I looked through the FAQ but didn’t see this. If a design is spread across different boards (like a separate power supply board or a display board or optional plug-in boards) because it can’t all fit on one board, how is it handled so that: 1) there is an entire schematic for overall design and simulation, and 2) there are separate schematics for troubleshooting and board layout? We use something else right now (very old software) and it’s an entirely manual and painfully slow process, but KiCad’s features would be nice to use if we can switch to it and make our lives easier.
Yes, this is how it’s done in KiCad
How are they tied together for overall design and simulations?
Well, one schematic = one layout, Kicad doesn’t support multiboard projects like Altium Designer does.
As to simulation (assuming SPICE), I suppose you can start with a combined schematic and, once simulated, split it logically into separate ones.
OK, that’s what I was wondering. Thanks.
Kicad has a much better price point than Altium 
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I tried to calculate benefit per dollar but it was a bit difficult.
I haven’t actually used simulation, but it may be difficult to create large schematic which works for both pcb design and simulation. At least you should create some new component libraries. For historical reasons the normal schematic diodes and simulation diodes aren’t compatible. Those who use simulation can tell you more and correct me if I’m wrong.
As for splitting a design, you should get familiar with hierarchical sheets if you aren’t already. You would design separate boards as separate projects and combine their schematics in another project as sub-sheets (they would point to files of the other projects). The main schematic top sheet would not have components but only subsheets. See Hierarchical or flat schematic design, what is best for me? (How to deal with multi page schematics?).
I again have to admit I haven’t tried this, so you have to try yourself or wait for other comments.
As of yet there is no “official” support for a project spread over multiple PCB’s, but it is a recurring topic and it can be done.
https://forum.kicad.info/search?q=multi%20pcb%20project
I also once did an experiment (no real PCB) in this direction, and it all seemed to work just right:
Apart from that, there are significant differences between a schematic made for simulation, and a schematic made for PCB design. For example for simulation you usually do not bother much with voltage regulators and connectors. Simulation is typically also done on relatively small parts of a project, such as an ADC front end, a HF mixer or other small identifiable block.
In my “multi PCB Project idea” I used a hierarchical design, and multiple sub project to organize different sheets for the PCB’s. Maybe one (or a few) of those can be tailored for simulation too. Or else copying parts of a design between a “Simulation Project” and a “PCB Project” is quite easy and straightforward.
Also, schematic entry is a relatively small task. The time consuming part is in the design itself. Reading and understanding datasheets, comparing specs of parts and figuering out what to make in the first place.
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