Introduction
EMI gaskets are used extensively by the electrical/electronic industry to assist in complying with the various EMI radiated emission requirements. These requirements include compliance to DoD TEMPEST and EMI, and FCC and EU EMI test limits. As a rule of thumb the radiated emission TEMPEST requirements are about two orders of magnitude (40 dB) more stringent than the DoD EMI requirements, and the DoD EMI radiated emission requirements are about two orders of magnitude (40 dB) more stringent than the FCC and EU EMI requirements. This means that in terms of difficulty, complying to the FCC and EU requirements is relatively easy. However, the expense can be high in terms of the percentage increase in the cost of manufacturing the equipment. The FCC requires that the manufacturers of the equipment that falls under their jurisdiction be responsible for compliance throughout the life of the equipment. As such, the cost of not complying for the life span of the equipment can be very costly (i.e., redesign and retrofit can become a catastrophic cost).
While many military and aerospace EMC issues may be addressed by operational changes, testing is still required to find weaknesses.
The cost of complying with the FCC (as well as DoD, EMI and TEMPEST) radiated emission requirements can be reduced to within acceptable limits by understanding the problems associated with the radiation and suppression of radiated electromagnetic waves. Because of the relatively low FCC compliance EMI radiated emission suppression levels, EMI gaskets are not always needed. However, the proper selection and use of EMI gaskets can often significantly reduce the expense associated with compliance costs. A significant aspect associated with the proper selection and use of EMI gaskets is to be prepared to use them if they are needed. If one is not prepared, then the driving force in selecting a gasket is the least cost to add the gaskets after the fact. In such cases, the cost can be, and usually is, exceedingly high. The paragraphs that follow describe the generation and propagation of electromagnetic (EM) waves from wires, the method used to shield the fields, low cost methods of implementing EMI gaskets and problems associated with obtaining reliable shielding throughout the life of the equipment.
The Generation, Propagation and Shielding of EM Waves
The equipment covered by FCC and EU requirements contains circuits, which generate RF energy that falls within the bandwidth of radios and other communication equipment. This energy travels on wires from one circuit to another, where the wires connecting the two circuits act as antennas. The energy emanating from the wires is transmitted out of the equipment in the form of electromagnetic (EM) waves. When the magnitude of the waves are a higher amplitude than is allowed by the specification limits we call it electromagnetic interference or EMI.
The fields, which radiate from wires are similar to the fields which radiate from electric dipole antennas. Figure 1 illustrates an EM field emanating from a transmission line pair.
We know from antenna theory that the impedance of the wave is equal to Ē/H where the relationship of H to Ē is approximately equal to the following:
R = Distance from radiating wire to point in question (meters)
When the wave of Figure 1 strikes a shielding barrier, a current JSI (i.e., surface current density on the incident side) is generated on the shield as illustrated in Figure 2. The current is equal to approximately two times the value of H in amperes/meter of the incident field (the field that radiates from the wire and strikes the barrier). The current in turn is attenuated by the skin depth of the barrier where the current on the transmitted side, JST, will generate another EM field. The magnitude of the “E” field in volts/meter emanating from the barrier will be JST (current density in amperes/meter on the secondary side) times the impedance of the barrier in ohms. The secondary field is what is detected by the test antenna.
If the shielding barrier has a joint in it, the current will flow across the joint creating a voltage which is equal to JSI times ZT (the current in amperes/meter times the transfer impedance of the joint in ohm-meters). A field will radiate from the joint as illustrated in Figure 3 and is observed by the test antenna. If the field so detected is above the limits specified by the requirements we must reduce the transfer impedance (ZT) of the joint. This can be accomplished by the use of additional fasteners or by the use of EMI gasket material.
Cost Effective Use of Gaskets
Commercial electronic equipment is generally housed in non-conductive die-cast or molded plastic cabinets. The cabinets are coated with a conductive material to provide the required shielding for compliance to FCC or VDE limits. This is usually accomplished by plating the inside of the cabinet with an electroless coating (aluminum, nickel, copper, tin, etc.) or with a conductive paint. This coating will reduce the EM fields penetrating the cabinet walls to within acceptable levels. However, the joints of the cabinet provide a convenient path for the EM fields to penetrate the cabinet. These fields are reduced to acceptable levels by providing conductive paths between the joint surfaces of the cabinet. This can be performed by the use of additional fasteners or by the use of EMI gasket material. The use of EMI gasket material can be a very cost effective means of obtaining the shielding at the joint surfaces. The cost of using EMI gasket material can be significantly less than the cost of using fasteners. However, to obtain the cost effective advantage, provisions must be made in the die or mold to provide room for the gasket material and methods of holding the gasket material in place.
There are two kinds of EMI gasket material that are recommended for cost effective use. These are as illustrated in Figures 4 and 5 and are as follows:
- Commercial grade convoluted spring EMI gasket material. The material is made from low cost stainless steel, and can be purchased in cut-to-size lengths for pennies per inch. The material can provide an EM bond of one milli-ohm per meter length, and can be held in place by the use of pinch bosses or retaining holes.
2. The commercial grade convoluted spring gasket material attached to a neoprene sponge elastomer. An adhesive backed tape is supplied with the elastomer, where the purpose of the elastomer and tape is to hold the EMI bonding material in place.
In using the convoluted spring gasket material, (or any similar EMI gasket material), a groove must be provided in the die or mold to house the gasket. The recommended groove is illustrated in Figure 6 where the width of the groove is about 35% wider than the gasket material and the depth is about 75% of the width (diameter) of the gasket material. Figure 7 also illustrates a method, which has been effectively used to protect the gasket.
The recommended diameter of the gasket material is between 0.06 and 0.15 inches (1.5 mm to 3.8 mm). Assuming a 25% maximum deflection of the gasket, this will accommodate a 0.015 to 0.037 inch gap (or unevenness) between the joint surfaces to be EM bonded. Please note! The purpose of the gasket is to provide a conductive path between the separate parts of the case. Therefore, care must be exercised to ensure that the conductive plating on the separate parts interface with the gaskets.
The grooves or configurations of Figures 6 and 7 provide a place for the gaskets to sit. However, provisions must be made to hold the gasket materials in place. This is accomplished by providing pinch bosses or retaining holes along the groove. The pinch bosses are illustrated in Figure 8 and retaining holes in Figure 9. Because the requirements are relatively easy to comply with, continuous gasketing throughout the length of the joint is not required (i.e., small segments along the length of the joint can be used effectively). The actual optimal length and number of segments of EMI gasket material will not be known until the EMI testing on a finished prototype equipment is completed. One (1) to 1-1/2 inch segments on one (1) or two (2) of the four (4) sides of a small cover is often sufficient. The grooves of Figure 6 and 7 must be placed in the die or mold during the early design phases. The pinch bosses or retaining holes can be placed in the die or mold after the EMI testing is completed and optimal required gasketing is known.
Please note! During EMI testing, the segments of EMI gasket material can be held in place using tape or other non-destructive methods of retainment.
In applying the gasket material to the unit case the following considerations should be applied.
- Pinch bosses
a) Cut the gasket material to the appropriate length (outside-to-outside distance between pinch bosses).
b) Push one end of the gasket material between one set of pinch bosses.
c) Stretch the gasket about 5% (to put the gasket under slight tension) and push the loose end into the other set of pinch bosses.
2. Retaining hole
a) Cut the gasket material to the appropriate length (distance between holes plus 0.4 inches).
b) Insert one end of the gasket into one hole.
c) Holding the inserted end in the hole stretch the gasket and insert the gasket into the other hole all the way to the bottom.
d) Release holding devices (i.e., fingers, etc.).
Note: A silicone RTV adhesive can be used to positively secure the two ends inside the hole.
The EMI gasket strip material that is attached to the neoprene sponge elastomer of Figure 2 uses adhesive backed tape to hold it in place. The standard thickness of the material is either 1/16, 3/32 or 1/8 inch. The recommended segments or lengths of gasket material are 1 to 1 1/2 inches long. The specific placement of the gasket segments can be determined during the EU or FCC EMI testing. However, provision must be made in the design of the cabinet to provide the required space for the gasket strip. Figures 10 and 11 illustrate two methods that have proven successful.
Reliability of Gasketed Joint
The FCC and EU require that compliance to the specification limits be for the life of the equipment. If a problem with a piece of equipment is detected and is proven to be due to inadequate design, then redesign and retrofit of all the equipment in the field can be required. By the proper selection and use of gaskets, these problems can be circumvented to a great extent.
Two basic problems can exist. These are: (1) the initial design is marginal and proves to be ineffective with time; and (2) the impedance (resistance) of the joint or gasket increases with time. Figures 12 and 13 illustrate work that was published by E. Grossart. The contents of Figure 12 illustrates that the surface conductivity of many materials used for shielding can be reduced with time. This means that the surface conductivity required for compliance to the FCC and/or EU radiated emission limits can be reduced with time. This can result in non-compliance with time.
The contents of Figure 13 illustrate: (1) common structural materials and subsequent plating; (2) materials that are commonly used in the manufacture of EMI gaskets; and (3) the compatibility of the two with each other.
Corrosion due to incompatibility of the surface plating and the gasket can significantly increase the resistance of the joint. This in turn could increase the radiated EMI from the unit case with time, creating future compliance problems. It is recommended that the contents of Figure 13 be used in selecting the joint surface plating and selection of gaskets for FCC and/or EU radiated emission EMI compliance.
Conclusion
The use of EMI gasket material can significantly reduce the cost of complying with the FCC and EU EMI radiated emission limits. The reduced cost results from using EMI gasket material in place of fasteners, where the EMI gasket material can cost as little as pennies per inch.
To use the gasket material in a cost effective means, provisions to hold and protect the gasket material must be designed into the mold or die.
These provisions consist of: (1) O-ring grooves and pinch bosses or retaining holes when using the convoluted spring gasket material; or (2) providing space between the various case sections to be EM bonded together when using the EMI strip gasket material.
Selected Bibliography
- Groshart, G., “Corrosion Control in EMI Design”, Symposium & Exhibition, Montrex, Switzerland, July 1977.
- Hellen, E., “Electromagnetic Theory”, John Wiley & Sons, New York, 1962.
- IT&T Corp., “Reference Data for Radio Engineers”, Stratford Press Inc., New York, 1964.
- Kunkel, G.M., “Introduction to Shielding of Electromagnetic Fields and the Application to EMI/RFI Gaskets”, IEEE International Symposium on EMC, October 1975.
- Kunkel, G.M., “An Overview of Problems Associated with the Design of Electromagnetic Shields”, IEEE International Symposium on EMC, Washington D.C., July 1986.
- Madle, P.J., “Transfer Impedance and Transfer Admittance Measurements on Gasketed Panel Assemblies, and Honeycomb Air Vent Assemblies”, IEEE 1977 International Symposium on EMC, Washington D.C., July 1976.
- Ott, H.W., “Noise Reduction Techniques in Electronic Systems”, John Wiley & Sons, New York, 1976.
- White, D.R.J., “Electromagnetic Shielding Materials and Performance”, Don White Consultants, Inc., Maryland 1975.