Sensor element for a capacitive sensor and method for producing the sensor element and the sensor

Abstract

Sensor element (1, 1a, 1b) for a capacitive sensor (5), comprising a tubular half-shaped base body (11, 11a, 11b) made of a plastic, wherein the base body (11, 11a, 11b) has - two spaced-apart electrode regions (21, 22) arranged on the outer side (A) of the base body (11, 11a, 11b), wherein at least one electrode material (3) is arranged on the electrode regions (21, 22), - a plurality of soldering points (41, 42, 43, 44) arranged in the longitudinal direction at the ends of the base body (11, 11a, 11b), wherein at least one soldering point (41, 42) is electrically connected to the first electrode region (21) and at least one soldering point (43, 44) is electrically connected to the second electrode region (22), characterized in that the sensor element (1, 1a, 1b) has a plurality of SMT positioning pins (111, 111a, 111b, 112, 112a, 112b) extending into its inner region (B).

Description

The present invention relates to a sensor element which can be SMD-equipped. The present invention furthermore relates to a method for producing the sensor element. The present invention also relates to a capacitive sensor which has two of the sensor elements. Finally, the present invention relates to a method for producing the capacitive sensor.Prior ArtCapacitive sensors, which can be used, for example, as fill level sensors, have at least two electrodes, which are usually produced from conductive metallic materials, such as brass or galvanized brass, by machining processing methods, such as turning or milling, or by means of deep drawing. These electrodes must have excellent electrical conductivity. In addition, its spatial shape must be adapted to the sensor housing, which protects the electrodes from a medium in which the capacitive sensor can be immersed for fill level measurement. Therefore, they are usually formed cylindrically. Quality fluctuations during the production of the sensor electrodes can lead to mounting errors of the electrodes on the printed circuit board due to fluctuating coaxiality; this results in an uneven distribution of the electric field in the capacitive sensor. For connecting the electrodes to the sensor electronics, which is usually arranged on an electrical printed circuit board, good solderableness of the electrodes is required. Manual soldering processes are, however, cost-intensive.Surface mounting techniques can be used to achieve higher precision and mounting speed, which allows a cost reduction due to the automation. For this purpose, metal components that can be equipped in an automated manner can be used. However, different electrode surfaces must consist of separate metal components. The electrode surfaces have a high weight.For different sensor applications, it may be necessary for the two sensor electrodes to have different areas. Accordingly, different electrodes must be kept ready as sensor components for the production of capacitive sensors.JP H10-23 971 A describes a sensor for measuring the level in a liquid tank. This has two tube-half-shaped base bodies which can be assembled to form a tubular sensor. One of the tube halves has a comb-shaped electrode which is connected to an electronic evaluation unit via an electrical contact.It is an object of the present invention to provide sensor elements which can be installed in a capacitive sensor in an automated manner by means of surface mounting techniques. A further object of the invention is to provide a method for producing such sensor elements. It is a further object of the invention to provide a capacitive sensor which can be manufactured using the sensor elements and whose electrodes are lighter than conventional all-metal electrodes. Finally, it is an object of the invention to provide a method for producing a capacitive sensor, which makes do with few starting components for production.Disclosure of the InventionIn one aspect of the invention, this object is achieved by a sensor element which has a tube-half-shaped base body. This base body consists of a plastic. In this case, a plurality of SMT positioning pins extend into its inner region. The inner region is understood here to mean that region which is partially enclosed by the base body. SMT positioning pins are understood to mean elements which make it possible, by means of surface mounting technology (SMT), to arrange the base body, for example, on a printed circuit board. The sensor element is therefore in particular a surface mounted device (SMD). Two electrode regions spaced apart from one another are arranged on the outer side of the base body. The outer side is understood to mean that side of the base body which is remote from the inner region. This is therefore the jacket surface of the tube half. At least one electrode material is arranged on the electrode regions, wherein the total layer thickness of the electrode materials is in particular in the range from 6 to 8 μm. Due to the spacing of the electrode regions, there is no electrical connection between the electrode material in the first electrode region and the electrode material in the second electrode region. A plurality of solder joints are arranged at the ends of the base body in the longitudinal direction. At least one solder joint is electrically connected to the first electrode region. At least one further solder joint is electrically connected to the second electrode region.In a surface mounting process, two such sensor elements may be connected into a complete tube, such that the electrode material in the first electrode regions forms a first annular electrode and the electrode material in the second electrode regions forms a second annular electrode. For this purpose, the first electrode region and the second electrode region are each in particular configured in the form of a half tube. The spacing of the two electrode regions is effected in particular by a likewise tube-half-shaped region which is arranged between the two electrode regions and on which no electrode material is arranged. The area of the electrode regions can be the same or different. In this way, the sensor element can be used both to produce capacitive fill level sensors for conductive media, which have two electrodes with the same dimensions, and to produce capacitive sensors for object detection of non-conductive objects, in which the dimensions of one electrode differ from the dimensions of the other electrode.The SMT positioning pins are preferably formed integrally with the base body. This enables simple production of the base body in a single working step. The SMT positioning pins then consist of the same plastic as the base body and are thus electrically non-conductive. However, this is also not necessary since the electrode materials can be connected in the electrode regions via the soldered joints.The plastic preferably comprises a thermoplastic polymer matrix in which molecules of at least one organometallic complex are enclosed. The polymer matrix can consist in particular of a polymer selected from the group consisting of acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polybutylene terephthalate (PBT), cycloolefin copolymer (COP), polyphenylene ether (PPE), polyamide (PA), polyphthalamide (PPA), polyethyleneimine (PEI), polyetheretherketone (PEEK) and mixtures and copolymers thereof, preference being given to polyamide. Furthermore, the polymer can be, in particular, a liquid crystal polymer (LCP), such as, for example, a liquid crystalline polyester. A laser direct structuring (LDS) process makes it possible to cleave metal atoms from their ligands, which can then serve as nuclei for subsequent reductive metallization. In this way, the base body receives a micro-rough surface in the electrode regions, on which the electrode material can be applied particularly firmly. The sensor element can be embodied in this way as a 3D MID (Molded Interconnect Device).The electrode material is preferably selected from the group consisting of copper, nickel, gold and alloys of these metals. These metals ensure high electrical conductivity on the one hand and are thus well suited as electrode materials. On the other hand, they can be applied to a LOS-structured surface in a simple reductive manner.In principle, it is sufficient if only one soldering location is present per electrode region, so that the electrode region can be electrically connected to a printed circuit board. In order to achieve a redundancy of this electrical contacting, however, it is preferred that at least two solder joints are electrically connected to the first electrode region and at least two solder joints are electrically connected to the second electrode region. In addition, in this way, a symmetrical arrangement of the electrode regions at a small distance from a printed circuit board to be connected to the sensor element can be provided, so that the automation of surface mounting by means of sensor elements according to the invention is simplified.Each solder joint is preferably designed such that it has a projection which is integrally connected to the base body and consists of the plastic. The projection is at least partially coated with the electrode material, wherein at least a part of the electrode material faces the inner region of the base body. When the base body is produced by means of injection molding, the projections can be produced in a simple manner in one piece with the remaining base body and the SMT positioning pins. The protrusions can be used as solder joints subsequently by applying the electrode material not only to the electrode regions but at the same time also to the protrusions. This can be prepared by laser structuring the projections together with the electrode regions. Parts of the electrode material on the projections facing the inner region of the sensor element face an electrical printed circuit board in a mounted capacitive sensor, so that they can be used for the electrical connection of the soldering points and thus also of the electrode regions to the printed circuit board.On its outer side between the two electrode regions, the base body preferably has at least one flattened region. Such a flattened region can be used in a surface mounting method as a receiving location in a "pick and place" process. By being arranged between the electrode region and thus lying in a region which is not coated with electrode material and will later also not serve as an electrode, it does not interfere with the electrode geometry.In a further aspect, the invention relates to a method for producing a sensor element. This comprises the production of a tube-half-shaped base body by means of injection molding of a plastic. Two electrode regions spaced apart from one another on the outer side of the base body are laser-structured. Finally, at least one electrode material is applied to the electrode regions by means of reductive metallization.During injection molding, a plurality of SMT positioning pins extending into the inner region of the base body are molded.The base body and its SMT positioning pins can be produced in particular in a cost-effective manner by means of single-component injection molding. For this purpose, a three-plate tool with four cavities can be used, for example.The plastic preferably comprises a thermoplastic polymer matrix in which molecules of at least one organometallic complex are enclosed. During laser structuring, the laser beam induces a physicochemical reaction in its focus. In this process, the complex bonds in the polymer matrix are broken up and metal atoms are cleaved off from their organic ligands. These can then serve as nuclei in the reductive metallization. As a result, all laser-structured regions can be metallized simultaneously in a simple manner without metal deposition occurring in the non-laser-structured regions.The reductive metallization takes place electrolessly. In this case, in particular one or more of the metals copper, nickel and gold can be deposited. The total layer thickness of all deposited electrode materials is in particular in the range from 6 to 8 μm.It is preferred that during injection molding at least one projection is formed at each end of the base body. Each projection is laser patterned together with the electrode regions. The electrode material is also applied to the protrusions, so that soldered joints are obtained which are electrically connected to the electrode areas via the electrode material.In yet another aspect, the invention relates to a capacitive sensor that has two sensor elements according to the invention. These are arranged in such a way that their base bodies form a tubular electrode carrier. Their first electrode regions are connected to form an annular first electrode and their second electrode regions are connected to form an annular second electrode. In this way, the capacitive sensor comprises the two cylindrical electrodes usually present in capacitive sensors. However, these consist not of solid metal, but only of metal coatings on the base body consisting of plastic. This leads to a weight reduction of the capacitive sensor compared to the conventional design.The capacitive sensor comprises a printed circuit board having a first side and a second side. SMT positioning pins of the first sensor element are arranged on the first side of the printed circuit board, and SMT positioning pins of the second sensor element are arranged on the second side. In contrast to conventionally produced capacitive sensors, the cylindrical electrodes of which have to be slid over a printed circuit board and which subsequently have to be positioned in a complicated manner in a manual soldering process, the SMT positioning pins of the sensor elements enable a rapid and precise arrangement of the electrodes on the printed circuit board.An electrical connection of the two electrodes to the printed circuit board can be effected in particular via the soldered joints. Whereas the two electrodes themselves, in contrast to all-metal electrodes, do not have a metal surface which faces the printed circuit board, but rather are separated from the printed circuit board by the electrode carrier, the solder joints make it possible to produce an electrical connection between the printed circuit board and the electrodes by means of conventional solder paste. Even if the electrode regions of the sensor elements in the capacitive sensor are connected to form ring-shaped electrodes, such electrical contacting does not take place only once per electrode, but at least once per electrode region, so that a high redundancy of the electrical contacting can be achieved.A further aspect of the invention relates to a method for producing a capacitive sensor. This comprises the provision of a printed circuit board, and the arrangement of a first sensor element according to the invention on a first side of the printed circuit board and the arrangement of a second sensor element on a second side of the printed circuit board. Here, SMT positioning pins of the first sensor element are arranged on the first side of the printed circuit board, and SMT positioning pins of the second sensor element are arranged on the first side of the printed circuit board. For this purpose, the sensor elements can be provided in an automated manner, for example, by means of a "tape and reel" method. In this case, the SMD components are chorded and optically oriented. Optionally, an electrical test of the two electrode regions of each sensor element can also be carried out. For the SMD mounting, a solder paste can be applied to the printed circuit board and subsequently, for example, vapor phase soldering or reflow soldering can be carried out. In this case, the first electrode regions of the sensor elements are connected to form an annular first electrode. The second electrode regions of the sensor elements are connected to form an annular second electrode.By soldering, the soldered joints of the first sensor element can be electrically connected to the first side of the printed circuit board and the soldered joints of the second sensor element can be electrically connected to the second side of the printed circuit board. In this way, a capacitive sensor can be manufactured quickly and automatically.Brief Description of the DrawingsExemplary embodiments of the invention are illustrated in the drawings and explained in more detail in the following description. FIG. 1 ashows an isometric representation of a sensor element according to an exemplary embodiment of the invention from its outer side. FIG. 1 bshows an isometric illustration of the sensor element according to FIG. 1 afrom its inner side. FIG. 2 is an isometric view of a circuit board that is a component of a capacitive sensor according to an embodiment of the invention. FIG. 3 shows an isometric view of several components of a capacitive sensor according to an exemplary embodiment of the invention. FIG. 4 shows a view of some of the components of the capacitive sensor shown in FIG. 3 from the direction denoted by IV in FIG. 3.Embodiments of the InventionFIGS. 1 aand 1 b show a sensor element 1 according to an exemplary embodiment of the invention from its outer side A and its inner side B. This sensor element 1 is produced by first producing, by means of one-component injection molding, a base body 11 made of the plastic Vectra E840i LDS (Celanese GmbH, Sulzbach Germany), which comprises a carbomonocyclic / carbopolycyclic polyester as liquid-crystalline polymer matrix, in which an organo-copper complex is finely distributed. This base body 11 has two parallel SMT positioning pins 111, 112 extending from its inner side and directed into its interior. These are arranged in a rotationally mirror-symmetrical manner with respect to the longitudinal axis of the base body 11. Two small projections 113, 114 are arranged at the first end of the base body 11 and two small projections 115, 116 are also arranged at its second end. These are positioned relatively close to the edges of these ends, respectively. Moreover, a large projection 117, 118 is arranged centrally at each of the ends. While the small protrusions 113, 114, 115, 116 are provided for forming solder joints, the large protrusions 117, 118 are formed to allow the areas that can be provided with metal layers functioning as electrodes to be enlarged. In the middle of the outer side A of the base body 11, a flattened region 119 is formed. Two half-tube-shaped electrode regions 21, 22 are laser-structured by means of an infrared laser. In this case, laser structuring of the small projections 113, 114, 115, 116 is also effected from both sides and laser structuring of the large projections 117, 118 is effected from the outside. The laser structuring produces a micro-rough surface in the laser-structured regions. Subsequently, reductive metallization of all laser-structured regions with a copper layer 4 μm thick is carried out. In a further metallization step, a nickel layer 3 μm thick and subsequently a gold layer 0.1 μm thick are applied to this. The electrode regions 21, 22 are metallized thereby to form half-tube-shaped electrode halves. The small projections 113, 114, 115, 116 form soldered joints 41, 42, 43, 44 connected to the electrode halves.In order to produce a capacitive sensor according to an exemplary embodiment of the invention, a printed circuit board 6 is provided, which is illustrated in FIG. 2. This has a first side 61 that can be seen in FIG. 2 and a second side 62 that cannot be seen in this figure. Two pairs of conductor tracks 631, 632, 633, 634 running parallel on opposite edges of the printed circuit board 6 extend on both sides 61, 62 of the printed circuit board 6, and two positioning depressions 641 a, 642 aare recessed from the printed circuit board on their first side. These contact the first pair of conductor tracks 631, 632. Mirror-image to this, two further positioning depressions 643 a, 644 aare recessed from the printed circuit board 6 in such a way that they contact the second pair of conductor tracks 633, 634. On the second side 62, four further positioning depressions 641 b, 642 b, 643 b, 644 bare arranged in mirror-image fashion with respect to the four positioning depressions 641 a, 642 a, 643 a, 644 ashown. A plurality of belted sensor elements 1 according to FIGS. 1 aand 1 bare provided and subsequently positioned on the printed circuit board 6 by means of an SMT method. This makes it possible to obtain the capacitive sensor 5 illustrated in FIGS. 3 and 4. This sensor element has a first sensor element 1 aand a second sensor element 1 b. A solder paste is applied to the side of its solder joints 41, 42, 43, 44 facing the inner side B of the sensor elements 1 a, 1 b. Subsequently, the first sensor element 1 ais placed on the first side 61 of the printed circuit board 6 in such a way that its first SMT positioning pin 111 aengages in one of the positioning depressions 641 aof the printed circuit board and its other SMT positioning pin 112 aengages in one of the other positioning depressions 644 a. The second sensor element 1 bis arranged on the second side 62 of the printed circuit board 6 such that its first positioning pin 111 bengages in a positioning depression 643 bof the second side 62 and its other positioning pin 112 bengages in one of the other positioning depressions 642 bof the second side 62 of the printed circuit board 6. The soldered joints 41, 42, 43, 44 of the two sensor elements 1 a, 1 bare soldered to the conductor tracks 631, 632, 633, 634 of the printed circuit board 6 by means of vapor phase soldering in such a way that the sensor elements 1 a, 1 bare firmly fixed on the printed circuit board 6. The two base bodies 11 a, 11 bof the sensor elements 1 a, 1 bform a common tubular electrode carrier 51 The electrode materials on the first electrode regions 21 connect to form a first annular electrode 52 The electrode materials on the second electrode regions 22 connect to form a second annular electrode 53 The first annular electrode 52 is electrically connected in each case by two of its soldered joints to a conductor track 631 and is connected by two further soldered joints to a further conductor track 633. The second annular electrode 53 is electrically connected to the two other conductor tracks 632, 634 by two of its soldering points in each case. By means of a non-metallized annular region of the electrode carrier 51, the two electrodes 52, 53 are electrically insulated from one another. As is shown in FIG. 3, the printed circuit board 6 can be fitted with further SMD modules. Together with the two sensor elements 1 a, 1 b, it is positioned in a housing 7, which protects them from the surrounding medium when used as a capacitive fill level sensor.Because the two electrodes 52, 53 are applied only as thin metallization layers on an electrode carrier 51 made of plastic, the capacitive sensor 5 is lighter than comparable conventional capacitive sensors. It can be produced with a high degree of automation by using SMT technology, wherein a high process reliability and functional density is ensured.

Claims

Sensor element (1, 1a, 1b) for a capacitive sensor (5), having a tube-half-shaped base body (11, 11a, 11b) which consists of a plastic, wherein the base body (11, 11a, 11b) has - two electrode regions (21, 22) which are spaced apart from one another and are arranged on the outer side (A) of the base body (11, 11a, 11b), wherein at least one electrode material (3) is arranged on the electrode regions (21, 22), - a plurality of soldered joints (41, 42, 43, 44) which are arranged at the ends of the base body (11, 11a, 11b) in the longitudinal direction, wherein at least one soldered joint (41, 42) is electrically connected to the first electrode region (21) and at least one soldered joint (43, 44) is electrically connected to the second electrode region (22), characterized in that the sensor element (1, 1a, 1b) has a plurality of SMT positioning pins (111, 111a, 111b, 112, 112a, 112b) extending into its inner region (B).Sensor element (1, 1a, 1b) according to Claim 1, characterized in that the SMT positioning pins (111, 111a, 111b, 112, 112a, 112b) are formed integrally with the main body (11, 11a, 11b).Sensor element (1, 1a, 1b) according to Claim 1 or 2, characterized in that the plastic comprises a thermoplastic polymer matrix in which molecules of at least one organometallic complex are enclosed.Sensor element (1, 1a, 1b) according to one of Claims 1 to 3, characterized in that the electrode material is selected from the group consisting of copper, nickel, gold and alloys of these metals.Sensor element (1, 1a, 1b) according to one of Claims 1 to 4, characterized in that at least two soldered joints (41, 42) are electrically connected to the first electrode region (21) and at least two soldered joints (41, 42) are electrically connected to the second electrode region (22).Sensor element (1, 1a, 1b) according to one of Claims 1 to 5, characterized in that each solder joint (41, 42, 43, 44) has a projection (113, 114, 115, 116) which is integrally connected to the base body and which consists of the plastic and is at least partially coated with the electrode material (3), wherein at least part of the electrode material (3) faces the inner region (B) of the base body (11, 11a, 11b).Sensor element (1, 1a, 1b) according to one of Claims 1 to 6, characterized in that the base body (11, 11a, 11b) has at least one flattened region (119) on its outer side (A) between the two electrode regions.Method for producing a sensor element (1, 1a, 1b) for a capacitive sensor (5), comprising the following steps: - producing a tube-half-shaped base body (11, 11a, 11b) having a plurality of SMT positioning pins (111, 111a, 111b, 112, 112a, 112b) extending into its inner region by means of injection molding of a plastic, - laser structuring of two electrode regions (21, 22) spaced apart from one another on the outer side (A) of the base body (11, 11a, 11b), and - applying at least one electrode material (3) to the electrode regions (21, 22) by means of reductive metallization.Method according to Claim 8, characterized in that the plastic has a thermoplastic polymer matrix in which molecules of at least one organometallic complex are enclosed, and in that metal atoms are cleaved from their ligands during the laser structuring.Method according to claim 8 or 9, characterised in that during the injection moulding a plurality of SMT positioning pins (111, 111a, 111b, 112, 112a, 112b) extending into the inner region of the base body (11, 11a, 11b) are formed.Method according to one of Claims 8 to 10, characterized in that during injection moulding at each end of the base body (11, 11a, 11b) at least one projection (113, 114, 115, 116) is formed, wherein each projection (113, 114, 115, 116) is laser-structured together with the electrode regions (21, 22) and the electrode material (3) is also applied to the projections (113, 114, 115, 116) in order thus to obtain soldered joints (41, 42, 43, 44).Capacitive sensor (5) comprising two sensor elements (1a, 1b) according to one of claims 1 to 7, which are arranged such that their base bodies (11a, 11b) form a tubular electrode carrier (51), such that their first electrode regions (21) are connected to form an annular first electrode (52) and their second electrode regions (22) are connected to form an annular second electrode (53), characterized in that the capacitive sensor (5) comprises a printed circuit board (6), wherein SMT positioning pins (111a, 112a) of the first sensor element (1a) are arranged on a first side (61) of the printed circuit board (6) and SMT positioning pins (111a, 112a) of the second sensor element (1b) are arranged on a second side (62) of the printed circuit board (6).Capacitive sensor (5) according to Claim 12, characterized in that the printed circuit board (6) is electrically connected to the first electrode (52) and to the second electrode (53) via the soldered joints (41, 42, 43, 44).Method for producing a capacitive sensor (5), comprising the following steps: - providing a printed circuit board (6), - a first sensor element (1a) according to one of Claims 1 to 7 on a first side (61) of the printed circuit board (6), - a second sensor element (1b) according to one of Claims 1 to 7 on a second side (62) of the printed circuit board (6), wherein the first electrode regions (21) of the sensor elements (1a, 1b) are connected to form an annular first electrode (51) and the second electrode regions (22) of the sensor elements (1a, 1b) are connected to form an annular second electrode (52), characterized in that SMT positioning pins (111a, 112a) of the first sensor element (1a) are arranged on the first side (61) of the printed circuit board (6), and SMT positioning pins (111b, 112b) of the second sensor element (1b) are arranged on the first side (62) of the printed circuit board (6).Method according to Claim 14, characterized in that the soldered joints (41, 42, 43, 44) of the first sensor element (1a) are electrically connected to the first side (61) of the printed circuit board (6) and the soldered joints (41, 42, 43, 44) of the second sensor element (1b) are electrically connected to the second side (62) of the printed circuit board (6).

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