For anyone involved in the electronics industry, the process followed for manufacturing Printed Circuit Boards (PCBs) is very important. This is because PCBs are the very basis of all electronic circuits, being used to provide the mechanical basis on which circuits are built. PCBs come in different forms—rigid, flexible, rigid flex, and High Density Interconnect (HDI), the difference being mainly in the materials used to fabricate them, and give them their ability to flex or remain rigid. To some extent, the PCB manufacturing process depends on the material being used, but the main stages in the PCB manufacturing process remain the same, irrespective of the nature of the PCB. However, before coming to the manufacturing stage, the designer has to make a few choices depending on the application. These are:
- Selecting the type of PCB required by the application
- Deciding whether the board will have single, double, or multiple layers
- Deciding the mechanical layout, the stackup, and the routing of tracks on different layers
- Producing relevant documents and files for the manufacturing process.
The above steps are important for the success of the final product, and its reliability when used in the application. For instance, if the application demands a component moves back and forth during operation, such as the head of a printer does, a flexible circuit must supply it. Most wearables are shrinking in size, and HDI technology is most suitable for the rigid-flex boards they use. At this stage, the designer selects the material for the PCB design. The complexity of the electrical circuit that goes into the design decides the number of layers on the PCB. With the area available on gadgets shrinking as they tend towards miniaturization, the density of PCBs also increases proportionately. The only option for the designer is to have multiple layers on the PCB to contain the design within the specified mechanical boundary. Depending on the nature of the application, the designer has to decide on the stackup, or the design of consecutive layers. For instance, if the application has high frequency circuits, the designer has to define the impedance and minimize crosstalk. For this, he may have to use power and ground layers alternately, with traces carrying signals in between. While doing this, the designer has also to decide on the width, spacing, and routing of traces, placing of vias and test pads, and more. Once he/she completes the design process, the designer produces an output in the form of standard documentation, which helps the manufacturer fabricate the specified design. This documentation can follow either the Gerber X2 format or the IPC-2581 format.
Selecting Suitable Material for Printed Boards
Material for Substrate
The most widely used material for rigid boards is based on glass fiber known as FR-4. Being reasonably priced, FR-4 also has an appropriate degree of stability under temperature variations, and does not break down easily. Other cheaper materials such as paper phenolic are also available and used for low cost commercial products. On the other end of the spectrum are substances such as Teflon or PTFE used for substrates, offering very low losses and a stable dielectric constant for high performance high frequency designs. Flexible, rigid-flex, and HDI PCBs typically use Polyimide based substrates, although Teflon and Kevlar are also used for high frequency and high performance applications.
Material for Cladding
A PCB requires copper traces and planes, and these originate in the form of copper cladding on the substrate—a thin sheet of copper bonded to the substrate. The designer can specify the thickness of the copper cladding, as the manufacturers make them in a few standard thicknesses. Selecting the copper on a circuit board is also critical to reaching and maintaining desired performance levels, especially for adhesion to the substrate and performance at high frequencies. Commercially, manufacturers use two types of copper foils for PCBs—Electrodeposited (ED) copper and Rolled-Annealed (RA) copper. The two types of copper foils have different forming processes, and undergo different treatments for improving and preserving adhesion to various substrate materials. For instance, formation of ED copper follows electrical deposition of copper on a slowly rotating polished stainless-steel drum, placed in a solution of copper sulfate. The copper foil is removed in a continuous roll, with the side against the drum providing the smoother finish. On the other hand, ingots of solid copper successively passing through a rolling mill produce RA copper foils. The two types of copper possess different qualities. ED copper used on PCB substrates are useful for applications where mechanical stress may be critical, while RA copper is suitable for applications involving thermal shock. When subjected to thermal cycling conditions, PCBs with ED copper may develop cracks in narrow conductors. The grain structure of PCB copper varies according to its manufacturing process. For instance, manufacturers offer ED copper foils in coarse, moderate, and fine grain structures, and is linked to the finish on the copper surface. Although a copper foil with a coarse structure also has a coarse surface finish, and enables a stronger bond between the copper foil and the dielectric laminate, it typically exhibits a greater insertion-loss performance at higher frequencies. Additionally, skin effect at higher frequencies worsens the insertion-loss performance of the rough surface of copper traces on a PCB.
Multilayer PCB 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 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.
Drilling and Etching the Central Panel
The panel is dried and is usually heated to remove excess moisture. Using details from the drilling files submitted by the designer, the fabricator proceeds to drill the necessary holes for the central layer. This starts with drilling the registration or locating holes on the periphery of the panel, and the CNC machine changes its drills to match the diameter for individual holes specified in the drill file. The registration holes are necessary for aligning subsequent layers. To obtain the correct pattern of tracks on the two sides of the panel, fabricators use a combination of a photographic process followed by a process of chemical etching. Typically, the copper surfaces on the drilled panel are covered with a thin layer of photoresist. Each side is then exposed to UV light through a photographic film or photo-mask detailing the optically negative pattern of tracks specified by the designer for that layer. UV light falling on the photoresist bonds the chemical to the copper surface, and the rest of the unexposed chemical is removed in a developing bath. This stage is usually supplemented with a visual inspection. When placed in an etching bath, the etchant removes the exposed copper from the panel. This leaves behind the copper traces hidden under the photoresist layer. During the etching process, concentration of the etchant and time of exposure are both critical parameters to obtain optimum results. Stronger than necessary concentration and a longer time of exposure can result in over-etching copper from under the photoresist, leading to tracks with widths thinner than specified by the designer. After successful etching, the photoresist is washed away to leave the necessary copper tracks on both sides of the central layer. Although the photographic process is very popular, other methods are also available. For low volumes or prototype PCBs, an etch-resistant ink may be transferred onto the copper surface using a silk screening process, with the silk screen representing the optically negative pattern of the tracks required. Another dry process uses a specialized, highly accurate milling machine to remove the unwanted copper from the surface of the panel. The machine uses inputs from the documents supplied by the designer to drive the automated milling head. However, as the process is time-consuming, it is suitable only for very small quantities of PCB.
Plated Through Holes
Holes in a PCB are necessary for connecting traces on one side to traces on the other. Sometimes, these holes may also be required to hold the leads of a leaded component, although this is becoming a rare requirement due to the availability of SMD components. Additionally, the PCB may require some holes to enable it to be mechanically mounted. To facilitate interconnection between the layers, PCB fabricators line the inner surface of the holes with a copper layer, using a plating process. On completion, these 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 type 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
Fabricators 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. Fabricators 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 such as Rush PCB Inc. use ultrasonic or laser drills in place of mechanical drills for achieving greater accuracy and reliability. The same process of photoresist and etching follows as above, and this leaves only the traces required by the designer for the specific layer. The drilled holes are then electroplated to provide the 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
Fabricators 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. However, this is a brittle layer, and not suitable for flexible PCBs. Therefore, flexible and rigid-flex PCBs use a polymer adhesive layer called coverlay for the purpose. The solder mask/coverlay protects the board from other contaminants as well, as the 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 consists of printing text and other idents on the PCB. Usually, this helps in identifying the board, marking component locations, and fault finding instructions. After a final inspection, the PCB is ready for dispatch.