The Ultimate Guide to PCB Manufacturing Process Flow
Before the advent of circuit boards, circuit connections were made by point-to-point wiring. However, as the service life of electronic products increases, the aging phenomenon of circuits becomes more and more detailed. This is a serious line reliability problem, and the rupture of the line will lead to an open circuit or short circuit of the line node. To deal with these issues, new PCB manufacturing technologies have been adopted. In this article, FS Technology will split each step of the circuit board fabrication process and give a detailed explanation and presentation. Next, let’s watch a video of the PCB manufacturing process from YouTube.
After watching the above video on the PCB manufacturing process, I hope that you have a deeper understanding of how to manufacture PCB boards. Now, we will further enhance your understanding of the process through the below sections of text.
PCB Manufacturing Process Flow Chart
With the ongoing development of science and technology, more and more types of PCBs are emerging. According to the number of faces and layers, PCBs can be divided into single-layer, double-layer and multilayer PCBs, as well as single-sided, double-sided PCBs, etc. The manufacturing process of different PCB types varies and the greater the number of sides, the more complex the PCB fabrication steps are. The following figures are PCB manufacturing flow charts of a variety of PCB types from FS Technology:
- Single-sided PCB manufacturing flow chart:
Single-layer boards are the simplest type of boards, so the manufacturing process is relatively simple, but as the number of stacked layers of printed circuit boards increases, the complexity and number of steps increases.
- Double-sided PCB fabrication flow chart:
The difference between the manufacturing steps of single-layer and double-layer PCB can be clearly found from the upper and lower comparison diagrams. In fact, PCB manufacturers do not need to carry out copper sinking and pattern plating when manufacturing single-layer panels, but circuit structures with more than two layers do.
- Multilayer PCB manufacturing process flow chart
We call a circuit structure with more than two layers a multilayer printed circuit board. As the number of layers increases, manufacturers need to add more inner layer processes and laminations when manufacturing these boards.
Complete PCB manufacturing process
The first step in making a PCB: Design
The design of a PCB is an important step in the PCB production process and it is usually referred to as PCB design or PCB layout. The boards should be strictly compatible with both the components that will be mounted/soldered on the board as well as the overall project that requires the PCBs and due to these needs, designers typically will create a PCB layout using PCB design tools. The PCB design steps can be subdivided into the following components:
- PCB Schematic: A diagram used to represent the connections of components on a circuit board. It can be said that the drawing of the schematic diagram is the focus of the circuit board manufacturing process and the bloodline of the circuit board. The quality of the schematic is closely related to the quality of the project.
- PCB Diagram: This refers to the installation diagram of the basic electronic components. With some knowledge of PCB boards, you may know that the PCB board is used to cover the metal foil on the underlying insulating plate, and then, the unnecessary parts of the metal foil are corroded. The remaining metal foil is subsequently used as the connections of the PCBA components as now, the components can be assembled according to the marks on the PCB diagram.
- BOM file: This refers to the procurement documents and for turnkey projects, these documents are required for PCBA component procurement, patch processing and soldering.
- Component Footprint Library: A prototype library of the components used on schematics.
- PCB package library: This refers to the external package library of chips, resistors, capacitors, etc. on the PCB board.
- CAM file: This file refers to the Geber and NC Drill files, which can be exported by all design software and are mainly used by PCB manufacturers.
Designing printed circuit boards requires the use of design tools and here at FS Technology, we recommend several popular applications including Altium, Eagle, KiCad, Cadence, OrCAD, Pads, etc.
NOTE: Before the PCB fabrication process, designers should inform their contract manufacturers about the version of the PCB design software used to design the circuit as it helps to avoid problems caused by differences. Once a PCB design is approved for production, designers can then export the design to a format supported by their manufacturer. The most commonly used file type is called a Gerber file.
The second step of the PCB manufacturing process: CCL Manufacturing
CCL is a substrate material which can be divided into rigid CCL and flexible CCL. The copper-clad laminate is key to determining the signal transmission speed, energy loss, and characteristic impedance in a circuit. It plays the role of interconnection conduction, insulation, and support in the PCB. The detailed CCL manufacturing process is as follows: PP cutting → pre-stacking → combination → pressing → disassembly → cutting inspection → packaging → storage → shipment. As the core of the circuit board, dust on the surface of the CCL may cause the final circuit to short circuit or open-circuit, so we need to add a cleaning link in the PCB manufacturing process.
The figure below is a detailed explanation of the inner layer of a 10-layer PCB. The printed circuit board is bonded by the copper-clad laminate and the copper film through the prepreg. The production sequence is to start from the middlemost core board, continuously stack them together, and then fix them.
The third step of the PCB fabrication process: Inner Line
After cleaning the CCL, we need to stick the photosensitive film on the surface of the PCB substrate to prepare for the subsequent image transfer. This film has a property that it cures when exposed to light. Therefore, we can use this film to form a protective film on the copper foil of the CCL.
In order to accurately place the stacking position of the PCB layout film, we need to insert the two-layer PCB layout film and the double-layer copper-clad laminate into the upper layer of the PCB layout film.
After the above preparations are completed, the photosensitive film is exposed to light and cured, and the purpose is to transfer the image of the substrate to the photosensitive film. There are light-transmitting and opaque parts in the film. When the photosensitive film passes through the photoreceptor, the UV lamp cures the photosensitive film in the light-transmitting position. The cured part is the inner circuit of the PCB we need, and the uncured part is removed with lye.
With the photosensitive film removed and the hardened resist covering the copper we want to keep, the CCL goes to the next stage: unwanted copper removal. Just like alkaline solutions remove resist, stronger chemicals will remove excess copper. A copper solvent solution bath removes all exposed copper. At the same time, the desired copper remains fully protected under the hardened layer of photoresist.
Not all copper plates are the same. Some heavier boards require larger amounts of copper solvent and different exposure times. Also, heavier copper boards require extra attention to track spacing. Most standard PCBs rely on similar specifications.
The fourth step of the PCB construction process: AOI Inspection
PCB testing includes many methods of testing such as visual inspection, AOI optical inspection, electrical inspection, functional testing, etc. Usually, AOI testing is used after SMT assembly is completed, but to ensure the yield of PCB, an AOI test is also required after the inner layer circuit manufacturing process is completed. This is because, when the circuit board enters the lamination process, even if the inner layer is found to be wrong, it cannot be modified. Therefore, the copper-clad laminate enters the AOI inspection equipment and after scanning, the equipment will transmit the data of the bad image to the VRS, and then the commissioner is responsible for overhauling.
Formal PCB production link: Lamination
As the name suggests, lamination refers to the bonding of the layers of a PCB together to connect the outer layers to the inner layers, a process that occurs in two steps: delamination and bonding. This step is an actual PCB manufacturing process, which means that the green board (or other colors) we see is formed after lamination. The basic form of PCB consists of laminates, the core materials of which are epoxy resin and glass fiber, and also known as substrate materials. The copper-clad laminate is the specific type of laminate used in PCB production, as mentioned above. In this link, we need to use a new raw material – prepreg. This material has good insulation and can help the bonding between the core boards and the outer copper foil.
In the inner layer structure of the PCB, the lower copper foil and two layers of prepreg are fixed in advance through the alignment hole and the lower iron plate. Then, the fabricated core board is placed in the alignment hole, and finally, the two layers of prepreg, one layer of copper foil and one layer of the pressure-bearing aluminium plate will cover the core board
After the lamination is completed, the printed circuit board clamped by the iron plate is placed on the holder, and then sent to a vacuum heat press for lamination. The high temperature will melt the epoxy resin in the prepreg, and finally, under the action of atmospheric pressure, the core board and the copper foil are fixed together.
After the above process is completed, the iron plate and the pressure-bearing aluminum plate on the upper layer of the PCB are removed. At this time, both sides of the PCB will be covered by a layer of smooth copper foil, and the lamination process is completed. It is important to note that, in addition to the pressure-bearing function, the aluminum plate also plays the responsibility of isolating different PCBs and smoothing the outer copper foil of the PCB.
The above steps are only used for the manufacture of circuit boards with more than four layers.
The sixth step in the PCB fabrication process: Drilling
The purpose of drilling holes in the stacked board is to connect the four layers of copper without contact, and all components mounted on the PCB rely on the precision of the precision drilling. The diameter of the drill is thinner than human hair, about 100 microns. To achieve precise drilling, we need to perform the following steps with the help of compact instruments:
First, we need to run through the entire printed circuit board, and then metallize the hole wall to conduct electricity. To find these tiny holes, precise positioning of the inner core of the PCB is required through sophisticated X-ray drilling equipment. After finding the hole position on the CCL, to ensure that the hole passes through the center of the hole when drilling, it is necessary to make a positioning hole on the PCB. Plus, in order not to tear the copper foil on the PCB when punching, it is necessary to cover a layer of aluminum plate on the upper and lower layers of the circuit board, and finally put the whole board into a punching machine for punching. To improve the efficiency of PCB manufacturing, we usually drill several identical PCB stacks together.
The seventh link of the PCB manufacturing process: Panel Plating
The connections between different layers of the circuit board are all through perforations. After drilling is completed, electroplating is required. FS Technology considers a good connection to be a copper film thickness greater than 25 microns on the hole wall, but since the hole wall is composed of non-conductive epoxy resin and fiberglass board, this makes it difficult. Therefore, we need to put the PCB into a copper plating bath containing sulfuric acid and copper sulfate, so that a layer of conductive material is deposited on the wall of the hole. When we conduct electricity into the solution, copper deposits on the surface of the conductive substance of the circuit board. After the copper plating is completed, tin plating is required, and the plating on the PCB surface is an etching barrier.
The process of transferring the outer layer PCB layout to the copper foil requires the use of photocopied film and photosensitive film, which is similar to the principle of the inner layer copper-clad laminate layout transfer. The difference is that in the transfer of the outer PCB layout, the position where the photosensitive film is cured is not the line. We need to first use copper and then tin the areas where there is no film. After the electroplating is completed, film stripping and tin stripping treatment are carried out. The circuit pattern is preserved on the PCB due to the protection of tin.
As mentioned earlier, the recommended thickness of a copper film should be greater than 25 microns. Therefore, in order to ensure that the holes have good electrical conductivity, the entire copper plating process is automatically controlled by a computer.
The eighth step of the circuit board manufacturing process: Secondary Inspection
For circuit boards, the most important aspect after production is the traces as we know that bare boards won’t work on their own. The working principle of PCBs is to realize the electrical connection between components through traces to complete various instructions, so it is necessary to perform secondary AOI detection on traces. The secondary inspection of PCB is facilitated in the same manner as the previous AOI inspection conducted, so I will not describe it too much here.
The Final Link of the PCB Manufacturing Process: Outer Layer Processing
The layers that make up a PCB include a silkscreen layer, solder mask, copper Layer, and substrate layers. The content of the inner layer has been described above but this section is mainly to explain the outer parts of the PCB layer – the silkscreen layer and solder mask layer.
Solder Mask Layer
The application of a solder mask is an important part of the PCB manufacturing process as it can protect the PCB board. Before applying the solder mask to both sides of the board, it is critical to clean the PCB and cover it with epoxy solder mask ink.
In terms of cleaning, FS Technology uses pickling, ultrasonic washing and other cleaning processes to remove board oxides and increase the roughness of the copper surface. After the cleaning process is completed, the manufacturing of the PCB solder mask begins. The process is as follows:
Cover the solder resist ink at the position that does not need to be soldered → Dry the solvent in the solder resist ink → Form a polymer by irradiating the hardened solder resist ink with a UV lamp → Remove the sodium carbonate solution in the unpolymerized ink → Secondary drying.
Silk Screen Layer
The near-finished circuit board receives inkjet writing on its surface to indicate all the important information related to the PCB.
Additional PCB Fabrication Processes: Paid Items
By reading the full text, you will find that the PCB process flow presentation provided by FS Technology has only 9 steps, while the PCB fabrication process flow chart is more complicated. In fact, what is displayed in the flow chart is the full range of services that the manufacturer provides to customers, including procurement, repair and payment services.
To add extra soldering capabilities to the PCB, we can electroplate them with gold or silver. At this stage, some PCBs can also receive hot air flat pads. Hot air levelling results in an even pad. This process results in the creation of a surface finish. FS Technology can handle many types of surface treatments according to the specific requirements of customers.
As a last resort, technicians can conduct electrical tests on the PCB although automated procedures will already confirm the functionality of the PCB and its consistency with the original design. At FS Technology, we offer an advanced type of electrical testing called flying probe testing, which relies on moving probes to test the electrical performance of each net on a bare circuit board.
The above are all the answers to the common question of “How is a PCB made step-by-step?” In fact, within the professional PCB manufacturing process, due to the different requirements of customers for different circuit board materials and different requirements for the PCB process, there will be slight gaps in the PCB board fabrication process. However, FS Technology will continue to update this topic in the PCB Knowledge Blog, and of course, if you have unique insights into the manufacturing steps of PCB, you can leave us a message at any time.