Printed circuit board manufacturing is the process of manufacturing bare printed circuit boards (PCBs) and populating them with electronic components. It includes all the processes to produce the full assembly of a board into a functional circuit board. In board manufacturing, multiple PCBs are grouped on a single panel for efficient processing. After assembly, they are separated (depaneled). Various techniques, such as silk screening and photoengraving, replicate the desired copper patterns on the PCB layers. Multi-layer boards are created by laminating different layers under heat and pressure. Holes for vias (vertical connections between layers) are also drilled. The final assembly involves placing components onto the PCB and soldering them in place. This process can include through-hole technology (in which the component goes through the board) or surface-mount technology (SMT) (in which the component lays on top of the board).
Design
Manufacturing starts from the fabrication data generated by computer aided design, and component information. The fabrication data is read into the CAM (Computer Aided Manufacturing) software. CAM performs the following functions:
Input of the fabrication data Verification of the data Compensation for deviations in the manufacturing processes (e.g. scaling to compensate for distortions during lamination) Panelization Output of the digital tools (copper patterns, drill files, inspection, and others) Initially PCBs were designed manually by creating a photomask on a clear mylar sheet, usually at two or four times the true size. Starting from the schematic diagram the component pin pads were laid out on the mylar and then traces were routed to connect the pads. Rub-on dry transfers of common component footprints increased efficiency. Traces were made with self-adhesive tape. Pre-printed non-reproducing grids on the mylar assisted in layout. The finished photomask was photolithographically reproduced onto a photoresist coating on the blank copper-clad boards.
Modern PCBs are designed with dedicated layout software, generally in the following steps:
Schematic capture through an electronic design automation (EDA) tool. Card dimensions and template are decided based on required circuitry and enclosure of the PCB. The positions of the components and heat sinks are determined. Layer stack of the PCB is decided, with one to tens of layers depending on complexity. Ground and power planes are decided. A power plane is the counterpart to a ground plane and behaves as an AC signal ground while providing DC power to the circuits mounted on the PCB. Signal interconnections are traced on signal planes. Signal planes can be on the outer as well as inner layers. For optimal EMI performance high frequency signals are routed in internal layers between power or ground planes. Line impedance is determined using dielectric layer thickness, routing copper thickness and trace-width. Trace separation is also taken into account in case of differential signals. Microstrip, stripline or dual stripline can be used to route signals. Components are placed. Thermal considerations and geometry are taken into account. Vias and lands are marked. Signal traces are routed. Electronic design automation tools usually create clearances and connections in power and ground planes automatically. Fabrication data consists of a set of Gerber format files, a drill file, and a pick-and-place file.
Panelization Several small printed circuit boards can be grouped together for processing as a panel. A panel consisting of a design duplicated n-times is also called an n-panel, whereas a multi-panel combines several different designs onto a single panel. The outer tooling strip often includes tooling holes, a set of panel fiducials, a test coupon, and may include hatched copper pour or similar patterns for even copper distribution over the whole panel in order to avoid bending. The assemblers often mount components on panels rather than single PCBs because this is efficient. Panelization may also be necessary for boards with components placed near an edge of the board because otherwise the board could not be mounted during assembly. Most assembly shops require a free area of at least 10 mm around the board.
Depaneling
The panel is eventually broken into individual PCBs along perforations or grooves in the panel through milling or cutting. For milled panels a common distance between the individual boards is 2–3 mm. Today depaneling is often done by lasers which cut the board with no contact. Laser depaneling reduces stress on the fragile circuits, improving the yield of defect-free units.
Copper patterning The first step is to replicate the pattern in the fabricator's CAM system on a protective mask on the copper foil PCB layers. Subsequent etching removes the unwanted copper unprotected by the mask. (Alternatively, a conductive ink can be ink-jetted on a blank (non-conductive) board. This technique is also used in the manufacture of hybrid circuits.)
Silk screen printing uses etch-resistant inks to create the protective mask. Photoengraving uses a photomask and developer to selectively remove a UV-sensitive photoresist coating and thus create a photoresist mask that will protect the copper below it. Direct imaging techniques are sometimes used for high-resolution requirements. Experiments have been made with thermal resist. A laser may be used instead of a photomask. This is known as maskless lithography or direct imaging. PCB milling uses a two or three-axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine (referred to as a 'PCB Prototyper') operates in a similar way to a plotter, receiving commands from the host software that control the position of the milling head in the x, y, and (if relevant) z axis. Laser resist ablation involves spraying black paint onto copper clad laminate, then placing the board into a CNC laser plotter. The laser raster-scans the PCB and ablates (vaporizes) the paint where no resist is wanted. (Note: laser copper ablation is rarely used and is considered experimental.) Laser etching, in which the copper may be removed directly by a CNC laser. Like PCB milling above, this is used mainly for prototyping. EDM etching uses an electrical discharge to remove a metal from a substrate submerged into a dielectric fluid. The method chosen depends on the number of boards to be produced and the required resolution:
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