Edited by David S. Hotter
Chris P. J. H. Borgmans
DSM Polymers-Research and Technology
Steven R. Gerteisen
DSM Engineering Plastics
How well an electromagnetic interference (EMI) shield protects electronic products from radiation largely depends on the materials used such as metal, conductive thermoplastic, or coated plastic. Other factors which are nearly as important include the product’s shape, thickness, relative position of the electronics, and the type of joints and holes the shield contains. Often, design decisions based on practical considerations such as size or available space, assembly, cooling, and mechanical and cosmetic features also influence these parameters.
As engineers continue to take advantage of the material and processing benefits of plastics, they also make it more challenging to shield electronics from EMI. While plastics may be easier to shape and help cut the weight of components, engineering resins alone can’t conduct electromagnetic waves away from electronic circuits. To form a conductive path for EM waves, engineers rely on conductive paint and plating, metallic inserts, and conductively compounded resins. Conductive resins offer the greatest design freedom because they don’t require secondary processing and aren’t limited by the capabilities of subsequent processing equipment such as paint-spray nozzles.
The first step in designing a conductive plastic part is to fully understand and define the product’s performance requirements. Next, calculate the amount of EMI attenuation needed so a suitable material may be selected. Size, location, mechanical function, and cosmetic requirements of the device help reduce the list of possible materials. But these are less critical and should be considered after the electromagnetic shielding needs have been satisfied.
Usually, the emission spectrum affecting the electronics is either known or can be measured. Subtracting the maximum allowable EMI (generally mandated by agencies such as FCC, VDE, EEC, or defined by performance requirements such as susceptibility limits) from the emission spectrum of the electronic source yields the frequency and amplitude of the interference needing attenuation.
How well a shield attenuates EMI depends on the wavelength of the radiation and the distance from the emission source. These factors determine whether the radiation approaches far-field conditions or is still in a near-field electric or near-field magnetic state when it reaches the shield. In extreme near-field conditions, when the EM radiation is almost entirely composed of either the electric or magnetic component, shielding effectiveness depends on the distance between emitter and shield. This is especially true for devices that preferentially attenuate either the electric or the magnetic component of the EM wave.
As the distance from the emitter increases (far field), the two components of the EM wave reach about the same magnitude. Beyond this point, shielding effectiveness is no longer affected by increased distance from the emitter, though the overall strength of the EM radiation will continue to decrease as a function of increasing distance.
The wave impedance is an important factor in determining shield effectiveness. The impedance (in ohms) is
Zw = E/H
where Zw is the wave impedance, E is the electric component, and H is the magnetic component of a plane electromagnetic wave. At far field, or equilibrium conditions Zw = 377 Ω. At near-field conditions for an electric dipole emitter Zw >> 377 Ω, and at near field for a magnetic dipole emitter Zw << 377 Ω.
The distance from the emission source at which far-field conditions are reached, R, is
R = λ/2 π
where l is the wavelength of the electromagnetic radiation. At short wavelengths (high frequencies), R is small and true far-field conditions are reached within a short distance from the source. At long wavelengths (low frequencies), R is larger and true far-field conditions are reached further from the source.
Shielding devices made from conductive plastics attenuate electric fields well and, therefore, are very effective in attenuating high impedance (Zw >> 377 Ω) EM radiation. However, conductive resins generally function poorly in attenuating low impedance (Zw << 377 Ω) EM radiation. It is also important to remember that wave impedance quickly reaches values greater than 100 Ω at distances appreciably below R.
In theory, thermoplastic resin can provide from 65 to 70 dB of electromagnetic interference attenuation or shielding effectiveness. However, in practice, it isn’t possible to attain EMI shielding effectiveness greater than 45 dB with conductive thermoplastic compounds. Shielding effectiveness, SE (in dB) is defined as:
SE = 10 logn (Pi/Pt)
where Pi is the energy of the incident wave and Pt that of the transmitted wave.
Choosing the right resin
Once the shielding requirements are understood, engineers must choose a conductive plastic resin to do the job. Shielding effectiveness can be translated directly into volume resistivity — a readily measurable material property — which can be related to the type and loading of conductive filler in the compound.
A nearly linear relationship exists between the volume resistivity and shielding effectiveness of compounded resins. The type of conductive filler used with resins also affects resistivity. The three most commonly used additives are carbon black, carbon fiber, and stainless-steel fibers.
Among the three fillers, stainless-steel fibers provide the lowest volume resistivity (highest conductivity) with the least amount of fiber loading. Using as little as 3% fiber content, stainless-steel-filled resins shield electronics from EMI. In contrast, carbon fibers and carbon black require 20% loading to achieve similar shielding effectiveness. However, carbon fibers have the added benefit of boosting material stiffness and strength, while carbon black is the least-expensive filler. One of the drawbacks of carbon (either black or fiber) is it makes it impossible to compound custom colors.
Of the three technologies, stainless steel fibers are generally preferred. The low fiber loading required for good conductivity (low volume resistivity) has the least effect on mechanical performance, letting the resins behave more like unreinforced than reinforced resins. This benefits designs using snap fits, living hinges, and molded-in custom colors, although some limitations still exist.
The conductivity of stainless-steel compounds depends on a network of fibers inside the molded part. The fibers are typically made from 302 stainless steel, with a diameter of 8 microns and an aspect ratio of about 750. Unlike glass or carbon, stainless-steel fibers are soft, which lets them disperse within the resin, forming an interconnected network.
Hole in one
A shielded enclosure usually has some type of joint or seam where the conductive-plastic parts mate. A typical clamshell enclosure, for example, contains a butt or overlap seam between the two halves. These joints or seams can lower the device’s shielding effectiveness as described by the circuit model. In this model, EM-induced currents flow freely through the conductive part. A joint or seam of higher resistivity than the bulk material restricts the free flow of EM induced currents and therefore reduces its effectiveness as a shield.
The model shows that current flow through the joint is determined by either the contact resistance or contact capacitance (depending on the frequency of the AC). At low frequencies, higher contact resistivity at the joint than in the bulk material inhibits current flow; however, at higher frequencies (>150-200 MHz), capacitance coupling across the mating surfaces makes the joint transparent to current flow.
With a low-volume percentage of conductive fibers, the resistivity of the joint is higher than that of the material on either side. In fact, the statistical probability of establishing good fiber-to-fiber contact at two mating surfaces is low. In reality the chances are even lower, because injection molding buries fibers below the surface leaving a resin-rich resistive skin that’s typically 20 microns thick. This skin is the reason why compounded resins typically display unexpectedly high surface resistivity.
Standard plastic-joint designs often don’t provide sufficient electrical contact to form across mating conductive pieces. This becomes a concern if the device needs to attenuate low-frequency EM radiation (<150-200 MHz). To lower the contact resistivity, engineers can modify designs using self-tapping screws, metal inserts, insert-molded pins, and sonic or vibration-welded seams.
The principle behind these techniques is simple: to form a conductive bridge between the bulk material in the two mating parts. In general, any method that breaks through the insulating skin and forms a stable contact to the stainless-steel network inside is effective.
Another less-effective, and often less-practical, design option would be to simply increase the contact surface area between mating parts. This would increase the probability of establishing electrical contact between fibers at the surface of the two halves.
Slots and holes used to ventilate or provide access to electronic devices are other design features that affect the shielding effectiveness of plastic enclosures, limiting the frequencies that can be contained. Holes and slots have two effects: they can be large enough for EM radiation to penetrate the shield unattenuated, or they can restrict the free flow of induced current in a shielding device.
Engineers can calculate the size of a hole that causes leakage, based on the wavelength of EM radiation. The wavelength of the highest frequency to be shielded can be assumed to be an approximation of the maximum hole size. In the case of restricting current flow, orienting a hole or slot in the direction of current flow has the least effect on current flow.
Design solutions for holes and slots are the same as would be applied to metallic shielding devices. Holes should be small enough to prevent unwanted frequencies from passing through the shield unattenuated. Slots can be oriented in the direction of current flow to minimize their impact. However, this applies only to cases where the induced current flow can be predicted, such as for shielding an emitter of known polarity and orientation.
|Design recommendations for EMI shielding with conductive plastic compounds |
1. Understand the nature of the EMI to be attenuated, plus its frequency, proximity, polarization, and strength.
2. Define the size and location limits of the device.
3. Determine the number and size of openings required to adequately cool the device or provide openings.
4. Select a filler that provides the best balance of conductivity and mechanical