JMEPEG (2000) 9:350-354 ©ASM International Materials for Electromagnetic Interference Shielding D.D.L. Chung (Submitted 25 October 1999; in revised form 5 January 2000) Materials for the electromagnetic interference (EMI) shielding of electronics and radiation soun;es are reviewed, with emphasis on composite materials and resilient EMI gasket materials, which shield mamly by reflection of the radiation at a high frequency. Keywords electromagnetic interference shielding, EMI shielding, composite materials, EMI gaskets 1. Introduction Electromagnetic interference (EMI) shielding refers to the re- flection and/or adsorption of electromagnetic radiation by a ma- terial, which thereby acts as a shield against the penetration of the radiation through the shield. As electromagnetic radiation, particularly that at high frequencies (e.g., radio waves, such as those emanating from cellular phones), tends to interfere with electronics (e.g., computers), EMI shielding of both electronics and the radiation source is needed and is increasingly required by governments around the world. The importance of EMI shielding relates to the high demand of today' s society on there- liability of electronics and the rapid growth of radio frequency radiation sources.[l- 9 1 The EMI shielding is to be distinguished from magnetic shield- ing, which refers to the shielding of magnetic fields at low fre- quencies (e.g., 60Hz). Materials for EMI shielding are different from those for magnetic fielding. 2. Mechanisms of shielding The primary mechanism of EMI shielding is usually reflec- tion. For reflection of the radiation by the shield, the shield must have mobile charge carriers (electrons or holes), which interact with the electromagnetic fields in the radiation. As a result, the shield tends to be electrically conducting, although a high con- ductivity is not required. For example, a volume resistivity of the order of I Q-cm is typically sufficient. However, electrical con- ductivity is not the scientific criterion for shielding, as conduc- tion requires connectivity in the conduction path (percolation in case of a composite material containing a conductive filler), whereas shielding does not. Although shielding does not require connectivity, it is enhanced by connectivity. Metals are by far the most common materials for EMI shielding. They function mainly by reflection due to the free electrons in them. Metal sheets are bulky, so metal coatings made by electroplating, electroless plating, or vacuum deposition are commonly used for shielding_P0- 25 1 The coating may be on bulk materials, fibers, D.D.L •. Chung, Composite Materials Research Laboratory, State UniversiWof New York at Buffalo, Buffalo, NY 14260-4400. 350--Volume 9(3) June 2000 or particles. Coatings tend to suffer from their poor wear or scratch resistance. A secondary mechanism of EMI shielding is usually absorp- tion. For significant absorption of the radiation by the shield, the shield should have electric and/or magnetic dipoles, which in- ternet with the electromagnetic fields in the radiation. The elec- tric dipoles may be provided by BaTi0 3 or other materials having a high value of the dielectric constant. The magnetic dipoles may be provided by Fe 3 0 4 or other materials having a high value of the magnetic permeability,noJ which may be enhanced by reduc- ing the number of magnetic domain walls through the use of a multilayer of magnetic filmsP 6 .2 7 1 The absorption loss is a function of the product C1,J1, whereas the reflection loss is a function of the ratio C1,/ J.L, where C1, is the electrical conductivity relative to copper and J.L, is the relative magnetic permeability. Table I shows these factors for various materials. Silver, copper, gold, and aluminum are excellent for reflection, due to their high conductivity. Superpermalloy and mumetal are excellent for absorption, due to their high magnetic permeability. The reflection loss decreases with increasing fre- quency, whereas the absorption loss increases with increasing frequency. Other than reflection and absorption, a mechanism of shield- ing is multiple reflections, which refer to the reflections at vliri- ous surfaces or interfaces in the shield. This mechanism requires the presence of a large surface area or interface area in the shield. An example of a shield with a large surface area is a porous or foam material. An example of a shield with a large interface area is a composite material containing a filler, which has a large sur- face area. The loss due to multiple reflections can be neglected Table 1 Electrical conductivity relative to copper ( cr,) and relative magnetic permeability (J.J..) of selected materialsU 19 1 Material 0", Silver 1.05 1 1.05 1.05 Copper 1 1 1 1 Gold 0.7 1 0.7 0.7 Aluminum 0.61 1 0.61 0.61 Brass 0.26 1 0.26 0.26 Bronze 0.18 1 0.18 0.18 Tin 0.15 1 0.15 0.15 Lead 0.08 1 0.08 0.08 Nickel 0.2 100 20 2 X 10- 3 Stainless steel ( 430) O.G2 500 10 4 X lQ- 5 Mumetal (at 1kHz) 0.03 20,000 600 1.5 X lQ-6 Superpermalloy (at 1kHz) O.Q3 100,000 3,000 3 X lQ- 7 Journal of Materials Engineering and Performance