Electronic – Designing a breakout for crenellated (castellated?) pads

If this is for production access only, then here's a trick I used once. The PCB (standard 0.062" thickness) was designed with SMT pads along one edge (both sides) so that a 2×7 0.100" JTAG header could be soldered to it, with the pins sticking out from the edge. This was useful for debugging prototypes, which needed more or less constant access to the JTAG (and weren't in cases).

In production, the connector is omitted, and I used a slightly modified DIP clip to access the pads for the connector. I just needed to bend the contacts inward a bit to get adequate friction and contact pressure.

Instead of the DIP clip, you could use a standard card edge connector as well.

The workflow in Altium which I got used to for 2 years was that when you make a custom component, you make all the stuff at once - the schematic symbol, then a footprint, and assign pins etc. which makes sense to me.

The workflow in KiCAD which I've been using now for about 1-2 weeks is totally different. You do the schematic, and have schematic libraries, and then you make a net list and assign footprints. The footprints may not exist yet, so you can make them at this point if you have not already made them. You can do altium-like workflow if you try really hard, but KiCAD does not make it easy, and documentation/YouTube videos are all using old versions so you can never really get a good concrete way of doing things. That is the glory of an ever-changing open source project.

There is a magical footprint wizard which has an icon up the top in the footprint editing program in KiCAD. The footprint wizard was useful to me to make SIP pin headers quickly. There IS a QFN footprint maker in the wizard, but maybe not a QFP - have a look.

Otherwise I suggest you learn how to make a footprint pad-by-pad, and using the datasheet by the manufacturer and their "suggested pay layout" which is almost guaranteed to be there, or it's a standard footprint diagram you can get from elsewhere.

The easiest way to do it in KiCAD is to set the "user grid" to the correct X and Y pitch so that you can just plop down the first set of pins, and then change the grid again to get you to the side set of pins, and then top, and then the remaining side. The problem with KiCAD is it's hard to measure stuff, and the grid settings are so awkward to get to, but you always have to use them - there may be hotkeys you can bind.