When should a Proteus PCB use microvias instead of ordinary through vias?

Begin with the escape-routing problem and the manufacturer's available constructions. A microvia is a fabrication feature with a defined layer span, not simply a small through hole. The achievable geometry depends on the dielectric thickness and process. Ask for a supported stackup and via structure before setting routing rules, because changing the stack later can invalidate the escape pattern.

Compare alternatives such as a larger package, different pin assignment where possible, more routing space or a different layer arrangement. Include assembly and inspection constraints as well as routing density. A technically routable design may still be expensive or difficult to yield. For a modest prototype, avoiding unnecessary HDI can simplify both procurement and fault diagnosis.

Once the construction is agreed, encode the permitted layer transitions and dimensions in the CAD rules and inspect the generated drill and fabrication documentation. The manufacturer must be able to distinguish the intended via types. Passing the CAD DRC means the design meets the rules you entered; it does not confirm that those rules describe a process the chosen supplier can manufacture.