It’s always challenging to design a product that is appropriate for an application and combines technical competence, ergonomic design, and an aesthetically pleasing appearance. Since the outer casing is the first thing seen, the appearance and feel convey immediate impressions of quality, strength, style, and function.
Generally, high-quality plastic enclosures can give products a pleasing appearance and ergonomic characteristics. A plastic enclosure, however, may be less attractive in situations where the amount of RFI generated by the internal circuitry requires some degree of attenuation.
The alternative is metal. While the intrinsic EMC properties are much better, there are drawbacks. Metal weighs more, complex shapes cannot be easily achieved, and it is not self-finished.
So must you choose between aesthetics and technical performance? Perhaps not. Stylish appearance and the requisite levels of EMC can be achieved by using screened plastic enclosures.
Many factors influence the EMC performance of a screened plastic enclosure: product design, method of screening, environment in which product will be used, required attenuation levels and frequency range, and cost. Because of the interrelationship of these factors, the best screening method is one that optimally accommodates the product’s intended application and operating environment.
Product Design
Screening performance will vary according to the design of the enclosure assembly. Is it clip-together or screw-together? Does it have front or rear panels? Does it have butt-together or tongue-and-groove joints? Are the moldings a loose or tight fit? Are the moldings detailed or complex?
Consider the product design relative to the anticipated screening method. Certain designs can increase or decrease screening effectiveness.
Screening Techniques
All methods of screening a plastic enclosure involve applying a conductive layer to its inner surfaces. Four different coating processes are commonly used, each with its strengths and weaknesses. Evaluating the attenuation resulting from each method helps you to select the most appropriate technique for the particular application. Copper/silver-loaded paint is suitable for the majority of applications requiring commercial levels of attenuation. For specialized or more demanding uses, other techniques can be applied as required.
The four main screening methods are:
Vacuum deposition.
Loaded paints.
Zinc arc spray.
Electroless plating.
Each method demonstrates some beneficial characteristics and some potential drawbacks. Table 1 summarizes various characteristics of the different screening processes applicable to plastic enclosures.
Vacuum Deposition
In vacuum deposition, a metal (normally aluminum) is melted in a vacuum chamber. The molten metal droplets sputter onto the surface of enclosures in the chamber, gradually building up a continuous metallic layer.
This process is ideal when very thin coatings are required over complex and detailed moldings. It is less effective if used on an enclosure with poorly fitting joints because the thin layer exhibits no gap-filling properties, making electrical continuity difficult to maintain.
RF leakage around joints and seams is a very real problem. Tooling costs are high because only very precise masks and fixtures will ensure that the metal is applied to only the inner surfaces of the enclosure.
Loaded Paints
Using loaded paints is the most cost-effective and efficient method. But the attenuation levels, although acceptable for commercial applications, are not as high as those achieved by some other techniques. A silicone-rubber electroform mask shields the external surfaces, and the metallic-loaded paints are applied using a wet spray process on either an automated or manual paint line. A finished coating thickness of 50 to 75 microns is applied to the inner surfaces of the enclosure, giving good coverage without obliterating fine details.
Zinc Arc Spray
Zinc arc spray results in a relatively thick layer over the molding surface. It is very effective in applications where a high magnetic field is expected. Attenuation in the H-field is good because of the direct relationship between material thickness and signal absorption. This process involves elevated temperatures so it is most suitable for use with polycarbonate enclosures that are better able to withstand the high temperatures involved. The thickness of the coating also is a drawback if fine details are present in the enclosure molding.
Electroless Plating
Electroless plating produces the best performance, but it is very expensive and not particularly suitable for high-volume applications. Masking is very difficult. If the enclosure is not properly masked, the screening material is deposited on both the internal and external surfaces. Then, a secondary finishing operation is required to remove the excess material or apply an overpainted finish.
Since colors are intrinsic to the materials used in plastic enclosures, the secondary operation is a major disadvantage. It is possible to selectively mask the target, but it also is very complicated and costly. This method can be justified only for applications where high attenuation levels are the overriding consideration.
Environmental Considerations
Because a product line may be used in many different environments, choose coatings with the most significant environmental factors in mind. Zinc arc spray and electroless plating provide good corrosion resistance. Vacuum deposition can better withstand humid conditions. In some cases, the adhesive qualities of the coating—to prevent coating degradation during the product’s final assembly or premature reductions in shielding effectiveness during use—may be the overriding consideration.
Attenuation Levels
Shielding performance will vary according to the product’s design, the screening material, and the application technique. While the enclosure manufacturer can specify the attenuation levels achievable by the product, this figure is irrelevant when specification limits are expressed as field strengths. You must consider the source strength and frequency range of the electromagnetic radiation generated by the circuitry housed by the enclosure before specifying the required attenuation levels.
Figure 1 shows the variation of attenuation levels on plastic enclosures shielded by different coating techniques. These results were achieved under test conditions in our in-house test facility.
Costs
Higher cost does not necessarily mean better performance for the intended application. Coatings vary in cost according to the materials used, the process complexity, and the batch quantity. Examine the overall product cost, the desired technical performance, and the application for the product.
The interrelationship of these factors and the wide variety of product applications make it impossible to develop fixed rules for the best screening method. For now, work with experts in the field who can offer independent advice—supported when possible by quantitative results from actual testing—to develop the best solution for a particular shielding problem.
About the Authors
George Ross is the marketing director at VERO Electronics. He has worked in the electromechanical industry for 20 years, 13 spent in design and engineering environments. Mr. Ross holds a Scottish National Certificate in mechanical engineering and a Scottish National Certificate in electronic engineering.
Richard Grant is the market development manager for subracks and enclosures at VERO Electronics Ltd., U.K. A 12-year veteran of the company, Mr. Grant received degrees in electronics and communications engineering and a diploma from the Chartered Institute of Marketing.
VERO Electronics, 5 Sterling Dr., Wallingford, CT 06492, (203) 949-1100.
Table 1.
|
COST/PROCESS |
Mechanical Stability |
Attenuation |
Sheet Resistivity |
Corrosion Resistance |
Adhesion |
Suitability For Complex Shapes |
Coating Uniformity |
||||||
|
High Cost |
|||||||||||||
|
Vacuum Deposition |
Average/ Good |
Good |
Low |
Good |
Average/ Good |
Average/Good |
Good |
||||||
|
Medium Cost |
|||||||||||||
|
Electroless Plating |
Good |
Good/ Excellent |
Low |
Good |
Good |
Very Good |
Good |
||||||
|
Zinc Arc Spray |
Good |
Excellent |
Low |
Good |
Good |
Average |
Poor |
||||||
|
Low Cost |
|||||||||||||
|
Loaded Plastics |
Poor |
Poor |
High |
Good |
N/A |
Poor |
N/A |
||||||
|
Copper/Silver Paint |
Good |
Average/ Good |
Low |
Average/ Good |
Good |
Average/Good |
Poor |
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Copyright 1998 Nelson Publishing Inc.
September 1998