Along with the growth of electronics in everything from small, portable devices to huge cabinets has come the increasing use of gaskets to eliminate emissions and protect the product from external interference. Gaskets continue to prove themselves worthy opponents to EMI/RFI, but there are more considerations than the level of attenuation they provide.
Manufacturability, durability, and application-specific issues have become important design criteria for gaskets. Mechanical fixing also is important, especially in high shear-load situations. And the recent rise of alternative shapes and attachment options presents some challenges. As a result, careful consideration must focus on operational forces necessary to maintain good contact between the base material and the gasket as well as the appropriate forces required to install it.
For consistent electrical contact, EMI shielding gaskets compress a certain amount to follow the dimensions of the surfaces to which they are attached. Increasing the amount of compression applied on a gasket improves the contact impedance. It also may result in higher shielding performance, which is why gasket shielding data can be somewhat understated. Because the application and the shape of the gasket can affect the performance of the part, theoretical information should be a guide, not the rule, to the part and material selection.
All gaskets need compression to function properly, with the spring effect of a gasket being a critical characteristic. In applications where considerable force is exerted on the gasket, compression set also is important. The more the product is opened and closed, the greater the need for a resilient material formulation.
Most manufacturers can provide test information on the amount of compression set their gaskets take, and these numbers are relatively independent of the shape of the part. To compress any gasket as recommended, usually there is a human operator involved. This leads to the issue of closing force.
Any electronic system with operating panels or doors needs enough shielding to meet the standards, yet it must allow easy operator access. Tests can be conducted to indicate the probable performance of a specific gasket profile in relation to the required closing force.
These tests determine compression load deflection (CLD). The compression force required by a gasket will depend on its material characteristics, shape, direction of the force, and direction(s) of deformation.
Table 1 gives the CLD and contact resistance (CR) for a 6-mm × 6-mm square- shaped profile. As can be expected, the higher the compression rate, the harder it is to compress the gasket. On the other hand, the CR improves.
Remember that the CR numbers listed in most catalogs are meaningless when considered as absolute values. Table 1 simply states the compression load required to get a consistent electrical bond across a joint and make a good EMI seal.
Typically, gasket manufacturers want to know how much force is required to get a CR reading consistently below a target set point; for example, 0.5 W or lower. The value of the resistance across the joint has little other meaningful use aside from providing information on the amount of compression recommended for each profile.
Because of material composition and profile shape, the CLD will vary between gasket types for a particular application. Let’s look at five examples, all sized to fit the same opening:
Conductive fabric on a urethane foam core (UFC).
Beryllium copper finger (BCF) stock.
Monel wire mesh on silicone elastomer tubing (SET).
Monel wire mesh on neoprene sponge (NSP).
Nickel/graphite loading on a d-shape elastomer tubing (DET).
To consider the recommended compression forces for each gasket, we have average compression from the recommended minimum to the maximum compression information provided by the gasket manufacturers. For comparison, the values are expressed in lb/lin. ft/% of deflection. The average compression is calculated using the following formula:Average Compression =
where X = percent of compression (at the recommended minimum/maximum).
Y = compression force (at the recommended minimum/maximum).
The results, shown in Table 2, are the forces required to compress a 1-ft length of a specified gasket 1% from its free height. Looking at these initial values, UFC has the lowest compression force. These gaskets have an open-cell structure and flexible conductive fabric that make it easy to compress. DET also has a low compression rate influenced by the geometry of the profile used.
BCF stock requires about five times more compression force than UFC and four times as much as D-shape tubing with loaded elastomer. SET and NSP both have the highest compression rates.
To find the total amount of force for a particular application, multiply the values by the length (in feet) needed for the application and by the recommended compression rate (30% in most cases). For example, the required closure forces for a cabinet door may be multiplied by a factor of 10 or more, depending upon the type of gasket, and may become a concern especially for large enclosures.
Each of these gaskets has advantages and disadvantages, such as closing force and shear forces. The five examples illustrate the importance of determining operational characteristics when considering EMI gaskets for a particular application.
Beyond the issues of CLD and compression set, the method in which the gasket is attached to the unit can be a concern. Today, the most popular attachment method is pressure-sensitive adhesive (PSA). These acrylic-based, high-tack adhesives work very well in static joints and dynamic joints where the cycle axis is perpendicular to the mounting plane of the gasket. Problems arise during shearing loads on the gaskets.
In the case of shearing loads, the compression loading forces are applied to the gasket in a direction parallel to the mounting surface of the gasket. In applications such as the sliding lid on a rarely opened PC, a very high-tack PSA will perform well, especially if the PC is used in ambient temperatures below 180oF. This joint, though functioning under shear force, can be considered static.
Under dynamic shear conditions such as gaskets on switching and router hub cards, the shear forces are constant and experienced in a bidirectional mode. This type of stress exceeds the capabilities of some PSA systems.
Enter mechanical fasteners—or better said—bring back mechanical fasteners. Not only do mechanical fasteners include screws, bolts, and pop rivets, but also clips, press-fit rivets, and kerf mounting systems.
For easy assembly, press-fit rivets and clips are popular. Most geometries of fabric-clad foam and beryllium-copper gaskets can easily accommodate press-fit rivets. Wire mesh and conductively loaded elastomers also can accept rivets, but usually require an add-on leg for attachment.
Clip attachment also is being used more in shear applications. Made of durable plastic or metal, the gaskets can attach easily with retention fins and remain securely fastened to the unit. Kerf mounting systems use the same duodurometer plastic-insert technology for applications where a slot can be designed into the enclosure.
The selection of an EMI gasket based on shielding effectiveness is just one step in designing a shielded enclosure. Gasket selection also should include how the application will be used and how often the contents will be accessed. For best results, be aware of all the characteristics of a particular gasket profile and its composite materials before making a selection.
About the Author
Shane Hudak is the EMI shielding products manager at Schlegel. He has more than eight years of experience in EMC test and measurement and EMI shielding strategies. Schlegel Systems, 1555 Jefferson Rd., Rochester, NY 14623 (716) 427-7200, www.schlegel.com.
Copyright 1998 Nelson Publishing Inc.