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PCB circuit boards are an indispensable core interconnection medium in almost all electronic devices. Structurally, it consists of an insulating substrate, conductive copper foil layers and multifunctional protective coatings, forming a precision composite physical structure. In the vast majority of commercial electronic products, the insulating substrate of the circuit board is made of FR4 material—a glass fibre-reinforced epoxy resin that combines excellent electrical insulation properties with outstanding mechanical stability, providing a solid platform for subsequent circuit etching and component mounting.
PCB circuit boards also serve as the fundamental mounting and connection platform for various electronic components such as resistors, capacitors, chips and connectors. Through precision-etched copper traces and metallised vias, the PCB circuit boards connects the electrical interfaces of all components in an orderly manner according to the design logic. It strictly adheres to the connection rules of the electronic schematic diagram, integrating disparate components into a complete, functional circuit system. In simple terms, a PCB is an integrated mounting base and wiring platform for electronic components.
The internal layered structure of multilayer PCBs
Multilayer PCBs are widely used in modern high-precision electronic devices such as smartphones, industrial control equipment and communication modules. The structure of a multilayer board is similar to a book with a spine—it is formed by stacking layers of insulating substrate and conductive copper layers and bonding them together under high temperature and pressure. Its core composition can be divided into two main categories: the base substrate and the multilayer functional structure.
Core Base Substrates
Core board: This is the fundamental supporting structure of the circuit board, made from an insulating substrate with copper cladding on both sides. It provides stable mechanical strength for the entire board and serves as the primary interlayer insulation.
Prepreg: A resin-impregnated semi-cured composite material. During the high-temperature lamination process, the prepreg melts under heat, filling the minute gaps between the conductive and insulating layers; once cooled and cured, it bonds the layers of the board tightly together into a single unit.
Complete Functional Layer Arrangement
1. Top Solder Mask Layer: Typically a protective layer of green ink (other colours can be customised), covering the surface copper conductors. It effectively prevents short circuits during component soldering, isolates the board from air and moisture, and prevents copper foil oxidation. ‘Windows’ are opened only at the pad locations, reserving areas for component soldering.
2. Top-layer silkscreen layer: Generally printed using white ink, this layer displays identification information such as component symbols, part numbers, polarity markings and brand logos, facilitating automated assembly, manual debugging and subsequent maintenance.
3. Top-layer signal layer: The outermost conductive copper foil layer of the pcb circuit board, etched with precision circuit traces, primarily used for the electrical connection of surface-mount electronic components.
4. Inner Signal Layers: Conductive layers hidden within the multilayer board; their internal traces handle the cross-connections of complex circuits, effectively saving space on the outer layers and increasing circuit integration.
5. Internal Power/Ground Layers: These are typically solid or partially segmented copper foil planes and constitute the core functional layers of high performance PCB circuit boards. The power layer provides a stable, continuous power supply to the entire board or independent circuit modules; the ground layer establishes a unified ground reference loop,suppressing electronic noise and optimising signal integrity, whilst also serving as electromagnetic shielding.
6. Vias: Metallised conductive holes that penetrate a single or multiple layers of the pcb circuit board, classified into three types: through-holes, blind vias and buried vias. They serve as key bridges connecting conductors on different layers to copper planes, enabling vertical interconnection between inner and outer layers.
7. Bottom Signal Layer: The outer conductive copper layer on the underside of the circuit board, which works in conjunction with the top signal layer to complete the routing layout on the bottom surface.
8. Bottom silkscreen layer: An optional functional layer used to print auxiliary identification markings on the underside of the board.
9. Bottom solder mask layer: Its function is identical to that of the top solder mask layer, providing protection against oxidation and solder short circuits.
Definition and Characteristics of PCB Circuit Boards Internal Circuits
The term ‘internal circuit’ of a pcb circuit boards refers to a hidden interconnection network comprising etched copper conductors on the inner signal layers, copper foil planes on the inner power/ground layers, and metallised vias that penetrate through all layers. This network strictly matches the connection logic of the electronic schematic and serves as the core physical foundation for circuit conductivity.
It is important to note that the pcb circuit boards itself does not contain active circuit components. Unlike integrated circuit chips (such as CPUs or memory), which integrate active components such as transistors onto a silicon wafer to perform logical operations, a pcb circuit board merely provides passive interconnection structures such as conductors, pads and vias, and cannot independently generate circuit functionality. In other words, a PCB is more like a city map criss-crossed with ‘roads’ and ‘interchanges’, whilst the actual tasks of control, amplification and computation are performed by the various components mounted upon it.
The Structure of a Complete PCB-Based Electronic Circuit
A fully functional electronic circuit relies on the coordinated operation of electronic components and the PCB’s interconnection network. Once components such as resistors, capacitors and chips have been soldered onto the pads provided on the pcb circuit boards, the copper traces and vias on the board’s surface and within its interior connect all the discrete components in series according to the design logic. The pcb circuit boards is responsible for providing conductive pathways and connection points, whilst the various electronic components perform specific functions such as current limiting, signal amplification, switch control and data processing. The two complement each other and are indispensable—only through perfect compatibility can a properly functioning electronic system be formed.
The internal circuitry of a standard PCB circuit board is, in essence, a high-precision, composite interconnection network: it consists of multiple layers of insulating substrate, internal copper signal traces, power supply and ground planes, and various conductive vias. As a passive interconnection medium for electronic circuits, the PCB circuit boards enables orderly electrical conduction between component pins, providing a robust physical foundation and electrical basis for the operation of electronic devices.