A METHOD OF ATTACHING A LEAD TO A METAL END CAP OF A SAFETY FUSE

Abstract: A method of attaching a lead (13) to a metal end cap (3) of a miniature type safety fuse (1) that is otherwise completed, by means of a soldering operation which comprises holding the lead (13) against the end cap (3), using the required amount of solder (9) and optionally flux and supplying a metered amount of energy to the area to be soldered only.

Description: This invention relates to a method of attaching a lead to a metal end cap of a miniature type safety fuse that is otherwise completed. For many uses, in particular in the electronics industry, it is desirable to have safety fuses of the miniature or subminiature type (commonly consisting of a generally cylindrical housing provided with two metal end caps, between which a fuse link is secured), which are provided with leads connected to the end caps. Hitherto two different methods have been used in practice for securing such so-called pig-tails to safety fuses. The first method consists in using additional end caps previously provided with a pig-tail for example by welding. Such caps already provided with a lead are pressed on to the end caps of the completed safety fuse. The interior dimensions of the additional end cap should be such that the end cap closely fits the end cap of the safety fuse. One disadvantage of this method is that an additional end cap and hence more material are required, while further the dimensions of the parts concerned should be accurately controlled to prevent the additional end cap from becoming detached from the safety fuse end cap in operation. Another method at present being employed is the use, during the manufacture of a safety fuse, of end caps already provided with a pig-tail, for example, by welding. One disadvantage of that method is that, as the manufacture of a safety fuse generally comprises soldering for securing the fuse link disposed in the housing to the end caps, there is the risk that the pig-tail becomes detached from the end cap. Furthermore, an automatic production process for the manufacture of safety fuses using end caps already provided with a pig-tail is not quite possible, at any rate it presents problems which do not occur when flat end caps are used. To avoid the problems encountered in the prior methods, it has already been considered to secure leads to completed safety fuses by welding such leads directly to the end caps of a safety fuse. Welding leads to an end cap of a safety fuse, however, is impossible, as welding gives rise to locally high temperatures, also on the inside of the end cap. As a result, the solder on the inside will locally melt and evaporate, owing to which the fuse element or fuse link is apt to become detached from the end cap. It is even possible that owing to the local excessive heating the solder explodes as it were and is thus blown far into the housing of the safety fuse. End caps for safety fuses are commonly made of nickel-plated brass, which nickel is commonly applied galvanically. Such end caps are of good quality and cheap. It is, however, very difficult, if not impossible, to produce a reliable connection between a lead and such an end cap by means of welding. This problem is additional to the above impossibility of welding to a completed safety fuse. The problem last mentioned can only be solved by selecting end caps of silver-plated copper, unplated copper and other materials. Such end caps are indeed used in the above methods of making safety fuses with pig-tails. It is an object of the present invention to provide a method by means of which so-called pig-tails can be attached to the end caps of completed safety fuses without the occurrence of any of the problems mentioned herein­ before and without any limitation in the choice of the material of the end cap. The object contemplated is achieved, according to the invention, by a method in which the attaching is effected by a soldering operation which comprises holding the wire against the end cap, using the required amount of solder and optionally flux, and supplying a metered amount of energy to the area to be soldered only. By means of the method according to the invention it has proved to be possible to attach leads to otherwise completed safety fuses without any damage to the fuse element or interference with the connection between the fuse element and the end cap. Now, it is already known that the amount of energy required for making a reliable connection between a wire and a metal surface using the minimum required amount of solder per unit volume is less than the minimum amount of energy for making a welded joint. It is to be expected, however, that, in a normal soldering process, so much heat is locally required that when a wire is soldered to an end cap of a completed safety fuse, the solder on the inside of the end cap at the position of the soldering area on the outside will yet melt, so that the fuse link or the fuse element is apt to become detached. However, by supplying a metered amount of energy to the area to be soldered only, in accordance with this invention, and naturally ensuring that the correct amount of solder and optionally flux are present at, or closely adjacent to, the areas to be connected, a good soldering joint can be effected without the risk of the solder within the end cap being melted, and hence the fuse element becoming detached. By controlling the supply of energy in accordance with the invention it can be achieved that just so much energy is supplied during just so long a period of time as to effect correct soldering. The supply of a metered quantity of energy to the area to be soldered only can be effected, in accordance with this invention, in several ways. Suitably, the energy is supplied, for example, by means of a pulsing laser. In a preferred embodiment of the method according to the invention, a pulsing Nd-YAG-laser providing laser pulses having a pulse energy of at least 10 Joules and a pulse duration longer than 10 milliseconds is used to solder tin-plated copper wires in a thickness of no more than 0.9 mm to nickel-plated end caps 0.2-0.25 mm thick. According to another embodiment of the method according to the invention, the metered amount of energy is supplied to the area to be soldered using resistance heating by means of discharge pulses from a capacitor. In yet another embodiment, the metered amount of energy is supplied to the area to be soldered by means of a pulsed arc discharge, with the area to be soldered being positioned in the focal point of a lense or mirror system to concentrate the energy in that point. It is noted that soldering using a laser to supply the required energy is known per se, for example, from European patent application 0 113 895. A system for soldering with short pulses, providing the required energy by resistance heating is known per se from Netherlands patent application 8301585. A method of soldering by means of metered concentrated infrared radiation is described in European patent 0 002 884. Netherlands patent application 7905090, finally, describes a welding method in which electrical arc heating is used by means of capacitor discharge. None of the publications referred to, however, relates to the attaching of leads to completed safety fuses, and these publications do not, therefore, suggest that welding while supplying a metered amount of energy does not, but soldering in that way does lead to a solution of the problems outlined. The method according to the invention can be used with or without using a flux. This depends on the nature of the metal of the parts to be connected, the soldering material and the ambient parameters, such as temperature, pressure, any surrounding gas and the like. The method according to the invention lends itself extremely well to mass production of safety fuses provided with leads. In a preferred embodiment of the method according to the invention the end of the lead to be attached is previously upset. Suitably, the amount of solder required can then be provided by putting a ring of solder­ ing material around the end of the lead, which ring is retained by the upset part of the end. The required amount of solder can also be suitably provided by previously attaching a drop of solder to the end of the lead. The invention is illustrated with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of a safety fuse and lead to illustrate one embodiment of the method according to the invention; and Fig. 2 is a similar view of some details to illustrate another embodiment of the method according to the invention. Fig. 1 shows a safety fuse 1 which must be provided with a lead 13. The safety fuse consists of a cylindrical glass housing 2 closed at opposite ends by end caps 3 and 4 fitting over the ends of housing 2. In housing 2, the fuse element 5, which in the case illustrated consists of a thin wire, is positioned diagonally, with the ends of wire 5 being flexed around the edge of housing 2 and clamped between the respective end caps 3, 4 and the outside of housing 2. Disposed in the end caps are solder plugs 6, 7, which provide for a firm connection of wire 5 to the end caps 3 and 4, respectively and for a sealing attachment, or at any rate a seal of housing 2 relative to the end caps 3 and 4. In the embodiment of the method according to the invention here illustrated, lead 13 is attached to safety fuse 1 as follows. Lead 13 has been previously upset, so that it is provided with a thickened part 8 at the end to be attached. Provided around lead 13 is a ring 9 of soldering material. The upset part 8 of lead 13 ensures that ring 9 cannot fall from the wire. Lead 13 is held with the upset part 8 in contact with the outer wall of end cap 3. Subsequently, from a laser source 10, a laser beam 11 is directed at the upset portion 8 of lead 13, the adjoining wall portion of end cap 3 and the ring 9 of solder. A pulsing laser source 10 is used and the laser beam is preferably directed at an angle to be parts referred to. Using a single laser pulse with a duration of approximately 15 milli­ seconds, a good joint is effected between lead and end cap. It is also possible, however, for the sake of certain­ ty, after using the single laser pulse, to rotate laser source 10 in the plane perpendicular to the plane of drawing, and to issue a second or even third laser pulse at a different location along the circumference of the upset portion 8 of the lead. If the use of a flux is desirable in the soldering operation according to the embodiment of the method of the invention as described above, this can be effected in a simple manner, for example, by dipping the end of the lead in thin-liquid flux. Fig. 2 illustrates a different embodiment of the method according to the invention. In that embodiment, a lead 13 with a flattened end is used. Using, for example, a soldering iron, a bead 12 of soldering material is secured to the end of wire 13. Securing wire 13 to end cap 3 is further effected as described with reference to Fig. 1. It will be clear that the soldering material can be supplied to the area to be soldered in a different manner from that illustrated in Fig. 1 or 2, for example in the form of wire. By means of the method according to the invention, leads of tin-plated copper with a diameter of 0.6 mm and of 0.8 mm were attached to safety fuses of the type illustrated in Fig. 1 with dimensions of 20 mm length and 5 mm diameter. The end caps of these safety fuses had a wall thickness of 0.25 mm and consisted of brass 63/37 with a galvanically applied nickel plating of 3-5 µm thick. A type of solder was used with a melting point higher than 300°C (silver-lead solder). It is noted that, from the point of view of the method according to the invention, solder with a lower melting point could also be used. With a view to subsequent uses of the safety fuses, for example, soldering into a print by means of a soldering bath, a lower melting point is not, however, always desirable. The energy was supplied to the area to be soldered from a pulsing Nd-YAG-laser with a pulse energy of 16-34 Joules. Using one laser pulse of 15 milliseconds, good joints were made. The use of two or three pulses spaced along the circumference of the joint also produced good connections. Best results were achieved using leads with an upset end. With leads having a flattened end only, however, acceptable joints were still realized. As good and acceptable joints were regarded the joints satisfying the requirements specified in the IEC publication 68-2-21, part 2.

Claims: 1. A method of attaching a lead (13) to a metal end cap (3) of a miniature type safety fuse (1) that is otherwise completed, characterized in that the attaching is effected by a soldering operation which comprises holding the lead (13) against the end cap (3), using the required amount of solder (9; 12) and optionally flux, and supplying a metered amount of energy (11) to the area to be soldered only. 2. A method as claimed in claim 1, characterized in that the metered amount of energy is supplied to the area to be soldered by means of a pulsing laser (10). 3. A method as claimed in claim 2, characterized by using a pulsing Nd-YAG laser providing laser pulses having a pulse energy of at least 10 Joules and a pulse duration longer than 10 milliseconds to solder tin-plated copper wires in a thickness of no more than 0.9 mm to nickel-plated end caps 0.2-0.25 mm thick. 4. A method as claimed in claim 1, characterized in that the metered amount of energy is supplied to the area to be soldered using resistance heating by means of discharge pulses from a capacitor. 5. A method as claimed in claim 1, characterized in that the metered amount of energy is supplied to the area to be soldered by means of a pulsed arc discharge, with the area to be soldered being positioned in the focal point of a lense or mirror system to concentrate the energy in that point. 6. A method as claimed in claims 1-5, characterized in that the end of the lead (13) is previously upset (8). 7. A method as claimed in claim 6, characterized in that the required amount of solder is provided by putting a ring of solder material (9) around the end of the lead (13), which ring (9) is retained by the upset part of the end (8). 8. A method as claimed in claims 1-6, characterized in that the required amount of solder is provided by previously attaching a drop of solder (12) to the end of the lead (13).

Inventors: VERMIJ, LEENDERT
Applicants: LITTELFUSE TRACOR B.V.
Application year: 1986
Main classification: H01H85
IPC codes: B23K1, H01H85, H01H69