|Printed circuit board (PCB)|
|Tuesday, 12 September 2006|
There are families of printed circuit boards that are classified according to board formation techniques or board selection standards. Printed circuit board families differ from one another in realizable wiring density. Printed circuit boards are classified into two sorts according to their substrates: rigid printed circuit boards and flexible printed circuit boards. The rigid printed circuit board is principally applied in a motherboard or a peripheral card of a computer, a communication product, such as a board of a mobile phone, or a board of a consumer product. The flexible printed circuit board is widely used, and the flexible printed circuit board can be divided into four sorts according to functions: lead line, printed circuit, connector, and integration function system. Printed circuit boards are also classified into single-sided PCBs, double-sided PCBs and multi-layer PCBs according to the number of wiring circuit surfaces. As the number of layers of a PCB increases, the ability to mount parts thereon increases greatly, so PCBs having many layers are applied to high precision products. A multi-layer PCB designates a PCB having three or more conductive patterns including a surface conductive pattern. The conductive patterns are attached to the respective layers of the PCB while being separated from each other by insulation materials between the respective layers.
A printed circuit board (PCB) is a circuit board fabricated by densely mounting a plurality of parts on a plate made of phenol resin or epoxy resin and densely forming curtailed circuits on the surface of the plate to connect the respective parts to each other. In printed circuit boards, integrated circuits, active components, passive components, and wiring are formed and/or placed onto one or both sides of an insulating base according to a predetermined pattern. A printed circuit board (PCB) typically contains a number of components such as memory chips, processors, I/O chips, other logic chips, and passive discrete components (resistors, capacitors, and the like). As electronic circuits become more complex and use a larger number of components, printed circuit boards must typically accommodate more electronic components in the same surface area. A single large printed circuit board is divided into a plurality of divisions, which are arranged horizontally at intervals or vertically at different levels to form a horizontal arrangement or vertical stack of divisional printed circuit boards, which are all electrically connected with the aid of connectors. Capacitors are typically embedded in panels that are stacked and connected by interconnection circuitry, the stack of panels forming a printed circuit board. The stacked panels can be generally referred to as innerlayer panels. Capacitors have been widely used in electronic circuits to control noise in power distribution. For example, decoupling capacitors have been widely used in printed circuit boards for controlling noise from active devices on printed circuit boards. The practice of embedding capacitors in printed circuit boards allows for reduced circuit size and improved circuit performance.
Standard flexible printed circuit boards comprise a flexible dielectric substrate having circuit lines attached to one or more surfaces thereof. A base material for flexible PCBs is an integral laminate composed of an insulating base film having high heat resistance and excellent electrical and mechanical characteristics and a metal foil bonded with an adhesive. The basic portion of a flexible printed circuit board is a film, which is used to support circuits composed of copper pieces. Characteristics required of the base material for flexible PCBs include adhesion, intimate contact capabilities, heat resistance, electrical properties, and processability. Electrical connections are made between such printed circuit boards and other electrically conductive components by means of conductive bumps which are in electrical communication with the circuit lines. Such bumps typically comprise a malleable metal such as gold, which readily bonds with the corresponding aluminum contact pads. When electrically connecting a flexible circuit with a printed circuit board, the bumps of the flexible circuit are pressed firmly against corresponding conductive contact pads of the printed circuit board in order to provide a reliable electrical connection. A clamping system is typically defined by a connector which provides the compression force necessary to maintain the desired mechanical and electrical contact between the bumps and the contact pads. The connector thus facilitates reliable electrical connection of the flexible circuit and the printed circuit board. Because the flexible printed circuit board has properties of lightness, soft, thinness, smallness, ductility, flexibility and high wiring density, the flexible printed circuit board can be three-dimensionally wired and the shape of the flexible printed circuit board can be changed according to space limitations. The flexible printed circuit board is therefore typically applied in tiny products, such as notebook (NB), mobile phone, personal digital assistant (PDA) and information appliance (IA).
A multilayer printed circuit board, which has different thickness in each portion, is known as a rigid-flexible printed circuit board. The rigid-flexible printed circuit board is composed of a rigid board portion and a flexible board portion. A multilayer PCB provides a plurality of electrically conductive layers separated by insulating dielectric layers. The conductive layers may comprise contiguous plane areas, or alternatively may comprise a complex pattern of point-to-point signal traces. A typical multilayer PCB has a ground layer which is normally formed vertically beneath a substrate so as to be substantially parallel to a signal trace and/or a power trace on the substrate. Generally, several of the copper layers are used to provide a reference voltage plane or ground plane. In addition, several layers of the copper are etched to form the lines that connect individual components. Copper lined through holes extend though the layers of the PCB to selectively connect the electronic components on the surface of the PCB to the reference planes and traces within the PCB and to selectively connect copper is traces on different layers to one another. A membrane printed circuit board inside the pushbutton structure utilized by a common computer or notebook computer primarily comprises an upper layer membrane, a lower layer membrane and an insulation layer arranged by stacking. Multi-layer printed circuit boards are widely used in modern electronics technology to interconnect various components in a compact and efficient manner. These components include active integrated circuits (ICs) or passive discrete components (resistors, capacitors, and the like).
Printed circuit boards (PCB) are flat boards that electrically connect different components of a circuit through copper tracks called traces. Conductive traces in different layers are typically connected using contact structures formed in openings in the dielectric layers. For example, printed circuit boards typically have several layers of conductive traces separated by dielectric layers. The conductive traces are used to electrically interconnect terminals of electronic devices mounted upon the PCB. The traces electrically connect the electronic devices to one another across, and sometimes through, the board. The electrically conductive traces on different board layers are electrically connected by vias. Vias are bored through the board layers and either metal-plated for the length of the bore or filled with a metallic plug. Conductive planes may be placed above or below the traces. The traces and the planes may be separated by a dielectric or non-conductive material. The conductive planes may be used for a plurality of different purposes. For example, the conductive planes may be used as an electrical ground plane functioning as an electrical ground return path. The conductive plane may also be used as an electrical power plane functioning as a power distribution path. Together, the trace and plane provide a complete electrical path to and from electrical components associated with a printed circuit board. Generally speaking, traces typically connect signal paths of electronic components associated with a printed circuit board, while planes typically connect power or ground paths of electrical components associated with a printed circuit board.
Printed circuit boards are conventionally referred to as motherboards and daughterboards. A motherboard is the main circuit board containing the primary components of a computer system. A motherboard is usually the principal board of a computer in that the electronic devices that direct the computer's operation, e.g., the central processing unit, memory, and basic controllers, are mounted to it. For this reason, the motherboard is sometimes also called the system board or main board. A daughterboard is a circuit board that attaches to a motherboard, or other type board, and provide additional or enhanced functions to a motherboard. A daughterboard is typically connected to a motherboard via a card edge connector, the socket portion of which is conductively mounted on the motherboard, and the pin portion of which is an edge of the daughterboard with conductive traces thereon. Printed circuit boards are used widely in the computer industry to support microelectronic devices for removable, electrical coupling to other components. For example, computer motherboards typically include expansion slots or sockets configured to receive a PCB that in turn carries microelectronic devices, such as memory chips. The memory capacity of the computer can be selectively enhanced by coupling the PCB to the expansion slot. Daughter boards are also typically shielded with metal fences, or cans, in a fashion analogous to that described for motherboards.
Printed circuit boards each having a plurality of electronic parts attached thereto is used in an electric or electronic apparatus. A variety of connectors are used for connecting divisional printed circuit boards. A printed circuit board (PCB) connector is often used to provide an electrical interface between a PCB and a cable. A right angle PCB connector is often used to minimize the space required by PCB connectors and to ease the installation of cables to the connector. A PCB is typically inserted into an expansion slot in order to place the electronic components thereon in electrical communication within the computer. The expansion slot is an opening in a computer where a printed circuit board can be inserted to add new capabilities to the computer. The PCB has electrical contacts that are in electrical communication with the electronic devices on the PCB. The PCB preferably has one or more bevels upon one edge in order to facilitate insertion thereof into an expansion slot for a digital computer, or other types of electronic devices. An electric contact is made of sheet of a metal such as copper alloy and comprises a leg, which contacts a conductor of a printed circuit board, and a connecting part, which is connected to the counterpart electric contact. This electric contact is surface-mounted on the printed circuit board by soldering the leg of the electric contact on the printed circuit board. An electric connector comprises such an electric contact and an insulating housing, which holds the electric contact.
The internal functions that a piece of electronic equipment performs are generally segregated by fiction onto certain boards. The motherboard typically also includes connectors for attaching devices to the piece's bus, or collection of wires over which electronic information is exchanged. Other types of boards commonly referenced include: backplanes, or circuit boards containing sockets into which other circuit boards may be plugged; expansion boards, which are any board that plugs into one of the equipment piece's expansion slots, and include controller boards, local area network (LAN) cards, and video adapters. Connectors used to assemble the daughterboards, which are removable, to the motherboards are referred to as backplane connectors. Nearly all personal computers, except for portable computers, contain expansion slots for adding more memory, graphics capabilities, and support for special devices. The boards inserted into the expansion slots are called expansion boards, expansion cards, cards, add-ins, and add-ons. Board mountable connectors are widely used to establish electrical connections between two separated PCBs. The ability to arrange electronic components on printed circuit boards having standard sizes allows for the easy configuration, assembly, repair and upgrading of electronic devices.
Electronic devices, such as communication devices, include printed circuit boards which emit electromagnetic waves. Printed circuit boards used in various types of data processing systems are typically mounted within a plastic or sheet metal housing structure. During operation of a printed circuit board electromagnetic radiation is generated. Electromagnetic radiation can interfere with data processing system operation. These electromagnetic waves generate noise or unwanted signals which interfere with the functionality of other electrical components and devices in the vicinity. With the miniaturization of electronic system and the development of high-frequency signal transmission technology, a flexible printed circuit connector used in such system needs to reliably transmit high-frequency signals. A shielding is commonly required to be provided for the FPC connector, to prevent electro magnetic interference (EMI) in relation to the external environment. Thus a reliable transmission of high-frequency signals within the system is ensured. Conventionally, such shielding is accomplished via a sheet metal housing used to enclose a printed circuit board. The sheet metal housing is normally grounded directly to a ground plane in the printed circuit board.
The printed circuit board is central to the electronic systems of modern devices, which typically have high signal speeds and operating frequencies. Printed circuit boards are used to provide for the highly reliable and robust connection of circuit components. Printed circuit boards with electrical components are widely used in the electronics industry in a variety of products including computers, televisions and telecommunication devices. The flexible printed circuit board can be used as a connection between a liquid crystal display (LCD), a floppy disk driver (FDD), a hard disk driver (HDD) or a compact disc read only memory driver (CD-ROM) to a motherboard; a connection between a component, such as slot or far-infrared ray transmission, to a motherboard in a high-end level notebook; tone push-buttons of a mobile phone or a personal digital assistant; and a connection between a microphone or numeral keys and a panel of a liquid crystal display. In many applications, such as cellular systems, personal communication systems, and land-based communication systems, printed circuit boards are connected to metal back plates to provide a metal-backed printed circuit board assembly.