It’s no secret there is an ongoing struggle in the electronics industry. Designers of today’s electronic products are pushing frequencies to higher and higher levels.
This is great news for consumers. Computers operating at 0.5 GHz now are available from all of the leading suppliers. Couple this processing speed with more than 100 MB of RAM and you have very powerful and fast machines that provide super realism and sound to those graphics-intensive video games many of us like to play.
But there is a downside. All this speed means more EMI. Traditional shielding and suppression techniques to eliminate unwanted interference may not be effective at these higher frequencies. The problem is compounded by the need to cram these high-performance electronics into very small packages.
According to Shane Hudak, product manager at Schlegel Systems, “Two technological trends continue to affect the EMI shielding industry. They are the push for increasing clock speeds and large-scale integration to minimize board area. These two trends show up in most major markets using EMI shielding products, from cell phone and palm-top computers to switching cabinets and router hub networks.”
Ernest Niemisto, former chief engineer at MMC Electronics America, added, “Since consumer products, such as Pentium III computers operating at 500 MHz and cordless phones at 1.8 GHz, have large potential volumes, design engineers need EMI suppression components to be smaller and less expensive.”
The 10-GHz Challenge
As can be expected, shielding-products manufacturers are gearing up to handle these higher frequencies. Typically, product development and testing have been concentrated at the 1-GHz level. However, with the new EMC test limit at 10 GHz, there is a flurry of activity to accommodate the new requirements.
“Traditional EMC solutions worked very well at the lower frequencies,” noted Lars Johnson, vice president of sales and marketing at APM. “As speeds increase, some of those components are failing or providing marginal results. An example is shielding components that have beryllium copper (BeCu) fingers. With slots required in the BeCu parts, the capability to provide extremely fine spaces is becoming a technical challenge and a physical limitation to shielding effectiveness,” he said.
Working with frequencies this high, however, can be unfamiliar territory. Only companies that specialize in UHF and microwave communications have a background in working with such high frequencies, according to Mr. Niemisto.
Think RF
One way to get a better feel for working at high frequencies is to “think RF.” At radio frequencies and beyond, components do not behave ideally.1
A high-frequency model or equivalent circuit often is used to describe the behavior of a component at high frequencies. Inductors and capacitors, for example, have a self-resonant frequency (SRF)—the crossover frequency at which a capacitor looks like an inductor and vice versa. In choosing these components for an application, choose an SRF that is five times the highest operating frequency of the system.
“The resistance of a conductor also is a function of frequency,” according to Mr. Hudak of Schlegel Systems. “Resistance varies with the conductor cross section, which is the skin depth times the conductor width. This phenomena is called skin effect and causes the resistance of a wire, PCB trace, or component to increase with frequency. This explains why beyond certain frequencies you only can carry energy through waveguides,” he concluded.
For magnetic materials used in inductors or ferrites, consider the effects of saturation with increasing frequency. Examine the properties of the dielectric material and the dissipation factor of a capacitor at high frequencies. You may find that its capacitive properties are seriously degraded. Also verify that high-frequency models, equivalent circuits, and data-sheet curves are supported by measurement data.
These measurements, of course, are different than those taken at DC or low frequency. RF equipment, such as impedance analyzers, spectrum analyzers, and s-parameter test sets, often is used. Since most of these measurements involve impedances, the Smith Chart comes in handy for quick, intuitive calculations.
Design the PCBs
As the probability of designing for EMC by default approaches zero, it becomes mandatory to adopt correct approaches from the outset. “With the trend toward miniaturization and higher speeds,” said Gary Fenical, product manager at Instrument Specialties, “engineers are forced to attack the source of the problem rather than trying to shield the entire product. Past shielding and suppression techniques may not work for newer applications. As a result, our company has more requests for on-board PCB shields, laminates, and form-in-place materials,” he added.
If you’ve ever compared the look and feel of a PCB designed for low-frequency applications with one designed for RF, the difference is apparent immediately. Since an EMI signal can propagate through air and couple to other circuits unintentionally, PCB layout is critical. Transmission-line and distributed-element effects also must be accounted for. PCB geometry is an integral part of the design, and PCB layout rules must be applied to all circuits where EMI control is essential.2
Use multilayer PCBs to separate analog and digital circuits and grounds and ground planes and to separate high- and low-frequency circuits. Pay a great deal of attention to inputs and outputs. On PCBs, I/O lines should be properly terminated. Unterminated lines and those with impedance discontinuities have reflections that generate common-mode signals prone to radiate or couple into other lines.
Terminations may include the use of low-pass RC filters on buffered I/O lines to slow down rise and fall times, pull-up resistors on bus lines, and transient suppressors for overvoltage, immunity, or lightning protection. Be sure that the 3-dB point for low-pass filters occurs at a frequency higher than the fastest signal on that line.
Install bypass capacitors close to the power pins on all chips. Keep all component leads as short as possible. Where multiple signals are in close proximity, such as on an analog-to-digital or a multiplexer chip, verify that channel crosstalk is attenuated sufficiently.
On-board PCB shielding also is available. There are even metal shields, or cans, that fit over individual chips. If chip temperature rise is a problem, investigate the use of shields that also may have heat-sinking capability. Perhaps you can use adhesive-backed, flexible ferrite material that may be cut and placed as needed.
“With the EMC test limit going to 10 GHz on telecommunications products, the cost of shielding becomes an issue,” said Jill Esposito, marketing communications specialist at Tecknit. “This could force designers to develop circuits and board layouts that minimize or eliminate the need for shielding. There is no question that these are application and design challenges,” she said.
Put the Chassis Together
Most EMI signals either penetrate or escape shielded enclosures through seams and openings that act like slot antennas and radiate EMI. For a chassis that requires cooling, the air vent usually will be the largest opening. Shielded vents are available to attenuate EMI and allow sufficient airflow.
At 10 GHz, form-in-place or flexible gaskets may provide a good seal and perform well if they are rugged enough for the application. The shielding effectiveness of EMC gaskets is tested either per MIL-I-83528 or the Transfer Impedance Method.3 Some gasket manufacturers specify the results of both tests.
“Fabric-clad foam gaskets address EMI shielding in the centimeter-wave region of the RF spectrum, from 3 GHz to 30 GHz,” said Mr. Hudak of Schlegel Systems. “The tightly woven metalized fabric, such as silver on nylon, acts like a continuous shield. The plating process, though, is critical to achieving that kind of performance,” he said.
The amount of power being switched influences the amount of unwanted EMI generation. Remember that a switching power supply can generate a great deal of EMI. Select a power supply that meets your EMC requirements. It should be contained in its own shielded and ventilated enclosure.
Use cables with the shield bonded to the connectors, not pigtailed, because they have too much inductance. Conductive tapes, ferrites, and shrink tubing provide additional shielding for cables and wires. Connectors are available with built-in EMI filters for each line.
Summary
In digital systems such as computers, significant emissions can occur at multiples of the master-clock frequency. In analog systems like cell phones, an unwanted emission may be a harmonic of the carrier frequency. In switching power supplies, usually it is related to the chopper frequency. The technique that works best suppresses the emission as close to the source as possible or prevents it from happening in the first place.
References
1. Paul, C., “Nonideal Behavior of Components,” 1998 IEEE Symposium on EMC, Workshop Notes.
2. Montrose, M., “EMI and the PCB, Fundamental Concepts and Design Techniques,” 1998 IEEE Symposium on EMC, Workshop Notes.
3. Kunkel, G., “Introduction to the Testing for the Shielding Quality of EMI Gaskets and Gasketed Joints,” 1992 IEEE Symposium on EMC, pp. 134-138.
EMC (Shielding/Suppression Products)
Mesh Shielding Tape
High-Flex™ Mesh EMI Shielding Tape holds up under repeated flex conditions and can be used for shielding cables or surfaces. Constructed of Electron® nickel-over-copper metallized polyester knitted fabric, it features a soft texture and an antifray coating to eliminate loose fibers. The material also is antifatigue and abrasion and corrosion resistant and will not tear or crease. High-Flex has a pressure-sensitive adhesive backing and comes in widths of 10 mm, 20 mm, and 50 mm in roll lengths of 20 m. APM, (800) 843-4556.
Small PCB Filter
The Miniature Filter line is available as solder-in and screw-in axial-lead components with diameters as small as 0.073″. Solder-in devices provide hermeticity; screw-in types are more easily replaced. The DC working voltages (WVDC) are 50, 100, and 200 V. The capacitance selection is 50 to 10,000 pF. Minimum insertion-loss values, measured according to MIL-STD-220, range from 25 dB to 60 dB at 10 GHz depending upon the model chosen. A gold finish, per MIL-G-45204, is standard. AVX, (843) 448-9411.
Surface-Mountable Shield
Standard Boldt Shields™ Surface-Mount Shields for PCBs are packaged in tape-and-reel formats for automatic installation by a variety of pick-and-place equipment. The EMI shields are available in many JEDEC sizes, including the 52-pin quad flat pack, 256-position ball grid array, and 84-pin plastic leadless chip carrier packages. BMI, (847) 934-4700.
EMI-Shielded Window
EmiClare™ GP 70 EMI Shielded Windows are made from two layers of UL 94 V-0-rated polycarbonate laminated around a center layer of metal mesh. The technique increases light transmission and eliminates text distortion on CRT displays. The minimum shielding effectiveness is 55 dB at 1 GHz. Front surfaces receive a nonglare hardcoat for scratch and chemical resistance. Standard thicknesses are 1.66 mm, 2.0 mm, and 3.0 mm. Chomerics/Parker Hannifin, (781) 935-4850.
Gasket Compound
ElectroForm Series 8558 Compounds are single-component, silicone-based EMI materials for form-in-place gasket applications. No mixing is required, and the compounds cure at room temperature. They accept plastic or metal substrates and provide a shielding effectiveness of >120 dB at 1 GHz. The low compression force needed to form gaskets made with these compounds accommodates mating surfaces that lack mechanical stiffness. Instrument Specialties, (717) 424-8510.
Flexible Ferrite Material
The Flexible EMI Absorption Ferrite materials suppress radiated emissions up to 8 dB in the 500-MHz to 5-GHz frequency range. They are suitable for manual or die cutting and attach to any plastic or metal surface via adhesive backing. Applications include attachment to individual components, enclosures, and cables and as suppression layers between PCBs. The materials have a UL 94 V-O inflammability rating and are available in several volume resistivities and dielectric strengths. INTERMARK (USA), (212) 629-3620.
Felt Shielding
Contex® Lossy Nonwoven Felts can be used to dampen cavity resonances and suppress EMI radiating from shielded-enclosure apertures. At 4.5-mm thick, they reduce radiated emissions by 10 to 20 dB at 0.5 GHz and up. The material can be die-cut to shape, meets UL 94 HB requirements for flame retardancy, and is available with adhesive backing. Contex also can be molded to shape by heating to 300°F, forming, and cooling. Marktek/Milliken, (314) 878-9190.
Quad TVS/Filter Chip
The EMCxxF-LC Series of quad TVS/filter chips protects four two-port networks when placed between the drivers and the 50-W lines. The chips come in an SO-16L package and contain filters with a 3-dB cutoff frequency of 500 MHz. They accommodate system voltages of 3.3, 5.0, 8.0, 12, and 15 VDC; provide a transient line protection >40 kV; and handle 300-W peak power in an 8/20-µs waveshape. The maximum clamping voltage typically is twice the supply voltage. ProTek Devices, (602) 431-8101.
Conductive Tape
CST Conductive Silver Tape has an average shielding effectiveness of 70 dB from 20 MHz to 10 GHz. The conductive adhesive backing will not crack after repeated flexing. It covers uneven surfaces and will not shrink at temperatures to 180°C (356°F). The silver also is galvanically compatible with a range of EMI gaskets. The 18-yard rolls are available in five widths. Schlegel Systems, (716) 427-7200.
Spiral Gasket
Ultra Quick-Shield Spiral Gaskets provide approximately 155-dB shielding at 1.0 GHz. They are wound from spring temper stainless steel and tin dip-plated for corrosion resistance and conductivity. The compression force ranges from 2 lb/in. to 30 lb/in. Cross-sectional diameters are 0.034″ to 1.000″. Ultra Quick-Shield is sold by the foot or cut to length. Spira Manufacturing, (818) 764-8222.
Perpendicular Gasket
The 250K4C070L Gasket provides EMI/RFI shielding to 100 dB for two perpendicular surfaces. The beryllium-copper finger stock offers continuous contact along its length and is available in strips from 1/2″ to 16″ long. Perpendicular clip-on contacts accommodate 0.060″-thick panels. The gasket can be used in high-temperature applications that do not support adhesive mounting. Tech-Etch, (508) 747-0300.
Conductive Ring Seals
O-SEALS are resilient O-rings of electrically conductive silicone elastomer with round or rectangular cross sections. The filler material is made of silver-plated inert, copper, or aluminum particles with volume resistivity ranges of 0.008 W · cm to 0.01 W · cm. Cross-sectional diameters of round O-SEALS are 0.070″ and 0.103″. Typical applications include jam nuts, interfacial seals, waveguide flange seals, cap seals, conductive moisture seals for sealing screws, and round military connector shell inserts. Tecknit, (908) 272-5500.
Shielded Shrink Tubing
Shrink-N-Shield has an outer jacket of polyolefin shrink tubing lined with Z-Cloth™ copper/nickel fabric with a shielding effectiveness averaging 85 dB from I MHz to 1 GHz. Tubing is available in diameters from 3/16″ to 1″ and provides a tight, flexible shield for wire bundles from 1/16″ to 7/8″ dia. The flame-retardant jacket meets the MIL-I-23053/5, Class I space, and clean-room outgassing standard. Zippertubing, (800) 321-8178.
Copyright 1999 Nelson Publishing Inc.
June 1999