PTC element and production process thereof

A method of manufacturing a PTC element comprising a pair of lead terminals bonded together by thermocompression with a matrix held therebetween comprises a matrix preparing step of preparing a matrix constructed by dispersing a conductive filler into a crystalline polymer; a terminal preparing step of preparing a pair of lead terminals holding the matrix therebetween, a surface of each lead terminal facing the matrix being formed with a plurality of anchor protrusions separated from each other; a flattening step of flattening the anchor protrusions formed in respective nonoverlapping areas in the pair of lead terminals kept from overlapping the matrix; and a thermocompression bonding step of holding the matrix between respective overlapping areas in the pair of lead terminals overlapping the matrix, and securing the pair of lead terminals and the matrix together by thermocompression bonding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a PTC (Positive Temperature Coefficient) element and a method of manufacturing the same.

2. Related Background Art

PTC elements have been known as elements for protecting circuit elements against overcurrents. The PTC elements are elements which drastically increase the positive temperature coefficient of their resistance value when reaching a specific temperature region. Known as one of such PTC elements is one disclosed in Patent Document 1 (Japanese Patent Publication No. HEI 5-9921).

SUMMARY OF THE INVENTION

In the PTC element disclosed in the above-mentioned Patent Document 1, a matrix having a positive resistance-temperature characteristic is constructed by a polymer and a conductive powder dispersed into the polymer, whereas a metal sheet having a roughened surface is bonded to the front face of the matrix such that the roughened surface comes into contact with the front face of the matrix, so as to be used as a terminal electrode. The surface in contact with the front face of the matrix is thus roughened in order to improve the bonding strength between the matrix and the metal sheet.

When the whole surface coming into contact with the front face of the matrix is roughened as in the PTC element disclosed in the above-mentioned Patent Document 1, however, the bonding strength may not fully be secured if the metal sheet acting as a terminal electrode is bonded to a connecting terminal such as external terminal by welding or soldering.

Therefore, it is an object of the present invention to provide a PTC element and a method of manufacturing the same which can improve the bonding strength when bonding a lead terminal extending from a matrix to another terminal.

The method of manufacturing a PTC element in accordance with the present invention is a method of manufacturing a PTC element comprising a pair of lead terminals bonded together by thermocompression with a matrix held therebetween, the method comprising a matrix preparing step of preparing a matrix constructed by dispersing a conductive filler into a crystalline polymer; a terminal preparing step of preparing a pair of lead terminals holding the matrix therebetween, a surface of each lead terminal facing the matrix being formed with a plurality of anchor protrusions separated from each other; a flattening step of flattening the anchor protrusions formed in respective nonoverlapping areas in the pair of lead terminals kept from overlapping the matrix; and a thermocompression bonding step of holding the matrix between respective overlapping areas in the pair of lead terminals overlapping the matrix, and securing the pair of lead terminals and the matrix together by thermocompression bonding.

In the present invention, the matrix is held between lead terminals having flattened the anchor protrusions formed in their nonoverlapping areas, and the lead terminals and the matrix are secured together by thermocompression bonding. Therefore, even when the matrix flows out to the nonoverlapping areas, for example, thus flowed-out part can easily be removed. Hence, the nonoverlapping areas are flattened without substantially leaving the matrix, whereby the lead terminals can favorably be bonded to other terminals.

Preferably, in the method of manufacturing a PTC element in accordance with the present invention, the anchor protrusions formed in the nonoverlapping areas are flattened by crushing in the flattening step. Crushing the anchor protrusions can flatten the nonoverlapping areas without generating unnecessary remnants.

The PTC element in accordance with the present invention is a PTC element comprising a matrix constructed by dispersing a conductive filler into a crystalline polymer, and a pair of lead terminals bonded together by thermocompression with the matrix held therebetween; wherein each of the pair of lead terminals has an overlapping area overlapping the matrix and a nonoverlapping area kept from overlapping the matrix; wherein the overlapping area in each of the pair of lead terminals is formed with an anchor protrusion having a larger diameter part and a smaller diameter part on a side closer to a root than is the larger diameter part; and wherein the anchor protrusion is flattened by crushing in the nonoverlapping area in each of the pair of lead terminals.

The present invention can easily flatten nonoverlapping parts of the lead terminals kept from overlapping the matrix, and thus can provide a PTC element whose nonoverlapping parts leave no matrix. This can improve the bonding strength at the time of bonding the nonoverlapping parts to other terminals.

Preferably, in the present invention, the overlapping area has a thickness of 60 to 140 μm, the nonoverlapping area has a thickness of 50 to 120 μm, and the anchor protrusion has an average height of 5 to 40 μm. When the thickness of the overlapping area is greater than 140 μm, the lead terminals become so thick that the thermal compression bonding between the matrix and lead terminals may become insufficient, thereby weakening the connecting strength between the matrix and lead terminals. Therefore, in view of the flattening, it will be preferred if the nonoverlapping area has a thickness of 120 μm or less. When the thickness of the nonoverlapping area is less than 50 μm, the strength of the lead terminals themselves decreases. Therefore, in view of the flattening of the nonoverlapping areas, it will be preferred if the overlapping area has a thickness of at least 60 μm. When the average height of the anchor protrusion is less than 5 μm, the anchor effect between the matrix and lead terminals cannot fully be exhibited, whereby the connecting strength between the matrix and lead terminals becomes weaker. When the average height of the anchor protrusion is greater than 40 μm, the strength of the anchor protrusion itself decreases, whereby the anchor protrusion may drop out of the lead terminals at the time of thermocompression bonding to the matrix.

The above-mentioned present invention can flatten the respective nonoverlapping areas in a pair of lead terminals without leaving the matrix there, and thus can favorably bond the lead terminals to other terminals. This can improve the bonding strength when bonding lead terminals extending from the matrix to other terminals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The findings of the present invention will easily be understood in view of the following detailed description with reference to the accompanying drawings which are given by way of illustration only. Embodiments of the present invention will now be explained with reference to the accompanying drawings. When possible, the same parts will be referred to with the same numerals without repeating their overlapping descriptions.

A PTC element which is an embodiment of the present invention will be explained with reference toFIG. 1.FIG. 1is a perspective view of a PTC element1. The PTC element1is a polymer PTC element comprising a pair of terminal electrodes12,14(lead terminals) and a matrix10.

The pair of terminal electrodes12,14are made of Ni or an Ni alloy, while having a thickness of about 0.1 mm. The pair of terminal electrodes12,14are arranged such that they partly overlap each other. The matrix10is arranged between their opposing parts, whereby the pair of terminal electrodes12,14hold the matrix10therebetween by their respective surfaces12s,14s. Therefore, the pair of terminal electrodes12,14are formed with overlapping areas121,141which overlap the matrix10and nonoverlapping areas122,142which do not overlap the matrix10.

The matrix10is formed by dispersing a conductive filler into a crystalline polymer resin. An Ni powder and a polyethylene resin which is a thermoplastic resin are preferably used as the conductive filler and the crystalline polymer resin, respectively. The matrix10is bonded under heat and pressure to the pair of terminal electrodes12,14.

FIG. 2is a side view of the PTC element1shown inFIG. 1. As shown inFIG. 2, the surfaces12s,14sof the terminal electrodes12,14holding the matrix10therebetween are formed with a plurality of anchor protrusions16,20and a plurality of flattened protrusions18,22. The anchor protrusions16,20are formed in the overlapping areas121,141, whereas the flattened protrusions18,22are formed in the nonoverlapping areas122,142. For the sake of explanation, the anchor protrusions16,20and flattened protrusions18,22are illustrated relatively greater inFIG. 2. The actual anchor protrusions16,20and flattened protrusions18,22are minute protrusions having a size which is hard to recognize by eyes. The same holds in drawings which will be used in the following explanations.

FIG. 3shows an enlarged side view of the terminal electrode12shown inFIG. 2. As shown inFIG. 3, each of the plurality of anchor protrusions16formed in the overlapping area121has a larger diameter part161and a smaller diameter part162. The larger diameter part161is provided on the leading end side in the direction along which the anchor protrusion16extends from the terminal electrode12, and is formed such that its outer periphery taken normal to this direction is greater than that of the smaller diameter part162. The smaller diameter part162is provided on the side closer to the root of the anchor protrusion16than is the larger diameter part161. The forms of the larger diameter parts161and smaller diameter parts162may vary among the anchor protrusions16. The larger diameter parts161and smaller diameter parts162may also have irregular outer peripheral forms instead of regular forms such as circles and ellipses.

The adjacent anchor protrusions16are arranged such as to be separated from each other. Therefore, the matrix10enters depressions17which are formed between the anchor protrusions16, whereby the terminal electrode12and the matrix10are secured together. When the terminal electrode12and the matrix10are secured together without forming the anchor protrusions16, the terminal electrode12is secured to the matrix10insufficiently, whereby the connecting strength between the matrix10and the terminal electrode12becomes extremely weak.

As shown inFIG. 3, each of the plurality of flattened protrusions18formed in the nonoverlapping area122has a larger diameter part181and a smaller diameter part182. The larger diameter part181is provided on the leading end side in the direction along which the flattened protrusion18extends from the terminal electrode12, and is formed such that its outer periphery taken normal to this direction is greater than that of the smaller diameter part182. The leading end of the larger diameter part181is formed with a flat surface181a. The smaller diameter part182is provided on the side closer to the root of the flattened protrusion18than is the larger diameter part181. The forms of the larger diameter parts181and smaller diameter parts182may vary among the flattened protrusions18. The larger diameter parts181and smaller diameter parts182may also have irregular outer peripheral forms instead of regular forms such as circles and ellipses.

The adjacent flattened protrusions18are arranged in contact with each other. The flattened surfaces181aof the flattened protrusions18continue with each other, thereby forming a substantially flat surface. Therefore, the matrix10does not substantially enter depressions19formed between the flattened protrusions18. Nevertheless, the flattened protrusions18are not completely in contact with each other, but may be separated from each other to such an extent that the bonding strength at the time of bonding the terminal electrodes12,14to other terminals is not substantially affected thereby.

Though a substantially flat surface is made by forming the flattened protrusions18in contact with each other in the nonoverlapping areas122,142in this embodiment, the embodiment is not limited to the one mentioned above as long as a substantially flat surface can be formed thereby. For example, the nonoverlapping areas122,142may be flattened by cutting or grinding.

A method of manufacturing the above-mentioned PTC element1will now be explained mainly with reference toFIG. 4, andFIGS. 5 to 8when necessary.FIG. 4is a view showing a procedure of the method of manufacturing the PTC element1in accordance with this embodiment.FIGS. 5 to 8are views showing the states of the terminal electrode12and matrix10under magnification in respective steps of the manufacturing method. As shown inFIG. 4, the method of manufacturing the PTC element1comprises a matrix preparing step (step S03), a terminal preparing step (step S02), a flattening step (step S03), and a thermocompression bonding step (step S04).

In the matrix preparing step (step S03), a matrix material to become the matrix10(seeFIGS. 1 to 3) is made and prepared. First, an Ni powder to become a conductive filler and polyethylene to become a matrix resin are kneaded, so as to form a block. This block is pressed into a disk, which is then cut, so as to yield a matrix material.

In the subsequent terminal preparing step (step S02), metal sheets to become the terminal electrodes12,14(seeFIGS. 1 to 3) are made and prepared. The surfaces12s,14sby which the terminal electrodes12,14(seeFIGS. 1 to 3) hold the matrix10(seeFIGS. 1 to 3) therebetween are formed with the anchor protrusions16,20(seeFIGS. 1 to 3). The anchor protrusions16,20are constructed by continuously forming the burl-shaped protrusions mentioned above. In the terminal electrode12, for example, the anchor protrusions16are formed in both of its overlapping area121and nonoverlapping area122as shown inFIG. 5. The same holds in the terminal electrode14, which is not depicted.

Returning toFIG. 4, in the flattening step (step S03), the anchor protrusions16,20formed in the nonoverlapping areas122,142(seeFIGS. 1 to 3) are flattened by crushing. In the terminal electrode12, for example, the anchor protrusions16formed in the nonoverlapping area122are crushed by a press, so as to yield the flattened protrusions18as shown inFIG. 6. The press moving amount in this case is 10 to 35 μm, more preferably 10 to 15 μm.

As mentioned above, the flattened protrusions18come into contact with each other, so as to be substantially flattened. In terms of the thickness of the terminal electrode, the average thickness of the nonoverlapping area122formed with the flattened protrusions18is smaller than that of the overlapping area121formed with the anchor protrusions16. The average thickness can be determined from the mass and specific gravity of a sample punched out by a predetermined area.

In this embodiment, for example, it will be preferred if the thickness after flattening is 60 to 140 μm, in the overlapping areas121,141, and 50 to 120 μm, in the nonoverlapping areas122,142. In this case, the average height of the anchor protrusions16,20is 5 to 40 μm. More preferably, the thickness after flattening is 95 to 100 μm, in the overlapping areas121,141, and 80 to 90 μm, in the nonoverlapping areas122,142. In this case, the average height of the anchor protrusions16,20is 5 to 20 μm.

When the thickness of the overlapping areas121,141is greater than 140 μm, the terminal electrodes12,14become so thick that the thermocompression bonding between the matrix10and terminal electrodes12,14may become insufficient, thereby weakening the connecting strength between the matrix10and terminal electrodes12,14. Therefore, in view of the flattening, it will be preferred if the nonoverlapping areas122,142have a thickness of 120 μm, or less.

When the thickness of the nonoverlapping areas122,142is less than 50 μm, the terminal electrodes12,14themselves decrease their strength, thereby bending in the nonoverlapping areas122,142and so forth, thus complicating their handling during and after their manufacturing process. Therefore, in view of the flattening of the nonoverlapping areas122,142, it will be preferred if the overlapping areas121,141have a thickness of at least 60 μm.

When the average height of the anchor protrusions16,20is less than 5 μm, the anchor effect between the matrix10and terminal electrodes12,14cannot fully be exhibited, whereby the connecting strength between the matrix10and terminal electrodes12,14becomes weaker. When the average height of the anchor protrusions16,20is greater than 40 μm, the strength of the anchor protrusions16,20themselves decreases, whereby the anchor protrusions16,20may drop out of the terminal electrodes12,14at the time of thermocompression bonding to the matrix10.

Returning toFIG. 4, in the thermocompression bonding step (step S04), the pair of terminal electrodes12,14(seeFIGS. 1 to 3) hold the matrix material (matrix) therebetween by their respective overlapping areas121,141(seeFIGS. 1 to 3), and the pair of terminal electrodes12,14(seeFIGS. 1 to 3) and the matrix10(seeFIGS. 1 to 3) are secured together by thermocompression bonding.

More specifically, as shown inFIG. 7, the terminal electrodes12and14(not depicted inFIG. 7) flattened in step S03hold therebetween the matrix material M prepared by step S03. At that time, the matrix material M is arranged so as to be held between the overlapping area121of the terminal electrode12and the overlapping area (not depicted inFIG. 7) of the terminal electrode14. Subsequently, the matrix material M is compressed by the terminal electrodes12and14while being heated, whereby the state shown inFIG. 8is obtained. Since the matrix material M flows out from the overlapping area121to the nonoverlapping area122as shown inFIG. 8, thus flowed-out part11is removed. Pressing may be effected either during or after the heating.

The above-mentioned method can yield the PTC element1in accordance with this embodiment. The anchor protrusions16,20are flattened by crushing in the flattening step, but may be flattened by cutting or grinding as well.

In this embodiment, the matrix material M (matrix10) is held between the terminal electrodes having flattened the anchor protrusions16,20formed in the nonoverlapping areas122,142, and the terminal electrodes12,14and the matrix10are secured together by thermocompression bonding. Therefore, even when the matrix material M (matrix10) flows out to the nonoverlapping areas122,142, for example, thus flowed-out part can be removed easily. Hence, the nonoverlapping areas122,142are flattened without leaving the matrix material M (matrix10), whereby the terminal electrodes12,14can favorably be bonded to other terminals by soldering or welding (spot welding in particular).