Printed circuit boards, or PCBs, are present in all but the simplest electrical devices. Without them the computer, the cell phone, and even the modern automobile, could not exist. Not as we know them. Without PCBs, electronic circuits in cell phones and DVD players would have to be made by hand. Some devices, like cell phones, might be impossible to make without printed circuits. The PCB makes mass produced electronics simple and quick. It also makes miniaturization easier. Even creating PCB’s can be automated for increased speed and efficiency. To make a PCB a conductor, usually copper, is laid on a board made from an insulating material. The board is usually fiberglass, but can be plastic or another insulating material. Holes in the board allow component placement on the board. After placement, the components are soldered to the board.
Constructing the Board
Printed circuit boards are usually made using a fiberglass-insulated base with copper tracings for the circuits. That is not the only possible combination, though. Everything from wood to cardboard to plastic will work as the insulator. Any conductor may work for the tracings, but in practice copper is the most common. Aluminum is relatively common, but not as common as copper. The board may have traces on one side, both sides, or in layers embedded within the insulator. Transistors, capacitors and other components are mounted in holes drilled into the board.
Modern PCB’s are usually made using laminates, copper-clad laminates, a material known as pre-reg, or copper foil. Laminates are the most common. There are several methods used to make the traces on the board, but they fall under two general categories: Subtractive and additive.
Subtractive processes start with a board coated with copper. A protective coating covers the copper. The coating might be designed in the pattern of desired traces, or coating may cover the entire board. If it covers the entire board then the protective coating will be removed in the areas that do not need copper. Whether the covering starts in the desired pattern, or covering the whole board, once the unneeded copper is exposed it is removed by etching, engraving, or milling. For high-resolution tasks, lasers are used.
The additive process places the traces on the board in the desired pattern. The process can be fully additive, or semi additive. Fully additive starts with blank laminate. Photosensitive film covers the laminate. The film is exposed in the pattern of the desired circuit and developed, and then the laminate is treated to create surfaces metals bond with. The board is plated with copper in the pattern of the desired circuit board. Removing the film leaves the copper pattern in place. The Semi Additive process uses a board already covered with a thin layer of copper. A reverse mask will expose the desired pattern, and more copper is added to the exposed pattern. The mask is removed, and then the thin layer of original copper, so that only the desired copper traces remain.
Two methods are used to connect components to the board, through-hole, and surface mount. Surface mount is the older technology. In through-hole connections, wire leads pass through holes in the board and soldered in place. Surface mount components have tabs where through-hole components have wire leads. That makes automation easier and component prices lower.
Printed Circuits are a marvel of modern technology. Without them we would live in a different world.
Sean McCann is a freelance blogger and copywriter based in Dublin, Ireland. He writes on behalf of several Irish businesses including Origin Manufacturing.