Isolation from electromagnetic interference is one of the key requirements for PCB designers. They employ various proven techniques, such as using dedicated ground vias, ground planes, and galvanic isolation, to reduce the impact of EMI on the PCB. Although these techniques may work for some applications, more is needed for applications that demand high levels of EMI isolation. This is when PCB designers consider multi-cavity solutions with high levels of EMI isolation from cavity to cavity. There are a few solutions that will do the job. Two of these solutions are a machined aluminum cover with a dispensed Form-In-Place (FIP) conductive gasket, and the other is a SnapShot® board-level EMI shield. How do these solutions work? What is the best solution for your application? Read this post to learn the answers.
Introduction to Form-in-Place Gaskets and SnapShot® Board Level EMI Shields
Understanding form-in-place EMI gaskets and SnapShot® Board-Level EMI shields will help you better appreciate the benefits of these solutions. This section offers an overview of these solutions.
- Form-in-Place Gaskets : These gaskets are created by dispensing a polymer compound through a needle on the lid. FIP conductive gaskets are used in applications where implementing traditional gasketing methods can be a challenge These gaskets help achieve small form factor, excellent adhesion, high shielding effectiveness, and complex geometries.
- SnapShot® Board Level EMI Shield : These multi-cavity shields, developed by XGR Technologies, feature a non-conductive, polyetherimide inner surface and conductive, tin-plated outer surface that assures superior shielding performance in a low-profile and ultra-lightweight solution. These board-level shields were introduced in 2002 and have been successfully implemented in military radios, avionics, drones, GPS devices, and many more.
Form-in-Place Gaskets vs. SnapShot® Board Level EMI Shields: What Do You Prefer
Compared to a traditional frame and lid style solution, form-in-place gaskets and SnapShot® Board Level EMI shields provide the same level of isolation from cavity-to-cavity as from the cavity to the external environment. Beyond this consideration, there are several other factors to consider when choosing machined metal covers with FIP vs. a SnapShot® shield. These include:
- Weight and Space : A machined metal cover with an FIP gasket can contribute to the PCB weight. The cover comprises most of the weight; however, adding the FIP gasket and the hardware required to affix the cover and compress the gasket are considerable weight adders. These together require significant board real estate and height. In contrast, the SnapShot® shielding technology is based on a very thin, thermoformed, extremely lightweight polymer. The attachment mechanism is inherent to the populated PCB and requires a very narrow trace width. This combination offers significant weight and space savings that can fit multiple orders of magnitude vs. a machined aluminum cover.
- Complexity and Cost : A machined cover with FIP gasket is a complex and costly solution. The cover is machined and the gasket is formed on the finished surface. This requires special tooling and advanced equipment. The process of gasket formation is lengthy and complex, and one has to consider several aspects, such as cure time, gasket material shelf life, and program optimization, to minimize inconsistencies from the starts and stops of dispensing. Additionally, there are extra considerations for storage and handling to ensure no damage is done to the gasket. Finally, there is a BOM consideration for the additional mounting hardware and added assembly time for the installation of the hardware. This typically includes a torque operation to ensure proper compression force is applied to the gasket. All these factors make it a complicated and expensive solution for businesses working in constrained budget. In contrast, the SnapShot® solution is rugged and simple. There are no special handling or storage requirements and the process for installation is a simple press operation. When you tally the simple supply chain, fewer components and straightforward press installation, it is easy to see the complexity and cost benefits of the SnapShot® shield solution.
- Shielding Effectiveness : For a cover with FIP, one must consider the inherent inconsistency of the shielding effectiveness this solution will realize.
The two factors that contribute to inconsistency are the initial deposition quality and the removal/reinstallation of the shield. As discussed previously, the dispensing process is complicated and plays a large role in determining the unit-to-unit isolation consistency. The deposition speed, start and stop locations, quality, and other dispensing parameters all contribute to the consistency of the gasket thickness at the interface. Inconsistencies in thickness lead to inconsistent compression characteristics, which, in turn, affect the final conductivity of the filled gasket. This is a significant consideration for determining the overall isolation capability of the system. As this can be variable, the resulting unit-to-unit isolation will also be variable. If the application is meant to allow for the removal and reinstallation of the shield, compression set of the gasket and the tendency of the gasket to stick to the mating surface of the PCB must be considered. Both conditions can damage the gasket and negatively alter its performance following a reinstallation. The SnapShot® EMI shields use redundant and durable mechanical interference connections at each SMT-mounted solder sphere located around the perimeter of the shielded cavity. This creates a consistent interconnect and, therefore, consistent isolation capability from unit to unit. Isolation levels do not change when a shield is removed and replaced. Overall, the SnapShot® shielding solution provides the most consistent isolation for initial and repeated installations.
In conclusion, SnapShot® board level shielding technology is a very attractive alternative to a machined aluminum cover with a FIP gasket. SnapShot® shields are significantly light weight and consume far less board real estate in the x, y, and z directions. SnapShot® shields are also far less complex and much less costly. Finally, the shielding effectiveness will be more consistent from device to device and over the lifetime of the device. Contact us today to discuss your project requirements. Our experts will guide you to the right solution.
