wisepowder: Improve PCB Shielding For Portable Devices

wisepowder

Miniaturization of handsets and other wireless devices creates scores of shielding challenges as high-frequency components become more closely spaced. As printed-circuit boards (PCBs) shrink, new electromagnetic-interference (EMI) shielding solutions must provide greater levels of interference suppression, but without significantly adding mass, weight, and cost to a device. Fortunately, a new shielding technology developed by W.L. Gore & Associates called snapSHOT™ shield, replaces bulkier soldered approaches with a snap-on metallized thermoformed shell that can be attached to a PCB by means of standard ball-grid-array (BGA) solder spheres.To get more news about Cavity PCB, you can visit pcbmake official website.

In understanding the shielding requirements of compact wireless designs, the shortcomings of traditional test methods (mostly based on military requirements) become apparent when applied to battery-powered portable wireless designs. Some test methods, such as ASTM D 49351 for planar shielding and a coaxial cell2 for EMI gaskets, for example were developed to characterize the materials that would ultimately comprise an EMI enclosure. But there remain no formal published test methods to evaluate the shielding effectiveness (SE) of a shield assembled to the PCB of a portable wireless device.

At present, two primary shielding approaches are used in cellular telephones: soldered perforated cans and plated covers with EMI gaskets (Fig. 1).Both try to create a complete shield around the PCB's components to ensure proper electrical performance and comply with regulatory requirements for EMI emissions and susceptance.

The goal of an EMI shield is to create a Faraday cage around the enclosed RF components using the six sides of a metallic box. The top five sides are created using a shielding cover or metal can, while the bottom side is achieved by using the ground plane within the PCB. In an ideal enclosure, no emissions would enter or exit the box. In reality, leaks do occur, such as from holes perforated into soldered cans that allow thermal heat transfer during solder reflow. Leaks can also occur from imperfections along an EMI gasket or solder attachments. Leaks are also possible from the spaces between ground viaholes used to electrically connect the shielding cover to the ground plane.

Shields designed for portable devices must be light in weight and low in cost, but they must also meet demanding mechanical and electrical requirements. Phenomenon such as cavity resonance, aperture radiation, and planar shielding are factors RF engineers face when designing shielding enclosures. The problem is further complicated by the fact that accurate EM field prediction from complicated PCB assemblies, particularly in the near field, is virtually impossible, forcing many engineers to build custom test fixtures to evaluate their designs.
To create the Faraday cage required for proper shielding, a metallic enclosure must be placed around and in close proximity to the components on a PCB. Unfortunately, this may have adverse effects on the performance of the components and the functionality of the circuit, with the greatest concern being enclosure (cavity) resonances at any of the PCB's operating frequencies. To study this, a simple test fixture was designed to mimic the effect of placing a metallic enclosure over RF components.