An electrical connector comprises an elongated flexible circuit having rigid strips bonded onto one or both sides and one or more ends thereof. The strips contain metallized castellations that are aligned with respective metal traces on the flex circuit and soldered thereto. The flexible portion, along with the attached rigid board(s) forms a plug assembly that mates with a socket having contact pins that engage with their respective metallized castellations, with the castellations providing a self-aligning function. The invention may be used to form a flex cable with connectors on one or both ends. Alternatively, the elongated flexible circuit may represent an extension of a larger flex circuit substrate that further contains other electronic devices mounted thereon. The socket and contact pins are preferably attachable to a motherboard by soldering or other means.