Designers of modern electronic products face many difficult issues in seeking to provide consumers with an attractive yet fully functional and reliable product. Designers must evaluate and select chips using the latest technology and optimize their placement on a printed circuit board (PCB). Mechanical components must be designed around the PCB to properly hold the board and allow consumer interaction while maintaining an aesthetically pleasing external appearance.

Designers also make certain assumptions about the need for EMI shielding. For instance, they can assume that their electronic product will pass EMC testing without the assistance of EMI shielding, or they can assume that EMC can only be achieved with the aid of EMI shielding. In the latter case, the internal surface of the plastic housing can be coated with a conductive paint or plated with a metal layer, or metal cans can be placed on the PCB.

From a manufacturing viewpoint, painting a plastic housing results in additional expense because plastic housings must be packaged and shipped to a painting vendor, then repackaged and shipped back to the assembly line. Often, painted plastic parts require a conductive gasket or conductive internal frame to ensure the continuity of the shielding structure.

Metal cans are popular for containing EMI, but their use must be taken into consideration during the initial design of the PCB. It is common to see a cellular telephone PCB with five or more metal cans, adding considerable weight to the mobile device. PCB design development and repair can be an issue with a soldered metal can because removal is problematic and many expensive chips may be damaged during can removal. In any event, even with high yields, repair costs for tightly integrated small devices with expensive components are significant.

The recovery and recycling of electronic product waste is rapidly becoming a major issue for designers and manufacturers. The European Commission Directive on Waste from Electrical and Electronic Equipment (WEEE) is significant and should be noted in the context of coming requirements for OEMs. The recycling of painted plastic housings causes additional expense, as the removal of conductive paint or metal plating from plastic is expensive. From an environmental viewpoint, the release of volatile organic compounds (VOCs), a natural byproduct of painting, is not desirable and is increasingly unacceptable to societies around the world.

Vacuum-Metalized Shields

The Insertible Shield from Shielding for Electronics (Sunnyvale, CA) is an example of a vacuum-metalized thermoformable structure (VMTS) composed of a polymer film substrate (typically 0.13–0.38 mm) that is first formed into the desired shape to facilitate its use within an electronics device, and then vacuum metalized with an aluminum layer (typically 1–6 µm, 99.9% pure). The process and design of vacuum-metalized thermoformable shields are patented. See Figure 1 for an example of an enclosure-level EMI shield for a business office network device. See Figures 2 and 3 for PCB-level EMI shields. See Figure 4 for an example of component-level shielding that can replace metal cans.

Figure 1. Enclosure-level EMI shielding with VMTS.

Figure 2. PCB-level EMI shielding with VMTS.

Figure 3. Board-level EMI shields.

Figure 4. Component-level shielding for a cell phone.

Vacuum metalization after thermoforming is required to ensure that the resulting conductive layer remains contiguous and fully effective as an EMI shield. In experiments done by first vacuum metalizing and then thermoforming, it was discovered that the metal layer severely cracked, losing its usefulness as an EMI shield. The thermoforming process results in high strains in localized areas of the substrate. These strains, which can be excess of 200%, easily exceed the tensile strain capability of the pure aluminum layer, estimated at 0.8%.

There is another reason for thermoforming before metalizing, and that reason relates to economics. Once the substrate is thermoformed, it must be die-cut to separate the part from the film sheet material. The material remaining after the parts are removed (known as the scrim) is returned to the film extruder; therefore, no waste is created from this part of the process. If the film material were metalized before thermoforming and die cutting, the scrim would constitute a waste material with a significant manufactured cost content. Therefore, from an economic viewpoint, metalizing after thermoforming is preferred.

The vacuum-metalized thermoformed shield may be properly grounded to the PCB's ground plane by one of the following methods:

Cooling can be provided by ventilation holes located on opposite sides of the shield. When the holes are properly sized, they can allow adequate airflow without impairing EMC. In general, a VMTS can be easily adapted to include heat sinks and other forms of cooling mechanisms and designs (e.g., heat pipes, ducts, etc.).

Shielding Effectiveness

Two major manufacturers of cellular telephone systems provided their cellular telephones with and without EMI shielding. In both cases, a replacement VMTS was designed, manufactured, installed, and tested. In the first test (see Figure 5), the increased (i.e., the delta) shielding effectiveness was determined. (FCC testing standards for radiated emissions were used.) The frequency range above 1700 MHz corresponds to the transmitting and receiving frequencies of the cellular telephone. The tests showed that the one-sided metalized VMTS provided 25 dB of increased shielding effectiveness, whereas the two-sided metalized VMTS provided 30 dB of improvement.

Figure 5. Shielding effectiveness of VMTS on a cell phone.

In the second test (see Figure 6), the existing shield provided a 40% reduction in radiated-emissions field strength. Tests A-1 through A-5 were conducted for various substrate materials, processing parameters, and assembly details. (As requested by the OEM, exact details of the shields are confidential.) The improvement ranged from 10% to 50%.

Figure 6. Electric field strength reduction of VMTS on cell phone, with original shielding normalized to 0%.

For most applications in mobile electronics, a shield that is metalized on only one side is adequate for shielding at higher frequencies (>500 MHz). In typical applications, the E-field shielding effectiveness is 45 dB at low frequencies (30 MHz), rising to 65 dB at higher frequencies. For enhanced shielding effectiveness, the shield can be metalized on both sides to create a double shield. The total shielding effectiveness of the double shield is much less than double that of a single shield; however, at frequencies of 100 MHz, there is an overall improvement in shielding effectiveness of over 3 dB, and at higher frequencies, the shielding effectiveness increases to more than 20 dB. This can be quite a significant improvement for marginal electronic devices.

The shielding effectiveness of vacuum-metalized aluminum was first determined by testing metalized resin injection-molded plastic substrates.¹ In these tests, comparisons were made between vacuum deposition and conductive painting using both silver and aluminum materials. A flat injection-molded plate was used as the substrate in all cases. Figure 7 shows the comparison between vacuum-deposited aluminum and a commercially available brand of conductive silver paint applied in accordance with the manufacturer's instructions. The final thickness of each conductive coating was not measured. Both specimens had a surface resistivity of approximately 0.03 W/square . The figure shows that the shielding effectiveness was virtually the same between the two samples.

Figure 7. Shielding effectiveness comparison.

Conclusion

Vacuum-metalized thermoformable shields can replace conductive paint on plastic enclosures and metal cans on circuit boards. The material allows the plastic housing to remain easily recyclable. The performance of the basic material is enhanced by the ability of the thermoformed structure to form tight seams that limit EMI. By designing in the EMC solution, the designer can take maximum advantage of this new technology.

Reference

1. Daryl Gerke, "Summary Report, Vacuum Deposition Shielding Effectiveness Tests," Kimmel Gerke Associates Ltd., March 1990.