Manufacturing method for a PTC thermistor

Abstract: PTC ( positive temperature coefficient ) thermistors of a novel configuration and a method for their manufacture are disclosed. The disclosed PTC thermistors have a PTC element (201) sandwiched between two electrodes (202, 203) for which leads (204, 205) are formed as an extension of each of the two electrodes protruding beyond the edge of the PTC element. Several methods for manufacturing this kind of PTC thermistor are described, whereby undue thermal and physical stress to the PTC composition are avoided. Additionally, by means of the described manufacturing methods, PTC thermistors having a variety of shapes and configurations can be fabricated.

Description: Background of the Invention The present invention relates to a method of manufacturing PTC (positive temperature coefficient) thermistors. PTC (positive temperature coefficient) thermistors are well known devices which have been employed in electronic circuits for over current protection and for thermal sensing. A conventional PTC thermistor is shown in Fig. 11. As can be seen in the illustration, the PTC thermistor SO has a composite structure of sandwiched PTC composition 1a between electrodes 2a and 3a. The above mentioned PTC element 1a is comprised of a PTC composition including polymers and conductive particles which demonstrates positive thermal coefficient resistance properties. The electrodes 2a, 3a are formed from sheet form metallic material, and each is provided with a respective lead 4, 5 connected thereto as shown in Fig. 11. For the manufacture of this type of PTC thermistor S0, the following method, for instance, can be applied. First of all, as is shown in Fig. 12, two relatively large metallic sheets 2, 3 each of which constitutes a plurality of the individual thermistor electrodes 2a, 3a respectively, are bonded to the opposing upper and lower surfaces of a sheet of PTC composition 1 which is to constitute a plurality of the individual PTC elements 1a, thereby forming a laminated PTC thermistor sheet 6. The above bonding of the metallic sheets 2, 3 to the PTC composition 1 is conventionally achieved using a conductive adhesive agent. Next, as shown in Fig. 13, the PTC thermistor sheet 6 thus fabricated is cut into small thermistor chips 7 of the desired form. Finally, to both the upper and lower electrode 2a, 3a of each thermistor chip, a respective lead 4, 5 is soldered or spot welded, thereby establishing an electrical connection between lead wire 4, 5 and the electrodes 2a, 3a, whereby the PTC thermistor S0 shown in Fig. 11 is fabricated. With the type of PTC thermistor SO shown in Fig. 11 and for the fabrication method thus described, several problems exist. These problems include the following : 1. It is necessary to prepare the leads 4, 5 from a separate metal sheet or metal wire from that used for the electrodes 2a, 3a. 2. A manufacturing process of connecting the leads 4, 5 to the electrodes 2a, 3a is necessary. 3. Application of heat and pressure to the thermistor chips 7 occurs when the leads 4, 5 are connected by soldering or spot welding. In particular, there is always the possibility that the added heat will deleteriously effect the PTC composition, for example resulting in change in the resistance properties of the composition, deterioration of the composition, weakening of the bond with the electrodes, etc.. 4. Variability in the quality of the electrical and physical connection between the leads 4, 5 and the electrodes 2a, 3a is likely to occur which also impairs the performance of the finished thermistor. EP-A-0 026 456 describes a PTC heat element comprising a PTC block sandwiched between two metal plates maintained against the opposite surfaces of the block by applied pressure. The metal plates overhang beyond the contours of the PTC block. US Patent No 4,327,351 relates to a laminate comprising a PTC conductive polymer sandwiched between two electrodes having protruding edge portions. The laminate is manufactured by feeding tape electrodes and heat-softened polymer through an opening defined by a set of rollers. Summary of the Invention In consideration of the above, it is an object of the present invention to provide a method of manufacturing PTC thermistors having simplified physical structures for which the electrical properties are consistent and can be selected to meet design requirements. The present invention provides a manufacturing method for PTC thermistors, the method including the steps of: (a) preparing a pair of electrode plates each having a contact surface, said contact surface having a lead portion and a non-lead portion; (b) fitting said pair of electrode plates into an injection mould so that said pair of electrode plates lie parallel with respect to one another and so that said contact surface of each said electrode plate faces and substantially overlaps the contact surface of the other electrode plate, said injection mould being such that said injection mould defines at least one hollow space between the facing contact surfaces of the two electrode plates, the portion of each electrode plate adjacent to said hollow space constituting a non-lead portion of said electrode plate, said injection mould occupying at least a portion of the area between said pair of electrode plates and adjacent to at least one electrode plate, the portion of said at least one electrode plate adjacent to said occupied area constituting at least one lead portion of one contact surface of said at least one electrode plate; (c) heating a PTC composition to a temperature above a solidification temperature of said PTC composition and below a thermal degradation temperature of said PTC composition, and maintaining said PTC composition at said temperature; (d) injecting said heated PTC composition into said hollow space in said injection mould between said non-lead portions of said contact surfaces of said pair of electrode plates; (e) allowing said injection mould, pair of electrode plates and injected PTC composition to cool, thereby forming a section of solid PTC composition between said pair of electrode plates, said section of solid PTC composition having at least two contact surfaces, so that the non-lead portions of the contact surface of each of said pair of electrode plates come to be bonded to a respective contact surface of said section of PTC composition, whereby a PTC thermistor is manufactured for which at least one lead portion of at least one of said electrode plates is caused to extend beyond said section of PTC composition. The present invention also provides a PTC thermistor made by the latter process. For PTC thermistors fabricated according to the method of the present invention, both electrodes of the PTC thermistor which are formed from corresponding metal sheets (or other suitable materials) have extensions integrally formed therein which function as electrical leads. Accordingly, it is possible to eliminate the need for separately prepared and attached electrical leads connected with the electrodes, and the above described problems associated therewith. Brief Description of the Drawings Fig. 1 is a schematic structural drawing illustrating an example of a first type of PTC thermistor which may be made by the method of the present invention. Fig. 2 is a schematic structural drawing illustrating an example of a second type of PTC thermistor which may be made by the method of the present invention. Fig. 3 is a schematic structural drawing illustrating an example of a third type of PTC thermistor which may be made by the method of the present invention. Figs. 4 and 5 are schematic structural drawings illustrating different examples of a fourth type of PTC thermistor which may be made by the method of the present invention. Fig. 6 is a schematic structural drawing illustrating an example of a fifth type of PTC thermistor which may be made by the method of the present invention. Figs. 7 through 9 are schematic structural drawings illustrating examples of a sixth type of PTC thermistor which may be made by the method of the present invention. Fig. 10 is an oblique view illustrating the manufacturing method according to the present invention. Fig. 11 is a schematic structural drawing illustrating an example of a conventional PTC thermistor. Figs. 12 and 13 are oblique views showing steps of a conventional manufacturing method for PTC thermistors. Detailed Description of the Preferred Embodiment In the following sections, preferred types of PTC thermistors which may be made by the manufacturing method of the present invention, together with the manufacturing method itself, will be described in detail with reference to the drawings. First of all, a first preferred type of PTC thermistor will be described with reference to Fig. 1. In Fig. 1, a schematic structural drawing illustrating an example of a PTC thermistor S1 in accordance with the first preferred type is shown. As can be seen in the drawing, the PTC thermistor S1 is made up of a block of PTC composition 101 which demonstrates positive thermal coefficient properties, sandwiched between two electrodes 102, 103. The block of PTC composition 101 is formed so as to have two opposing surfaces which have an equal and substantially greater surface area than that of any of the other surfaces of the block of PTC composition 101. These two surfaces having the greatest surface area are the surfaces which contact the electrodes 102, 103. The PTC thermistor S1 shown in Fig. 1 differs from the conventional PTC thermistor S0 shown in Fig. 11 in that, for the PTC thermistor S1 shown in Fig. 1, the surface area of one side of each of the electrodes is greater than the surface area of the surface of the block of PTC composition 101 with which it is in contact. Thus, a portion of each electrode 102, 103 extends beyond the edges of the block of PTC composition 101, the extending portion of each electrode thereby forming a respective lead portion 104, 105. As mentioned above, the block of PTC composition 101 is formed from a PTC composition which demonstrates positive thermal coefficient properties. This PTC composition may be an organic substance. As an example, the PTC composition may be formed from a resin composite material including a resin matrix in which carbon black or some similar substance which is an electrical conductor is dispersed. The electrodes 102, 103 of the present invention as well as the lead portions 104, 105 formed thereof are fabricated from a metal which is a good electrical conductor, for example, nickel or copper sheet material. Additionally, the electrodes 102, 103 and leads 104, 105 may be fabricated from a thin layer of highly conductive metal leaf applied to a base plate formed from an insulating material. Other examples include grid electrode material, mesh electrode material, or braided electrode material. Furthermore, suitably conductive non-metallic materials may be applied as well. For the purposes of the present invention, the term "contact portion" of the electrode means the portion of the electrode 102, 103, a substantial portion of which is in contact with the block of PTC composition and the term "lead portion" means a portion of the electrode which is free from contact with the block of PTC composition. Typically, the lead portion of the electrode extends beyond the periphery of the block of PTC composition with which the electrode is in contact. For the purposes of the present invention, the term "single continuous electrode having a lead portion integrally formed with a contact portion" means an electrode such as illustrated in Fig. 1 (as well as in the other types of PTC thermistor illustrated in Figs. 2 to 9) wherein the electrode is formed from a sheet comprising a contact portion and at least one extension integrally formed therewith which functions as a lead portion. Thus, the single continous electrode having a lead portion integrally formed with a contact portion can be formed without the need for a separately prepared and attached electrical lead connected to a contact portion as is necessary for the conventional PTC thermistor described in conjunction with Fig. 11. For purposes herein, the lead portions 4, 5 of the conventional thermistor of Fig. 11 are not deemed "integrally formed" with the electrodes 2a, 3a since they are formed from separately prepared and attached conductive materials. The lead portions of the devices made by the method of the present invention provide that the devices can be connected to wires or other components of electrical systems using known techniques such as solder, conductive adhesives, mechanical means or other techniques without encountering the problems associated with the prior art devices. In Fig. 2, a schematic structural drawing illustrating an example of a PTC thermistor S2 in accordance with this second preferred type is shown. The PTC thermistor S2 shown in Fig. 2, differs from the PTC thermistor S1 of the first type shown in Fig. 1 in that, for the PTC thermistor S2, only a portion of each of the electrodes 202, 203 extends beyond the edges of the block of PTC composition 201, thereby forming leads or lead portions 204, 205 as tongue-like projections, each extending from an edge of its respective electrode 202, 203. By forming the electrodes 202, 203 with the above mentioned tongue-like projections, the manufacturing steps can be considerably simplified. Furthermore, with this kind of structure, connecting the PTC thermistor S2 with other components within an electrical circuit is much simplified. Both the contact portions of the electrodes 202, 203 and the lead portions 204, 205 have been shown in Fig. 2 as having a square or rectangular shape. The present type of thermistor is not so limited, however, and both the contact portions of the electrodes 202, 203 and the leads 204, 205 can be of any desired outline. The contact portions of the electrodes 202, 203, for example may be semicircular in shape with their respective lead portions 204, 205 extending from the flat side of the semicircle outline. In Fig. 3, a schematic structural drawing illustrating a PTC thermistor S3 in accordance with a third preferred type is shown. The PTC thermistor S3 shown in Fig. 3, differs from the PTC thermistor S2 of the second type shown in Fig. 2 in that, for the PTC thermistor S3, the portion of each of the electrodes 302, 303 extending beyond the block of PTC composition 301, thereby forming the lead portions 304, 305, is considerably wider than the lead portions 204, 205 of the PTC thermistor S2, so that the lead portions 304, 305 are the same width as the side of the respective electrodes 302, 303 from which they project. In Figs. 4 and 5, schematic structural drawings illustrating two examples of a PTC thermistor S4, PTC thermistor S4a and PTC thermistor S4b, in accordance with this fourth preferred type are shown. The PTC thermistors S4a, S4b shown in Figs. 4 and 5 respectively, differ from the PTC thermistor S2 of the second type shown in Fig. 2 in that, for the PTC thermistor S4a shown in Fig. 4, the lead portions 404, 405 extend from adjacent sides of the PTC thermistor S4a from the contact portions of their respective electrodes 402, 403, and are thus perpendicular to each other. In the case of the PTC thermistor S4b shown in Fig. 5, the lead portions 404, 405 extend from opposing sides of the PTC thermistor S4b from the contact portions of their respective electrodes 402, 403, and are thus parallel. With a structure in which the leads project from different sides of the PTC thermistor, as is the case with the PTC thermistors S4a and S4b of the present type, connecting the PTC thermistors S4a and S4b with other components within an electrical circuit is even further simplified compared with the previously-described types of PTC thermistors. In Fig. 6, a schematic structural drawing illustrating a PTC thermistor S5 in accordance with a fifth preferred type is shown. The PTC thermistor S5 shown in Fig. 6 differs from the PTC thermistor S4b shown in Fig. 5 in that, for the PTC themistor S5, the block of PTC composition 501 as well as the contact portion of electrodes 502, 503 are circular shaped. By fabricating a PTC thermistor S5 in which the block of PTC composition 501 and the contact portion of electrodes 502, 503 are circular or ellipse shaped, it becomes possible to pack the PTC thermistor S5 and surrounding components in an electrical circuit more densely, and thus facilitates practical applications of the device where a compact design is desirable. In Figs. 7 to 9, schematic structural drawings illustrating a PTC thermistor S6, S7, and S8 in accordance with a sixth preferred type which may be made by the method of the present invention are shown. The PTC thermistors S6, S7 and S8 of the sixth type are based on PTC thermistors S2 of the second type, and PTC thermistors S4a and S4b of the fourth type respectively. In each case, circular connection holes 608, 609 are provided in the distal portion of each tongue-like projecting lead portion 604, 605 of each PTC thermistor. The connection holes 608, 609 are provided to facilitate connections with wires and other components in an electrical circuit, using solder, screws, rivets, etc.. In the following section, there will be described the manufacturing method according to the present invention, by which the PTC thermistors of any of the above-described six types can be fabricated. In the manufacturing method according to the present invention, a block of PTC composition 701 is fabricated from PTC composition exhibiting positive temperature coefficient properties. The block of PTC composition 701 is formed so as to have two opposing surfaces which have an equal and substantially greater surface area than that of any of the other surfaces of the block of PTC composition 701. As illustrated in Fig. 10, this block of PTC composition 701 is sandwiched between two electrodes 702, 703 so that each electrode 702, 703 is in contact with one of the two surfaces of the block of PTC composition 701 having the greatest surface area. It should be noted that to alter certain electrical and/or physical characteristics in accordance with the present invention, the electrodes can alternately be placed in contact with surfaces of the PTC composition other than those having the greatest surface area. By using electrodes 702, 703 which have a larger footprint than does the surface of the block of PTC composition 701 which they contact, it is possible to manufacture any of the PTC thermistors of the first six preferred types by using an appropriately shaped block of PTC composition 701 and appropriately shaped electrodes 702, 703. According to the manufacturing method of the present invention, electrodes 702, 703 are fabricated so as to have a suitable shape and suitably large surface area as described above from a metal or other material which is a good electrical conductor, for example, copper sheet material. The electrodes 702, 703 may be fabricated from a thin layer of highly conductive metal leaf applied to a base plate formed from an insulating material. Other examples include grid electrode material, mesh electrode material, or braided electrode material. Furthermore, suitably conductive non-metallic materials may be applied as well. The PTC composition 701 is formed of a composite resin material. The electrodes 702, 703 are maintained in a fixed position having a desired gap therebetween and injection molding methods are used to directly extrude the PTC composition 701 between the electrodes 702, 703. This method forms the block of PTC composition 701 and achieves adhesion in one operation. For the various PTC thermistors manufactured by the manufacturing method of the present invention, the resistance properties of the respective PTC thermistors can be finely adjusted to meet design requirements. Thus for example, by varying the total volume of the block of PTC composition, or the total surface area of the PTC composition that is in contact with the electrode plates in the manufactured PTC thermistor, it is possible to vary the resistance and other electrical properties of the manufactured PTC thermistor. In the case of the thermistors of the sixth type as shown in Figs. 7, 8 and 9, holes were provided in the leads for facilitating connection to other components. It is perfectly acceptable to include an operation for drilling, chemically etching or otherwise forming this kind of hole as is known in the art in the manufacturing method of the present invention. While the thermistors and the manufacturing method therefor described herein have generally concerned PTC thermistors having two lead portions, it should be understood that it is not the intent of the inventors to exclude PTC thermistors having other than two lead portions. For example, for certain surface mounted applications, it could be feasible to employ a PTC thermistor having only one lead portion. Although the particular types of PTC thermistor discussed herein illustrate the lead portion of the electrode as being coplanar with the contact portion, it will be understood that according to the present invention, the lead portion need not be coplanar with the contact portion. The lead portion, so long as it is integrally formed with the contact portion, can be formed in a non-coplanar (e.g. bent) relationship with the contact portion. Alternatively, the lead portion, if originally integrally formed coplanar with the contact portion, also can be altered from a coplanar relationship with the contact portion, whether such alteration is accomplished before or after the electrode is joined to the PTC composition.

Claims: A manufacturing method for PTC thermistors, the method including the steps of: (a) preparing a pair of electrode plates each having a contact surface, said contact surface having a lead portion and a non-lead portion; (b) fitting said pair of electrode plates into an injection mould so that said pair of electrode plates lie parallel with respect to one another and so that said contact surface of each said electrode plate faces and substantially overlaps the contact surface of the other electrode plate, said injection mould being such that said injection mould defines at least one hollow space between the facing contact surfaces of the two electrode plates, the portion of each electrode plate adjacent to said hollow space constituting a non-lead portion of said electrode plate, said injection mould occupying at least a portion of the area between said pair of electrode plates and adjacent to at least one electrode plate, the portion of said at least one electrode plate adjacent to said occupied area constituting at least one lead portion of one contact surface of said at least one electrode plate; (c) heating a PTC composition to a temperature above a solidification temperature of said PTC composition and below a thermal degradation temperature of said PTC composition, and maintaining said PTC composition at said temperature; (d) injecting said heated PTC composition into said hollow space in said injection mould between said non-lead portions of said contact surfaces of said pair of electrode plates; and (e) allowing said injection mould, pair of electrode plates and injected PTC composition to cool, thereby forming a section of solid PTC composition between said pair of electrode plates, said section of solid PTC composition having at least two contact surfaces, so that the non-lead portions of the contact surface of each of said pair of electrode plates come to be bonded to a respective contact surface of said section of PTC composition, whereby a PTC thermistor is manufactured for which at least one lead portion of at least one of said electrode plates is caused to extend beyond said section of PTC composition.

Inventors: HAYASHI MORIO, ISSHIKI SETSUYA, KURODA MASAKAZU, KUROSAWA YUKIHIKO, YAMADA MAKOTO, HAYASHI, MORIO, ISSHIKI, SETSUYA, KURODA, MASAKAZU, KUROSAWA, YUKIHIKO, YAMADA, MAKOTO
Applicants: FUJIKURA LTD, FUJIKURA LTD.
Application year: 1990
Main classification: H01C1
IPC codes: H01C7, H01C1