Electromagnetic interference (EMI) is a constant challenge in modern electronics. This is particularly true in industries like automotive, military & defense, aerospace, and electronics. Proper EMI shielding is critical for regulatory compliance and performance of critical vehicle, sensor, and control systems.
Many applications use EMI gaskets for shielding. Made from specialty elastomers with flexibility, low compression set, corrosion resistance, and electrical conductivity, gaskets are designed to seal an enclosure against physical contaminants and moisture as well as electromagnetic waves. Selecting the correct EMI gasket for an application is a highly technical decision that directly impacts compliance, shielding effectiveness, and cost.
Here, we’ll look at some important considerations when selecting EMI gaskets including key industry standards, material selection, and application-specific factors.
Understanding EMI Gasket Standards
There are several standards that apply to EMI gaskets to ensure that they contain internally-generated interference and block external interference from neighboring devices and signals. Because the gaskets are often used in applications exposed to harsh conditions and temperatures, other standards govern sealing performance.
MIL-DTL-83528
This military standard applies to electronics designed for aerospace or defense use. It primarily defines electrical properties and test methods, such as shielding performance impulse survivability. It also touches on mechanical properties of the gasket such as compression set, chemical resistance, vibration resistance, and temperature performance. Satisfying MIL-DTL-83528 standards enhances reliability under intense operational conditions.
NEMA Enclosure Ratings
National Electrical Manufacturers Association (NEMA) ratings define the ingress protection capabilities of a specific enclosure, and can also be applied to EMI gaskets. NEMA ratings range from Type 1 for basic indoor environments to Type 4X for washdown and hazardous locations and beyond. NEMA requirements influence the choice between solid and non-conductive sealing materials in addition to conductive/sealing solutions.
IP (Ingress Protection) Ratings
Like NEMA ratings, IP ratings, as defined by IEC standard 60529, designate the degree of protection against solids/dust and liquids/water a gasket provides. Because most EMI gasket applications require both EMI shielding and ingress projection, material and cross-sectional geometry need to be optimized for both.
UL JMST/UL50 Certification
Materials covered by UL in this category for use as components of complete electrical equipment submitted for investigation by UL. The basic standard used to investigate products in this category is UL 157, “Gaskets and Seals.” Products in this category are not required to comply with the full property profile test requirements specified in UL 157. Where the individual Recognitions indicate compliance with a specific UL end-product standard, the test parameters, such as exposure time and temperature, used to investigate the product may differ from those specified in UL 157. Common applications where this certification is important include electric signs, enclosures for electrical equipment, room air conditioners, industrial control equipment, luminaires, and heating and cooling equipment.
Material Selection Considerations
Material selection is a critical aspect of specifying EMI gaskets because it impacts both cost and performance. Silicone, fluorosilicone, or EPDM are often used as flexible base materials that create a physical barrier against particles and moisture, with conductive fillers that provides EMI shielding protection.
There are several conductive filler categories, such as:
- Silver particles: Silver provides the highest level of shielding and conductivity of nearly any conductive particle. Further, its conductive properties are not compromised by the effects of oxidation. However, silver can encounter challenges when subjected to dissimilar galvanic materials.
- Silver-plated particles: These materials offer a valuable compromise between the performance of pure silver particles and other galvanically compatible materials. They also can offer substantial cost savings over the pure silver particles. They are suitable for high-frequency and/or miniature electronics, and provide excellent conductivity and high shielding effectiveness.
- Nickel-graphite: This material is often billed as a cost-effective and reliable alternative for many commercial shielding applications; however, it is important to carefully consider the actual long term conductive performance of these particles when specifying them. There are also concerns about regulatory restrictions regarding nickel use in products. Silver-plated particles often provide superior solution for applications where nickel products are under consideration.
- Carbon fillers: These products offer a low cost alternative to those listed above; however, it is important to consider that their shielding/conductive properties are significantly lower.
Flame Retardancy Requirements
No matter the application, flame retardancy is a safety requirement in all enclosed electronic assemblies. Aerospace, defence, and industrial applications typically mandate that materials be self-extinguishing. When safety is a concern, it is critical to specify conductive elastomers that comply with standards like UL94HB, UL94V1/2, or UL94V0, which ensure that the chosen conductive elastomer will not be a combustion accelerant.
Application-Specific Factors
Some applications require additional considerations, based on operating and environmental conditions, such as:
- Galvanic compatibility. The conductive filler must be galvanically compatible with the metal material of the enclosure, and the gasket’s mechanical design must prevent the dissimilar metals from corroding each other and reducing shielding effectiveness.
- Compression force. To avoid damaging fastener points and enclosure walls, gaskets must provide the required sealing capacity at an acceptable closure force (i.e., the ease with which the enclosure door can be securely closed).
- Environmental exposure. Factors such as temperature, chemical exposure, UV radiation, or ozone exposure around the enclosure can damage or degrade the polymer gasket base over time. Not all gasket materials withstand the same environmental challenges equally well, so it’s important to consider which factors are the most significant in the specific application and specify materials accordingly. For example, silicone has superior temperature stability compared to EPDM in high- or low-heat environments. In contrast however, EPDM is resistant to a different spectrum of chemicals than silicone.
Choose Vanguard for Reliable EMI Gaskets
Vanguard Products Corporation provides standard and custom EMI gaskets made from a range of high-performance elastomers. We also offer engineering support and can help you determine suitable materials for different application requirements. Our products are manufactured in the USA and we are ISO 9001:2015-certified for quality system management.
Request a video consultation with us, or contact us to learn more about our products and capabilities for your next EMI shielding project.