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APRIL 2018 I FLEX007 MAGAZINE 55 imide, polyester) with 25 mm of adhesive (e.g., acrylic, modified epoxy) on either side to bond copper foil to the surface of the base polymer. Thinner and thicker core materials can be pro- cured both with and without adhesive. It is recommended that designers check with their flex vendors for both their recommendations and the availability of the chosen material. While the discussion so far been limited to flexible circuit core material, rigid materials are employed in the fabrication of rigid-flex cir- cuits. Of course, any of the myriad core mate- rials used in rigid multilayer circuits are also available to make rigid-flex circuits. However, once again, it is advisable to check with the flex manufacturer for advice as to what options are most common and readily available. Separation Distance Between Flex Circuit Cores When a product requires two or more cores, there is a need to define in the specification what the spacing requirements are between cores. The spacing can impact product performance (physical and electrical) and, most obviously, thickness. In some designs, the spacing between flex circuit cores may be filled with dielectric material, but with other designs the dielectric between flex cores in the flex area may be omit - ted to assure maximum flexibility (Figure 2). If the core layers must be unbonded, this should be noted in the documentation. Those areas where bonding is to be avoided should be identified in the design artwork package. The unbonded areas must have a coverlayer applied to each exposed side (Fig- ures 2 and 3). In laminated areas, it is not required and arguably a liability when plated through-hole reliability through the assembly process is considered. Obviously, in areas where interconnection is required between multiple layers of internal circuits, a dielectric is required as shown in Figure 2. In the next installment we will continue this three-part series by addressing circuit layup symmetry, designing for bending, and controlled impedance. FLEX007 Dave Lackey is vice president of business development at American Standard Circuits. Anaya Vardya is CEO of American Standard Circuits. Related Content Visit I-007eBooks to download your copy of American Standard Circuits' micro eBooks today: The Printed Circuit Designer's Guide to Flex and Rigid-Flex Fundamentals E The Printed Circuit Designer's Guide to Fundamentals of RF/Microwave PCBs E Figure 3: Completed PCB flex belts separated.