Page 39 - ConnectorSupplier.com 2019 Mil/Aero eBook

3. EMC Shielding
Electromagnetic compatibility (EMC) shielding protects a signal from outside electromagnetic energy that can disrupt the signal and prevents emissions from the signal itself from interfering with external electronics. EMC shielding can control both electromagnetic interference (EMI) and radio frequency interference (RFI). EMI/RFI shielding requirements are usually defined as a measure of allowable resistance (dBs) within a certain frequency range.
Effective shielding in electrical wire interconnect systems is a multidisciplinary approach that not only depends on connector component selection, but also on conductive plating, wire and cable shielding, individual and overall cable shield grounding, the use of conductive gaskets or fittings, and more. Because the entire transmission path — including cabling, connectors, printed circuit board (PCB) sockets, input/output (I/O) ports, etc. — is vulnerable, it is important to view EMC shielding in terms of a “follow-the-wire” approach to interconnect design.
Poorly shielded connectors (e.g., those with bad plating or an out-of-tolerance mating interface) can adversely affect the electrical performance of a system, and this problem can rarely be overcome by augmenting the shielding in some other part of the system. Intermateability between connector solutions from different manufacturers (or the lack thereof) is a significant concern in this regard, as resistance qualification testing can never be performed on all of the many combinations of connectors, sizes, platings, and so on between manufacturers. As such, end- users are well advised to address shielding requirements early in the electrical wiring interconnect system (EWIS) design process to prevent costly surprises.
Selected EMC shielding must meet the needs of the specific electronics and frequencies involved. For example, Cat 6A shielded, twisted-wire-pair copper cable capable of supporting up to 10Gb/s can incorporate either braided mesh or foil along the length of the cable. In extremely noisy environments, EMC shielding can involve multiple layers — both a foil and a braid — to reflect energy and carry induced current produced by EMI to ground.
Workmanship is critical at the terminations. Braid materials must be properly spread and bonded with the connector. In addition, the connector should terminate around the entire circumference of the shield material, the backshell material should be compatible with the cable, and solder lumps and artifacts should be eliminated. A military standard dating from 1981 (MIL-STD-1344A, Method 3008) covers emissions from a multi-contact connector-pair interface. An effective follow-the-wire approach to EMC shielding can create a low-impedance path that allows induced current to go to ground and effectively prevents EMI/RFI from compromising signal integrity.
4. Shock and Vibration
All mechanical systems experience shock and vibration at some point in their existence. Depending on the application, sometimes the shock and vibration associated with shipping and handling can be greater than that of the intended platform; so, all environments must be carefully considered.
Shock is typically associated with rapid acceleration or deceleration. For example, if a system is dropped onto a rugged structure, a force is imparted on all the mass of the system, which is dependent on the acceleration, deceleration, and mass of each component. This instantaneous force load stresses all the mechanical components and, if not adequately compensated for, can result in either plastic deformation or fracture, which can compromise the mechanical and electrical integrity of the interconnect system.
A typical example of this for an interconnect system would be the mounting flange of a receptacle connector breaking due to the cantilever load created by the mating plug connector, accessory, and associated unsupported cable. Shock also causes mechanical components to oscillate at their natural frequency, which is a function of the mass, spring, and dampening characteristics of the system, and typically results in a short duration of sinusoidal vibration.
Vibration is the continuous input of oscillating mechanical energy. It can be sinusoidal, such as that generated by the rotation of a motor that isn’t perfectly balanced or like the vibration mode associated with a mobile phone, or it can be random, like what automobile passengers feel when traveling over a road with randomly located cracks, lumps, and potholes. The dynamics of a system can amplify the dynamic acceleration of such vibration, especially at the natural frequency of the system.
The impact on the interconnects within said systems could include fatigue failure of the receptacle connector mounting flange or contact intermittency due to contact fretting. Fretting is the cyclical micro-motion of mated contacts, which causes localized wear of the electrical contact interfaces. This type of wear is exacerbated
39