Introduction to Printed Circuit Boards (PCBs)

Introduction to PCBs

• PCBs, or Printed Circuit Boards, serve as flat insulating surfaces where printed wiring and miniaturized components are connected in a specific design and attached to a common base.
• Components connect to the board and printed wiring is on the reverse side
• This technique significantly reduces interconnecting wiring to lines of conducting material like copper, silver, or gold, deposited on an insulating circuit board, excluding high power leads and cabling
• PCBs are easily adapted as plug-in units, leading to a substantial reduction in the size of electronic equipment by eliminating terminal boards, fittings, tie points, and wires

Preceding Technology

• Early circuits relied on point-to-point wiring, which was laborious and prone to failures at wire junctions and short circuits due to aging insulation
• PCBs are now found in almost every electronic device and not only keep components in place but also provide electrical connections between them
• Electronic devices have become more complex with more components, PCBs have become densely populated with wiring and components

Basic PCB Construction

• PCBs are made from various substances, typically a flat laminated composite of non-conductive substrate materials with copper circuitry layers
• PCBs range from a simple copper layer on one side with components on the other, to complex circuits with two layers or high-density applications with fifty or more laminated layers
• The flat composite surface supports components soldered or attached to the PCB, while copper conductors connect the electronic components

Base or Substrate Material

• The base material of a PCB, known as the substrate or laminate, consists of phenolic paper, epoxy paper, or epoxy glass
• The metal foil, or cladding, is usually copper, can also be other metals
• The substrate board is made first, then the cladding is bonded to it
• The substrate is an insulating, non-flexible material, with thin wires on the surface made of copper foil etched away to form a network
• These wires, called the conductor pattern or tracks, provide electrical connections for the components
• Basic PCBs feature a copper circuit pattern or foil on one side, with holes drilled through pads or terminals for component leads
• Leads are pushed through the holes and soldered to the copper foil to complete the circuit

Solder Mask and Silk Screen

• The green or brown color of a PCB comes from the solder mask, an insulating and protective coat that prevents corrosion and unintended solder attachment on copper wires
• A silk screen, or legend, is printed on top of the solder mask,consisting of text and symbols in white to label the locations for mounting components

Through-Hole Technology

• Components mounted on one side of a board with legs soldered on the opposite side are called through-hole PCBs or THT
• THT components take up significant space and require a drilled hole in the PCB for each leg
• Legs occupy space on both sides of the board, and their connection points are fairly large
• THT components offer better mechanical connection to the PCB than surface-mounted devices
• Connectors for cables and devices needing to withstand mechanical stress are typically THT

THT Component Mounting

• THT components are fastened to the PCB by soldering their legs to the conductor pattern
• Basic single-sided PCBs have components on one side and the conductor pattern on the opposite side, requiring holes for component legs to penetrate the board
• Legs are soldered to the PCB on the opposite side of where the components are mounted
• The top and bottom sides of a single-sided PCB are referred to as the "component side" and "solder side," respectively

Surface-Mounted Technology

• Surface Mounted Technology (SMT) involves soldering component legs to the conductor pattern on the same side of the PCB as the mounted component
• SMT doesn't require holes in the PCB for each component leg
• Surface-mounted components can be placed on both sides of the PCB
• SMT components are smaller than THT components, allowing for denser PCBs
• Most main board components are now SMT due to their smaller size and lower cost
• Soldering SMT components manually is difficult due to the small size of the connection points and component legs
• Machines handle most of the assembly of these components

PCB Terminology

• PCBs often have distinct labeling, with foil patterns varying in shape depending on the circuit's function.
• Heat sinks dissipate heat from components and should have large areas
• Voltage and ground lines or planes are long and slender and follow a path to provide power
• Terminals or pads are points with drilled holes for accommodating component leads
• Conductors or runs are thin foil strips between components
• Edge connectors are part of the foil that connects a circuit board to a special plug

PCB Additional Components

• Component sockets allow components to be removable from the PCB after manufacturing
• The socket is soldered to the board, allowing the component to be inserted and removed without soldering

Edge Connector

• Edge connectors connect one PCB to another
• Edge connectors consist of small uncovered copper pads along one side of the PCB, part of the conductor pattern
• The edge connector on one PCB is inserted into a matching connector (slot) on another PCB to connect PC graphic cards, sound cards, and similar products to the main board

PCB Types: Single-Sided Boards

• Most basic boards have components mounted on one side and the conductor pattern on the opposite side
• Single-sided PCBs have conductor pattern on one side, with limited wire routing capabilities
• Single-sided boards are used in very primitive circuits

Double-Sided Boards

• Double-sided boards feature a conductor pattern on both sides, requiring electrical connections between the patterns called "vias"
• A via is a metal-filled hole in the PCB connecting conductor patterns on both sides
• Double-sided PCBs offer twice the surface area for the conductor pattern; and more suited for complex circuits because wires can cross by routing them on opposite sides

Multi-Layer Boards

• Multi-layer PCBs consist of a sandwich of conducting and insulating layers for complex circuits
• The stack of layers alternates between a non-conducting dielectric layer and copper foil that has been patterned to form the circuit connections