Understanding the PCB Manufacturing Process – A Comprehensive Overview
Printed circuit boards are manufactured through several steps, depending on their complexity. Each step has its challenges and impact on the final product.
Engineers carefully review the PCB blueprint for potential errors or missing components. Once they approve the design, it proceeds to print.
A particular plotter printer prints the plan onto a piece of laminate material. The inner layers use black ink to mark copper pathways, while the outer layers have clear ink.
Substrate
The substrate is the laminated electrical insulator that is the foundation for a PCB. It must withstand high temperatures, humidity, and stress while sustaining its electrical properties. Specifically, it needs low loss due to polar molecules that interfere with the path of electric currents or signals throughout the board. That is why designers opt for lower-loss materials like PTFE or Rogers 4350B instead of traditional FR-4.
An initial schematic that shows the logical connectivity between components is used to create a PCB. Prototyping and production preparation come next to guarantee that the data and components are ready for large-scale PCB manufacturing. Then, the fabrication process takes place using etching and drilling. It includes forming holes known as vias and through holes, which establish electrical connections between different substrate layers.
It is also essential that the substrate be woven tightly to prevent the formation of unintentional shorts between copper layers. Other key factors include its glass transition temperature, or Tg, decomposition temperature, or Td.
Inner Layer
The inner layer is made from a laminate board with copper pre-bonded to both sides of the structure. A layer of photo-reactive chemicals covers the laminate and hardens when exposed to ultraviolet light. Registration holes are drilled into the film to help align the blueprints and the printed board.
Next, the outer layers of the PCB are coated with a photoresist and imaged. Then, they are plated in the same way that the inner layers were plated in step four. The difference is that the outer layer receives a tin coating to help protect the copper.
Before the final PCBs go out for assembly, they must be cleaned to remove all contaminants and ensure the accuracy of the through-holes (PoH) that are required by your submitted files. The holes are then drilled using CNC machines, and the drilling parameters like drill speed and diameter are carefully controlled to match your specifications.
Outer Layer
Once the PCB’s inner layer is cleaned, the outer layers are coated in photoresist. The photoresist is imaged in the same way as the earlier steps, and it’s plated with tin to guard the copper on the outside of the PCB.
Unlike standard blueprints, these printed plans are reversed: black ink represents the copper pathways, while clear ink shows non-conductive areas of the PCB. It helps the fabricator match the design and the actual print when aligning them for production. A set of registration holes in the film helps with this alignment process.
Now that all the layers are coated, they’re ready for alignment and optical inspection. Technicians place the layers in a machine that compares them to the original Gerber file. If no defects exist, the layers move on to the next step. The layers are then drilled with CNC machines, and the vias and through holes are plated. The PCB goes through continuity testing with automated test equipment, and it’s shipped back to the contract manufacturer for component assembly.
Copper Layer
In additive and subtractive PCB fabrication methods, copper is used to carry signals to and from the inner layers of the board. The copper can be a complete layer of copper foil or a pre-plated copper coating on the bare substrate.
Once the outer layer is imaged with photoresist and etched, it undergoes a chemical treatment to safeguard the copper from corrosion. A surface finish is then applied based on design specifications.
Then, the layered PCB is fed into a lamination press—the heat and pressure cause the prepregs to melt and bond to form a single cohesive multi-layered structure. The alignment of these layers is critical as vias connect circuits across the PCB. Registration holes are drilled through the layers to align them and keep them secure while in the press. It is done by a computer-guided machine known as an optical punch. An automated optical inspection (AOI) machine inspects the resulting alignment and registration holes. Those who pass AOI move forward in the process.