Computer-aided design provides an interface between the PCB designer and the computer. The combination of a graphic terminal (video display unit), an input device and a functional keyboard gives the designer an automated drawing board, which brings about a significant improvement in productivity. In recent years, there has been a phenomenal growth in the availability of software for the design of printed circuit boards.
Early software programs were simple geometric editors allowing only the placement and routing of tracks. However, they were interactive and it was, therefore, easy to erase, shift and replicate the
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components and blocks of the circuitry. They had some severe limitations as they did not allow grids other than 100 mil or components on both sides of the board. With the developments in software, schematic entry was added to the geometric editor, which, in turn, allowed automatic routing. Today, automatic placement and routing is possible, as the software library now contains not only standard footprints of individual devices and integrated circuits, but full electrical data on standard product lines (such as TTL and CMOS packages), so that simulation can be undertaken easily on digital circuits. In addition, net lists, drill sizes and other relevant data are automatically generated.
A CAD system with various possibilities offers tremendous advantages over manual methods of designing. An important advantage is the reduced time for the layout procedure. Also, in many cases, the capability to make circuit modifications simple simultaneously provides a completely updated production documentation.
With the assistance of CAD, higher package densities can be achieved and complex circuitry with a larger number of ICs per board are realized, which can hardly be arranged by a manual design. The resulting patterns are constantly of the same high precision and of a consistent quality. In a multi-layer board design, especially, interactive CAD plays an important role in the design process.
5.7.1 System Requirements
Figure 5.25 shows a generic block diagram of a CAD system. The various components of the CAD system are discussed below.
Drill data on tape Paper tap punch Printer
Input device
Digitiser
Computer system Software database Interactive station
Photo Plotter Schematic
assy. drawing
Netlist Drill data
Master film Out put
preparation tool Mouse
Fig. 5.25 Generic block diagram of a CAD system (NTTF Notes)
5.7.1.1 Hardware
A CAD system comprises both hardware and software, wherein the hardware is the most visible component. Depending upon the user requirement, the hardware may range from a personal computer (PC) to a workstation. A workstation, in general, has more computing power and offers a network environment. In a networked system, all users can share the common database to component libraries and peripheral devices like pen plotters and printers, etc. The PC or workstation configuration is usually governed by the CAD software. The following PC hardware is, however, adequate for work of the PCB design packages:
CPU : Intel Pentium IV Processor 2.0 GHz or better with 512 KB cache Memory or better;
Bus architecture : Integrated Graphics, 3 PCI and AGP;
Memory : 128 MB 266 MHz DDR RAM upgradable upto 2.0 GB Master Hard Disk Drive : 40 GB Ultra DMA 100 HDD PCI Dual Channel Bus Master FDD : 1.44 Floppy Disk Drive (3.5") Internal
Monitor : 15" SVGA Digital Colour Monitor (Support 1024 ¥ 768 NI Resolution)
Video Controller : on board 4 MB or better Dynamic Video Memory Keyboard : 104 Keys Keyboard cherry type
Ports : 2 USB Ports, 2 Serial Ports, 1 Parallel Port,
CD ROM : 48X or better CD ROM DRIVE
The above configuration provides a good PCB design CAD workstation environment.
Data Entry Devices: The keyboard is the most commonly used data entry device. In addition, some graphic input devices such as light pens, touch screens, joysticks, track balls, mice and digitizers allow operators to enter data such as lines and points in the graphic form. Graphic input devices are also used to select items from a menu.
Output Devices: Hard copies are required for various purposes, including preview, file storage, reports, presentations, finished drawings and precision photo-tools. Normally, an A-1 size plotter can meet most of the requirements, but it is expensive. Therefore, an A3/A4 plotter is adequate.
Two basic types of electromechanical pen plotters are used in CAD systems. In the flat bed X-Y plotter, servo-controlled pens or stylus are moved in two axes over flat stationary sheets of paper. In the drum or roll type of plotters, the pen or stylus moves in one axis while the paper moves in another axis on a revolving drum. In photoplotters, a moving light head transmits a focused beam of light through an aperture onto a photo-sensitive film or paper. The aperture selection and motion of the light head are under the control of software. Photoplotters generate high resolution images and are a primary source of high quality PCB artworks from the CAD system.
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The printer is used to generate the netlists, and bill of materials. The most commonly used printers are inkjet or laser printers.
5.7.1.2 Software
The selection of an appropriate computer-aided design software is often difficult and many articles keep appearing in professional journals to assist people in their selection. There are several considerations for selecting a suitable software for the intended applications, which include the capabilities of the software and its cost. In general, PCB design needs, and factors such as user interface, learning time, help provisions, speed of use, etc., should be taken into account.
Since making a schematic is the first step in designing a PCB, it is necessary to analyse the attributes of the schematic editor, which may include schematic capture programme, libraries supplied with the software, scaling of symbols, netlist generation, online packaging, automatic bus connection, etc.
Since a good placement forms the backbone of a well-designed PCB, it is important that the placement tools offered by the software should facilitate placement of the components in the most optimal manner and in the shortest possible time. Routing percentage, definition of spacing parameters, placement control, matrix placement, re-entrant autoplace, rotation controls, tools for ascertaining optimal placement, independent viewing of bus connections, and component search during placement form the basics of these tools. The time needed to finish a board with a combination of auto and manual routing, throughput, routing percentage, degree of control, and support of post- routing optimization form the guidelines for evaluation of an auto-routing tool.