PCB Edge Fringing
When proper PCB layout design techniques such as trace routing, stack-up assignment, decoupling, bypassing, and proper termination are implemented, the radiation from the PCB itself can be minimized. However, there are several other mechanisms of the PCB assembly remaining that can still be radiation sources, including the components themselves, cavity resonance effects of the power/signal return layers, and the edges of the PCB. Edge effects can be particularly burdensome since it is the board edges that are in such close proximity to the chassis and hence these radiation fields can induce currents into the chassis frame.
There are numerous studies and papers which have been written that discuss various approaches or techniques pertaining to reducing radiation edge effects from the PCB. The most common approach is termination. The problem with many of these techniques is that they can use additional components and valuable PCB real estate without actually reducing the energy. In many cases, energy can be reflected possibly creating additional internal cavity resonance effects and coupling to internal vias, also resulting in increased radiation.
The use of MAST Technologies cavity resonance absorber material applied along the edge of the printed circuit board reduces the edge radiation from the printed circuit board without using additional board real estate for more components or traces. It also reduces the possibility of board resonance problems by dissipating the energy and not reflecting the energy back into the interior of the board.
Reduce PCB Trace Radiation
By placing MAST Technologies’ surface wave absorber material directly on top of a microstrip trace, the engineer can effectively reduce the fields emanating from the top side of the trace. This can be a particularly troublesome coupling mechanism if the traces are located on the bottom side of the board laying adjacent to the bottom of the chassis enclosure. The coupling of the field to the chassis may cause currents to flow into the chassis and set up circulating currents within it. These circulating currents can then cause radiation from any slots, seams or apertures in their path. Placing absorber with pressure sensitive adhesive (PSA) on the traces reduces the field coupling to the chassis. The effect on the trace impedance is minimal since the absorber material is high impedance (> 10 kohm) and it can also be conveniently placed directly on top of the trace without any additional mounting or holding mechanisms. This approach was used on a switch box and produced 4-6 dB of reduction in radiated emissions at 6 GHz. MAST’s surface wave absorbers are best used for this type of application.