SSP makes EMI shielding materials and products (i.e. EMI gaskets and EMI O-rings) from electrically conductive silicones that we make in Ballston Spa, New York (USA). This article is a basic but practical guide for engineers who need to design reliable electronics.
Introduction
Electromagnetic interference (EMI) is a critical concern in modern electronics, especially as devices become smaller, faster, and more densely integrated. Effective electromagnetic compatibility (EMC) design requires mitigation strategies that minimize unwanted emissions and susceptibility to external fields. Shielding remains one of the most robust approaches, providing predictable attenuation across broad frequency ranges.
Keep reading to learn about shielding principles, the electrical properties of common materials, and key design considerations for engineers tasked with meeting EMC performance and compliance requirements.
Shielding Principles in EMC
EMI shielding operates by reflecting and absorbing incident electromagnetic fields. The shielding effectiveness (SE) is typically expressed in decibels (dB) and represents the ratio of field strength without shielding to that with shielding in place. A well-designed shield can provide attenuation from 20 dB for low-performance housings to more than 100 dB for precision enclosures.
Key mechanisms include:
Reflection loss – Dominant at higher frequencies, governed by material conductivity (σ).
Absorption loss – More relevant at lower frequencies, proportional to shield thickness and material permeability (μ).
Multiple reflections – Occur in thin shields, contributing additional attenuation at mid-to-high frequencies.
Common Shielding Materials and Applications
Selecting a material requires balancing conductivity, permeability, weight, and manufacturability.
Copper (σ ≈ 5.8 × 10⁷ S/m): Excellent electrical conductor with SE >100 dB for thin foils at GHz frequencies. Used in PCB shielding cans, cable braids, and chassis enclosures.
Aluminum (σ ≈ 3.5 × 10⁷ S/m): Lightweight with good SE, though prone to galvanic corrosion. Common in aerospace and automotive housings where weight is critical.
Steel: Provides both conductivity and high relative permeability (μr), making it effective for low-frequency magnetic shielding (<100 kHz). Galvanized and stainless variants are used in industrial cabinets.
Nickel Alloys: High μ materials such as Mu-metal (μr > 20,000) deliver superior low-frequency magnetic shielding, critical for protecting sensors and precision instrumentation.
Conductive Coatings: Silver- or nickel-filled paints can provide 60–80 dB attenuation on plastics, often used in handheld electronics.
Conductive Fabrics: Metal-coated textiles with surface resistances <0.1 Ω/sq are increasingly used in flexible electronics and cable overbraids.
Practical Design Considerations
Engineers should account for multiple design variables to ensure reliable shielding:
Frequency Range: Higher frequencies (>30 MHz) emphasize conductivity and enclosure continuity, while lower frequencies require materials with higher μ.
Enclosure Integrity: Even a 1 mm gap can significantly degrade SE at GHz frequencies. Use beryllium copper gaskets, EMI fingerstock, and conductive adhesives to maintain continuity.
Shield Thickness: For high-frequency fields, skin depth (δ = √(2/ωμσ)) determines effective thickness. At 1 GHz in copper, δ ≈ 2 μm, so thin foils suffice; at low frequencies, thicker materials are required.
Grounding Strategy: Shields must be bonded to a low-impedance ground to prevent re-radiation. Improper grounding can reduce SE by >20 dB.
Thermal and Mechanical Constraints: Vents, seams, and fasteners should be designed to allow airflow and assembly without compromising EMI performance. Honeycomb vents provide up to 90 dB attenuation while allowing cooling.
Validation: EMC testing per CISPR 32, IEC 61000-4, or FCC Part 15 should be conducted early in development. Adjustments may include material substitution or additional gasketing.
Conclusion
For engineers, EMI shielding is not simply a compliance measure but a design parameter that directly impacts product reliability and performance. By understanding material properties, frequency-dependent shielding mechanisms, and enclosure design practices, it is possible to achieve effective attenuation and regulatory compliance. Incorporating these considerations early in the design process reduces risk, shortens certification timelines, and ensures robust EMC performance in real-world environments.
Contact SSP for EMI shielding materials and fabricated products.
