What shielding effectiveness is required to pass CISPR 25 Class 5 at 30 MHz to 1 GHz?

Class 5 radiated emission limits for broadband signals are 34 dBµV/m at 30–54 MHz (quasi-peak) and drop to 32 dBµV/m above 150 MHz. A shield can providing ≥40 dB of shielding effectiveness over 30 MHz–1 GHz, combined with a low-impedance ground return (<5 mΩ DC, <10 mΩ at 1 GHz), is typically sufficient margin for 6 dB headroom.

How does the λ/2 cavity resonance of a shield can limit usable bandwidth?

A rectangular cavity resonates at the half-wavelength of its longest internal dimension. A 30 mm × 20 mm × 3 mm can shows first TE mode resonance near 7.5 GHz. Above this, shielding effectiveness collapses by 20–30 dB. Mitigation requires internal partitions, RF-absorbing foam on the lid inner surface, or reducing maximum internal dimension below λ/10 of the highest concern frequency.

What is the minimum contact pitch for the shield can ground fence to suppress slot radiation?

Ground contact pitch must not exceed λ/20 at the highest frequency of concern. For 6 GHz suppression, this equals 2.5 mm maximum pitch. POCONS two-piece shield can frames standardize on 2.0 mm pitch solder tabs to maintain ≥50 dB attenuation through 8 GHz.

Crosstalk-Driven EMI Failures: Shield Can Design for Automotive and RF-Dense PCBs

Q: How does the λ/2 cavity resonance of a shield can limit usable bandwidth?

A rectangular cavity resonates at the half-wavelength of its longest internal dimension. A 30 mm × 20 mm × 3 mm can shows first TE mode resonance near 7.5 GHz. Above this, shielding effectiveness collapses by 20–30 dB. Mitigation requires internal partitions, RF-absorbing foam on the lid inner surface, or reducing maximum internal dimension below λ/10 of the highest concern frequency.

Q: What is the minimum contact pitch for the shield can ground fence to suppress slot radiation?

Ground contact pitch must not exceed λ/20 at the highest frequency of concern. For 6 GHz suppression, this equals 2.5 mm maximum pitch. POCONS two-piece shield can frames standardize on 2.0 mm pitch solder tabs to maintain ≥50 dB attenuation through 8 GHz.

Executive Summary

Crosstalk between adjacent traces and radiated emissions from switching regulators, clock drivers, and RF front-ends remain the dominant EMI failure modes on dense mixed-signal PCBs. At frequencies above 300 MHz, near-field coupling from unshielded sections of an otherwise compliant board will reliably fail CISPR 25 Class 5, IEC 61000-4-3 at 10 V/m, and ISO 11452-4 BCI tests. The root cause is almost never the IC itself but the absence of a Faraday enclosure with a low-impedance RF return. This application note specifies shield can geometry, ground fence pitch, spring contact selection, and reflow profile for POCONS two-piece shield cans, SMD pan nuts, and pogo-pin spring contacts engineered to deliver ≥60 dB attenuation from 200 MHz to 6 GHz with demonstrated repeatability across high-volume SMT assembly.

Technical Specifications & Attenuation Data

Shielding effectiveness (SE) of a PCB-level enclosure is governed by three loss mechanisms: absorption loss (A), reflection loss (R), and multiple-reflection correction (B). For thin-wall nickel-silver or tin-plated steel cans in the 0.15–0.20 mm range, reflection loss dominates below 1 GHz while absorption dominates above 3 GHz. The limiting factor in practical designs is never the wall material but the seam impedance between can body and PCB ground plane, which is a direct function of contact pitch and contact normal force.

| Parameter | Specification | Standard | |-----------|--------------|----------| | Shielding effectiveness, 30 MHz–200 MHz | ≥50 dB | MIL-STD-285 / IEEE 299 | | Shielding effectiveness, 200 MHz–6 GHz | ≥60 dB | IEEE 299.1 | | Shielding effectiveness, 6 GHz–18 GHz | ≥45 dB | IEEE 299.1 | | Can wall material | Nickel silver C7521, 0.20 mm | ASTM B122 | | Sheet resistance, post-plate | ≤2 mΩ/sq | ASTM B193 | | Ground fence contact pitch | 2.0 mm nominal | Internal / λ/20 at 6 GHz | | Spring contact resistance | 15–30 mΩ at 100 mA | IEC 60512-2-1 | | Spring contact normal force | 40–80 gf | MIL-STD-1344 | | Insertion cycles (pogo pin) | ≥10,000 | IEC 60512-5-2 | | Radiated emission limit, CISPR 25 Class 5 | 32–34 dBµV/m | CISPR 25:2021 | | Bulk current injection test level | 100 mA, 1 MHz–400 MHz | ISO 11452-4 | | Reflow peak temperature | 245–250 °C | J-STD-020E |

The frequency-dependent SE values above assume a two-piece can with ≥2.0 mm contact pitch, seam contact resistance ≤5 mΩ per contact, and a continuous ground return plane on layer 2 of the PCB stackup with no splits under the can footprint.

Common Design Pitfalls

Insufficient ground via stitching under the frame solder pad. Root cause: the RF return current under the can perimeter travels through the nearest ground via, and sparse stitching (>3 mm pitch) creates a magnetic loop that re-radiates. Consequence: 15–25 dB degradation of SE above 500 MHz and failure of CISPR 25 Class 5 radiated emissions between 400–900 MHz. Mitigation: stitch ground vias at 1.5 mm pitch along the entire frame footprint, drill diameter 0.25 mm minimum, connected directly to layer 2 ground.
Cavity resonance at λ/2 of longest internal dimension. Root cause: a rectangular cavity formed by the can and PCB behaves as a resonator at f = c/(2·L). Consequence: for a 40 mm × 25 mm can, first resonance at 3.75 GHz produces a narrowband SE notch down to 20 dB. Mitigation: insert a soldered or press-fit internal partition to split the cavity, or apply a 1 mm thick RF absorbing foam (e.g., carbon-loaded polyurethane) to the lid inner surface; partition reduces effective L by half and pushes resonance above 7.5 GHz.
Unequal thermal mass between frame and lid causing lid warp. Root cause: two-piece designs with heavy lids and thin frames cool at different rates, producing a residual gap of 0.05–0.15 mm along one edge. Consequence: slot antenna formation, 20 dB SE loss at frequencies where slot length approaches λ/2. Mitigation: specify frame and lid wall thickness within 0.02 mm of each other, enforce flatness ≤0.05 mm per IPC-A-610 Class 3, and validate with peel-tested gold-plated contact fingers on the frame rim.
Spring contact normal force degradation after thermal cycling. Root cause: pogo pin internal spring stress relaxation after 1000 cycles of −40 °C to +125 °C per ISO 16750-4. Consequence: contact resistance rises from 20 mΩ to 200+ mΩ, creating a non-linear RF path and intermodulation. Mitigation: specify spring contacts with beryllium-copper C17200 spring material (not music wire), preload ≥50 gf, and require contact resistance measurement after thermal aging per manufacturer qualification report.
Shared ground between shield can return and digital logic return. Root cause: routing digital return currents through the same plane region used as the shield can RF return creates common-impedance coupling. Consequence: digital switching noise appears as broadband emission radiated by the can itself. Mitigation: define a dedicated shield-return island on layer 2, connected to the main ground plane only at a single point beneath the power entry filter.

PCB Footprint & Soldering Profile Guidelines

The frame solder pad shall be 0.8 mm wide minimum for 0.20 mm wall cans, with a courtyard clearance of 0.5 mm to adjacent component bodies and 0.3 mm to adjacent copper features. Paste aperture shall be 90% of pad area with a 1:1 stencil-to-pad ratio for the initial version; tuning to 85% may be required if solder bridging is observed during first-article inspection. Stencil thickness 0.12 mm (4.7 mil) stainless steel, laser-cut and electropolished, is the POCONS-recommended default. Pad geometry follows IPC-7351 Level B (nominal) for the rectangular perimeter, with internal isolated pads matching any internal partition contact points.

Reflow profile per J-STD-020E for SAC305 paste: preheat ramp 1.5–2.5 °C/s from 25 °C to 150 °C, soak zone 150–180 °C for 60–90 seconds, ramp-to-peak 1.5–3.0 °C/s, peak 245–250 °C (not to exceed 260 °C), time above liquidus 217 °C shall be 45–75 seconds, cooling ramp ≤4 °C/s. For two-piece designs, frame is reflowed during the main SMT pass and lid is installed via snap-fit or secondary selective reflow at reduced peak of 235 °C. IPC-7711/7721 procedures apply to any post-reflow rework of the frame. All soldered frames shall be inspected per IPC-A-610 Class 3 for automotive and medical applications, Class 2 minimum for consumer.

SMD pan nut integration requires a through-hole clearance of 0.1 mm over the nut outer diameter and a solderable annular ring of 0.4 mm minimum to resist mechanical torque without pad lifting. Pan nuts shall be installed before lid placement and torqued to manufacturer specification (typically 0.15–0.25 N·m for M2.5 threads).

Recommended POCONS Components

For designs targeting CISPR 25 Class 5, ISO 11452, or commercial CISPR 32 Class B compliance, POCONS USA supplies three component families engineered as a system.

Custom Two-Piece Shield Cans (POCONS-TPSC series). Nickel-silver C7521 construction with tin-lead-free plating, 0.20 mm wall, 2.0 mm ground fence pitch, available in standard footprints from 10 × 10 mm to 60 × 40 mm and fully custom geometries with 4-week tooling. Delivers the ≥60 dB attenuation from 200 MHz to 6 GHz specified above and solves cavity resonance via optional integrated partitions. See /products/shield-cans/.

SMD Pan Nuts (POCONS-SPN series). Surface-mount captive fasteners rated to 0.25 N·m torque with solderable steel base and M2–M3 thread options. Enable removable lids for rework access while maintaining mechanical pressure on the frame-to-lid RF seam. See /products/smd-pan-nuts/.

Spring Contacts and Pogo Pins (POCONS-SC series). Beryllium-copper C17200 construction, gold-over-nickel plating, 15–30 mΩ contact resistance, 50 gf nominal preload, and 10,000-cycle qualification per IEC 60512-5-2. Deployed as lid-to-frame RF bonding contacts, board-to-board shield return paths, and test-point access for post-assembly compliance verification. See /products/spring-contacts/.

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