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

Executive Summary

Automotive DC-DC converters — particularly buck regulators operating between 100 kHz and 2.2 MHz — generate conducted emissions that routinely fail CISPR 25 Class 5 limits without dedicated PCB-level containment. The primary failure mode is common-mode current coupling from the switching node through parasitic capacitance to the input harness, producing broadband noise from 150 kHz to 108 MHz that exceeds the voltage-method limit line by 10–25 dB. Board-level shield cans, when designed with low-impedance ground contact and proper internal cavity management, suppress radiated coupling to adjacent traces and reduce the conducted emissions burden on input-line filters by 15–30 dB across the CISPR 25 frequency range. POCONS USA's two-piece shield cans and precision spring contacts provide the mechanical and electrical interface required to achieve this suppression in volume automotive production.

Technical Specifications & Attenuation Data

CISPR 25 (Edition 5, 2021) defines conducted emissions limits across four frequency bands using either the voltage method (artificial network) or the current probe method. Class 5 — the most restrictive classification and the de facto requirement for Tier 1 automotive OEMs — imposes limits that leave minimal margin for switching converters without shielding.

The voltage method, per CISPR 25 Clause 6.4, measures RF voltage across a 50 Ω artificial network (AN) inserted in each supply line. For a Class 5 device, the quasi-peak limits are approximately 18 dBμV at 530 kHz, dropping to ~6 dBμV at 1.8 MHz, then holding near 12–18 dBμV through the FM broadcast band (76–108 MHz). A typical automotive buck converter based on commodity controller ICs generates raw conducted emissions of 50–80 dBμV at the switching fundamental, with harmonic content extending to 200 MHz and beyond.

The engineering delta between raw emissions and the Class 5 limit line — often 40–60 dB — must be closed by a combination of input filtering, layout optimization, and shielding. Shield cans address the portion of conducted emissions caused by near-field coupling: the electromagnetic field from the switching loop radiates into adjacent PCB traces and the wire harness, bypassing the input filter entirely. Without a shield can, even an aggressively designed multi-stage LC filter cannot achieve Class 5 compliance because the filter's insertion loss is circumvented by radiated coupling.

The following table summarizes the shielding effectiveness requirements and achievable performance for POCONS two-piece shield cans with spring contact interfaces:

| Parameter | Specification | Reference Standard | |-----------|--------------|-------------------| | Shielding effectiveness, 150 kHz–30 MHz | ≥25 dB (magnetic near-field dominant) | IEEE 299.1 (small enclosures) | | Shielding effectiveness, 30 MHz–200 MHz | ≥45 dB | IEEE 299.1 | | Shielding effectiveness, 200 MHz–1 GHz | ≥60 dB | IEEE 299.1 | | Shielding effectiveness, 1 GHz–6 GHz | ≥55 dB | IEEE 299.1 | | Shield can material | Tin-plated cold-rolled steel (CRS), 0.2 mm | — | | Sheet resistance | ≤0.7 mΩ/sq (tin-plated CRS, 0.2 mm) | — | | Relative permeability (μr), CRS | 100–200 (below magnetic saturation) | — | | Spring contact resistance | ≤2 mΩ per contact point | EIA-364-06 | | Spring contact cycle life | ≥500 mating cycles at rated deflection | EIA-364-09 | | Total shield ground impedance (20 contacts) | ≤100 μΩ at DC, ≤5 mΩ at 1 GHz | — | | Operating temperature range | −40 °C to +125 °C | AEC-Q200 environment | | Corrosion resistance, tin plate | ≥96 hours salt spray, no red rust | ASTM B117 |

The material choice matters. Tin-plated CRS provides both magnetic shielding (μr > 100) below ~10 MHz and conductive shielding (σ = 5.8 × 10⁶ S/m for steel substrate) at higher frequencies. At 1 MHz, the skin depth in CRS is approximately 0.2 mm — meaning a 0.2 mm wall provides roughly one skin depth of attenuation. At 100 MHz, the skin depth drops to ~20 μm, and the 0.2 mm wall provides approximately ten skin depths — yielding absorption loss alone exceeding 80 dB. The practical limit on shielding effectiveness is not the wall material but the contact impedance at the shield-to-ground interface.

For applications requiring enhanced low-frequency magnetic shielding — such as enclosing inductors with significant fringe fields below 1 MHz — mu-metal or nickel-iron alloy shield cans offer μr > 20,000 in the initial permeability region. POCONS manufactures these on a custom basis for automotive and medical applications where low-frequency magnetic containment is critical.

Common Design Pitfalls

1. Insufficient ground pad copper area beneath spring contacts. The PCB ground pad for each spring contact must provide a low-inductance path to the ground plane. A pad connected by a single via to an inner ground layer introduces approximately 0.5–1.0 nH of parasitic inductance per via, which at 500 MHz represents 1.5–3 Ω of impedance — effectively an open circuit for RF shielding current. Mitigation: provide a minimum 1.0 mm × 1.5 mm pad per contact with three or more vias of 0.3 mm diameter stitched directly to the ground plane. Maintain unbroken copper pour beneath the entire shield can footprint on the outermost ground layer.

2. Shield can cavity resonance at the half-wavelength of the longest internal dimension. A 30 mm × 20 mm × 5 mm shield can has a fundamental cavity resonance at approximately c / (2 × 0.030) = 5 GHz for the TE₁₀₀ mode. If the enclosed circuit generates significant energy near this frequency — common with high-speed clock harmonics or GHz-band switching transients — the shield can amplifies rather than attenuates the emission. Mitigation: for enclosures with internal dimensions exceeding 15 mm, add one or more internal partition walls (fence structures) to reduce the longest uninterrupted dimension below λ/2 at the highest frequency of concern. POCONS two-piece shield cans support integral partition walls at no additional tooling cost when specified during design.

3. Apertures and slots in the shield wall exceeding λ/20 at the highest frequency of concern. Any opening in the shield — whether an intentional ventilation slot, a gap at the lid-to-frame interface, or clearance for a component lead — radiates as a slot antenna. A 3 mm continuous gap along one edge of the shield becomes an efficient radiator above 5 GHz (where λ/20 = 3 mm). In conducted emissions testing this re-radiated energy couples to harness conductors inside the CISPR 25 test setup. Mitigation: maintain spring contact spacing ≤3 mm (λ/20 at 5 GHz) around the full perimeter. For lid-to-frame interfaces, use POCONS spring contacts with ≤2.5 mm pitch to ensure gap lengths remain below λ/20 through 6 GHz.

4. Routing high-di/dt traces beneath or through the shield can boundary. When a switching node trace or high-current gate drive trace crosses the shield can perimeter, it creates a common-mode antenna: the trace acts as a driven element and the shield boundary as a ground plane edge. The conducted emissions contribution from this single trace can exceed the entire Class 5 budget. Mitigation: all connections to shielded circuits must enter through filtered feedthroughs, or the trace must be routed on an inner layer with continuous ground reference above and below. No switching-node copper should exist within 2 mm of the shield can perimeter on any layer.

5. Neglecting thermal management inside the shielded enclosure. A sealed shield can with 0.2 mm CRS walls has minimal thermal conductivity to ambient air. A 2 W power dissipation inside a 30 mm × 20 mm × 5 mm can raises the internal air temperature by approximately 30–50 °C above the board temperature without thermal vias or heatsinking. Elevated junction temperatures shift switching frequency, increase leakage currents, and degrade filter component values — all of which alter the emissions profile from the characterized state. Mitigation: place an array of thermal vias (0.3 mm diameter, 1.0 mm pitch) beneath dissipating components, connecting to an exposed copper pad on the bottom layer. Specify POCONS shield cans with a thermal pad feature — a bare copper or silver-plated contact area on the shield lid that interfaces with the component's thermal pad through a compressible thermal interface material.

PCB Footprint & Soldering Profile Guidelines

Pad Geometry

The shield can frame footprint consists of a continuous perimeter pad with discrete spring contact landing pads. POCONS provides component-specific footprint recommendations, but the following general guidelines apply to standard two-piece shield cans:

• Perimeter pad width: 1.5 mm minimum, centered on the shield frame wall
• Pad-to-component courtyard clearance: 0.5 mm from the outer edge of the perimeter pad to the nearest non-shielded component courtyard
• Via stitching: 0.3 mm finished hole diameter, 0.6 mm pad diameter, maximum 2.0 mm pitch along the entire perimeter, connected to the primary ground plane
• Solder paste aperture ratio: 70–80% of pad area for frame soldering; reduce to 60% for spring contact pads to prevent solder wicking up the contact
• Stencil thickness: 0.12 mm (5 mil) for standard pads; consider 0.10 mm (4 mil) locally for fine-pitch spring contact arrays to control solder volume

The shield frame is soldered to the perimeter pad during the primary reflow cycle. The removable lid attaches via spring contacts that engage with dedicated landing pads inside the frame perimeter.

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

| Phase | Parameter | Value | |-------|-----------|-------| | Preheat ramp | Rate | 1.0–2.5 °C/s | | Soak zone | Temperature | 150–200 °C | | Soak zone | Duration | 60–120 s | | Reflow peak | Temperature | 245 ± 5 °C | | Time above liquidus (TAL) | Duration | 40–70 s (217 °C liquidus) | | Cooling | Rate | ≤4 °C/s (−6 °C/s max) |

The tin-plated CRS shield frame is compatible with standard lead-free reflow profiles per IPC J-STD-001 Class 2 or Class 3 workmanship requirements. The frame's thermal mass is significant — a 30 mm × 20 mm × 5 mm CRS frame weighs approximately 3–5 g — and may require extended soak times or modified zone settings to achieve uniform wetting. Verify solder joint quality per IPC-A-610 Class 2 minimum: continuous wetting along ≥75% of the frame-to-pad interface.

For rework, the shield can frame is removable with hot-air rework stations at 350–380 °C nozzle temperature per IPC-7711/7721 guidelines. Spring contacts on the lid require no soldering and can be removed and replaced without board-level rework.

Recommended POCONS Components

Custom Two-Piece Shield Cans

The POCONS custom two-piece shield can system is purpose-built for automotive and industrial applications requiring CISPR 25 or CISPR 32 compliance. The solderable frame provides permanent, low-impedance grounding to the PCB, while the removable lid enables post-reflow access for programming, debugging, and inspection — critical during the automotive PPAP process.

• Series: POCONS TS-CRS and TS-MUM (mu-metal) series
• Material options: Tin-plated CRS (0.15–0.30 mm), nickel-silver, mu-metal
• Custom dimensions: Any rectangular or L-shaped footprint from 5 mm × 5 mm to 80 mm × 80 mm, heights from 2.0 mm to 12.0 mm
• Integral partitions available for cavity resonance management
• Explore options: /products/shield-cans/

Precision Spring Contacts / Pogo Pins

POCONS spring contacts deliver the electrical interface between the shield lid and the soldered frame. Each contact provides ≤2 mΩ resistance with a rated deflection of 0.3–0.8 mm, accommodating PCB warpage and manufacturing tolerances inherent in automotive assemblies.

• Series: POCONS SC-A (standard automotive) and SC-H (high-frequency, gold-plated)
• Contact pitch: 1.5 mm, 2.0 mm, and 2.5 mm standard; custom pitch available
• Spring force: 0.3–1.0 N per contact (application-dependent)
• Cycle life: ≥500 cycles (SC-A), ≥5,000 cycles (SC-H)
• Explore options: /products/spring-contacts/

SMD Pan Nuts

For applications requiring mechanical fastening of the shield lid in high-vibration environments (ISO 16750-3), POCONS SMD pan nuts provide a soldered threaded insert on the PCB that accepts a machine screw through the shield lid. This eliminates reliance on spring contact retention force alone in environments exceeding 30 g random vibration profiles.

• Series: POCONS PN-M2 and PN-M2.5
• Thread sizes: M1.6, M2.0, M2.5
• Material: Brass, nickel-plated; stainless steel available
• Soldering: Compatible with standard lead-free reflow profiles
• Explore options: /products/smd-pan-nuts/

Design Review Service

POCONS USA offers complimentary shield can design reviews for automotive OEMs and Tier 1 suppliers working toward CISPR 25 compliance. Submit your PCB layout (Gerber, ODB++, or Altium/KiCad native format) and emissions test data to applications@poconsusa.com. Our RF engineering team will recommend shield can dimensions, material selection, spring contact placement, and partition wall locations optimized for your specific switching converter topology and emissions profile.

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