Understanding EMI Shielding: Its Significance in Design

What is EMI Shielding? 

 

EMI shielding involves employing specific manufacturing processes and materials in electronic devices and equipment to safeguard signals from external electromagnetic interference and to prevent the signals produced from causing disruption to nearby components.

 

Why is EMI Shielding Important?

 

Electromagnetic interference (EMI) poses a threat to a wide array of critical electronic devices, systems, and applications, ranging from medical and military equipment to mass transit systems and industrial controls. This interference, stemming from both natural phenomena and human-made sources, can lead to temporary malfunctions, data loss, system failures, and in severe cases, even fatalities.

 

Understanding the sources of EMI and its impact is crucial for engineers and designers. Recognizing how electromagnetic energy (EME) within the operational environment might cause interference is essential. An EMI shield's role is to mitigate these interferences, especially in the radio frequency (RF) spectrum, which spans from 3 kHz to 300 GHz. RF waves are fundamental to radio technology but can also disrupt wireless communication by transmitting interfering signals. Without proper EMI shielding considerations, designs may fall short in providing necessary protection against these electromagnetic fields, compromising the reliability and safety of the devices.

 

Sources of EMI

 

Railroad and mass transit systems are susceptible to electromagnetic interference (EMI) due to several application-specific sources, including:

 

• Emissions from the control and propulsion systems of trains
• Switching of high voltage contacts
• Contact with third rail shoes
• Train signaling and control systems

 

Medical equipment is also vulnerable to electromagnetic interference (EMI), with potential sources unique to healthcare environments, such as:

 

• Electrical and electronic devices used in surgical units
• Life support machinery, including ventilators and infusion pumps
• Patient monitoring and assistance gear
• Diagnostic and therapeutic X-ray equipment

 

Military assets and critical infrastructure are exposed to electromagnetic interference (EMI) threats that include, but are not limited to, Intentional EMI (IEMI), often referred to as "electronic warfare," and other specific menaces such as:

 

• High-altitude nuclear electromagnetic pulse (HNEMP)
• High-power microwave weapons
• Electromagnetic bombs (E-bombs)
• Electromagnetic pulse (EMP) cannons

 

While some electromagnetic interference (EMI) threats, such as high-altitude nuclear electromagnetic pulses or electromagnetic bombs, might seem extreme, it's crucial for engineers to evaluate all potential EMI risks. This comprehensive assessment ensures the integration of suitable protection measures into their gasket designs to safeguard against a wide spectrum of EMI threats.

 

EMI Shielding Gaskets

 

EMI shielding gaskets protect electronics from electromagnetic interference, traditionally crafted from metal sheets like aluminum, copper, and steel, shaped to fit electronic enclosures. While effective, these metals can deform under sealing pressures, potentially compromising the shield.

 

Current advancements in EMI shielding include flexible metal screens, wires, foams, and metallic ink coatings for electronic enclosure interiors. Among these, particle-filled silicone stands out, marrying metal's electrical benefits with silicone rubber's flexibility. This combination is particularly valuable for designers facing diverse sealing and insulation tasks.

 

For instance, ruggedized touchscreens often utilize silicone-based EMI gaskets filled with metal particles. These gaskets not only reduce EMI emissions and provide conductivity but also seal against environmental extremes without hindering touchscreen functionality or durability against mechanical shocks. Cost and fabrication simplicity are crucial considerations for gasket designers across various sectors.

 

Conductive Silicones

 

Particle-filled silicone gaskets are a solution for demanding applications, but it's essential to evaluate if these conductive elastomers align with all the requirements of your project. Questions about their cost-effectiveness and manufacturability arise, considering the compromises that might come with incorporating a high volume of metal particles. These include potential hardness or brittleness, constraints on part sizes due to mold dimensions, and concerns about the material thickness for sleek electronic designs.Historically, the cost of particle-filled silicones, especially those using silver-aluminum as the filler, was also a deterrent, particularly when silver prices spiked.

 

Despite past skepticism over their usability, advancements have made particle-filled silicones more appealing. The military's MIL-DTL-83528 specification highlighted silver-aluminum's significance in EMI shielding, but the rising cost of silver prompted a search for alternatives. Now, designers have access to less expensive options like silver-copper, silver-glass, and notably, nickel-graphite silicones. These nickel-graphite options, cost-effective and compliant with MIL-DTL-83528's shielding requirements, offer a viable solution for achieving strong EMI protection without the premium price of silver-based elastomers.

 

EMI Materials

 

Recent advancements in silicone compounding have made particle-filled elastomers capable of meeting stringent EMI shielding demands while fulfilling other project criteria. Nickel-graphite silicones, for instance, are now offered in varying softness levels—30, 40, and 45 durometer (Shore A), making them suitable for enclosure gaskets. For environments requiring resistance to fuels and chemicals, higher-durometer fluorosilicone-based elastomers, available in 50, 60, and 80 durometers, provide a robust solution.

 

These modern materials incorporate sufficient metal filler to ensure effective EMI shielding and electrical conductivity, supporting reliable and economical manufacturing processes. Particle-filled silicones maintain their shape during cutting, ensuring connector holes align correctly and enhancing tear resistance, a crucial feature for gaskets with thin walls. Designers can opt for adhesive-backed versions for simpler installation. For applications needing Z-axis conductivity, these silicones work well with electrically-conductive adhesives, boosting shielding effectiveness.

 

While various durometers of nickel-graphite silicones cater to different requirements, certain applications necessitate reinforced materials for extra strength. Options include 65-durometer elastomers reinforced with nickel-coated mesh and lower-durometer versions paired with conductive fabric layers. These reinforcements enhance conductivity and material robustness, preventing brittleness and tearing in EMI gasket production.