Sensor Element and Method for Producing a Sensor Element

Abstract

A sensor element and a method for producing a sensor element are disclosed. In an embodiment the sensor element is configured to be secured on a printed circuit board by pressure sintering, wherein a structural form of the sensor element is designed such that an exposure to pressure of the sensor element during the pressure sintering is compensated.

Claims

1-14. (canceled)

15. A sensor element for temperature measurement, wherein the sensor element is configured to be secured on a printed circuit board by pressure sintering, and wherein a structural form of the sensor element is designed such that an exposure to pressure of the sensor element during the pressure sintering is compensated.

16. The sensor element according to claim 15, wherein the sensor element has at least two electrodes, and wherein the sensor element is designed such that a compressive loading occurring during the pressure sintering is dissipated to the printed circuit board in an intermediate region between the electrodes.

17. The sensor element according to claim 16, wherein the sensor element has a ceramic main body, wherein the electrodes are arranged on an outer area of the main body, wherein the main body has a projection, and wherein the projection is arranged between the electrodes.

18. The sensor element according to claim 17, wherein the electrodes is arranged on a common outer area of the sensor element, and wherein the common outer area represents an underside of the sensor element.

19. The sensor element according to claim 17, wherein the projection is designed as a base, and wherein the projection protrudes between the electrodes out of the outer area of the sensor element.

20. The sensor element according to claim 17, wherein the projection forms an integral part of the main body.

21. The sensor element according to claim 16, wherein the sensor element has a ceramic main body and comprises a ceramic carrier material, wherein the main body is formed on the carrier material, wherein the carrier material has a projection, and wherein the projection is arranged between the electrodes.

22. The sensor element according to claim 21, wherein the electrodes are arranged on different end faces of the sensor element.

23. The sensor element according to claim 22, wherein the electrodes are caps of the end faces.

24. The sensor element according to claim 21, wherein the projection is designed as a base, and wherein the projection projects between the electrodes out of an outer area of the sensor element.

25. The sensor element according to claim 21, wherein the projection forms an integral part of the carrier material.

26. The sensor element according to claim 15, wherein the sensor element has a T-shaped structural form.

27. A method for producing a sensor element, the method comprising: producing NTC powder to form a ceramic main body; pressing the NTC powder using a pressing mold, wherein the pressing mold is designed in such a way that the pressed main body has a projection; sintering the pressed main body; and applying electrodes to an underside of the main body, the electrodes being separated from one another by the projection.

28. A method for producing a sensor element, the method comprising: providing a ceramic carrier material, the carrier material having a projection; at least partially printing the carrier material with an NTC paste to form an NTC layer; and applying electrodes on opposite end faces of the system comprising the NTC layer and the carrier material, the electrodes being separated from one another by the projection.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The sensor element and the method are explained in more detail below on the basis of exemplary embodiments and the associated figures.

[0039] The drawings described below should not be regarded as true to scale. Rather, for better representation, individual dimensions may be shown as increased or reduced in size or even distorted.

[0040] Elements that are the same as one another or perform the same function are provided with the same designations.

[0041] FIG. 1 shows a sensor element in a first embodiment;

[0042] FIG. 2 shows a sensor element in a further embodiment;

[0043] FIG. 3 shows a sensor element in a further embodiment; and

[0044] FIG. 4 shows an embodiment of the sensor element from FIG. 3.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0045] FIG. 1 shows a sensor element 1, in particular a sensor chip. The sensor element 1 is preferably designed for measuring a temperature. The sensor element 1 has two electrodes 2. The sensor element 1 comprises a ceramic sensor material. The sensor element 1 has a ceramic main body 7. The main body 7 comprises the ceramic sensor material.

[0046] The sensor material is an NTC (negative temperature coefficient) ceramic. For example, the ceramic has a perovskite structure. In particular, the ceramic may be based on the system YCaCrAlO with various dopings. Such a sensor element 1 is particularly suitable for high-temperature applications. Alternatively, in particular in the case of lower application temperatures, the sensor element 1 may comprise a ceramic with a spinel structure. For example, the ceramic may be based on the system NiCoMnO with various dopings.

[0047] The sensor element 1 is designed to be secured on a printed circuit board under pressure, for example, by means of Ag sintering. Ag sintering involving exposure to pressure is not possible in the case of the conventional SMD NTC sensors and alternative flip-chip structural forms because of the flexural loads occurring that exceed the intrinsic strength of the components. The flexural loading is caused by the component that lies on the terminal pads on the mother board being pressed from above. Especially in the case of DCB boards, between the terminal pads there is a relatively deep trench, which corresponds to the thickness of the electrode layer on the DCB board and is generally several 100 m.

[0048] To compensate for the compressive loads occurring during Ag sintering, the sensor element 1 from FIG. 1 has a base 3. The sensor element 1 is designed in a T-shaped form. According to the exemplary embodiment from FIG. 1, in particular the ceramic main body 7 is designed in a T-shaped form. In this exemplary embodiment, the horizontal line of the T forms an upper side of the sensor element 1 or of the main body 7. The vertical line of the T represents the base 3. The base 3 protrudes out of an underside of the sensor element 1 or main body 7. The base 3 is an integral part of the main body 7.

[0049] The sensor element 1 with the base 3 may either be produced directly by pressing with a suitable pressing mold or be cut in a T shape from a pressed blank or a substrate produced from NTC sheets. This can be realized by sawing, grinding, laser cutting or other suitable machining operations.

[0050] The metallization is applied as a thin-film or thick-film electrode. The electrodes 2 are separated or spatially separated from one another by the base 3.

[0051] The electrodes 2 are applied on the underside to the right and left of the base 3, in particular by means of sputtering, vapor deposition or screen printing. The electrodes 2 are arranged on the same outer area (here the underside) of the sensor element 1.

[0052] To achieve good adhesive attachment in the Ag pressure sintering process, an electrode that likewise consists of Ag is of advantage. However, other electrode materials, such as, for example, Au, Cu, Al, etc., may also be used, as long as they are Ag-sinterable or can be processed by some other standard process.

[0053] The production of thin-film electrodes may be performed by sputtering or vapor deposition. In this case, in a first embodiment the base electrode consists of a nickel layer, which may comprise fractions of vanadium, or in a second embodiment of two layers, the lower layer comprising chromium or titanium and the second layer consisting of nickel, which likewise may comprise fractions of vanadium.

[0054] The base electrode may be protected by a covering layer consisting of an oxidation-inhibiting metal such as, for example, silver, gold, copper, aluminum, etc. This covering electrode may either just serve for protecting the nickel base electrode from corrosion (oxidation) or else be advantageous or even necessary for contacting. In the case of a connection by means of Ag sintering with finely dispersed silver pastes, for example, a silver covering electrode is indispensable.

[0055] The thickness of the base electrode is less than 10 m, advantageously less than 3 m, ideally less than 0.5 m. The thickness of the covering electrode may be up to 1 m, in exceptional cases up to 20 m.

[0056] The production of thick-film electrodes may be performed by a screen printing process with subsequent firing. The pastes used may contain Ag or any admixtures.

[0057] The final geometry is produced by a cutting process. In the case of very closely toleranced resistances, a trimming process may be performed for setting the resistance at nominal temperature by partial laser ablation. The contacting of the sensor element 1 with respect to the DCB board or the printed circuit board or the mother board may be performed by means of Ag sintering, soldering or adhesive bonding.

[0058] According to this exemplary embodiment, the sensor element 1 (pressed blank) is produced, for example, in the following way:

[0059] In a first step, NTC powder is produced. This comprises, for example, initial weighing, wet pre-grinding, drying, screening, calcining, wet after-grinding and spraying. After that, the pressing of the granular sprayed material is performed. The pressing mold is in this case designed such that a T-shaped main body is created during the pressing.

[0060] The decarburizing of the pressed blank follows in a further step. After that, the pressed blank is sintered.

[0061] The application of Ni/Ag thin-film electrodes 2 to the undersides to the right and left of the base 3 is performed by means of sputtering technology, as described above. The electrodes 2 are separated from one another by the base 3.

[0062] To improve the long-term stability of the ceramic, in a further step a thin, nonconducting protective layer, which consists, for example, of ceramics, glasses, plastics or metal oxides, may be applied over the unmetallized region. This can be achieved by sputtering, vapor deposition, lithography or printing and firing.

[0063] After that, the electrical measuring of the resistances of the individual sensor elements 1 with base 3 at nominal temperature is performed. For setting the resistance, the metalized substrates are electrically measured in advance. The geometry of the NTC sensor chips with base is defined on the basis of the measurement data obtained in advance. Since the length is fixed, the width remains as a variable setting parameter.

[0064] In a further step, a trimming of the individual sensor element 1 with base 3 to the required resistance value is performed by grinding the full surface area of one side.

[0065] For particularly closely toleranced resistances at nominal temperature, the resistance of the individual components can be set by the additional trimming process (also see in this respect FIG. 4). In this case, ceramic material or electrode material is partially removed, for example, by laser cutting or grinding, in such a way that the resistance is adapted by changing the geometry.

[0066] A visual inspection and random control measurement follow in a final step.

[0067] FIG. 2 shows a reinforced sensor element 1 with base 3. The construction corresponds substantially to the sensor element 1 from FIG. 1. However, the sensor element 1 has an additional ceramic layer 4 on the upper side (the side opposite from the base 3). The main body 7 is arranged on the ceramic layer 4. In particular, the ceramic layer 4 covers the upper side of the main body 7 preferably completely. The ceramic layer 4 serves as mechanical reinforcement for processes with particularly high stresses. The construction is realized by pressing granular material or stacked sheets. The NTC ceramic is pressed either together with the ceramic layer 4 or one after the other. The production of the sensor element 1 according to FIG. 2 is otherwise performed as described in connection with FIG. 1.

[0068] FIG. 3 shows a carrier material 5 with base 6 printed with sensor material (NTC layers). In this exemplary embodiment, the NTC layers represent the main body 7.

[0069] The NTC layers are printed onto the ceramic carrier material 5. The carrier material 5 is designed in a T-shaped form. In particular, the carrier material 5 has the base 6, the base representing the vertical line of the T. The base 6 is an integral part of the carrier material 5.

[0070] The ceramic carrier material 5 consists on the basis of, for example, Al.sub.2O.sub.3, ZrO.sub.2, ATZ or ZTA materials or MgO. The carrier material 5 may be brought into an appropriate form before or after being printed with NTC paste and individually separated after the sintering, or already take the form of a single part.

[0071] The electrodes 2 are applied to the end faces or side faces. In particular, the electrodes 2 are applied as caps. This allows the contacting of this structural form on the printed circuit board or mother board or the DCB board. Since the sensor material (the NTC layer) is not however in direct contact with the pads, the cap form of the electrodes 2 is required to ensure contacting of the NTC layer.

[0072] FIG. 4 shows a trimmed NTC sensor on a carrier material 5 with base 6. Sensor material has been removed by the trimming process for setting the resistance at nominal temperature. The removal is performed by partial laser ablation, as already described above.

[0073] According to all of the exemplary embodiments shown in FIGS. 1 to 4, the sensor element 1 has a T-shaped structural form. The high forces acting on the component during the pressure sintering are compensated by the structural form chosen, and the mechanical flexural loading is reduced to a minimum. In particular, the compressive loading during the Ag sintering is for the most part dissipated to the mother board in the intermediate region of the electrodes 2, whereby instances of damage due to flexural loading during mounting are avoided.

[0074] The use of ceramic carrier materials on the basis of, for example, Al.sub.2O.sub.3, ZrO.sub.2, ATZ or ZTA materials or MgO can lead to a further increase in the mechanical stability.

[0075] For use on mother boards or DCB boards, the sensor elements 1 shown in FIGS. 1 to 4 may be sintered onto the conductor tracks. This may be performed under pressure or without pressure. Mounting by adhesive bonding or soldering continues to be applicable.

[0076] The description of the subjects specified here is not restricted to the individual specific embodiments. Rather, the features of the individual embodiments canas far as technically feasiblebe combined with one another in any desired manner.