There’s no doubt that achieving the CE Marking, a prerequisite to selling products in Europe, is of growing importance to electronics manufacturers who must compete globally. “Many products that previously were not required to pass EMI testing at all now must meet new standards to compete in the European markets,” observed José Loveira, technical support specialist at Spira Manufacturing. “This has generated a demand for low-cost shielding solutions for many companies that just aren’t accustomed to dealing with this type of requirement.”
By anticipating likely EMC problems during the design phase, you often can manage EMI/RFI economically, without resorting to additional shielding. Designs should follow these guidelines:
Reduce signal slew rate.
Turn off data buses when not needed.
Use properly terminated striplines or microstrips for fast data lines on PCBs. Along with constraining EM fields, transmission lines maintain signal integrity. This preserves noise margins in fast switching circuits which, in turn, accommodates higher levels of EMI without malfunction.
“EMI problems will worsen significantly due to faster rise-time signals as we implement IC technology using deep submicron (<0.2 m m) design rules with copper technology,” said Ron Demcko, applications engineering manager at AVX. “Sooner or later, we will realize that today’s high-speed digital circuits must be treated with RF/transmission-line design rules.”
Use multilayer PCBs with ground planes. This is strongly recommended for fast digital systems. For very sensitive analog circuitry, large ground planes can reduce performance because they add too much local capacitance.
Consider using laminated DC buses to provide distributed capacitance to earth and to reduce power supply series inductance.
Treat sensitive signals as differential rather than single-ended.
Provide adequate power supply decoupling capacitors.
Use feedthrough filters as control lines passing from one module to another.
Apply ferrite beads and bricks to ordinary wires and flat cables to attenuate EMI. An EMI-absorbing ferrite pad can be glued to the top of fast ICs. Common-mode interference on data lines can be attenuated by connectors with ferrite liners.
Generally, avoid creating EMC problems.1,2 If that’s not practical, then try to contain or suppress emissions and reduce susceptibility of sensitive circuitry by using local shielding.3
Finally, contain emissions and reject interference by putting your electronic product in a shielded metal box. This may be your only choice if, for example, you are integrating a system and have no control over the design and the EMC performance of the individual instruments.
EMC design challenges were summed up by Gary Fenical, senior EMC engineer at Instrument Specialties. “Many of our customers now attack their RF noise problems at the board level to eliminate shielding the entire system. Suppressing the noise at the component level also helps keep RF noise off cables, establishing less need for shielded cables.”
Higher-Speed Devices
The trends toward higher-speed semiconductor devices and smaller plastic enclosures for cellular phones, notebook computers, and paging systems compound the EMC problems confronting designers. “Since all of these systems radiate energy in the FCC-controlled spectrum of 30 MHz to 40 GHz, it is necessary to provide specified levels of filtering for all unwanted signals,” commented Jack Van Den Heuvel, director of strategic marketing at California Micro Devices. “As a result, suppliers have introduced a multitude of passive filtering components that can provide the necessary filtering for the frequency spectra of interest.”
“Radiated emission levels are proportional to the square of the frequency, and the radiated susceptibility of a circuit is linearly related to frequency. As a result, a 10× increase in speed results in a minimum 10× increase in susceptibility,” explained Ron Brewer, vice president of EMC technical services at Instrument Specialties. “If faster logic families are required to achieve the speed increases, then the total radiated susceptibility will be increased by more than 10× because the faster logic devices have lower noise margins.”
Cost of Compliance
As usual, cost is a factor. Circuit integration and product miniaturization have driven down product price while providing greater functionality. An example is the cordless phone.
“As frequency increases with the newest technology, the current technology is reduced in price,” according to Ernest Niemisto, chief engineer at MMC Electronics America. “The advancement in technology has dropped the price of the 49-MHz phone from more than $100 to less than $50 and the 900-MHz phone from $200+ to under $100.”
A reminder that maintaining very low assembly costs is part of the EMC-related design was added by Ray Borgersen, marketing manager at Tecknit. “Miniaturization, the use of higher frequencies, and designs that facilitate easy assembly are among the technological and economical trends affecting the EMI component industry. These factors create significant application challenges for the EMI component manufacturer and can prompt the need for customer design modifications to accommodate the required EMI shielding material.”
To avoid the cost of a separate metal shield, perhaps you can use a conductive plating or coating to improve the EMC performance of your product’s plastic case. If a shield is being used to improve susceptibility of sensitive circuitry, consider the capacitive loading effect of the shield. Leaving more space around circuits improves the situation at the expense of a larger shield and more PCB area.
Cooling and Shielding
Higher functionality often is accompanied by greater power dissipation. Even if the power remains the same in a next-generation product, it becomes more difficult to provide sufficient cooling in a smaller package size while still controlling EMC. One solution is lightweight, board-mounted shielding, perforated to allow airflow while reducing emissions from fast switching circuitry. If your design uses a fan, rather than deal with both a filter and a screen, you can use a one-piece shielded air filter.
“Even though the electronics have shrunk, the heat load actually can increase considering how much more can be packed into the enclosure,” said Dan Krupp, R&D engineer at Universal Air Filter. “Using enclosure ventilation systems leaves a gaping hole for EMI/RFI problems that could sink an attempt at full CE certification.”
If a gasket will be used to seal doors and covers, make sure the mechanical tolerances of your enclosure allow adequate gasket compression. Rated shielding effectiveness is achieved at a specified deformation—often 20% or 30%. However, too much compression not only may strain enclosure hinges and latches, but also may cause a permanent set of the gasket material which reduces long-term effectiveness.
Gasket types include all-metal, conductive yarn-over-elastomer, conductive elastomer, and composite metal/elastomer. Remember, if you can slip a business card between two parts of an enclosure that should contact each other, RF signals can radiate from the opening.
Because of the many trade-offs among the conflicting requirements of circuit performance, space, power, susceptibility and emissions limits, and cost, suppression and shielding must be treated as integral parts of the design process, not as afterthoughts.
References
1. Violette, M., “The 10 Basic Steps to Successful EMC Design, Part One: Steps One to Five,” EE-Evaluation Engineering, December 1997, pp. 66-74.
2. Violette, M., “The 10 Basic Steps to Successful EMC Design, Part 2: Steps 6 to 10,”
EE-Evaluation Engineering, January 1998, pp. 70-80.
3. O’Shea, P., “Squelch EMI Events With Shielding and Suppression Components,” EE-Evaluation Engineering, June 1997, pp. 64-76.
NOTE: These articles can be accessed on EE’s TestSite at www.nelsonpub.com/ee/. Select EE Achieves and use the key word search.
Suppression Products
SMT Feedthrough Components
Offer Tailored Performance
Both the standard range and the KNF range of SMT feedthrough capacitors feature controlled series-to-parallel inductance ratios and provide broadband HF attenuation. In systems with good grounds, the standard low series inductance feedthrough capacitor is recommended. For systems with less well-defined grounds, the KNF range with higher series inductance forms an effective LC filter. A separate range of feedthrough varistors ordinarily functions as capacitors and clamps high- voltage inputs up to 25 kV. AVX, (919) 878-6200.
Compliant EMI Gaskets
Compress With Low Force
The SOFT-SHIELD® 5000 Series of EMI gaskets requires less than 1 lb/in. closure force to achieve 40% deflection. Comprising a self-terminating conductive fabric jacket over polyurethane foam, the gaskets comply with UL 94V-0 flammability requirements. They provide >90 dB of shielding from 30 MHz to 1 GHz and >75 dB at 10 GHz and include rectangular, square, and D cross sections. Gaskets are offered in cut lengths, as peel-and-stick parts and die cuts, and as picture-frame and custom shapes. Chomerics/Parker Hannifin, (781) 939-4163.
Plastic Coating Material Is
Conductive, Resists Abrasion
CHO-SHIELD® 2052 Coating consists of silver-plated-copper particles dispersed in a one-component, acrylic resin system. The ready-to-spray coating provides EMI shielding on plastic enclosure materials such as polycarbonate, ABS, PC-ABS, Noryl, and PVC. It is stable at high and low temperatures, in high humidity, and in moderate salt fog environments. CHO-SHIELD 2052 can be applied with all commercially available technologies and meets the adhesion requirements of UL746. Chomerics/Parker Hannifin, (781) 939-4163.
Industrial Connectors Achieve
Metal-to-Metal EMI Barrier
The EMI Series Multipole Connectors have six to 24 contacts and provide 70 to 90 dB of EMI attenuation. The metal hood surrounding the connector plugs into a silver-plated lamellar insert in the base, forming a complete metal-to-metal shield. EMI-resistant cable glands shield the wire entry positions. In addition to EMI attenuation, the mated connector-base assembly supports IP65 wash-down and dust-tight resistance and has a 240-h salt-spray rating. Wieland Electric, (800) 943-5263.
Miniature Elastomer Gasket
Shields Thin-Wall Assemblies
The ElectroForm System automatically dispenses conductive elastomer directly onto metal or plastic electronic housings. EMI/RFI gaskets formed in this manner are 0.026″ to 0.071″ wide and 0.022″ to 0.060″ high, require 1.5 lb/in. closure force for 15% deflection, and seal against dust and moisture. They feature silver-plated aluminum or silver-plated copper conductive fillers. The gaskets provide >90 shielding at 100 MHz and have a volume resistivity of <0.01 W · cm. Instrument Specialties, (717) 424-8510.
Large Beryllium Copper
Gaskets Snap Into Place
Symmetrically-slotted gaskets address large-enclosure shielding applications including network servers, workstation towers, and central office equipment. They snap into small rectangular holes in material from 0.03″ to 0.09″ thick, and their solid top permits sliding contact without snagging. The gaskets provide up to 110 dB of shielding at 1 GHz and are available in standard and UltraSoft® low-compression versions in lengths of 16″, three fingers, and five fingers. Instrument Specialties, (717) 424-8510.
Compliant Knit Gasket
Is Soft and Nonabrading
Ultra-Soft Knit EMI Gaskets consist of close-knit metallized nylon yarn over a UL-94 HF-1 flame-retardant, low-density, polyester foam core. Closure force is 8 lb/ft for 30% deflection. The gaskets provide 85-dB shielding from 10 Hz to 1 GHz. At 20% deflection, the silver-plated or tin-plated gaskets provide surface resistivity of 0.051 W /in.2 or 0.341 W /in.2, respectively. The temperature range is -40°F to 225°F, and compression set is 10% at 50% deflection. Instrument Specialties, (717) 424-8510.
Bonded-Metal Gasket Offers
Low Friction and Resistance
Flexi-Shield EMI Gaskets feature a continuous metal spiral bonded to silicone tubes (low-force series) or cords. Benefits include low friction in sliding-contact applications. Stainless steel or beryllium-copper spirals with optional tin plating shield from 100 to 130 dB at 1 GHz. The 0.063″ to 0.125″ dia gasket material is available by the foot, cut to length, or formed into O-rings. Spira Manufacturing, (818) 764-8222.
SMT Ferrite Beads Have
High DC Current Rating
FBMH Series Chip Beads with current ratings of 2 A have impedances from 250 W to 1,600 W at 100 MHz. Parts rated to 4 A have lower high-frequency impedances. FBMJ beads have 4-A or 6-A ratings and impedances from 28 W to 110 W at 100 MHz. FBMH and FBMJ sizes range from 0603 through 1810 with higher current and higher impedance parts in the larger packages. The intended application is reduction of radiated and conducted HF noise on power supply lines. Taiyo Yuden (USA), (847) 925-0888.
Flexible Stick-On Gasket
Has Low Deflection Force
The TECKSOF 2000™ Series Gaskets consist of nickel-copper-metallized fabric wrapped over a UL 94 V-O-rated neoprene core. They provide 90-dB shielding to 100 MHz with 20% deflection. The operating temperature is -40°F to 122°F, surface resistivity is <1 W /sq, and compression set is 21% maximum at 25% deflection. Three cross sections are available: 0.250″ × 0.375″ rectangular, 0.140″ × 0.250″ D-shaped, and 0.125″ × 0.750″ flat. The gaskets have laminated adhesive strips and are supplied in 3’, 6’, or custom lengths. Tecknit EMI Shielding Products, (908) 272-5500.
Elastomer Combines Sealing
And Effective Shielding
Available in sheets, tubes, round, D, P, U, and rectangular styles, NC-Consil® Silicone Elastomer Gaskets typically provide 100-dB shielding at 1 GHz. The dark gray material has a nickel-coated graphite conductive filler. The recommended compression ranges are from 7% to 15% of the original height for rectangular strips and 20% to 60% for tubing and P shapes. Volume resistivity is 0.1 W •cm, and the temperature range is -55°C to 160°C. Tecknit EMI Shielding Products, (908) 272-5500.
Conductive Flexible Adhesive
Bonds, Seals, and Shields
TECKBOND™-NC is a silicone rubber filled with nickel-coated graphite particles for applications where the bond thickness is <0.016". Common uses are the bonding of silicone gaskets to metal. The cured material remains flexible, resilient, waterproof, and conductive from -55°C to 200°C. Volume resistivity is <0.5 W •cm. Joined parts generally can be used after 72 hours, but curing time varies with humidity for this RTV moisture-cured compound. Tecknit EMI Shielding Products, (908) 272-5500.
Modular Jacks Have
Integral Filter and Shield
A ferrite housing in the PCRJ-1 Series of RJ-45, -14, and -11 size filtered jacks provides 45-dB common-mode attenuation and >100-kW impedance at 100 MHz while passing differential signals up to 50 MHz without attenuation. The PCRJ-2 Series offers 20-dB attenuation over a broader frequency range with 1-kW impedance at 100 MHz. Differential signals up to 150 MHz pass without attenuation. A one-piece metal shield increases EMI and ESD protection. Coilcraft, (847) 639-6400.
Thin-Film Termination
Replaces 43 Components
The Super 1284 is an SMT thin-film-on-silicon network that uses P/Active™ technology to provide filtering and termination for the IEEE 1284 parallel port. The circuit integrates 25 resistors and 17 capacitors and has output-pin ESD protection to 8 kV with respect to the human body model. Significant board area is saved by the 28-pin QSOP package, and reliability is similar to that of an IC. California Micro Devices, (408) 263-3214.
SMT Filter Combines Ferrite
And Dielectric Materials
The LFA20 Series of SMT 0805-size filters has cutoff frequencies of 10, 22, 47, 100, and 220 MHz. Made from a composite of ferrite and dielectric materials, the nonpolarized filters are equivalent to a ladder-type circuit with fast roll-off and steep insertion-loss characteristics. Ratings include a maximum of 25 V and 100 mA, temperature from -40°C to 85°C, and a maximum 6-W DC resistance. The filters are available in tape-and-reel packaging. MMC Electronics America, (847) 577-0200.
Toroidal Chokes Have
Small Footprint
Current ratings from 0.3 to 10 A and inductances from 0.7 to 100 mH are features of the RN 142, RN 143, RN 152, and RN 242 families of chokes for use in DC-DC converters and switch-mode power supplies. The VDE- and UL-approved PC-mounting ferrite-core chokes suppress 100 kHz to 3 MHz common-mode interference, but pass differential-mode signals from DC to 1 kHz. Dual current-compensated windings prevent core saturation when handling high peak currents. Schaffner EMC, (800) 367-5566.
DC/DC Converter Chokes
Reduce Switching Noise
A series of self-leaded SMT common-mode chokes has 0.225- to 1.590-mH inductance and a -30°C to 120°C temperature range. It handles 1.22-A load current with 200-mW resistance to 14 A with 8 mW resistance. Maximum attenuation occurs between 2 and 3 MHz for most models. Package sizes range from 1.4-gm Polecat 0.5″ × 0.5″ × 0.215″ to 14-gm BigFoot 1.22″ × 1.00″ × 0.50″. Pulse Engineering, (619) 674-8100.
Air Filter Enhances
Cooling and Shielding
Aluminum-framed Dual EMI Air Filters support a UL 94 HF-1 flame-retardant, open-cell, polyurethane foam material between metal mesh panels. The filters have been tested in accordance with MIL-STD-285 and are certified to deliver greater than 30-dB attenuation from 10 kHz to 1 GHz. Options include a clear chromate coating of all aluminum components to improve shielding. Universal Air Filter, (800) 541-3478.
Cylindrical Filters Retain
Loss at High Frequencies
A series of 0.625″ dia × 1.875″ to 1.5″ dia × 3.75″ EMI/RFI power filters meets MIL-STD-220 insertion loss and MIL-F-15733 environmental requirements. Insertion loss is 45 to 90 dB from 1 MHz to 1 GHz. Current ratings are 0.1 to 20 A from 200 to 600 VDC over the -55°C to 85°C temperature range. LC filters with the L input at the threaded end or at the opposite end and symmetrical PI filters are available. Rtron, (847) 679-7180.
Filters Address Office
And Factory Applications
Single-stage, single-phase, chassis-mount F.AI and F.AM Series LC Filters support 1-A to 40-A load currents. The two-stage F.AK and F.AR Series handle 0.5 A to 20 A with higher attenuation. The F.AS Series adds symmetrical differential-mode inductance to suppress switch-mode power supply noise. The F.xx Series products are designed for 250 VAC to 400 Hz and meet VDE 0565-3, SEV 1055, UL 1283, and CSA 22.2 No. 8 standards. Arcotronics, (011) 44 1327-351515.
Filters Have High Current
And Attenuation Ratings
The tin-plated SN Series of three-phase, multistage filters handles load currents from 8 A to 300 A in 480/520-V, 50/60-Hz applications. Over 90-dB insertion loss and 77-mA maximum leakage current are achieved in the 39 × 390 × 210 mm, 30-A model. In-house-supplied self-healing X and Y capacitors are approved to UL, VDE, SEMKO, NEMKO, and EN132400 standards. Connections are via terminal blocks. Optional threaded terminals are available on the 16-A to the 100-A models. Arcotronics, (011) 44 1327-351515.
Shields Allow Access
While Reducing Noise
CBS Style Circuit-Board Component Shields comprise an inverted tray-shaped top plugged into a board-mounted fence with formed clips at regular intervals. The sizes of the tin-plated steel or phosphor-bronze shields range from 0.5″ × 0.5″ to 6″ × 24″ and heights from 0.13″ to 2″. The standard tin-plated steel thickness is 0.015″. Options include perforations for cooling, dimples for increased retention, custom shapes, and access holes. Leader Tech, (813) 855-6921.
Gasket Shields and Spaces
Close-Tolerance Assemblies
Shielding effectiveness greater than 80 dB over the frequency range of 100 MHz to 10 GHz is provided by a range of custom-designed, molded, conductive, elastomer EMI gaskets. With the silver/copper-filled silicone elastomer’s formulation, dimensional tolerances within ±0.005″ can be achieved. Gaskets can be supplied with an integral, rigid support that eases installation with cross sections as small as 0.040″ × 0.040″. Holes, locator pins, and conductive adhesive mounting are design options. Custom Designed Shielding (CDS), (978) 388-8406.
Copyright 1999 Nelson Publishing Inc.
January 1999