The manufacture of printed circuit boards involves both the addition and removal of materials. A non-conductive material is used as the backing of the board. The board is coated with a conductive material such as copper, aluminum or nickel. circuit boards are generally divided into three types; multi-layered, single-sided or double-sided. The type used will be determined by density and spatial requirements and complexity of the circuits.
Drilling and Scrubbing the Board
After plating the backing with a conductive material, holes are drilled in the backing to allow conduction between layers and to mount electronic components. After the drilling, the board is scrubbed to remove any small conductor particles. Conductive material, such as copper, can be recovered and recycled from the waste water by using centrifuge or filteration processes. If copper is left in, its mixture with other waste can dissolve it and make recycling difficult, as well as add to pollutants going to the treatment plant.
Etching the Board
The board is cleaned and etched to promote better adhesion for the next step. This is followed by an additional layer of conductor. The holes that were previously drilled are not conductive. A process such as electrolysis copper plating can be used to coat the holes with conductor. An alkaline pH solution is necessary for this process and requires the addition of solutions made from acidic materials, such as copper sulfate, to balance the pH.
Obtaining Final Thickness
Photo-imaging is used to create the final circuit design. Final thickness is obtained by electroplating copper to the printed circuit. A thin layer of pure tin or lead solder resists are applied to protect the final circuits in the next etching. The plating of resistant tin or lead is removed to expose copper that will not be part of the final circuit. Ammonia-based or sulfuric acid/hydrogen peroxide solutions are used to etch away the unwanted copper.
Alternative Resists
Another form of resists come from using photosensitive or other volatile organic compounds. These resists may be applied wet or dry. The photosensitive compounds harden when exposed to ultraviolet light. The liquid resists can be applied by various techniques including squeegee, roller, silk screen or spray. The liquid can be applied to one side, both sides or a specific area of the surface. The use of light as a hardener makes finer circuits possible. Dry film comes in sheets that can be quickly applied using lamination and pressure.
Multi-Layer Printed Circuit Board Panel
Multi-layer panels are formed by assembling together inner layer cores. The panels are assembled as a book consisting of layers of copper foil with alternating layers of epoxy sheets known as pre-preg. The book is placed in a lamination press and subjected to high heat and pressure. This melts the pre-preg and produces a bond. The panel is cured in a oven, and trimmed and buffed. Necessary holes are then drilled.
