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Shielding Materials Help Prevent Last-Minute Test Failures

Wed, 12/09/2009 - 12:17pm
David Garagnani, Application Engineer, W. L. Gore & Associates, Inc., www.gore.com

David-g-webElectronics packaging solutions are developed on many levels, from the board architecture all the way up to the mechanical enclosure. The individual elements that compose these solutions need to work together so that the device can pass compliance tests. While designing printed circuit boards for new electronic devices, engineers should consider electromagnetic interference (EMI) shielding solutions early. One of the main issues the packaging engineer encounters is that the individual components may pass EMI testing, but as the components are combined into subsystems or into the final product, the device under test (DUT) fails.

Signal harmonics and interactions among subsystems in the DUT can be difficult to foresee and often do not present themselves until final compliance testing is done prior to shipping the product. Any type of failure at this point is costly because it will result in product modification, delayed shipments, and potential customer dissatisfaction.

The challenge for the design/packaging engineer is to incorporate sufficient component-level isolation using shielding products that minimize crosstalk. While most engineers are familiar with strategies that reduce EMI issues at the board level — for example, adding extra ground planes, isolating power signals, and carefully locating components — these solutions become more difficult, and in some cases more costly, as new features are added and electronic devices get smaller. For example, when cameras were first added to cell phones, the module was on a separate board connected to the main board via flex circuit. Board-to-board isolation became the EMI challenge here.

To predict potential crosstalk and harmonic issues, engineers can turn to modeling software. Although EMI modeling software is based on method of moments and other widely accepted techniques, these models can be time-consuming to develop. Simple geometries quickly turn into complex models when small mechanical apertures (or discontinuities) are accounted for. These features of small-scale mechanical enclosures can have very large effects on overall EMI performance. At the system level, the use of models can be time-consuming and unreliable. Skilled technicians performing brute force testing is the typical route used in the industry today to determine an EMI solution.

Incorporating EMI shielding materials as part of the initial design is the most cost-effective way to prevent these last-minute issues during testing. For example, by assuming that you will be using surface mount technology (SMT) ground pads to protect the final product, you can incorporate ground pads at the outset, giving you the option during testing to evaluate whether you need a full complement or some subset of ground pads (Figure 1). On the other hand, if you encounter compliance issues as you are preparing to launch a new product, your options are more limited; however, there are solutions that can be incorporated without requiring a complete product redesign. One of our customers’ products was running at a very high frequency and needed cabinet-level shielding. By inserting a peel-and-stick shielding material in the cabinet’s seams, the customer was able to eliminate the slot antenna effects.

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The goal is to find the most cost-effective solution that fits the design envelope while minimizing unwanted interference. A wide variety of gasketing and shielding products are available on the market today. Selecting the best option for a particular application depends on a range of application-specific issues:

* materials selected for the covers and housings
* environmental factors
* product performance
* product volume
* installation technology

Material Selection
The type and thickness of an enclosure’s material can have a significant impact on potential EMI solutions. You want to construct a completely conductive Faraday cage around the device to prevent any energy transfer in or out. Enclosures usually serve this purpose for larger devices, but smaller devices — such as a cell phone or GPS — may have a separate board-level shield (BLS) within the enclosure to provide shielding.

Good design principles dictate using a housing material that resists corrosion. Two factors play into this consideration — the galvanic compatibility of materials that contact one another and the use of protective coatings. First, because dissimilar metals more readily react with moisture in the air, you should select materials that have the lowest potential voltage between them on the galvanic scale. Second, engineers often use a coating to protect metallic components like the housing. The best approach is to select a conductive coating for covers and enclosure components.
For example we have found that chromate coatings used over aluminum for corrosion control add a nonconductive layer that can prevent the substrate of the enclosure from engaging the gasket or mating cover. This insulative chromate layer defeats conductive sealing efforts and can contribute to adverse EMI effects. If you select a nonconductive coating for protection, then you should design the enclosure and cover such that the mating surfaces do not have any coating, if possible. Enclosure seams and mating surfaces can be masked off during the coating or painting process and exposed once the process is complete. If non-ideal coatings are necessary to protect the housing, you can use an interface material that will pierce the coating under pressure — such as a gasket that contains sharp particles of conductive media or oriented wire suspended in an elastomer matrix.

When selecting the cover or housing material, you should consider its mechanical strength. The cover needs to make full contact with the mating surface, compressing any gasket material and maintaining a Faraday shield. If the cover material is thin or flexible, such as plated plastic, gaps may occur in locations where the cover meets the circuit board or the enclosure (at points away from the screws, where the cover bows). These gaps can lead to slot antenna effects that can radiate EMI energy.

Another consideration is to select stable materials that do not degrade or exhibit compression set over time. Changes in the material have a direct impact on a component’s performance as an electrical seal. This is important for gaskets, ground pads, and other flexible components used to provide continuity between variable gaps. For filled-type gasket materials, you need to consider both the matrix that holds the conductive material and the filler used. The matrix needs to remain dimensionally stable so that the conductive filler can maintain its location and contact throughout the bulk material. After extensive accelerated life testing, Gore has found that using nickel or carbon in a PTFE matrix provides a reliable, long-lasting electrical seal.

In more challenging gasket formats, a high performance material’s conductivity can allow a smaller interface that still maintains the connection between the board and the cover. A smaller interface area allows more space for components or a smaller overall package. The smaller the gasket trace, the more challenging the shielding product is to handle and install accurately. So when selecting the material, you should consider the EMI solution’s format (size) as well as its installation method.

Environmental Factors
The environment in which the electronic device will be used is another area that should be carefully thought out when selecting a shielding solution. As the electronic industry moves in the direction of ruggedized products, environmental standards like Ingress Protection (IP) are considered. For example, IP65 is a common level of protection required for portable devices. Enclosures or devices meeting this IP level have basic splash and dust protection. Before deciding on a particular material set for shielding, be sure to determine whether the product needs to comply with a specific standard for such things as liquid immersion/splash, flame retardance, dust, acoustic performance, and service temperature. These are a few of the environmental needs that direct the type of shielding solutions you should consider.

Product Performance
Overall product performance is based on expected life cycle, electrical performance, and mechanical performance. When selecting the right EMI material, you need to consider whether the product will be used once, for five years, or indefinitely. Anticipated length of service can affect the end-product cost because more durable materials are generally more expensive.

When considering electrical performance for EMI materials and gaskets used to seal enclosures, manufacturers typically provide shielding effectiveness (SE) data. Much of this testing is ultimately derived from the MIL-DTL-83528 standard with modifications. Data from this testing method can vary depending on the vendor’s modifications to the standard. The trace width of the test specimen can have a large impact on system performance. SE really depends on a variety of application-specific factors, such as the housing material, the gasket material, gasket width, and the amount of compression or force applied to the gasket material. This and similar methods should only be considered a baseline performance comparison because these factors are unique to each electronic application. Therefore, it is important that you review the SE data for each gasket or shield completely, including the list of testing modifications noted by the manufacturer.

Ground pad performance is driven by several factors: surface area of the pad, the type of compression, the dwell time after compression (i.e., the amount of time required for the material to return to its original form), and the pad’s ability to make full contact with components of different heights. As the market moves toward using more sophisticated ground pads made of multiple materials and laminates, evaluating their electrical performance becomes more complicated. Ground pads made of homogeneous materials can be evaluated using the traditional volume resistivity equation1; however, this equation does not take into account the interactions among non-homogeneous materials used in some ground pads.

For example, homogeneity is lost once an adhesive layer is added to bond these materials to a substrate. For composite ground pads, calculating the Z-axis resistance at a target compression provides a better assessment of its electrical performance. This test measures the amount of resistance that passes through all of the layers from the top of the ground pad to the bottom, including any adhesives. At Gore, we use a micro-ohmmeter and four-point probe setup to determine resistance for these measurements (Figure 2). This procedure ensures accurate single-digit milliohm resistance measurements on small highly conductive material samples.

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Compression is the key to consistent electrical and mechanical performance of flexible EMI materials. It is essential that the material sufficiently engages adjoining parts to minimize surface contact resistance. Additionally some materials require a compressive force (i.e., the force required to deflect a material to operating thickness) to become fully conductive and ensure proper functionality of the EMI shielding design. Recovery (i.e., the distance that a material rebounds after compression) and softness are also important attributes. Materials need to recover both at the same rate as surrounding materials and to the same position to account for flex in a product’s housing. If this does not happen, then slot antennas can temporarily occur. If the material is not soft enough, then board flexing can occur. For example, an LCD screen can become distorted if the EMI gasket puts too much pressure on the display. At Gore, our testing indicates that soft materials with large working ranges perform best in these types of situations.
Product Volume
 
When selecting an EMI shield, you should also consider the quantity that you will need during the product’s manufacturing life. For example, if you are manufacturing over one million cell phones per year, you can consider designing a customized board-level shield specifically for your product. However, if you only need 75 gaskets for a specialized program, it would be more cost-effective to use readily available materials as long as they meet your compliance requirements. These standard materials could then be custom cut to fit your application.

The quantity needed also affects the gasket design when it comes to shape. Gaskets can be created in all shapes and sizes, but if only a small quantity is needed, it may be more cost-effective to combine several standard profiles/extrusions to form custom geometry rather than design a custom-shaped gasket. Large sizes and custom shapes also limit the type of materials that can be used in the manufacturing process. For example, if you need an intricate plan view shape, you should consider sheet products. High-performance materials, such as plated foams and filled elastomers, are good choices for intricate gaskets (Figure 3). Less complex geometries can be constructed from products like fabric-over-foam profiles.

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 The third issue related to product volume is the manufacturing process itself. Such issues as confidentiality constraints, product lead time, and total product cost can drive the selection of the best EMI shielding solution. If you work with highly proprietary technology, you may prefer to select a solution that can be implemented directly on your manufacturing line. Also, a gasket installed in-line “just in time” may be a viable option when evaluating the total cost of ownership. For example, consider a casting made at another location with form-in-place (FIP) applied by the casting manufacturer versus purchasing a gasket that can be installed on-site as part of your manufacturing process. In addition to accounting for lead time and possible repair, there are additional shipping costs associated with protecting a casting with an installed FIP gasket.
Installation Technology

ec911if100e-webOptions for installing EMI shielding materials have steadily evolved. Now more sophisticated and complex gaskets and shields can cost-effectively be installed automatically. Probably the most familiar alternative for automated installation is standard surface mount technology (SMT) equipment, which is already available at most electronic manufacturing sites. Another alternative is label machine technology, which can place adhesive-backed materials (Figure 4). This technology is ideal for installing a range of materials, from simple rectangular designs to intricate designs that have features like islands and multi-level gaskets. These automated technologies improve reliability, decrease labor costs, and allow smaller gaskets to be accurately placed with minimum operator contact.

 

Conclusion
Many factors affect the choice of the best EMI shielding materials for an application, including the materials used in the product’s enclosure, the environment in which the product will be used, the performance requirements for the shielding materials, the quantity of products to be manufactured, and the installation equipment preferred. Evaluating the need and type of EMI shielding materials as part of the initial design is the most cost-effective way to prevent last-minute failures during compliance testing.

 

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