A feedthrough terminal assembly for an active implantable medical device includes a conductive terminal pin or leadwire, a feedthrough filter capacitor having a first set of electrode plates conductively coupled to the terminal pin or leadwire, and a second set of electrode plates conductively coupled to a housing, ferrule or ground plane of the active implantable medical device, and a non-conductive mullion disposed relative to the terminal pin or leadwire for increasing creepage distance between the terminal pin or leadwire and another conductive element, creating a tortuous path that increases resistance to arcing/flashover.