Low Frequency Shielding
Mobile-handset features such as address book, voice recognition, and text messaging require digital devices that operate in the low MHz region. These devices generate low-frequency noise that can interfere with the wireless device's transceiver function. Therefore, the digital sections of the PCB require EMI shielding.
Using a full EMI gasket stops all radiation, but, as one U.S. handset manufacturer learned recently, full EMI gasketing is unnecessary. Low frequency EMI is characterized by very large wavelengths, which impede its ability to leak out of openings in the EMI shield. However, grounding the shield with GORE-SHIELD™ SMT RF grounding pads in certain critical spots around the perimeter adequately contains lower frequency radiation more cost-effectively than a full-perimeter EMI gasket. The handset manufacturer approached the EMI gasketing problem by populating only one side of the PCB with digital devices. This design allowed use of a single, stamped-metal shield over the entire board.
Only 12 GORE-SHIELD SMT RF grounding pads were needed to contain the low-frequency noise from the digital electronics. This approach yielded a 50 percent cost reduction compared to a full-perimeter EMI gasket solution.