Flex Circuit Shielding Design Options
In December of 2014 I visited the Radiological Society of North America (RSNA) conference in Chicago. As you can imagine, MRI and X-ray equipment filled the convention center. Attending the show gave me the opportunity to speak to radiology designers. Since the equipment they are designing “radiates” with waves of electrons, the underlying electronics have to be super protected for fear of interference. When discussing flex designs, EMI and shielding circuits is the number one design concern.
Shielding may not be your company’s number one design concern when thinking about your interconnect designs. But if you have to shield circuits for EMI, then you will need to depend on your supplier to assist you with their favorite shielding technique and experience.
The Basics: What is EMI and Do Flex Circuits Radiate?
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. Shielding requirements for commercial electronics generally range from 40–60 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).
And yes, flex circuits will radiate.
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.
Flex shielding methodology
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. Here are some common and not so common practices for shielding a flex circuit.
- Copper-clad—adding additional copper layers to the circuit—the least flexible method.
Figure 1: Very thick outside copper layers supressing radiation.
- 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.
Figure 2: Outer layer cross hatch for EMI suppression. Benefit: More flexible than pure copper layers.
- Silver paste/epoxy—achieved by applying a conductive paste over the outer layers of a circuit. The Kapton® covercoat 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.
Figure 3: Silver paste screened over a coverlay showing the via stitches down to a ground line—another coverlay goes on top to protect the silver.
- EMI Thermoplastic Shielding film—latest technique. 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 the older technologies. North American flex shops who use this material are pushing their customers in this direction. It is the thinnest and most flexible. It is easy to apply and creates a 360 degree shield similar to a silver paste or stitched copper via.
Figure 4: Material stack of the thermoplastic shielding film.
Figure 5: Thermoplastic shielding film applied to circuits. Easy for your vendor to apply, but material is still expensive.
Shielding—Design for Manufacturing—a Review of what we learned
Most of the North American flex suppliers can provide all four solutions mentioned below, but they will steer you to the technique they are most comfortable with. The most important design decisions for shielding flex circuits are flexibility/bendability and cost.
Here are my impressions:
- Adding layers of copper to shield, while adding potentially excellent shielding characteristics, adds cost, weight and thickness, and can prove to be a disaster when trying to bend the circuit. This method requires via stitching to prevent “leakage.”
- Adding cross-hatch layers can provide excellent shielding characteristics again, but still adding layers, weight, and will affect flexibility. This also requires via stitching to prevent “leakage.”
- Conductive paste allows better flexibility and lighter weight, and better coverage of shielding. Requires via stitching.
- Thermoplastic shielding film is slick stuff. Easy to apply, thin, light, and bendable. It allows no side leakage. It was originally made for cellphones so the material has been proven, but is more expensive (by 25%) than shielding with normal/older techniques.
Mike Morando is VP of sales and marketing for PFC Flexible Circuits Limited.