I have some prototype boards that I would like to modify. The modification includes removing a section of circuit and replacing it with another circuit. My theory on how to do this is to make an adapter board that will hold my new circuit that will have castellated holes on the perimeter. The perimeter holes will align with existing SMD pads on the main board in order to make the necessary connections.
My question is are there recommended land patterns for castellated holes that I could use to guide the design of my adapter board? In this case I would be designing the castellated holes to match existing land patterns. Of course this is not optimal but I hope that for a prototype it will work. Most of the mounting points on the main board are one side of 0402 resistor lands. I'm guessing that I should use a hole that is slightly smaller than the existing pad. For example, to connect to a 0.024" square pad I'll use a 0.020" castellated hole. Does this sound reasonable?