January 19, 2015
PFC exhibited at the RSNA (Radiological Society of North America) conference in Chicago the first week in December. As you can imagine, MRI and Xray equipment manufacturers require designs with EMI in mind, so many of the prospects visiting the PFC booth were interested in some of the new technologies used in shielding. Based on the interest from the RSNA conference, we decided to do a special web feature on EMI shielding. Don’t miss reading the end of the article where some of the newer shielding techniques are discussed.
What is EMI?
Electromagnetic radiation that adversely affects circuit performance is generally termed EMI, or electromagnetic interference. Many types of electronic circuits are susceptible to EMI and must be shielded to ensure proper performance. Conversely, emissions radiating from sources inside electronic equipment may threaten circuits within the same or nearby equipment. To protect the performance integrity of electronic equipment, electromagnetic emissions from commercial equipment must not exceed levels set by the FCC, VDE and other organizations. Further standards set EMI levels to which electronic equipment must itself be immune.
What is EMI shielding?
Shielding is the use of conductive materials to reduce radiated EMI by reflection and/or absorption. Shielding can be applied to different areas of the electronic package from equipment enclosures to individual circuit boards or devices. Effective placement of shielding causes an abrupt discontinuity in the path of electromagnetic waves. At low frequencies, most of the wave energy is reflected from a shield’s surface, while a smaller portion is absorbed. At higher frequencies, absorption generally predominates. Shielding performance is a function of the properties and configuration of the shielding material (conductivity, permeability and thickness), the frequency, and distance from the source to the shield.
What does grounding have to do with EMI shielding?
Grounding issues affect both safety and EMI emissions. Conductive components are grounded to protect equipment users from electric shock. If a system is properly grounded, all conductive elements which a user might touch are theoretically at zero potential. Shielding against EMI emissions is commonly provided by a conductive enclosure. The separate parts of the enclosure must be electrically bonded together and grounded for the shielding to work. Disruptions in the conductive continuity between parts adversely affect shielding performance. Proper grounding of FPCs & PCBs and shielding enclosure components is also a method for reducing board-generated EMI. However, improper or ineffective grounding may actually increase EMI emission levels, with the ground itself becoming a major radiating source
Shielding requirements for commercial electronics generally range from 40 to 60 dB. Finding a system’s overall shielding needs involves determining the radiated emission spectrum of the equipment, and the specifications the unit must meet (e.g. FCC Part 15).
Where is EMI shielding required?
Uses for EMI shielding abound in computers, medical devices, telecommunications, and many other types of electronic equipment. As new emission and protection requirements are placed on these devices, the importance of shielding grows.
Shielding methods using Flex Circuits
Shielding a flex circuit can be accomplished through multiple methods. Shields are designed and used for EMI and ESD considerations as well as signal integrity methodology.
PFC recommends and provides four different types of shielding methods:
- Copper clad– adding additional copper layers to the circuit- the least flexible method.
- 2. Copper cross hatch– adding additional copper layers and etching them to create a cross hatch design which allows more flexibility than standard Copper clad layers.
Benefit- More flexible than pure copper layers.
3. Silver paste/Epoxy– achieved by applying a silver paste over the outer layers of a circuit. The Kapton cover coat has holes. Silver paste is sprayed on the covercoat- and the silver paste drains down the holes and makes contact with the copper ground inside the circuit. This technique allows a slightly more flexible design than the copper clad solution mentioned above.
Benefit: more flexible. Downside: higher cost
- EMI Thermoplastic Shielding film- This technology is the newest and most flexible shielding technique. The ultra-thin, ultra-flexible materials were developed for the cell phone market. The manufacturing process has been dramatically improved making it only slightly more expensive than older technologies. PFC has seen a spike in applications using the Thermoplastic Shielding Films.
Material stack of the thermoplastic shielding film
Benefit: Most flexible. Downside: Potentially most expensive solution
Shielding – Design for Manufacturing
PFC can and have provided all the above shielding methods. The important decisions for shielding are flexibility and cost. These two factors need to be weighed. Early involvement/engagement with your flex manufacturer is essential.