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Printed Circuit Boards (PCBs)

by: Nov 29,2013 6578 Views 0 Comments Posted in Engineering Technical

A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. PCB's can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductor on different layers are connected with plated-through holes called vias. Advanced PCB's may contain components - capacitors, resistors or active devices - embedded in the substrate.

History

Appeared before the printed circuit board, the interconnection between electronic components and rely on wires directly connected to form a complete circuit. Now, just as an effective circuit breadboard experimental tools exist, and the printed circuit boards in the electronics industry has become occupied the position of absolute rule.

Early 20th century, people in order to simplify the production of electronic devices, reducing the wiring between electronic components and reduce the production cost, etc., and began studying ways to replace the printed wiring methods. Three decades, there have been proposed to engineer a metallic conductor for wiring on the insulating substrate. The most successful 1925, American Charles Ducas on the insulating substrate of the printed wiring pattern, and then to electroless plating, a conductor for wiring successfully established. Until 1936, the Austrian Paul Ai Sile (Paul Eisler) in the UK published a foil membrane technology, he uses a printed circuit board in a radio device; while in Japan, to help Miyamoto hi to spray attached wiring Law "metabolic re co nn Law blowing wiring method (Patent No. 119384) "successful patent applications. And both the methods and Paul Eisler today's most similar to the printed circuit board, such a practice is known as the subtractive method, the unnecessary metal is removed; while Charles Ducas, Miyamoto Kinosuke practice with only the required wiring, called the additive method. Nevertheless, because it was electronic parts heat, with both substrates is difficult to use, so that's not practical to make a formal, but also make the printed circuit technology further.

In 1941, the United States talc painted on copper paste for wiring to make proximity fuses.
In 1943, a large number of Americans using the technology in the military radio.
In 1947, for the manufacture of epoxy substrate began. While NBS began to study the formation of printed circuit technology coils, capacitors, resistors and other manufacturing techniques.
In 1948, the United States formally recognized the invention for commercial purposes.
Since the 1950s, a large number of low calorific transistor replaced the vacuum tube position, printed circuit board technology began to be widely used. And then etched foil technology into the mainstream.
In 1950, the Japanese use on a glass substrate with silver paint for wiring; phenolic resin and paper phenolic substrates (CCL) on copper foil for wiring.
1951, polyimides, then the heat resistance of the resin and further, that makes a polyimide substrate.
In 1953, Motorola developed the law through hole plating double panel. This method can also be applied to the latter part of the multi-layer circuit board.
Printed circuit boards are widely used in the 1960s, after 10 years, the technology is increasingly mature. Since the double-sided available Motorola multilayer printed circuit boards appeared that the ratio of the wiring area of ​​the substrate is more improved.
1960, V. Dahlgreen printed circuit with a metal foil affixed to a thermoplastic plastics, creating flexible printed circuit board.
In 1961, the United States Hazeltine Corporation plated through hole reference method to produce plywood.
1967, published one additional layer method "Plated-up technology".
1969, FD-R manufacturing a polyimide flexible printed circuit board.
In 1979, Pactel published one additional layer method "Pactel law."
1984, NTT developed film circuit "Copper Polyimide Act."
In 1988, Siemens has developed a Microwiring Substrate growth-layer printed circuit board.
In 1990, IBM developed a "surface layer line growth" (Surface Laminar Circuit, SLC) growth layer printed circuit board.
1995, Panasonic developed ALIVH growth layer printed circuit board.
In 1996, Toshiba has developed B2it growth-layer printed circuit board.
The increase in the number of layers of the printed circuit board solutions are proposed in the late 1990s, growth-layer printed circuit board also officially be a lot of practical, until now.

Design

CAM350 Software

Printed circuit board artwork generation was initially a fully manual process done on clear mylar sheets at a scale of usually 2 or 4 times the desired size. The schematic diagram was first converted into a layout of components pin pads, then traces were routed to provide the required interconnections. Pre-printed non-reproducing mylar grids assisted in layout, and rub-on dry transfers of common arrangements of circuit elements (pads, contact fingers, integrated circuit profiles, and so on) helped standardize the layout. Traces between devices were made with self-adhesive tape. The finished layout "artwork" was then photographically reproduced on the resist layers of the blank coated copper-clad boards.

Modern practice is less labor intensive since computers can automatically perform many of the layout steps. The general progression for a commercial printed circuit board design would include:
1.Schematic capture through an electronic design automation tool.
2.Card dimensions and template are decided based on required circuitry and case of the PCB. Determine the fixed components and heat sinks if required.
3.Deciding stack layers of the PCB. 1 to 12 layers or more depending on design complexity. Ground plane and power plane are decided. Signal planes where signals are routed are in top layer as well as internal layers.
4.Line impedance determination using dielectric layer thickness, routing copper thickness and trace-width. Trace separation also taken into account in case of differential signals. Microstrip, stripline or dual stripline can be used to route signals.
5.Placement of the components. Thermal considerations and geometry are taken into account. Vias and lands are marked.
6.Routing the signal traces. For optimal EMI performance high frequency signals are routed in internal layers between power or ground planes as power planes behave as ground for AC.
7.Gerber file generation for manufacturing. In the design of the PCB artwork, a power plane is the counterpart to the ground plane and behaves as an AC signal ground, while providing DC voltage for powering circuits mounted on the PCB. In electronic design automation (EDA) design tools, power planes (and ground planes) are usually drawn automatically as a negative layer, with clearances or connections to the plane created automatically.

Manufacturing


The basic components of PCB

Line and Pattern: line is between the original conduction as a tool in the design of the design will be another large copper surface as the ground and power planes. Line and the plane is the same make.
Dielectric layer: to keep the insulation between the lines and layers, commonly known as the substrate.
Through Hole/Via: two levels can vias over each conductive line, the larger the vias used as a plug part, and another nPTH is usually used as a surface -mount position, when assembled with screws.
Solder Resistant/Mask: not all have to eat tin on the copper surface parts, and therefore non- food tin region, will be printed eat tin copper surface layer of isolation material ( usually epoxy), avoid eating a tin of non-short-circui. Depending on the process, divided into green oil, oil, blue oil.
Legend /Marking/Silk screen: This non- necessary structure, the main function is marked in the name of the parts on the circuit board , the position of the box, easy assembly after repairs and identify with.
Surface Finish: As the copper surface in the general environment, it is easily oxidized , resulting in not on the tin (solder bad), and therefore be protected eat tin copper surface. There are ways to protect HASL, ENIG, Immersion Silver, Immersion Tin, OSP, methods have advantages and disadvantages, collectively referred to as surface finish.

PCB Substrate

The baseplate's insulation portion as a classification of the substrate, the common raw materials, wood, glass fiber, as well as all kinds of plastic plates. And PCB manufacturers are generally in a glass fiber, the fabric material, and an insulating portion of the resin composition, an epoxy resin and a copper foil and then pressed into a "prepreg" used.

Circuit design and common substrates and the main ingredients are:
FR-1 phenolic cotton paper, which base known as Bakelite (higher economy than FR-2)
FR-2 phenolic cotton,
FR-3 Cotton paper, epoxy resin
FR-4 Woven glass, epoxy
FR-5 glass cloth, epoxy resin
FR-6 matte glass, polyester
G-10 glass cloth, epoxy resin
CEM-1 tissue, epoxy resin (flame retardant)
CEM-2 tissue, epoxy resin (non-plenum)
CEM-3 glass cloth, epoxy resin
CEM-4 glass cloth, epoxy resin
CEM-5 glass cloth, Polyester
AIN aluminum nitride
SIC silicon carbide

PCB Metal coating

In addition to the metal coating on the wiring board, the circuit board is now in place for soldering electronic components. Furthermore, due to different metals price and therefore directly affect the cost of production. Further, each metal solderability, contact, etc. of different resistance values​​, which will directly affect the device performance.

Common metal coating are:
Copper
Sn

5 to 15m in thickness is usually
Pewter (tin or copper alloy)
the solder thickness is usually from 5 to 25m, the tin content of about 63%
Gold
usually only plated Interface
Silver
usually only the interface plated or silver alloy overall also.

PCB layers classification

single sided (one copper layer)
Single sided PCB assembly requires components to be assembled on only one side of the circuit board and is the least complex of PCB surface mount assembly processes.

double sided (two copper layers)
Double-sided circuit boards have copper tracking and component pads (land pattern) on the top and bottom side of the PCB, and often each side is interlinked by through-hole vias.

Multi-Layer Boards
Multilayer printed circuit board is composed of more than three layers of conductive graphics layer and insulation layer alternately by laminated gluing together and form of the PCB, and meet the design requirements stipulated conductive graphics interconnection between the layers. It has high density, small volume, light weight, high reliability, etc, output value is the highest, fastest growing type of PCB products.

PCB characteristics

Through-hole technology



Through-hole (leaded) resistorsThe first PCBs used through-hole technology, mounting electronic components by leads inserted through holes on one side of the board and soldered onto copper traces on the other side. Boards may be single-sided, with an unplated component side, or more compact double-sided boards, with components soldered on both sides. Horizontal installation of through-hole parts with two axial leads (such as resistors, capacitors, and diodes) is done by bending the leads 90 degrees in the same direction, inserting the part in the board (often bending leads located on the back of the board in opposite directions to improve the part's mechanical strength), soldering the leads, and trimming off the ends. Leads may be soldered either manually or by a wave soldering machine.

Through-hole PCB technology almost completely replaced earlier electronics assembly techniques such as point-to-point construction. From the second generation of computers in the 1950s until surface-mount technology became popular in the late 1980s, every component on a typical PCB was a through-hole component.

Through-hole manufacture adds to board cost by requiring many holes to be drilled accurately, and limits the available routing area for signal traces on layers immediately below the top layer on multilayer boards since the holes must pass through all layers to the opposite side. Once surface-mounting came into use, small-sized SMD components were used where possible, with through-hole mounting only of components unsuitably large for surface-mounting due to power requirements or mechanical limitations, or subject to mechanical stress which might damage the PCB.

Surface-mount technology


Surface mount components, including resistors, transistors and an integrated circuitSurface-mount technology emerged in the 1960s, gained momentum in the early 1980s and became widely used by the mid-1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the PCB surface, instead of wire leads to pass through holes. Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies with much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labour costs and greatly increasing production rates. Components can be supplied mounted on carrier tapes. Surface mount components can be about one-quarter to one-tenth of the size and weight of through-hole components, and passive components much cheaper; prices of semiconductor surface mount devices (SMDs) are determined more by the chip itself than the package, with little price advantage over larger packages. Some wire-ended components, such as 1N4148 small-signal switch diodes, are actually significantly cheaper than SMD equivalents.

Circuit properties of the PCB
Each trace consists of a flat, narrow part of the copper foil that remains after etching. The resistance, determined by width and thickness, of the traces must be sufficiently low for the current the conductor will carry. Power and ground traces may need to be wider than signal traces. In a multi-layer board one entire layer may be mostly solid copper to act as a ground plane for shielding and power return. For microwave circuits, transmission lines can be laid out in the form of stripline and microstrip with carefully controlled dimensions to assure a consistent impedance. In radio-frequency and fast switching circuits the inductance and capacitance of the printed circuit board conductors become significant circuit elements, usually undesired; but they can be used as a deliberate part of the circuit design, obviating the need for additional discrete components.

About PCBway

Since 2003 PCBWAY has been the leading PCB quick turn manufacturer specializing in both Prototype and Production quantities, Initially produced single-sided and double-sided printed circuit boards for the consumer electronics market. PCBWAY is ranked among the top 4 board fabricators in asia and is well-known for its expedited turn time capabilities and its reliable best on-time shipping record.

Today, we have over 450 operators with high modern facilities to manufacture multi-layer PCB up to 12 layers. Backing up with a group of professional engineers, and well established quality system. EVERGREEN has grown to become a major PCB manufacturer in Asia to serve in diverse customers base such as electronics appliance, communication, educational electronics, power supplies, Automationsetc.

Our mission is to become one of leading PCB manufacturer that provide in high quality product with total customer satisfaction.

For more information about PCB, or to learn more about the online quote and ordering process, please visit //www.PCBway.com.

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