Electrically conductive gaskets are used to improve EMI shielding and electrical bonding in metal enclosures, primarily through RF bonds. These gaskets can achieve resistances as low as 2.5mΩ when used with metal bonding straps or screws in combination with flat or O-ring gaskets, especially where traditional bonding methods aren’t feasible. However, these connections can exhibit an unexpected increase in electrical resistance over time. This issue is often linked to gasket instability due to over-compression, particularly in flat gaskets, which typically allow a maximum deflection of about 15% of their original thickness. Variations in gasket thickness can lead to over-compression, which in turn causes an increase in resistance, even when metal-to-metal contact is maintained.
Optimizing EMI Shielding and Electrical Bonding with Conductive Gaskets
Electrically conductive gaskets are essential for improving EMI shielding in metal or metallized enclosures, as well as enhancing electrical bonding and, in certain cases, providing structural grounding.
These gaskets primarily serve as RF bonds rather than electrical safety connections, which are intended to carry high currents during a short circuit or malfunction. Resistance levels can be as low as 2.5mΩ, which is relatively easy to achieve with metal bonding straps linking the structural components. However, in scenarios like those involving typical Mil-C-38999 connectors, using straps may not be practical. Instead, screws paired with a conductive flat or O-ring gasket are often used to bond the connector to the housing.
A challenge with these connections is the unexpected increase in electrical resistance over time. Initially, the resistance might be low, but it can gradually rise to three or four times its original value over several days or weeks, even in stable environments.
This issue is often attributed to gasket instability, particularly in flat gaskets, where over-compression can be a problem. Conductive flat gaskets generally allow for a maximum deflection of around 15% of their original thickness. However, maintaining the correct compression without over-tightening fasteners is difficult due to thickness variations of ±10% to ±20%. This problem also occurs with jam-nut or O-ring seals, where resistance can increase despite metal-to-metal contact.
Gasket Selection
Flat conductive EMI gaskets cut from sheet material have limitations, but these can be addressed in two ways:
1. Use an O-ring or D-ring in a groove.
2. Use a molded “flat” gasket with features and compression stops, either on the housing, connector, or within the gasket itself.
For O-rings, two key factors are important, similar to flat gaskets:
– The contact area provided by the O-ring.
– The metal-to-metal surface contact area (excluding screw threads).
For instance, with a size 19 connector, an O-ring with a 40mm inside diameter and a 1.78mm cross-section could have a contact area of around 257mm² and a thickness of about 1.45mm, resulting in a gasket resistance of approximately 17mΩ. To achieve resistance below 10mΩ, the metal-to-metal contact area must be calculated. With ideal contact at 5mΩ/in², a contact area of 0.25in² (1.6cm²) is needed, which is difficult to achieve. Therefore, either bare metal-to-metal contact is necessary, risking damage to the chromate surface, or a bonding strap must be used.
The second option—replacing a flat gasket with a molded one, potentially with compression stops—offers a more reliable solution, albeit at a higher cost.
