Military connectors need conductive O-rings that provide reliable levels of EMI shielding and resist compression set.
EMI/RFI O-rings for military connectors are donut-shaped gaskets that combine reliable environmental sealing with proven levels of shielding against electromagnetic interference (EMI) and radio frequency interference (RFI). They are not the same as flat, washer-style EMI gaskets. Rather, EMI/RFI O-rings are designed for tough environments where temperature, pressure, and tolerances could cause other types of EMI gaskets to fail.
Military Connectors and Gasket Compression Set
With military electronics, EMI/RFI conductive O-rings are used with the following types of interconnects:
- panel connectors
- cylindrical connectors
- power supply connections
- connector backshells
Because the conductive O-rings for these connectors are subjected to compressive forces, they must withstand compression set: permanent deformation that can occur when a gasket is jammed into a tight space. If an EMI/RFI O-ring remains crushed after installation, this deformation may result in dimensions that are different from part drawings. Durometer, a measure of hardness, is important because of its relationship to compression. With conductive silicones, both harder and softer durometers are available.
EMI Elastomers and Fillers for Conductive O-Rings
For the base material, EMI/RFI O-rings can use either silicone or fluorosilicone. Both can withstand water, ozone, ultraviolet light (UV), and a wide range of temperatures. Fluorosilicones add fuel and solvent resistance. Fill materials for these conductive compounds include silver, silver-aluminum, silver-copper, silver-glass, and nickel-graphite. Filler materials differ in terms of cost, conductivity, and typical volume resistivity (VR) – an indirect measure of shielding effectiveness.
The table below describes EMI gasket materials from Specialty Silicone Products (SSP).
Filler Type | Cost | Electrical Conductivity | Typical VR (ohms/cm) |
Silver | $$$$$ | Extremely Conductive | .0009 |
Silver-Aluminum | $$$$ | Super Conductive | .003 |
Silver-Copper | $$$$ | Super Conductive | .003 |
Nickel-Aluminum | $$$ | Very Conductive | .1 |
Nickel-Graphite | $$ | Very Conductive | .1 |
Note: For military electronics, SSP provides elastomers that meet MIL-DTL-83528 requirements.
Molding vs. Bonding for EMI/RFI O-Rings
EMI/RFI O-rings can be compression molded or cut and bonded. Molded EMI/RFI O-rings don’t require bonding, which reduces the risk of EMI leakage. However, molding requires tooling that adds costs and extends projects. Bonded EMI/RFI O-rings from SSP use tooling that’s faster and less expensive to produce. At our Ballston Spa, New York (USA) manufacturing facility, we build the tools and extrude the EMI cords that you need in accordance with our ISO 9001:2015 quality management system (QMS).
The bonding method that SSP uses is more reliable than other joining methods and enables us to control EMI O-Ring quality. Our hot splicing method applies heat and pressure to a conductive silicone with a durometer that’s similar to the EMI cord itself. Other types of bonded EMI O-rings may use a non-conductive RTV silicone adhesive that lacks an EMI filler, or a non-silicone glue that creates a “hard spot” because it dries to a much harder durometer than the EMI cord itself.
Choose Bonded EMI/RFI O-Rings from SSP
With our in-house capabilities, SSP can supply you with more prototypes of bonded EMI O-rings than you’d get if you waited for a single molded one. Whether it’s for prototyping or production, our tooling costs are lower and the lead times are faster than you’ll get with molded parts. To request a quote or for more information, contact SSP on-line or email Dominic Testo, SSP’s Business Development Manager.
