The present invention relates to a method of joining an insulated wire to a conductive terminal and more particularly to a method of joining suitable for integrally joining an insulated wire as unbare to a conductive terminal and forming a metallic joining conductive layer between a core wire of the insulated wire and the conductive terminal.
The present invention to a method of joining an insulated wire such as a high heat resisting copper wirer relates having heat resisting temperature of 155.degree.-180.degree. C., for example an amide-imide-coated wire, a low heat resisting copper wire having heat resisting temperature of 130.degree. C., for example a polyester-coated wire to a conductive terminal such as Cu, Cu alloy comprising at least one of brass, bronze, Cu-Ni and Cu-Ni-Ag, and soft steel, SUS steel etc..
Thermo-compression connecting has been employed for connecting an insulated wire to a U-shaped conductive terminal as described in Japanese Patent Publication No. 18940/1975.
In this method, a current cannot flow because the insulated wire is covered with an insulating coating. For this reason, the insulated wire was connected to the U-shaped conductive terminal by placing the insulated wire in a U-shaped groove of a previously formed conductive terminal, interposing the conductive terminal between an upper electrode and a lower electrode, pressing the conductive terminal, causing a current to flow in both electrodes to bring about heat buildup in the U-shaped conductive terminal, and carbonizing the insulating coating of the insulated wire by means of the electric heating.
In this method, since a resistance welder is employed, not only resistance welding time is short but also a metallic joining conductive layer is not formed between the core wire of the insulated wire and the conductive terminal. In order to compensate for this drawback, the connection between the core wire of the insulated wire and the conductive terminal was conducted by utilizing calking of the conductive terminal through application of force.
The above-described connecting method through mechanical calking brought about a problem with respect to a remarkable lowering in the fatigue resistance as well as in the electrical properties when the joined body was used for a long period of time.
In the above-described conventional mechanical calking connection method, no consideration was given to the formation of a metallic joining conductive layer between the core wire of the insulated wire and the conductive terminal. Therefore, the conventional connecting method had problems also with respect to the mechanical connecting strength, vibration, and electrical properties derived from the vibration. That is, since the conventional connecting method mainly aims at the removal of an insulating coating of the insulated wire, no metallic joining conductive layer can be attained between the core wire of the insulated wire and the conductive terminal.
Under these circumstances, the present inventors have expected that when a joining assistant which melts during carbonization of the insulating coating of the insulated wire is present around the joining area, the molten joining assistant wets a Cu wire as a core wire of the insulated wire which a clean surface has been exposed through the removal of the insulating coating of the insulated wire and then reacts therewith, thereby attaining a metallic joining conductive layer between the core wire of the insulated wire and the conductive terminal.
Further, the present inventors have made various studies on the kind of the insulating coating of the insulated wire and the joining assistant and, as a result, have found that for example, when the insulated wire is an amide-imide-coated wire, a P-containing solder, BAg-1 (JIS), BAg-2 (JIS), etc. which melt at a temperature of 600.degree. to 750.degree. C. are suitable as the joining assistant.
More specifically, in the present invention, a joining assistant having a melting point corresponding to the carbonization temperature (or the peeling temperature) of the insulating coating of the insulated wire is applied to the conductive terminal, and a metallic joining conductive layer between the core wire of the insulated wire and the conductive terminal is attained simultaneously with the peeling of the insulating coating of the insulated wire.
As an improvement of the above stated thermocompression connecting method, a joining method of the insulated wire to the conductive terminal has been employed in, for example Japanese Patent Laid-Open No. 199575/1986. This joining method is adopted merely for the conductive terminal made of Cu.
In this method, a joining assistant such as Cu-Sn alloy, or Cu-Zn alloy is used for forming as a metallic joining conductive layer between a Cu core wire of an insulated wire and Cu conductive terminal. However, these alloys are joined in general with a flux. Even flux is used, this flux may evaporate during supplied current, the effect according to flux function becomes small. Namely, there can not obtained a joining body having high joining strength.
Further the above stated joining assistant has no element for working as flux therein, accordingly the wetting property of the joining assistant is bad.