Board Edge Electromagnetic Radiation Containment and Absorbing Device
Original Publication Date: 2009-Nov-24
Included in the Prior Art Database: 2009-Nov-24
Described is a solution to the problem of radiation of electromagnetic emissions from board edges. The solution consists of providing a containment shield along with an internal absorbing material.
The invention involves creating an edge shield to reflect electromagnetic energy back toward the card edge but having an intermediate absorber material to convert the electromagnetic energy to heat. The absorber material may be of the homogeneous or graded permeability type to obtain a broadband response. In addition, the material edge may be rough (not unlike cones that line an anechoic chamber) to reflect the wave at the material interface and direct it in a scattering pattern to obtain more absorption. In order to fully contain the edge emission, the shield contacts the top and bottom of the circuit board and may contact the internal logic or chassis ground to complete the enclosure. The contact need not be totally continuous but must be contacted at distances that are shorter than 1/20th of a wavelength. Likely the best would be a strip along the circuit board that continuously contacts the edge shield. Without the absorber material, only reflection occurs that merely moves the problem to another location on the board to be absorbed, radiated, or possibly interfere with vias connecting circuity to nets throughout the board (likely most of the cross-talk and emissions generation is caused by vias that have displacement current as a return path). Figure 1 shows a cutout of a circuit board just showing the edge with the invention installed.