Whether you’ve been tasked with sealing an enclosure around electronics, or you’re a highly enthusiastic mechanical engineer and really curious about why electromagnetic compatibility (EMC) matters, this article is for you. It covers the following aspects:
- The methods of how devices can be effectively sealed against electromagnetic interference (EMI).
- Strategies to rethink the problem from a systems level to create compliant devices.
Let’s start with some jargon:
- Electromagnetic Interference (EMI) is the disturbance (usually undesired) caused by an external source effecting an electrical circuit (e.g., within a medical device) by electromagnetic induction, electrostatic coupling, or conduction. The effect from EMI may degrade device performance or even stop it from functioning.
- This leads us to Electromagnetic Compatibility (EMC), which is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment, while also ensuring that they don’t negatively affect equipment in their proximity. Engineers accomplish this by limiting the unintentional generation, propagation, and reception of EM energy, which can lead to EMI.
- Electromagnetic Shielding (Shielding) is the practice of reducing the electromagnetic field with barriers of conductive and/or magnetic materials, whether as a covering on cables, sheet metal cover on PCB components, or enclosure and gaskets to shut in/out disturbance of and to a system.
- Electrostatic Discharge (ESD) is the sudden flow of electricity between two electrically charged objects caused by contact or a dielectric breakdown. While ESD is unrelated to EMI, some EMI components can double as protection for ESD by shunting excess charges away from sensitive, covered components.
- Emissions are when EM radiation is emitted from individual components or from the device itself.
- Immunity is the ability for a device to resist external EM radiation.
Systems Level Considerations
After reviewing the definitions above, you may see why EMI considerations matter in design. But what are the acceptable thresholds? This all depends on the region for which you are designing. The IEC-61000 family of standards are generally used, but a careful review of regulations for a given region is merited before design. FDA’s perspective on EMC can be found here. For pre-scanning methods that can help engineers get closer to EMC before expensive external testing (which takes weeks), please see my colleague’s first and second blogs on the topic.
Mechanical Design Considerations
EMI can be addressed first by electrical engineers (EE) in their designs through using component-level strategies of mitigating emission and immunity. These include PCBA-integrated solutions such as grounding planes and cages to cover sensitive components. For cabling, this includes annular shielding layers and clip-on ferrites. After these steps, in a general case, the system will still require shielding to protect from external sources. This is where the mechanical engineer becomes involved. Deriving the requirements for shielding from an EE, the mechanical engineer can work toward sealing for EMC.
Requirements can involve:
- Acceptable gap width (between or within parts).
- Ground interface point (for earth or ground reference within system).
- Reliability (e.g., can the shielding be scratched and defeated?).
- Seal/enclosure conductivity levels.
- Serviceability (for getting into internal components and/or retrofitting shields upon damage).
- Thermal considerations.
If your EE colleague needs guidance on acceptable gap geometry and width, Laird Technologies’s Gary Fenical provides guidelines in his white paper.
Many EM shields are made of sheet metals. Considering process capability, sheet metal forming does well at both low and high volume from a cost and dimensional tolerance perspective. This helps because early-stage testing can identify design improvements and confirm assumptions. The potential for enclosures to be carried through to the late stage and product level device is generally high. When used in external environments, sheet metal enclosures also come with the added benefit of being robust to external physical forces and wear over time.
An example of sheet metal being used both for mechanical protection and to mitigate EMI is with desktop computers, where cases are made from powder-coated mild steels and aluminum. When placed onto a PCB directly, shields can be fixed with soldered through holes, clips, fasteners, soldered pads, or even heat stakes (if on a plastic subsurface).
Many types of materials can be leveraged for EMI shields, including:
- Copper alloys (e.g., nickel silver alloy 770, copper beryllium).
- Steel: zinc/tin plating.
- Stainless steel.
- Aluminum alloys (e.g., silver-aluminum per MIL-DTL-83528).
- Nickel alloys.
- Silver-coating on a variety of alloys.
If a device is enclosed within a plastic casing, there are ways to leverage the plastic case itself as a means to meet EMC requirements. By adding a conductive coating to the plastics after fabrication, the effect can be an EMI opaque shield. This is done using several methods: vacuum metallization, arc and flame spraying, conductive film application, or plating. Both external or internal surfaces of components can be coated, but it is best applied to internal surfaces owing to external surfaces’ potential to be worn through and the need for the EM shield’s surface to be continuous with known gap width thresholds. If the process can be successfully employed, an integrated shielding solution can be achieved without adding additional components, such as sheet metal, to an assembly.
A third solution is the use of conductive fingers, meshes, films, tapes, and gaskets. These can be manually applied as part of device assembly, and are especially useful for larger devices where part interfaces are less reliably defined. For early prototype testing, these methods are modular and can be used to patch EM gaps as they are detected, with the learnings being pushed to future device prototypes.
All part-to-part interfaces will experience some gap. This can be closed off through several methods. If the parts are sheet metal, part interfaces can be designed to create controlled gap widths according to the maximum allowable width. These can be non-fastening features or separate components, such as tabs, fingers, domes, and gaskets. Fastening methods can also be leveraged to seal, including welding/soldering/brazing, rivets, PEMs/screws, conductive epoxies, and formed seams.
Gaskets designed for EM-sealing purposes are made up of a variety of materials. Some are woven conductive threads (usually nickel-coated copper) over foams, allowing them to be compliant and squeezed between part interfaces. Others are copper-alloy fingers, allowing for compatibility through the fingers springing when compressed and maintaining contact across a gap. Another option is an elastomeric material filled with conductive fiber or particulate, which conducts at certain levels when compressed an amount prescribed by the manufacturer. Others still are metalized or metal foam, integrating the conductive properties into the foam itself. An added benefit of these components is that they are able to accommodate gaps within or between components when required by the design, whether for parts that may shift during use (e.g., thermal expansion/contraction, forces from being handled, etc.), or from vibration.
One of the most reliable and manufacturing friendly solutions is to create contact between parts with spring-like tabs integrated into the part features. This means parts can be reliably sealed and scaled at high volumes with no additional processes or components.
EMC should be considered first from an EE’s perspective, addressing concerns at the PCB level. MEs can get involved when enclosing the PCB or device. This is a collaborative effort. Both specialties should work through planning and execution. The ability to leverage a range of materials and components for EM enclosures allows engineers to make informed choices to meet the EMC requirements of a device. When considered at early stages, engineers can make EM an integral part of the design approach.