Printed Circuit Board
A printed circuit board (PCB) is a laminated structure consisting of conductive and insulating layers. PCBs have two primary functions. The first is to mechanically support electronic components in designated locations, typically by means of soldering. The second is to provide electrical connections between component terminals through a predetermined arrangement of conductive paths.[1] Each conductive layer contains a pattern of copper traces, pads, and other features that form part of the electrical circuit. Connections between layers are made using vias, including plated-through holes and other types of interlayer connections.
Overview
A basic PCB consists of a flat sheet of insulating material and a layer of copper foil laminated to the substrate. Chemical etching removes unwanted copper to produce conducting lines called tracks or circuit traces, pads for component connections, and solid conductive areas used for grounding, power distribution, or electromagnetic shielding.[2] The tracks function as wires fixed in place and are insulated from each other by air, solder mask, and the board substrate material.
The surface of a PCB may have a protective coating that reduces copper corrosion and helps prevent solder bridges between adjacent conductors. This coating is commonly called solder resist or solder mask. The solder mask leaves selected pads and other conductive features exposed for soldering or electrical contact.
Design
Initially, PCBs were designed manually by creating a photomask on a clear Mylar sheet, usually at two or four times the actual size. Starting from the schematic diagram, component pads were laid out on the Mylar, and traces were routed to connect them. Rub-on dry transfers of common component footprints increased efficiency. Traces were made with self-adhesive tape, while pre-printed non-reproducing grids assisted in layout. The finished photomask was photographically reduced and used to transfer the circuit pattern onto a photoresist coating on copper-clad boards.
Modern PCB design is performed using electronic design automation (EDA) software. The process involves schematic capture, component placement, trace routing, and the definition of copper layers, vias, and board dimensions. Design rules specify manufacturing constraints such as minimum trace widths, conductor clearances, and hole sizes. For multilayer boards, the layer stack-up is also defined to meet electrical and mechanical requirements.
Manufacturing
PCB manufacturing begins with the preparation and verification of fabrication data. Copper-clad laminates are processed using photolithographic imaging and chemical etching to form the required circuit patterns. In multilayer boards, the inner circuit layers are inspected, aligned, and laminated together with insulating prepreg under controlled heat and pressure.
After lamination, holes are drilled mechanically or, in the case of certain microvias, by laser. Hole walls are cleaned and metallized through copper deposition and electroplating to establish electrical connections between layers. The outer circuit patterns are then formed using additional imaging, plating, and etching processes.[3]
Solder mask is applied to the board surfaces, leaving designated connection pads exposed. A surface finish, such as hot air solder leveling (HASL), electroless nickel immersion gold (ENIG), or organic solderability preservative (OSP), protects the exposed copper and maintains solderability. Component markings may be added by silkscreen printing. Finished boards undergo inspection and electrical testing to detect open circuits, short circuits, and other manufacturing defects.
Assembly
In assembly, the bare board is populated (or "stuffed") with electronic components to form a functional printed circuit assembly (PCA), commonly called a printed circuit board assembly (PCBA).[4]
In through-hole technology (THT), component leads are inserted into holes surrounded by conductive pads and are soldered to establish mechanical and electrical connections. In surface-mount technology (SMT), components are placed directly onto pads or lands on the PCB surface. Solder paste is generally applied to the pads before placement, temporarily holding the components in position until the solder is melted during reflow soldering.
Depending on the assembly requirements, inspection and testing may be performed to identify soldering defects, incorrect component placement, or electrical faults.
Protection
PCBs intended for harsh operating environments may have a conformal coating applied after the components have been soldered. The coating helps protect the assembly against moisture, contamination, and corrosion, reducing the risk of leakage currents or electrical shorts caused by condensation.
Early protective coatings included wax-based materials. Modern conformal coatings commonly use acrylic, silicone, polyurethane, or epoxy formulations and may be applied by spraying, dipping, brushing, or selective coating equipment.
Conformal coatings can make inspection, repair, and component replacement more difficult because the coating may need to be removed before servicing. The degree of difficulty depends on the coating material and the removal method.
Circuit Properties
Each trace consists of a flat, narrow section of copper whose resistance is determined by its width, thickness, length, and operating temperature. Its resistance must be sufficiently low for the current the conductor is intended to carry. Power and ground traces may need to be wider than signal traces to limit voltage drop and conductor heating.
In a multilayer board, an entire copper layer may be used as a ground plane to provide a low-impedance return path and help control electromagnetic interference. For microwave and high-speed circuits, transmission lines can be arranged as microstrip or stripline structures with controlled dimensions and dielectric properties to achieve a specified characteristic impedance.
High-density interconnect (HDI) PCBs use fine conductors and small vias, often including microvias, to support higher interconnection density than conventional PCB structures.
