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Conductive coatings are becoming increasingly popular for the control of EMI in plastic enclosures. For esthetics, cost, and weight considerations, manufacturers of electronic equipment like to use plastic enclosures. Unfortunately, plastic provides no shielding unless measures are taken to provide a conductive surface - most commonly, conductive coatings. But the benefits of conductive coatings are not without their downsides. Let’s look at the various conductive coatings and finish up with some words on how to make the shield work.

Coating Types
The demands for plastic coatings are quite different from the coatings on metal. Coatings for metal enclosures are intended primarily for corrosion protection. Conductivity need not be real high, as the base metal provides more than enough conductivity for most applications. All you really need is enough conductivity for current to get through to the base metal. With metal coatings for plastic, however, the coating carries all the current, so conductivity of the coating is very important.

The three popular conductive coatings in the electronics business today are electroless plating, vacuum plating, and conductive paints. In reviewing the literature over the years, we have found such a wide variance reported in the conductivity of one or the other coatings that we have to believe there have been substantial errors in the testing process. We have not had occasion to do our own testing, but our experience is that any of the coating types provides good shielding, plenty good enough for most commercial applications. Thus, the decision for selecting one over the other is usually made for reasons other than shielding effectiveness. We will outline some of the common issues below.

How much shielding is needed?
Shielding needs vary widely with the application, but generally, shielding effectiveness needs are:

Commercial - 30 dB (30x)
Military - 60 dB (1000x)
Special - 90 dB (30,000x)

In practice, the primary limitation in shielding effectiveness is with the penetrations (seams, holes, and wire penetrations). 30 dB of shielding is fairly easy to achieve, given some attention to the openings. Once you get to 60 dB, the need for closure is considerable, and you need to take a lot of care to make sure you don’t have leaks. 90 dB of shielding takes extreme care.

But the key point is that the shielding effectiveness of most enclosures is limited by breaches in the shield, that is, the holes and wire entries. There is no need to have a shield material with significantly greater effectiveness than that provided by the overall enclosure. Thus, if you need 30 dB of shielding, a 40 dB coating is sufficient, and will work about as well as an 80 dB coating.

Shielding Effectiveness
Resistivity of the coating is usually specified in ohms/square (for a given thickness, the resistance of a square of material is independent of the size of the square). For reasonably good conductors, reflective shielding effectiveness is approximately:

SE = 20*log(Zw/4*Zb),

where Zw is the impedance of the incident wave (377 ohms for a plane wave), Zb is the impedance of the barrier in ohms/square, and the shielding effectiveness, SE, is in dB.

As an example, a poor coat of conductive paint might have a resistance of 0.1 ohm/square. Plugging this into the shielding equation, we find that the shielding effectiveness is nearly 60 dB, more than adequate for most shielding applications.

This is an approximate relationship, and ignores the absorption effects (for thin coatings, absorptive shielding doesn’t become significant until quite high frequencies, anyway). Also note we haven’t accounted for skin depth, which diminishes at high frequencies, increasing the effective resistance and decreasing shielding effectiveness. Nevertheless, for comparison purposes, this relationship shows that you don’t need a real good conductor to get plenty of shielding.

Having said that, let’s look at the characteristics of the popular coatings.

Electroless Plating
Electroless plating has been the standard by which other coatings are judged. The coating is a pure metal, is thin and very uniform. The process starts with etching the plastic, then immersing it in a fluid, where the plating material, usually copper, bonds to the plastic. This is usually followed by an overcoat of nickel to provide corrosion protection. The result is a highly conductive, stable and durable surface.

This process is limited to those plastics that can be etched. This requirement is not highly restrictive, as etching can be done with good, readily available plastics, but there are plenty of plastics that can’t be etched. As it is an immersion process, it does not rely on line of sight for coating, a clear benefit where the plastic is molded in complex shapes, but not a significant factor in most common enclosures.

Masking requires an extra step, because it involves masking the area not to be coated, then spraying with a base coat, and then commencing with the coating. (We haven’t yet figured out how spraying the base coat avoids the line-of-sight concern, but that is a relatively minor issue.) Alternately, you can coat the entire enclosure, then paint over the finished areas; this is cheaper, but may not be a desirable approach.

This process is mostly associated with large volumes, like personal computers. This process is quite popular, accounting for upwards of half of the coating market.

Vacuum Plating
Vacuum plating starts with a treated plastic that is placed in a vacuum chamber where a metal, usually aluminum, is evaporated on to the plastic. This is essentially a line of sight process, but there is some back scatter, so coating does penetrate the nooks and crannies a little. But you still want to design your enclosure to minimize blind spots. The coating is not as uniform as electroless plating, but is usually adequate unless you have a complex shape. Vacuum plating can be done on a variety of plastics (except polyethylene or urethane), so you are relatively free to select the plastic of your choice.

Although we haven’t encountered significant problems with materials compatibility, aluminum is relatively vulnerable to galvanic corrosion - so you need to consider material compatibility if you are mating with a different metal, or protect with an overcoat. Aluminum tends to evaporate when placed in contact with certain metals, and the coating is so thin that it doesn’t take long to disappear completely at the mating surfaces.

Vacuum plating is well suited to small quantities and short turnaround. If you are in a hurry, you might be able to persuade your supplier to turn around your job in a few days, making it nice for prototyping. The market share for vacuum plating is small, but new uses are being developed.

Conductive Paints
Conductive paints, being mixed with a nonconductive carrier, have lower conductivity than electroless and vacuum plating and are not as good a conductor. They depend on lots of small conductors making contact with each other. The paints are applied in the same manner of common paints, using a spray gun or even a brush, and can be applied to just about any surface that will accept paint. Aerosol sprays are readily available, so coating is easy.

Just about any conductive metal can be used, most commonly copper, nickel, silver, and combinations thereof. Copper has excellent conductivity but is vulnerable to corrosion. Silver has excellent conductivity and corrosion characteristics, but is expensive. Nickel has relatively low conductivity, but has excellent corrosion resistance and leaves a fairly hard surface (not as hard as electroless plating, however), but the conductivity is still good enough for many applications. As shielding demands continue to increase, we are seeing less use of nickel in favor of silver covered copper and hybrids, to increase the conductivity while minimizing copper corrosion effects.

You can get conductive paints in gallon cans or in spray cans. This coating is ideal for after-the-fact fixing. Whether you are on the test floor or in the field and find yourself in need some shielding, you can haul out the spray can and quickly make a shield. Just a word of warning – the conductive materials are in suspension, and have a nasty habit of settling to the bottom of the can and staying there, even with vigorous shaking. We advise rotating and shaking the can periodically during storage. Be sure to check the conductivity of the coating after spraying, since it is not always certain that the conductor has made it out of the bottom of the spray can.

Which technique is best?
With all that said and done, we find that any of the approaches can be effective and that, in many applications, there is no clear EMI difference. Cell phones, for example, might use any of the three, depending on the manufacturer. Your primary considerations would be applicability to your base material, operating environment, corrosion protection, cost, quantities, and response time. In some cases, you will find a clear technical advantage of one over the other. In other cases (notably the cell phone), they can be used pretty much interchangeably, so cost becomes a driving issue.

Roughly speaking, the costs of conductive coatings are, from high to low:

Selective silver paint
Selective electroless copper/nickel
Electroless and electrolytic copper/nickel all sides (including paint outside)
Selective vacuum deposited aluminum
Vacuum deposited aluminum all sides (including paint outside)
Selective copper paint
Selective nickel paint

Clearly, if your shielding needs are minimal, nickel paint would be a good choice.

Where does the shield fail?
The key point here is that the overwhelming reason for shield failure in a plastic enclosure is due to reasons other than conductivity of the material. First and foremost, the principal causes of failure are due to openings in the shield, principally the seams, followed by holes. Let’s take a look at a typical scenario.

In a new design, you decide that the plastic box won’t meet your EMI needs; you need shielding, either to contain emissions or increase immunity. Sadly, you will probably end up with two situations. First is that addition of the shielding doesn’t solve your problem. This is quite probable, unless your packaging engineer is attuned to the needs of shielding. But, in all probability, the designer has provided a minimum of contact points, and a simple butt or lap joint, as shown in Figure 1. If you have passed that hurdle, your mechanical designer masks off the plastic with a generous gap, to make sure the unsightly conductive coating doesn’t show to the outside world. Of course, it is completely useless as a coating, but you have retained the cosmetic appearance.

Figure 1: Conductive coating does not mate

Figure 2: ESD to conductive coating

So you need to go back and make sure you have closed up the seams. While this may seem to be an easy task, remember you need contact between the mating surfaces at numerous points, and this only occurs where there is positive contact, such as is found at the fasteners or due to the springiness of a gasket or even the shielding material. Herein lies the problem with most plastic boxes; they are designed by packaging designers, who provide a few snap contact points and perhaps a few screws, but who don’t take steps to ensure continuous positive contact.

Shielding effectiveness for openings or slots is given by:

SE = 20*log (λ/2l),

Where l = wavelength in meters, l is longest dimension of opening in meters and SE is shielding effectiveness in dB.

At 1 GHz, your opening must be less than an inch, if you want to get any shielding at all. Obviously, this distresses the packaging designer.

Once you have fixed these deficiencies and passed your emission or radiated immunity test, you are done, right? Sadly, no. Now you need to revisit the ESD test. Where previously, you had little problem with ESD, you now have a conductive seam that the ESD can reach. So now you need to find a way to cope with the discharge directly to the conductive seam, which increases the need for shielding effectiveness, specifically closure of the seams. Otherwise, you need to recess the metalization far enough that discharge cannot occur.

The next set of problems to address is the penetrations, specifically the wires for power and signal. If they penetrate the shield without making contact with the metal coating, they will effectively bypass the shield. Interference currents on the wire will stay on the wire, penetrating the shield from either side without regard to the quality of the shield material. You need to intercept those currents by shunting them off to the shield, either by terminating the cable shield or the filter capacitor to the enclosure shield. Unlike a metal enclosure, where contact can be made on either side, contact with the conductive coating can usually only be made on the inner surface.

The final problem is the inadvertent breaches in the shield itself, caused by inadequate coating or breaks in the shield due to flexing. Deep recesses may get little or no coating material. Sharp inside corner conduction may break when the material is flexed (see Figure 3), including bosses, ribs, tongue and groove, or any internal sharp edges that would otherwise be inconsequential in unshielded enclosures.

Figure 3: Coating cracks when flexed; minimized with rounded corners

Designing for Coating Effectiveness
The box needs to be designed to accommodate the coating. Make no mistake about it. Packaging designers do not like the constraints shielding places on the enclosure design. You’ll need to bird dog them through the design process, or you’ll get to do it over again. The designer needs to be firmly convinced that shielding continuity is imperative, otherwise it doesn’t work. Here are a few guidelines that you can take to your packaging designer.

Conductive mating surfaces need to make contact at frequent intervals around the perimeter – plating must go right to the mating surface. Gasketing is preferred, so material flexing needs to be minimized, using ribs or flanges –
cell phones make a good model. If you can’t use gasketing, avoid butt or simple lap joints; tongue and groove joints are much better.
Opening dimensions need to be minimized. It is the longest dimension of the opening that is important, and usually this is a seam. Our rule is that the longest dimension should be less than 1/20 wavelength of the highest frequency of interest. At 1 GHz, this is about 1/2 inch.
Avoid sharp edges. Keep transitions smooth and use rounded corners with at least 1 mm radius. Keep plated surface smooth as possible.
Avoid deep recesses.

Other Shielding Techniques
Although this article has focused on coating techniques, there are two other plastic shielding techniques that should be mentioned, thermoformed inserts and conductive plastic.

Thermoformed inserts mate a conductive fabric or screen directly to thermoformed plastic during the molding process. Obviously, you need thermoform plastic for this to work. The outcome is a conductor laminated to the plastic. There are constraints to the shape to be molded (for example, compound curves cannot be readily handled). The conductive fabric approach hit the streets with a bang a few years ago and went nowhere. Now we see screen replacing the fabric. The advantage is that cost is quite low, if you can live with the form constraints.

Conductive plastic impregnates a conductive filament in the resin and is molded. The result is a moderately conductive plastic. The shielding effectiveness is good enough for many commercial applications, but there is another bigger problem - the surface conductivity is very low. In the process of molding, the conductive material sinks below the surface. If you measure the conductivity with a meter, you will get a high impedance reading. In order to reach the filaments, you need to scuff the surface with a coarse sandpaper to expose some of the filaments - even then, you need to have quite a large surface area in order to uncover enough metal to get adequate conduction for good shielding performance. In practice, the only real good way to get at the filaments is to drive a cleat directly into the plastic. This looks like an elegant solution, but we haven’t seen a good solution to the surface contact problem. The most common uses are larger surfaces where size is not a prime consideration.

Conclusions
All three common shielding coatings, electroless plating, vacuum plating, and conductive paints provide effective shielding. The choice will be application specific. Electroless plating and conductive paints dominate the market, but vacuum plating is increasing in popularity.

Whichever coating is selected, good enclosure design is key. Seams and holes need to be minimized and wire penetrations need to be bonded or filtered to the shield. Package design needs to be monitored closely.

Actually, the cell phone industry has pretty well handled the coated plastic issue. The key is to provide a stiff enough surface that it is feasible to insert a gasket and expect it to close. But cell phones are designed by radio engineers, and they know how to shield. Most analog and digital engineers could take some lessons from the radio engineers. n

William D. Kimmel, PE, and Daryl D. Gerke, PE, are founding partners of Kimmel Gerke Associates, Ltd., an engineering consulting firm specializing in EMI/EMC design, troubleshooting and technical training. They can be reached via their web site at www.emiguru.com.