KiCad progress

I am learning to use KiCad with increasing speed, and it is certainly good to have a FOSS cad system that is available for Windows and Linux.

So far so good. However, the mapping of schematic to layout has problems.

Taking the example of a diode, the schematic (drawing) symbol looks like an arrow with a bar across the sharp end, with a connection at each end. One of the possible physical shapes for this can be SOT23, a shape used by several different components, including transistors, mosfets, and double diodes. The standard numbering of the three connections on a SOT23 goes, unsurprisingly, 1 2 3. A normal single diode connects to numbers 1 and 3, while 2 is unused.

Mapping the connections from the symbol to the physical shape requires that the pin numbers match, so if a diode is to use a standard SOT23 shape, its connections would have to be numbered 1 and 3. However, if the diode is to have another physical shape, then pin numbers 1 and 2 might be needed, or maybe even 2 and 3. A single schematic symbol would not work for this approach – but one of the intended strong points of KiCad is that one can concentrate on the schematic until that level of theoretical design is dealt with, and only then consider the detailed physical shape of the parts used. If a different (but similar looking) schematic symbol had to be used for different shaped parts, that level of simplicity doesn’t exist.

The alternative is to have separate layout shapes called, for example, SOT23-diode, SOT23-mosfet, and so on. If there is ever a need to change the dimensions slightly, for example to accommodate a change in soldering process, then all these separate footprints will need to be edited. Meanwhile, back at the diode, with a system like that, the diode connections would be labelled A for anode and K for cathode (don’t ask), and the diode footprint library would have pads labelled A and K.

Most electronics CAD systems have a third type of element, often called a device. That is a file that maps which connections on a schematic symbol match with which connections on the layout footprint, so that for any type of component there is just one schematic symbol, and for any layout footprint shape, there is only one of them regardless of how many different types of part use it.

I am still trying to decide whether there is a logical way to use KiCad, or whether it is (very reluctantly) necessary to purchase an upgrade to my existing EDWIN software in the foreseeable future. Edwin is good, but CAD software is usually very costly, so I am using a version from 1999 which will only work on Windows 7 using the virtual machine, and which I have so far failed to make work under WINE.