Fuse element, a fuse, a method for producing a fuse, SMD fuse and SMD circuit

Abstract

The invention relates to a fuse element (12_1; 12_2), comprising two connecting contacts (24_1, 24_1; 24_2, 24_2) and an interposed conductive track (26_1; 26_2), wherein the conductive track (26_1; 26_2) has a reduced line-cross-section, in relation, to the connecting contacts (24_1, 24_1; 24_2, 24_2) at least in some sections, further comprising at least one overlay (16_1; 16_2, 16_2), wherein the fuse element (12_1; 12_2) and the overlay (16_1; 16_2, 16_2) each comprise materials which undergo diffusion when a predetermined ambient temperature is exceeded and when an electric current is conducted by the fuse element (12_1; 12_2). The invention further relates to a fuse (TO) having such a fuse element (12_1; 12_2) and a base support (14), wherein the fuse element (12_1; 12_2) is disposed on a surface of the base support (14).

Claims

1. A method for producing a fuse (10), comprising the steps of: providing at least one fuse element (12; 12, 12) having two connecting contacts (24, 24) and an interposed conductive track (26), such that the conductive track (26) has a reduced line cross-section in relation to the connecting contacts (24, 24) at least in some sections; providing a base support (14); providing the fuse element (12; 12, 12) with at least one overlay (16; 16, 16), wherein the fuse element (12; 12, 12) and the overlay (16; 16, 16) are each selected from materials which undergo diffusion when a predetermined ambient temperature is exceeded and when an electric current is conducted by the fuse element (12; 12, 12), and wherein the fuse element is being connected to an external component by a reflow soldering process without the fuse element triggering at a reflow soldering process temperature occurring in this process, wherein the reflow soldering process temperature is higher than the predetermined ambient temperature, and arranging the at least one fuse element (12; 12, 12) on the base support (14), wherein the at least one overlay (16_1) is arranged within the conductive track (26_1) adjacent to one of the connecting contacts (24_1, 24_1) of the fuse element (12_1).

2. The method of claim 1, wherein the line cross-section of the conductive track (26) of the provided at least one fuse element (12; 12, 12) increasingly converges gradually to the line cross-section of the connecting contacts (24,24).

3. The method of fuse claim 2, wherein the at least one overlay (16; 16, 16) is arranged at least in sections within the conductive track (26) in a region of the gradually increasing line cross-section.

4. The method of to claim 2, wherein the at least one overlay (16; 16, 16) is arranged in a region of the conductive track (26) with a gradually increasing line cross-section adjacent to a section of the conductive track (26) with minimal line cross-section.

5. The method of claim 1, wherein the material of the fuse element (12; 12, 12) comprises copper and the material of the overlay (16; 16, 16) comprises tin.

6. The method of claim 1, wherein the support (14) is made of an electrically insulating material and, wherein the at least one fuse element (12; 12, 12) is arranged on a surface of the base support (14).

7. The method of claim 6, wherein the at least one fuse element (12, 12) is arranged on opposite surfaces of the base support (14).

8. The method of claim 6, further comprising providing two base contacts (18, 18) which are each electrically connected to connecting contacts of the at least one fuse element (12, 12) and which are located on opposite ends of the base support (14).

9. The method of claim 6, further comprising coating the at least one fuse element (12; 12, 12) with a protective lacquer (22; 22, 22), especially a protective polymer lacquer.

10. A method of producing an SMD fuse, comprising the method of claim 6.

11. A method of producing an SMD circuit, comprising the method of claim 10.

12. The method according to claim 1, wherein the step of providing the fuse element (12; 12, 12) with the at least one overlay (16; 16, 16) comprises arranging the overlay (16; 16, 16) in at least one recess (20; 20, 20) introduced into the conductive track (26).

13. The method of to claim 12, wherein the at least one recess (20; 20, 20) is oriented in a continuously transverse manner in relation to the longitudinal direction of the conductive track (26).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention are explained below in closer detail by reference to drawings, wherein:

(2) FIG. 1 shows a fuse in accordance with the invention in a perspective view;

(3) FIG. 2 shows a sectional view of the fuse according to the invention which is shown in FIG. 1;

(4) FIG. 3 shows a fuse element according to a first embodiment of the invention in a perspective view;

(5) FIG. 4 shows a sectional view of the fuse element according to the first embodiment of the invention as shown in FIG. 3;

(6) FIG. 5 shows a fuse element according to a second embodiment of the invention in a perspective view, and

(7) FIG. 6 shows a sectional view of the fuse element according to the second embodiment of the invention as shown in FIG. 5.

DETAILED DESCRIPTION

(8) With respect to FIGS. 1 and 2, a fuse 10 in accordance with the invention comprises two fuse elements 12, 12, which are each arranged on surfaces of a base support 14 which are opposite of each other as seen in the longitudinal direction of the fuse 10. The base support 14 consists of an electrically insulating material, which is permanently resistant even at high temperatures of up to 300 C. for example. Rogers4000 material is used in an especially preferably way as the material of the base support 14. The fuse elements 12, 12 resting on the base support 14 are each provided on their surface facing the exterior side with an overlay 16, 16. The overlays 16, 16 each extend in a region of the fuse elements 12, 12 which extend transversely to the direction of the current.

(9) The respective ends of the opposite fuse elements 12, 12, which are also known as the connecting contacts, which are situated on one plane as seen in the longitudinal sectional direction of the fuse 10, are electrically connected to each other via base contacts 18, 18. Said base contacts 18, 18 are used as connections of the fuse 10 for conducting an electric current in the longitudinal direction of the fuse 10. The fuse elements 12, 12 and the base contacts 18, 18 are made of copper for example. As soon as the current conducted through the fuse 10 exceeds a predetermined or defined current quantity (current threshold value), one of the fuse elements 12, 12 will melt or fuse in the conventional way. As a result of the thus reduced line cross-section, the further fuse element will thus also melt or fuse directly. The current path is thus interrupted.

(10) In addition to this protection from overcurrent, the fuse 10 also offers protection from overtemperature. The aforementioned overlay 16, 16 comes to bear in this case. If the ambient temperature exceeds a predetermined temperature threshold value of 200 C. for example and if in addition an electric current flows through the fuse elements 12, 12, a diffusion process is activated in accordance with the invention in which the atoms of the material of the fuse element (copper) diffuse into the material of the overlay 16, 16. The material of the overlay 16, 16 is made of tin as the diffusion partner for this purpose. In this example, a copper-tin alloy is formed by the diffusion of the copper atoms into the tin overlay. As will be explained below in closer detail, the overlay 16, 16 is filled into a recess 20, 20 introduced into the material of the fuse element 12, 12 for amplifying the diffusion process.

(11) Once the ambient temperature has reached or exceeds the predetermined temperature threshold, the copper layer diffuses completely into the tin layer. A high-resistant diffusion zone with high power loss of P=I.sub.n.sup.2R is produced even at nominal current. In this case, the melting temperature of the diffusion zone decreases from 1080 C. to approximately 500 C.

(12) The diffusion zone is designed by a respective selection of design parameters such as extension, selection of material etc in such a way that the reduced melting temperature of approximately 500 C. is already reached at relatively low currents and the current circuit is thus interrupted reliably by triggering or fusing the fuse element 12, 12 at the position of the diffusion zone. As a result, the fuse 10 also triggers at predetermined ambient temperatures (overtemperature), e.g. more than 200 C., when no overcurrent is flowing. The functionality and the advantage of the fuse 10 will be explained below by examining the fuse elements in closer detail.

(13) FIGS. 3 and 4 show detailed views of a fuse element 12_1 according to a first embodiment of the invention in a perspective view and in a sectional view, respectively. The fuse element 12_1 is integrally assembled of two connecting contacts 24_1, 24_1 and a conductive track 26_1 arranged between the connecting contacts 24_1, 24_1, wherein the conductive track 26_1 has a line cross-section which is continuously reduced in relation to the connecting contacts 24_1, 24_1. The line cross-section of the conductive track 26_1 is constant over the entire extension of the conductive track 26_1. Furthermore, the connecting contacts 24_1, 24_1 have a relatively large area in relation to the conductive track 26_1. As a result of this configuration, a substantially higher amount of heat dissipation is provided in the region of the connecting contacts 24_1, 24_1 than in the region of the conductive track 26_1 itself. As seen in the longitudinal direction of the fuse element 12_1, the temperature is highest approximately in the middle of the conductive track 26_1 and decreases in the direction toward the two connecting contacts 24_1, 24_1.

(14) As seen in a top view, the exterior shape of the fuse element 12_1 assumes an H-profile. The connecting contacts 24_1, 24_1 are formed in a rectangular way, wherein the connecting contacts 24_1, 24_1 can also assume other shapes as long as generally, as seen in a plane perpendicularly to the longitudinal direction of the fuse element, the line cross-section of the conductive track 26_1 is reduced in relation to the line cross-section of the connecting contacts 24_1, 24_1. For example, the fuse element 12_1 is formed by punching out an integral material (e.g. copper). Alternatively, the fuse element 12 can be formed by cutting, e.g. by means of laser.

(15) The support 16_1 is filled into the recess 20_1 which is introduced into the material of the conductive track 26_1. Although not shown in FIGS. 3 and 4, recesses with respectively filled overlay is can be provided on both end sections of the conductive track 26_1, at the converging points to the connecting contacts 24_1, 24_1. One of the design parameters for setting or defining the temperature threshold is indicated by the line cross-section of the conductive track 26_1 which is reduced in the region of the recess 20_1. The temperature threshold is reduced increasingly with decreasing line cross-section of the conductive track 26_1 (copper). As a result, the geometric shape of the recess 20_1 generally provides a possibility for setting or defining the temperature threshold. Once the outer temperature exceeds this temperature threshold, this leads to melting or fusing of the fuse element 12_1 in the region of the conductive track 26_1. The quantity of the material of the overlay 16_1, which means the quantity of tin, acts as a further design parameter for setting or defining the temperature threshold. A further design parameter for setting or defining the temperature threshold is indicated by the selection of the material composition of the two diffusion partners. In addition to the diffusion partners of copper and tin that are presented here, further suitable diffusion parts can also be selected.

(16) For the purpose of protecting the fuse element 12_1 against damaging exterior influences, it can be coated with a protective lacquer 22_1, e.g. a protective polymer lacquer (see FIG. 4).

(17) FIGS. 5 and 6 show detailed views of a fuse element 12_2 according to a second embodiment of the invention in a perspective view and a sectional view, respectively. The fuse element 12_2 is integrally made of two connecting contacts 24_2, 24_2 and a conductive track 26_2 arranged between the connecting contacts 24_2, 24_2, similar to the configuration of the fuse element 12_1 of the first embodiment as shown in FIGS. 4 and 5.

(18) The fuse element 12_2 according to the second embodiment differs from the fuse element 12_1 according to the first embodiment in such a way that the line cross-section of the conductive track 26_2 at the two end sections approaches the greater line cross-section of the connecting contacts 24_2, 24_2 in a step-by-step manner. In contrast to the first embodiment, the line cross-section of the conductive track 26_2 is not constant over the entire extension of the conductive track 26_2.

(19) As is shown in FIG. 5, the line cross-section increases linearly. When seen in a top view, the conductive track 26_2 comprises sections in the shape of an isosceles trapeze at its two ends. When seen in a top view, the exterior shape of the fuse element 12_2 thus assumes a bone-shaped profile. The line cross-section of the conductive track 26_2 can also alternatively increase in a non-linear manner, as a result of which the end sections of the conductive track 26_2, when seen in a top view, are provided with a geometric shape which differs from the isosceles trapeze.

(20) The connecting contacts 24_2, 24_2 are further formed in a rectangular manner as seen in the top view, wherein they can also assume other geometric shapes as long as the line cross-section of the conductive track 26_2 decreases continuously toward the centre of the fuse element 12_2 in relation to the line cross-section of the connecting contacts 24_2, 24_2.

(21) In contrast to the fuse element shown in FIGS. 3 and 4, the fuse element 12_2 according to the second embodiment comprises two recesses 20_2, 20_2, which are introduced into the material of the conductive track 26_2. The recesses 20_2, 20_2 are each arranged in those regions of the conductive track 26_2 in which the line cross-section decreases, as described above. In other words, the recesses 20_2, 20_2, when seen in a top view, are each arranged on the obtuse tips of the trapezoidal end sections of the conductive track 26_2.

(22) Overlays 16_2, 16_2 are respectively filled into the recesses 20_2, 20_2. As a result of the line cross-section of the conductive track 26_2 which is thus reduced in the region of the recesses 20_2, 20_2 and in addition of the respective trapezoidal geometry of the end sections of the conductive track 26_2 as seen in the top view, one of a plurality of design parameters is indicated for setting or defining the temperature threshold. One possibility to set or define the temperature threshold is generally provided by choosing the geometrical shape of the recesses 20_2, 20_2. The fuse element 12_2 is coated with a protective lacquer 22_2 for protection against damaging external influences.

(23) The fuse 10 shown in FIGS. 1 and 2 can be equipped on one side or both sides with one or several fuse elements 12_1 of the first embodiment (see FIGS. 3 and 4) or one or several fuse elements 12_2 of the second embodiment (see FIGS. 5 and 6). Combinations are also possible.

(24) All told, a reliable fuse 10 for thermal and simultaneously electrical monitoring of power transistors arranged adjacent to each other is thus created. One advantage is that the fuse 10, despite the thermal fuse feature, is capable of being soldered via a direct reflow soldering process onto the circuit board without triggering. Since no current flows through the fuse element 12 in the course of this reflow soldering process, the high temperatures occurring in this process will not trigger the fuse element 12. Only in the operating state, i.e. when conducting a current such as a nominal current for example, will the fuse element 12 also trigger at overtemperatures, which can be lower than the temperatures occurring during the reflow soldering process.

(25) As a result, an SMD fuse that has previously not existed is thus created, which can be placed and soldered automatically on SMD basis. As a result of the small form factor of the SMD fuse, it can advantageously be positioned especially close to a component that strongly develops heat, e.g. a power transistor. Once this component assumes a temperature which exceeds a predetermined temperature threshold, which is caused for example by a defect in the component itself or a defect in the circuit, the SMD fuse will trigger rapidly, as a result of which the current flow to said defective component is reliably interrupted.

(26) The fuse 10 has the smallest possible form factor (e.g. 0201, 0402, 0603, 1206, 1812, 2010, 2512, 4018 etc). Furthermore, the fuse 10 shows high pulse loading capability because the fuse element 12 is fixed to the base support 14.

(27) A multilayer construction with one or several fuse elements 12; 12, 12 in parallel connection is enabled. The fuse element 12; 12, 12 is completely protected from environmental influences by a protective lacquer 22; 22, 22. The use at maximum ambient temperatures of up to 280 C. is possible through a respective selection of the previously mentioned design parameters. Currents in the range of a few mA up to several hundred A can similarly be secured. As a result of the small form factor, the fuse 10 can advantageously be positioned especially close to electrical components that produce a large amount of heat, e.g. power transistors. This allows good thermal coupling, through which increased temperatures can be detected immediately, e.g. an increased temperature of the power transistor which is caused by a malfunction of the power transistor. Since the fuse 10 triggers immediately upon exceeding the defined overtemperature, the risk of a fire hazard is thus eliminated. In comparison with conventionally known thermal fuses the fuse 10 generally offers substantial improvements with respect to reliability, costs, size, weight, workmanship, pulse resistance, vibration resistance, response behaviour etc.

(28) A fuse 10 is created which improves and expands previously known properties of fuses with respect to current/time behaviour, temperature behaviour, pulse strength, breaking capacity, insulation resistance, i2t values, material and production costs.