2. CAD Software
Numerous CAD Software packages are available for PCB design. Mostly schematics capture and PCB layout are integrated in a single software package. Some of the popular freeware PCB CAD software packages are DipTrace, EagleCAD, KiCAD, ExpressPCB, gEDA, Fritzing etc. Some of these freeware packages have design limitations and one would have to buy the full version to use all the features.
The best among these is without doubt KiCAD, which is an open source software, available for a wide range of operating systems and completely free. DipTrace and EagleCAD are pretty capable PCB design packages too, but their free version have limitations on either the number of pins or the number of layers or both. To use all the advanced features, one would have to buy the full version.
Among the high end PCB design software, Altium Designer (Formerly Protel PCB), Cadence Allegro, Mentor Graphics PADS PCB and Xpedition. Among them, my vote goes to Altium Designer, for tight integration between schematics and PCB, ease of use, 3-Dimensional capabilities, integrated EMI analysis etc.
3. PCB Design Flow
[ Click above image for a larger view ]
The process flow for a generic Printed Circuit Board design is shown above. It is assumed that the component selection and Schematic capture is complete and the PCB layout design is all set to begin.Setting up the workspace area is the first task a PCB designer performs. It begins with setting a Grid and the measurement units. This is generally called a “snap grid” since the cursor snaps to the grid while moving around the workspace. Using a grid is important because it makes symmetrical placement of components easier and also simplifies layout of traces without uniform clearances.
The choice of units is between imperial and metric units. Imperial units have been traditionally followed by PCB designers around the world. Also a lot of electronic components are manufactured with imperial pin spacing.
This is followed by defining the board outline. Many CAD software allows importing a CAD drawing file as the board outline. If this is not available, the outline has to be created manually.
4. Footprint Creation
Once Schematic capture is complete, an output of the process is the Bill of Materials (BOM) document. Using the BOM, the PCB designer can begin creation of footprints. Footprints of standard packages will already be available in the PCB design software libraries. Those which are not available will have to be created by the designer using the corresponding datasheets as reference. Assigning PCB footprints to components at the schematic capture stage itself will make the job of the PCB layout designer much easier. Ensuring that the footprints created comply with IPC-7351, will make sure that there are no assembly issues at least as far as footprints are concerned.
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Constraint Definition
