PCB manufacturing is a process of fabricating real-life electronic printed circuit boards. The PCB fabrication follows a complex procedure that involves several steps that work together to make sure that the PCB functions well. Mainly the PCB is divided into two categories single or double layers and multilayers. The manufacturing process for single-layer and double-layer PCBs is similar and simple. While the manufacturing process of multilayer PCBs is a bit complex and requires special knowledge and experience to design and fabricate. Hence, complex multilayer PCBs may involve up to 20-steps for manufacturing.
Each step in the PCB manufacturing process is of great value and requires skilled hands to execute. Minor negligence in the manufacturing stage can negatively affect the performance of the PCB. Due to the accuracy needed for the manufacturing process and the importance of proper handling, it is crucial to select an experienced PCB manufacturer that provides a wide range of services like NextPCB. At Next PCB, we perform multiple quality checks on our manufactured PCBs to ensure the delivery of a well-functioning circuit board.
A flowchart describes the stages of a process visually. To aid your understanding regarding the PCB manufacturing process, NextPCB is representing a flowchart.
Please attach the PCB manufacturing flowchart here.
As stated above PCB manufacturing process involves several stages depending on the complexity and unique features of the circuit design. Understanding the manufacturing process helps designers develop a deep knowledge of how PCB fabrication impacts the performance of the circuit board.
An understanding of the PCB manufacturing process and its impacts on the output enables PCB designers to utilize such techniques that go well with the fabrication process. So, now that we know the importance of being familiar with the procedure of PCB production. Let's get its in-depth knowledge.
Schematic design is the first stage of PCB production. In schematic design, the engineer describes the building blocks of the several circuits integrated on a PCB. A schematic design narrates from where an electrical signal will begin, where it will connect with other components, and where it will get end. The schematic drawings use symbols of electronic components that visually represent the components and help layout designers to draw the blueprint of the printed circuit board.
After the completion of schematic drawings, PCB designing begins. PCB designing starts with complete planning and appropriate know-how of the functions of the circuit board, as the design parameters of a circuit board affect its electrical properties. The designer starts drawing a layout design by following the schematic drawings. The layout is the second stage of PCB production; it begins with outlining and declaring the board size and involves component placement on the board. At the time of component placement, the designer ensures that all the components are well-placed so that the signals routing around them do not cross each other. After the component placement, the designer creates a netlist by assigning each pad its dedicated net. A netlist may describe logic, connections between components, and hierarchical relationships. Further, the netlist helps in structuring the circuit board.
After the placement of components and generation of the netlist, it's time to route the signal traces. Now at this stage, the designers have two options for routing the PCB. Number one is auto-routing, which is possible due to netlist, and offered by the PCB designing software; the other one is manual routing which a designer does by himself to meet specific design parameters.
To lay out the PCB blueprint, the designers have a wide range of PCB designing software, for example, Altium, Altium 365, Extended Gerber and Fusion 360.
Once the routing of the PCB is complete, the designer checks it against the schematic and all other aspects of PCB designing. This stage of designing is crucial to ensure; that there are no errors in the circuit board because once the PCB is manufactured, it is almost impossible to make changes in the design and get the desired output without modifying the PCB in external ways. Hence, the designers check them in different ways to find and eliminate the error, if any.
As soon as the testing gets ends, the Gerber file of the circuit design is sent to the PCB manufacturer, where the actual manufacturing begins. When the PCB design file arrives at the manufacturing house, the manufacturer first checks the file for the Design for Manufacturing (DFM) test. The DFM test reports if the design meets the minimum manufacturing requirements for the circuit board or not.
At this stage, the manufacturer starts his part of the work by printing the Gerber files onto laminates. But before getting printed onto the laminates, the Gerber files are printed onto a transparent sheet and make "films" known as the photo negatives of the PCB design or photo tools. For printing the Gerber files onto transparent sheets, the manufacturer uses a special kind of printer known as a plotter. The plotter uses two colours of ink, clear and black, to print photo tools; these different colours of ink have individual functions.
Clear Ink: The plotter use clear ink to print the conductive paths of the printed circuit board.
Black Ink: The area printed with black ink refers to the non-conductive space of the PCB.
Note: For some reason, the functionality of ink is inversed for the outer layers of the PCB. This means that for outer layers, the clear ink denotes the area required to remove, while the black ink denotes the conducting paths of the circuit.
The number of films required for a PCB depends on its layer count. However, for the printing of the solder mask, the solder mask needs its "films." So, for a two-layer PCB, we need to print four films, and for a 4-layer PCB, we need to print six films.
Once the printing of films is complete, the manufacturer aligns all the photo tools and punches some holes through them, known as registration holes. Later these holes help align the photo tools with the laminate and fibreglass sheets.
Lamination is a stage in PCB production at which the manufacturer stacks the copper layers, prepreg and laminates together and aligns them between two thick plates of steel and screws that plates together to generate pressure. Then the ready stack rests in a high-heat oven for curing, which is a process that melts the prepreg, so it works as an adhesive to bind the cores and copper layers together.
But before lamination, the laminates undergo a lengthy procedure consisting of several processes. So, let's have a look at them.
So, once the photo tools are printed, they are ready to get converted onto the laminates. Before the conversion of photo tools onto the laminates, the laminates are pressure washed to eliminate any dust or dirt to avoid issues like short circuits and open circuits. Then a coat of photo-sensitive material known as photoresist is applied to the laminate.
Now, to convert photo tools onto laminate, the manufacturer uses a UV printer. The UV printer has pins on its surface that works as a guide to align the photo tools with the laminate, as the laminate and photo tools have holes punched into it previously. Now, on the printer surface, the operator sandwiches a laminate between two photo tools and then lid down the printer. Then the printer starts its operation, and the area of photo tools printed with black, which represents the non-conducting parts of the circuit, absorbs the UV light and remains un-harden. Whereas the conducting surface of the circuit board, which is printed with clear ink, lets the UV light pass through it and gets polymerized or hard.
At the stage of image development, the UV-exposed laminates are pressure washed with an alkaline solution that removes the unharden layer of photoresist material from the laminates.
After removing the unharden layer of resist, the laminates are again pressure washed with an alkaline solution that etches the exposed copper from the laminates. Now at this stage, we get the exact same circuit design printed onto the laminates as we lay out in the software.
After the etching, the dry film is chemically removed from all over the laminate, which leaves the whole circuit exposed.
Before processing the finished laminate for drilling, it is significant to confirm that there are no errors in the laminates because, later, it is almost impossible to make changes in the inner layers of the PCB. To eliminate human errors, the manufacturer uses Automatic Optical Inspection(AOI). The AOI uses data provided by the Gerber files to check for errors. If any error is found, it is immediately shared with the concerned department to correct the mistake. Otherwise, the process of PCB manufacturing carries on without any disturbance.
If the PCB is a multi-layer printed circuit board, after the Automated Optical Inspection and prior to the lamination, the inner laminates are chemically treated to make the surface of the laminates a little rough to aid the adhesion of the laminates during the curing process and increase laminate bond strength. At NextPCB, we use organic chemistry to oxide the laminates, which leaves the copper a dark brown in colour. However, there are other methods available too, that provide different colours.
The Aims behind Brown Oxide.
(1) Roughening the copper surface to increase the surface area in contact with the resin.
(2) Increase the wettability of the copper surface to the flowing resin.
(3) Make the copper surface passivated to avoid an adverse reaction.
The lamination is the process of binding cores, prepreg and copper foil together under high heat and high pressure. At this stage, the high heat melts the prepreg, and pressure helps the laminates to bond together and make a strong bond. After the lamination, we get a perfectly stacked board.
After the lamination, the panel is ready for drilling vias and holes for through-hole components. Before drilling the holes, an X-Ray machine is used to locate and mark the spots where holes have to drill. Once the marking is complete, the laminates go for drilling. To avoid errors, the drilling of holes is a computerized process.
The normal-size holes are drilled with the standard machinery, but drilling tiny vias and holes requires special machinery.
The drilling process causes some uneven raised in the edges of the metal. Burr represents the same elevated copper. Whereas the deburr is the mechanical process of smoothing the elevated copper area. This process also clears the holes from the burr and provides smooth holes for electroplating.
Desmear is a process of removing a thin layer of resin that usually appears on the inner copper layers due to heat and drilling. The desmear only applies to multi-layer circuit boards and enhances the electrical connectivity of the signal traces.
A more comprehensive answer to why we should execute desmear is that it exposes the copper rings that need to be interconnected in each layer. Further, the bulking agent can improve the pore wall structure and enhance the adhesion of copper plating.
KMnO4 is an important desmear raw material known as a debinding agent.
After the deburr and desmear, the walls of the holes become unconducive, which is why electroless copper deposition is necessary. Electroless copper deposition refers to the depositing of a specific amount of copper onto the walls of holes through a chemical process to make them conductive. The thickness of electroless copper deposits ranges between 45-60 millionths of an inch (microinch).
The process of handling the outer layers of the printed circuit board is the same as for the inner laminates. The outer layers use the same photoresist that the inner layers use. The photoresist material covers the entire board, including the drilled holes and vias.
Copper plating is a process of depositing copper on the exposed copper circuit through an electrical process. The thickness of deposited copper depends on the needed final finish of the circuit board. However, the most commonly deposited amount of copper is 1mil (0.001).
After the electroplating of copper, the manufacturer electroplates tin on the exposed copper circuit. So now, the question is, why do we need to electroplate tin on top of copper? The tin works as etch resist during the etching of the outer layers. This means that during etching, the alkaline solution will only be able to dissolve tin, and the copper beneath the tin will remain intact.
At this stage, we remove the developed photoresist from the outer layers of the circuit board; that we applied in the previous step. While removing the resist, the tin plating remains intact. After removing the resist strip, all the holes covered through it are now clean. It is the first step in the SES (strip-etch-strip) process.
After removing the resist strip, NextPCB etches the exposed copper area that the tin was not covering. By the end of the etching, we only have the pads and signal traces of copper covered with tin plating.
At this stage, NextPCB also removes the tin plating through a chemical process, which leaves the copper beneath it exposed. It is the last step in the series of the SES (strip-etch-strip) process.
After the Strip-Etch_Strip process, for better electric connectivity, before applying the solder mask, it is necessary to confirm that all the signal tracks, pads, through-holes and vias are clean and free of oxidation. When cleaning the laminates, the manufacturer also scrubs the panels with pumice to remove oxidation and make the laminates a bit rough to enhance the adhesion of the solder mask.
At this stage, the finished panel is coated with a liquid photo-imageable solder mask that thoroughly covers the laminate. After the application of the solder mask, the PCB is dried but not fully cured and exposed to intense UV light using a film tool. Then the PCB goes through another process that exposes the copper pads and holes.
Silkscreen is the process of printing relevant information, such as the component number, placement position and company logo on the PCB. The silkscreen requires a specific ink. This specific ink usually has a standard colour of white, but the colour may vary according to the requirements.
Standard printed circuit boards (PCBs) possess a copper layer on the circuit board. To prevent the copper layer from oxidation and damage, it is necessary to protect it. Otherwise, it will affect the quality of the performance and subsequent soldering. The method that the manufacturers used to protect the exposed copper pads and holes is simple and called surface finish. There is a variety of surface finishes available but the most commonly used protective layers are: hot air levelling (HASL), organic coating (OSP), electroplated nickel gold (plating gold), chemically immersed nickel gold (ENIG), gold finger, immersed silver (IS) and immersed Tin (IT) and so on.
It is a type of finish for PCBs in which the whole PCB is supposed to dip in melted solder so that the exposed copper gets enough coat of solder. However, during this process, some exposed copper areas get excessive solder coating, which is later removed by passing the PCB through hot air. The PCB experience hot air with intense pressure from both the front and back sides. This blast of air melts and removes the excess copper and levels it by spreading it all over the surface.
There are two types of Hot Air Leveling, lead and lead-free. However, only lead-free Hot Air Leveling is RoHS compliant.
Organic coating PCB finish involves coating a liquid organic compound on the exposed copper that bonds with it and preserves it until the soldering. The organic compounds used for OSP usually belong to the azole family. For example, benzotriazoles, imidazoles, and benzimidazoles.
Gold finger: The edges of the connector, known as fingers, need an extra efficient finish, so the manufacturer deposits gold in that area through electroplating. PCBs with gold finger plating always perform longer and prevent connector edges from deterioration.
Once the process of surface finish is complete, the boards are cut according to the marked size on a CNC machine or router. The different manufacturers use individual methods to cut PCBs, such as V-cut machines and stencils. After cutting, the circuit boards are checked for cleanliness, sharp edges, burrs and other fabrication requirements. Chamfers, slots, countersinks and bevels are added during the rout & fabrication processes.
Electrical testing of the PCB is among the last stages of PCB manufacturing. The manufacturer performs electrical tests to check the PCB for electrical shorts and discontinued signal tracks. There are multiple methods available to test the boards. Usually, the manufacturer loads the main file or the data provided by the Gerber file in the machine, and then the testing machine automatically checks the PCB. NextPCB offers three types of PCB tests to ensure the manufacturing quality of the printed circuit board.
Flying Probe test:
According to the manufacturers, the most effective method of PCB quality check is the flying probe test. In the flying probe test, two probes work on the circuit board, touch the signal tracks and verify them against the Gerber file. The flying probe test helps identify whether all signal tracks have been copied accurately on the laminates and copper foil during production.
The universal on-grid test is excellent for the printed circuit board with multiple points to check. Such as backplanes, server circuit boards and motherboards. Usually, the universal on-grid test is performed on circuit boards with large areas.
Dedicate PCB testing is another affordable PCB testing method. The dedicated adapters in the machine pass a voltage to the probes, which are then transferred to the PCB. This way, the PCB is checked against continuity and other parameters.
Visual inspection of the printed circuit boards is the last step of PCB manufacturing. During the visual inspection, a person inspects the PCBs under a magnifying glass to confirm that the PCB is up to the mark and meets the industry specifications and the quality standards of NextPCB. At this stage, the visual inspector also verifies the physical dimensions and holes of the PCB.
Finally, the circuit boards that pass the visual inspection are shrunk-wrapped and delivered to their owners with all necessary certificates and other documents.
The manufacturing process of a printed circuit board has a tremendous effect on its performance. A PCB manufacturing process may contain twenty stages depending on the complexity of the circuit design. The standard boards are easy to manufacture, but PCBs with complex and unique features require special skills, observant handling and special machinery to carry out the procedure.
The manufacturing process of a metal-backed PCB, such as an aluminium-backed PCB is different from a fibreglass epoxy-based PCB, such as FR-4 PCB. However, both types of PCBs require skilled labour and careful handling.