harness or adjacent victim circuits. The relevant metric is insertion loss measured per CISPR 25 Annex A methods: the reduction in voltage-method emissions at the LISN port when the shield is installed versus removed. POCONS two-piece shields consistently deliver 15–25 dB of insertion loss improvement across the 30–108 MHz FM band, which is typically the hardest region to pass due to the overlap with converter switching harmonics in the 100–500 kHz fundamental frequency range (60th–200th harmonics fall directly in-band).

Common Design Pitfalls

1. Insufficient ground pad copper area creating inductive return paths. The most frequent root cause of underperforming shield cans is narrow ground pads (< 0.8 mm width) connected to internal ground planes through a single via per pad. Each via adds approximately 0.5–1.0 nH of inductance. At 500 MHz, 1 nH presents 3.14 Ω of impedance, which is orders of magnitude above the milliohm-level contact resistance of the shield wall itself. The consequence is that return currents flow through the PCB ground plane rather than through the shield perimeter, creating a common-mode loop area that radiates. Mitigation: use ≥1.0 mm pad width, place a minimum of two vias per pad stitched to all internal ground layers, and maintain an unbroken ground copper pour extending at least 0.5 mm beyond the courtyard on all layers.

2. Excessive aperture size on the shield fence or lid seam. Two-piece shield cans inherently have a seam between the fence (soldered frame) and the lid (clip-on cover). If the lid contact points are spaced more than λ/20 at the highest frequency of concern, the seam radiates. For a 2.5 GHz requirement, λ/20 = 6 mm, meaning lid retention clips or spring fingers must be spaced no wider than 6 mm apart. POCONS two-piece designs use continuous spring-finger perimeters with an effective contact pitch of 2.5 mm, maintaining SE integrity to 6 GHz and beyond.

3. Internal cavity resonance excited by switching harmonics. A shield can with internal dimensions of L × W × H will exhibit cavity resonance at f_res = (c/2) × √((m/L)² + (n/W)² + (p/H)²), where c is the speed of light and m, n, p are mode indices. A 30 × 20 × 5 mm cavity has a TE₁₀₁ resonance at approximately 9.0 GHz, which is above most CISPR 25 requirements. However, a 50 × 40 × 8 mm cavity resonates at approximately 4.2 GHz (TE₁₀₁), within the extended test range. At resonance, the shield can amplifies internal fields rather than attenuating them. Mitigation: if the longest internal dimension exceeds 35 mm and the test requirement extends above 3 GHz, partition the cavity with an internal divider wall. POCONS offers shield cans with integrated partition slots for exactly this purpose.

4. Neglecting thermal management inside the shielded volume. Enclosing a DC-DC converter dissipating 0.5–2.0 W in a sealed metal can raises the internal ambient temperature by 15–30 °C depending on the airflow conditions and can wall emissivity. This elevated temperature increases MOSFET Rds(on), shifts LC filter component values (especially ceramic capacitor Class II dielectrics, which lose 30–40% of capacitance at +125 °C), and degrades the conducted emissions margin. Mitigation: specify ventilation apertures on the lid face (perpendicular to the dominant radiation polarization), sized below λ/20 at the highest test frequency. A 3 mm diameter hole array with 4 mm pitch provides adequate airflow while maintaining ≥50 dB SE up to 6 GHz.

5. Solder joint voiding under shield fence pads due to outgassing. Large, continuous ground pads trap volatiles during reflow, producing voids that exceed the IPC-A-610 Class 3 limit of 25% void area. Voided joints exhibit higher contact resistance that worsens with thermal cycling, creating an intermittent emissions failure that passes in the lab but fails in production. Mitigation: segment continuous ground pads into discrete pads with 0.5 mm relief gaps, and use a crosshatch solder paste pattern with 50% area coverage to provide outgassing channels.

PCB Footprint & Soldering Profile Guidelines

Pad Geometry

The shield fence footprint consists of a perimeter ring of discrete pads on the top copper layer. POCONS provides component-specific land pattern recommendations with each shield can, but the following general rules apply to all two-piece SMD shields:

• Pad width: 1.0 mm minimum, 1.5 mm recommended for shields wider than 30 mm per side
• Pad length (along wall): 2.0 mm minimum per discrete pad
• Pad pitch: ≤5.0 mm center-to-center; 3.0 mm preferred for applications above 3 GHz
• Courtyard clearance: 0.3 mm from pad edge to nearest signal trace on the same layer; 0.15 mm to adjacent ground copper fill
• Via stitching: minimum two vias per pad, 0.3 mm finished hole diameter, connecting to all ground layers
• Solder paste aperture: 70–80% of pad area, using a 0.127 mm (5 mil) stencil thickness; for 0.150 mm stencils, reduce aperture to 60–70% to prevent bridging
• Solder mask: maintain solder mask web of ≥0.1 mm between pads; do not use solder mask-defined pads on the shield perimeter

Corner pads require special attention. The shield fence corners experience the highest mechanical stress during thermal cycling and board flex. Extend corner pads by 0.5 mm in each direction beyond the nominal fence footprint, and add a third via at each corner for mechanical anchoring.

Reflow Soldering Profile

POCONS shield cans are compatible with standard SAC305 lead-free reflow profiles per IPC/JEDEC J-STD-020. The following profile parameters are recommended:

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

Shield cans are classified as large thermal-mass components. Place them after fine-pitch ICs in the reflow profile optimization sequence. If the shield can shares a reflow zone with moisture-sensitive components rated MSL 3 or higher, ensure the peak temperature does not exceed the MSL derate curve per J-STD-020. POCONS CRS and nickel-silver shields are rated for three reflow cycles at 260 °C peak without degradation of the tin plating or spring temper.

For rework, the two-piece design eliminates the need for hot-air or IR rework of the shield to access components underneath. Simply unclip the lid, rework the component, and replace the lid. The fence remains soldered in place. This reduces rework cycle time from 15–20 minutes (single-piece shield removal and re-soldering) to under 2 minutes.

Recommended POCONS Components

Custom Two-Piece Shield Cans

The POCONS two-piece shield can system consists of a solderable perimeter fence and a snap-on lid with integrated spring fingers. This architecture provides full shielding effectiveness with tool-free lid removal for debug, rework, and in-circuit test access. Available in CRS tin-plated and nickel-silver, with standard heights from 2.0 mm to 8.0 mm and custom footprints from 10 × 10 mm to 60 × 50 mm. Internal partition walls are available for cavity segmentation in larger shields. Ideal for enclosing automotive DC-DC converters, clock generators, and RF front-end modules where CISPR 25 or ISO 11452-2 compliance is required.

Product line: Custom Two-Piece Shield Cans

SMD Pan Nuts

POCONS SMD pan nuts provide a threaded, surface-mount fastening point for shield can lids in applications requiring positive mechanical retention beyond spring-clip force. The pan nut solders to the PCB ground pad and provides a low-impedance ground connection (< 2 mΩ) while serving as a mechanical anchor. Tin-plated brass construction maintains solderability and corrosion resistance across the automotive temperature range. Available in M1.6 and M2.0 thread sizes.

Product line: SMD Pan Nuts

Spring Contacts / Pogo Pins

For board-to-board shielding interconnects and test fixtures, POCONS precision spring contacts deliver consistent contact force (30–100 gf) and contact resistance (≤3 mΩ initial, ≤10 mΩ after 100,000 cycles) across the full automotive temperature range. Gold-plated beryllium copper construction ensures long-term reliability in high-vibration environments per SAE J1211. Use these as grounding contacts between shield cans and chassis ground planes, or as RF test probe interfaces in production ICT fixtures.

Product line: Spring Contacts & Pogo Pins

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