Managing Interference with EMI Shielding
What is EMI Shielding?
Electromagnetic interference (EMI) shielding is when conductive or magnetic materials act as barriers to block unwanted electromagnetic signals from entering the field. It impedes stronger signals from affecting its electronic surroundings and protects the more sensitive ones from escaping.
EMI itself is a disturbance that affects the operation of an electrical circuit, transmission channel or system. It can decrease or completely interrupt the performance of a circuit, and in extreme cases, even cause damage to the equipment.
This is caused by either:
- Conduction – generated by physical contact between the affected circuits and source
- Radiation – caused by induction, which is the motion of a conductor across a magnetic field or by a change in magnetic flux in a magnetic field.
EMI is also called radio frequency interference, or RFI, when the interference is in the radio frequency spectrum.
How Does it Manage Interference?
All integrated circuits are both potential sources and recipients of EMI, especially when the printed circuit board (PCB) contains physically larger components such as heat sinks, connecting cables, and transformers. On-board mitigation techniques include the use of surge arresters or transzorbs (transient absorbers), decoupling capacitors, and more.
Reducing the interference at its source.
Isolating the offending circuits by filtering, grounding, and shielding.
Increasing the immunity of any susceptible circuits.
Did you know we are exposed to EMI in our daily lives?
Common examples include:
- Lightning discharges
- Solar storms
- Power lines
- Fluorescent lights
- Consumer electronics
- TV & Radio
- Cellular Devices
- Wi-Fi & Bluetooth
Techniques for Electromagnetic Interference
Isolation through physical shielding is a non-invasive suppression technique to help protect electronics from EMI. As shields are not inserted into the circuit itself, this method has many advantages, including:
- No effect on the operation of the digital system
- No timing problems
- No waveform distortion
- Reduction of crosstalk
- Emission suppression
- Resolution of susceptibility (immunity) problems
The first approach consists of simple metal housings for devices fabricated from steel, copper or aluminum. Downsides include weight, cost, a trade-off of durability vs. thickness/mass, and susceptibility to leakage when any deformation of any type occurs.
The second approach brings screens, wires, metal-infused foams, and coatings containing metallic particles. These would typically adhere to the inside of the enclosure for electronics. Each has its own advantages and disadvantages.
EMI Shielding Gaskets
A consistent problem occurs at EMI gaskets, where the joining of surfaces introduces gaps into the shielding coverage. Material costs and ease-of-fabrication are important considerations for gasket designers in many different industries.
Metallic-particle-filled materials, such as silicone, are able to combine the sealing properties of silicone rubber with the electrical properties of metals, making them among the top choices for gaskets. Unlike older compounds, newer materials are able to ensure both effective shielding and electrical conductivity.
Today’s electronic designers can specify particle fills that trade-off costs. While silver and silver-hybrid particles were common in early iterations, more recently developed nickel-graphite silicones offer a higher performance with cost savings.
Select silicone gaskets have a minimum shielding effectiveness of 100 dB at RF frequencies between 20 and 10,000 Hz, which meets requirements of industry-standard MIL-DTL-83528.
These conductive shielding silicones are:
- Cost-effective, reliable fabrication
- Strongly resist stretching or deformation
- Are tear-resistant
- Able to withstand harsh environments and allow connector holes to align properly
Adhesive backings can also be applied for easier installation. Additionally, for shielding applications where Z-axis conductivity is required, particle-filled silicones can support the use of electrically-conductive adhesives.
As an example, some touchscreens designed for use in extreme desert, arctic or marine environments have gaskets made of metallic-particle-filled silicones. These gaskets provide electrical conductivity and environmental sealing. While shielding EMI emissions are radiated by the contained circuit, it also protects the circuit from outside interference.
The gaskets are soft enough to avoid interfering with the touchscreen function while cushioning the device from mechanical shock. Particle-filled silicone elastomers can meet environmentally-demanding requirements.
For example, because nickel-graphite silicones are available in 30, 40, and 45 durometers (Shore A), they’re soft enough for enclosure gaskets. Other, higher-durometer shielding elastomers that use fluorosilicone as the base elastomer can resist fuels and chemicals. These fluorosilicone compounds come in 50, 60, and 80 durometers (Shore A) for applications that require EMI gaskets made of harder materials.
Various higher-durometer, nickel-graphite silicones are available, but some EMI gasket applications require reinforcement for added strength. That’s why EMI materials include products such as 65-durometer elastomer reinforced with an internal nickel-coated mesh. Lower-durometer, nickel-graphite silicones can also be reinforced with an inner layer of conductive fabric for added conductivity and material strength, which helps to prevent brittleness and tearing during EMI gasket fabrication.
Our system can significantly reduce installation cost of an EMI gasket by up to 60%.
The durable, highly conductive seals have low compression set, ensuring years of effective shielding and mechanical performance.
EMI Shielding Attributes
Excellent Shielding Effectiveness
Effectiveness of selected K.R. Anderson FIP gaskets exceeds 100 dB between 200 MHz and 12 GHz.
Gasket Application Programmable in 3 Axes
Full 3-axis motion of the application technology accommodates uneven surfaces that are common in castings or injection-molded parts. The result is enhanced control of the gasket cross-section.
Denser Packaging is Possible
FIP gaskets can be applied to walls or flanges as narrow as 0.030” (0.76 mm), and don’t require mechanical retention. Compared with groove and friction-fit designs, the positional accuracy and self-adhesive properties of our gaskets will typically save 60% or more space. This frees additional board space, and allows for smaller overall package dimensions.
Small Cross Sections, Complex Geometries
Virtually any gasket bead path can be programmed using our application technology. In addition to simple straight lengths, the system applies continuous 360⁰ perimeter gaskets in combination with any required number of internal subpaths that form “T” joints with the perimeter seal. The system produces reliable junctions between bead paths that provide continuous shielding and environmental sealing.
Secure Gasket Adhesion
FIP gaskets exhibit adhesion to a variety of common housing substrates, including:
- cast aluminum, magnesium or zinc alloys with various platings
- nickel-copper plating on plastic stainless steel (300 series)
- Vacuum metalized aluminum
Low Closure Force Not a Problem
FIP gasket materials are ideal for low deflection force designs, or those whose mating surfaces have low mechanical rigidity. Nominal deflection of 30% using a mechanical compression stop is recommended. Deflection below 20% or above 40% is not recommended.
Tight Dimensional Control and Terminations
FIP gasket beads are dispensed with an accuracy of 0.001 inch (0.025 mm). and a cross-sectional height tolerance of 0.006 inch (0.15 mm). This innovative technology produces clean bead ends minimizing the “tail” characteristic of other processes. The key is precise management of flow rate of material through the nozzle, material viscosity and dispensing speed.
Note: Gasket cross-section and tolerances will vary slightly at the site of ‘start’ or ‘stop’ events in the dispense bead length.
High levels of Quality Control
K.R. Anderson has the capability to perform automated dimensional verification of gasket bead placement and height for statistical process control, using fully programmable optical coordinate measuring technology and vision systems. Electrical resistance of cured gasket material is tested with a multimeter capable of measuring to 0.001 ohm. Typical Cp and Cpk values are approximately 1.5 of circuit suppression/hardening and shielding.
Applications for Shielding Electromagnetic Interference
Gaskets including Form-In-Place Gaskets
KraFAB custom gaskets from a variety of material formats and formulations that meet your specific application needs.
The best material options are based on the application demands including shielding level required, EMI/RFI frequency range, potential for galvanic corrosion, closure force, interface design, area available for the gasket, production volumes, assembly requirements and the working environment of the finished device.
Electrically Conductive Paints
KraFAB sells and supports product selection and application of Chomerics Cho-Shield EMI Shielding Paints.
Cho-Shield Electrically Conductive Paints are designed to adhere to a variety of substrates including various metals, plastics, aluminum, and composites. The Cho-Shield Electrically Conductive Paints are available in polyurethane, acrylic, epoxy and polyester and silver, silver plated copper, or nickel conductive particles to meet your application requirements.
KraFAB sells and supports product selection and application of Chomerics PREMIER™ Electrically Conductive EMI Shielding Thermoplastics. PREMIER™ is a blend of PC/ABS thermoplastic polymer alloys and conductive fillers engineered for stable electrical, mechanical, and physical performance. The conductive filler technology utilizes nickel plated carbon (Ni-C) fibers. Higher shielding versions incorporate Nickel-Graphite (Ni-C) powder to increase performance.
Microwave Absorber Materials
KraFAB sells and supports product selection and application of Chomerics CHO-MUTE elastomer based microwave absorber materials. CHO-MUTE elastomer based absorber materials are designed to the reduce of unwanted electromagnetic radiation from electronic equipment and minimize cavity to cavity cross coupling, and microwave cavity resonances. These materials provide RF absorption performance over a broadband frequency range from 500 MHz to 18 GHz.
EMI Engineered Laminates
Engineered laminates offer design flexibility to reduce weight and space requirements while providing the needed shielding. Our Parker Chomerics Engineered Laminates are typically constructed using a conductive layer, an insulative layer and an adhesive layer. They are often integrated with gaskets and/or thermally conductive materials to maximize performance.
EMI Cable Wrap and Knitted Wire Mesh
KraFAB sells and supports product selection and application of Chomerics SHIELD WRAP™ Knitted Wire Mesh Tape. Chomerics SHIELD WRAP™ Knitted Wire Mesh Tape is a highly flexible and conformable mesh meant to be wrapped around cables and harnesses.
Electrically Conductive Sealants
KraFAB sells and supports product selection and application of Chomerics CHO-BOND Electrically Conductive Sealants, Gap Fillers and Caulks. These polymer systems allow for larger bond lines and fillets. They are based on a variety of chemistries including silicone, polyolefin, polyisobutylene, polyurethane and polythioether. The electrically conductive particles include silver, silver plated copper, silver plated glass, silver plated aluminum, stabilized-copper and nickel graphite. The silver plated aluminum and nickel graphite particles provide corrosion resistance in specific applications.
Electrically Conductive Adhesives
KraFAB sells and supports product selection and application of Chomerics Cho-Bond Electrically Conductive Adhesives. The Cho-Bond family of Electrically Conductive Adhesives are designed to adhere to a variety of substrates while providing the needed level. The Cho-Bond Electrically Conductive Adhesives include epoxy and silicone polymer systems that offer the option of rigid or flexible bondlines.
Metal Foil Tapes
KraFAB sells and supports product selection and application of Chomerics CHO-FOIL metal foil tapes. The tapes include copper, embossed copper, tin-plated copper and nickel plated silver fabric. They are coated with an electrically conductive acrylic pressure sensitive adhesive (PSA).
KraFAB sells and supports product selection and application of Chomerics EMI Shielding Honeycomb vents. The vents can be framed or frameless. The variables involved in choosing a vent include airflow, attenuation, installation method, durability, aesthetics, environmental exposure, gasketing, air filtration, size, flame exposure and Foreign Object Debris (FOD).
Electrically Conductive Grease
KraFAB sells and supports product selection and application of Chomerics CHO-LUBE® 4220 Electrically Conductive Grease. CHO-LUBE® 4220 is a silver filled conductive grease designed for surface–to-surface, particularly metal-to-metal, sliding contact areas requiring continuous electrical paths.
Electrically Conductive Shrink Tubing
KraFAB sells and supports product selection and application of Chomerics CHO-SHRINK Conductive Heat Shrinkable Shielding. CHO-SHRINK tubing is a heat-shrinkable polyolefin tubing that provides shielding for cables, connectors, and connector terminations.
EMI Shielding Design Support
Contact K. R. Anderson for design and manufacturing engineering support. We are one of the original Authorized Chomerics Distributor, Fabricator, and Form-In-Place gasket Application Centers in the United States.