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78 DESIGN007 MAGAZINE I FEBRUARY 2022 materials allow for denser designs because components and traces can be placed and routed in three dimen- sions, eliminating physical connectors. is reduces cost and impedance dis- continuities while increasing reliability. Rigid-flex designs also have improved e l e c t r o m e c h a n i c a l f u n c t i o n a l i t y, including dynamic bending, vibration and shock tolerance, heat resistance, and weight reduction. eir increased shock and vibration tolerance makes flexible designs popu- lar within medical, automotive, mili- tary, and aerospace applications that necessitate dependable field operation. e ability to bend portions of the cir- cuitry facilitates dense constructions, leading to widespread use in flat panel displays and cellular devices, while also making wearable technology possible. Rigid-Flex Materials e materials used in rigid-flex are a bit different than the cores and prepregs we introduced in Chapter 1. Figure 7.1 shows the different types of flexible/ rigid-flex PCBs according to the IPC- 6013 specification. • Type 1 are single-sided flexible printed boards containing one conductive layer, with or without stiffeners. • Type 2 are double-sided flexible printed boards containing two conductive layers with PTHs, with or without stiffeners. • Type 3 are multilayer flexible printed boards containing three or more conduc- tive layers with PTHs, with or without stiffeners. • Type 4 are multilayer rigid and flexible material combinations containing three or more conductive layers with PTHs. • Type 5 are flexible or rigid-flex printed boards containing two or more conductive layers without PTHs. Copper in rigid-flex designs is adhered to flex dielectrics in various ways. Adhesive may be added where copper is bonded directly onto the base material. Stiffeners are sometimes added to reinforce a flex area for component placement or routing holes. Copper can either be electrodeposited (ED, less flexible, lower cost) or rolled annealed (RA, more flexible, higher cost). Flex planes oen have a hatched pattern etched into them because the reduc- tion in copper makes them more flexible. Coverlays are flexible materials, typically on the outside of a flex substack. ey protect and insulate the flex circuitry on the surfaces, pre- venting it from liing. Coverlays are typically made from acrylic, polyimide, or polyester. A typical coverlay construction, as shown in Fig- ure 7.1, has a polyimide dielectric on top and Figure 7.1: Flex/rigid-flex PCB types.