What is the PCB Fabrication Process?

A Printed Circuit Board (PCB) is a flat board made of a non-conductive substrate material with electrically conductive pathways and components attached to it. They form the core of modern electronics by providing a structural base for electronic components and establishing electrical connectivity between them. These boards are an essential part of electronic devices ranging from simple devices like calculators to complex systems like computers and smartphones.

PCB Fabrication Process

Step 1 – Imaging and Printing the Design

The process of manufacturing a PCB starts with the PCB design files usually created in the Gerber format. Following the Design for Manufacturability (DFM) examination to ensure the PCB design is within manufacturable limits, a special printer called a plotter printer is used to generate photo films for each layer of a PCB. These films are called photomasks and they show the conductive pathways and also the regions of the board where no conductor is present. Areas free of copper are shown with clear ink while areas where copper is present are shown in black. In the case of a simple double-sided board, 4 films are needed – 2 for copper layers and 2 for solder mask layers. To make sure that the orientation of various layers during future manufacture will not change excessively, registration holes are also drilled through these films which help keep the layers in proper orientation during the final production stage.

Step 2 – Creating the Substrate

The substrate, which is commonly composed of insulating materials like fiberglass and epoxy resin, gives the necessary stiffness to the PCB. It is made by passing these materials through an oven. This core substance then gets cured into a hard, non-conductive substrate to which copper and other layers can be applied. The thickness of the core depends on design specifications and applications.

For flexible PCBs, Polyimide, Polyester (PET), or Liquid Crystal Polymer (LCP) are commonly used as substrates. They are not passed through an oven to be cured so that they retain their flexible property.

Click here to learn more about Flexible PCB materials.

Step 3 – Printing the Copper Traces

A thin layer of copper is bonded to one or both sides of the substrate board. This copper layer will later be etched to form the copper traces.

A layer of photoresist (photosensitive material) is applied to a copper-clad substrate board. The design is aligned to the photoresist and exposed to UV light using photomasks prepared in Step 1. The light passes through the clear parts of the photomask and hardens the photoresist in those areas. In the areas covered by the black parts of the photomask, the photoresist remains soft and soluble. The pattern hardens only where the UV light has been directed, allowing for the exact outlines of the circuitry to be formed.

The purpose of the UV exposure step is to position the board and the film with respect to each other and to cure the photoresist in the areas intended for copper retention. After this process, the board undergoes alkaline washing to strip off the areas of uncured photoresist while preserving the copper circuit trace patterns. Before proceeding further, a technician examines the board for any defects.

The outer layers of a multilayer PCB receive an additional layer of thin tin guard after copper traces are printed to protect the copper from being etched off. The tin guard also protects the copper layer during soldering. This is followed by an additional layer of copper plating, reinforcing conductivity.

Using a chemical etching solution, the PCB is washed to remove copper in areas not protected by the hardened photoresist. The etching process leaves only the essential copper traces required for the circuitry. This process, often called inner layer etching, varies depending on board size and copper thickness.

Step 4 – Layering

Once the etching process is done, the individual layers of a PCB will be properly aligned and stacked. Registration holes created during imaging help in the alignment. To maintain accuracy, a punching machine inserts pins through these holes onto the layers while using an Automated Optical Inspection (AOI) machine to check every layer for faults. This check prevents any issues from arising before the layers are glued together.

The layers of the PCB are subjected to a mounting process and bonded with epoxy which acts as an adhesive. A metal press applies heat to the layers under pressure for them to fuse. In this process, which is also referred to as layer-up and bonding, a layer of thin copper foil (with etchings for the copper traces) which is to remain inactive is placed in the center of the two insulating layers. The epoxy resin softens under heat allowing the entire PCB to encapsulate and form a solid unit.

Step 5 – Drilling

This process establishes the foundation for vias and enables connectivity between different PCB layers. High-precision computer-controlled drilling machines bore holes through the PCB layers. All components slated to come later, such as copper linking via holes and leaded aspects, rely on the precision of drill holes. Typical drill sizes can reach as small as 100 microns, critical for the circuit’s functionality. Once drilled, the PCB is further profiled to remove excess material around the edges.

Click here to learn more about PCB vias.

Step 6 – PCB Plating

The drilled holes are filled with copper through a series of chemical baths, connecting the different layers electrically. These chemical baths deposit a thin layer of copper, approximately one micron thick and completely cover the walls of the holes. Additionally, the entire panel receives a new thin layer of copper and also covers all the new holes.

Step 7 – Solder Mask Application

A solder mask is applied to protect the PCB’s surface and also marks areas where soldering is not required. In this process, an ink epoxy and solder mask film mixture is applied to the surface and then exposed to a UV blast, which penetrates through a solder mask photo film. The covered portions remain unhardened and will be removed followed by an oven-baking process to ensure adhesion.

Step 8 – Silkscreening

Important information, including logos, warning labels, component IDs, pin locators, and labels, is printed onto the board using silkscreening. This step provides essential guidance during assembly and aids in component identification. Once applied, the PCB passes through one final coating and curing stage.

Step 9 – Surface Finish

In surface finishing, a conductive material, such as gold, silver, or tin, is applied to the exposed copper areas. This increases the quality/bond of the solder to the PCB, improves solderability and protects the copper from oxidation. Common finishes include:

• Immersion Silver: Low signal loss, RoHS compliant but oxidizes over time.
• Hard Gold: Durable with a long shelf life but expensive.
• ENIG (Electroless Nickel Immersion Gold): A popular choice with excellent durability.
• Hot air solder leveling (HASL): is cost-effective, long-lasting, and reworkable. Also available in an RoHS-compliant Lead-free variant.
  

The correct material depends on the design specifications and the customer’s requirements.

Step 10 – Testing

PCB testing verifies the functionality and design adherence of each board. Electrical tests check for short circuits, open circuits, and other potential issues. This crucial stage ensures the PCB meets quality standards before final assembly.

Step 11 – Profiling

Until this stage, the printed circuit boards are one construction panel for the panelization process. Profiling involves cutting the PCB from a larger panel into individual boards and is also called depanelization. Techniques like scoring and V-grooving are used to make precise cuts along the board edges, allowing easy separation. This step also profiles any excess material around the PCB’s edges.

Step 12 – Final Quality Check

After profiling, each PCB undergoes a thorough visual inspection. The quality check ensures that all design requirements are met and confirms the PCB is free from defects. Approved PCBs are packaged for shipping or further assembly.