The present invention relates to thermal fuse and method of manufacture thereof. In particular, it relates to a thermal fuse having a spring element soldered to a ceramic substrate in which the solder melts under electrical overload causing relaxation of the spring to break the electrical contact.

Thermal fuses are in widespread use, particularly in telecommunications equipment. The heat generated by a short circuit in the lines melts a soldered connection and breaks the contact as soon as the softening/melting temperature of the solder alloy is reached.

In order to ensure that contact is reliably broken after the solder melts, it is necessary that the fuse be under mechanical tension in normal operation. This ensures that the spring contact moves safely out of the conducting solder pad when the solder melts.

An example of an arrangement of a known thermal fuse in which spring action is provided by an M-shaped element is shown in Figures 3a-3c.

The main part of the fuse (shown in normal operating conditions in Figure 3b) is an approximately M-shaped component 4 (shown undeformed in Figure 3a). This component may be produced from a thin metal strip or continuous reels of wire with spring properties by cutting and shaping the wire with a tool. The formed component 4 is then placed on the substrate 1 and attached by heating pads 2, 3, which have solder paste on. Only part of the wire lying on the substrate 1 is held by the solder and the joint is strong enough to take the stress arising in the component 4 when it is formed into the shape of Figure 3b by a lateral force.

However, as the solder heats up beyond the softening temperature, the limb 6 of the component 4 separates from the solder pad 3 and the current in the component 4

is interrupted.

Alternatively, the component of the fuse may be manufactured from a "memory alloy" and initially has the shape shown in Figure 3c and is plastically deformed to the shape of Figure 3a. In this shape the component is attached to the substrate such that only the locality of the joint is heated. A subsequent heating of the whole component 4 (e. g. as a result of electrical overload) causes the component 4 to try to return to its original form, leading to the condition shown in Figure 3b and, after softening of solder at 2, 3 to the blowing of the fuse, as in Figure 3c.

However, such a fuse can only operate satisfactorily over a range of currents. This is because at higher currents the component itself heats up due to the current flowing through it and, as a result, loses it springiness before the solder melts. Therefore, even though the solder melts, the spring action of the component is insufficient to isolate it from the connection pad. As a consequence the ceramic substrate can reach dangerously high temperatures and the connection is still not broken. This results in an obvious safety hazard.

The present invention proposes to overcome the above disadvantage.

One aspect of the present invention provides a thermal fuse arranged to provide an electrical current path between a pair of conductors, comprising a member which comprises a fuse element and a spring means, said member being supported at two points, said spring means being outside the current path.

The fuse element and spring means are preferably formed integral with each other.

Preferably, at least one of the points of support comprises a solder connection such that under electrical overload the solder melts releasing the spring tension

to break the current path which may comprise the fuse element attached by solder connections to both of the conductors at one end of the member. A second end of the member, remote from the first end, may be permanently fixed as an anchor point, e. g. by solder or adhesive.

Alternatively, the fuse element may be attached to the conductors at the first end of the member and a further fuse element may be provided at the second end. A third conductor is then respectively connected to the pair of conductors by the two fuse elements. Thus, the spring means is short-circuited by the third conductor.

A second aspect of the present invention also provides a thermal fuse arranged to provide an electrical current path between a pair of soldered pads, the fuse comprising a member, which member comprises a pair of limbs between which is located a spring element, each limb being attached to a respective support, at least one of the supports being constituted by the soldered pads.

Furthermore, the present invention provides a method of manufacturing a thermal fuse, the method comprising:

(a) forming a member which comprises a fuse element and a spring means;

(b) arranging the fuse element to provide an electrical current path between a pair of conductors such that the member is supported at two points; and

(c) imparting a spring tension to said spring means between said two points but outside the current path.

The member may be formed of an M-shaped metal strip.

The present invention will now be explained in more detail by means of the following description of preferred embodiments and with reference to the following drawings, in which:

Figure 1 shows a thermal fuse according to a first embodiment of the present invention;

Figure 2 shows a thermal fuse according to a second embodiment of the present invention; and

Figures 3a-3c show a prior art thermal fuse arrangement.

The overall arrangement of the first embodiment of the present invention is shown in Figure 1. The fuse comprises a ceramic substrate on which is formed conductive tracks 17 and 19 and conductive pad 13. The conductive tracks 17 and 19 are arranged in line with conductive pads 12 and 21 respectively formed at the adjacent ends of the tracks 17 and 19.

The fuse further comprises an approximately M-shaped component 14 manufactured from a thin metal strip. One limb 15 of this component is attached to the ceramic substrate via the conductive pads 12, 21 of tracks 17 and 19 by the use of solder paste. A second limb 16 of the component 14 is also attached by solder connection to the separate conductive pad 13. Alternatively, in place of a soldered anchor point at pad 13, anchoring to the substrate may be effected by means of adhesive. The tracks 17, 19 are covered with an insulating coating. Thus, for example, limb 15 of the component 14 cannot short-circuit to the conductor 19.

In the second embodiment shown in Figure 2, the ceramic substrate has mounted thereon a pair of parallel conductive tracks 23 and 25 and a third track 27 perpendicular to tracks 23 and 25 and remote therefrom. The two ends of tracks 23 and 25 which are adjacent the third track 27 are provided with respective conductive pads 26 and 28. Each end of track 27 adjacent conductive pads 26 and 28 are also provided with respective conductive pads 29 and 31. The M-shaped component 14 is then attached to the four pads 26, 28, 29 and 31 using solder. Again, all three conductor tracks 23, 25, 27 are covered with an insulating coating.

The fuses of Figures 1 and 2 are manufactured as

follows.

The conductive tracks are formed on the ceramic substrate 11. Solder paste is placed on the areas of the conductive tracks. The M-shaped component is formed by either method described with reference to Figures 3a-3c. The element may comprise a rectangular cross-section, for example, a width of 3mm and thickness 0.2mm.

The component 14 is placed on the substrate (by means of an automatic placement machine) and is attached by heating the solder paste on the conductive pads. The component 14 is then plastically deformed by application of a lateral force or by thermal treatment as in the prior art arrangement (Figure 3b).

In the fuses of Figures 1 and 2, only part of the M-shaped element is held by solder in the locality of the conductive pad. These joints are strong enough to take the stress arising from the deformation of the component 14.

Operation of the fuse with reference to Figure 1 will now be described. Figure 1 shows the thermal fuse in normal operation (component 14 is shown undeformed for simplicity, although it is understood that in normal operation this component would be deformed as shown in Figure 3b). Current flows through the fuse via tracks 17 and 19 and flows through only part of the component 14 connected between the pads 12 and 21. During electrical overload the ceramie substrate overheats causing the solder to melt releasing the component and blowing the fuse.

Since current only flows through the wire of the component 14 connected between the pads 12 and 21, the remaining part of the component lying between the pads 21 and 13 does not heat up directly due to current flow and consequently lose its spring tension. Therefore, for operation with larger currents, the springiness

within the component is not lost and the fuse can still function effectively.

With reference to the fuse shown in Figure 2, current flows through the tracks 23, 25 and 27 and through part of the component 14 connected between pads 26 and 29 and between pads 28 and 1. As in the fuse illustrated in Figure 1, electrical overload causes the solder to soften and the spring release of the component, thus breaking the electrical connection and blowing the fuse.

As in the first embodiment, current only flows through a part of the component 14. In this case through the parts of the component 14 connected between pads 26 and 29 and between pads 28 and 31. The majority of the component, however, lying across pads 29 and 31 has no current flowing and, therefore, does not heat due to current effects. The M-shaped component, therefore, retains its springiness and, thus, is effective at larger currents.

In the light of this disclosure, modifications of the described embodiments as well as other embodiments, all within the scope of the appended claims will now become apparent to persons skilled in the art.