Knowledge of shielding options and how to implement them into initial product designs is vital to producing an electronic packaging product that satisfies several important design objectives. A properly applied shielding solution will meet standards for electromagnetic compatibility (EMC), as well as cost restrictions and the specific needs of high-power and high-speed electronics.
The key for designers is identification of shielding goals early in the prototype phase and the use of techniques that provide flexibility for future product upgrades. Doing this can reduce incremental design costs and prevent an expensive retrofit should additional shielding be required.
The properly designed electronic package performs five essential functions. It provides the structure that physically supports the circuit boards. For proper operation, it cools the electronic device to the correct temperature range and air quality. The package supplies and distributes power. It also accommodates communication with other electronic devices via I/O cabling. Finally, it furnishes proper shielding to make the electronic device electromagnetically compatible with other electronic devices.
The ideal EMC package is a seamless and solid sphere manufactured from highly conductive materials, such as copper, aluminum, or silver. But, this solution isn't practical because it provides shielding at the expense of the package's essential five functions. A more feasible packaging solution maximizes shielding efficiency economically without hindering cooling, power distribution, or I/O.
Electronic devices emit electromagnetic radiation during the natural course of their operation. This energy has the ability to travel either as radiated energy, or as a current when it encounters a conductor. Conductors can radiate energy too. Consequently, the electronic devices emit electronic noise or interference, and so do the cables attached to them.
The noise transmitted by one electronic device may be picked up by another as "bogus" signals, which interfere with the latter device's proper operation. Hence, this noise is referred to as electromagnetic interference (EMI). This interference becomes a serious problem when it affects "mission critical" equipment, like a medical apparatus or a navigational computer on an aircraft.
To prevent such interference from occurring, electronics must be designed for EMC. There are two dimensions to EMC—immunity and emissions. A properly packaged device is resistant to EMI from neighboring devices, or in other words, it has achieved immunity. When it doesn't radiate energy above specified levels to its neighboring devices, controlled emissions have been achieved.
Determining Shielding Needs
With some engineering judgment early in the design phase, the product design can be simplified, costs can be minimized, and an effective packaging solution can be realized. To optimize the shielding solution in a packaging design, several issues must be addressed. The engineer needs a thorough understanding of shielding requirements for the product, and the latest shielding technology and practices.
Plus, one should know what other important features are required by the design. To define shielding requirements, engineers should consider certain criteria. They should know the key frequencies. Engineers have to be aware of what levels of attenuation in decibels are required at the key frequencies to achieve the required level of shielding. They will also need to know which devices are the big noise generators and where these generators will be located. And, engineers must keep in mind the requirements of U.S. and international standards.
When considering the desired level of shielding, there are two important aspects—frequency and effectiveness. Because the behavior of a device is frequency-dependent, one needs to determine both the magnitude of shielding required and the frequencies where those levels need to be attained. Designers should identify those components—such as a power amplifier or switching power supply—that broadcast noise at relatively high power at specific frequencies.
Identifying those frequencies and the locations of their sources will help designers anticipate problems and provide the most effective solution later during product development. Key frequencies are determined by the application and its environment. For example, many regional Bell operating companies try to ensure that central office equipment doesn't interfere with or suffer from interference from community police and fire radio communications. As a result, telecom equipment shielding must be designed with the relevant radio frequencies in mind.
In addition to defining shielding effectiveness and frequency objectives, designers must determine the other mechanical requirements of the design. These include cooling, power distribution, I/O, and mechanical interface, as well as the important issues of aesthetics and access. Each of these factors influence the enclosure's ability to shield against EMI.
Cooling typically involves moving air through openings in the walls of the enclosure, which means the engineer must design-in open areas to accommodate the necessary air flow. If active or forced-air cooling is required, the noise conducted over the power lines to the blower fans must be reviewed too.
Power distribution usually implies some sort of power entry plus a socket strip, bus bar, or discrete wiring in order to get power to the devices in the system. The engineer's greatest concern in this area deals with preventing the power distribution network from receiving radiated emissions and conducting that noise, as well as the background noise of devices attached to the network, and ultimately causing interference.
Large cutouts for cable entry and egress are commonly used for device I/O. This, however, also affects shielding. To accommodate cable entry, the engineer runs the risk of opening up a hole large enough to enable radiated energy to escape. The opening might allow the equipment to broadcast noise from attached cables too.
Mechanical interface broadly refers to the range of issues where each device, as a module of the whole, gets integrated into the final assembly. In addition, it deals with other mechanical requirements, like access and aesthetics. For example, the central office GR-1089CORE (Bellcore NEBS) specifies that a design must attain level 3 compliance if the equipment passes emissions and immunity tests with the doors open.
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