A locking device for connectors is provided that uses a threaded seal latch application using a ferrule. The locking device secures leads in electrical connectors, particularly connectors involved in medically implantable devices. The locking device utilizes a threaded nut, pre-assembled onto a lead cable with a ferrule. The lead cable is inserted into and engages with a threaded sleeve, the threaded sleeve serves as an opening for the in-line stack inside a housing. The threaded nut engages the threaded sleeve, and the ferrule along the lead cable is encased therebetween. The ferrule is pushed into and against the inner surface of the threaded sleeve by the threaded nut, tightly securing the ferrule in place, and thereby locking the lead cable in the desired position.