Reflow-Compatible Two-Piece Shield Cans: Thermal Profile & Pad Design for EMI Compliance

Executive Summary

Board-level EMI shield cans fail most often not at the RF bench but on the reflow line. A frame that warps during peak reflow lifts one corner 80–150 µm off its pad, opening a slot antenna that radiates the exact harmonics the shield was specified to contain. This note addresses reflow-induced coplanarity loss and slot leakage in two-piece SMT shield cans used for automotive infotainment and ADAS radar modules subject to CISPR 25 Class 5 radiated emissions limits and ISO 11452-2 immunity requirements from 200 MHz to 6 GHz. POCONS USA two-piece shield frames with matched snap-on lids, SMD pan nuts for grounded lid attachment, and BeCu spring contacts for internal compartmentalization are engineered to survive lead-free reflow while holding ≥60 dB shielding effectiveness through L-band and S-band.

Technical Specifications & Attenuation Data

Shielding effectiveness of a two-piece can is governed by the frame-to-PCB seam, the frame-to-lid seam, and the aperture count. Leakage dominates above the frequency where the longest aperture approaches λ/20. For a 12 mm unfenced gap, useful attenuation collapses near 1.25 GHz, which is why fence-pad continuity and lid spring-finger pitch are the two specifications that matter most on the datasheet.

| Parameter | Specification | Standard | |-----------|--------------|----------| | Shielding effectiveness, far field | ≥60 dB from 200 MHz to 3 GHz; ≥45 dB from 3 to 6 GHz | IEEE 299 / MIL-STD-285 (scaled) | | Frame material | Nickel silver C7521 (0.20–0.30 mm) or SPTE tin-plated steel | ASTM B122 / JIS G3303 | | Frame sheet resistance | ≤3 mΩ/sq (nickel silver); ≤5 mΩ/sq (SPTE) | Four-point probe, 23 °C | | Frame-to-PCB coplanarity | ±0.10 mm over footprints ≤25 mm; ±0.15 mm over 25–50 mm | IPC-A-610 Class 3 | | Lid spring-finger pitch | ≤3 mm (target λ/20 at 5 GHz) | Internal DFM | | Lid-to-frame contact force | 0.8–1.5 N per finger after 10 mate cycles | POCONS TP-SC-04 | | Pogo/spring contact resistance | ≤50 mΩ initial, ≤80 mΩ after 10,000 cycles | EIA-364-23 | | Peak reflow temperature | 245–260 °C, TAL 60–90 s | J-STD-020 / IPC/JEDEC | | Radiated emissions target | ≥10 dB margin to CISPR 25 Class 5 | CISPR 25:2021 | | Radiated immunity target | 100 V/m, 200 MHz–2.7 GHz, no performance degradation | ISO 11452-2 | | Conducted immunity (BCI) | 100 mA, 1 MHz–400 MHz | ISO 11452-4 | | MIL-grade option | ≥80 dB H-field below 1 GHz via mu-metal liner | MIL-STD-461G RE102 |

The 60 dB floor is the working number for automotive and industrial radio compliance. Missing it typically traces to four root causes, not a material deficiency.

Common Design Pitfalls

1. Discontinuous ground fence under the frame wall. A tented via-in-pad array with 0.8 mm pitch creates an effective slot every 0.8 mm. Root cause: layout treats the fence as a ground net, not a current return boundary. Consequence: leakage at harmonics of 10 GHz and degraded return loss for internal traces crossing the wall. Mitigation: solid copper pour under the full frame footprint, 0.5 mm wider than the wall on both sides, with stitching vias every 1.5 mm (≤λ/20 at 10 GHz) tied to the reference plane.

2. Reflow warp on long frames. Frames with any dimension >30 mm exhibit thermal bow of 100–200 µm when the peak exceeds 250 °C. Root cause: CTE mismatch between a 0.25 mm stamped frame and the FR-4 it is paste-bonded to. Consequence: corner solder joints lift during cooldown, producing a radiating slot. Mitigation: stiffen the frame with internal cross-walls at ≥20 mm pitch, or specify nickel silver over SPTE for lower modulus-weighted bow.

3. Lid spring fingers designed for aesthetics. Many reference designs use 4–6 mm finger pitch. Root cause: finger pitch chosen from mechanical CAD without an RF crosscheck. Consequence: above 3 GHz, SE drops 10–15 dB versus a 2–3 mm pitch lid. Mitigation: specify finger pitch ≤3 mm for any product targeting 5 GHz Wi-Fi, UWB, or 5 GHz radar harmonics, and require a minimum 0.8 N contact force per finger after 10 mate cycles.

4. Internal cavity resonance left untreated. An empty 30 × 20 × 4 mm cavity resonates at TE101 near 8.5 GHz. Root cause: the shield is treated as a pure boundary, not as a resonator. Consequence: a 15–25 dB peak in radiated emissions coincident with a cavity mode, often mistaken for a clock harmonic. Mitigation: subdivide the cavity with a partition wall that pushes the lowest mode above the highest harmonic of concern, or apply a thin (0.5 mm) magnetic absorber sheet to the lid interior.

5. SMD standoff nuts placed outside the seam current path. Root cause: mechanical grounding screws are treated as chassis-bond features only. Consequence: the lid-to-frame seam carries the induced shield current with no low-impedance return to PCB ground near the corners, where current density is highest. Mitigation: place a grounded SMD pan nut within 5 mm of each corner, pad-connected to the ground pour with a minimum of three 0.3 mm stitching vias.

PCB Footprint & Soldering Profile Guidelines

Fence pad geometry is the single most sensitive footprint feature. For a 0.25 mm wall thickness, specify a continuous pad 0.65 mm wide, centered on the nominal wall location with ±0.05 mm placement tolerance. Courtyard clearance to adjacent components is 0.50 mm minimum to allow for inspection and rework nozzle access. Soldermask is defined 0.05 mm off the pad edge on the inside and outside of the wall. For frame perimeters over 20 mm on a side, segment the paste aperture into 1.5–2.0 mm dashes with 0.3 mm gaps — this gives flux volatiles a vent path and prevents solder beading that propagates under the wall and short-circuits fine-pitch components inside the cavity. Stencil is laser-cut 0.12 mm thick electropolished stainless, aperture-to-pad area ratio at 1:1, with an aspect ratio above 0.66 to meet IPC-7525B paste release.

The reflow profile is the second-order determinant of yield. A J-STD-020-compliant ramp-to-peak profile is specified as follows: preheat ramp 1.5–2.5 °C/s from 25 to 150 °C, soak zone 150–190 °C for 60–90 s to activate flux and equalize thermal mass across the frame, ramp to peak at 1.0–2.5 °C/s, peak 245–250 °C for SPTE steel frames or 250–260 °C for nickel silver, TAL (above 217 °C liquidus for SAC305) 60–90 s, and forced-convection cooling at 2–4 °C/s to prevent brittle intermetallic growth. For frames with dimensions above 30 mm, reduce peak to 245 °C and TAL to 60–70 s to hold coplanarity inside the ±0.10 mm envelope required for Class 3 joints under IPC-A-610. Profile every new board at five thermocouple locations: two frame corners, the frame centroid, the heaviest adjacent component, and an open-copper reference — corner-to-centroid ΔT should not exceed 8 °C at peak.

Reworkable two-piece cans benefit from a lower-profile second reflow when the lid is installed after board test. Specify a localized hot-air reflow at 230 °C spot temperature for 30 s, or use SMD pan nuts with screw-on lids to skip the second thermal excursion entirely.

Recommended POCONS Components

Custom Two-Piece Shield Cans — Nickel silver or tin-plated steel frames in 0.20–0.30 mm wall thickness, paired with snap-fit or screw-down lids. Frame-and-lid system delivers ≥60 dB SE to 3 GHz and survives 260 °C peak reflow without coplanarity loss on footprints up to 50 mm. Specify when the module requires post-assembly RF tuning, firmware-gated rework access, or multiple lid variants over a shared frame. /products/shield-cans/two-piece/

SMD Pan Nuts — Tin-plated brass or stainless standoff nuts, reflow-compatible to 260 °C, provide grounded screw-attach points for removable lids and chassis bonding. Part-number series PCN-M2 and PCN-M2.5, pick-and-place compatible, packaged on tape-and-reel. Place within 5 mm of frame corners for low-impedance seam current return. /products/smd-hardware/pan-nuts/

Spring Contacts / Pogo Pins — Gold-plated BeCu spring probes, contact resistance ≤50 mΩ initial and ≤80 mΩ after 10,000 cycles per EIA-364-23, working stroke 0.5–1.5 mm. Use as internal cavity partitions to break resonant modes above 6 GHz, or as lid-to-frame contact elements where a continuous spring-finger edge is not mechanically feasible. Series PSC-050 through PSC-250 cover 0.5 to 2.5 mm pitch. /products/spring-contacts/pogo-pins/

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