Conformity Magazine - December 2008 - (Page 38)

• The coating resistance can be specified, either in ohms per square or point-to-point ohms resistance. In this case, plating or paint thickness need not be specified. • Coating adhesion is commonly measured according to the ASTM D3359 standard. This standard is based on tape test and a 1-5 scale for the amount of coating removed during the tape test (5 is no metal removed, and 1 is complete removal of the plating). Cost Trade-offs of Alternative EMI Shielding System The recurring unit cost and upfront non-recurring expense (NRE) for tooling and fixturing for the different EMI shielding systems are highly dependent on part design, size and complexity. If plating all surfaces of a part is feasible, that will have the lowest unit cost and NRE. Selective plating and conductive paint costs can vary based on complexity of the part design and pattern for the coating. Table 5 outlines basic cost considerations for each option but, to obtain an accurate cost for the EMI shielding application, the requirements must be analyzed, and the processing details, production cost and tooling/fixturing cost need to be defined for the specific part. Also, as is the case in most industries, unit volumes have sizable impact on unit cost. Shielding Requirements and Shielding Effectiveness Designing for EMI shielding is best done up front, early in the product design cycle. Designing the electronics, the plastic enclosure and EMI shielding system simultaneously as opposed to sequentially can reduce the time-to-market as well as production cost. The basic EMI shielding principal, also known as the Faraday Cage Principal, is well understood; however, applying the principal to electronic equipment design can present challenges to engineers. Ultimately, compliance to regulations as well as performance in the end use environment needs to be confirmed with fully assembled systems being subjected to EMI testing in a lab. A few guidelines for EMI shielding coatings can assist the engineer in specifying a coating that will meet the requirements. EMI can be divided into two broad categories, magnetic and electric. Magnetic interference is generally viewed as less than 30 MHz frequency, while electric interference is 30 MHz and above. To shield longer wavelength magnetic interference, thicker coatings that include magnetic materials such as nickel are preferred. For electric interference, thin conductive coatings comprised of copper or silver typically perform best. In many cases, the conductive metal thickness can be as thin as 1 micron for applications with frequency >30 MHz. All-Over Plating Generally lowest unit cost and NRE option Autocatalytic Selective Plating Moderate unit cost and NRE due to 2-step process (coat with catalyst and plate) 2-Shot Molding Highest NRE due to mold tool complexity to mold 2-shot part design. Plating cost low (similar to allover plating) Peelable Plating Resist Cost to apply & remove resist can be high depending on masking complexity. Plating cost low (similar to all-over plating) Conductive Paints Moderate unit cost and NRE. 1-step process to apply paint. Paint material cost can be high, which can make parts with large surface area costly. Table 5: Cost considerations for various shielding options Attenuation (dB) Coating System All-Over Plating Selective Plating Copper Paint Thickness 40-400 µ” (1.0-10.0 µm) 80-200 µ” (2.0-5.0 µm) 0.001-.0015” (0.025-.375 mm) (ohms/square) 30 MHZ Resistivity 5-50 25-100 25-100 15-50 15-50 90 77 65 78 70 100 MHz 108 73 63 73 71 300 MHz 104 71 59 72 70 1 GHz 120 71 70 69 62 5 GHz 113 60 81 85 70 10 GHz 87 63 63 82 70 Copper-Silver 0.0008-.001” Paint (0.020-.25 mm) Silver Paint 0.0005-.001” (0.0125-.025 mm) Table 6: EMI shielding effectiveness of conductive coatings 38 Conformity DeCember 2008

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Conformity Magazine - December 2008 Contents Editor's Note FCC Warns Regulatory Fee Payers About Potential "Phishing" Commission Prowls the Web to Enforce Its Regulations First DTV Rollout Initiated in Wilmington, NC New Standards List Released for the EU's Directive on the Safety of Toys ESD Association Call for Papers ESDA. JEDEC Announce Standards Development Cooperation Commission Releases Data on Local Telephone Competition The Future of ESD Testing ESD Open Forum Emerging Issues in Standards: China RoHS: One Year and Counting Challenges in Testing: Revisiting 10-Meter and 3-Meter Radiated Emission Measurements Design Fundamentals: Meeting the Shielding Challenges of Injection-Molded Plastics Focus On...Microwave Absorbers Buyer's Guide FDA Recalls Automated External Defibrillators FDA Revises Guidance on Medical Device Tracking Guidance Document Safety of Passive Implants Released FDA Offers Guidance on Marketing Diagnostic Ultrasound Systems EU Commission Issues Correction on Machinery Directive Standards Other CPSC Actions in the News Product Reviews IEC Standards Update UL Standards Update Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - December 2008

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