I need to design a modular circuit and I was thinking about making smaller identical pcbs and solder them on a bigger board (also for module reusability, hyerarchical and modular assembly). The first approach I was thinking is the castellation but I saw that it is a non-standard design option for most pcb manufacturers. Is it possible to use the second side of the pcb (unpopulated) to attach the same balls of a BGA package? What are the drawbacks? It should also use less space on the bigger board than castellation pads.