Technology for high quality factor membrane filters in organic multilayer PCBs

This article presents technology for a membrane (structured sheet metal) band pass filter with a high quality factor integrated in organic multilayer printed circuit boards (PCB). Creating a cavity in the multilayer board by milling and subsequently copper-plating, the loss of the filter resonators is reduced. The membrane fabricated from sheet steel is silver-plated to obtain a very high conductivity. The connection of the membrane and the PCB is established with a soldering process. A microstrip transition from the substrate to the cavity is included in the filter design. In order to validate the approach an interdigital band pass filter with five resonators at 5GHz and 12% bandwidth was implemented, showing a minimum measured insertion loss of 0.57 dB.