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112 DESIGN007 MAGAZINE I APRIL 2021 ucts now require high layer counts and high- density interconnect (HDI) design techniques. As layer count increases, so does the need for control in design generation to accommodate manufacturing process realities. It is also worth noting, while on the topic of layer count, that stiffness increases as a cube of thickness. at is, if one doubles its thickness, the stiffness goes up eightfold (2 3 = 8), and thus small increases in thickness due to increases in layer count can greatly decrease circuit flexibility. e converse is also true, of course. e following are some key concerns to be understood and addressed in the design process relative to flex layer count. As is the case with any multilayer construc- tion, core thickness must be provided with the assumption that copper is clad on at least one surface. e core thickness is generally under- stood to be the thickness of the dielectric material between the copper layers. e core mate- rial can be a simple single-sided piece of copper clad polymer, or it can be clad with copper on both sides. Many different core thicknesses are commonly avail- able for flexible circuits, but the most common is 75 mm, typi- cally comprised of 25 mm of base polymer (e.g., polyimide, poly- ester) with 25 mm of adhesive (e.g., acrylic, modified epoxy) on either side to bond copper foil to the surface of the base polymer. inner and thicker core materials can be procured both with and without adhesive. It is recommended that design- ers check with their flex vendors for both their recommendations and the availability of the cho- sen material. While the discussion so far has 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. e spacing can impact product per- formance (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 Figure 2: Bonded vs. unbonded flex areas.