Printed circuit boards provide mechanical and electrical support for electronic devices. They are used to connect electronic components using conductive pathways and signal traces from copper sheets which are laminated onto a non-conductive substrate. PCBs are crucial for electronic devices to function properly, ranging from household appliances to smartphones and computers, to high-tech machines.
The PCB manufacturing process is quite complex, requiring multiple steps which all contribute to the development of the board. Each critical step is performed with meticulous attention to detail, to ensure that the PCB prototype or batch of PCB assembly is without defects.
The process starts with design and review stages, using PCB circuit board design CAD tools, and goes on through the production of the boards. To avoid human error and facilitate speed, many steps are computer-guided and machine-driven. This is necessary to prevent short circuits or incomplete circuits. For quality assurance, the boards are subjected to strict testing at various points of fabrication, and finally as complete boards, before they are packaged and shipped for delivery.
The initial step of the PCB manufacturing process is the design. This will be the schematic layout that guides the fabrication process of the board. It is a plan that is laid out in line with all the requirements of the intended board. This design is usually carried out using computer software. Some commonly-used PCB design software includes Altium Designer, KiCad, Eagle, OrCAD, and Pads, but the most common is a software called Extended Gerber, also known as IX274X.
Extended Gerber is a remarkable tool for PCB design because it also functions as an output format. It encodes vital information required by the designer, such as the number of solder masks needed, copper tracking layers, drill drawings, and the other pieces of component notation.
When the Gerber Extended software has successfully encoded the design blueprint for the PCB, it performs oversight algorithms on all aspects of the design to ensure that no errors go undetected. At this point, the designer also scrutinizes elements relating to track width, board edge spacing, trace and hole spacing, and hole size.
Once the designer is satisfied with the assessment, the finished design is sent over to a PCB fabrication house for the production of the PCB prototype. Here, the fabricator performs another check known as a Design for Manufacture (DFM) check. This is to ensure that the design fulfills the minimum requirements for the tolerances needed for manufacture.
This is a key step in the printed circuit board fabrication process. It involves checking the design for potential defects or flaws such as PCB layout errors, violations of the manufacturing guidelines, and verifying the design's adherence to the required specifications.
An engineer performs a design review by examining every part of the PCB design to ensure there are no missing components or incorrect structures. Once the engineer clears the design, it can proceed to the printing phase.
Some vital questions for this review include the following:
Does the design meet the specified project requirements?
Is the design manufacturable?
Are we sure to avoid common design mistakes?
Black ink to show the conductive copper traces and circuits of the PCB. The same colors are employed on the outer layers. After the film is printed, they’re lined up and a hole, known as a registration hole, is punched through them using a punch machine. The registration hole is used as a guide to align the films later on in the process.
Here is where the actual PCB production begins. A copper foil layer or coating is applied on the piece of laminate material where the PCB design has been printed. Then the copper is pre-bonded to the laminate which functions as the structure for the PCB. Next, the copper is etched away to reveal the earlier discussed design blueprint.
Following the above, a photo-sensitive film called the resist is used to cover the laminate panel. The resist contains photo-reactive chemicals that harden from exposure to ultraviolet light. This light is administered after the resist and laminate have been aligned using the holes punched in earlier.
The UV light passes through the sheer parts of the film, solidifying the photoresist and revealing the pathways of copper. Black ink prevents any light from getting to the areas that aren’t meant to harden so that they can be removed later on. After this, the bare board is washed with an alkaline solution and then pressure washed to remove the excess photoresist and other unwanted materials on the surface. Then the board is left to dry.
The next step is to remove any unwanted copper on the core or inner layers of the PCB. This is usually done by covering the required copper on the PCB panel and applying a chemical solution to the rest of the board so that it may etch away the unwanted copper. The chemical etching process removes all unprotected copper from the PCB, so it is crucial to properly cover the necessary amount of copper on the board. The hardened photoresist is not affected by this exercise.
For multilayer printed circuit boards, there are additional steps in the fabrication process to account for the added layers in the design. These extra steps are closely similar to those used during single-layer PCB production, but the stages are repeated for each layer of the board. Another exception in multilayer PCBs is the replacement of copper coating with copper foil between the layers.
The procedure for inner layer imaging is essentially the same as PCB design printing. A plotter printer prints the design and produces a film. The solder mask for the inner layer is also printed out. These two are then lined up to receive a registration hole which aids in proper alignment with the laminate material for the inner layer that has been infused with copper.
The design prints on a plotter printer to create a film. The solder mask for the inner layer also prints out. After aligning both, a machine creates a registration hole in the films to help keep the films lined up properly with the layers later. Then comes the introduction of ultraviolet light which exposes the film or resist, causing the white-colored areas to harden into the printed pattern.
As previously discussed, etching strips off the excess copper from the board. At this point, the areas covered by light ink have hardened. This hardened material protects the copper beneath, ensuring that it remains on the board after etching.
First, the board is washed with alkaline to remove any leftover resist that didn’t harden. Next, the technician will etch off the excess copper from the exposed non-conductive areas by dipping the board into a copper solvent to remove the exposed copper.
Resist stripping is essentially cleaning off any remaining resist on the copper of the PCB inner layer. This prevents interference with the copper that would affect its conductivity. Completing this step allows the manufacturer to inspect the basic design of the PCB layer.
In this step, the layers are aligned so that a hole can be punched through them using the registration holes as a guide. To execute this, a computer-controlled machine known as an optical punch is used. This automated machine is also responsible for inspecting the hole and alignment, which is the next step in the board fabrication process.
Inner layer automated optical inspection involves the use of a computer to carefully examine the inner layer to search for incomplete patterns or resist that may remain on the surface. If the machine detects any unwanted elements, the board is taken back a few steps for etching and stripping, but in the absence of such detection, it proceeds to the next step in the process.
Applying oxide to the PCB inner layer ensures that the copper foil bonds properly with the insulating epoxy resin layers between the inner and outer layers of the circuit board.
In the layup step, a machine is used to align, heat, and bond the layers of a multilayer PCB together with a copper foil layer and insulating material between the inner and outer layers. These machines are usually steered by computers because the alignment of the layers and bonding must be precise to ensure a perfect structure for the printed circuit board.
Lamination applies heat and pressure to dissolve the bonding epoxy between the layers. When p PCBs are properly laminated, their layers will hold tightly together with effective insulation between them.
This is where an X-ray is used to ensure proper placement of the drill bit when drilling multilayer boards after lamination. These holes are important for connecting the layers of the multilayer PCB. So they must be accurately placed and sized while taking the rest of the layer and the other layers into consideration. After the X-ray alignment of the layers, the board is drilled, and then reaches the subsequent steps common to both single and double-sided PCB production.
This step is to place the layers for alignment. The earlier-drilled holes are to align the inner and outer layers. This is done with the aid of an optical punch machine that drills a pin through the holes to keep the layers of the PCB lined up. After this, another machine inspects the board to ensure that it is devoid of errors.
After the optical punch stage, another machine is introduced to carry out an optical inspection on the PCB to make sure it has no defects. This automated optical inspection is crucial because once the layers are placed together, any errors on the board cannot be fixed. For optimum assurance of perfection, the AOI machine compares the PCB with the Extended Gerber design, which serves as the model for the manufacturer.
After the PCB has passed the optical inspection, which is inclusive of machine and human inspection performed by the technician, it is cleared for the remaining steps of PCB manufacture.
The PCB laminating process is done in two steps: the lay-up step and the laminating step. This fusing can only occur when the layers are ascertained as being free of defects.
To fuse the layers, metal clamps are used on a special press table. Each layer fits onto the table using a specialized pin. A layer of pre-coated epoxy resin known as prepreg is placed on the table’s alignment basin. Then a substrate layer is placed over the prepreg resin, followed by a copper foil layer and more sheets of prepreg resin.
Lastly, one more copper layer is applied, which is known as the press plate. A mechanical press is then used to press the layers together. Pins are punched through the stack of layers to ensure they are properly aligned and secured.
After this, the PCB goes to the laminating press, which applies heat and pressure to the layers, using a pair of heated plates. This causes the epoxy to melt inside of the prepreg so that it combines with the pressure to fuse the layers. Once the PCB layers are pressed together, the top press plate and the pins from earlier are then removed, allowing the PCB to be freely pulled out. 
Before drilling starts, an X-ray machine is used to pinpoint the drill spots. Then, registration holes are drilled so that the PCB stack-up can be secured in place before the actual intended holes are drilled. To drill these holes, a computer-guided drilling machine is used to make the holes that expose the substrate and inner panels, using the file from the Extended Gerber design as a model. Any remaining copper is removed after the drilling is complete.
The copper plating process happens after the board has been drilled. A chemical solution is applied to fuse all the layers of the PCB. Then the board is thoroughly cleaned and bathed in a series of chemicals. These chemicals also coat the panel with a thin layer of copper, which pours into the drilled holes from the topmost layer on which it is deposited.
This stage is similar to step four above, as the board receives another layer of photoresist. Only this time around, the application is just on the outer layer. After this coating, the board is sent for imaging, where UV light hardens the photoresist and the unwanted parts are removed afterward.
Following all these, the outer layer is plated with a thin copper layer, as was previously performed with the interior layer, then it receives a thin layer of tin to help protect the copper of the outer layer from being etched off. This is known as tin plating.
During the outer layer etching, the earlier mentioned tin guard protects the needed copper from the chemical solution used to perform the etching.
Recall that it was noted earlier that the inner layers use dark ink for conductive areas and clear ink for non-conductive parts, while the reverse is the case for the outer layers. Thus the copper of the outer layer has light ink that allows for the tin guard to protect it. This is the main difference between the inner layer and outer layer etching. After the remnant copper and resist have all been removed, the layer is set for inspection.
Automatic optical inspection is as important for the outer layer as it is for the inner layer. Performing this inspection ensures that the layer is taking form just as the design dictates and that there are no harmful copper remnants from the etching stage that would create defects on the circuit board like erroneous electrical connections causing short circuits.
The panels usually receive meticulous cleaning before the solder mask is applied. An ink epoxy and solder mask film are applied on each panel. Then a concentration of UV light indicates where the solder mask needs removal, while the required solder mask is baked onto the board. This is done by putting the board into an oven to cure the mask. The solder mask provides the green color usually found on a circuit board, as well as protection from the damaging effects of oxidation and corrosion.
Silkscreen application is vital because this is where vital information is directly printed onto the board. This process is also called legend printing. The data may include warning labels, part numbers, company ID numbers, logos of manufacturers, etc.
This printing is usually done with an inkjet printer, after which surface finish application commences on the PCB.
Finishing the PCB involves plating the board's surface with conductive materials.
A surface finish is vital for creating a reliable connection between the PCB and the electronic component. The major function of a surface finish is to provide a solderable surface for efficiently soldering components to the PCB and to protect any exposed copper from oxidizing due to air exposure.
Surface finishes are now usually lead-free, in accordance with Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives. They include:
ENIG (Electroless Nickel Immersion Gold)
ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)
Electrolytic Nickel/Gold - Ni/Au (Hard/Soft Gold)
White Tin or Immersion Tin
OSP (Organic Solderability Preservatives)
Organic solderability preservative (OSP): RoHS compliant, cost-effective, short shelf life
Lead-Free HASL (Hot Air Solder Leveling)
The correct material is determined by the design specifications and the manufacturing budget.
At this stage, a series of electrical tests are performed on different areas of the board to make sure it meets the required functionality. Electrical testing must follow the standards of IPC-9252, Guidelines, and Requirements for Electrical Testing of Unpopulated Printed Boards. The PCB is mainly tested for circuit continuity and isolation.
The circuit continuity test checks for any opens (disconnections) in the PCB, while the circuit isolation test checks the isolation values of different parts of the PCB, looking for shorts in the circuitry. Apart from ensuring functionality, the tests also seek to verify how well the initial PCB design accounted for the manufacturing process.
Another common test to check PCB functionality is the “bed of nails” test. To administer this test, several spring fixtures are attached to the test points on the circuit board. The spring fixtures then supply an enormous amount of pressure to the test points on the circuit board (usually about 200g of pressure) to see how well the board tolerates the high-pressure contact at its test points.
For small to medium-volume production, a flying probe test is commonly used to check electrical contacts. The flying probes testers utilize moving heads to make contact with the copper lands and holes to verify the electrical connectivity of the circuit board.
For this step, the fabricator needs to identify the shape and size of each individually printed circuit board cut from the construction board. This data is usually contained in the Gerber files for the design. Profiling functions to steer the routing out process by programming where the machine should make the scores on the construction board.
Routing out, or scoring, makes it easier to separate the boards in the circuit board assembly. A CNC machine or router creates several small pieces along the edges of the board. These edges allow the board to quickly detach or break off without damage.
Another available option is to use A v-groove. This is where a machine creates v-shaped cuts along the sides of the board. In either case, the boards will cleanly separate without cracking.
The board is subjected to inspections and quality checks at several points in the PCB fabrication process. This is for the crucial purpose of ensuring that the finished product is a high-functioning circuit board devoid of defects. Here is another checkpoint for the printed circuit board assembly (PCBA) to be inspected before it is packaged and delivered. Many aspects of the board's structure need to be checked to verify the following:
That the hole sizes are correct on all layers and meet the design requirements.
Board dimensions are exactly like those in the design specifications.
that the boards do not have dust or debris on them.
That the finished boards are devoid of burrs or sharp edges.
That all boards that failed electrical reliability tests go through repair and retesting. 
This is the last stage of the PCB manufacturing and assembly process. During packaging, the printed circuit boards are covered with sealing material to keep out dust and other elements. Then the sealed boards are moved into containers that protect them from damage during shipping and delivery. 
The printed circuit board (PCB) manufacturing process is an extensive and intricate procedure, where each step must be carefully executed to ensure optimum performance of the finished board. The fabrication process for single, double, or multilayered PCBs are essentially the same, however, the stages are repeated for each layer of a multilayered board.
The design specifications and board requirements may extend the process to contain 20 steps or more, according to the complexity involved. It is ill-advised to skip any step or cut back on the processes as this could bear negative consequences for the circuit board's performance. Strict adherence to the required procedures would yield boards quality boards that meet functionality standards.
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