What ground pad width is required under a shield can wall to avoid impedance discontinuities above 1 GHz?

A continuous ground pad of at least 1.5 mm width with vias on ≤2 mm pitch to the reference plane keeps inductance below ~0.3 nH per segment, preserving attenuation above 1 GHz. Narrower pads (<1.0 mm) shift the first cavity resonance down by 8–12% and degrade shielding effectiveness by 6–10 dB at the λ/2 mode.

How does shield can seam leakage compare between one-piece and two-piece (frame plus lid) constructions?

One-piece cans typically deliver ≥70 dB attenuation up to 3 GHz but require full rework on service access. Two-piece cans with a soldered frame and removable lid using beryllium copper spring fingers retain 55–65 dB to 6 GHz while enabling field rework. Seam leakage is dominated by finger pitch: 2 mm pitch yields ≥60 dB at 3 GHz; 4 mm pitch drops to ~45 dB.

For automotive infotainment modules, which POCONS part family supports CISPR 25 Class 5 compliance with rework access?

POCONS Custom Two-Piece Shield Cans with integrated BeCu spring fingers, paired with SMD Pan Nuts for mechanical tie-down and Spring Contacts for board-to-can grounding, have been deployed on Class 5 head-unit designs passing 150 kHz–2.5 GHz radiated limits with ≥6 dB margin.

PCB Grounding Topology and Shield Can Integration for CISPR 25 Compliance

Q: How does shield can seam leakage compare between one-piece and two-piece (frame plus lid) constructions?

One-piece cans typically deliver ≥70 dB attenuation up to 3 GHz but require full rework on service access. Two-piece cans with a soldered frame and removable lid using beryllium copper spring fingers retain 55–65 dB to 6 GHz while enabling field rework. Seam leakage is dominated by finger pitch: 2 mm pitch yields ≥60 dB at 3 GHz; 4 mm pitch drops to ~45 dB.

Q: For automotive infotainment modules, which POCONS part family supports CISPR 25 Class 5 compliance with rework access?

POCONS Custom Two-Piece Shield Cans with integrated BeCu spring fingers, paired with SMD Pan Nuts for mechanical tie-down and Spring Contacts for board-to-can grounding, have been deployed on Class 5 head-unit designs passing 150 kHz–2.5 GHz radiated limits with ≥6 dB margin.

Executive Summary

Radiated emissions failures at the 150 kHz–1 GHz CISPR 25 bands and the 1–6 GHz ISO 11452-2 immunity bands are almost never a shield-material problem — they are a PCB grounding problem that the shield can inherits. A 60 dB shield with a 15 nH ground return path behaves like a 30 dB shield above 500 MHz. This application note addresses the specific failure mode of ground loop impedance under shield can walls, quantifies the consequences in terms of cavity resonance and seam leakage, and specifies the PCB footprint, via stitching, and SMD hardware selection needed to realize datasheet shielding performance. POCONS Custom Two-Piece Shield Cans, SMD Pan Nuts, and BeCu Spring Contacts are the component set specified throughout, targeting CISPR 25 Class 4/5 and MIL-STD-461 RE102 compliance for automotive, industrial CNC, and avionics hardware.

Technical Specifications & Attenuation Data

The following parameters apply to POCONS standard tin-plated cold-rolled steel (CRS) and nickel-silver shield cans populated over a four-layer FR-4 stack-up with a dedicated ground reference on layer 2.

| Parameter | Specification | Standard | |-----------|--------------|----------| | Shielding effectiveness, CRS 0.20 mm | ≥65 dB, 30 MHz–1 GHz; ≥55 dB, 1–6 GHz | IEEE 299 / MIL-STD-285 | | Shielding effectiveness, nickel-silver 0.18 mm | ≥70 dB, 30 MHz–1 GHz; ≥60 dB, 1–6 GHz | IEEE 299 | | Sheet resistance, tin-plated CRS | ≤5 mΩ/sq | ASTM B193 | | Contact resistance, BeCu spring contact | 20–50 mΩ initial; ≤80 mΩ after 10,000 cycles | MIL-STD-1344 2.6 | | Seam leakage, 2 mm finger pitch | ≤−60 dB at 3 GHz | IEC 61000-4-21 reverberation | | Permeability, nickel-silver | μr ≈ 1 (non-magnetic) | Vendor datasheet | | Permeability, CRS (for LF magnetic) | μr ≈ 200 at 1 kHz | ASTM A773 | | Operating temperature range | −40 °C to +125 °C | AEC-Q200 | | Reflow compatibility | Pb-free, 260 °C peak, 3× cycles | J-STD-020 |

Compliance target bands that drive these numbers: CISPR 25 Class 5 radiated limits (150 kHz–2.5 GHz), ISO 11452-2 immunity (200 MHz–18 GHz), IEC 61000-4-3 (80 MHz–6 GHz), and MIL-STD-461 RE102 (10 kHz–18 GHz). For aerospace programs, MIL-STD-461 CE102 and CS114 drive the common-mode choke and shield-to-chassis bond resistance, which should be ≤2.5 mΩ DC.

Common Design Pitfalls

Insufficient ground pad copper under shield walls. Root cause: designers route ground as a narrow trace (≤0.5 mm) or segment it with silkscreen keepouts. Consequence: the return path inductance under the can wall becomes comparable to the shield impedance at the resonant frequency, and the enclosure radiates through the gap. Rule: continuous copper pour ≥1.5 mm wide directly under every wall, with ground vias at ≤2 mm pitch tying to the reference plane. Above 3 GHz, drop to 1 mm pitch.
Cavity resonance from internal dimension neglect. Root cause: shield can cavity forms a rectangular resonator with first mode at f = c/2L where L is the longest internal dimension. A 40 × 25 × 4 mm cavity resonates near 3.75 GHz. Consequence: 10–20 dB shielding degradation at the resonant peak and its harmonics. Mitigation: partition large cans with internal walls, or apply conductive foam absorber (≥10 dB/cm insertion loss) on the lid interior for cavities whose longest dimension exceeds λ/4 at the highest operating frequency.
Single-point grounding of a removable lid. Root cause: designers assume one or two tabs are adequate for the lid return. Consequence: lid behaves as a slot antenna at odd multiples of λ/4. Rule: contact spacing ≤λ/20 at the highest frequency of interest — at 6 GHz, that is 2.5 mm maximum between spring finger contact points. POCONS standard BeCu finger strips at 2.0 mm pitch satisfy this through 6 GHz with margin.
Sharing shield ground with high-di/dt switching returns. Root cause: SMPS return currents routed through the same copper pour the shield wall solders to. Consequence: shield wall becomes an antenna driven by switching harmonics. Rule: partition the ground pour under the shield from SMPS return via a moat of ≥0.5 mm, bridged only at a single controlled-impedance tie point at the connector reference.
Paste aperture over-reduction causing cold joints at shield solder tabs. Root cause: designers apply generic 1:1 paste ratio to large shield solder lands. Consequence: insufficient solder volume yields intermittent 200–500 mΩ tab joints that rectify EMI currents and re-emit harmonics. Rule: paste aperture ratio 85–90% on shield tabs ≥2 mm², stencil thickness 0.127 mm (5 mil); verify with X-ray on first-article for voiding ≤25%.

PCB Footprint & Soldering Profile Guidelines

Footprint geometry for POCONS two-piece cans follows these defaults unless the mechanical drawing specifies otherwise. Solder tab pad length equals the tab dimension plus 0.4 mm toe extension and 0.2 mm heel; pad width equals tab width plus 0.25 mm per side. Courtyard clearance is 0.5 mm beyond the outer wall projection to accommodate reflow self-alignment and avoid neighbor-component collision. SMD pan nut footprints require a central clearance hole aligned to the nut bore within ±0.1 mm and a ground pad annulus ≥0.75 mm around the nut perimeter for torque reaction.

Stencil design uses 0.127 mm (5 mil) laser-cut stainless with electropolish. For shield tabs and pan nut lands, paste aperture ratios of 0.85–0.90 produce reliable fillets without bridging. The area ratio must exceed 0.66 per IPC-7525B to guarantee release.

Reflow profile for Pb-free SAC305 with POCONS tin-plated components:

Preheat ramp: 1.0–2.0 °C/s from 25 °C to 150 °C
Soak zone: 150–190 °C, 60–120 s
Reflow ramp: 1.0–3.0 °C/s through liquidus (217 °C)
Peak temperature: 240–250 °C at the shield can body (measure with thermocouple on the wall, not adjacent PCB)
Time above liquidus (TAL): 45–90 s
Cooling ramp: ≤4 °C/s

Large shield cans act as thermal mass; increase soak dwell by 15–20 s versus a bare SMT assembly to avoid cold joints on inner tabs. Reference J-STD-001 Class 3 for aerospace and automotive acceptance, and IPC-7711/7721 for any field rework of two-piece lid removal and replacement.

Recommended POCONS Components

Custom Two-Piece Shield Cans — Soldered frame with removable BeCu spring-finger lid. Frame material CRS or nickel-silver, 0.18–0.25 mm wall. Delivers ≥60 dB attenuation through 6 GHz with rework access. Specify cavity partitions when the longest internal dimension exceeds 25 mm at operating frequencies above 3 GHz. /products/shield-cans/

SMD Pan Nuts — Reflow-capable threaded standoffs for mechanical tie-down of lids, antennas, or chassis bonds. M2 and M2.5 threads standard, tin-plated CRS body, 6 kgf pull strength. Used to enforce lid contact pressure independent of solder fillet integrity, critical for vibration environments (DO-160, MIL-STD-810). /products/smd-pan-nuts/

Spring Contacts / Pogo Pins — BeCu gold-plated spring-loaded contacts for board-to-can, board-to-chassis, and board-to-board grounding. 20–50 mΩ contact resistance, 100,000-cycle life at 2 N working force. Deployed between shield can and EMI gasket to chassis to enforce ≤2.5 mΩ bond resistance required by MIL-STD-461 CS114. /products/spring-contacts/

Specifying these three component families together — soldered frame for baseline attenuation, pan nuts for mechanical reliability, spring contacts for chassis bonding — closes the three failure modes that dominate EMI compliance retest cycles: seam leakage, contact resistance drift, and ground loop impedance.

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