What peak reflow temperature is safe for nickel-silver and tin-plated CRS shield frames without warpage?

Peak temperature should be held to 235–245 °C for SAC305 paste with a Time Above Liquidus (TAL) of 45–75 seconds. Exceeding 250 °C peak or 90 s TAL on frames thinner than 0.20 mm produces measurable warpage (>75 µm across a 25 mm span) and degrades coplanarity, opening RF leakage gaps at the frame-to-PCB interface.

How does reflow-induced frame warpage affect EMI attenuation above 1 GHz?

Frame lift of 100 µm at any point along the perimeter creates a slot antenna. At 3 GHz (λ = 100 mm), a 10 mm long, 0.1 mm gap exhibits aperture leakage that drops measured shielding effectiveness from ≥60 dB to roughly 30–35 dB per CISPR 25 radiated emissions testing. Coplanarity must be held under 50 µm for sustained ≥55 dB SE through 6 GHz.

Can two-piece shield cans be reflowed in the same profile as adjacent BGAs and 0201 passives?

Yes, when the frame is specified for full SAC305 compatibility and the soak zone is tuned to 150–180 °C for 60–90 s. POCONS two-piece frames are designed with thermal mass matched to typical mixed-technology boards so that a single profile satisfies BGA wetting, 0201 tombstoning margins, and frame solder joint formation simultaneously.

Reflow Profile Optimization for Two-Piece SMD Shield Cans: Coplanarity, Solder Joint Integrity, and RF Seal Continuity

Executive Summary

Two-piece SMD shield cans deliver their rated shielding effectiveness only when the perimeter solder joint between the frame and the PCB ground ring is metallurgically continuous and the frame remains coplanar within tens of micrometers. The dominant in-line failure mode is reflow profile mismatch: excessive ramp rate, insufficient soak, or peak temperature drift that produces frame warpage, partial wetting, and intermittent ground contact. The resulting micron-scale apertures behave as slot radiators above 1 GHz and collapse compliance margins under CISPR 25, CISPR 32, and ISO 11452-2 radiated emissions and immunity tests. This application note specifies the reflow profile envelope, PCB pad geometry, and inspection criteria required to preserve ≥55 dB shielding effectiveness from 200 MHz to 6 GHz when assembling POCONS Two-Piece Shield Cans, SMD Pan Nuts, and board-mount Spring Contacts.

Technical Specifications & Attenuation Data

The shield can is an electromagnetic enclosure whose performance is bounded by the worst aperture along its perimeter, not by the bulk material. Material conductivity, plating quality, and joint continuity each contribute to the total transfer impedance presented to a circulating surface current. The table below captures the operating envelope for POCONS two-piece SMD frames in tin-plated cold-rolled steel (CRS) and nickel-silver (NS) variants.

| Parameter | Specification | Standard | |-----------|--------------|----------| | Shielding effectiveness (frame + lid, sealed) | ≥60 dB, 200 MHz – 2 GHz; ≥55 dB, 2 – 6 GHz | IEEE 299 / MIL-STD-285 derived | | Frame material (standard) | Nickel-silver C7521, 0.20 mm | ASTM B122 | | Frame material (alternate) | Tin-plated CRS, 0.15–0.25 mm | ASTM A879 | | Sheet resistance (NS, post-reflow) | ≤2.5 mΩ/sq | IPC-TM-650 2.5.14 | | Coplanarity (post-reflow, perimeter) | ≤50 µm across any 25 mm span | JEDEC JESD22-B108 (adapted) | | Solder joint void area (perimeter wetting) | ≤25 % per IPC-A-610 Class 2; ≤15 % Class 3 | IPC-A-610H | | Peak reflow temperature (SAC305) | 235–245 °C | J-STD-020E | | Time Above Liquidus (217 °C) | 45–75 s | J-STD-020E | | Spring contact resistance (pogo, signal) | ≤30 mΩ at 100 mA | ASTM B539 | | Compliance test reference | CISPR 25 Class 5, ISO 11452-2, IEC 61000-4-3 | — |

The 60 dB floor at 2 GHz assumes a continuous perimeter solder joint with no aperture exceeding λ/20 in maximum dimension. For a 6 GHz upper bound this constrains any single break to under 2.5 mm — a constraint that is met by paste deposition, not by mechanical contact alone, which is why two-piece frames are reflowed rather than press-fit.

Common Design Pitfalls

Insufficient ground pad copper area on the frame footprint. A perimeter pad narrower than the frame foot plus 0.20 mm on either side produces inadequate thermal mass for paste melt and starves the joint. The observable consequence is a ring of partial wetting visible under a 5× scope and a 10–15 dB drop in SE between 1 and 3 GHz. Mitigation: specify pad width = (frame foot width) + 0.40 mm, with continuous ground copper underneath and at least four stitching vias per 10 mm of perimeter to the chassis ground plane.
Aggressive ramp-to-peak rate exceeding 3 °C/s. Thin-wall frames (≤0.20 mm) absorb heat asymmetrically across long sides, producing differential expansion and post-reflow bow. The result is a frame that lifts 80–150 µm at the midpoint of its longest side, opening a slot antenna whose resonance falls inside CISPR 25 monitored bands. Mitigation: cap ramp from soak to peak at 1.5–2.5 °C/s and verify with a thermocouple bonded to the frame centerline.
Cavity resonance at λ/2 of the longest internal dimension. A 50 mm × 25 mm × 4 mm cavity resonates near 3.0 GHz (TE101). If the shielded circuit contains a clock or LO harmonic at that frequency, internal Q rises and emissions through any aperture grow by 15–25 dB. Mitigation: add absorber material (carbonyl-iron-loaded foam) on the lid interior, or partition the cavity with an internal frame wall to push the lowest cavity mode above the highest concern frequency.
Stencil aperture ratio below 0.66 on perimeter pads. Long, narrow perimeter pads with 0.12 mm stencils produce paste release problems, leading to localized opens. Mitigation: use 0.13–0.15 mm stencil thickness with home-plate or rounded-rectangle apertures, target area ratio ≥0.70, and inspect post-print with SPI to a height tolerance of ±20 µm.
Mixing lead-free frame assembly with downstream SnPb rework. Reworking a SAC305-attached frame with a SnPb iron at 320 °C locally re-melts the joint, draws flux residue into the seam, and leaves a contaminated, high-impedance interface. Mitigation: rework only with hot-air at controlled 250 °C, replace the entire frame rather than spot-touching, and re-test SE on a coupon.

PCB Footprint & Soldering Profile Guidelines

The frame footprint should provide a continuous ground ring 0.40 mm wider than the frame foot, with solder mask defined openings to control paste registration. Courtyard clearance of 1.0 mm on all exterior sides keeps neighboring 0402 and 0201 passives clear of the lid removal tool path. Stitching vias on the perimeter should be 0.30 mm finished hole diameter, tented on the component side, on a pitch no greater than 2.5 mm. This via cage suppresses the slot-line mode that would otherwise propagate along the gap between the inner and outer ground pours.

Stencil thickness of 0.13 mm with home-plate apertures sized at 90 % of pad length and 100 % of pad width yields an area ratio above 0.70 for typical 0.50 mm wide perimeter pads. Paste type 4 (20–38 µm) is preferred for fine perimeter geometries; type 3 is acceptable on frames with foot widths above 0.60 mm.

The reflow profile envelope for SAC305 with POCONS two-piece frames:

Preheat ramp: 1.0–2.5 °C/s from ambient to 150 °C.
Soak zone: 150–180 °C for 60–90 s, allowing flux activation and thermal equalization between the frame and adjacent BGAs.
Ramp to peak: 1.5–2.5 °C/s from 180 °C to peak.
Peak temperature: 235–245 °C measured at the frame centerline; do not exceed 245 °C on frames thinner than 0.20 mm.
Time Above Liquidus (217 °C): 45–75 s.
Cooling rate: 2–4 °C/s from peak to 180 °C; faster cooling embrittles the joint, slower cooling coarsens the IMC layer beyond 3 µm and reduces mechanical robustness during lid removal.

Profile verification per J-STD-001 should use thermocouples bonded to (a) a perimeter solder joint, (b) the frame centerline, and (c) an adjacent BGA package. All three must remain inside the SAC305 envelope simultaneously. AOI and X-ray inspection per IPC-A-610H Class 2 or Class 3, with void area limits enforced on the perimeter joint, complete the process control loop.

Recommended POCONS Components

Custom Two-Piece Shield Cans — The frame is reflowed with the rest of the SMT load in a single thermal cycle, and the lid is press-fit after final test or rework. Standard offerings in nickel-silver C7521 and tin-plated CRS, 0.15–0.25 mm wall, with internal partitions available for cavity-mode suppression. Specify per the footprint and profile envelope above for sustained ≥55 dB SE through 6 GHz. /products/shield-cans/

SMD Pan Nuts — Reflowable captive nuts for chassis grounding screws and lid retention features. Identical thermal profile compatibility with the shield can frames; co-planarity controlled to ≤50 µm to avoid lifting adjacent perimeter pads during reflow. /products/smd-pan-nuts/

Spring Contacts / Pogo Pins — Board-mount and lid-mount spring contacts maintain low-impedance ground continuity between the lid and the frame, or between the shielded module and a chassis. Contact resistance ≤30 mΩ at 100 mA, rated for ≥10,000 mating cycles, suitable for field-serviceable enclosures where the lid must be removed without re-reflow. /products/spring-contacts/

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