To gain a better understanding of how Printed Wiring Boards (PWBs) fit into electronic systems, it's useful to examine the packaging hierarchy of electronic systems. The Institute for Interconnecting and Packaging Electronic Circuits (IPC) has identified eight system elements listed below in ascending order of size and complexity.
Category A includes fully processed active and passive devices, such as bare or uncased chips and discrete capacitors, resistors, or their networks.
Category B comprises all packaged devices (active and passive) in plastic or ceramic packages, as well as connectors, sockets, and switches.
Category C involves substrates that interconnect bare or uncased chips (components of Category A) into a separable package, including multichip modules (MCMs), chip-on-boards (COBs), and hybrids.
Category D encompasses all kinds of substrates that interconnect already packaged components from Categories B and C, including rigid PWBs, flexible and rigid-flexible PWBs, and discrete-wiring boards.
Category E includes backplanes made with printed wiring and flexible circuits, which interconnect PWBs but not components from the preceding groups.
Category F covers intraenclosure connections, including harnesses, ground and power distribution buses, RF plumbing, and coax or fiber optic wiring.
Category G comprises system assembly hardware, card racks, mechanical structures, and thermal control components.
Category H includes the entire integrated system with all its bays, racks, boxes, and enclosures, as well as auxiliary and support subsystems.
From this list, it's clear that PWBs are located in the center of the hierarchy and are the most important and widely used element of electronic packaging. Categories F, G, and H are mainly used in large mainframes, supercomputers, central office switching, and some military systems. As miniaturization and portability of electronic products become more important, the first five categories are often the focus of design, with careful consideration given to the trade-offs in selecting among them.
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