阅读说明：本技术 电子器件 (Electronic device ) 是由 阿尔弗雷德·霍弗里希特 弗朗茨·林纳 于 2020-02-25 设计创作，主要内容包括：本发明涉及一种器件,所述器件具有基体(9)和至少一个外部电极(1),所述外部电极通过连接材料(4)固定在所述基体(9)上,其中所述基体(9)和所述外部电极(1)具有不同的热膨胀系数,所述热膨胀系数确定临界温度,在超过所述临界温度时,所述基体(9)和所述外部电极(1)的连接会承受机械应力,所述机械应力引起所述器件的损坏,其中所述连接材料(4)具有低于临界温度的熔点。(The invention relates to a component having a base body (9) and at least one external electrode (1) which is fixed to the base body (9) by means of a connecting material (4), wherein the base body (9) and the external electrode (1) have different coefficients of thermal expansion which determine a critical temperature above which the connection of the base body (9) and the external electrode (1) is subjected to mechanical stresses which lead to a destruction of the component, wherein the connecting material (4) has a melting point below the critical temperature.)

1. A kind of device is disclosed, which comprises a substrate,

the component has a base body (9) and at least one external electrode (1) which is fixed to the base body (9) by means of a connecting material (4),

wherein the base body (9) and the external electrode (1) have different coefficients of thermal expansion, which determine a critical temperature above which the connection of the base body (9) and the external electrode (1) is subjected to mechanical stresses which cause damage to the device,

wherein the connecting material (4) has a melting point below a critical temperature.

2. The device as set forth in claim 1, wherein,

wherein the connecting material (4) is a brazing material.

3. The device as set forth in claim 2, wherein,

wherein the brazing material has tin and bismuth.

4. The device according to claim 2 or 3,

wherein the brazing material has a tin content of between 35 and 50 wt.% and a bismuth content of between 50 and 65 wt.%.

5. The device of any of the preceding claims,

wherein the connecting material (4) has a melting point below 150 ℃.

6. The device of any of the preceding claims,

wherein the connecting material (4) has a melting point below 140 ℃.

7. The device of any of the preceding claims,

wherein the base body (9) comprises a ceramic material.

8. The device of any of the preceding claims,

wherein the substrate (9) has at least one electrode with silver.

9. The device according to the preceding claim,

wherein at least one of the external electrodes (1) is fixed to at least one electrode (2) of the base body (9) by means of the connecting material (4).

10. The device of any of the preceding claims,

wherein the base body (9) and at least one of the external electrodes (1) are pressed against one another by means of a prestressing element (10).

11. The device of any of the preceding claims,

wherein the connecting material (4) is arranged on a surface of the base body (9) and forms a wetting angle with the surface, in which wetting angle the connecting material (4) is held in a liquid state between the base body (9) and the external electrode (1).

12. The device of any of the preceding claims,

wherein the connecting material (4) has a higher resistivity than the base body (9) and the external electrode (1).

13. The device of any of the preceding claims,

wherein at least one of the external electrodes (1) has copper,

or

At least one of the external electrodes (1) has invar,

or

At least one of the external electrodes (1) has a layer structure of copper-invar-copper.

14. The device of any of the preceding claims,

wherein the component has a second external electrode (1) which is fixed to the base body (9) by means of the connecting material (4).

15. The device of any of the preceding claims,

wherein the device is a thermistor, capacitor or varistor.

16. The device of any of the preceding claims,

wherein the device is a PTC heating element or a starting current limiter with NTC ceramic.

Technical Field

The application of relatively high specific power to the device can cause uneven or asymmetric temperature heating of the device. In this case, some regions of the component can be heated more strongly than other regions of the component. Examples of devices in which such uneven or asymmetric temperature heating occurs are PTC heating elements and starting current limiters with NTC ceramics.

Background

Due to the thermal expansion of the component, an uneven or asymmetrical temperature heating causes high mechanical stresses, since the more strongly heated regions expand more strongly than the less strongly heated regions. Mechanical stress can lead to device failure or damage. For example, the connection material, the ceramic, and/or the external electrode may be broken due to mechanical stress. In this case, the electrical contact between the ceramic and the external electrode is interrupted by a break and may interfere with the function of the component.

Even in devices that do not experience mechanical stress due to uneven or asymmetric temperature heating, similar mechanical stress may be generated due to different coefficients of thermal expansion of different components of the device. Accordingly, there is also a risk of damage in these components in the event of excessive heating.

Disclosure of Invention

It is an object of the invention to provide an improved device which has a reduced probability of being damaged, for example when heated.

The object is achieved by a device according to claim 1. Advantageous embodiments of the device are the subject matter of the dependent claims.

A device is provided having a base body and at least one external electrode. At least one external electrode is fixed to the substrate by a connecting material. The base body and the external electrodes have different coefficients of thermal expansion, which determine a critical temperature above which the connection of the base body and the external electrodes is subjected to mechanical stresses that cause damage to the device. The joining material has a melting point below the critical temperature.

Since the aggregate state of the joining material can change from solid to liquid state before the critical temperature is reached, it is possible to exclude: when heating the component, the component is heated to a critical temperature at which the connection of the base body to the at least one external electrode is established, and the base body and the external electrode are simultaneously connected to one another by the connecting material. Rather, the joining material melts before the critical temperature is reached. Any mechanical stresses which may occur during heating due to different thermal expansion coefficients or due to an asymmetrically or unevenly performed heating process can be eliminated immediately by melting of the connecting material. Accordingly, the low melting point of the connecting material ensures: when the critical temperature is reached, no damage of the connection by the connecting material occurs, since at this point in time the connection can no longer be produced by the connecting material. In its liquid state, the connecting material cannot mechanically fix the at least one external electrode to the substrate.

Possible damaging stresses can thus be eliminated in the device. The connecting material can be designed to solidify again after the heating phase in the cooling phase and to reestablish the connection between the at least one external electrode and the base body.

The component can be, for example, a ceramic component. The device can be an electronic device. The device can be an electrical device. The device can be an active or passive device. The device can be a ceramic multilayer device. Alternatively, the component can have a single ceramic layer. The component can be provided for Surface Mounting (SMD) component.

In particular in ceramic components, the external electrode with metal or consisting of metal and the base body with ceramic material can have different coefficients of thermal expansion from one another. The base body of the ceramic component can in this case consist essentially of a ceramic material. The different coefficients of thermal expansion from each other result in the mechanical stresses set forth above. It is therefore advantageous to use joining materials having a melting point below the critical temperature, in particular in ceramic components.

The "critical temperature" can be referred to herein as the temperature at which the connection between the substrate and the external electrode is subjected to mechanical stress that can cause damage to the device. The mechanical stress may be caused by different expansions of the outer electrode and the base body. The critical temperature is thus essentially determined by the coefficients of thermal expansion of the external electrode and the base body, which in turn are related to the materials of the external electrode and the base body. The shape and thickness of the external electrode and the base body and the type of the connection portion also affect the critical temperature. The critical temperature of the connection of the at least one external electrode to the base body can be determined by neglecting: when the melting point is exceeded due to melting of the connecting material, the connection may open.

The joining material can be a brazing material. Accordingly, the external electrode and the base body can be connected to each other by a connecting material via a soldered connection. The soldered connection enables a reliable electrical connection and a reliable mechanical connection of the two connecting partners.

The brazing material can have tin and bismuth. In particular, the brazing material can consist of tin and bismuth. The brazing material can have a tin content of between 35% by weight and 50% by weight and a bismuth content of between 50% by weight and 65% by weight. Preferably, the tin fraction lies between 40% and 45% by weight. Preferably, the proportion of bismuth lies between 55% and 60% by weight. For example, the brazing material can have 42 wt% tin and 58 wt% bismuth.

A solder material with a mixture of tin and bismuth in the above-mentioned mixing ratio is very suitable as a connecting material between the at least one external electrode and the substrate. Due to the mixing ratio of tin and bismuth, the melting point of the brazing material is below the critical temperature common to devices.

The brazing material is capable of establishing a reliable electrical and mechanical connection of the external electrode to the substrate at temperatures below its melting point. If the brazing material is heated to a temperature higher than the melting point, the brazing material can be rapidly melted, so that the connection portions are rapidly separated by the connection material. Thus, the risk of mechanical damage can be quickly eliminated.

The melting point of the connecting material can be below 150 c, preferably below 140 c. These temperatures are below the usual critical temperatures for connecting the external electrodes to the substrate. For example, the melting point can be 138 ℃. For example, for a joint material having a braze material of 42 wt% tin and 58 wt% bismuth, a melting point of 138 ℃ results.

The melting point of the connecting material can be greater than 100 c, preferably greater than 120 c. It is not necessary to melt the connecting material at temperatures below 100 c, or below 120 c, since at these temperatures no mechanical stress is expected between the substrate and the at least one external electrode, which is so great that it may cause device damage.

The substrate can have at least one electrode with silver. The electrodes can be comprised of silver. The electrodes can be applied, for example, by means of screen printing. The electrodes can be thinner than the external electrodes. The electrodes can be metallizations by means of which the ceramic layer of the base body can be brought into electrical contact with at least one external electrode. The at least one external electrode can be fixed to the at least one electrode of the base body by means of a connecting material.

The base body and the at least one external electrode can be pressed against one another by means of the prestressing unit. The prestressing element can be designed here to continue the electrical contacting of the base body with the external electrode when the connecting material has melted. The prestressing element can also cause a mechanical fixing of the external electrode on the base body. The prestressing element can fix at least one external electrode to the base body in such a way that no damage of the component occurs even when the external electrode and the base body are heated to a temperature above a critical temperature. For example, the prestressing unit can be designed to effect a relative movement between the outer electrode and the base body. The prestressing unit can be designed to allow only a relative movement in which the at least one outer electrode is moved over a distance relative to the base body which is significantly smaller than the lateral extent of the outer electrode and the base body. For example, the distance can be less than one percent of the lateral extension of the outer electrode. Such small movement amplitudes are sufficient to eliminate mechanical stresses and can also be small enough that the external electrode and the base body remain in electrical contact with each other and abut against each other in a sufficiently large area.

The prestressing unit can have a spring, for example. The prestressing unit can apply a spring force to the at least one outer electrode, via which spring force the outer electrode can be pressed against the base body.

The connecting material can be arranged on a surface of the base body and form a wetting angle with said surface, in which wetting angle the connecting material remains in a liquid state between the base body and the external electrode. Accordingly, it is possible to exclude: the connecting material flows out of the matrix after it has melted. After the heating phase, the connecting material can always be in its position between the base body and the external electrode and can solidify in a later cooling phase, and the external electrode is fixed again on the base body and is in contact with the latter.

The connection material can have a higher resistivity than the base body and the external electrode. Accordingly, the connecting material can be heated more strongly than the base body and the external electrode. Thus, the connecting material is able to reach its melting point before the substrate or the outer electrode is heated to the critical temperature.

The at least one external electrode can have a layer structure of copper or invar or copper-invar-copper. Copper is a proven robust material for the outer electrode. The copper invar compound has a coefficient of thermal expansion similar to that of the base. Since the coefficients of expansion of the external electrode with the copper invar compound and the substrate, for example with a ceramic material, are similar to one another, mechanical stresses occur to a small extent, so that the critical temperature can be higher than in a component with an external electrode made of copper.

The device can have a second external electrode fixed to the substrate by a connecting material. The second external electrode can be fixed to the base body in the same manner as the first external electrode. The connection of the second external electrode to the base body can also be melted when the melting point of the connecting material is reached.

The device can be a thermistor, a capacitor or a piezoresistor. The device can be a PTC heating element or a starting current limiter with NTC ceramic. In the last-mentioned devices, inhomogeneous or asymmetrical heating is not uncommon, so that it is advantageous to use connection materials with melting points below the critical temperature, in particular in these devices.

Drawings

Preferred embodiments of the device are explained below with reference to the drawings.

Fig. 1 shows a schematic diagram of a device.

Detailed Description

Fig. 1 shows a device. In the embodiment shown here, the device is a thermistor, in particular an NTC thermistor.

The device has a base body 9 with a disc 3 of NTC material. The base body 9 also has two electrodes 2. The electrodes 2 are arranged on opposite sides of the disc 3. The electrode 2 has silver or consists of silver. The electrode 2 is very thin. The electrodes 2 can be applied to the disk 2, for example, by means of a screen printing method. The invention is not limited to the basic body 9 shown in fig. 1. For example, the substrate 9 may have a multilayer structure.

For electrically contacting the device, the device has two external electrodes 1. The external electrode 1 can have copper or consist of copper. The first external electrode 1 is connected to one of the electrodes 2 of the base body 9 via a connecting material 4. The second external electrode 1 is connected to the other electrode 2 of the substrate 9 via the connecting material 4.

The joining material 4 is a brazing material. Accordingly, the outer electrode 1 is connected to the base body 9 by a soldered connection. The brazing material is a tin-bismuth compound. In particular, the brazing material can have 42 wt% tin and 58 wt% bismuth. The brazing material can have a melting point of 138 ℃.

The outer electrode 1 is connected to a voltage source 5 via a feed line 6. A voltage can be applied between the outer electrodes 1 via a voltage source 5. Correspondingly, the voltage source 5 can apply a voltage to the component via the outer electrode 1.

In such devices, mechanical stresses can occur when the device is heated above a critical temperature due to the risk of the mechanical stresses causing device damage. The external electrode 1 and the base 9 have different thermal expansion coefficients from each other. This generates a critical temperature at the connection between the external electrode 1 and the base body 9. If the outer electrode 1 and the base body 9 are now heated, they expand to a different extent due to their different coefficients of thermal expansion, so that mechanical stresses arise. Here, a temperature at which a mechanical stress occurring at the connection portion of the external electrode 1 and the base body 9 becomes so large as to cause damage of the device is defined as a critical temperature. Damage can occur, for example, in the form of fractures.

The connecting material 4 used here is selected such that its melting point is below the critical temperature of the connection of the base body 9 with the external electrode 1. Accordingly, the connection material 4 melts before the device is damaged by excessive heating. Possible mechanical stresses are immediately eliminated by melting of the connecting material 4. Accordingly, the component is designed in such a way that it is not damaged by mechanical stresses which occur as a result of overheating beyond the critical temperature of the component.

The device also has a pre-stressing element 10. The prestressing unit 10 guarantees: when the connecting material melts, the base body 9 and the external electrode 1 are held in contact with each other. The prestressing unit 10 presses the outer electrode 1 against the base body 9.

The prestressing unit 10 has two springs 7 and a support 8. The support 8 surrounds the device or is constituted by two elements arranged on opposite sides of the device.

The first spring 7 is arranged on the side of the base body 9 on which the first external electrode 1 is arranged. The first spring 7 is clamped between the support 8 and the first outer electrode 1. The first spring 7 presses the first outer electrode 1 against the base body 9 with a clamping force. The second spring 7 is arranged on the opposite side of the base body 9, i.e. on the side on which the second external electrode 1 is arranged. The second spring 7 is disposed between the support 8 and the second external electrode 1. The second spring 7 presses the second external electrode 1 against the base body 9 with a clamping force. The clamping forces exerted by the first and second springs 7 on the base body 9 act in opposite directions, so that the base body 9 and the two outer electrodes 1 are pressed together. If the connecting material 4 melts, the external electrode 1 remains in electrical and mechanical contact with the base body 9, since the prestressing element 10 ensures: the outer electrode 1 is pressed against the substrate 9.

The connection of the external electrode 1 to the base body 9 is designed such that the connecting material 4 remains in its position between the base body 9 and the respective external electrode 1 even after the connection has melted. To achieve this, a connecting material 4 with a corresponding wetting angle can be selected. The corresponding wetting angle can be present here in the liquid state of the connecting material. By a suitable choice of the wetting angle, it is ensured that the connecting material 4 does not flow out of the base body 9. In this case, the largest possible wetting angle is selected.

The device can be designed such that, when heating the device, the connecting material 4 is heated more quickly than the outer electrode 1 and the base body 9. In particular, the connection material 4 can have a higher resistivity than the external electrode 1 and the base body 9. Accordingly, it is ensured that the connecting material 4 is heated to a temperature above its melting point before the outer electrode 1 and the base body 9 are heated to the critical temperature, and may be subjected to excessive mechanical stress due to the different coefficients of expansion.

As the device cools, the connecting material 4 solidifies again below the solidification temperature. Since the connecting material 4 is held in a liquid state at a position where it is located between the external electrode 1 and the base body 9, the connecting material 4 connects the external electrode 1 and the base body 9 to each other again after it is cured.

Since the aggregate state of the connection material 4 between the base body 9 and the external electrode 1 changes before heating to the critical temperature and, in particular, the connection material 4 becomes liquid, no destructive mechanical stresses occur in the component. Since the connecting material 4 solidifies again after the cooling phase and mechanically connects the outer electrode 1 and the base body 9, a good mechanical and electrical connection results which is remelted in the next heating cycle.

Furthermore, if the joining material 4 is a brazing material, the aging mechanisms, which otherwise might limit the service life of the device with the brazed joint, can be eliminated by the brazing material becoming liquid.

List of reference numerals:

1 external electrode

2 electrode

3 disks

4 connecting material

5 Voltage Source

6 feeder line

7 spring

8 supporting piece

9 base body

10 prestressing unit