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86 DESIGN007 MAGAZINE I FEBRUARY 2020 result, every multi-board design combines electronic design automation (EDA) with mechanical engineer- ing design (MCAD). The Sum Is Greater Than the Whole Good troubleshooting techniques involve con- sidering a system as indi- vidual parts rather than as a whole. The same tech- niques apply to your work with multi-board PCB de- signs. Each board consists of a single unit that has its own lifecycle. Some product designs may use a single PCB design for multiple functions or for multiple devices. Others may interconnect multiple PCB designs to produce a complete, fully functional system. Depending on the product design, a single PCB design may have multiple uses. No matter the method, the multi-design represents an overall approach to system design. Multi-board designs feature rigid-flex PCB technologies. Combining flexible circuits and rigid circuits extends the benefits of individual technologies. While rigid PCBs carry most of the components, flexible PCBs serve as inter- connecting pieces between the rigid boards. In addition, the combination of rigid and flex re- duces complexity and cost, saves weight, and improves reliability. Let's Be Flexible Here While some applications use static flexible circuits that have minimal movement, oth- er products use dynamic, flexible circuits that move frequently. The two types of flexible cir- cuits use different types of materials and re- quire different methods for construction. For example, dynamic, flexible circuits require a much longer flexing lifespan and require elon- gated grains in the copper. As we consider the construction of flexible circuits, we begin with a stackup of flexible substrate material and copper. A combination of adhesives, heat, and pressure laminates the materials together. The substrate consists of a polyimide, or a strong, flexibly thermosetting polymer. PCB manufacturers use different types of poly- imides based on application requirements. Giv- en the need for flexibility, the circuits rely on rolled and annealed copper along with acrylic adhesives that combine softness with higher coefficients of thermal expansion. The different types of flex circuits include the Type 1 single layer with one conductive layer sandwiched between two layers of poly- imide, the Type 2 double layer that has two conductive layers, and the Type 3 multilayer flexible PCB that uses three or more conduc- tive layers. The rigid part of a rigid-flex circuit uses standard PCB design and manufacturing processes. Along with prepreg, the board typically con- sists of an FR-4 substrate, conductive copper layers, the solder mask, and identifying in- formation. Type 4 multilayer rigid-flex boards have three or more conductive layers. Because most designs feature rigid sections that have a different set of layers than the flexible sec- tions, multilayer rigid-flex designs do not have a consistent set of layers across the entire de- sign. The circuit consists of the inner layers of multiple circuits attached together with adhesive. While thermal conductive layering under floors may differ in size to your boards, considerations are similar.