EMC & Circuit Protection

Going plastic

reflection. Adjusting the blend of the two types of fillers enables conductive thermoplastics to

achieve SE greater than 85dB. This is comparable to the performance of a

An emerging class of conductive thermoplastic EMI-shielding materials promises cost-of-ownership savings for designers, according to Mark Carter

E

MI-aware product design is an essential discipline as product developers seek to satisfy market

demands and take advantage of technical innovations. Examples include consumer demand for feature- rich mobile handsets and other wireless devices using standards such as Wi-Fi and Bluetooth often require sensitive circuitry to be positioned close to RF emitters, which can cause interference. Important technical trends include the almost total reliance on switched-mode power supplies and voltage regulators, and the increasing adoption of class-D amplifiers in audio products ranging from personal media players to home theatre equipment. Both types of devices deliver advantages in terms of efficiency, performance and miniaturisation, but employ pulse- width modulation and hence can produce interfering harmonics at up to radio frequencies. In addition, compatibility standards

such as the EU’s Electromagnetic Compatibility (EMC) directive, as well as various industry-specific standards for equipment such as communication and networking systems specify various limits for EMI emissions and susceptibility. To satisfy these various demands, designers require effective techniques to prevent devices from emitting excessive EMI and to provide protection from potentially damaging external sources of EMI. A number of approaches are

available to product designers seeking

to manage the EMI performance of a new design. Sensitive circuitry can be physically positioned as far away as possible from an EMI source such as the RF circuitry of a mobile handset, for example. However, the benefits can be limited, particularly as consumers demand ultra-miniature devices packed with value-added features. It may simply be impossible to ensure adequate separation from the RF circuitry for this alone to provide meaningful protection.

Shielding techniques

Electromagnetic shielding is an effective EMI countermeasure, and can be used either to protect sensitive components from external radiation or to contain the emissions from sources such as RF or high-frequency switching circuitry. Two common approaches are to place a purpose-designed metal shield around the circuitry, or to apply a metallic coating to a plastic shield. The shield may be a dedicated component or may be an integral part of the product’s chassis or enclosure. The shield must also be grounded to provide proper EMI protection. Removing the battery cover of a

mobile phone can often reveal EMI shielding around the wireless components, which may be implemented as a large metal cover attached to the printed circuit board using tamper-proof screws or similar fixings. On the other hand, devices that are required to operate in extremely electrically noisy environments, such as an automotive electronic-control unit (ECU), which can be susceptible to alternator noise as well as switching noise from the multitude of systems now present even in mid-market

vehicles, are often completely enclosed inside a metal case. Metal shields are typically produced

by casting or pressing. Casting allows designers to create complex multi- compartment geometries, and thereby separate analogue and digital circuitry within a single housing. However, tooling costs can be relatively high, and constraints imposed by casting processes can limit designers’ options and lengthen the development cycle. The high tooling costs, combined with long lead times, are also unsuitable for building small production runs and prototype batches. A pressed shield can be easier to design and produce, but complex shapes or multi-compartment designs can require a number of separate sub-assemblies. Alternatively, a metallic coating can

be applied to a plastic housing. A number of processes are available to achieve this, such as applying conductive paint loaded with metallic particles. An alternative is to use electroless plating to deposit metallic copper over a base layer applied to the plastic. A nickel protective layer may then be added. The coating can be applied selectively to specific areas, or generally to an entire moulding. Metallised plastic enclosures can provide high levels of Shielding Effectiveness (SE), but the thickness of the metallisation must be closely controlled. For this reason, plating processes typically call for specialised competencies that moulding companies often do not retain in- house. Hence, metallisation not only increases cost owing to the additional process but can also complicate the supply chain and contribute to longer lead times.

Cost of ownership

Conductive thermoplastic is a newer technology that enables the costs associated with plating or coating processes to be eliminated. Commercially available materials,

such as Chomerics’ PREMIER, are engineered for stable electrical, mechanical and physical performance. The base PC/ABS thermoplastic polymer alloy is mixed with conductive fillers comprising nickel- plated carbon fibres and nickel-plated graphite powder. The inherent material properties of both nickel and carbon result in high energy dissipation and create a material that is paramagnetic, enabling the moulding to provide shielding by absorption as well as

14 April 2010

Components in Electronics

metallised plastic shield. When used with standard injection-

moulding processes, a proprietary dispersion technology ensures even dispersal of filler particles throughout the moulding’s geometry. This avoids resin-rich areas, which are prone to EMI leaks, and also brittle resin-poor areas that can break under mechanical stress. The carbon fibres have the additional advantage of being able to bend and flow around and into cavity details without clogging or breaking. For applications that are weight

sensitive, such as in handheld devices or vehicle electronics, the thermoplastic’s density is less than half that of aluminium and less than one- quarter that of steel. This, combined with the ability to mould walls as thin as 2mm (or sometimes less, depending on the grade of material used), enables designers to achieve significant weight savings compared to a metal casting or stamped shield. The thermoplastic can usually be connected directly to a suitable ground. Design considerations are based

around shielding of the 'noisy' components. The first consideration is to select the optimum material grade for the application, taking into account structural and environmental properties as well as attenuation. Best practice, generally, is to avoid thin-wall sections, although some localised areas can be made smaller depending on the component size and shape. This can be verified using a simple moldflow and structural analysis. Other best-practice design guidelines include using thicker sections around problematic areas, building-in overlap between interconnecting walls, and leaving space for gaskets if required. The general properties of conductive

thermoplastics are similar to those of a glass-filled polymer, depending on the material specification. PREMIER PEI-140 allows mouldings to be designed according to similar parameters as 20% glass-filled Ultem. The dispersion and flow properties allow tooling designs using draft angles of around 1.5 - 2.0°. A minimum wall thickness of 2mm is recommended for mouldings using PEI- 140. Drawn by the prospect of valuable

savings in cost of ownership, as well as reduced lead-times and performance enhancements, designers are specifying various grades of PREMIER conductive thermoplastics into an increasing variety of applications worldwide.

Chomerics | www.chomerics.com

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