PCB Shield Can Design for Power Supply EMI Filtering Compliance

Executive Summary

Switched-mode power supplies generate broadband conducted and radiated emissions spanning 150 kHz to beyond 6 GHz — frequencies where ferrite beads lose impedance, common-mode chokes saturate, and PCB trace routing becomes an unintentional antenna. When filter-only approaches fail to close the last 10–20 dB margin gap at radiated emission boundaries defined by CISPR 25 Class 5, CISPR 32 Class B, or MIL-STD-461 RE102, board-level shield cans provide deterministic attenuation without redesigning the filter topology. This application note addresses the engineering integration of POCONS two-piece shield cans, spring contacts, and SMD retention hardware with power supply filter stages to achieve repeatable compliance margins across production volumes.

Technical Specifications & Attenuation Data

The shielding effectiveness (SE) of a board-level enclosure is governed by three loss mechanisms: reflection loss at the impedance boundary, absorption loss through the wall material, and aperture leakage through seams, ventilation slots, and contact gaps. For tin-plated cold-rolled steel (CRS) at 0.20 mm wall thickness — the standard POCONS shield can material — the dominant limitation above 1 GHz shifts from material absorption to aperture and contact impedance.

POCONS two-piece shield cans are manufactured from C2600 phosphor bronze and tin-plated CRS with the following measured characteristics:

| Parameter | Specification | Test Standard | |-----------|--------------|---------------| | Shielding effectiveness (200 MHz – 1 GHz) | ≥60 dB | IEEE 299 (modified for PCB-mount) | | Shielding effectiveness (1 GHz – 6 GHz) | ≥40 dB | IEEE 299 | | Wall material thickness | 0.15 – 0.30 mm | — | | Wall conductivity (tin-plated CRS) | 8.7 × 10⁶ S/m | ASTM B193 | | Relative permeability (CRS, DC) | μᵣ ≈ 300 | — | | Spring contact resistance (initial) | ≤2 mΩ | EIA-364-06 | | Spring contact resistance (after 10K cycles) | ≤5 mΩ | EIA-364-09 | | Solder joint shear strength (SMD pan nut) | ≥30 N | IPC-9701A | | Operating temperature range | −40 °C to +125 °C | — | | RoHS / REACH | Compliant | 2011/65/EU |

Skin Depth and Absorption Loss

At 100 MHz, the skin depth in tin-plated CRS is approximately 2.7 μm. A 0.20 mm wall provides roughly 74 skin depths at this frequency, yielding absorption loss exceeding 130 dB — far beyond what is measurable. The practical SE limitation is therefore entirely determined by aperture control and contact quality, not wall thickness.

Contact Impedance Budget

The total shield effectiveness can be modeled as:

SE_total = SE_material − 20·log₁₀(λ / 2πd) − L_contact

Where d is the maximum aperture dimension, λ is wavelength, and L_contact represents the contact impedance loss term. For a shield can with 40 mm maximum internal dimension, the first cavity resonance occurs at approximately 3.75 GHz (λ/2 = 40 mm). Below this frequency, maintaining L_contact below 3 dB requires total perimeter contact resistance below 10 mΩ — achievable only with dedicated spring contacts or continuous solder fillets.

Common Design Pitfalls

1. Insufficient ground pad copper area creates inductive return path. When the shield can fence footprint sits on a narrow ground trace rather than a continuous ground pour, the inductance of the return path increases by 1–5 nH per mm of trace length. At 2 GHz, even 2 nH represents 25 Ω of impedance — effectively an open circuit for shield currents. The observable consequence is a 15–25 dB degradation in SE above 1 GHz, often manifesting as a sharp resonant peak in radiated emission scans. Mitigation: Provide a minimum 0.5 mm wide continuous ground copper ring on the shield can mounting layer, stitched to the ground plane with vias at ≤λ/20 spacing (≤2.5 mm pitch for 6 GHz compliance). All ground pad segments must connect to the same ground plane copper island with no splits or routing channels crossing underneath.

2. Aperture slots from lid-to-fence gap exceed λ/2 at test frequency. Two-piece shield cans inherently have a perimeter gap between the fence (soldered base) and the removable lid. If this gap exceeds 0.5 mm uniformly, or if localized deformation creates a 2 mm gap at any point, the slot antenna effect produces radiated leakage that can exceed the CISPR limit by 6–12 dB at frequencies where the slot length approaches λ/2. Mitigation: Specify lid retention features with ≤0.3 mm nominal gap. POCONS shield can clips (PTC series) provide controlled mechanical preload that maintains gap consistency across thermal cycling. For frequencies above 3 GHz, add internal spring contact fingers at ≤3 mm spacing along the longest lid edge.

3. Power supply switching node routed outside the shielded volume. The highest dV/dt node in a buck converter — the switch node connecting the high-side FET drain to the inductor — is the dominant source of radiated emissions above 100 MHz. If this trace exits the shield can boundary to reach an external inductor, the shield provides zero attenuation for the primary emission source. Mitigation: Enclose the complete power stage — input capacitors, FETs, gate driver, inductor, and output capacitors — within a single shield can volume. If the inductor height exceeds the shield can clearance, use a taller custom can rather than routing the switch node externally. POCONS manufactures custom shield cans from 2.0 mm to 15.0 mm internal height to accommodate power inductor profiles.

4. Shared via fence between analog and digital ground partitions. When a shield can fence via row is shared between a digital ground plane and an analog ground plane that are intentionally split, the fence creates an unintended low-impedance bridge between the two domains. This couples digital switching noise into the analog ground reference, defeating the partition strategy. The observable consequence is increased noise floor on ADC channels co-located with the shielded supply. Mitigation: Route the shield can fence exclusively on one ground domain. If the shield must span both domains, use a dedicated shield ground pour that connects to the system ground at a single defined point, maintaining the intentional split topology.

5. Reflow-induced solder wicking starves the fence joint. During reflow, solder paste on the shield fence pads can wick up the fence wall by capillary action, leaving a dry joint at the pad-to-PCB interface. This increases contact resistance from the design target of ≤5 mΩ to 50–500 mΩ, catastrophically degrading SE above 500 MHz. Mitigation: Use a paste aperture ratio of 60–70% on fence pads (reduced from the typical 80–90% for standard SMD components). Specify Type 4 solder paste (20–38 μm particle size) to control paste volume. POCONS SMD pan nuts (PPN series) provide a mechanically independent ground contact that is immune to solder wicking failure modes, serving as a reliable backup ground path.

PCB Footprint & Soldering Profile Guidelines

Fence Pad Geometry

The shield can fence requires a continuous perimeter pad on the PCB surface layer. POCONS provides component-specific footprint libraries, but the general design rules are:

• Pad width: 1.0 mm minimum, centered on the fence wall landing (0.5 mm inside, 0.5 mm outside)
• Courtyard clearance: 0.25 mm from pad edge to nearest copper feature on the same layer
• Paste aperture: 60–70% area ratio relative to copper pad, using a home plate or window pattern to control paste volume
• Stencil thickness: 0.12 mm (5 mil) for standard fence pads; 0.10 mm (4 mil) if solder wicking is observed during process validation
• Ground via stitching: 0.3 mm finished hole diameter, 0.6 mm pad diameter, placed at ≤2.5 mm pitch along the fence perimeter, offset 0.3 mm outboard of the fence wall center line
• Solder mask: 0.05 mm mask-defined opening oversize on all fence pads; no solder mask between fence pad and via pads to allow continuous wetting

SMD Pan Nut Footprint (PPN Series)

POCONS SMD pan nuts require a dedicated pad geometry per IPC-7351B land pattern guidelines:

• PPN0121 (M1.2): Pad diameter 2.4 mm, paste aperture 1.8 mm diameter (56% area ratio)
• PPN0168 (M1.6): Pad diameter 3.0 mm, paste aperture 2.2 mm diameter (54% area ratio)
• Thermal relief: None — full copper connection to ground plane required for EMI performance
• Keepout zone: 1.0 mm radial clearance from pad edge for component placement; 0.5 mm for copper features

Reflow Soldering Profile

The following profile is validated for POCONS shield can assemblies using SAC305 solder paste 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 | | Ramp to peak | Rate | 1.0 – 2.5 °C/s | | Peak reflow | Temperature | 245 ± 5 °C | | Time above liquidus (TAL) | Duration | 40 – 70 s | | Cooling | Rate | ≤4.0 °C/s (≤6.0 °C/s max) |

Critical process note: Shield can fence joints have significantly higher thermal mass than discrete SMD components due to the continuous metal perimeter acting as a heat sink. Verify soak zone duration is sufficient for the fence to reach thermal equilibrium before the reflow ramp. Insufficient soak time is the primary root cause of cold fence joints in production. Use thermocouple profiling per IPC-7530 with at least one thermocouple directly on the fence pad at the point farthest from the board edge.

Post-Reflow Inspection

Per IPC-A-610 Class 2 (Class 3 for automotive and medical):

• Fence joints: Minimum 75% pad wetting on visual inspection; no visible voids exceeding 25% of joint area on X-ray
• SMD pan nut joints: Full circumferential fillet required; toe fillet height ≥ 0.5 mm
• Spring contact insertion: Verify seated height within ±0.1 mm of specification; contact force ≥ 0.3 N per contact point

Recommended POCONS Components

Custom Two-Piece Shield Cans (PES Series)

The POCONS PES series two-piece shield cans are the primary enclosure solution for power supply filter isolation. The two-piece architecture separates the soldered fence (base) from the removable lid, enabling rework and debug access without desoldering — critical during EMC pre-compliance iteration. Available in tin-plated CRS and mu-metal configurations for applications requiring both electric and magnetic field attenuation below 100 MHz.

• Application: Enclosing complete SMPS stages including input filter, switching regulator, and output filter to contain both conducted emissions coupling to adjacent circuits and direct radiated emissions from switch node and inductor
• Customization: Any rectangular geometry from 5 × 5 mm to 80 × 80 mm footprint, 2.0 – 15.0 mm internal height, with optional internal partitions for multi-stage filter isolation within a single enclosure
• Product page: Two-Piece Shield Cans

Shield Can Clips (PTC Series)

POCONS PTC series clips provide the mechanical retention force that maintains lid-to-fence contact integrity over the product lifetime. The clip geometry generates controlled normal force on the lid perimeter, ensuring the perimeter gap remains below the 0.3 mm threshold required for SE above 3 GHz.

• Application: Lid retention on two-piece cans where thermal cycling (−40 to +125 °C) would otherwise cause lid loosening due to differential CTE between shield material and PCB laminate
• Key specification: ≥1.0 N retention force per clip, maintaining contact through 1,000 thermal shock cycles per JESD22-A104
• Product page: Shield Can Clips

C-Clip Contact Terminals / Spring Contacts (PMT Series)

For applications requiring serviceable ground connections or board-to-board shielding continuity, POCONS PMT series spring contacts deliver ≤2 mΩ contact resistance with a controlled wipe action that breaks through surface oxides on each mating cycle. These contacts are essential where the shield can lid must be repeatedly removed for field service while maintaining EMI compliance after reassembly.

• Application: Ground contact between shield lid and PCB ground pads; board-to-board shield continuity in stacked module assemblies; test point access through shield boundaries
• Key specification: Contact resistance ≤2 mΩ initial, ≤5 mΩ after 10,000 cycles; current rating ≥1 A continuous
• Product page: Spring Contacts

SMD Pan Nuts (PPN Series)

POCONS PPN series surface-mount pan nuts serve dual functions: mechanical fastening for shield can lids in high-vibration environments, and low-impedance ground anchors that supplement the fence solder joint. In automotive applications subject to ISO 16750-3 vibration profiles, SMD pan nuts eliminate the risk of lid detachment that clip-only retention cannot fully address.

• Application: Mechanical shield retention under vibration (automotive, industrial, aerospace); supplementary ground contact to reduce total perimeter resistance; mounting point for external cable shields and grounding straps
• Key specification: PPN0168 (M1.6) solder joint shear strength ≥30 N; thread engagement ≥ 2 threads minimum for screw retention
• Product page: SMD Pan Nuts

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