How to manufacture an NTC sensor

Abstract

The present invention relates to a method for manufacturing an NTC sensor, including a method for assembling an NTC sensor and a method for coating an NTC sensor, which method includes several process steps in a specific order. The present invention further relates to an NTC thermistor element (4).

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing an NTC sensor, including a method for assembling an NTC sensor and a method for coating an NTC sensor, which method includes several process steps in a specific order. The present invention further relates to an NTC thermistor element. [Background technology]

[0002] Already available NTC thermistor temperature sensors that use a plastic coating are manufactured using conventional thermally controlled assembly and coating techniques.

[0003] The electrical resistance of the NTC thermistor material changes with changes in temperature. In particular, the resistance of the NTC thermistor decreases with increasing temperature. The NTC thermistor material is incorporated into an electrical circuit by a connecting wire. The plastic housing can protect the thermistor from environmental influences.

[0004] An example of an NTC thermistor element is known from the document DE 102005017816 A1. Summary of the Invention

[0005] The object of the present invention is to improve the known manufacturing method for NTC sensors.

[0006] The method involves several manufacturing steps.

[0007] In a first aspect, the present invention relates to a method of assembling an NTC sensor comprising several steps.

[0008] In one step, an NTC thermistor element (NTC stands for Negative Temperature Coefficient) and connecting wires having terminals for contacting the NTC thermistor element are provided.

[0009] The NTC thermistor elements may be configured in a disk or block shape adapted for use in a surface mounted device (SMD).

[0010] The NTC thermistor element may include an NTC ceramic material that acts as an electrical resistor whose resistance varies depending on the internal temperature of the material and that may be self-heated by applying an electric current.

[0011] The NTC thermistor element may further comprise two electrodes configured on opposing surfaces of the NTC ceramic material. The electrodes electrically contact the NTC thermistor element and allow the NTC thermistor element to be incorporated into an electrical circuit. The electrodes may comprise a material exhibiting good electrical conductivity, such as a metal, a precious metal, or a metal alloy.

[0012] Alternatively, the NTC thermistor element can be designed as a multi-layer component, which comprises several layers of NTC ceramic material and internal electrodes arranged between these layers, the internal electrodes comprising a metallic conductive material.

[0013] The multilayer component is formed as a stack of a number of ceramic layers and internal electrodes and preferably has an overall cuboid structure.

[0014] In the described embodiment, the internal electrodes can be electrically contacted by external electrodes on the surface of the multilayer component. The external electrodes are preferably applied to two opposite sides of the multilayer component. The external electrodes can be cap-shaped and can cover different sides of the multilayer component. Preferably, the internal electrodes laminated in the multilayer component are alternately connected to two oppositely located external electrodes, respectively.

[0015] The external electrodes comprise metallization layers, for example of silver or gold, which are applied, for example, by screen printing and subsequent heat treatment.

[0016] The electrical resistance of such multi-layer components can be very precisely defined, so that resistance tolerances of less than 1% can be achieved.

[0017] The connecting wire is a conductive wire comprising a conductive metal or metal alloy. The connecting wire may be insulated by a polymer coating. At least a portion of the end of the connecting wire may not be insulated by a coating.

[0018] In a further step, the NTC thermistor element is self-heated by application of a current. The self-heating is maintained during the following steps.

[0019] In a first step, a solder paste is dispensed onto the ends of the connecting wires. The solder paste may contain different metals such as lead, tin, zinc, silver, copper, gold, antimony, and bismuth. Preferably, the solder paste may be lead-free. Additionally, the solder paste may be impregnated with a fluxing agent.

[0020] After heating, the dispensed solder paste may impart a flame-like shape to the end of the connecting wire, that is, the head of the connecting wire.

[0021] In a second step, connecting wires are applied to the NTC thermistor element. In particular, the connecting wires can be applied to the electrodes of the NTC thermistor element on two opposite sides of the element. The connecting wires are applied to the NTC thermistor element such that the solder paste is in direct contact with the NTC thermistor element, in particular with the electrodes.

[0022] In a third step, the heat generated by the NTC thermistor element melts the solder paste, which forms solder bonds between the NTC thermistor element, particularly the electrodes, and the connecting wires, particularly the ends.

[0023] A closed electrical circuit is thereby formed including a closed electrical connection between the end of the first connecting wire, the first solder joint, the first electrode of the NTC thermistor element, the NTC thermistor material, the second electrode of the NTC thermistor element, the second solder joint, and the end of the second connecting wire.

[0024] The steps described may be performed in the given order, or in any other order, as applicable. Here and below, the designations "first," "second," etc. do not dictate a necessary order of process steps.

[0025] In one embodiment, the NTC thermistor element is heated to over 200°C.

[0026] Such temperatures can be reached without any change in the properties of the NTC thermistor material.

[0027] Unlike when heating an NTC thermistor material in an oven, the phase composition of the NTC thermistor material does not change significantly during the described self-heating process. Thus, the subsequent time-consuming aging process can be omitted or significantly shortened. The following coating process can be carried out immediately after the soldering process, without any additional heat treatment.

[0028] Preferably, the current for heating the NTC thermistor is applied to the NTC thermistor by the connecting wire itself.

[0029] In an alternative embodiment, the current is applied by an auxiliary electrode applied to the NTC thermistor, the auxiliary electrode being applied temporarily and reversibly.

[0030] Auxiliary electrodes may be applied on two opposing surfaces of the NTC thermistor element.

[0031] The auxiliary electrode may be included in a holding structure to which the NTC thermistor element is secured during manufacture.

[0032] The auxiliary electrode may be in contact with an electrode of the NTC thermistor element.

[0033] This forms a closed electrical circuit including a closed electrical connection between the first auxiliary electrode, the first electrode of the NTC thermistor element, the NTC thermistor material, the second electrode of the NTC thermistor element, and the end of the second auxiliary electrode.

[0034] In the above process, the heat generated by the NTC thermistor element melts the solder paste, forming a solder joint between the NTC thermistor element and the connecting wire, and then the auxiliary electrode is removed to stop the self-heating of the NTC thermistor element.

[0035] In one embodiment, the solder joint between the connecting wire and the NTC thermistor element exhibits high strength: the solder joint can withstand a tensile force of at least 6N, preferably up to 7N, and even more preferably up to 8N.

[0036] In one embodiment, two connecting wires are applied onto two opposing surfaces of the NTC thermistor element, in particular onto two opposing electrodes arranged on the opposing surfaces of the NTC thermistor element.

[0037] In one embodiment, the solder paste is impregnated by dipping the connecting wires into a reservoir containing the fluxing agent.

[0038] The solder paste is impregnated with a fluxing agent before the wire is applied onto the NTC thermistor element and before the solder paste is melted.

[0039] Thus, no fluxing agent is wasted by unwanted impregnation of the NTC thermistor element. Preferably, the solder paste is impregnated by dipping the connecting wires into a sponge having the fluxing agent.

[0040] In this manner, the fluxing agent can be applied in a targeted manner with minimal waste of fluxing agent.

[0041] In the soldering of metals, fluxing agents serve several purposes: they remove oxidized metals from the surfaces being soldered, they seal out air, thereby preventing further oxidation, and they improve the wetting characteristics of the liquid solder.

[0042] In one embodiment, the solder paste melts and forms solder bonds in less than 30 seconds.

[0043] During this time span, the NTC thermistor element and the surface of the NTC thermistor element to which the solder paste has been applied can be heated to a predetermined temperature, causing the solder paste to melt and form a solder joint.

[0044] The temperature required depends on the composition of the solder paste. The NTC thermistor element may be configured to be heated to above 200°C, preferably between 200°C and 400°C, by application of electrical current.

[0045] The solder paste is melted directly by the self-heated NTC thermistor element to which the solder is applied, thus eliminating thermal pre- and post-processing steps.

[0046] Therefore, the soldering process can be shortened compared to the conventional method.

[0047] In a second aspect, the invention relates to a method for coating an NTC sensor comprising several steps.

[0048] The method of the second aspect may further comprise all the steps of the method of the first aspect.

[0049] A method of coating an NTC sensor includes providing an NTC sensor comprising an NTC thermistor element and a connecting wire secured onto the NTC thermistor element.

[0050] The NTC thermistor element and connecting wires may be processed as described in the first aspect. An NTC sensor may be provided by carrying out the process of the first aspect.

[0051] In one step, the NTC thermistor element is self-heated by application of an electric current. The self-heating is maintained during the following process steps.

[0052] In the first step, the NTC thermistor element is immersed in the coating material.

[0053] The coating raw materials can be provided as polymer powders or as resins. Typical coating materials can include polymers such as perfluoroalkoxyalkanes (PFA), Teflon, polyurethanes (PU), polyamides (PA), polyimides (PI), silicones, polyesters, polyacrylates, epoxy polymers, resins, and epoxy resins.

[0054] The coating material may be stored in a coating feed reservoir, such as a tub.

[0055] In a second step, the coating material is melted by the heat generated by the NTC thermistor element. The amount of heat required depends on the coating material used.

[0056] In a third step, a coating layer is formed surrounding the NTC thermistor element and adjacent portions of the connecting wires.

[0057] The coating layer is formed from molten coating raw material. Preferably, the coating layer comprises only one single layer of polymeric material.

[0058] By the disclosed process steps, the coating layer can be applied with a minimal demand for heating energy and coating raw material. Only the raw material adjacent to the self-heated surface of the NTC thermistor element melts and forms the coating layer. The further coating raw material provided in the reservoir remains in the form of powder or resin.

[0059] In one embodiment, the NTC thermistor element is heated to above 170°C.

[0060] Such temperatures can be reached without any change in the properties of the NTC thermistor material.

[0061] Unlike when heating an NTC thermistor material in an oven, the phase composition of the NTC thermistor material does not change significantly during the described self-heating process, so the subsequent time-consuming aging process can be omitted or significantly shortened.

[0062] Furthermore, due to the targeted heating of only a small amount of coating material, the thermal post-treatment steps for hardening and tempering the coating material can be reduced to a time span of less than 2 hours, preferably less than 1 hour.

[0063] The required temperature depends on the composition of the coating material. The NTC thermistor element may be configured to be heated by application of electric current to a temperature of greater than 80°C, or greater than 100°C, or greater than 170°C, preferably between 80°C and 350°C.

[0064] In one embodiment, the current is applied by using connecting wires secured to the NTC thermistor element.

[0065] Preferably, two connecting wires are applied onto two opposing surfaces of the NTC thermistor element, in particular onto two opposing electrodes arranged on the opposing surfaces of the NTC thermistor element.

[0066] This forms a closed electrical circuit including a closed electrical connection between the first connecting wire, the first electrode of the NTC thermistor element, the NTC thermistor material, the second electrode of the NTC thermistor element, and the second connecting wire.

[0067] In one embodiment, the coating material melts and forms a coating layer in less than 30 seconds.

[0068] During this time span, the NTC thermistor element and the surface of the NTC thermistor element having the coating material applied thereto can be heated to a predetermined temperature, causing the coating material adjacent to the surface to melt and form a coating layer.

[0069] In another embodiment, the coating material melts and forms a coating layer in less than 50 seconds.

[0070] By choosing a short time span, only one coating layer is formed, thus minimizing the demand for raw materials.

[0071] The coating material is melted by a self-heated NTC thermistor element once the material is applied, so no thermal pre- or post-treatment steps are required to form the coating layer.

[0072] Therefore, the coating process can be shortened compared to the conventional process.

[0073] In a third aspect, the present invention relates to a method for manufacturing an NTC sensor comprising any one of the assembly method steps and any one of the coating method steps as described with respect to the first and second aspects.

[0074] The manufacturing method of the first aspect may include all the steps of the method of the first aspect, and may further include all the steps of the method of the second aspect.

[0075] The first two aspects of the invention may also include all the steps of the third aspect.

[0076] The method of manufacturing the NTC sensor comprises several assembly steps and several coating steps.

[0077] In particular, the manufacturing method includes the following steps, which may be performed in a given order:

[0078] In one step, an NTC thermistor element and a connecting wire having an end for contacting the NTC thermistor element are provided.

[0079] In the next step, the NTC thermistor is self-heated by application of a current, preferably by the connecting wire itself, or alternatively by using an auxiliary electrode. Self-heating is maintained during the following three steps.

[0080] In a first step, solder paste is dispensed onto the ends of the connecting wires.

[0081] In a second step, connecting wires are applied to the NTC thermistor element.

[0082] In a third step, the solder paste is melted by the heat generated by the NTC thermistor element. This forms a solder joint between the NTC thermistor element and the connecting wire.

[0083] After the third step, any auxiliary electrodes may be removed, stopping self-heating of the NTC thermistor element.

[0084] In the following, the NTC sensor comprising the NTC thermistor element and the connecting wire fixed on the NTC thermistor element is further processed.

[0085] In the fourth step, the NTC thermistor element is self-heated by application of a current. The current can be applied by a connecting wire. The self-heating can be maintained until the next step.

[0086] In the fifth step, the NTC thermistor element is immersed in the coating material.

[0087] In the sixth step, the coating material is melted by the heat generated by the NTC thermistor element.

[0088] In a seventh step, a coating layer is formed surrounding the NTC thermistor element and adjacent portions of the connecting wires.

[0089] Self-heating of the NTC thermistor element can then be stopped.

[0090] No further thermal post-treatment step is required.

[0091] Alternatively, a thermal post-treatment step may be performed to cure and / or temper the coating. These thermal post-treatment steps may be performed in less than an hour.

[0092] In one embodiment, several NTC thermistor elements are handled simultaneously. In a preferred embodiment, at least five NTC thermistor elements are handled simultaneously. Simultaneous processing is possible due to simplifications of conventional manufacturing processes.

[0093] Simultaneous handling allows for mass production of NTC sensors.

[0094] The present invention further relates to an NTC sensor element comprising an NTC thermistor element, two connecting wires fixed to opposite sides of the NTC thermistor element by solder joints, and a single layer coating completely surrounding the NTC thermistor element and the solder joints.

[0095] In one embodiment, the solder joint between the connecting wire and the NTC thermistor element exhibits high strength. The solder joint can withstand a tensile force of at least 6 N. This strength is at least comparable to that of conventional solder joints. Preferably, the solder joint exhibits high strength up to 7 N, and even more preferably up to 8 N.

[0096] The NTC sensor element may be manufactured by the method according to any one of aspects 1-3 of the present invention.

[0097] The NTC sensor element may further be configured according to any one of the aspects 1-3.

[0098] In one embodiment, the NTC thermistor element has a block shape.

[0099] In a further embodiment, the NTC thermistor element has a disk shape.

[0100] The dimensions of the NTC thermistor element may not exceed 30mm x 30mm x 5mm, preferably not exceeding 3mm x 3mm x 1mm.

[0101] The disks or blocks may have a circular, oval, rectangular or square shape with or without rounded corners.

[0102] The NTC thermistor element may include an NTC ceramic material that acts as an electrical resistor whose resistance changes depending on the internal temperature of the material. The NTC ceramic material may be self-heated by applying an electric current.

[0103] The NTC thermistor element may further comprise two electrodes configured on opposing surfaces of the NTC ceramic material. The electrodes electrically contact the NTC thermistor element and allow the NTC thermistor element to be incorporated into an electrical circuit. The electrodes may comprise a material exhibiting good electrical conductivity, such as a metal, a precious metal, or a metal alloy.

[0104] The thickness of the current path, which is the dimension of the NTC thermistor element from one electrode to the other, may not exceed 5 mm (including the dimensions of the electrodes therein), and preferably may not exceed 1 mm.

[0105] The surface of the NTC thermistor element perpendicular to the direction of the current path may have a circular, elliptical, rectangular or square shape and may not exceed 30 mm x 30 mm, preferably not exceed 3 mm x 3 mm.

[0106] The connecting wire is a conductive wire comprising a conductive metal or metal alloy. The connecting wire may be insulated by a polymer coating. At least a portion of the end of the connecting wire may not be insulated by a coating.

[0107] Hereinafter, the embodiments of the present invention will be described in more detail with reference to the accompanying drawings. Similar or obviously identical elements in the figures are marked with the same reference numerals. The figures and the proportions in the figures are not to scale. It should be noted that the present invention is not limited to the following embodiments. [Brief description of the drawings]

[0108] [Figure 1] FIG. 1 shows a first embodiment of a connecting wire. [Diagram 2]FIG. 2 shows a first embodiment of a connecting wire having a solder bump. [Diagram 3] FIG. 3 illustrates the temperature and electrical resistance profile inside the NTC thermistor element during the soldering process. [Figure 4] FIG. 4 shows a first embodiment of an NTC thermistor element before the coating process. [Diagram 5] FIG. 5 shows the temperature and electrical resistance profile inside the NTC thermistor element during the coating process. [Figure 6] FIG. 6 shows a first embodiment of a coated NTC thermistor element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0109] An example of a manufacturing process for an NTC (negative temperature coefficient) thermistor element will now be described with reference to the drawings. The NTC thermistor element 4 is used in an NTC sensor.

[0110] In a first process section, the NTC thermistor element 4 is provided with a connecting wire 1. The connecting wire 1 is provided by soldering on two opposite surfaces of the NTC thermistor element 4. On the two opposite surfaces of the NTC thermistor element 4, two electrodes 5 are formed. The connecting wire 1 is applied to the surfaces of the electrodes 5.

[0111] First, a connecting wire 1 is prepared, as shown in Figure 1. The connecting wire 1 is mainly made of a material with good electrical conductivity, such as a noble metal, such as silver or copper, or a corresponding alloy. The wire 1 may have a coating insulating layer 2 of a non-conductive polymeric material. The insulating layer 2 is absent at the end of the wire 1, where the solder paste is then applied.

[0112] In a first manufacturing step, the wires 1, preferably five pairs of two wires 1, are clamped in a holding device. In this way, five sensors can be produced simultaneously. Now, the ends of the wires 1 that do not have a coating are processed. Solder bumps 3 are applied to the free ends of the wires 1 via a syringe. By applying the solder paste in a targeted manner, the amount of solder required can be minimized. The excess solder material that is then discarded is reduced.

[0113] The wire ends with solder bumps 3 applied are shown in FIG.

[0114] In a next process step, the wire 1 with the applied solder bumps 3 is immersed in a reservoir of fluxing agent, which may preferably be in the form of a sponge containing the fluxing agent, and then the wire 1 is pressed, solder bumps 3 first, against the sponge, so that the solder bumps 3 are impregnated with the fluxing agent.

[0115] The NTC thermistor elements 4 are provided in a holder, preferably several elements at a time, for example five elements.

[0116] A wire 1 with solder bumps 3 is applied on two opposite surfaces comprising electrodes 5 of an NTC thermistor element 4 .

[0117] A current of up to 1 A is applied to the NTC thermistor element 4 via the wire 1, depending on the resistance value and resistance temperature (RT) curve of the NTC ceramic material, and the NTC thermistor element 4 self-heats.

[0118] Self-heating of the NTC thermistor element to above 200° C. causes the solder bumps 3 to melt and form a solid solder connection between the NTC thermistor element 4 and the wire 1 .

[0119] Figure 3 shows the temperature profile during the soldering process.

[0120] During the period T1, the NTC thermistor element 4 is preheated above 120°C. At a temperature between 120°C and 210°C the fluxing agent is activated. In this temperature profile, a temperature of 120°C is reached after about 5 seconds (T1). At a temperature of about 220±10°C the solder paste liquefies. After a further 11 seconds (T2) the maximum temperature of 270±5°C is reached.

[0121] The maximum temperature of the soldering process is held for 14 seconds (T3) to form the solder bumps 3.

[0122] After a total time of 30 seconds, the soldering process is finished and the self-heating of the NTC thermistor element is stopped, i.e. the current applied for self-heating is switched off.

[0123] Since the NTC thermistor element 4 by definition has a high electrical conductivity at high temperatures, the electrical resistance also shown in Figure 3 varies inversely with temperature. During the soldering process at 270 °C, the electrical resistance decreases according to the corresponding RT curve.

[0124] After the soldering process, the NTC sensor has the configuration shown in Figure 4. Two wires 1 have their free ends attached to electrodes 5 on opposite faces of an NTC thermistor element 4 via solder bumps 3.

[0125] In the following, the coating process is carried out, which is explained with reference to the temperature diagram of Fig. 5. The NTC thermistor element 4 is self-heated by applying an electric current. For this purpose, an electric current can be applied via the connecting wire 1 attached hereto.

[0126] In a first step, the NTC thermistor element 4 is preheated to a temperature of 140° C. within 11 seconds (S1).

[0127] The preheated NTC thermistor element 4 is then immersed in a reservoir containing the coating material. The coating material is in powder or resin form. In the described example, the coating material may be in powder form. The immersion operation lasts for about 3 seconds (S2). By immersing in a powder of coating material that is cooler than the NTC element, the NTC thermistor element 4 is cooled relatively quickly to about 50° C. The coating material has a temperature of about room temperature (around 25° C.).

[0128] The NTC thermistor element 4 in the reservoir of coating material is then heated again (S3) to approximately 170°C for 3 seconds, melting the coating material powder adjacent to the NTC thermistor element 4 and forming a single coating layer 7 around the NTC thermistor element 4 and the connecting wire 1 and the adjacent portion of the connecting solder bump 3.

[0129] The NTC thermistor element 4 with the applied coating layer 7 is then lifted out of the reservoir of coating material within about 3 seconds (S4), whereby the NTC thermistor element 4 is cooled to about 160° C. The coating layer 7 is then applied onto the NTC thermistor element 4. After about 20 seconds, the actual coating process is completed. The coating layer 7 is then thermally post-treated to be strengthened.

[0130] In a pre-curing step, the NTC thermistor element 4 with the coating layer 7 applied thereto is heated to 190° C. in 10 seconds (S5). The curing temperature of about 195±5° C. is then maintained for 30 seconds (S6). After about 40 seconds, the curing process is completed and the self-heating of the NTC thermistor element is stopped.

[0131] In this way, the NTC sensor element shown in FIG. 6 is obtained, with the NTC thermistor element, the adjacent solder bump 3 and the adjacent non-insulated part of the connecting wire 1 being surrounded within a coating 7.

[0132] The above-described method allows for a uniform thin application of the coating material, so that the NTC thermistor element 4 is completely and uniformly surrounded after only one layer of coating material has been applied. Therefore, no further application of a coating layer 7 is necessary. This means that the time and costs required to produce the coating can be significantly reduced.

[0133] The thin single layer coating 7 also eliminates the need for additional time-consuming thermal post-treatment steps: the thin coating 7 can be cured within the 40 seconds mentioned above.

[0134] If necessary, further post-processing steps can be performed. These steps, such as hardening and tempering, take place in less than an hour to achieve the required properties of the coating 7.

[0135] In conventional processes, the time required for such post-treatment steps is typically 8 hours for the hardening process and 72 hours for the tempering process. In conventional processes, more than two or even five layers of coating material are often required because of the difficulty in applying the material uniformly.

[0136] An exemplary NTC thermistor element as shown in FIG. 6 is described below.

[0137] The NTC thermistor element 4 has a cuboid shape. The NTC thermistor element 4 includes an NTC thermistor ceramic 6 having a cuboid shape.

[0138] Electrodes 5 for electrical contact are provided on two opposite surfaces of the NTC thermistor ceramic 6. The overall dimensions of the NTC thermistor element 4 do not exceed 3 mm x 3 mm x 1 mm.

[0139] Here, 1 mm is a measurement of the thickness of the NTC thermistor element from the surface to which one electrode 5 is applied to the surface to which the other electrode 5 is applied. This thickness corresponds to the length of the current path that an applied current follows within the NTC thermistor element.

[0140] The surface perpendicular to the current path has dimensions not exceeding 3 mm x 3 mm. In general, the long side of the surface points in the same direction as the connecting wire 1 attached to the element, and the short side points in a direction perpendicular to the direction in which the connecting wire 1 extends.

[0141] The electrode 5 is preferably an electrode comprising a material including at least one of silver, gold, copper, nickel, platinum, or palladium, and is applied to the thermistor material, for example by screen printing or sputtering.

[0142] The connecting wires 1 are attached to the electrodes 5 via solder bumps 3. The solder bumps 3 are made of a solderable material including a variety of suitable metals such as germanium, tin, silver, lead, antimony, bismuth, gold, zinc, copper, etc. Preferably, the solderable material is lead-free.

[0143] The connecting wire 1 has, for example, a diameter of 0.25 mm and comprises a material with good electrical conductivity, such as copper. The connecting wire 1 is covered by an electrical insulating layer 2, except for its ends.

[0144] The diameter of the insulated wire 1 is about 0.5 mm. Preferably, high temperature resistant plastics are used for the insulation 2, such as polyetheretherketone (PEEK), PFA, polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), PA, PI or similar plastics.

[0145] Alternatively, non-insulated wire may be used.

[0146] The NTC thermistor element 4 and adjacent portions of the wire 1, particularly the non-insulated portions, are surrounded by a single layer coating 7, for example a coating layer 7 comprising an epoxy resin or other epoxy polymer material. Further possible coating materials include PFA, Teflon, or fluorinated epoxy materials.

[0147] The solder joint between the connecting wire 1 and the NTC thermistor element 4 soldered according to the above-described method exhibits a high strength at least comparable to that of a conventional solder joint. The solder joint can withstand a tensile force of at least 6N, preferably up to 7N and even more preferably up to 8N.

[0148] Because of the short processing times described in the present method, a continuous serial production can be operated in the manufacture of the NTC thermistor elements 4 instead of a conventionally operated batch-type production run.

[0149] This means that the size of the manufacturing equipment required can be significantly reduced, so that, for example, instead of producing hundreds of elements simultaneously in a batch fashion, a sufficiently high production output can be achieved by simultaneously producing five elements each in series.

[0150] Furthermore, by omitting the thermal pre-treatment and post-treatment, part of the manufacturing equipment can also be omitted.

[0151] For example, the ovens used for the conventional aging steps of thermistor ceramic materials and for the hardening and tempering steps of coating layers can be omitted or can be significantly smaller in size.

[0152] For example, the production line for the new method is 3.5 x 2 m in length and width and includes three modules for soldering, coating, and post-thermal treatment. The modules are small because only five elements need to be manufactured in parallel. The post-thermal treatment module includes a small oven to accommodate the thermistor elements for one hour.

[0153] On the other hand, a conventional production line, for example, is 14 x 5 m in length and width and comprises five modules for soldering, ageing, coating, hardening and tempering. The modules are relatively large, since to achieve the same production capacity as the new method, several hundred elements must be produced in parallel. The post-heat treatment module for hardening and tempering comprises a huge oven to accommodate several hundred thermistor elements for up to 80 hours. [Explanation of symbols]

[0154] 1 Connecting Wire 2. Insulation layer 3 Solder Bumps 4 NTC thermistor elements 5 electrodes 6 Thermistor Ceramic 7 Coating Layer

Claims

1. 1. A method of assembling an NTC sensor, comprising: providing an NTC thermistor element and a connecting wire having an end for contacting the NTC thermistor element; self-heating the NTC thermistor element by applying an electric current during the following steps: dispensing solder paste onto the ends of the connecting wires; applying the connecting wire to the NTC thermistor element; melting the solder paste with heat generated by the NTC thermistor element, thereby forming a solder joint between the NTC thermistor element and the connecting wire; The method includes:

2. The NTC thermistor element is heated to above 200° C. The method of claim 1.

3. The current is applied by an auxiliary electrode applied to the NTC thermistor element. The method of claim 1.

4. Two connecting wires are applied to two opposite sides of the NTC thermistor element; The method of claim 1.

5. The solder paste is impregnated by dipping the connecting wires into a reservoir containing a fluxing agent. The method of claim 1.

6. the solder paste melts and forms the solder joint in less than 30 seconds; The method of claim 1.

7. 1. A method for coating an NTC sensor comprising: Providing an NTC sensor comprising an NTC thermistor element and a connecting wire fixed onto the NTC thermistor element; self-heating the NTC thermistor element by applying an electric current during the following steps: dipping the NTC thermistor element into a coating material; melting the coating material with heat generated by the NTC thermistor element; forming a coating layer surrounding the NTC thermistor element and adjacent portions of the connecting wire; The method includes:

8. The NTC thermistor element is heated to above 170°C. The method of claim 7.

9. The current is applied by using the connecting wires. The method of claim 7.

10. The coating material melts and forms the coating layer in less than 30 seconds. The method of claim 7.

11. 1. A method for manufacturing an NTC sensor, comprising: an assembly step according to the method of any one of claims 1 to 6; A coating step according to the method of any one of claims 7 to 10; The method includes:

12. Multiple NTC thermistor elements are handled simultaneously; The method of claim 11.

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