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.
Another important issue involves understanding the applicable EMC standards that must be met. Be aware that many standards differentiate telecom, medical, and commercial electronics. Furthermore, these standards aren't static documents. Recently, there has arisen much activity in this area as international, national, and industrial organizations attempt to harmonize their respective requirements. Until these efforts produce mutual recognition agreements, the path to compliance will require an understanding of many standards.
Several state-of-the-art shielding techniques are available. Before package design begins, the designer should have a good understanding of these packaging options. Armed with this knowledge, the designer then begins the design process by defining the role that shielding will play in the package design. Once this has been completed, the designer is ready to begin selecting the correct shielding techniques.
When shielding requirements are well defined, it's possible to optimize the EMC performance of the design. On the other hand, if many questions about shielding remain, it's recommended that the designer incorporate as much shielding flexibility as possible into the package design.
One way to achieve maximum flexibility is through standard packaging products that have upgradable shielding provisions. An example is the Schroff-brand multipac chassis, offered by Pentair Electronic Packaging. This 19-in. rackmount enclosure for horizontal board mounting features built-in shielding. Plus, if further shielding is required, it can be upgraded easily by adding a standard shielding gasket (Fig. 1).
A balanced solution accomplishes shielding at several levels of the mechanical hierarchy. It's important that these different levels be applied in an optimum fashion because shielding technology typically adds significant cost to packaging fabrication.
Determining Shielding Needs
Circuit boards or noisy discrete devices may be housed in shielded plug-in units or small packages. These, in turn, may be packaged in a shielded case or rackmount subrack. Finally, overall shielding effectiveness can be improved through the use of a shielded cabinet. In this way, shielding responsibilities are distributed where necessary, rather than being centralized. Hence, this approach is called distributed shielding.
By identifying noisy equipment early in the design cycle, the offending devices can be isolated and their noise reduced. The entire problem doesn't have to be eliminated on one level. With the creation of subsequent barriers to the radiated emissions, a very good level of shielding can be obtained with relatively little expense.
Common elements can be incorporated into a design so that shielding techniques of one level will be compatible with all of the possible "upper levels" in which they might be used. Some systems will require a combination of all the shielding techniques discussed here, including distributed shielding, upgradability, gasketing, and contact clips.
Most enclosures can be reduced to a smaller-sized box with doors or removable panels, cutouts for cabling, and perforations for ventilation. When doing so, the challenge lies in making the new design meet EMC specifications. Wherever a removable panel exists, there's a seam that needs to be sealed. In this case, gasketing is normally used because the spacing between the matting surfaces is difficult to control (Fig. 2).
Stainless-steel spring fingers and nickel-impregnated silicon cord are two popular shielding materials (Fig. 3). Some designers, though, opt for a gasket in order to avoid the multiple fasteners that otherwise are required to obtain the same result. Fastener overuse can be costly and cause a detrimental impact on the aesthetics of the product.
Through a number of ways, a packaging design can be made upgradable with respect to shielding. For example, in one enclosure design that included side extrusions, the extrusions were designed to accept shielding material. The same design included specially stamped stainless steel clips to make contact between the vertical edges of the front panel and the side extrusions. The clips were treated as options for improving the shielding performance of the case as needed.
In another situation, in order to achieve a lower-cost solution, a front panel was assembled directly to the side extrusions relying on direct contact to ensure continuity. A flat top cover made contact along the side through the use of silicon cord, while contact was made along the front and rear by using an adhesive, stainless-steel serrated strip.
In packaging VMEbus systems or other standardized bus-structured systems, the shielding challenge is to shield between each removable circuit board. Covering the entire front with a single seamless front-panel shield solves the problem. But, this blocks access and interferes with cables. The best solution is to shield between each front panel.
Obtaining a tight, EMI-resistant seal between front panels, however, is difficult when the panels are designed to slide in and out. In this situation, designers must account for the build-up of spring force across the subrack. We have overcome this problem by utilizing the front rail to align the front panels. This technique absorbs the compression forces so that they don't accumulate.
Shielding vertical 19-in. steel cabinets presents unique problems, particularly with the large doors associated with this type of enclosure. One requirement is that these doors be shielded around their perimeter. Another is design flexibility. It should be possible to shorten the doors for cable entry. Shortened doors become necessary when power and I/O cabling cannot be run through the top or bottom of the enclosure.
The type of gasket used is an important consideration in the selection of an EMC enclosure. Seals on doors must be sturdy enough to withstand many opening and closing cycles. Keeping in mind that the shielding effectiveness of the enclosure was determined under laboratory conditions, it's important to consider how effective the seal will be after the enclosure has been in the field.
Some seals lose effectiveness over time, as the shielding material suffers from a mishap or loses resiliency. Gaskets must be stable and durable for a long time, especially those used to seal doors. Doors should be properly aligned. Similarly, gasket seals that are compressed and relaxed each time the door is cycled need to be directly compressed, rather than compressed in a sweeping motion. This is because a sweeping motion can cause the shielding material to become deformed or pulled off. In addition, gaskets which haven't been properly utilized can develop a memory and might eventually form unreliable seals.
Metal-finger-style gasketing, such as that fabricated of beryllium copper (BeCu), can become deformed easily. It may become caught or accidentally bent during use as well. Also, this type of gasketing requires an excessive compression force that makes it difficult to latch or unlatch doors.
Knowing the capabilities of gaskets, other shielding options, and how to apply them grants designers the best shot at creating a balanced packaging solution that meets all of the project's design goals cost effectively. This knowledge is valuable for retrofits as well as for new product designs. Optimized packaged solutions, though, are achieved when shielding concerns are addressed at the beginning of the product development cycle. With high power equipment, faster electronics, and new European standards like the CE Mark, which focuses the spotlight on shielding, EMC is an aspect of packaging product design that can no longer be viewed as an afterthought.