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110 DESIGN007 MAGAZINE I APRIL 2021 trolled impedance designs and signal integrity issues. Several different systems have been developed over the years to help assure that there is no uncertainty in the order of the cir- cuit layers in the final construction. e fab- ricators engineering staff can provide recom- mendations if needed. Note the thickness and construction of each core in Figure 1. Coverlayer Requirements Not Properly Called Out or Defined Coverlayer and cover coat are terms normally reserved for flexible circuit constructions and they are by default a defining structural ele- ment of both flex and rigid flex circuits. Cov- erlayers serve as a flexible solder mask of sorts, protecting the delicate circuits from damage and potential wicking of solder along circuit traces, while leaving open access to design fea- tures where interconnections are to be made to components by soldering. It is important to determine the thickness of a coverlayer to allow for maximum flexibility when desired and ensure you have chosen a coverlayer with a sufficient amount of adhe- sive on it to accommodate the copper weight. Coverlayers are also of importance in the design of areas where the circuit is to be bent either just one time, intermittently, or dynami- cally, millions or even billions of times over its useful life. e latter case, the dimensions and make of the flexible circuit coverlayer is critical. In dynamic flex circuits, there is need to balance the number of flexible materials on the sides of the conductors where flexing is to occur. It is important to know and understand that there are different types of materials avail- able for use as coverlayer materials, and that there is no single, ideal solution. ese material choices include materials that are laminated to the copper circuits using heat and pressure; materials that can be laminated and then pho- toimaged, like solder mask, to define points of connection; and materials that are simply screen printed on to seal traces, while leaving open features of interest for further processing or for making interconnections. Number of Flex Layers e clear majority of flexible circuits have just one or two metal layers. However, an increasing number of high-performance prod- Figure 1: Example of four-layer flex construction.