Appliance Design - January 2008 - (Page 45)

PLASTICS Source Absorption (Heat) Incident Field Absorption Reflection Transmission An illustration detailing absorption, reflection and transmission of electromagnetic emissions. Reflection Internal External Transmission Waves can be blocked from leaving the electronic device as well being blocked from entering the device. nutshell, the directive states that the devices are “secure in, and cause no harm to, the environment into which the device will be placed,” says Larry Rupprecht, Manager of Conductive Materials Development, RTP Co., Winona, Minn. Most regulations describe the maximum allowed level of emission within the frequency range from 30 MHz to 1 GHz, which is the range at which most electronic equipment operate. However, the frequency range used by electronics is expanding to higher frequency ranges such as 10 Ghz or even 40 Ghz, according to Eric Jiang, product manager of Faradex, SABIC Innovative Plastics, EMI Shielding Compounds, SABIC Innovative Plastics is formerly GE Plastics, with U.S. headquarters in Pittsfield, Mass. Designing an electrically quiet circuit board is a good practice for reducing emissions, but by itself does nothing to shield the elec- tronics to interference from other nearby or far-away systems. Unfortunately, one of the all-too common problems today is that engineers focus their attention on the control of emissions, while ignoring immunity, says Montauti. This is acceptable design practice when the product is limited to the American market, but it becomes a liability when the product is sold on the international market, he says. While metal parts block EMI inherently, plastic parts must rely on the addition of some conductive material to stop EMI from coming in and going out. The three basic means of achieving this include: 1. Embedding the raw polymer materials with conductive particles such as carbon, metallic fibers, or other conductive fillers, thereby turning the molded plastic part into a shield. 2. Installing a metal or metallized barrier component inside the molded plastic housing that conforms to its shape, or alternately installing the barrier only around the pertinent electronic circuit board. 3. Applying a thin conductive film to the inside of the plastic housing by painting, electroplating, or physical vapor deposition (PVD). Each of these various shielding methods have their pluses and minuses and tradeoffs may need to be made. What is optimal for one application may not be optimal for another, so designers need to understand how those pros and cons apply to their particular application. “The best way to reduce the level of emission is to aim for an optimal design of electronic device, including the design of circuits, placement of components, and filters,” says Jiang. “When these methods are not adequate to meet the requirements, an enclosure can be used such as a Faraday cage around the electronic device to shield against EMI.” A Faraday cage, which is also known as a Faraday shield, is a term for an enclosure formed by a metal or conduct- WaveZero's Form/Met EMI suppression process creates a form fitting shield that conforms to the shape of the housing. www.applianceDESIGN.com Designed to conform to the inner contours of a product's enclosure, the enclosure level shield from WaveZero encapsulates a PCB in a pseudo-Faraday cage. applianceDESIGN January 2008 45 http://www.appliancedesign.com

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Appliance Design - January 2008 Contents Editorial Shipments/Forecasts News Watch Makers of Commercial Refrigeration Equipment Turn to Variable-Speed Motors to Improve Energy Efficiency. New Motor Design and Control Scheme Makes Brushless Motors More Economically Feasible for a Wider Range of Applications. External Gas Molding, a Relatively New Technique, Solves Many of the Challenges Associated with the Injection Molding of Large Plastic Parts. Plastic Housings Offer Many Benefits: EMI Protection Isn't One of Them. But, New Materials and Techniques are Solving that Problem. Choosing the Optimal Strategy for Effective EMI Shielding Depends on Both Mechanical and Electrical Requirements Ethernet-Enabled Microcontroller Makes Networking Schemes Easier and More Cost Effective to Implement The Ever-Changing Array of Microcontroller Memory Options Makes it Imperative to Keep Abreast of What's Available to Determine the Optimal Choice for an Application. A Vast Range of Components and Finished Products will be Exhibited at the Country's Preeminent HVAC/R Show, AHR Expo, which is Scheduled for Jan. 22-24 in New York. Advertiser's Index Association Report: PRBA

Appliance Design - January 2008

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