Circuit Board Design
(Printed) Circuit Boards (PCB), or Printed Wiring Boards (PWB), are part of a commercial evolution from the days when electronic components were bigger and assembled on large chassis.
The circuit board is a flat sheet of material usually of either fibreglass (high quality) or resin bonded paper (budget). Holes for mounting components with wire ends are surrounded by copper areas known as pads. These pads are connected, as defined by the circuit, by copper traces or tracks that take the place of the historic wires.
In this way, the basic circuit board becomes a mechanical holder for electronic parts and the wiring system for all the interconnections between them. Conventional wires and connectors are still required to reach out to other parts that for size, weight or practicality cannot be located on the circuit board. This makes the circuit board an extremely valuable foundation item, whose value cannot be under-estimated.
The continual reduction in the size of electronic devices has resulted in smaller and in some cases cheaper components. The concept of Surface Mount Devices (SMD) describes not only another advance in manufacture but also a change to the concept of circuit board design. Surface Mount Devices are often the preferred package for new low power devices. Power devices that generate heat or deal with high currents often require “conventionally” packaged parts.
Surface Mount devices have no wire ends. Instead many of the simplest parts are like very small bricks with metal end caps. The metals end caps “sit” on copper pads with no holes and are literally soldered to the surface. Surface mount chips have a variety of designs but still there are no “legs” to go though the circuit board.
Increasingly the circuits are more dense, requiring more components and more connections to be squeezed into a small space. Often this requires designs with tracks on both sides of the circuit board and specialised manufacture to provide copper paths (plated through holes) between top and bottom surfaces.
PCs, mobile phones and other dense applications use sandwich constructions that involve numerous layers. These are called multi-layer boards. Another recent development involves the use of holes for connectivity (vias) that are less than 0.3mm in diameter. These are no longer drilled but created by laser and called micro-vias. The advanced technology is perceived to cost more but if the manufacture process is properly understood it can actually save money.
The layers are not just about simple A to B connections. The speed of circuits and the critical nature of performance mean that some connections require a field or plane of copper. These planes are often used both for connectivity and shielding or noise reduction. The circuit board is then not only a mechanical and wiring device it becomes a total environment in which the circuit operates.