Flight Hardware Packaging Design for Stringent EMC Radiated Emission Requirements
- Wednesday, 01 May 2013
This design can be used for any electronic package that needs to meet stringent electromagnetic interference/ electromagnetic compatibility (EMI/EMC) environments.
This packaging design approach can help heritage hardware meet a flight project’s stringent EMC radiated emissions requirement. The approach re quires only minor modifications to a hardware’s chassis and mainly concentrates on its connector interfaces. The solution is to raise the surface area where the connector is mounted by a few millimeters using a pedestal, and then wrapping with conductive tape from the cable backshell down to the surface-mounted connector. This design approach has been applied to JPL flight project subsystems.
The EMC radiated emissions requirements for flight projects can vary from benign to mission critical. If the project’s EMC requirements are stringent, the best approach to meet EMC requirements would be to design an EMC control program for the project early on and implement EMC design techniques starting with the circuit board layout. This is the ideal scenario for hardware that is built from scratch. Implementation of EMC radiated emissions mitigation techniques can mature as the design progresses, with minimal impact to the design cycle. The real challenge exists for hardware that is planned to be flown following a built-toprint approach, in which heritage hardware from a past project with a different set of requirements is expected to perform satisfactorily for a new project. With acceptance of heritage, the design would already be established (circuit board layout and components have already been pre-determined), and hence any radiated emissions mitigation techniques would only be applicable at the packaging level. The key is to take a heritage design with its known radiated emissions spectrum and repackage, or modify its chassis design so that it would have a better chance of meeting the new project’s radiated emissions requirements.
This design approach addresses radiated emissions leaking mainly from connectors. Based on a history of multiple EMC tests performed at JPL, D-connectors and micro-D connectors are known for compromising radiated emissions and causing test failures. There are two potential areas where radiated emissions can leak through: (1) the gap between the connector and the chassis (where the connector is mounted to), and (2) the gap between where the connector receptacle and the cable plug mates to. Both areas need to be shielded in order to reduce radiated emissions from leaking through these gaps. For this design, all D-connectors and micro-D connectors are mounted on an elevated surface area, known as the pedestal. The pedestal and the chassis are one piece of metal. For circuit cards that have the connector mounted on the card, this surface pedestal may not be possible. The alternative approach would be to implement a gasket to seal the gap between the chassis and the connector. Once the cables and connectors are securely mated, conductive tape is wrapped from the backshell (metal to metal contact) of each cable all the way to the raised pedestal onto the chassis, where the connector is mounted. In this process, the tape fully encloses the backshell, connector interface, and parts of the metallic raised chassis. This effectively seals any potential radiated emissions breach coming from the connectors, including from the gap between the chassis and the connector, and from the gap between the plug and the receptacle.
The novelty of this packaging design approach is to make limited changes to heritage design and increase its chance to meet a project’s stringent radiated emissions requirement. Without employing a raised surface or using a pedestal, the act of using conductive tape to seal the leakages from the connectors becomes harder, because the tape may not fully enclose the gap between the connector and the chassis. Having some elevation gives that additional surface area for the conductive tape to fully enclose the gap. The alternative to the pedestal would be to implement a gasket design as mentioned in the previous paragraph.
With this approach to packaging, D-connectors and micro-D connectors that have long been considered a weak point for radiated emissions can be improved by using conductive tape and raising the surface area of where the connector is mounted. This gives enough surface area for the tape to fully enclose the gap between the connector and chassis, and between the cable plug and connector receptacle. This is a simple solution to reduce the impact of radiated emissions leakage from D-connectors and micro-D connectors.