|EMI shielding solutions|
|Monday, 15 January 2007|
The electromagnetic interference (EMI) is a complicated problem in the computer manufacturing industry. The interference sources include the CPU (central processing unit), motors, inverters, relays, switches, transistors, amplifiers, power supplies or other exchange circuits. Desktop PC's are generally housed in a relatively large chassis containing a main printed circuit board or motherboard and other components that are incorporated into or connected to the motherboard. The components may be located inside or outside of the chassis. Typical internal components include a power supply, a central processing unit (CPU), random access memory (RAM), a mass storage device such as a magnetic disk drive, expansion cards connected to a bus on the motherboard, and various peripherals mounted on "rails" in "bays" within the chassis and electrically connected to the motherboard or an associated expansion card by a ribbon cable. Peripheral component interface (PCI) circuit boards or cards are often used to connect computer peripheral devices to, for example, a computer motherboard. Typical peripherals include user input devices such as a keyboard, a mouse, a microphone, a joystick, a graphics tablet or a scanner) and user output devices such as speakers a printer, and a video display device. As a by-product of normal operation, those electronic componenents in the computer cases frequently emit undesirable electromagnetic radiation. Unnecessary voltages or currents exist among various internal systems of the compact electronic information product and produce many wide-frequency noises which are normally called the EMI (electromagnetic interference). In the computer industry there is a continuing trend for size reduction of components and increases in their operating capabilities. Moreover, as the capabilities of the electronic devices have increased, the number of electronic components, such as PCI boards, required to be located within a computer has also generally increased. The reduced sizes of the electronic devices coupled with the increased number of electronic components located within the electronic devices have increased the need for improved EMI shielding. A portable computer is a self-contained personal computer which can be easily moved to and operated at various locations. Portable computers are often referred to as laptop or notebook computers. Electromagnetic interference (EMI) is a significant problem in high performance notebook computers. The high clock rate of the CPU of a high performance notebook computer is associated with high-frequency signal components. These high-frequency signal components may generate electromagnetic waves which propagate to other portions of the notebook computer or to neighboring electronic circuits and produce deleterious electromagnetic interference (EMI). Notebook computers incorporate electromagnetic shielding to reduce electromagnetic emissions in order to meet regulatory standards on such emissions. Portable computers also use thermal transfer mechanisms to cool the CPU which accounts for almost half the power dissipated within the computer system.
Electromagnetic radiation can adversely effect circuit performance, and radiate from electronic equipment to threaten circuits in nearby equipment. Designers of all electronic devices are under constant pressure to reduce physical size, weight and cost of the devices, and improve the electrical performance of such devices. This is particularly true of mobile devices such as cellular, personal communication system and cordless telephones, personal digital assistants (PDAs) and other portable radio communication devices. EMI shielding may be particularly important or critical at one specific assembly level as a function of the electronic device being considered. Electromagnetic shielding is typically used to protect electrical equipment from unwanted electromagnetic radiation or EMI. Many systems such as processors require at least some shielding for proper operation or to meet EMI requirements for emissions and immunity. As circuit speeds and sensitivities increase so will the need for improved shielding. EMI protection is particularly important in small, densely packaged, sensitive electronic applications operating at high frequencies. Electromagnetic shielding often provides protection for electrical equipment by reducing unwanted signals to levels that do not adversely affect equipment. This is achieved by both reflecting and absorbing the radiation signals. Reflection depends on a permeability and conductivity of the shielding material, and a frequency and wave impedance of the signal. Generally, the reflectance of a shielding material increases with frequency. The remaining signal passing through a shielding material is reduced by absorption. To meet EMI regulations, most electronic equipment currently employs a combination of two approaches commonly referred to as source suppression and containment. Source suppression attempts to design components and subsystems such that only essential signals are present in signal interconnections, and that all non-essential radio frequency (RF) energy is either not generated or attenuated before it leaves the component subsystem. Containment attempts to place a barrier around the assembled components, subsystems, interconnections, etc., so that any unwanted electromagnetic energy remains within the boundaries of the product, where it is dissipated harmlessly. Likewise, EMI shielding prevents any externally generated electromagnetic signals from entering the device and interfering with its operation. Such electromagnetic signals often develop as a field or as a transient within the radio frequency band of the electromagnetic spectrum. Cell phones in particular are required by law to adhere to electromagnetic compatibility (EMC) limits. To provide maximum protection for radiated EMI, shielding methods are implemented at more than one "assembly" level. For example, shielding may be provided at the component level, the printed circuit board (PCB) level, the shelf level or the cabinet level, or a combination thereof as appropriate. There are also many regulatory requirements imposed on computer systems limiting the level of electromagnetic emissions that they produce. Significant efforts are taken to reduce electromagnetic interference (EMI) and electromagnetic radiation (EMR) and virtually every country has a regulating agency that controls the marketing and sale of electronic equipment that do not pass stringent requirements for EMI/EMR, whether radiation or intercepted by an electronic device. Electromagnetic compatibility (EMC) means that electric and electronic devices are compatible with similar devices in their vicinity in terms of shielding against EMI caused by the electromagnetic (EM) energy levels so that EMI does not adversely affect their operations. EMC is defined as the ability of a device to function properly in its intended electromagnetic environment and not to be a source of electromagnetic pollution to that environment. Because of the faster and more powerful computer systems evolving, EMI levels associated therewith are consequently changing and becoming relatively more difficult to satisfy.
As mentioned above, to reduce or attenuate the effects of EMI, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device and to insulate the device or other target devices from other source devices. When an electronic device is coupled to the electronic assembly, the electronic device typically covers the opening through which it is connected, thereby inhibiting EMI emissions from that opening. Shielding is designed to prevent both ingress and egress of electromagnetic energy relative to a housing or other enclosure in which the electronic equipment is disposed. Present day EMI shielding solutions typically involve the use of conductively painted plastic housings, conductive gaskets, and/or metal cans that are affixed to a printed circuit board by soldering or similar methods, some of which are semi-permanent. Since such enclosures often include vent openings and gaps or seams between adjacent access panels and around doors, effective shielding is typically important to prohibit the gaps in the enclosure from permitting transference of EMI therethrough. EMI shielding gaskets are used to electrically seal gaps in metallic enclosures that encompass electronic components. The gaps between the panels, hatches, etc., and a housing provide an undesired opportunity for EMI/RFI to pass through the shield. The gaps also interfere with electrical currents running along the surfaces of the housing from EMI/RFI energy, which is absorbed and is being conducted to ground. The EMI/RFI shielding gaskets include an electrically conductive element, such as a conductive filler, which is bound by a binding agent and helps to prevent external EMI from interfering with an electronic device having a shielding gasket. Specialized EMI gaskets have been developed for use in shielding small gaps in electronic enclosures. These include metal spring fingers, wire mesh, fabric-over-foam, and conductive elastomers. Electronic assemblies often include an enclosure that defines a physical opening through which an electronic device is coupled to the electronic assembly. EMI shielding vent panels are used for shielding electronic devices from electromagnetic interference. Typically, the vent panels utilize a filtering media being an electrically conductive (e.g., metallic) honeycomb structure, which is particularly effective in dissipating EMI from electronic equipment. One common method that has been used to shield electromagnetic radiation has been to use the chassis (or enclosure) to contain the electromagnetic energy within the chassis and block the energy from escaping. This is achieved by enclosing the electronics within a grounded conductive box. Integrated circuit (IC) chip carriers are specialized circuit panel structures that are frequently used to attach IC's to circuit boards. Chip carriers provide high density, complex interconnections between the IC and the circuit board. Separately attached peripheral or edge shielding is used to address cross-wise EMI emissions. Such shielding may be in the form of conductive tape or foil. For computer systems, EMI requirements are generally complied with by enclosing the computer system in an enclosure made of metal or other conductive material. EMI shielding for electronic circuitry is commonly formed by a housing or enclosure associated with the particular circuitry. In a computer system, for example, the enclosure which houses the system processor, random access memory, and related devices commonly includes structures which connect together to form an EMI shield. Other shielding may be used within the primary enclosure for shielding particular circuits in the system. Human exposure to electromagnetic radiation can be minimized through utilization of an EMI shield. Specially manufactured clothing or fabric comprised of conductive elements can be used to provide such shielding.