MIL-STD-461 CE101/CS116 Compliance: Shield Can Design for Conducted Emissions Control

Executive Summary

Qualifying commercial-grade (COTS) hardware against MIL-STD-461F/G and RTCA DO-160 Section 22 exposes a specific PCB-level failure mode: conducted and radiated emissions escaping through seams, apertures, and unshielded high-impedance nets at frequencies where chassis-level gasketing is ineffective. CE101 (30 Hz–10 kHz conducted emissions on power leads), CS116 (damped sinusoidal transients 10 kHz–100 MHz), and DO-160 Sec. 22 pin/cable injection all require that the board-level enclosure behave as a Faraday cavity with deterministic ground-return impedance below 10 mΩ across the test band. POCONS two-piece shield cans, SMD pan nuts, and beryllium-copper spring contacts provide the mechanical and electrical interface engineers need to close the margin between COTS silicon and military/avionics limit lines without a full redesign.

Technical Specifications & Attenuation Data

Board-level shielding effectiveness (SE) is governed by the aperture model — the lowest leakage path dominates total SE regardless of wall material. For MIL-STD-461G RE102 and RS103 (200 V/m from 2 MHz to 18 GHz), POCONS standard shield cans target SE ≥ 80 dB below 1 GHz and ≥ 60 dB through 6 GHz when mounted with ≤5 mm ground-pad pitch. The tin-plated cold-rolled steel (SPCC) base material provides the permeability (µr ≈ 200 at 1 kHz) required to attenuate the low-frequency magnetic component of CS114 bulk cable injection, while the tin plating maintains contact resistance below 10 mΩ over 500 thermal cycles per MIL-STD-202 Method 107.

| Parameter | Specification | Standard | |-----------|---------------|----------| | Shielding effectiveness, 30 MHz–1 GHz | ≥ 80 dB | IEEE-299 (enclosure), MIL-STD-461G RE102 | | Shielding effectiveness, 1 GHz–6 GHz | ≥ 60 dB | MIL-STD-461G RS103, DO-160 Sec. 20 | | Base material sheet resistance | ≤ 4 mΩ/sq | ASTM B193 | | Plating: matte tin over nickel strike | 2.5–8 µm Sn / 1.3 µm Ni min | ASTM B545, J-STD-001 | | Ground-pad contact resistance | ≤ 10 mΩ initial, ≤ 20 mΩ post-aging | MIL-STD-202 Method 307 | | Spring contact (pogo pin) resistance | 20–50 mΩ at 100 mA | MIL-STD-1344 Method 3004 | | Spring contact current rating | 2–5 A continuous (series-dependent) | UL 60950-1 | | Operating temperature | −55 °C to +125 °C | MIL-STD-810 Method 501/502 | | Aperture cutoff (λ/2 rule, longest slot ≤ 3 mm) | Effective to 50 GHz | Waveguide-below-cutoff theory | | Seam impedance (lid-to-frame) | ≤ 5 mΩ DC, ≤ 50 mΩ at 1 GHz | MIL-STD-461G bonding |

For CE101 compliance on 28 VDC aircraft power buses, the conducted emissions limit is 110 dBµA at 30 Hz dropping to 80 dBµA at 10 kHz. Achieving this margin requires that the shield can form a continuous return path for switching-converter ground currents; a discontinuous ground ring introduces a loop inductance of ~1 nH per millimeter of gap, which at 10 kHz still produces negligible impedance but at CS116's 100 MHz upper limit presents 0.6 Ω per millimeter — enough to couple transient energy directly into sensitive analog nets.

Common Design Pitfalls

1. Insufficient ground-pad copper area under the shield can wall. Root cause: layout engineers place minimum-width (0.3 mm) ground traces under the can perimeter. Observable consequence: at frequencies above 500 MHz, the inductive return path creates a λ/4 resonant stub, producing a 15–20 dB SE notch. Mitigation: pour a continuous ground ring ≥ 1.5 mm wide under the full can footprint, stitched to inner ground planes with vias on ≤ 2.5 mm pitch.

2. Aperture length exceeding λ/20 at the highest test frequency. Root cause: ventilation slots or component-access cutouts sized for thermal requirements without EMC review. Consequence: at 6 GHz (λ = 50 mm), any aperture longer than 2.5 mm radiates as a slot antenna. Mitigation: subdivide large openings into multiple round or square apertures ≤ 2 mm, maintaining open area for airflow while preserving cutoff behavior.

3. Lid-to-frame contact discontinuity on two-piece cans. Root cause: mechanical tolerance stack-up between stamped lid dimples and frame rails. Consequence: intermittent 30–50 dB SE degradation under thermal cycling per DO-160 Sec. 5. Mitigation: specify POCONS spring-finger frame geometry with ≥ 4 contact points per linear centimeter and 0.15–0.25 mm interference fit.

4. Cavity resonance at λ/2 of the longest internal dimension. Root cause: empty cavity with no absorber or dissipative loading. Consequence: internal field enhancement of 10–15 dB at the cavity's first TE mode, which re-radiates through every aperture simultaneously. Mitigation: line the lid interior with a thin (≤ 0.5 mm) carbon-loaded absorber, or populate the cavity such that no clear dimension exceeds λ/4 at the highest emission frequency of concern.

5. Unfiltered I/O traces crossing the shield boundary. Root cause: signal traces routed directly from inside the can to external connectors without feedthrough capacitance. Consequence: CS116 injected transients couple onto internal nets with no attenuation, bypassing the shield entirely. Mitigation: place 100 pF–10 nF feedthrough or three-terminal capacitors within 2 mm of the can wall, with their ground terminal soldered to the can's ground ring directly.

PCB Footprint & Soldering Profile Guidelines

Shield-can footprints on 1.6 mm FR-4 require a continuous solder pad 1.2–1.5 mm wide matching the can's external wall thickness, with a 0.5 mm courtyard clearance to the nearest component body. Paste aperture ratio should be 90% of the pad area for stencil thicknesses of 0.127 mm (5 mil), reducing to 80% for 0.100 mm stencils to prevent solder bridging at wall-to-wall intersections. For stepped stencils near fine-pitch BGAs adjacent to the can, maintain the can aperture at 0.127 mm thickness to guarantee sufficient solder volume for a continuous ground seal.

Reflow profile per J-STD-001 and IPC-7711/7721, validated for POCONS tin-plated cans: preheat ramp 1.5–2.5 °C/s to 150 °C, soak 150–180 °C for 60–90 s, ramp to peak at 2–3 °C/s, peak reflow 235–245 °C for SAC305, time above liquidus (TAL) 45–70 s, cooling rate ≤ 4 °C/s. Exceeding TAL of 90 s degrades the tin plating and increases post-reflow contact resistance by 3–5× at the lid interface. For two-piece cans, the frame is reflowed with the board; the lid is installed after functional test and requires no secondary reflow — this is the primary reason avionics programs specify two-piece geometry over one-piece cans, as it preserves rework access for DO-160 requalification after ECOs.

Recommended POCONS Components

Custom Two-Piece Shield Cans — Frame-and-lid geometry with spring-finger contact retention. Part number family PCN-2P-[L]x[W]x[H]-[plating]. Solves the field-serviceability requirement for DO-160 and MIL-STD-461 qualified assemblies: the frame reflows with the board; the removable lid permits rework, firmware update access, and post-deployment ECO implementation without desoldering. Specify for any program requiring requalification cycles. See /products/shield-cans/

SMD Pan Nuts — Reflow-compatible threaded fasteners for securing absorber pads, grounding straps, or secondary shield layers to the primary can. Part number family PCN-PN-M[size]-[plating]. Provides mechanical attachment points for cavity absorber installation addressing Pitfall #4 above, and enables strap-bonding the can to chassis for CS116/CS117 ground-return control. See /products/smd-pan-nuts/

Beryllium-Copper Spring Contacts / Pogo Pins — Board-to-lid and board-to-chassis interconnects with 20–50 mΩ contact resistance and 2–5 A current rating. Part number family PCN-SC-[series]. Essential for continuous ground return between stacked shielded assemblies and for providing the low-impedance bypass path that suppresses CS116 damped sinusoid coupling into sensitive victim circuits. See /products/spring-contacts/

---

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