Patent Publication Number: US-2007103269-A1

Title: Surface-mounted thermistor and manufacturing method thereof

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a thermistor changing a resistance value depending on temperature, and more particularly to a surface-mounted thermistor being mounted on a PCB (printed circuit board) and having a function of protecting other circuit elements.  
      2. Description of the Related Art  
      Among conductive materials, some materials change their specific resistances depending on temperature. A resistance element made using such materials is generally called ‘a thermistor’, which are representatively classified into an NTC (negative temperature coefficient) resistance element whose resistance decreases according to the rise of temperature and a PTC (positive temperature coefficient) resistance element whose resistance increases according to the rise of temperature.  
      In particular, the PTC resistance element passes a current at a low temperature such as a normal temperature since its resistance is low, while it interrupts a current if a temperature of the material is increased due to an excess current or a surrounding temperature is increased because its resistance increases as much as 1,000 to 10,000 times of its origin state. Thus, the PTC resistance element is commonly used as an element being mounted on a PCB for protecting other circuit elements by restraining an excess current.  
      Meanwhile, since various circuit elements are mounted on the PCB, the thermistor suffers from many restrictions in its mounting location and structure, particularly in the trend of these days, as described below in brief.  
      The thermistor is generally configured so that electrodes are respectively laminated on upper and lower surfaces of a layer made of PTC material. The thermistor configured as above is mounted by soldering an electrode, formed on the lower surface of the thermistor, to an electrode pad previously formed on a PCB surface. At this time, a separate wire is required for connecting the electrode formed on the upper surface of the thermistor to the electrode pad on the PCB, which also needs a relevant procedure and a space on the PCB. Thus, in order to avoid such disadvantages, the electrode on the lower surface of the thermistor is formed not on the entire area of the lower surface but on a region except a partial portion, and a metal pattern separated from the electrode on the lower surface is formed in the partial portion. The metal pattern and the upper electrode are electrically connected through a side of the PTC material layer. Then, when the thermistor is mounted on the PCB, the electrode and the metal pattern on the lower surface of the thermistor may be soldered to respective electrode pads, thereby not requiring a separate wire or space.  
      However, such a thermistor has some problems as follows.  
      First, a so-called Tombstone or Manhattan phenomenon occurs. When a thermistor is mounted on the PCB, the thermistor in which a solder is coated on the metal pattern and the electrode on the lower surface thereof is arranged on the electrode pad, and then the solder is reflowed by heating. Due to the applied heat, the PTC material and the electrode material of the thermistor are expanded. However, they have different thermal expansion coefficients, and in particular the thermistor having a configuration that the metal pattern and the electrode on the upper surface are connected through a side of the PTC material layer is structurally asymmetric, so right and left stresses are not uniform, thereby making the thermistor become inclined on the plane of PCB. As a result, physical and electrical reliability of the solder is significantly deteriorated. In order to lessen these problems, U.S. Pat. No. 6,380,839 suggested a thermistor structure in which thermal stress relief areas are formed in the electrodes on the upper and lower surfaces, but it does not give a fundamental solution.  
      In addition, the connection between the electrode on the upper surface and the metal pattern on the lower surface through a side of the PTC material layer drops physical and electric reliability. That is to say, the side of the PTC material layer is seriously influenced by an expansion pressure of PTC materials due to the heat applied during the solder reflow and due to the temperature increased while the thermistor is used, and if such a stress makes a crack in the connection portion formed on the side of the PTC material layer, this crack is propagated along the side of the PTC material layer, thereby probably cutting the electric connection.  
      In the prior art including the above U.S. patent, the thermistor is manufactured as follows. First, a plurality of long slits are formed in parallel on a sheet configured that metal films such as aluminum foils are coated on both sides of a PTC material layer, and then the electrodes or the metal patterns on the upper and lower surfaces are electrically connected through the slits. This slit becomes the aforementioned side of the PTC material layer later. A desired electrode pattern is formed between the slits, and then the processes such as solder resist coating and solder coating are conducted to successively make a plurality of thermistors between the slits. Finally, the sheet is cut in a direction perpendicular to the slits, thereby mass-producing thermistors of the same configuration.  
      However, if such long slits are formed in the sheet, a portion between the slits may droops down due to the gravity during the manufacturing process or be twisted due to the heat generated in the process. This results in that the pattern is inexactly formed in the electrode patterning process or the solder resist coating process, thereby increasing a failure rate.  
     SUMMARY OF THE INVENTION  
      The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a thermistor having an excellent physical and electric reliability and ensuring smooth flow of current at a normal temperature.  
      Another object of the present invention is to provide a thermistor configured to be capable of giving a fundamental solution to the aforementioned Tombstone phenomenon and ensuring a connection portion of electrodes on upper and lower surfaces through a side of a thermistor material layer to endure a crack well.  
      Another object of the present invention is to provide a method for manufacturing a thermistor, which enables mass production of thermistors without causing twist or increasing a failure rate during its procedure.  
      In order to accomplish the above object, a thermistor according to the present invention is configured so that electrode patterns formed on each of upper and lower surfaces of a film resistance element are separated but engaged to each other with a non-conductive gap interposed therebetween, so the Tombstone phenomenon caused by a asymmetric structure is fundamentally eliminated.  
      That is to say, a thermistor according to an aspect of the invention includes a film resistance element changing a resistance depending on temperature; first and second electrodes formed on one surface of the film resistance element so as to be electrically separated from each other; third and fourth electrodes formed on the other surface of the film resistance element opposite to one surface so as to be electrically separated from each other; a first connection portion for electrically connecting the first electrode and the third electrode; and a second connection portion for electrically connecting the second electrode and the fourth electrode, wherein the first and second electrodes have shapes symmetrically engaged to each other with a non-conductive gap interposed therebetween, while the third and fourth electrodes have shapes symmetrically engaged to each other with a non-conductive gap interposed therebetween.  
      Preferably, the first and second electrodes have a pattern rotationally symmetric to a pattern of the third and fourth electrodes so that the pattern on the surface of the thermistor is coincided when the thermistor is turned over.  
      In addition, the non-conductive gaps between the first and second electrodes and between the third and fourth electrodes may have various shapes such as crank, rectangular wave, zigzag or wave.  
      Meanwhile, a thermistor according to another aspect of the invention is configured so that electrode patterns formed one of both surfaces of the film resistance element are symmetrically engaged as mentioned above, and a connection portion for connecting the electrode on both surfaces surrounds a part of, not all of, the side of the film resistance element so as to prevent a crack of the connection portion, if happens, from being propagated.  
      That is to say, the thermistor according to this aspect of the invention includes a film resistance element having grooves on one side and the other side opposite to one side, the film resistance element changing a resistance depending on temperature; first and second electrodes having shapes symmetrically engaged to each other on one surface of the film resistance element with a non-conductive gap interposed therebetween; third and fourth electrodes formed on the other surface of the film resistance element opposite to one surface so as to be electrically separated from each other; a first connection portion for electrically connecting the first electrode and the third electrode with surrounding one side except the groove formed in the film resistance element; and a second connection portion for electrically connecting the second electrode and the fourth electrode with surrounding the other side except the groove formed in the film resistance element.  
      On the other hand, in order to accomplish the above object, the present invention provides a method for manufacturing a thermistor, in which a plurality of elongated through holes are formed in a film resistance sheet in a matrix shape, and the thermistor is formed in each region between the through holes.  
      That is to say, the method for manufacturing a thermistor according to one aspect of the invention includes preparing a sheet configured that metal films are laminated on both surfaces of a film resistance element which changes a resistance depending on temperature; forming a plurality of through holes having a predetermined width and a length longer than the width in the sheet in a matrix pattern; electrically connecting the metal films laminated on both surfaces of the sheet through a side wall of the through hole; forming an electrode pattern by patterning the metal film; and cutting the sheet on which the electrode pattern is formed so that a region between adjacent through holes in a width direction of the through hole becomes one unit.  
      In addition, the object of the present invention may be obtained in a way of forming circular or oval through holes in a film resistance sheet, electrically connecting electrodes on both sides through the through hole, and then making the electrode patterns on both sides be rotationally symmetric.  
      That is to say, the method for manufacturing a thermistor according to another aspect of the invention includes preparing a sheet configured that metal films are laminated on both surfaces of a film resistance element which changes a resistance depending on temperature; forming a plurality of circular or oval through holes in the sheet in a matrix pattern; electrically connecting the metal films laminated on both surfaces of the sheet through a side wall of the through hole; forming an electrode pattern by patterning the metal films; and cutting the sheet in which the electrode pattern is formed so that a region between adjacent through holes in one direction along the arrangement of the through holes becomes one unit, wherein, in the electrode pattern forming step, for each metal film formed on both surfaces of the sheet, the metal film is partially removed in each region between adjacent through holes in one direction so as to make the electrode pattern into two parts which are separated but symmetrically engaged to each other and to make the electrode pattern coincided when the sheet is turned over. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other objects and aspects of the present invention will become apparent from the following description of embodiments with reference to the accompanying drawing in which:  
       FIG. 1   a  is a perspective view showing a thermistor according to an embodiment of the present invention;  
       FIG. 1   b  is a plane view showing an electrode pattern of the thermistor shown in  FIG. 1   a;    
       FIG. 2   a  is a perspective view showing a thermistor according to another embodiment of the present invention;  
       FIG. 2   b  is a sectional view of the thermistor shown in  FIG. 2   a , taken along the B-B line;  
       FIG. 3  is a plane view showing a thermistor according to still another embodiment of the present invention;  
       FIGS. 4   a  to  4   c  are plane views showing various examples of the electrode pattern applied to the embodiments of the present invention;  
       FIGS. 5   a  and  5   b  are sectional views taken along the  5 - 5  line of  FIG. 4   a ;  
       FIGS. 6   a  to  6   g  respectively depict plane view and partial sectional view together for illustrating a method for manufacturing a thermistor according to an embodiment of the present invention;  
       FIG. 7  is a plane view for illustrating a method for manufacturing a thermistor according to another embodiment of the present invention; and  
       FIG. 8  is a plane view for illustrating a method for manufacturing a thermistor according to still another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Hereinafter, preferred embodiments of the present invention will be described in detail referring to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.  
       FIG. 1   a  is a perspective view showing a thermistor according to an embodiment of the present invention, and  FIG. 1   b  is a plane view showing a pattern of an electrode  20  of the thermistor  100  shown in  FIG. 1   a.    
      Referring to  FIGS. 1   a  and  1   b , the thermistor of the present invention includes a film resistance element  10  changing its resistance depending on temperature, electrodes  20  respectively laminated on the upper and lower surfaces of the resistance element in a predetermined pattern, connection portions  30  for electrically connecting the electrodes on the upper and lower surfaces, solder resists  40  for covering the upper and lower surfaces of the thermistor, and solders  50  formed to surrounding both sides of the thermistor.  
      To describe the thermistor in more detail, the resistance element  10  is made of polymer in which conductive particles are dispersed to give PTC or NTC characteristics electrically. The polymer may adopt polyethylene, polypropylene and ethylene/propylene polymers, and the conductive particles may adopt carbon black or other metal particles.  
      The electrodes  20  formed on the upper and lower surfaces of the resistance element  10  are made of metal such as aluminum, copper or copper alloy, and the electrodes  20  are patterned in a predetermined pattern. The electrode pattern formed on the upper surface of the resistance element  10  is separated into two parts but engaged to each other with a non-conductive gap of a crank shape interposed therebetween, as shown in  FIG. 1   b . In addition, the electrode pattern formed on the lower surface of the resistance element  10  is also similarly separated into two parts but engaged to each other with a non-conductive gap of a crank shape interposed therebetween. That is to say, the electrode pattern of this embodiment is formed in a rotationally symmetric pattern which gives a coincident pattern when the thermistor  100  is turned over. Thus, the Tombstone phenomenon is fundamentally eliminated, and the thermistor of this embodiment may be more easily mounted on PCB since there is no need to distinguish upper and lower surfaces of the thermistor.  
      The connection portions  30  are made of metal such as copper or its alloy, and formed in the same way as the pattern of the electrodes  20 . The connection portions  30  cover the electrodes  20  respectively and electrically connect the electrodes on the upper and lower surfaces with surrounding the sides of the thermistor  100 .  
      The solder resists  40  are formed upon the electrode pattern at center portions of the upper and lower surfaces of the thermistor  100  together with the non-conductive gaps  14 , respectively. Thus, in the solder forming process described later, the solder  50  is not formed at a region where the solder resist  40  is formed. Meanwhile, the perspective view of  FIG. 1   a  shows the solder resist  40  is partially cut away so that the electrode  20  and the connection portion  30  formed below the solder resist  40  are partially revealed, for easy understanding.  
      The solder  50  commonly made of Sn/Pb plating is formed in contact with the connection portion  30  to surround the side of the thermistor  100 . The solder  50  is soldered in contact with an electrode pad formed on PCB when the thermistor  100  is mounted on the PCB, thereby acting as a terminal for ensuring supply of electric current to the electrode  20  of the thermistor.  
      Meanwhile, on both sides of the thermistor, though not particularly limited to a specific shape, substantially semicircular grooves  72  are formed. The resistance element  10 , the electrode  20  and the connection portion  30  are exposed through the groove  72 . That is to say, the connection portion  30  is formed not on the entire side of the thermistor  100  but a region of the side except the groove  72 . Thus, though a crack is arisen in the connection portion  30  at the side of the thermistor  100  during a heat-applying process such as the solder reflow process or while the thermistor is in use, it is possible to prevent the crack from propagating through the entire side. In addition, the groove  72  may be useful for examining inferiority of the connection portion. That is to say, when the connection portion is examined through a vision instrument, an electrically-connected part having a big contrast to light and an electrically-unconnected part are positioned in one side, thereby facilitating the examination.  
      On the while, though the electrode pattern shown in  FIG. 1   b  is shaped so that two parts are engaged to each other with the non-conductive gap  14  of a crank shape interposed therebetween, the electrode pattern of this embodiment is not limited to that case. For example, various shapes such as a rectangular wave shape, a zigzag shape (or, a chopping wave shape) and a waved shape may be used, as shown in  FIGS. 4   a  to  4   c.    
      Here, it is important that the non-conductive gap has a width smaller than a thickness of the resistance element with keeping a horizontal and rotational symmetric shape of the electrode pattern. By adopting such a horizontal and rotational symmetric shape of the electrode pattern, the Tombstone phenomenon is fundamentally eliminated, and the smaller width of the non-conductive gap ensures smooth current flow Ig (see  FIGS. 5   a  and  5   b ) between adjacent electrode patterns on the upper or lower surface of the resistance element, thereby flowing sufficient current at a normal temperature. (that is, during normal operation of the thermistor).  
      In the end, in the fact that the electrode pattern is shaped so that a first electrode  22   a  and a second electrode  22   b  adjacent on the basis of the non-conductive gap  14  as a border are engaged to each other, the adjacent electrodes to which different voltages are applied configure a kind of resistor together with the resistance element. In addition, since the first and second electrodes are alternatively arranged on the basis of the border, when the electrodes are seen as a whole, the electrodes are configured like that a plurality of resistors having alternative polarities are arranged in parallel, thereby reducing overall resistance.  
      In addition, an amount of electric current flowing through the thermistor at a normal temperature is varied depending on a connection manner of the connection portion  30  which electrically connects the electrode  20  formed on the upper surface of the resistance element  10  and the electrode  20  formed on the lower surface. The connection portion  30  is formed on the side of the resistance element  10  as mentioned above, and some cases are taken into consideration in the following description: a case that the connection portion connects the electrodes formed on the upper and lower surfaces of the resistance element at both sides of the thermistor, seen from the figure, as for the electrode pattern shown in  FIG. 4   a ; and a case that the connection portion connects the electrodes formed on the upper and lower surfaces of the resistance element at the upper and lower surfaces of the thermistor, seen from the figure.  
       FIGS. 5   a  and  5   b  are sectional views schematically showing current flows in those cases. In  FIG. 5   a , the connection portion  30  connects the first electrode  22   a  formed on the upper surface of the resistance electrode  10  and the third electrode  22   c  formed on the lower surface as well as the second electrode  22   b  formed on the upper surface and the fourth electrode  22   d  formed on the lower surface, at both sides of the thermistor in  FIG. 4   a . In this case, if voltage is applied to the first and second electrodes  22   a  and  22   b  (strictly speaking, voltage is applied to the third and fourth electrodes  22   c  and  22   d  formed on the lower surface of the thermistor to be mounted on PCB), an electric current Ig is flowed through a surface layer of the resistance element between the adjacent electrodes (or, through the non-conductive gap) formed on the upper and lower surfaces of the resistance element  10 , and at the same time an electric current Ir is also flowed between the opposite electrodes on the upper and lower surfaces with the resistance element  10  interposed therebetween in a thickness direction of the resistance element  10 , as shown in  FIG. 5   a.    
      In  FIG. 5   b , the connection portion  30  connects the first electrode  22   a  formed on the upper surface of the resistance element  10  and the fourth electrode  22   d  formed on the lower surface as well as the second electrode  22   b  formed on the upper surface and the third electrode  22   c  formed on the lower surface. In this case, if voltage is applied to the first and second electrodes  22   a  and  22   b  (strictly speaking, voltage is applied to the third and fourth electrodes  22   c  and  22   d  formed on the lower surface of the thermistor to be mounted on PCB), an electric current Ig is flowed through a surface layer of the resistance element between the adjacent electrodes (or, through the non-conductive gap) formed on the upper and lower surfaces of the resistance element  10 . However, the opposite electrodes with the resistance element  10  interposed therebetween are already electrically connected through the connection portion  30  to have the same polarity, so an electric current does not flow in a thickness direction of the resistance element  10 . Thus, the connection structure of  FIG. 5   b  has limited paths for electric current, compared with that of  FIG. 5   a.    
       FIG. 2   a  is a perspective view showing a thermistor according to another embodiment of the present invention, and  FIG. 2   b  is a sectional view of  FIG. 2   a , taken along the B-B line. Now, configuration of the thermistor  200  according to this embodiment will be described with reference to  FIGS. 2   a  and  2   b , focused on differences to the thermistor  100  of the former embodiment.  
      The thermistor  200  of this embodiment is different from the former embodiment in the point of configuration of its sides and a connection portion  32 . That is to say, while the connection portion  30  of the former embodiment electrically connects the electrodes formed on the upper and lower surfaces through the sides of the thermistor  100  except the grooves  72  formed in the sides, the connection portion  32  of this embodiment electrically connects the electrodes  20  formed on the upper and lower surfaces through grooves  72  formed in the sides of the thermistor  200 . In addition, a solder  52  is accordingly formed only on the groove  72 . Other parts than the sides have the same configuration and are made of the same materials as the former embodiment.  
      According to this embodiment, like the former embodiment, configuration of the thermistor  200  is horizontally and rotationally symmetric, so the Tombstone phenomenon is fundamentally eliminated, and there is no need to distinguish upper and lower surfaces of a thermistor when the thermistor is mounted on PCB.  
      In addition, since the connection portion  32  is formed in the curved groove  72 , a crack, if happens, is hardly propagated. Moreover, since electrically connected parts and electrically unconnected parts are shown from the same side, it is easy to examine inferiority of the connection portion.  
      Meanwhile, the configuration of the thermistor according to the aforementioned embodiments may be variously modified. For example, though it is described that the same electrode pattern is formed on the upper and lower surfaces in the above embodiments so that the electrode pattern is coincided when the thermistor is turned over, the electrode patterns on the upper and lower surfaces may have different shapes. In addition, the electrode pattern on the lower surface which is to be mounted on PCB may have a simply-faced pattern as shown in  FIG. 3  as an example, not the aforementioned pattern in which right and left patterns are engaged to each other, in order that the thermistor may be more stably contacted with a larger area of the electrode pad formed on PCB.  
      In addition, though it is described in the former embodiments that the semicircular groove  72  is formed in the side of the thermistor  100  and  200 , the groove may be modified differently to have various shapes such as a semi-oval, “V”-type or rectangular shape. Moreover, though it is described in the former embodiments that the connection portion  30  and  32  electrically connects the electrodes on the upper and lower surfaces with surrounding the side except the groove  72  or the inner wall of the groove  72 , it is also possible to electrically connect the electrodes on the upper and lower surfaces through the entire planar side of the thermistor without forming the groove itself.  
      Furthermore, though it is described in the former embodiments that the connection portion  30  and  32  is formed to surround the side of the thermistor, other modifications may be possible. For example, it is possible to form a through hole which is vertically penetrating the resistance element  10  in the thermistor, and then form a connection portion through the through hole. In addition, it is also possible that, in the step of preparing a sheet on which the resistance element  10  and the metal films  20  (or, the electrodes) are laminated in the thermistor manufacturing procedure described later in connection with  FIG. 6   a , metal wires (not shown) contacted with the metal films  20  formed on the upper and lower surfaces of the resistance element are positioned between adjacent resistance elements, which is a polymer sheet, to form a connection portion. In a word, configuration and shape of the connection portion are not specially limited if the electrode patterns formed on the upper and lower surfaces of the thermistor are horizontally and rotationally symmetric to eliminate the Tombstone phenomenon.  
      Now, a method for mass production of the thermistor  100  and  200  described above according to the present invention will be described in detail.  
       FIGS. 6   a  to  6   g  are diagrams for illustrating the method for manufacturing the thermistor  100  (see  FIG. 1   a ) according to the first embodiment of the present invention, in each of which a plane view is located in a left region and a partial sectional perspective view taken along the C-C line is located in a right circle.  
       FIG. 6   a  shows a sheet of a film resistance element cut into a suitable size. The sheet is configured so that metal films  20 , which will be electrodes of the thermistor later, are laminated on both surfaces of an NTC or PTC resistance element  10  whose resistance value changes depending on temperature. This sheet may be made by electrolytic- or electroless-plating of a metal such as aluminum or copper onto upper and low surfaces of a polymer sheet in which conductive particles are dispersed, or by compressing metal foils on both surfaces of the polymer sheet. The sheet made in such a way is then cut into a suitable size and washed for easy handling in a following process.  
      Subsequently, as shown in  FIG. 6   b , a plurality of through holes  12  having predetermined width and length are perforated. The plurality of through holes  12  are arranged in longitudinal and lateral directions, that is, in a matrix pattern. A thermistor is formed in each region between adjacent through holes in a width direction of the through hole  12  (or, a longitudinal direction in the figure). Meanwhile, the through holes  12  are formed at regular intervals in a longitudinal direction, differently from the long slits successively formed in one direction in the conventional sheet. Thus, the sheet is not drooped down or twisted along a longitudinal direction of the through holes in the succeeding processes.  
      Then, as shown in  FIG. 6   c , copper or its alloy is plated on the entire surface of the sheet in which the through holes  12  are formed to electrically connect the metal films  20  on the upper and lower surfaces through the through holes  12 . The plating film  30  of copper or its alloy formed in such a way becomes the aforementioned connection portion.  
      Then, as shown in  FIG. 6   d , the metal film  20  and the plating film  30  formed in each region between the through holes  12  are patterned to form a predetermined electrode pattern and a predetermined connection portion pattern. Specifically, a photo resist is coated on the sheet of  FIG. 6c , and then exposed to light and developed into a desired pattern, and then the plating film  30  and the metal film  20  are subsequently etched with the formed photo resist pattern as an etching mask so that the resistance element  10  is exposed in a crank shape as shown in  FIG. 6   d . After that, the photo resist pattern is removed to form a sheet as shown in  FIG. 6   d.    
      In this procedure, an exposed region of the resistance element  10  becomes the aforementioned non-conductive gap  14 . At this time, though it is depicted and described that the non-conductive gap  14  has a crank shape, the shape of the non-conductive gap  14  may be changed into various shapes such as a rectangular wave shape, a zigzag shape and a waved shape as shown in  FIGS. 4   a  to  4   c . In addition, the non-conductive gaps  14  formed on the upper and lower surfaces of the sheet may have the same shape or different shapes.  
      Subsequently, as shown in  FIG. 6   e , a solder resist  40  is formed on a region, requiring insulation, on the upper and lower surfaces of the sheet together with the non-conductive gap  14 . After all, the solder resist  40  is formed on a region except an area on and near the through hole  12 . The solder resist may be formed in various ways such as screen printing, liquid photolithography, laminating of a dry film or the like.  
      Then, Sn/Pb plating is conducted on the sheet on which the solder resist  40  is already formed, thereby forming the solder  50  on the inner circumference of and near the through hole  12  except the region where the solder resist  40  is already formed as shown in  FIG. 6   f . This solder is to be acted as a terminal connected to the electrode pad on PCB by means of the solder reflow method when the thermistor is mounted on PCB later.  
      Finally, as shown in  FIG. 6   g , the sheet is cut along a cutting line  60 , thereby making each region between adjacent through holes in a width direction of the through hole  12  into a unit thermistor. At this time, a middle portion of the side wall of the through hole  12  is cut in a circular or oval shape  70 , thereby completing the thermistor having the groove  72  in its sides as shown in  FIG. 1   a . Cutting the sheet may be realized by means of a sawing process or a sheet metal working using a mould. Meanwhile, if the circular or oval shape perforating process is excluded in  FIG. 6   g , it is also possible to manufacture a thermistor configured that the electrodes on the upper and lower surfaces are connected through the entire sides of the thermistor.  
       FIG. 7  is for illustrating a method for manufacturing the thermistor  200  (see  FIG. 2   a ) according to the second embodiment of the present invention. The following description is focused on different factors to the thermistor manufacturing method of the first embodiment.  
      As mentioned above, the thermistor  200  of  FIG. 2   a  has a side configuration that the electrodes on the upper and lower surfaces are connected through the semicircle or semi-oval groove  72 , differently from the thermistor  100  of  FIG. 1   a . In order to manufacture such a thermistor  20 , circular or oval through holes  16  are formed in the sheet as shown in  FIG. 6   a , instead of the elongated through hole  12  of  FIG. 6   b , and then, after conducting the connection portion forming process, the electrode patterning process and the solder resist and solder forming process, the sheet is cut along the cutting lines  60  and  62  shown in  FIG. 7 .  
      Meanwhile, in the example of  FIGS. 6   g  and  7 , since the same non-conductive gaps  14  of a crank shape are formed in each area between the through holes  12  and  16 , the sheet is wasted as much as certain margins in upper and lower portions than the through hole, when seen from the figure. It is because the through holes are arranged at regular intervals, without forming long slits successive in a vertical direction, so that the sheet is not twisted during the thermistor manufacturing procedure.  
      As for the fact, the thermistor  200  of this embodiment may be configured so that through holes  16  are originally not successive, differently from the through holes  12  of  FIG. 6   g  which are resulted from discontinuous forming of long slits which are originally successive. Thus, like  FIG. 8 , instead of forming the non-conductive gaps  18  on a region where each thermistor is formed to have the same shape, it is possible to successively form gaps  18  by turning over the crank shape in a right or left direction with alternating in a vertical direction on the drawing, and then cut the sheet along the cutting lines  60  and  62 , thereby manufacturing the thermistor  200 . As a result, there is no margin between the cutting lines  60  which is to be wasted, thereby eliminating waste.  
      The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
     APPLICABILITY TO THE INDUSTRY  
      The thermistor of the present invention structurally has a symmetric shape, so it is possible to fundamentally prevent the Tombstone phenomenon caused by asymmetric structure. In addition, since the electrodes on the same surface of the thermistor to which different voltages are applied are arranged to be engaged to each other with the non-conductive gap interposed therebetween, a current flow is increased, thereby improving a resistance characteristic of the thermistor at a normal temperature.  
      Moreover, since the groove is formed in the side of the thermistor and the electrodes on the upper and lower surfaces are electrically connected through the groove or through the side except the groove, the thermistor of the present invention prevents a crack, if generated in the connection portion, from being propagated along the side of the thermistor, thereby capable of providing a thermistor with high reliability.  
      Furthermore, according to the method for manufacturing a thermistor, since the elongated through holes are arranged in the sheet in a matrix pattern longitudinally and laterally at regular intervals, it is possible to prevent the sheet from being twisted during the thermistor manufacturing procedure.