Issue link: https://iconnect007.uberflip.com/i/1496727
66 DESIGN007 MAGAZINE I APRIL 2023 sided core has an identical shape. However, multiple-layer flex is likely to have variations in shape for each layer or core. Flexible PCBs require a thin film insulator over the conduc- tors know as a coverlay. Unlike rigid PCB sol- der masks, coverlays are die cut much like the flexible layers they insulate. e stackup of a multi-layer flexible PCB can vary across the PCB area. is is especially true with multi- layer flex where layer shape varies among the collection of layers or cores. A flexible ground or power plane area is typically crosshatched versus solid for rigid PCBs. e crosshatch reduces potential for fracture of conductors. Alternatively, flexible layers can be shielded with a layer of copper or silver foil. Masking and screening over flexible layers is not rare, but uncommon. Rigid-flex PCBs are obviously a combination of rigid and flexible materials. Rigid-flex is, in essence, a hybrid PCB combination of mate- rials and processes from both rigid and flex- ible PCBs. e two material types are gener- ally processed separately and bonded together later in the fabrication process. e layer Contrasting Rigid PCBs With Flex and Rigid-flex Some designers design flexible PCBs as sim- ple bendable circuit boards, but there are vast differences between rigid and flexible. Both technologies produce an electrical intercon- nect function, but are manufactured using dif- ferent types of materials and processes. ey also have varying applications. No need to design a rigid-flexible PCB for the mother- board of a desktop PC, but rigid-flex is required for most medically implanted devices. A typical rigid PCB is comprised of electro- deposited copper-clad fiberglass substrates bonded together. While there are variations on materials used to bond substrates, it is com- monly sheets of cloth pre-impregnated with uncured epoxy. is bonding material com- position is not engineered to be flexible. e copper is chemically etched to create a circuit pattern. e hardness of the bonded substrates requires mechanical routing to trim the raw PCBs. All layers of the PCB are commonly identical in size and shape unless cavities, embedded components, or other such exotic construction is present. e rigid PCB layer stackup is identical across the entire PCB area. Solder mask and legend are almost always applied. Flexible PCBs are comprised of rolled annealed copper over flexible polyimide sub- strates. Flexible layers or cores are produced with or without adhesives. Adhesiveless flex is prevalent in applications requiring higher performance, while those with adhesives are oen found in low layer-count applications. e most common usage is copper foil lami- nated to a substrate with epoxy or acrylic adhe- sive. Both substrate material and adhesive are engineered for bending to minimize trace fracture. Like rigid PCBs, a chemical etching process is used to create a circuit pattern. e flexible nature of the materials requires die cutting or "blanking" rather than mechani- cal routing. Each layer of a flexible double- Mark Gallant