PCBs or printed circuit boards are used for a number of functions in electronic devices. These boards are made from layers of materials (e.g., fiberglass, phenolics) that are laminated together. Copper patterns are etched across the surface where electronic parts are mounted rigidly.
PCB fabrication is a multi-step process that is mostly carried by machines. These processes include the following:
A board designer creates a layout of the PCB, where details of parts to use, type of board, number of layers and a host of other information are written.
The design or board data is placed in a photo plotter where it is transformed into pixel image. Lasers print the board data on a film, which is used during PCB fabrication. One film is generated for each layer of the PCB. These films are registered with each other to provide perfect alignment of the different PCB layers.
Printing inner layers (for multilayer boards)
Next in the process is the creation of the inner layers of the printed circuit board. This starts with a panel of laminate, which is an epoxy resin and fiberglass core with pr-bonded copper foil on both sides. The copper is cleaned and then layered with photosensitive photo resist film. The first film (from the printed protocols) is loaded into the printing machine followed by the coated laminate panel and the second film. The printer hardens the photo resist through the first layer of clear film using powerful ultraviolet (UV) lamps. On the laminate, there will be areas of unhardened photo resist. This is removed by spraying the laminate with alkali solution.
The panel is then pressure-washed and thoroughly dried.
All unwanted copper foil on the panel is removed by subjecting it to potent alkaline solution, which erodes or etches at the exposed copper.
Registration and AOI
Punching the registration holes is needed in aligning the inner layer to the outer layers of the PCB. The inner layer is loaded into an optical punch machine, which arranges the registration targets in the copper pattern and punches the registration holes. Then, the panel is subjected to an automated optical inspection system, which checks the registration holes against the design made at the beginning of the PCB fabrication process.
Lay-up and bond
Next, the outer layers of the PCB are prepared. Two sheets of prepreg (sheets of glass that have already been pre-impregnated with uncured epoxy) and thin foil of copper are placed on a heavy steel base plate. The inner layer (pretreated panel) is placed carefully over the alignment pins. Another 2 sheets of prepreg, copper foil and aluminum press plate is placed. Three layers of base plate are created this way. The stacks are then pinned together. A number of these stacks are then loaded into a bonding press, where pressure and hot press plates bond the different layers of the printed circuit board.
Next, the press is unloaded. The top plate in the stack is de-pinned and removed. The aluminum press plates are also removed, which creates a smooth copper finish.
Next in the PCB fabrication process is drilling holes where lead components would later pass through.
A very thin copper layer is chemically deposited on the walls of the drilled holes.
Imaging the outer layers
Next, the outer layers are imaged in much the same way as the inner panel was printed.
Next in the process is copper electroplating of the board, which is necessary to enable conductivity within the board. The copper plated through the holes and on the pads and tracks are then covered with a thin tin layer.
Etching the outer layers
All unwanted copper from electroplating is etched away. The layer of tin over the plated areas serves as etch resist so that only the desired copper plated areas are left after the etching process.
A solder mask is applied to protect the copper surface.
These are the basic PCB fabrication steps. A few more steps are added after the last step, depending on the end use of the PCB and design specifications.