Introduction

Printed Circuit Boards (PCBs) are the backbone of modern electronics, providing the physical foundation for electronic components and circuits. Understanding the components and fabrication process of PCBs is crucial for engineers, designers, and anyone interested in electronics.

PCB Fabrication Process

The manufacturing process of a Printed Circuit Board (PCB) undergoes multiple steps to translate the design blueprint into a professional-looking board. Typically conducted within a specialized PCB fabrication facility, this intricate process employs various techniques such as etching, drilling, plating, and integrates Computer Numerical Control (CNC) technology for high precision.

The copper layer undergoes a series of steps multiple times. It is coated with a photosensitive film, exposed to ultraviolet light through a mask representing the circuit pattern, and subsequently developed. This process exposes the circuit pattern, allowing for selective copper removal through chemical etching. The end result is a circuit pattern revealed in copper, with holes drilled for through-hole components and vias, followed by plating to protect the copper and enhance conductivity.

The culmination of the fabrication process involves the application of a solder mask, which serves to shield the circuitry, prevent solder bridges, and enhance the overall robustness of the PCB. Additionally, a silkscreen layer is printed onto the board, providing essential labels and markings for component identification.

Steps Involved in PCB Fabrication Process

Step 1 – Imaging and Printing the Design

Step 2 – Creating the Substrate

Step 3 – Printing the Inner Layers

Step 4 – Ultraviolet Light Blasting

Step 5 – Removing the Unwanted Copper

Step 6 – Layer Alignment and Inspection

Step 7 – Laminating and Bonding the Layers

Step 8 – Drilling

Step 9 – PCB Plating

Step 10 – Outer Layer Imaging

Step 11 – Tin Plating

Step 12 – Final Etching

Step 13 – Solder Mask Application

Step 14 – Silkscreening

Step 15 – Surface Finish

Step 16 – Testing

Step 16 – Profiling

Step 16 – Final Quality Check

Step 1 – Imaging and Printing the Design

PCB printing begins after designers submit the Gerber files, and manufacturers conduct a DFM check. A special printer, called a plotter printer, is used to print the design of the PCB. It produces a photo film of the PCB that displays the details and layers of the board. Two ink colors are used on the inner layer of the board for this process.

Clear Ink to show the non-conductive areas.

Black Ink to show the conductive copper traces and circuits.

The same colors are used for the outer layers, but their meanings are reversed. Each layer of the PCB and solder mask receives its own clear and black film sheet. In total, a two-layer PCB requires four sheets: two for the copper layers and two for the solder mask. To proceed further, all the film layers must be perfectly aligned with each other. To achieve perfect alignment of all films, registration holes should be punched through all films. These holes will align with the registration pins (a predefined structure) in the subsequent step of the imaging process.

Step 2 – Creating the Substrate

The substrate, which is the insulating material (epoxy resin and glass fiber) that holds the components on the structure, begins to form by passing the materials through an oven. It is the rigid core of the PCB that provides robustness and strength to the onboard circuitry.

Step 3 – Printing the Inner Layers

This step in the PCB manufacturing process marks the beginning of creating the actual PCB. The process commences with the fundamental form of a PCB, consisting of a laminate board made from the substrate material. Simply put, it can be described as a mask layer utilized to pattern photosensitive material onto the copper. In this step:

● The PCB design is printed onto the laminate board

● Copper is pre-bonded on both sides of the laminate board

● Next, the laminate board is covered with a photosensitive film called the resist.

This photosensitive film is made from photo-reactive chemicals that harden when exposed to ultraviolet light (the resist) and covers the structure. This process ensures an exact match between the photo films and the photoresist. The films are placed onto pins that secure them in place over the laminate panel.

Step 4 – Ultraviolet Light Blasting

The film and board align and receive a blast of UV light. The light passes through the clear parts of the film, hardening the photoresist on the copper underneath. The black ink from the plotter prevents the light from reaching the areas not meant to harden. Only hardened areas are retained as copper pathways; the rest of the board will be etched away in the next step.

After this, the board is washed with an alkaline solution, which dissolves and removes the unhardened photoresist. A final pressure wash is used to remove any residue left on the surface. Subsequently, the board is dried. A technician examines the boards to ensure that no errors occur during this stage.

Step 5 – Removing the Unwanted Copper

This process removes the extra copper from the circuit board, leaving the copper beneath the hardened photoresist intact. A chemical solution, similar to the alkaline solution, eats away the unwanted copper. This step can also be referred to as inner layer etching.

The time and solvent required for etching may vary depending on the size of the boards. Larger boards often require more time and/or solvent. Finally, the board now glistens with only the copper substrate necessary for the PCB.

Step 6 – Layer Alignment and Inspection

The newly cleaned layers will need to be inspected for alignment. The holes drilled earlier help align the inner and outer layers. An optical punch machine drills a pin through the holes to keep the layers lined up. After the optical punch, another machine will inspect the board to ensure there are no defects. From here on out, you will not be able to correct any missed errors. In the inspection process:

● The designer will use an Automated Optical Inspection (AOI) machine for inspection.

● The AOI machine compares the PCBs with the original design from Gerber.

● The AOI machine scans the layers with a laser sensor and conducts an electronic comparison.

● The defective circuit boards are discarded at this stage.

● The process is repeated for the outer layers after imaging and etching them.

Step 7 – Laminating and Bonding the Layers

Now, you will see the board taking shape as the layers are fused together. Metal clamps hold the layers together as the laminating process begins. This process involves sandwiching the outer layer of PCBs (made from fiberglass pre-coated with epoxy resin) and the layer of thin copper foil (with etchings for the copper traces). The process is carried out on a specialized press table using metal clamps and is also known as layer-up & bonding.

● The operator places the stack on the laminate press with proper alignment.

● The bonding press computer controls the laminate press.

● The computer will heat press plates and apply pressure according to calibrations, fusing the PCB layers.

● The epoxy melts inside the prepreg, which, along with pressure, fuses the layers together.

● After removing the top press plate and the pins, the technician will pull out the printed circuit board. This process fuses all the layers together.

Step 8 – Drilling

Drilling is considered the most critical step in the PCB manufacturing process. It establishes the foundation for vias and enables connectivity between different PCB layers.

All components slated to come later, such as copper linking via holes and leaded aspects, rely on the precision of drill holes. The holes are drilled to a hair's width, with the drill achieving a diameter of 100 micrometers, while the average hair diameter is 150 micrometers. Due to its very thin diameter, the highest precision is required in PCB drilling.

That’s why leading professional manufacturers like JLCPCB tend to use computer-controlled PCB drilling machines. These machines can drill holes as small as 100 microns in diameter, using air-driven spindles that turn at 150,000 RPM. At this speed, you might think that drilling happens in a flash, but there are many holes to bore. An average PCB contains well over one hundred bore intact points. During drilling, each one needs its own special moment with the drill, so it takes time.

● Before drilling, an X-ray locator locates the drill spots

● A board of buffer material is placed beneath the drill target to ensure clean drilling.

● First, registration or guide holes are drilled to secure the PCB stack.

● A computer-controlled machine drills the target using the original design as a guide.

● After drilling, the excess layer of copper around the edges is removed through profiling.

Step 9 – PCB Plating

The board is now ready to be plated. PCB plating is the process of filling the drilled holes with copper, allowing the current to pass from one layer to another in the PCB by connecting them electrically. The process involves a series of chemical baths.

● Cleaning the PCB panel thoroughly

● Placing the panel in a series of chemical baths that deposit a thin layer of copper, approximately one micron thick

● Controlling the PCB plating process using computers

The copper baths completely cover the walls of the holes. Additionally, the entire panel receives a new thin layer of copper. Most importantly, the new holes are covered.

Step 10 – Outer Layer Imaging

In Step 3, we applied photoresist to the inner layers of the panel. In this step, we repeat the same process for the outer layers of the panel. The process is conducted in a sterile environment to prevent any contamination.

● Pins secure black ink transparencies and prevent misalignment

● The PCB panel, after being coated with photoresist, enters the yellow room.

● The yellow and UV light blast hardens the photoresist

● The panel then passes into a machine that removes the unhardened resist, which is protected by the black ink opacity.

Finally, the outer plates undergo inspection to ensure that all undesired photoresist was removed during the previous stage.

Finally, the outer plates undergo inspection to ensure that all undesired photoresist was removed during the previous stage.

Step 11 – Tin Plating

We electroplate the panel with a thin layer of copper again as described in Step 10. The exposed sections of the panel from the outer layer photoresist stage receive the copper electroplating. Following this, a thin tin guard is applied to the board. The tin serves to protect the copper of the outer layer from being etched off.

Step 12 – Final Etching

The same chemical solution used previously removes any unwanted copper beneath the resist layer. The tin guard layer safeguards the necessary copper. This process readies the PCB panel for AOI (Automated Optical Inspection) and soldering.

● A layer of copper is applied using the electroplating method.

After the initial copper baths, tin electroplating is used to protect the copper in the critical area.

● The PCB board undergoes Automated Optical Inspection (AOI) to ensure the copper layer meets the desired specifications.

● The tin guard is not removed from the copper because it protects the copper from oxidation and also facilitates proper soldering of panels.

Step 13 – Solder Mask Application

This application is essential as it adds a protective layer to the exterior surfaces of a printed circuit board, preparing it for the soldering process. Essentially, it marks the areas where soldering is not required.

● The PCB panel is cleaned to remove impurities or unwanted copper.

● An ink epoxy and solder mask film mixture is applied to the surface.

● The boards are exposed to a UV blast, which penetrates through a solder mask photo film.

● The covered portions remain unhardened and will be removed.

Finally, the circuit board is placed in an oven and baked onto the board.

Step 14 – Silkscreening

Silkscreening is a vital step because this process is responsible for printing critical information onto the board. Once applied, the PCB passes through one final coating and curing stage. This stage typically includes:

● Warning labels

●  Logo or symbols

● Component ID

● Pin locators and other markings

Step 15 – Surface Finish

The nearly complete PCB panels require a coating of conductive material, generally as per the customer’s specifications. This increases the quality/bond of the solder to the PCB. This process is known as surface finishing. Here is a list of some commonly used conductive materials for surface finishing:

Immersion silver: Low signal loss, lead-free, RoHS compliant. The finish can oxidize and tarnish, but strong anti-surface tarnish can solve this problem.

●In cases where the surface is not protected, immersion silver has a short shelf life.

●Not suitable for multiple assembly processes

Hard gold : is durable, has a long shelf life, is RoHS compliant, lead-free, expensive compared to other surface finishes due to its complex process, and is not re-workable.

Hot air solder leveling (HASL) : is cost-effective, long-lasting, and reworkable, but it contains lead and is not RoHS compliant.

Lead-free HASL : is cost-effective, lead-free, RoHS compliant, and reworkable, but it is not suitable for multiple reflow/assembly processes.

● The process requires the use of a carcinogen called Thiourea

● Difficult to measure the thickness

Electroless nickel immersion gold (ENIG): is one of the most common finishes. It has a long shelf life, is RoHS compliant, but is more expensive than other options.

The correct material depends on the design specifications and the customer’s budget. However, applying such finishes creates an essential trait for the PCB. The finishes allow an assembler to mount electronic components. The metals also cover the copper to protect it from oxidation that can occur with exposure to air.

Step 16 – Testing

PCB testing is also a critical step in the manufacturing process. We will use various testing methods to ensure that the PCBs are functional and conform to the original design specifications. Before the PCB is considered complete, a technician will perform an electrical test on the board to confirm that it functions according to the original blueprint designs.

Step 17 – Profiling

Profiling is essentially the last step in the PCB manufacturing process. Until this stage, the printed circuit boards are one construction panel. Using the original design files, the PCBs get sliced into individual boards. In simple words, we can say this is the board cutting process. There are two most common ways to split PCB boards:

● Scoring: Also known as routing out, this method involves cutting several small tabs around the edges of the circuit boards.

● V-Groove: In this method, the CNC machine creates V-shaped cuts along the sides of the PCB boards.

You can easily break off the PCB boards after profiling, regardless of the method used.

Step 18 – Final Quality Check

After profiling, each printed circuit board undergoes a final visual inspection and quality check. The manufacturer will package and ship error-free PCBs after the final examination.

Conclusion

Understanding Printed Circuit Board Manufacturing Process is essential for anyone involved in the design, production, or use of electronic devices. This guide has provided a comprehensive overview of the key elements and steps involved in PCB manufacturing.

About JLCPCB

JLCPCB is a leading manufacturer of high-quality PCBs. The company has extensive experience in manufacturing a wide range of PCBs, from simple single-sided boards to complex multilayer boards. JLCPCB uses state-of-the-art equipment and processes to ensure that its PCBs meet the highest quality standards.

JLCPCB Factory Walkthrough Video