11-12-2012, 05:38 PM
An Introduction to Printed Circuit Boards
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Syncopated Systems regularly designs and develops products incorporating electronic circuits. In doing so, one of its most fundamental tasks is the physical implementation of those circuits through the design of printed circuit boards and the outsourcing of their fabrication and assembly. Each time it performs these tasks, as summarized in this article, Syncopated demonstrates expertise in the disciplines of computer science, electrical engineering, mechanical engineering, component engineering, purchasing and project management.
Printed circuit boards create electrical connections between electronic components – and sometimes components themselves – in the most cost-effective manner for many applications including general-purpose computers and consumer products.
By recognizing and understanding applicable rules of physics and available manufacturing processes and costs, skilled printed circuit designers may create circuits that behave as desired at minimal total cost to produce.
This article surveys common types of printed circuit boards and the history of their development, as well as basic design processes, considerations and tools needed to create them.
Printed Circuit Board Defined
A printed circuit board, commonly abbreviated p.c.b., consists of at least one rigid planar electrically-insular substrate laminated with at least one electrically-conductive layer (usually metal foil), which is printed with a pattern designed to make specific desired connections between the pins of components mounted or otherwise attached to it.
Other Printed Circuits
Derived from p.c.b. fabrication processes are those used to create flexible printed circuits and semiconductor integrated circuits (commonly called silicon "microchips" or "chips"). Though detailed description of these circuits falls outside the scope of this article, they are summarized below.
Flexible Printed Circuits
Flexible printed circuits, as the name implies, are similar to printed circuit boards in having one or more conductive metal layers printed on an insular substrate, but the substrate used is a thin sheet of a flexible plastic suchMylar (a brand of biaxially-oriented polyethylene terephthalate, or boPET) polyester film. Because the substrate plastics used often have melting points well below that of the solder used to connect components to rigid printed circuit boards, connections to flexible printed circuits are usually made via crimped contacts and/or friction-locked connectors. Flexible printed circuits are often used in electromechanical devices such as keyboards, disc drives and liquid-crystal displays.
Integrated Circuits
Integrated circuits (abbreviated i.c.s) are fabricated on a much smaller scale using similar printing techniques.
Though they are created using processes very similiar to and ulitmately derived from p.c.b. processes, detailed description of flexible printed circuits and integrated circuits falls outside the scope of this article.
Nomenclature
Despite the industry's history of more than 100 years of electrical engineering and 50 years of printed circuits, there is still little accepted commonality in the names used to describe printed circuits.
Some refer to a printed circuit board as a printed wiring board, abbreviated p.w.b.; it is a naive term, implying failure to recognize the electrical properties of the board's substrate and patterns printed thereupon. As the density of printed circuit board integration increases, the values of these properties become much less negligible, so the term p.w.b. should be deprecated.
Some organizations, allegedly including I.B.M., insist upon using the word "card" in place of "board", apparently to help differentiate electrical circuits from lumber. Both words adequately describe the planarity of the substrate, so both are acceptable and may be used interchangeably. However, the word "board" is more commonly used and therefore more commonly understood, so its usage should be preferred.
Layers
A printed circuit board is constructed through the combination of several planar layers that are themselves formed through additive and subtractive processes. The simplest printed circuit boards are formed by placing a conductive layer on an insulator and removing (via chemical or mechanical means) the conductive material from areas in which it is not desired.
Insulators
A printed circuit board substrate is an electrical insulator that provides the p.c.b. its rigid planar structure. The materials used for p.c.b. substrates and their flame-retardance ("f.r.") ratings vary, from FR-0 pressed cellulose (paper) to ceramic tile, but FR-4 epoxy fiberglass is most commonly used in the United States.
Conductors
Each metal layer consists of metal foil (such as copper, Cu) laminated to the substrate which is printed using a photographic process (described in more detail below) with a resistive mask, etched with acid (such as ferric chloride, FeCl3), then the mask and any residual acid are washed off. Connections between multiple metal layers were first made through pins soldered on top and bottom and later through plated-through holes; a connection between metal layers without using the pins of a component are made via a hole called a via (pluralized "vias").
Solder Masks
Solder masks are usually green, though they may be any color. Solder masks assist averting solder bridges (which cause undesired short circuits) in the assembly process by block the flow of liquified solder when components are populated onto the printed circuit board. Most modern printed circuit boards include top and bottom solder mask layers, which have been historically silkscreen-printed but are now more commonly printed using liquid photo-imageable (l.p.i.) processes.
Component Legends
Legend layers, as with solder masks, have been historically silkscreen-printed but are now more commonly printed using l.p.i. processes; they are still often (and usually incorrectly) referred to as "silkscreen" or just "silk" layers. Legend layers are usually the outermost layers printed over the solder mask, if present. Often white, though they may be any color (usually contrasting that of the solder mask and/or substrate onto which it is printed), they identify the intended location and orientation of components to be mounted to the board, and often include other information that may be of service to those assembling, troubleshooting or using the assembly built on the board.