An overview of the multilayer PCB fabrication process.
The actual process of PCB fabrication can begin on receipt of the necessary documentation from the designer. These data include the choice of materials for the substrate and cladding, the number of layers and stackup, the mechanical layout, and the routing. The documentation must provide individual details for each layer of the PCB.
Preparing the central panel. The fabrication process starts with obtaining the copper-clad substrate. For a multilayer board, copper will be on both sides of the substrate, which forms the innermost or central layer. Usually, such copper-clad substrates are supplied in sizes of standard dimensions, with the panel sized to match the specific mechanical layout. Otherwise, the fabricator will resize the panel the necessary dimensions by means of a shearing process. Depending on the size and total number of discrete PCBs to be made, the panel may be dimensioned to contain multiple PCBs: An 18" x 24" panel might, for instance, contain four 4" x 4" PCBs. The copper cladding is usually provided with a thin layer of protective coating to protect the surface from oxidation. This protective layer must be removed by immersing the panel in a weak acid bath.
Drilling and etching the panel. The panel is dried and typically heated to remove excess moisture. Then the fabricator begins drilling holes for the central layer, guided by CAD files provided by the designer. The process begins with drilling the registration, or locating, holes at the periphery of the panel, and the CNC machine changes its drills to match the diameter of individual holes specified in the drill file. The registration holes are necessary for aligning subsequent layers.
Fabricators use a photographic process followed by chemical etching to obtain the correct pattern of traces on both sides of the panel. The copper surfaces of the drilled panel are typically covered with a thin layer of photoresist. Each side is then exposed to UV light through a photographic film or photomask that details the optically negative pattern of traces specified by the designer for that layer. UV light exposed to the photoresist bonds the chemical to the copper surface, and the rest of the unexposed chemical is removed in a developing bath. (An alternate method involves use of the CAM file to guide a laser to generate the image on the substrate.) This stage is usually followed by visual inspection.
In the etching bath, the etchant removes exposed copper from the panel. This leaves behind the copper traces, hidden under the photoresist layer. The concentration of the etchant and time of exposure are critical to obtaining optimum results during etching. Stronger-than-necessary concentration and excessive dwell time can result in over-etching copper from beneath the photoresist, resulting in traces with widths thinner than specified by the designer. After successful etching, the photoresist is washed away to leave the desired copper traces on both sides of the central layer.
Although the photoimaging process is very popular, other methods are also available. For low-volume production, an etch-resistant ink may be transferred to the copper surface using a silk-screening process, with the silkscreen representing the optically negative pattern of the traces.
Another “dry” process uses a specialized, highly accurate milling machine to remove unwanted copper from the surface of the panel. The machine uses inputs from the CAD files supplied by the designer to drive the automated milling head. However, because the process is time-consuming, it is suitable only for very low volumes of PCB production.
Plated through-holes (PTH). To facilitate interconnection between layers, PCB fabricators line the inner surface of the holes with a copper layer using a plating process. Once completed, these are called plated-through holes (PTH). A round of visual inspection and electrical continuity testing at this stage verifies the process.
PTHs are necessary for connecting traces on one side of the PCB to traces on the opposite side. These holes may also be required to hold the leads of a leaded component (e.g., axial or radial), although this is becoming less common due to the availability of SMT components. Additionally, the PCB may require some holes to enable it to be mechanically mounted.
For single- or double-sided PCBs, the panel now proceeds to solder masking (or coverlay or coverfilm for flexible PCBs, used to encapsulate and protect the external circuitry of a flexible PCB).
Adding layers. Fabricators add subsequent copper layers to the central layer with a layer of insulation between each. For rigid PCBs, this insulation is usually the prepreg, while for flexible PCBs, this is an adhesive layer.
Multilayer boards will have further layers added to the central or innermost layer. Fabricators add additional insulating and copper layers to each side of the central layer, using heat and pressure to bond them. The copper surfaces on both sides then undergo the same treatment of protective layer removal, followed by drilling, but this time, the drilling depth is controlled so that the copper on the innerlayers remains undamaged. PCB manufacturers use laser or mechanical drills for this process. The same process of applying photoresist and etching follows as above, and this leaves only the traces required by the designer for each specific layer. The drilled holes are then electroplated to provide the necessary connections. Usually, a round of visual inspection and electrical testing follows. For additional layers, the process is repeated. If no additional layers are required, the panel now proceeds to the solder masking/coverlay process.
Solder mask/coverlay and surface finish. Fabricators protect areas of the PCB not intended for soldering by covering them with a protective layer. For rigid PCBs, this is the familiar green layer of solder mask. However, this layer is brittle and not suitable for flexible PCBs. Therefore, flexible and rigid-flex PCBs use a polymer adhesive layer called “coverlay” for the purpose. As mentioned, the flexible circuit coverlay serves the same function as solder mask used on a rigid PCB. The difference with a flex coverlay is the flexibility and durability it provides. The solder mask/coverlay protects the board from contaminants, and from the environment of the assembly process.
To enable leaded or SMT components to be added to the PCB, the solder mask/coverlay has openings at appropriate places, exposing the copper surface. To prevent the exposed copper from oxidizing, fabricators add tin or tin-lead solder, gold, silver, or other combinations of metals to achieve a surface finish. Typically the designer specifies the finish to use.
Legends. The last step in the manufacturing process consists of printing text and other identifiers and legends on the PCB. This can help identify the PCB, marking component locations, and fault-finding instructions. After a final inspection, the PCB is now ready to be populated with electronic components.