Printed Circuit Board Fabrication Process
The actual process for PCB fabrication can begin on receipt of the necessary documentation from the designer regarding the proper choice of materials for the substrate and cladding, the number of layers and PCB stackup, the mechanical layout and routing. The documentation must have individual details for each layer of the PCB.
Preparing the Central Panel
The fabrication process starts with obtaining the copper clad board, with the specified substrate material and copper cladding. For a multilayer board, the cladding will be on both sides of the substrate, and this forms the innermost or central layer. Usually, such copper clad boards come in large sizes of standard dimensions, and the necessary panel of a size matching the mechanical layout specified has to be cut out by shearing the board. Depending on the individual size and total number of PCB units to be made, the panel may have to be dimensioned to hold multiple PCB units. The copper cladding usually comes with a thin coating of protective layer to protect the surface from oxidation, and the protective layer must be removed by immersing the panel in a bath containing the solution of a weak acid.
Plated Through Holes
Holes are used for connecting traces on one side to traces on the other. Some holes may be required to be mounted on the PCB board mechanically. To facilitate interconnection between the layers, PCB manufacturers line the inner surface of holes with a copper layer, using a plating process, and that’s why they are called plated-through holes or PTH. A round of visual inspection and electrical continuity testing at this stage verifies the process. For single or double-layer PCBs, the panel now goes for solder masking (coverlay for flexible PCBs) to cover those parts of the tracks that will not be soldered, and surface finish for the exposed tracks and pads. Multilayer boards will have further layers added on to the central or innermost layer.
Adding on Subsequent Layers
Manufacturers add subsequent copper layers to the central layer with a layer of insulation in between each. For rigid PCBs, this insulation is usually the prepreg, while for flexible PCBs this is an adhesive layer. Manufacturers add insulation and copper layers on to each side of the central layer, using heat and pressure to bond them together. The copper surfaces on both sides now undergo the same treatment of protective layer removal followed by drilling. Only this time, the drilling depth is controlled so that the copper on the inner layers remains undamaged. Eminent PCB manufacturers like EPCB use ultrasonic or laser drills instead of mechanical drills in order to get greater accuracy and reliability. The drilled holes are then electroplated to provide necessary connections. Usually, a round of visual inspection and electrical testing to verify the process follows. If no further layers are required, the panel now goes for the solder masking/coverlay process. For additional layers, the above process is repeated.
Solder Mask/Coverlay and Surface Finish
Manufacturers protect areas of the PCB not to be soldered by covering them with a protective layer. For rigid PCBs, this is the usual green layer of solder mask. The solder mask/coverlay protects the board from other contaminants as well, as PCB goes through the assembly process. To enable leaded or SMD components to be added to the board by soldering, the solder mask/coverlay has openings at appropriate places, exposing the copper surface. To prevent the exposed copper from oxidizing, fabricators tin or plate them with solder, plate them with gold, or use other combinations of different metals to achieve a surface finish, as specified by the designer.
The last step in the PCB manufacturing process is to print text, UL logo, PCB manufacturer, and other indentify. Usually, this helps in identifying component location and fault finding instructions. Next is the final inspection, after this, a well-functioned printed circuit board is made.