Patent Publication Number: US-9844950-B2

Title: Thermal head and thermal printer provided with same

Description:
TECHNICAL FIELD 
     The present invention relates to a thermal head and a thermal printer provided with the thermal head. 
     BACKGROUND ART 
     Various thermal heads have been proposed as printing devices for facsimile machines, video printers, and the like. For example, a known thermal head includes a substrate, a heat-generating portion disposed on the substrate, an electrode disposed on the substrate and electrically connected to the heat-generating portion, a driver IC disposed on the substrate and electrically connected to the electrode, and a covering member covering the driver IC (see, for example, PTL 1) 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 8-281990 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, with the thermal head described in PTL 1, a recording medium, such as thermal paper or the like, passes along a surface of the covering member, which covers the driver IC, while being in contact with the surface. Because the driver IC generates heat as the thermal head is driven, heat of the driver IC is conducted to the recording medium through the covering member, and it is probable that a printed image has a nonuniform density. 
     Solution to Problem 
     A thermal head according to an embodiment of the present invention includes a substrate, a heat-generating portion disposed on the substrate, an electrode disposed on the substrate and electrically connected to the heat-generating portion, a driver IC disposed on the substrate and electrically connected to the electrode, and a covering member covering the driver IC. In plan view, a center line of the driver IC extending in a main scanning direction is located farther than a top portion of the covering member from the heat-generating portion. 
     A thermal head according to another embodiment of the present invention includes a substrate, a heat-generating portion disposed on the substrate, an electrode disposed on the substrate and electrically connected to the heat-generating portion, a circuit board electrically connected to the electrode, a driver IC disposed on the circuit board and electrically connected to the electrode, and a covering member covering the driver IC. In plan view, a center line of the driver IC extending in a main scanning direction is located farther than a top portion of the covering member from the heat-generating portion. 
     A thermal printer according to an embodiment of the present invention includes the thermal head described above, a conveying mechanism that conveys a recording medium onto the heat-generating portion, and a platen roller that presses the recording medium against the heat-generating portion. 
     Advantageous Effects of Invention 
     With the present invention, the probability of heat of the driver IC being conducted to a recording medium can be reduced. As a result, the probability of occurrence of nonuniform density in an image printed by the thermal head can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view of a thermal head according to a first embodiment. 
         FIG. 2  is a cross-sectional view taken along line I-I of  FIG. 1 . 
         FIG. 3  is a schematic view illustrating a state in which the thermal head illustrated in  FIG. 1  is performing printing. 
         FIG. 4( a )  is an enlarged plan view of the vicinity of a covering member, and  FIG. 4( b )  is a cross-sectional view illustrating a contact state in which a recording medium is in contact with a covering member during printing. 
         FIG. 5  is a schematic diagram illustrating the structure of a thermal printer according to the first embodiment. 
         FIGS. 6( a ) and 6( b )  illustrate a thermal head according to a second embodiment,  FIG. 6( a )  is an enlarged plan view of the vicinity of a covering member, and  FIG. 6( b )  is a cross-sectional view illustrating a state in which a recording medium is in contact with the covering member during printing. 
         FIG. 7  is a plan view of a head base body of a thermal head according to a third embodiment. 
         FIG. 8  is a cross-sectional view taken along line II-II of  FIG. 7 . 
         FIG. 9  is a plan view of a thermal head according to a fourth embodiment. 
         FIG. 10( a )  is a cross-sectional view illustrating a state in which a recording medium is in contact with a covering member of a thermal head according to a fifth embodiment during printing, and  FIG. 10( b )  is a sectional view illustrating a state in which a recording medium is in contact with a covering member of a thermal head according to a modification of the thermal head of  FIG. 10( a )  during printing. 
         FIG. 11  is a cross-sectional perspective view of a thermal head according to a seventh embodiment. 
         FIG. 12  is a schematic view illustrating a state in which the thermal head illustrate in  FIG. 11  is performing printing. 
         FIGS. 13( a ) and 13( b )  illustrate a thermal head according to an eighth embodiment,  FIG. 13( a )  is an enlarged plan view of the vicinity of a covering member, and  FIG. 13( b )  is a cross-sectional view illustrating a state in which a recording medium is in contact with the covering member during printing. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Hereinafter, a thermal head X 1  will be described with reference to  FIGS. 1 to 4 . The thermal head X 1  includes a heat sink  1 , a head base body  3  disposed on the heat sink  1 , and a flexible printed circuit  5  (hereinafter, referred to as “the FPC  5 ”) connected to the head base body  3 . The FPC  5  is not illustrated in  FIG. 1 . Instead, a region in which the FPC  5  is disposed is represented by a chain line. Likewise, a protective layer  25  and a covering layer  27 , which are not illustrated, are represented by chain lines. 
     The heat sink  1  has a plate-like shape that is rectangular in plan view. The heat sink  1  is made of, for example, a metal material, such as copper, iron, or aluminum. The heat sink  1  has a function of dissipating a part of heat that is generated by heat-generating portions  9  of the head base body  3  and that does not contribute to printing. The head base body  3  is bonded to the upper surface of the heat sink  1  by using a double-sided tape, an adhesive, or the like (not shown). 
     The head base body  3  has a plate-like shape in plan view, and components of the thermal head X 1  are disposed on a substrate  7  of the head base body  3 . The head base body  3  has a function of performing printing on a recording medium (see  FIG. 3 ) in accordance with an electrical signal supplied from the outside. 
     The FPC  5  is electrically connected to the head base body  3  and includes an insulating resin layer and a plurality of printed wires patterned in the insulating resin layer. The FPC  5  is a circuit board having a function of supplying an electric current and an electrical signal to the head base body  3 . One end of each of the printed wires is exposed from the resin layer, and the other end of each of the printed wires is electrically connected to a connector  31 . 
     The printed wires of the FPC  5  are connected to connection electrodes  21  of the head base body  3  via a bonding material  23 . Thus, the head base body  3  and the FPC  5  are electrically connected to each other. Examples of the bonding material  23  include a solder and an anisotropic conductive film (ACF), which is composed of an electrically insulating resin and electrically conductive particles mixed in the resin. A reinforcing resin plate (not shown), which is made of a phenolic resin, a polyimide resin, a glass epoxy resin, or the like, may be disposed between the FPC  5  and the heat sink  1 . 
     In the example described above, the FPC  5 , which is flexible, is used as a printed circuit board. Instead, a hard circuit board may be used. Examples of a hard printed circuit board include a circuit board made from a resin substrate, such as a glass epoxy substrate or a polyimide substrate. 
     Hereinafter, each component of the head base body  3  will be described. 
     The substrate  7  is made of an electrically insulating material such as alumina ceramic, a semiconductor material such as single-crystal silicon, or the like. 
     A heat storage layer  13  is disposed on the upper surface of the substrate  7 . The heat storage layer  13  includes a base  13   a  and a protruding portion  13   b . The base  13   a  extends over the entire area of the upper surface of the substrate  7 . The protruding portion  13   b  extends in strip-like shape in the direction in which the plurality of heat-generating portions  9  are arranged, and has a substantially semi-elliptical cross section. The protruding portion  13   b  functions to appropriately press a recording medium, which is to be printed, against the protective layer  25  disposed on the heat-generating portions  9 . 
     The heat storage layer  13  is made of glass having low thermal conductivity and temporarily stores a part of heat generated by the heat-generating portions  9 . Therefore, the heat storage layer  13  can reduce the time required to increase the temperature of the heat-generating portions  9 , and functions to increase the thermal responsivity of the thermal head X 1 . The heat storage layer  13  is formed, for example, by applying a predetermined glass paste, which is obtained by mixing glass powder with an appropriate organic solvent, to the upper surface of the substrate  7  by using a known screen printing method or the like; and by firing the glass paste. 
     An electrically resistive layer  15  is disposed on the upper surface of the heat storage layer  13 . A common electrode  17 , individual electrodes  19 , and the connection electrodes  21  are disposed on the electrically resistive layer  15 . The electrically resistive layer  15  is patterned in the same shape as the common electrode  17 , the individual electrodes  19 , and the connection electrodes  21 . The electrically resistive layer  15  has exposed regions, in which the electrically resistive layer  15  is exposed, between the common electrode  17  and the individual electrodes  19 . As illustrated in  FIG. 1 , the exposed regions of the electrically resistive layer  15  are arranged in a row on the protruding portion  13   b  of the heat storage layer  13 , and the exposed regions serve as the heat-generating portions  9 . The plurality of heat-generating portions  9 , which are illustrated in a simplified manner in  FIG. 1  for convenience of description, are disposed, for example, at a density of 100 to 2400 dpi (dot per inch). 
     The electrically resistive layer  15  is made of, for example, a material having relatively high electric resistance, such as a TaN-based, TaSiO-based, TaSiNO-based, TiSiO-based, TiSiCO-based, or NbSiO-based material. Therefore, when a voltage is applied to the heat-generating portions  9 , the heat-generating portions  9  generate heat by Joule heating. 
     As illustrated in  FIGS. 1 and 2 , the common electrode  17 , the plurality of individual electrodes  19 , and the plurality of connection electrodes  21  are disposed on the upper surface of the electrically resistive layer  15 . The common electrode  17 , the individual electrodes  19 , and the connection electrodes  21  are made of any one of electroconductive metals, such as aluminum, gold, silver, and copper, or made of an alloy of such metals. 
     The common electrode  17  includes a main wiring portion  17   a , sub-wiring portions  17   b , and lead portions  17   c . The main wiring portion  17   a  is disposed so as to extend along a long side of the substrate  7 . The sub-wiring portions  17   b  are disposed so as to respectively extend along one short side and the other short side of the substrate  7  and are connected to the main wiring portion  17   a . The lead portions  17   c  are disposed so as to individually extend from the main wiring portion  17   a  toward the heat-generating portions  9  and connect the main wiring portion  17   a  and the heat-generating portions  9  to each other. One end of the common electrode  17  is connected to the plurality of heat-generating portions  9  and the other end of the common electrode  17  is connected to the FPC  5 . Thus, the common electrode  17  electrically connects the FPC  5  and the heat-generating portions  9  to each other. 
     One end of each of the individual electrodes  19  is connected to a corresponding one of the heat-generating portions  9  and the other end of each of the individual electrodes  19  is connected to one of driver ICs  11 . Thus, the individual electrodes  19  electrically connect the heat-generating portions  9  to the driver ICs  11 . The individual electrodes  19  divide the plurality of heat-generating portions  9  into a plurality of groups and electrically connect the heat-generating portions  9  in each group to one of the driver ICs  11  corresponding to the group. 
     One end of each of the connection electrodes  21  is connected to one of the driver ICs  11 , and the other end of each of the connection electrodes  21  is connected to the FPC  5 . Thus, the connection electrodes  21  electrically connect the driver ICs  11  and the FPC  5  to each other. The plurality of connection electrodes  21  connected to each of the driver ICs  11  include a plurality of wires having different functions. 
     As illustrated in  FIG. 1 , the driver ICs  11  are disposed on the substrate  7  so as to correspond to each group of the plurality of heat-generating portions  9 , and is connected to the other end of each of the individual electrodes  19  and the one end of each of the connection electrodes  21 . The plurality of driver ICs  11  are arranged in the main scanning direction. The driver ICs  11  have a function of controlling the state of an electric current applied to the heat-generating portions  9 . As each of the driver ICs  11 , a switching member including a plurality of switching elements may be used. 
     The electrically resistive layer  15 , the common electrode  17 , the individual electrodes  19 , and the connection electrodes  21  are formed, for example, by stacking material layers for these components successively on the heat storage layer  13  by using a known thin-film forming technique such as sputtering, and then processing the stacked body to have a predetermined pattern by using a known photoetching process or the like. The common electrode  17 , the individual electrodes  19 , and the connection electrodes  21  can be simultaneously formed by the same process. 
     As illustrated in  FIGS. 1 and 2 , the protective layer  25  is disposed on the heat storage layer  13  on the upper surface of the substrate  7 . The protective layer  25  covers the heat-generating portions  9 , a part of the common electrode  17 , and a part of the individual electrodes  19 . For convenience of description, the protective layer  25  is not illustrated in  FIG. 1 . Instead, a region in which the protective layer  25  is formed is represented by the chain line. 
     The protective layer  25  protects the covered areas of the heat-generating portions  9 , the common electrode  17 , and the individual electrodes  19  from corrosion due to adhesion of moisture or the like included in the atmosphere or from abrasion due to contact with a recording medium on which printing is to be performed. The protective layer  25  can be formed by using SiN, SiO, SiON, SiC, SiCN, diamond-like carbon, or the like. The protective layer  25  may include a single layer or multiple layers of such materials. The protective layer  25  can be formed using a thin-film forming technology, such as sputtering, or a thick-film forming technology, such as screen printing. 
     As illustrated in  FIGS. 1 and 2 , the covering layer  27  is disposed on the base  13   a  of the heat storage layer  13  on the upper surface of the substrate  7 . The covering layer  27  partially covers the common electrode  17 , the individual electrodes  19 , and the connection electrodes  21 . For convenience of description, a region in which the covering layer  27  is formed is represented by a chain line in  FIG. 1 . 
     The covering layer  27  protects the covered areas of the common electrode  17 , the individual electrodes  19 , and the connection electrodes  21  from oxidation due to contact with the atmosphere or from corrosion due to adhesion of moisture or the like included in the atmosphere. The covering layer  27  can be formed by using a resin material, such as an epoxy resin or a polyimide resin, and a thick-film forming technique such as screen printing. 
     Openings (not shown), for exposing the individual electrodes  19  and the connection electrodes  21  connected to the driver ICs  11 , are formed in the covering layer  27 . The individual electrodes  19  and the connection electrodes  21  are connected to the driver ICs  11  through the openings. 
     Referring to  FIGS. 1 to 4 , a covering member  29  will be described in detail. 
     The covering member  29  is disposed so as to cover the driver ICs  11  and is disposed so as to cover the entirety of the driver ICs  11 . The covering member  29  protects the driver ICs  11  by covering the driver ICs  11 . The covering member  29  also protects connection portions at which the individual electrodes  19  and the connection electrodes  21  are connected to the driver ICs  11 . 
     The covering member  29  has a first edge  29   b  and a second edge  29   c  extending in the main scanning direction. The first edge  29   b  of the covering member  29  is disposed closer to the heat-generating portions  9 , and the second edge  29   c  of the covering member  29  is disposed farther from the heat-generating portions  9 . 
     The covering member  29  has a rectangular shape with rounded corners in plan view, and has a semi-elliptical shape having a top portion  29   a  at the center thereof in cross-sectional view. The top portion  29   a  is a portion of the covering member  29  that is located farthest from the substrate  7  in the thickness direction of the substrate  7 . 
     As illustrated in  FIG. 4 , a center line L 1  of the covering member  29  extending in the main scanning direction (hereinafter, referred to as “the center line L 1 ”) passes through the top portion  29   a . A center line L 2  of each driver IC  11  extending in the main scanning direction (hereinafter, referred to as “the center line L 2 ”) is located farther than the top portion  29   a  of the covering member  29  from the heat-generating portions  9 . 
     The center line L 1  is a line that is equidistant from the first edge  29   b  and the second edge  29   c  and extends in the main scanning direction. The center line L 2  is a line that is equidistant from a pair of long sides of the driver IC  11  and extends in the main scanning direction. 
     As illustrated in  FIG. 3 , the recording medium P is conveyed in a conveying direction S while being in contact with a surface of the covering member  29 . To be specific, as illustrated in  FIG. 4( b ) , the recording medium P is conveyed on the top portion  29   a  of the covering member  29 , and heat of the driver IC  11  is conducted to the recording medium P through the covering member  29  while the recording medium P is conveyed on the covering member  29 . 
     In plan view, the thermal head X 1  has a structure in which the center line L 2  is located farther than the top portion  29   a  of the covering member  29  from the heat-generating portions  9 . Therefore, the distance between the top portion  29   a  of the covering member  29  and the driver IC  11  can be increased. 
     Thus, the volume of the covering member  29  located between the driver IC  11  and the recording medium P can be increased. As a result, the probability of heat of the driver IC  11  being conducted to the recording medium P can be reduced, and the probability of occurrence of nonuniform density on the recording medium P can be reduced. Accordingly, the probability of occurrence of nonuniform density in an image printed by the thermal head X 1  can be reduced. 
     On the thermal head X 1 , the recording medium P is conveyed in the conveying direction S from the driver IC  11  toward the heat-generating portions  9 . Therefore, the center line L 2  is disposed upstream of the top portion  29   a  of the covering member  29  in the conveying direction S. Accordingly, the probability of occurrence of nonuniform density in an image printed by the thermal head X 1  can be reduced. The recording medium P may be conveyed in the opposite direction. That is, the conveying direction S of the recording medium P may be a direction from the heat-generating portions  9  toward the driver IC  11 . 
     In the thermal head X 1 , the center line L 1  passes through the top portion  29   a  of the covering member  29 . That is, the top portion  29   a  is disposed on the center line L 1 . Thus, the covering member  29  has a shape that is gently curved from the top portion  29   a , which is on the center line L 1 , to the first edge  29   b  and the second edge  29   c , and the shape of the covering member  29  can be stabilized. 
     In plan view, the thermal head X 1  has a structure in which the entirety of each driver IC  11  is located farther than the top portion  29   a  of the covering member  29  from the heat-generating portions  9 . In other words, in the thermal head X 1 , in plan view, the driver IC  11  is not disposed below the top portion  29   a  of the covering member  29 . 
     Therefore, the volume of the covering member  29  located below the top portion  29   a  can be increased. Thus, the probability of heat of the heat-generating portions  9 , which has been conducted to the substrate  7 , being conducted to the recording medium P through the covering member  29  can be reduced. 
     As illustrated in  FIG. 4 , in a cross-sectional view seen in the main scanning direction, the thermal head X 1  has a structure in which, when a first distance La is defined as the distance from the center of gravity of the driver IC  11  to the top portion  29   a  and a second distance Lb is defined as the shortest distance from the center of gravity of the driver IC  11  to the surface of the covering member  29 , the second distance Lb is smaller than the first distance La. That is, a portion of the surface of the covering member  29  is disposed at a distance smaller than the first distance La from the center of gravity of the driver IC  11 . 
     Therefore, heat generated in the driver IC  11  is more easily dissipated from the surface of the covering member  29  than is conducted to the top portion  29   a . As a result, the amount of heat conducted to the top portion  29   a  can be reduced. 
     The center of gravity of the driver IC  11  is equidistant from the surface of the driver IC  11  and is the center of gravity when the shape of the driver IC is regarded as a rectangular parallelepiped. 
     The covering member  29  can be formed by using, for example, a resin material, such as an epoxy resin or a silicone resin. As the resin material, a thermosetting resin, a thermosoftening resin, a UV curable resin, or a two-part resin can be used. 
     The covering member  29  can be made, for example, by using the following method. 
     First, the common electrode  17 , the individual electrodes  19 , the connection electrodes  21 , and the heat-generating portions  9  are formed on the substrate  7 . Next, the protective layer  25  is formed on the heat-generating portions  9  by sputtering. Next, the covering layer  27  is formed by printing. Openings (not shown), in which the driver ICs  11  are to be disposed, are formed in parts of the covering layer  27 . The driver ICs  11  are disposed in the openings, and the driver ICs  11  are electrically connected to the individual electrodes  19  and the connection electrodes  21  by welding, ACF, or wire bonding. 
     Next, a resin material to become the covering member  29  is applied to each driver IC  11 , and the resin material is dried and cured by heat, thereby forming the covering member  29 . The resin material may be applied by printing by using a mask. 
     Next, a thermal printer Z 1  will be described with reference to  FIG. 5 . 
     As illustrated in  FIG. 5 , the thermal printer Z 1  according to the present embodiment includes the thermal head X 1  described above, a conveying mechanism  40 , a platen roller  50 , a power-supply device  60 , and a control device  70 . The thermal head X 1  is mounted on a mounting surface  80   a  of a mounting member  80  disposed in a case (not shown) of the thermal printer Z 1 . In  FIG. 5 , to facilitate understanding of the structure of the thermal printer Z 1 , the thermal head X 1  and the mounting member  80  are enlarged. 
     The conveying mechanism  40  includes a driving unit (not shown) and conveying rollers  43 ,  45 ,  47 , and  49 . The conveying mechanism  40  conveys a recording medium P, such as heat-sensitive paper or image-receiving paper onto which ink is to be transferred, in the direction S illustrated in  FIG. 5  onto the protective layer  25 , which is located on the plurality of heat-generating portions  9  of the thermal head X 1 . The driving unit has a function of driving the conveying rollers  43 ,  45 ,  47 , and  49 . For example, a motor can be used as the driving unit. The conveying rollers  43 ,  45 ,  47 , and  49  can be formed, for example, by coating cylindrical shafts  43   a ,  45   a ,  47   a , and  49   a , which are made of a metal such as stainless steel, with elastic members  43   b ,  45   b ,  47   b , and  49   b , which are made of butadiene rubber or the like. Although not illustrated, if the recording medium P is image-receiving paper or the like onto which ink is to be transferred, an ink film is conveyed together with the recording medium P to a space between the recording medium P and the heat-generating portions  9  of the thermal head X 1 . 
     The platen roller  50  has a function of pressing the recording medium P against the protective layer  25  located on the heat-generating portions  9  of the thermal head X 1 . The platen roller  50  is disposed so as to extend in a direction perpendicular to the conveying direction S of the recording medium P. Both ends of the platen roller  50  are supported so that the platen roller  50  can rotate while pressing the recording medium P against the heat-generating portions  9 . The platen roller  50  can be formed, for example, by coating a cylindrical shaft  50   a  made of a metal such as stainless steel with an elastic member  50   b  made of butadiene rubber or the like. 
     The power-supply device  60  has a function of supplying an electric current for causing the heat-generating portions  9  of the thermal head X 1  to generate heat and an electric current for operating the driver ICs  11 . The control device  70  has a function of supplying a control signal, for controlling the operation of the driver ICs  11 , to the driver ICs  11  to selectively cause the heat-generating portions  9  of the thermal head X 1  to generate heat as described above. 
     As illustrated in  FIG. 5 , the thermal printer Z 1  performs a predetermined printing operation on the recording medium P by selectively causing the heat-generating portions  9  to generate heat by using the power-supply device  60  and the control device  70  while pressing the recording medium P against the heat-generating portions  9  of the thermal head X 1  by using the platen roller  50  and conveying the recording medium P onto the heat-generating portions  9  by using the conveying mechanism  40 . If the recording medium P is image-receiving paper or the like, printing on the recording medium P is performed by thermally transferring ink of an ink film (not shown) conveyed together with the recording medium P to the recording medium P. 
     Second Embodiment 
     Referring to  FIG. 6 , a thermal head X 2  will be described. The thermal head X 2  includes a covering member  129 , which differs from the covering member  29  of the thermal head X 1 . Hereinafter, the same members will be denoted by the same numerals. 
     The covering member  129  has a structure in which the center line L 1  does not pass through a top portion  129   a , and a perpendicular line L 3  that passes through the top portion  129   a  (hereinafter referred to as “the perpendicular line L 3 ”) is located at a position different from that of the center line L 1 . As illustrated in  FIG. 6( b ) , the perpendicular line L 3 , passing through the top portion  129   a , is located at a position farther than the center line L 1  from the heat-generating portions  9 . As a result, in the thermal head X 2 , in plan view, the top portion  129   a  of the covering member  129  is located farther than the center line L 1  from the heat-generating portions  9 . 
     Therefore, the position at which the recording medium P and the covering member  129  contact each other can be disposed upstream in the conveying direction S. Thus, it takes a certain time for the recording medium P to be conveyed onto the heat-generating portions  9 , and therefore heat can be dissipated while the recording medium P is conveyed. As a result, the probability of occurrence of nonuniform density in an image on a recording medium printed by the thermal head X 2  can be reduced. 
     In the thermal head X 2 , the center line L 2  is located at a position farther than the perpendicular line L 3  from the heat-generating portions  9 . Therefore, the probability of heat of the driver IC  11  being conducted to the recording medium P can be further reduced. 
     That is, in the thermal head X 2 , the perpendicular line L 3  is disposed at a position farther than the center line L 1  from the heat-generating portions  9 , and the center line L 2  is disposed at a position farther than the perpendicular line L 3 , passing through the top portion  129   a , from the heat-generating portions  9 . As a result, the covering member  129  and the recording medium P can be made to contact each other on the perpendicular line L 3 , which is located upstream of the center line L 1  in the conveying direction S. Moreover, the driver IC  11  can be disposed upstream, in the conveying direction S, of a contact point at which the covering member  129  and the recording medium P contact each other. As a result, the probability of occurrence of nonuniform density in an image printed by the thermal head X 2  can be further reduced. 
     In plan view, the center line L 2  may be disposed between the center line L 1  and the perpendicular line L 3 . Also in this case, conduction of heat from the driver IC  11  to the recording medium P can be suppressed. 
     Third Embodiment 
     Referring to  FIGS. 7 and 8 , a thermal head X 3  will be described. 
     In the thermal head X 3 , a covering member  229  is integrally disposed on the plurality of driver ICs  11 . The covering member  229  includes first regions R 1  in which the driver ICs  11  exist in the cross direction of the main scanning direction and second regions R 2  in which the driver ICs  11  do not exist in the cross direction of the main scanning direction. In other words, the covering member includes the first regions R 1  in which the driver ICs  11  exist when seen in the conveying direction S, which is the sub-scanning direction, and the second regions R 2  in which the driver ICs  11  do not exist when seen in the sub-scanning direction S. The first regions R 1  and the second regions R 2  extend in the sub-scanning direction. 
     The covering member  229  includes first edges  2  and second edges  4  in the first regions R 1  and includes first edges  6  and second edges  8  in the second regions R 2 . The first edges  2  of the first regions R 1  are continuous with the first edges  6  of the second regions R 2 . The second edges  4  of the first regions R 1  are continuous with the second edges  8  of the second regions R 2 . 
     In the thermal head X 3 , the covering member  229  has a structure in which the second edges  8  of the second regions R 2  are located closer than the second edges  4  of the first regions R 1  to the heat-generating portions  9 . Therefore, a contact state in which the covering member  229  and the recording medium P are in contact with each other varies in the main scanning direction. 
     That is, as the recording medium P is conveyed, the contact state changes from a state in which the recording medium P is in contact with only the first regions R 1  to a state in which the recording medium P is in contact with the first regions R 1  and the second regions R 2 . As a result, the covering member  229  functions to remove a crease from the recording medium P, and the thermal head X 3  can perform precise printing. 
     As illustrated in  FIG. 8 , in the thermal head X 3 , the height of the covering member  229  of the first regions R 1  from the substrate  7  is larger than the height of the covering member  229  of the second regions R 2  from the substrate  7 . 
     Therefore, when the recording medium P is conveyed onto the covering member  229 , gaps  10  are generated between the covering member  229  of the second regions R 2  and the recording medium P. Thus, the recording medium P is conveyed on the covering member  229  in a state in which the gaps  10  are formed thereon. As a result, the contact area between the recording medium P and the covering member  229  can be reduced, and the probability of occurrence of sticking of the recording medium P can be reduced. 
     In the thermal head X 3 , the covering member  229  has a shape such that the distance between the first edges  2  of the first regions R 1  and the heat-generating portions  9  is substantially the same as the distance between the first edges  6  of the second regions R 2  and the heat-generating portions  9 . Therefore, the first edges  2  of the first region R 1  and the first edges  6  of the second region R 2  are arranged along a substantially straight line in the main scanning direction. 
     As a result, the recording medium P, which has been conveyed on the covering member  229 , is removed from the covering member  229  in a state in which the recording medium P extends uniformly in the main scanning direction, and the recording medium P can be conveyed to the heat-generating portions  9  in a state in which the recording medium P extends uniformly in the main scanning direction. In particular, this is effective when conveying a recording medium P having a low rigidity. 
     The sentence “the distance between the first edges  2  of the first regions R 1  and the heat-generating portions  9  is substantially the same as the distance between the first edges  6  of the second regions R 2  and the heat-generating portions  9 ” means that, including a manufacturing error, the distance between the first edges  2  of the first regions R 1  and the heat-generating portions  9  is in the range of 0.95 to 1.05 times the distance between the first edges  6  of the second regions R 2  and the heat-generating portions  9 . 
     The thermal head X 3  can be made, for example, by using the following method. As with the thermal head X 1 , after disposing the driver ICs  11  on the head base body  3 , the covering member  229  can be made by applying a resin material, to become the covering member  229 , by using a dispenser and curing the resin material by heat. 
     At this time, the nozzle positions of the dispenser for applying the resin material of the covering member  229  are disposed closer than the driver IC  11  to the heat-generating portions  9  so that the driver ICs  11  are disposed at positions farther than the center of the covering member  229  from the heat-generating portions  9 . Moreover, the amount of the resin material applied to the first regions R 1  is made larger than the amount of the resin material applied to the second regions R 2 . 
     The covering member  229  can be made by using the method described above. The nozzle positions of the dispenser for forming the second regions R 2  may differ from those for forming the first regions R 1 . For example, the thermal head X 3  may be made by disposing the nozzle positions of the dispenser for forming the second regions R 2  closer than those for forming the first regions R 1  to the heat-generating portions  9 . Instead of using a dispenser, the covering member  229  may be made by printing by using a mask. 
     The distance between the first edges  2  of the first regions R 1  and the heat-generating portions  9  need not be substantially the same as the distance between the first edges  6  of the second regions R 2  and the heat-generating portions  9 . The height of the covering member  229  of the first regions R 1  from the substrate  7  need not be larger than the height of the covering member  229  of the second regions R 2  from the substrate  7 . 
     Fourth Embodiment 
     Referring to  FIG. 9 , a thermal head X 4  will be described. In the thermal head X 4 , the FPC  5  is disposed adjacent to the second edges  4  and  8  of the covering member  229 , and a resin layer  512  is disposed on the FPC  5  and the second edges  4  and  8  so as to extend from the FPC  5  to the second edges  4  and  8 . In other respects, the thermal head X 4  is the same as the thermal head X 3 . 
     The resin layer  512  is provided in order to increase the strength of bond between the head base body  3  and the FPC  5 . In particular, the resin layer  512  increases the strength of bond between the head base body  3  and the FPC  5  in the thickness direction of the head base body  3 . The resin layer  512  can be made from a resin layer material, such as an epoxy resin or a silicone resin. A thermosetting resin, a thermosoftening resin, a UV curable resin, or a two-part resin can be used as the resin layer material. 
     The thermal head X 4  has a structure in which the second edges  8  of the second regions R 2  are located closer to the heat-generating portions  9  than the second edges  4  of the first regions R 1  and the resin layer  512  is disposed on the FPC  5  and the second edges  4  and  8  so as to extend from the FPC  5  to the second edges  4  and  8 . Therefore, when applying a resin layer material to from the resin layer  512 , surplus of the resin layer material flows into gaps between the second edges  8  of the second regions R 2  and the FPC  5 . As a result, the probability of the resin layer material flowing out of the thermal head X 4  can be reduced. 
     In particular, if the resin layer material has surplus in a central region in the main scanning direction, the surplus may flow onto the FPC  5 . However, with the thermal head X 4 , the surplus resin layer material can flow into the gaps between the second edges  8  of the second regions R 2  and the FPC  5 . 
     Fifth Embodiment 
     Referring to  FIG. 10 , a thermal head X 5  and a thermal head X 6 , which is a modification of the thermal head X 5 , will be described. In  FIG. 10( b ) , a tangent line to the driver IC  11  extending from a top portion  429   a  is represented by a broken line. 
     In the thermal head X 5 , a covering member  329  includes a plurality of bubbles  412   a . In other respects, the thermal head X 5  is the same as the thermal head X 2 . The thermal head X 6 , which is a modification, differs from the thermal head X 5  in the disposition of bubbles  412   a  formed therein. 
     In the thermal head X 5 , the covering member  329  includes the plurality of bubbles  12 . Therefore, the thermal conductivity of the covering member  329  can be reduced, and heat of the driver IC  11  is not easily conducted in the covering member  329 . As a result, the probability of heat of the driver IC  11  being conducted to the recording medium P can be reduced, and the probability of occurrence of nonuniform density in an image printed by the thermal head X 5  can be reduced. 
     The thermal head X 6  includes the plurality of bubbles  412  in a covering member  429 , and some of the bubbles  412  (bubbles  412   a ) are disposed between the driver IC  11  and the top portion  429   a . Therefore, the bubbles  12  function as a heat-insulating layer between the driver IC  11  and the top portion  429   a , and heat of the driver IC  11  is not easily conducted to the top portion  429   a . As a result, the probability of occurrence of nonuniform density in an image printed by the thermal head X 6  can be reduced. 
     The sentence “the bubbles  412  are located between the top portion  429   a  and the driver IC  11 ” means that the bubbles  412   a  and  412   b  are included in the covering member  429  located in a region (hereinafter, referred to as “the region”) surrounded by the top portion  429   a  and the tangent line of the driver IC  11  extending from the top portion  429   a . The entirety of the bubble  412   b  need not be disposed in the region as in the case of the bubble  412   b , and it is sufficient that a part of the bubble  412   b  is disposed in the region. 
     The thermal heads X 5  and X 6  can be made, for example, by using the following method. When using a two-liquid thermosetting resin to form the covering members  329  and  429 , the covering members  329  and  429  including the bubbles  12  and  412  can be formed by increasing the viscosity of each of a base resin and a curing agent and by agitating the base resin and the curing agent in the highly viscos state. 
     A resin material of the covering members  329  and  429  may include a foaming agent. The surface of the driver IC  11  may be treated so that the bubbles  12  and  412  can be formed around the driver IC  11 . 
     Sixth Embodiment 
     Referring to  FIGS. 11 and 12 , a thermal head X 7  will be described. 
     The thermal head X 7  includes a heat sink  1 , a head base body  3 , an FPC  5 , and a connector  31 . The head base body  3  is disposed on the heat sink  1 . The FPC  5  is disposed adjacent to the head base body  3  on the heat sink  1 . The connector  31  is disposed below the FPC  5  adjacent to the heat sink  1 . 
     The driver IC  11  is disposed on the FPC  5 . Terminals (not shown) of the driver IC  11  are connected, through a plurality of wires  14 , to printed wires (not shown) of the FPC  5  or to connection electrodes (not shown) of the head base body  3 . Although not illustrated in the figures, as in the thermal head X 1 , a plurality of the driver ICs  11  are arranged in the main scanning direction. 
     A covering member  529  is disposed on the plurality of driver ICs  11  so as to extend in the main scanning direction. The covering member  529  is disposed on the FPC  5  and the head base body  3  so as to extend from the FPC  5  to the head base body  3 . Therefore, a first edge  529   b  is disposed on the head base body  3 , and a second edge  529   c  is disposed on the FPC  5 . 
     As illustrated in  FIG. 12 , in the thermal head X 7 , in plan view, the driver IC  11  is located farther than a top portion  529   a  of the covering member  529  from the heat-generating portions  9 . Therefore, the distance between the top portion  529   a  of the covering member  529  and the driver IC  11  can be increased. 
     Thus, the amount of the covering member  529  disposed between the driver IC  11  and the recording medium P can be increased. As a result, the probability of heat of the driver IC  11  being conducted to the recording medium P can be reduced, and the probability of occurrence of nonuniform density in an image printed by the thermal head X 7  can be reduced. 
     Seventh Embodiment 
     Referring to  FIG. 13 , a thermal head X 8  will be described. 
     The thermal head X 8  differs from the thermal head X 1  in the disposition of the driver IC  11  in a covering member  629 . Other parts of the thermal head X 8  are the same as those the thermal head X 1 , and description of such parts will be omitted. 
     In plan view, the thermal head X 8  has a structure in which the center line L 2  is disposed farther than the center line L 1  from the heat-generating portions  9  and a part of the driver IC  11  is disposed below a top portion  629   a . That is, the thermal head X 8  has a structure in which the distance between the center line L 1  and the center line L 2  in the sub-scanning direction is smaller than the distance from the center of gravity of the driver IC  11  to the surface of the driver IC  11 . 
     Also in this case, the volume of the covering member  629  disposed between the driver IC  11  and the recording medium P can be increased. As a result, the probability of heat of the driver IC  11  being conducted to the recording medium P can be reduced, and the probability of occurrence of nonuniform density in an image printed by the thermal head X 8  can be reduced. 
     Thus, as long as the center line L 2  is located farther than the top portion  629   a  of the covering member  629  from the heat-generating portions  9 , the probability of occurrence of nonuniform density in an image printed by the thermal head X 8  can be reduced. That is, it is sufficient that more than 50% of the driver IC  11  is located farther than the top portion  29   a  of the covering member  29  from the heat-generating portions  9 . 
     The present invention is not limited to the embodiments described above, and the embodiments may be modified in various ways within the spirit and scope of the present invention. For example, the thermal printer Z 1  described above includes the thermal head X 1  according to the first embodiment. The thermal printer Z 1  is not limited thereto, and the thermal printer Z 1  may include any one of the thermal heads X 2  to X 8 . A combination of the thermal heads X 1  to X 8  according to the embodiments may be used, and such embodiments are assumed to be described in the present description. That is, features of the thermal heads X 1  to X 6  and X 8  and features of the thermal head X 7 , in which the driver IC  11  is disposed on the FPC  5 , may be used in combination. 
     It is not necessary that the center lines L 2  of all of the driver ICs  11  mounted in the thermal head X 1  be disposed farther than the top portion  29   a  from the heat-generating portions  9 . That is, the center lines L 2  of some of the driver ICs  11  mounted in the thermal head X 1  need not be disposed farther than the top portion  29   a  from the heat-generating portions  9 . It sufficient that the center lines L 2  of 60% or more of the driver ICs  11  mounted in the thermal head X 1  are disposed farther than the top portion  29   a  from the heat-generating portions  9 . Also in this case, the probability of occurrence of nonuniform density in an image printed by the thermal head X 1  can be reduced. 
     In order to reduce the probability of occurrence of nonuniform density in an image printed by the thermal head X 1 , it is most preferable that all of the driver ICs  11  mounted in the thermal head X 1  are disposed farther than the top portion  29   a  of the covering member  29  from the heat-generating portions  9 . 
     In the thermal head X 1 , the heat storage layer  13  includes the protruding portion  13   b , and the electrically resistive layer  15  is disposed on the protruding portion  13   b . However, the structure is not limited thereto. For example, without providing the protruding portion  13   b  in the heat storage layer  13 , the heat-generating portions  9  of the electrically resistive layer  15  may be disposed on the base  13   a  of the heat storage layer  13 . Alternatively, without forming the heat storage layer  13 , the electrically resistive layer  15  may be disposed on the substrate  7 . 
     In the thermal head X 1 , the common electrode  17  and the individual electrodes  19  are disposed on the electrically resistive layer  15 . However, the structure is not limited thereto as long as both of the common electrode  17  and the individual electrodes  19  are connected to the heat-generating portions  9  (the electrically resistive layer  15 ). For example, the common electrode  17  and the individual electrodes  19  may be disposed on the heat storage layer  13 , and the electrically resistive layer  15  may be formed only in a region between the common electrode  17  and the individual electrodes  19  to form the heat-generating portions  9 . 
     The thermal heads X 1  to X 8  are planar heads in which the heat-generating portions  9  are disposed on the main surface of the substrate  7 . However, the present invention may be applied to a real-edge-type head in which the heat-generating portions  9  are disposed on an end surface of the substrate  7 . In the example described above, the head base body  3  is electrically connected to the outside through the FPC  5 . However, the connector  31  may be directly electrically connected to the head base body  3 . Thin-film heads including the heat-generating portions  9 , which are formed by using a thin-film forming technology, have been described above. However, the present invention may be applied to a thick-film head including heat-generating portions  9  formed by using a thick-film forming technology. 
     REFERENCE SIGNS LIST 
     
         
         
           
             X 1  to X 8  thermal head 
             Z 1  thermal printer 
             R 1  first region 
             R 2  second region 
             S conveying direction (sub-scanning direction) 
               1  heat sink 
               2  first edge of first region 
               3  head base body 
               4  second edge of first region 
               5  flexible printed circuit board 
               6  first edge of second region 
               7  substrate 
               8  second edge of second region 
               9  heat-generating portion 
               11  driver IC 
               12  bubble 
               13  heat storage layer 
               15  electrically resistive layer 
               17  common electrode 
               19  individual electrode 
               21  connection electrode 
               23  bonding material 
               25  protective layer 
               27  covering layer 
               29 ,  129 ,  229 ,  329 ,  429 ,  529 ,  629  covering member 
               29   a ,  129   a ,  229   a ,  329   a ,  429   a ,  529   a ,  629   a  top portion 
               29   b ,  129   b ,  229   b ,  329   b ,  429   b ,  529   b ,  629   b  first edge 
               29   c ,  129   c ,  229   c ,  329   c ,  429   c ,  529   c ,  629   c  second edge 
               412  resin layer