A feedthrough assembly, as well as a method of forming a feedthrough assembly, including a metallic ferrule, and a biocompatible, non-conductive, high-temperature, co-fired insulator engaged with the metallic ferrule at an interface between the ferrule and the insulator. The insulator includes a first surface at the interface and a second surface internal to the insulator. At least one conductive member may be disposed at the second surface, wherein at least the first surface of the insulator is devoid of surface cracks greater than 30 μm. The first surface of the insulator may also be devoid of a surface roughness greater than 0.5 μm.