The present invention relates to sub-miniature electrical fuses which comprise a fusible element extending between two supporting conductors. More specifically the invention is concerned with sub-miniature fuses intended for mounting on a printed circuit board and in which the supporting conductors extend through an insulating base member and form both a support for the fusible element to one side of the base member and connections to the circuit board at the other side of the base member. The two supporting conductors are preferably in the form of connecting pins arranged parallel to each other and the fusible element comprises some type of fuse wire.

The invention is particularly directed to the problem of forming reliable, effective and economic connections between the fusible element and its supporting conductors. Hitherto such connections have generally been made by the use of a solder. However, the soldering operation may necessitate * the use of undesirable corrosive fluxes and even so it is sometimes difficult to obtain an effective solder _^ connection. Moreover solder iε expensive and adds significantly to the cost of the fuse. Flux residues may also affect the operation of the fuse, besides which under some conditions, a solder connection can r.elt before the fusible element operates, for example when soldering the connecting pins of the fuse on to a printed circuit board.

As one alternative, purely mechanical crimping of the fusible element to its terminals has been employed. However, the electrical connection so formed iε unreliable and can place unacceptable stresses on the fusible element, particularly if it is a very thin wire.

A combination of mechanical crimping and soldering has also been proposed but the above mentioned disadvantages associated with the use of solder still remain.

A further method of joining the fusible element to its supporting connections employs electrical resistance welding which can result in good mechanical and electrical connections, but difficulties are encountered due to the widely differing combinations of metallic materials which are required to be joined and also due to the very small size of the fusible element. In particular the reduction in cross sectional area or necking of the fusible element caused by the welding operation can result in premature operation of the fuse.

It iε an object of the present invention to provide an improved method for fixing a fusible element ^■ to its supporting conductors which does not necessitate the use of a solder and which is economic and suitable for a high speed of manufacture.

From one aspect the invention provides a method of attaching a fusible element to a supporting conductor in which the supporting conductor is first shaped to hold the fusible element and energy is then directed on to the shaped portion of the supporting

conductor so as to cause it to melt over and around the fusible element.

The shaping of the supporting conductor may be effected by a crimping operation such that it lightly holds the adjacent portion of the fusible element. The direction of the necessary energy on to the shaped supporting conductor iε effected by some kind of non-contact welding process, for example laser beam welding, electron beam welding or ion beam welding. As diεtinct from electrical resistance welding which requires direct physical contact with both the fusible element and the supporting conductor, the method of the present invention does not require any such direct physical contact and moreover the welding energy iε directed solely on to the supporting conductor and not on to the fusible element.

From another aspect therefore the invention provides a method of attaching a fusible element to a terminal electrode in which a portion of the electrode is crimped on to the fusible element and the mechanical connection so formed iε welded by means of energy beam welding directed solely on to the terminal electrode in - the region of the crimp so that a portion of said electrode melts and welds to the fusible element. It has been found that the method according to the invention can be employed to form effective and reliable connections between a fusible element and- its supporting conductors in an economic manner, even when difficult materials have to be connected. This is largely because it iε only the material of the supporting conductor which receives the welding energy and which subsequently melts to form the weld.

The invention further provides sub-miniature fuses produced by the methods diεcloεed herein.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which Figures 1 to 3 illustrate one manner of carrying out the method according to the invention.

Figure 1 illustrates a base member 1 of plastics material through which extend two parallel terminal members 2. These have flattened regions 2a to one side of the base for supporting a fuse wire and form connecting pins 2b at the other side of the baεe for soldered connection to a printed circuit board. Figures 2a and 2b illustrate respectively in end and side elevation how the flattened portions 2a are folded over at 3 and lightly crimped on to the opposite ends of a length of fuse wire 4 supported by the terminal members 2. Welding energy, for example a laser beam iε now directed on to the crimped portionε of the terminal members so as to cause them to melt over and around the ends of the fuse wire, as shown at 5 in Figure 3, thereby resulting in a weld of high integrity. The proceεε aε described can be carried out in about 0.2 seconds.

It will thus be seen that the method of the preεent invention iε both faεt and forms a highly reliable mechanical and electrical connection. Ho solder iε employed, therefore it iε cheap and energy beam welding can be precisely controlled both as to energy delivery and position. Moreover the method is non-contacting so there is no electrode wear aε occurε with electric resistance welding and in solder operations.

Aε an example, the method of the invention haε been employed εucceεsfully to attach a fusible element of "Nikrothal LX" nickel chrome alloy to tinned copper supporting conductors.