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as worst-case tolerance and timing analysis. It can be run on a design before physical layout begins as well as after layout is completed. Some examples of circuit analyzers are SPICE and PSPICE. SPICE and PSPICE build mathematical models of each circuit and then perform thousands of complex calculations to predict how the circuit will respond to input signals. Most suppliers of CAD systems also offer their own adaptations of these analysis tools. 14.3.1.6 Impedance Predicting Tools. These tools are used to examine the cross section, trace sizes, and materials properties of a PCB to ensure that the resulting circuit impedance is within allowable limits or to interactively adjust these parameters to achieve a desired final impedance.This is an essential step in the design of the PCB itself. Most suppliers of CAD systems intended for high-speed design supply some form of on-line impedance analysis tools as part of their systems. 14.3.2 CAD Tools CAD tools are used to turn the electrical circuit described by the schematic into a physical package or PCB. CAD tools are typically operated by PCB designer specialists who are skilled in the areas of PCB manufacture and assembly rather than by electrical engineers. These tools are fed netlists, component lists, wiring rules, and other layout information by schematic capture or CAE tools. In their simplest form, they allow the designer to create the pad patterns for the component leads and PCB shape and then manually connect the component leads with copper traces. The most sophisticated CAD tools can automatically determine the optimum location of each component on the PCB (autoplacement) and then automatically connect (autorouting) all leads while following high-speed layout rules. This is accomplished by providing the CAD tool with a table of rules specifying which components must be located in groups or near connectors as well as by specifying how much space must be maintained between neighboring traces, the maximum length allowed between points on a net, etc. The outputs of CAD tools are the information files needed to fabricate, assemble, and test the PCB assembly. These are test netlists, photoplotting files, bills of materials, pick-and-place files, and assembly drawings. CAD tools are made up of circuit routers, placement tools, checking tools, and output file generation tools. 14.3.2.1 Placement Tools. Placement tools are used to arrange the components on a PCB surface. Placement tools tend to be part of a complete CAD system rather than a module purchased separately. The inputs to a placement tool are:
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Component list or bill of material Netlist or manner in which the components connect to each other Shapes, sizes, and spacial arrangement of the component leads Shape of the PCB with areas into which components cannot be placed (keepouts) Instructions concerning fixed locations for components, such as connectors Electrical rules, such as maximum and minimum distance between points on a net Thermal rules, such as which parts must be kept apart or near sources of air flow
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Placement tools range from completely manual to fully automatic. All have some form of graphical feedback to the designer that assesses the quality of the placement in terms of its ability to be routed or connected in the desired number of signal layers. Most have spacing rules that ensure that the components have enough room between them for successful assembly, rework, and testing. 14.3.2.2 Routers. Routers are the part of a CAD system that makes the physical connections between the components as specified by the netlist.A router operates on the PCB netlist
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and placement after the placement step has been completed. Routers range from the completely manual, in which case the designer specifies where wires are to be located by using a graphical display and a mouse or light pen, to fully automatic, where a specialized software program takes the netlist, the placement, the spacing rules, and the wiring rules and makes all the decisions necessary to completely connect all components. The principal advantage of manual routing lies in the fact that the designer can tailor every connection to his liking. The principal disadvantage of manual routing is the fact that it is slow and time consuming, often taking several minutes to completely route and check a single net. Automatic or autorouters solve this speed problem. However, the ability to control the detailed shape of each net is limited by the ability of the autorouter to follow wiring rules. Some advanced autorouters are able to comply with very complex wiring rules. A significant problem with autorouters lies in the fact that they may not find ways to successfully route all of the wires. When this occurs, the designer must add more wiring space in the form of more layers or attempt to complete the routing manually. An important feature of a good autorouter is its manual routing option, as it substantially affects the ease with which this often necessary finishing operation is completed. Nearly all routers have a suite of checking tools that ensure that the final route matches the netlist and that all of the spacing rules have been followed. Routers come in several forms and can be purchased as an integral part of a CAD system or as modules to add onto CAD systems. Some router types are: Gridded Router. This type of router operates by placing wires on a predefined grid pattern. The routing surface is divided into a uniform grid that provides a proper gap between traces when wires are routed on every grid line. It is the first form of both manual and autorouter offered with CAD systems. The primary disadvantages of gridded routers are that it is difficult to manage more than one trace width without losing wiring density and it requires end points of nets to be on the routing grid in order to connect to them successfully. Offgrid component connections typically have to be made by hand and checked by hand. Gridless Router. This form of router does not depend on a grid to locate wires on a surface. Instead, it places as many wires in a space as will fit and still maintain the spacing rules established by the design engineer to ensure proper electrical performance while optimizing manufacturability. Multiple trace widths on the same layer are handled easily by this type of router. Once the routing job is completed, the router divides up any unused space equally. The advantage of this technique lies in its ability to optimize manufacturability by keeping spaces as large as possible. The disadvantage of this type of router lies in the fact that it usually depends on a given wiring layer being all horizontal or all vertical a real disadvantage in SMT applications where components do not need through-holes to connect them but a very powerful router for designs with a very regular array of high-pin-count parts, such as big CPUs and massively parallel processors. This router type is the workhorse of very high complexity digital designs where there is a great deal of regularity and a need to achieve predictable spacings and trace lengths for speed and performance reasons. Shape-based Router. This type of router recognizes shapes already placed in a wiring surface and routes wires to avoid them. Spacing between wires and other objects, such as vias, used to change layers and component pads is maintained as the router places a wire in a space. This router is becoming the workhorse of SMT-based designs. 14.3.2.3 Checking Tools. These tools verify that the routed PCB complies with rules such as spacing between traces and trace and holes by comparing the actual spacings found in the finished artwork to rules provided by the designer. They also ensure that all nets are completely connected and are not connected to objects they should not be, such as other nets and mechanical features on the PCB, by comparing the routed results to data supplied by the CAD system. Some checking tools also check to ensure that transmission line rules are followed and that coupling from neighboring traces is within limits. Checking tools are usually an integral part of a CAD system. 14.3.2.4 Output File Generators. Once a PCB has been routed and all connectivity verified as accurate, the CAD system holds this information in a neutral form specific to the way
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