Edited by David S. Hotter

Gary Shawhan
Market Development Manager
Enthone-OMI Inc.
New Haven, Conn.

Plastics have quickly become the material of choice for electronic enclosures because they can easily and inexpensively form complex shapes and consolidate parts in devices such as laptop computers, cellular phones, and televisions, to name a few. However, since plastics are electrical insulators and transparent to electromagnetic radiation, take extra precautions to shield interior components from electromagnetic interference (EMI). To protect electronics housed in plastics, use conductive coatings such as platings, paints, and vacuum- deposited metals.

Conductive paint and plating are the principal coatings used today. Paints are made conductive with pigments containing silver, nickel/silver, nickel, or silvered copper. These paints are sprayed onto plastic parts manually, in a semiautomated production setup, or will full robotics. Electroless plating coats parts by immersing plastic parts in a defined sequence of chemical process steps. The final steps involve electroless copper and electroless nickel plating. Electroless plating does not require electrical current to deposit metal on the plastic part. Instead, it chemically deposits metal uniformly onto all activated surfaces of a part. Electroless plating can be done selectively or completely cover over all surfaces of the plastic part.

Conductive EMI coatings offer excellent conductivity with good adhesion, durability, and environmental resistance. Proper application of these processes provides excellent uniformity at a low cost. Too often, however, coatings become an afterthought with enclosure designs. To get the most performance out of conductive coatings, components should be designed with the constraints of the plating process in mind. By following basic molding practices, engineers can seamlessly integrate the advantages of injection molding with their choice of conductive coatings.

Joints and seams: Without a continuous conductive path between overlapping walls and mating surfaces, slot antennas develop letting high-frequency electromagnetic fields leak through joints and seams.

Slot antennas also form when molded enclosures are warped in any way. Radio-frequency energy leaks from gaps that approach onehalf the shortest wavelength (highest frequency) of the fundamental operating frequency or the component’s highest harmonic frequency. The effects of leakage should be considered up to the seventh harmonic. By using tighter tolerances, electroless plating prevents slot-antenna formation by thoroughly coating enclosures and recessed areas with a conductive coating, while at the same time permitting tight, continuous joints. For maximum shielding, use the following guidelines:

• Use tongue and groove instead of butt joints to boost EMI shielding by increasing contact area and minimizing gaps between mating parts.

• Design grooves with rounded edges instead of V-shapes. Vshaped grooves are potential sites for stress concentrations and, therefore, may create discontinuities in coatings.

• Maintain electrical contact between mating surfaces to a maximum distance of l/6. For example, for a frequency of 1 GHz, the maximum distance should be 2 in.

Fastening: Injection-molded plastic enclosures use snap-fits and contact fingers in place of traditional fastening methods such as screws. Besides making assembly and disassembly easier, these fasteners must also provide a path to conduct electrical charge. Electroless plating and robotically applied conductive paints work well for complex-shaped fasteners because they reach surfaces regardless of the relative shape or size and withstand exposure to extreme temperatures and humidity.

Coatings alone, however, don’t guarantee EMI protection. Properly selected conductive-coating processes provide the durability and wear resistance to maintain continuity when snap fits are used. The electroless nickel outer layer provides a hard, durable finish that bonds strongly to plastic and stands up to this constant pressure. It also resists scratches, abrasion, and fretting wear from assembling and disassembling housings.

Ribs, bosses, and standoffs: Ribs, bosses, and standoffs strengthen enclosures and function as mounting sites for subassemblies. Complex designs such as four-sided standoffs, notched ribs, and hollowed bosses are a challenge for EMI shielding. Complex geometries with recesses and cavities can be difficult to paint without the aid of automation or robotics. Electroless plating provides very uniform coverage in such featured areas.

Vents and apertures: Vents designed into plastic enclosures help cool internal electronics, while apertures provide access for cables and connectors. If not designed properly, each of these features may leak EMI.

The key to properly shielding enclosures around openings is controlling the size and depth of the opening in relation to the operating frequency of the device. The relationship is the basis for an EMI shielding technique known as the wave-guide effect (See Calculating cutoff frequency) which can either support or attenuate the EMI.

When designing vents and apertures, use the following guidelines:

• To avoid antenna effects and attenuate EMI, make sure the distance across an opening is less than half of the signal wavelength (typical values fall in the range from 1⁄4 to 1⁄5 the expected wavelength).

• Use a greater number of small openings rather than one large opening. It is better to have several rows of smaller vents than one row of large vents.

• Space vents 0.25 in. from each other and leave 0.5 in. between rows.

• Frequency requirements for both emitted and incoming EMI must be considered for enclosure designs. Leakage potential increases as frequencies increase (or wavelength decreases), which reduces the allowable discontinuity length.

• Leakage also depends on EMI wave impedance and direction. It is important to define whether a wave is predominantly electric, magnetic, or planar, as well as the distance between the radiating source and the opening.

Grounding: With faster processing speeds, today’s electronics produce electronic fields that build and fade rapidly, which can lead to interference. Adequate grounding is necessary to control the electromagnetic charges. More compact designs, as well as the increased use of electronic devices, also make field interactions a concern.

Engineers commonly rely on fasteners to create a common ground between mating parts. To maximize contact between two parts, use bosses to enclose screw threads. This helps increase clamping forces as well as protecting fasteners from oxidation.

Coating requirements for grounds are similar to those for snap fits and contact fingers. Maintaining surface conductivity is the key to effective shielding. If the coating’s conductivity decreases due to oxidation or corrosion, resistance increases across mating surfaces which will lead to EMI.

Ground designs range from relatively simple to fairly complex depending on the enclosure configuration. However, for more complex designs, electroless plating or automated paint systems are needed to provide uniform coating coverage. By using properly applied conductive coatings, extensive ground-plane strapping, clips, gaskets, and bonding straps can be reduced.

Gasketing: Use gaskets between mating parts, followed by the application of conductive coating to handle EMI. Uniform coverage of the conductive coating not only helps ground parts, but also eliminates the need for gaskets. The selection of a given coating process should take into consideration its ability to fill the narrow gaps left between mating parts.

CALCULATING CUTOFF FREQUENCY
The relationship between the size and depth of a gap, seam, or hole, and the device’s operating frequency affects a components ability to shield EMI. The relationship between these parameters is known as the wave-guide effect. Wave guides such as vents and holes have a cutoff frequency, vc, below which it becomes an attenuator, according to:
nc (Hz) = c/λc
where λc = 2 times the maximum dimension (for slots)
= 1.7 times the diameter (for holes) and
c = speed of light = 3.0 × 108 m/sec.

When the signal frequency is below the cutoff frequency of the wave guide — in other words the signal wavelength is longer than the wave-guide cutoff wavelength — the theoretical attenuation of the wave guide is:
SE (dB) = 27.3 d/w (for slots)
= 32.0 d/D (for holes)
where d = depth of the opening
w = width of the opening
D = diameter of the hole

For example, a 0.25-in.-diameter hole has a cutoff frequency of 2.8 GHz and a corresponding cutoff wavelength of 0.43 in. A 1-in. slot has a v = 600 MHz and a λ = 2 in.

To preventing EMI leaking from vents and apertures, use the following guidelines:
λ< 2d — radiation passes freely
λ = 2d — cutoff frequency, no EMI shielding
λ> 2d — effective shielding
Multiple vent spacing should be at least ½λ (λ/2 > d).

© 2010 Penton Media, Inc.