Specifying Impedance Requirements in Rigid Flex PCB’s

Controlled impedance requirements are popular in today’s electronic designs, and rigid flex designs are no exception. However, rigid flex designs have dissimilar and unique material characteristics that need to be accommodated and modeled in your design. The dielectric materials in the flexible sections of a rigid flex board are different than rigid laminates, and offer better electrical performance. The cover layers and bondplys also offer better electrical performance than the hardboard layers. These materials have varying dielectric values and should be modeled in software designed to predict how each dielectric, reference plane, and circuit relate to one another. Trying to do this in a rigid flex design, using free online impedance calculators, almost always returns false values. There are just too many interactions to use single value calculators. Added to this, some of the material suppliers give global dielectric values when the value can differ due to core thickness, resin content and signal speed.