Draft:Printed Circuit Board

Printed Circuit Board

A printed circuit board (PCB) is a structure used to mechanically support and electrically connect electronic components through conductive tracks, pads, planes and other features formed from layers of conductive material, most commonly copper, laminated to or between insulating substrates. PCBs are fundamental to modern electronics and are used in products ranging from simple consumer devices to computer motherboards, industrial control equipment, communication hardware, medical electronics and aerospace systems.

A PCB without mounted components is often referred to as a bare board. After electronic components have been attached and soldered, the resulting assembly is commonly called a printed circuit board assembly (PCBA) or printed wiring assembly (PWA). NASA terminology, for example, distinguishes the PCB itself from a printed wiring assembly consisting of the PCB, components and associated materials.

Printed circuit boards allow a large number of electrical connections to be produced repeatedly in a compact structure. Their manufacture combines aspects of engineering, materials science, chemical processing, mechanical drilling, imaging, plating and electrical testing.

Construction

Most conventional rigid PCBs consist of alternating conductive and insulating layers bonded together into a laminated structure. Copper foil is commonly used for the conductive layers because of its high electrical conductivity, ductility and suitability for fabrication and soldering.

The insulating material between conductive layers is known as the dielectric. Glass-reinforced epoxy laminates are widely used in rigid printed boards. International standards in the IEC 61249 series define requirements for several classes of copper-clad and unclad materials used in printed boards.

A finished board may contain several types of features:

• Conductors or traces carry electrical signals and current across a layer.
• Pads provide conductive surfaces for component terminals, vias and test points.
• Planes are larger copper areas commonly used for power distribution, grounding or electromagnetic reference.
• Vias electrically connect conductors on different layers.
• Plated through-holes pass through the board and normally contain a conductive metal coating on the wall of the hole.
• Blind vias connect an outer layer to one or more internal layers without passing completely through the board.
• Buried vias connect internal layers and are not visible from the outer surfaces.
• Microvias are small vias commonly associated with high-density interconnect construction.

Surface-mount land geometries and their relationship to soldering, inspection and manufacturability are addressed by standards such as IPC-7351.

Types

Single-sided and double-sided boards

A single-sided PCB has conductive circuitry principally on one side of an insulating substrate. A double-sided board has conductive patterns on both sides, with electrical connections between the two sides normally provided by plated holes or other interconnection techniques.

These structures remain useful where circuit density is modest and manufacturing simplicity is important.

Multilayer boards

A multilayer PCB contains multiple conductive layers separated by dielectric materials and bonded into one structure. Internal layers may carry signals, power or ground connections, allowing considerably higher interconnection density than a single- or double-sided board.

The performance and qualification requirements for rigid boards, including multilayer constructions with plated through-holes, blind vias, buried vias and microvias, are covered by IPC-6012.

Flexible and rigid-flex boards

Flexible printed circuits use thin, bendable dielectric materials instead of the comparatively rigid laminates used in conventional boards. They are related to the broader field of flexible electronics and may be designed to bend during installation or, in some applications, repeatedly during operation.

Rigid-flex construction combines rigid printed-board sections and flexible sections in one interconnected structure. IPC-6013 specifies qualification and performance requirements for flexible and rigid-flex printed boards.

High-density interconnect boards

High-density interconnect (HDI) boards use features such as microvias, fine conductor geometries, sequential build-up layers and blind or buried interconnections to increase connection density.

HDI construction is particularly useful when a design must route a large number of connections within limited board area. Research on high-density PCB structures has examined microvia fabrication and reliability under thermal cycling, humidity ageing and repeated reflow exposure.

Industry publications also discuss the increasing use of HDI techniques for compact, high-interconnection electronic products.

Design

PCB design converts an electrical circuit into a physical arrangement of components and interconnections. Modern boards are generally designed with computer-based engineering tools. Engineering applications increasingly use computer-aided systems for circuit schematics, printed-board layout and other forms of electronic design automation.

The design process normally includes placement of components, routing of conductive connections, definition of board dimensions and layer structure, and preparation of manufacturing information. The physical layout must also account for fabrication tolerances, assembly requirements, electrical performance and inspection.

General printed-board design practices are covered by the IPC-2220 family of standards. NASA identifies IPC-2221, IPC-2222 and IPC-2223 among standards commonly used to guide design of high-reliability printed circuit boards.

Electrical considerations

At low frequencies, PCB interconnections can often be treated primarily as conductors joining circuit nodes. As operating frequency and signal edge rate increase, the geometry and material properties of the board become increasingly important.

Transmission behaviour is influenced by factors such as conductor dimensions, dielectric thickness, dielectric properties and the position of nearby reference planes. High-speed layouts may therefore use controlled-impedance structures, continuous reference planes, differential routing and carefully designed transitions between layers.

IPC test methods include procedures for determining properties relevant to high-frequency operation, including dielectric constant, dissipation factor, propagation and signal loss.

Power distribution and thermal performance also influence PCB design. Copper thickness, conductor width, plane geometry, component losses and available cooling paths can affect conductor temperature and board temperature.

Manufacturability

A PCB layout must be compatible with the capabilities of the intended fabrication process. Important manufacturing constraints can include minimum conductor width and spacing, drilled-hole diameter, annular-ring dimensions, layer registration, copper thickness and dielectric spacing.

More complex structures generally require tighter process control. In high-reliability applications, manufacturability and material selection are commonly considered together during design rather than evaluated only after layout completion. NASA's PCB assurance guidance similarly treats material properties, design criteria and manufacturing readiness as related aspects of board reliability.

Manufacturing

PCB fabrication varies with board type, layer count, materials and required feature sizes, but manufacture of a conventional multilayer rigid board typically involves a sequence of imaging, etching, lamination, drilling, plating, surface finishing and inspection operations.

Imaging and etching

The conductor pattern for each layer is transferred to copper-clad material using a patterned imaging process. Selected copper is then removed to leave the required conductive geometry.

Accurate control of this stage is important because variation in etching can change conductor width and spacing. These dimensions can affect both manufacturability and electrical performance.

Lamination

For multilayer boards, individual inner layers are aligned with insulating bonding materials and pressed together under controlled temperature and pressure. The resulting laminated panel forms the basic multilayer structure.

The thermal properties of the laminate are important during later processing. IEC 61189-2-804, for example, defines methods for determining time to delamination of base materials and printed boards at elevated temperatures.

Drilling and metallisation

Holes are produced for component leads, vias and mechanical features. Mechanical drilling is commonly used for conventional through-holes, while laser processes are widely associated with smaller microvia structures.

Where an electrical connection is required through a drilled hole, the hole wall is metallised and plated to produce a conductive path between layers.

Inspection of plated structures is important because discontinuities, inadequate plating and defects at the connection between the via barrel and internal layers can affect reliability. IEC 61189-3 provides test methods for printed-board interconnection structures.

IEC also publishes inspection methods addressing the appearance of gold, nickel and copper plated surfaces used in printed wiring boards.

Solder mask and surface finish

A protective polymer coating known as solder mask is normally applied over most exposed circuitry while leaving selected pads and contacts accessible. The mask reduces accidental solder bridging during assembly and provides protection to the underlying conductor surfaces.

Exposed copper surfaces intended for soldering or electrical contact are normally given a suitable surface finish. The choice of finish depends on factors such as component type, assembly process, required shelf life, contact performance and reliability requirements.

Assembly

During PCB assembly, electronic components are mechanically positioned and electrically connected to the fabricated board.

Components may be installed using through-hole mounting, surface mounting, or a combination of both. In modern high-density electronics, surface-mounted components allow large numbers of devices to be placed on relatively small board areas.

Solder paste is commonly deposited on selected surface-mount pads before component placement. The assembly then passes through a controlled heating process in which the solder melts and forms joints between component terminals and PCB lands.

IEC 61189-5-3 specifies test methods for soldering paste used in printed-board assemblies, while IEC 61189-5-2 covers soldering flux. IEC 61189-5-4 addresses solder alloys and wire for printed-board assemblies, including lead-free soldering.

As component pitches and deposited solder volumes become smaller, finer solder-paste particles may be required. IEC 61189-5-301 provides test methods for fine-particle solder pastes intended for high-density printed circuit board applications.

The ability of the board and solder joints to tolerate the thermal conditions of assembly can also be tested. IEC 61189-5-601 specifies reflow solderability and reflow heat-resistance methods for organic rigid printed boards.

Inspection and testing

Bare printed boards and assembled boards may be inspected at several stages of production.

Visual and dimensional inspection can identify surface defects, conductor damage, registration problems and other readily observable conditions. IPC-A-600 provides illustrated acceptance criteria for externally and internally observable features of printed boards.

Electrical testing may be used to detect unwanted open or short circuits. Cross-sectional inspection, often called microsectioning, can be used to examine internal structures such as plated holes, layer registration and interconnections. IPC's TM-650 collection includes methods for microsectioning, electrical measurement and environmental evaluation of printed-board materials and structures.

IEC 61189-1 similarly establishes general test methodology for electrical materials, printed boards and interconnection structures.

Inspection of assembled boards may also include automated optical inspection, X-ray inspection and electrical or functional testing. A review of automatic PCB inspection techniques describes visual inspection as one of the principal approaches used in PCB quality assurance and surveys the growing application of automated and artificial-intelligence-based inspection methods.

Reliability

PCB reliability depends on interactions among board materials, geometry, manufacturing processes, assembly conditions and the operating environment.

Potential failure mechanisms include conductor cracking, plated-hole fatigue, delamination, insulation degradation, corrosion and damage associated with repeated thermal expansion and contraction.

Boards used in aerospace and other high-reliability systems are therefore subject to particularly detailed material, fabrication and quality-control requirements. NASA's GSFC-STD-8001 establishes requirements and recommendations intended to ensure that high-reliability PCBs are appropriately designed, procured, produced and used in mission hardware.

IPC-6012 defines qualification and performance requirements for rigid printed boards and includes provisions for structures such as plated through-holes, blind and buried vias, microvias and multilayer constructions.

The IEC 61189 family also includes mechanical, electrical, chemical and environmental test procedures intended to provide repeatable evaluation methods for printed boards and related materials.

Environmental considerations

PCB manufacture and assembly use metals, polymers, process chemicals and soldering materials, making material selection and waste management important environmental considerations.

Restrictions on hazardous substances have affected the composition of electronic equipment and associated manufacturing processes. In the European Union, Directive 2011/65/EU, commonly known as the RoHS Directive, restricts the use of specified hazardous substances in electrical and electronic equipment.

The transition to lead-free assembly has in turn influenced solder materials, process temperatures and reliability testing. IEC standards for solder alloys, solder paste and printed-board reflow testing include methods applicable to lead-free processes.

Applications

Printed circuit boards are used throughout the electronics industry. A motherboard, for example, is a PCB that connects the processor, memory, storage interfaces and peripheral subsystems of a computer.

Other applications include communication equipment, industrial electronics, vehicles, household appliances, instrumentation, medical devices and aerospace hardware. PCB manufacturing also forms part of the wider electronics and semiconductor manufacturing ecosystem.

In India, organisations such as Bharat Electronics manufacture a variety of professional electronic systems, while the Semi-Conductor Laboratory and projects such as SHAKTI form part of the country's wider electronic hardware and semiconductor research infrastructure. These activities are related to the broader electronics and semiconductor manufacturing industry in India.

Standards

Standards frequently referenced in printed-board design, manufacture, assembly and inspection include:

• IPC-2220 series – printed-board design.
• IPC-6012 – qualification and performance requirements for rigid printed boards.
• IPC-6013 – qualification and performance requirements for flexible and rigid-flex printed boards.
• IPC-A-600 – acceptance criteria for printed boards.
• IPC-7351 – land-pattern design for surface-mounted components.
• IPC-TM-650 – test methods for printed-board materials and structures.
• IEC 61189 series – testing of electrical materials, printed boards and assemblies.
• IEC 61249 series – materials for printed boards and other interconnecting structures.