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Grounding and shielding methods in electronics: Pros and cons of CPSAs and other techniques
Electronic devices of all types are becoming smaller and more powerful. They function at higher frequencies and components are often designed much closer together in dense packages. As a result, engineers all over the world are searching for more effective and efficient materials for well-functioning electrical systems.
Here is a look at some of the most common, along with some advanced grounding and shielding techniques
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Mechanical fasteners
Fasteners such as metal screws have been used to ground components and seal metal cages for many years.
Pros: Metal fasteners for grounding and shielding provide excellent mechanical strength and can be reliable for directing electrical current.
Cons: Mechanical fasteners can be susceptible to corrosion and vibration. They require thicker metal structures for stability, and drilling of tapped holes – which may not always be possible on small components or under tight space constraints. Metal fasteners also do not readily allow for access in cases like shield can lids, making repairs and upgrades a hassle.
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Soldering
This process of flowing melted filler metal into joints has been used for decades to establish mechanically stable, conductive bonds between components. Soldering has been practical to date because the melting point of solder is traditionally cooler than that of the surrounding material.
Pros: Solder can contain optimal conductive metals like tin, silver and copper, making it easy and efficient to create consistent ground planes across electrical systems.
Cons: The latest lead-free solder can have a melting point as high as 220°C (428° F). This is not optimal for sensitive electronics. Soldered joints are also permanent, making rework inconvenient and potentially costly.
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Spring pins
Usually made from spring steel, spring pins combine compression and radial force to hold two components together. When compressed, tension builds within the pin. When the tension is released, it applies force between components to help prevent shear, slippage and vibration.
Pros: Spring pins are a good choice for applications requiring electrical contact at a lower force vs. permanent attachment methods. They do not usually require any additional hardware, and they can be removed and replaced relatively easily.
Cons: Spring pins are not generally used for applications requiring high adhesion. They are more susceptible to axial force vs. other methods. Also, they are generally soldered onto the board at one end (see soldering above).
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Electrically conductive foams
Foams embedded with conductive materials are highly compressible,
Pros: Conductive foams are lightweight alternatives to mechanical fasteners like screws and bolts. Their high compressibility makes them ideal for filling wide and irregular gaps – particular challenges for grounding and shielding. Foam gasket material is often laminated to conductive adhesive for strong and consistent grounding paths, as well as effective EMI shielding at bond lines. Foams can also be used with pick and place and other automated assembly equipment.
Cons: Foams can be more sensitive to chemicals and other harsh environments and can degrade over time.
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Evaporative metal coatings
Technicians apply a material such as liquid metal to the board itself. This material, for example silver paste, contains a solvent which evaporates when the metal is heated. This leaves a very thin, even conductive layer on the surface being coated.
Pros: This technique is good for grounding and shielding smaller-scale, board-level components. It can provide an extremely uniform film coating for consistent performance, and it can create very strong adhesion to the substrate.
Cons: Evaporative coating requires very specialized equipment. It can therefore be a slow and costly process, especially for low-frequency devices requiring thicker metal and multiple coatings. Evaporative coatings also do not always accept add-on solutions such as adhesive foils.
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Bonding adhesives
Generally comprised of two-part liquid epoxies, these conductive adhesives are usually mixed and applied using specialized dispensing equipment and cured either using heat or at room temperature.
Pros: Unlike mechanical fasteners, adhesives can create a solid bond across the entire contact surface. They also form permanent bonds at much cooler temperatures vs. soldering.
Cons: Epoxies require precise mixing and application, as even tiny bond lines can allow for EMI leakage in today’s small and powerful electronics – especially at high frequencies.
Conductive pressure-sensitive adhesives (CPSAs)
The use of CPSAs is becoming increasingly common in such electronic applications as a grounding and EMI shielding material. CPSAs are generally supplied as tapes or laminated to a conductive foam to make a conductive gasket. 3M has been developing CPSA materials since the 1980s.
Pros: Pros: CPSAs are thinner and more lightweight vs. mechanical fasteners. Supplied in tape format, they are very simple to use – peel-and-stick application means they don’t require high temperatures or special skills and equipment. CPSAs can provide high shielding effectiveness and reliable grounding in small spaces. They can be used in automatic assembly equipment as well. Acrylic adhesives are common for CPSAs; 3M has introduced a polyolefin CPSA that delivers the same benefits as acrylic with a longer shelf life. Learn more about 3M™ Electrically Conductive Single-Sided Tape 5113SFT here.
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More about CPSAs from 3M: offered as adhesive transfer tapes for ultra-conformable designs, single-coated tapes for shielding & covering, and double-coated tapes for grounding two substrates and shielding in the bond line gap. Fabric and foil tapes with various conductive filler types are available in multiple product constructions to meet different design requirements. These CPSA’s can be easily die-cut, enabling the tape to make a grounding path with contact areas that may not be possible with fasteners or even solder. CPSAs are also available applied to gasket material which delivers increased conductivity with compression – an excellent alternative to spring pins or elastomers where gaps vary across bond lines.
Conductive metal fillers in the adhesive help reinforce grounding and prevent EMI leakage through the bond lines. Fillers are available in various metals including nickel, silver and gold among others, and are dispersed throughout the adhesive for high conductivity. This makes them more effective than standard electrically conductive tapes for grounding and EMI control vs. standard electrical tape applications.
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Another advantage of CPSAs is that they can be used relatively late in the design process. Engineers can quickly optimize device performance without adding more fasteners (and the extra weight and space) or solder joints. They can also help avoid costly rework or even the need to redesign the entire system because of EMI or stray current.
3M is an industry leader in the development of electrically conductive tapes with conductive pressure-sensitive adhesives. We have conductive tapes and gaskets with a broad range of backings and conductive fillers.
3M can help engineers choose the right CPSA for their specific applications. For more information and to contact a 3M expert, visit our EMI Shielding and Grounding page.
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