Page 47 - 2019 Mil/Aero eBook
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Filters — Filtered connectors play a critical role in managing and suppressing EMI/RFI and surge spikes and supporting the proper performance of devices and equipment. Some connector designs provide standard filtering capabilities, including individual, isolated pin filtering for high-frequency noise, built-in ground plane barriers in the connector inserts, and filtering at the face of system boxes. A more effective connector signal barrier enhances these traditional filter attributes by offering system designers complete flexibility in defining or revising individual circuit capacitance, ground, and electromagnetic pulse (EMP) performance during the design and development phase. Many connector designs also incorporate shield cans on the PCB to protect the circuitry from signal interference. Connectors designed with proper shielding and EMI gaskets can typically satisfy EMI requirements. Although, electromagnetic interference can also be reduced by adjusting the capacitance value of a connector. Connectors with capacitance values up to 50,000pF can significantly reduce the signal noise traveling through a device to help improve overall system performance.
Chip-on-Flex Technology — Fragile ceramic-planar-array block capacitors have been replaced with state- of-the-art flexible circuits in applications that require the utmost protection, such as mission-critical military and aerospace systems. Chip-on-flex (CoF) technology is suitable for applications that require lower working voltages, typically below 200VDC for high-frequency filtering and below 100VDC for low-frequency filtering, and its frequency rejection response is most effective to 200MHz. Designed to increase durability and reliability in harsh-environment applications, these circuits are comprised of individual chip capacitors that are surface- mounted on a pad adjacent to (but not soldered to) the feed-through contacts and can help relieve the stress at certain points that are vulnerable to thermal shock and vibration for up to 1,000 cycles. CoF filter connectors use off-the-shelf chip capacitors mounted on the flex circuits for filtering. This type of filter connector has standard filtering capabilities that include filtering at the face of system boxes, plane barriers in connectors, and high- frequency noise. Chip-on flex technology also allows engineers to package capacitors for filtering, as well diodes and metal oxide varistors (MOVs) for transient voltage surge (TVS) protection, within a single connector housing, resulting in a very compact and lightweight high-performance design.
Cable Assemblies — High-performance, bonded cable assemblies at the cable interface offer another line of defense against EMI/RFI. Sealing connector shells and using cables with high shield coverage that allows for up to 80dB of attenuation offers excellent protection from EMI. Cables with high shield coverage and special overmold features provide very robust, yet cost-effective solutions for protecting against RF emissions. The two most common shields used in cable assemblies are foil wraps and metal braids. Metal braids are often made of tin, silver, or nickel over copper, but special braids for applications that require non-magnetic material, such as radar applications, are also available and can be made from materials including tin/copper alloys with an iron core (SnCuFe).
ITT Cannon’s KJ 38999-Style Chip-on-Flex (CoF) Connector protects critical circuits from electrical interference by providing a stray signal barrier upstream and away from electronic devices without impacting system function and performance.
3. Materials and Plating
The most effective defense against EMI/RFI is filtering at the connector interface, with shielding materials as the next level of signal integrity. Experienced interconnect solutions engineers and designers frequently recommend the use of multiple shielding solutions for more complex needs and custom or especially sensitive applications.
Shielding materials are frequently acrylic-based sprays, but can also be paints or brush-on coatings applied to electronic housings. These shields contain extremely pure metals, such as copper, silver, and nickel, to help reduce or eliminate the effects of EMI/RFI. Different materials help shield critical interconnects in different ways. For example, soldier-worn connectors must be designed with features that allow for termination with 360° shielding, including banding platforms, cable banding platforms, EMI springs, and conductive O-rings.

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