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APRIL 2018 I FLEX007 MAGAZINE 53 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 The clear majority of flexible circuits have just one or two metal layers. However, an increasing number of high-performance prod- 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 accommo- date manufacturing process realities. It is also worth noting, while on the topic of layer count, that stiffness increases as a cube of thickness. That is, if one doubles its thickness, the stiff- ness goes up eightfold (2 3 = 8), and thus small increases in thickness due to increases in layer count can greatly decrease circuit flexibility. The converse is also true, of course. The fol- lowing are some key concerns to be understood and addressed in the design process relative to flex layer count. along circuit traces, while leaving open access to design features where interconnections are to be made to components by soldering. It is important to determine the thickness of a coverlayer to allow for maximum flexibil- ity when desired, and ensure you have cho- sen a coverlayer with a sufficient amount of adhesive 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 dynamically, millions or even billions of times over its useful life. The latter case, the dimen- sions and make of the flexible circuit cover- layer is critical. In dynamic flex circuits, there is need to balance the amount of flexible mate- rials on the sides of the conductors where flex- ing is to occur. It is important to know and understand that there are different types of materials available for use as coverlayer materials, and that there is no single, ideal solution. These material choices include: materials that are laminated to the copper circuits using heat and pressure; materials that can be laminated and then pho- RIGID SECTION FLEX SECTION RIGID LAMINATE CORE 10 MILS COVERLAY 1 MIL POLYIMIDE ADHESIVE 1 MIL ACRYLIC 1 OUNCE COPPER Adhesiveless Polyimide Flex 1 mil RIGID SECTION LAYER 1 LAYER 2 LAYER 3 LAYER 4 RIGID LAMINATE CORE 10 MILS RIGID LAMINATE CORE 10 MILS RIGID LAMINATE CORE 10 MILS NO-FLOW PREPREG 6 MILS NO-FLOW PREPREG 6 MILS NO-FLOW PREPREG 6 MILS NO-FLOW PREPREG 6 MILS ADHESIVELESS POLYIMIDE FLEX 1 MIL 1/2 OZ. COPPER 1/2 OZ. COPPER 1/2 OZ. COPPER 1/2 OZ. COPPER COVERLAY 1 MIL POLYIMIDE MASK MASK MASK MASK ADHESIVE 1 MIL ACRYLIC 1 OUNCE COPPER Figure 1: Example of four-layer flex construction.