Patent Publication Number: US-2023150273-A1

Title: Thermal head and thermal printer

Description:
RELATED APPLICATIONS 
     The present application is a National Phase of International Application No. PCT/JP2021/013395, filed Mar. 29, 2021, and claims priority based on Japanese Patent Application No. 2020-065150, filed Mar. 31, 2020. 
    
    
     TECHNICAL FIELD 
     Embodiments of this disclosure relate to a thermal head and a thermal printer. 
     BACKGROUND OF INVENTION 
     Various kinds of thermal heads for printing devices such as facsimile machines and video printers have been proposed in the related art. 
     Furthermore, a thermal head on which an electrode containing glass is applied is known (e.g., Patent Document 1). 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP 2011-110751 A 
     SUMMARY 
     In an aspect of an embodiment, a thermal head includes a substrate, an electrode, and a gap. The electrode is located on the substrate. The gap is located between the substrate and the electrode. The thermal head includes glass in an inner portion of the gap. 
     In an aspect of an embodiment, a thermal printer includes the thermal head described above, a transport mechanism, and a platen roller. The transport mechanism transports a recording medium on a heat generating part located on the substrate. The platen roller presses the recording medium on the heat generating part. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view schematically illustrating a thermal head according to an embodiment. 
         FIG.  2    is a cross-sectional view schematically illustrating the thermal head illustrated in  FIG.  1   . 
         FIG.  3    is a plan view schematically illustrating a head base illustrated in  FIG.  1   . 
         FIG.  4    is an enlarged cross-sectional view of a region A illustrated in  FIG.  2   . 
         FIG.  5    is an enlarged cross-sectional view for describing a shape of a main surface of the substrate. 
         FIG.  6    is an enlarged cross-sectional view of a region B illustrated in  FIG.  2   . 
         FIG.  7    is an enlarged cross-sectional view of a region C illustrated in  FIG.  2   . 
         FIG.  8    is a plan view illustrating a main portion of a thermal head according to a variation of the embodiment. 
         FIG.  9    is a cross-sectional view taken along line E-E illustrated in  FIG.  8   . 
         FIG.  10    is a cross-sectional view taken along line F-F illustrated in  FIG.  8   . 
         FIG.  11    is a schematic view of a thermal printer according to an embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of a thermal head and a thermal printer disclosed in the present application will be described below with reference to the accompanying drawings. Note that this invention is not limited to each of the embodiments that will be described below. 
     EMBODIMENTS 
       FIG.  1    is a perspective view schematically illustrating a thermal head according to an embodiment. In the embodiment, a thermal head X 1  includes a heat dissipation body  1 , a head base  3 , and a flexible printed circuit board (FPC)  5  as illustrated in  FIG.  1   . The head base  3  is located on the heat dissipation body  1 . The FPC  5  is electrically connected to the head base  3 . The head base  3  includes a substrate  7 , a heat generating part  9 , a drive IC  11 , and a covering member  29 . 
     The heat dissipation body  1  has a plate-like shape and has a rectangular shape in plan view. The heat dissipation body  1  has a function of dissipating the heat generated by the heat generating part  9  of the head base  3 , especially heat not contributing to printing. The head base  3  is bonded to an upper surface of the heat dissipation body  1  using a double-sided tape, an adhesive, or the like (not illustrated). The heat dissipation body  1  is made of, for example, a metal material such as copper, iron, or aluminum. 
     The head base  3  has a plate-like shape and has a rectangular shape in plan view. The head base  3  includes each member constituting the thermal head X 1  located on the substrate  7 . The head base  3  performs printing on a recording medium P in accordance with an electrical signal supplied from the outside (see  FIG.  11   ). 
     A plurality of drive ICs  11  are located on the substrate  7  and arranged in a main scanning direction. The drive ICs  11  are electronic components having a function of controlling a conductive state of the heat generating part  9 . A switching member including a plurality of switching elements inside may be used for the drive IC  11 . 
     The drive IC  11  is covered by a covering member  29  made of a resin such as an epoxy resin or a silicone resin. The covering member  29  is located across the plurality of drive ICs  11 . The covering member  29  is an example of a sealing material. 
     The FPC  5  is electrically connected to the head base  3  at one end and is electrically connected to a connector  31  at the other end. 
     The FPC  5  is electrically connected to the head base  3  using an electrically conductive bonding material  23  (see  FIG.  2   ). An example of the electrically conductive bonding material  23  may include a solder material or an anisotropic conductive film (ACF) in which electrically conductive particles are mixed into an electrically insulating resin. 
     Hereinafter, each of the members constituting the head base  3  will be described using  FIGS.  1  to  3   .  FIG.  2    is a cross-sectional view schematically illustrating the thermal head illustrated in  FIG.  1   .  FIG.  3    is a plan view schematically illustrating the head base illustrated in  FIG.  1   . 
     The head base  3  further includes the substrate  7 , a common electrode  17 , an individual electrode  19 , a first electrode  12 , a second electrode  14 , a terminal  2 , a heat generating resistor  15 , a protective layer  25 , and a covering layer  27 . Note that, in  FIG.  1   , the protective layer  25  and the covering layer  27  are omitted.  FIG.  3    illustrates wiring of the head base  3  in a simplified manner, and in  FIG.  3   , the drive IC  11 , the protective layer  25 , and the covering layer  27  are omitted. In  FIG.  3   , a configuration of the second electrode  14  is illustrated in a simplified manner. 
     The substrate  7  has a rectangular shape in plan view. A main surface (upper surface)  7   e  of the substrate  7  includes a first long side  7   a  that is one long side, a second long side  7   b  that is the other long side, a first short side  7   c , and a second short side  7   d . The substrate  7  is made of an electrically insulating material such as an alumina ceramic or a semiconductor material such as monocrystalline silicon. 
     The substrate  7  may include a heat storage layer  13 . The heat storage layer  13  protrudes from the main surface  7   e  in the thickness direction of the substrate  7 , and extends in a strip shape in a second direction D 2  (the main scanning direction). The heat storage layer  13  functions to cause the recording medium to be printed to be favorably pressed against the protective layer  25  located over the heat generating part  9 . The heat storage layer  13  is located below the heat generating part  9  (the heat generating resistor  15 ) as illustrated in  FIG.  2   . Although not illustrated, the heat storage layer  13  is located below the heat generating part  9  (the heat generating resistor  15 ) at the same position as the heat generating part  9  (the heat generating resistor  15 ) in plan view in  FIGS.  1  and  3   . Note that the heat storage layer  13  may be located not only in the region immediately below the heat generating part  9  (the heat generating resistor  15 ), but also in a wider region including the region immediately below the heat generating part  9 . Hereinafter, the portion on the main surface  7   e  in which the heat storage layer  13  is not located may be referred to as a “non-disposition area of the heat storage layer  13 ”. 
     Note that the heat storage layer  13  may include an underlying portion. In this case, the underlying portion is a portion located in the entire area of the heat storage layer  13  on the main surface  7   e  of the substrate  7 . 
     The heat storage layer  13  contains, for example, a glass component. The heat storage layer  13  temporarily stores some of the heat generated by the heat generating part  9 , and thus the time to increase the temperature of the heat generating part  9  can be shortened. This functions to enhance the thermal response properties of the thermal head X 1 . 
     The heat storage layer  13  is made by, for example, applying a predetermined glass paste obtained by mixing glass powder with an appropriate organic solvent onto the main surface  7   e  of the substrate  7  using a known screen printing method or the like, and firing the main surface. Note that the substrate  7  may have only an underlying portion as the heat storage layer  13 . 
     The common electrode  17  is located on the main surface  7   e  of the substrate  7  as illustrated in  FIG.  3   . The common electrode  17  is made of an electrically conductive material, and examples thereof include at least one metal selected from aluminum, gold, silver, and copper, or an alloy of these metals. 
     The common electrode  17  includes a first common electrode  17   a , a second common electrode  17   b , a third common electrode  17   c , and a terminal  2  as illustrated in  FIG.  3   . The common electrode  17  is electrically connected in common to the heat generating part  9  including a plurality of elements. 
     The first common electrode  17   a  is located between the first long side  7   a  of the substrate  7  and the heat generating part  9 , and extends in the main scanning direction. The plurality of second common electrodes  17   b  are located respectively along the first short side  7   c  and the second short side  7   d  of the substrate  7 . Each of the plurality of second common electrodes  17   b  connects the corresponding terminal  2  and the first common electrode  17   a . Each of the third common electrodes  17   c  extends from the first common electrode  17   a  toward a corresponding element of the heat generating part  9 , and a part of the third common electrode  17   c  extends through the heat generating part  9  to the side opposite to the heat generating part  9 . The third common electrodes  17   c  are located at intervals in the second direction D 2  (the main scanning direction). 
     The individual electrode  19  is located on the main surface  7   e  of the substrate  7 . The individual electrode  19  contains a metal component and thus has electrical conductivity. The individual electrode  19  is made of, for example, a metal such as aluminum, nickel, gold, silver, platinum, palladium, or copper, and an alloy of these metals. The individual electrode  19  made of gold has a high conductivity. A plurality of individual electrodes  19  are located in the main scanning direction and each of them is located between adjacent third common electrodes  17   c . As a result, in the thermal head X 1 , the third common electrodes  17   c  and the plurality of individual electrodes  19  are alternately arranged in the main scanning direction. Each individual electrode  19  is connected to an electrode pad  10  at a portion close to the second long side  7   b  of the substrate  7 . 
     The first electrode  12  is connected to the electrode pad  10  and extends in the main scanning direction. The drive IC  11  is mounted on the electrode pad  10  as described above. 
     The second electrode  14  extends in the main scanning direction and is located over a plurality of first electrodes  12 . The second electrode  14  is connected to the outside via the terminal  2 . 
     The terminal  2  is located on the second long side  7   b  side of the substrate  7 . The terminal  2  is connected to the FPC  5  via the electrically conductive bonding material  23  (see  FIG.  2   ). In this way, the head base  3  is electrically connected to the outside. 
     As an electrode material of the above-described individual electrode  19  and the first electrode  12 , a conductor paste containing a metal component and a glass component having a particle size from about 0.01 to 10 μm, for example, in an organic solvent can be used. The individual electrode  19  and the first electrode  12  can be made by forming a material layer constituting each electrode on the substrate  7  using, for example, a screen printing method, a flexographic printing method, a gravure printing method, a gravure offset printing method, or the like. Note that a thickness of each of the individual electrode  19  and the first electrode  12  is, for example, approximately from 0.5 to 5 μm. The above-described electrodes may be formed, for example, by sequentially layering the electrodes using a known thin film forming technique such as a sputtering method, and then processing the layered body into a predetermined pattern by using known photoetching, or the like. 
     For the material layer constituting the individual electrode  19  and the first electrode  12 , for example, a conductor paste containing a metal component and a glass component having a particle size of approximately from 0.01 to 10 μm in an organic solvent can be used. 
     The above-described first common electrode  17   a , the second common electrode  17   b , the third common electrode  17   c , the second electrode  14 , and the terminal  2  can be formed by forming a material layer constituting each electrode on the substrate  7  using, for example, a screen printing method. A thickness of each of the first common electrode  17   a , the second common electrode  17   b , the third common electrode  17   c , the second electrode  14 , and the terminal  2  is approximately from 5 to 20 μm. By forming the thick electrode in this manner, the wiring resistance of the head base  3  can be reduced. Note that the portion of the thick electrode is illustrated by dots in  FIG.  3   , and this also applies to the following drawings. 
     The heat generating resistor  15  is located across the third common electrode  17   c  and the individual electrode  19  and spaced apart from the first long side  7   a  of the substrate  7 . A portion of the heat generating resistor  15  located between the third common electrode  17   c  and the individual electrode  19  functions as each element of the heat generating part  9 . Although each element of the heat generating part  9  is illustrated in a simplified manner in  FIG.  3   , the elements are located at a density from, for example, 100 dpi to 2400 dpi (dot per inch) or the like. 
     The heat generating resistor  15  may be formed, for example, by placing a material paste containing ruthenium oxide as a conductive component on the substrate  7  including the patterned various electrodes in a long strip-like shape elongated in the main scanning direction using a screen printing method or a dispensing device. 
     The protective layer  25  is located over the heat storage layer  13  formed on the main surface  7   e  of the substrate  7  (see  FIG.  1   ) and covers the heat generating part  9 . The protective layer  25  is located extending from the first long side  7   a  of the substrate  7  but separated from the electrode pad  10  and extending in the main scanning direction of the substrate  7 . 
     The protective layer  25  has an insulating property and protects the covered region from corrosion due to deposition of moisture and the like contained in the atmosphere, or from wear due to contact with the recording medium to be printed. The protective layer  25  can be made of, for example, glass using a thick film forming technique such as printing. 
     The protective layer  25  may be formed using SiN, SiO 2 , SiON, SiC, diamond-like carbon, or the like. Note that the protective layer  25  may be a single layer or be formed by layering a plurality of protective layers  25 . The protective layer  25  such as that described above can be formed using a thin film forming technique such as a sputtering method. 
     The covering layer  27  is located on the substrate  7  such that the covering layer partially covers the common electrode  17 , the individual electrode  19 , the first electrode  12 , and the second electrode  14 . The covering layer  27  protects the covered region from oxidation due to contact with the atmosphere or from corrosion due to deposition of moisture and the like contained in the atmosphere. The covering layer  27  can be made of a resin material such as an epoxy resin, a polyimide resin, or a silicone resin. 
     The main portion of the thermal head X 1  according to an embodiment will be described in detail with reference to  FIGS.  4  and  5   .  FIG.  4    is an enlarged cross-sectional view of a region A illustrated in  FIG.  2   .  FIG.  5    is an enlarged cross-sectional view for describing a shape of the main surface of the substrate. 
     The substrate  7 , the individual electrode  19 , the protective layer  25 , and the covering layer  27  are located in the region A as illustrated in  FIG.  4   . 
     The individual electrode  19  is located on the substrate  7 . A gap  20  is located between the substrate  7  and the individual electrode  19 . 
     The main surface  7   e  of the substrate  7  is an uneven surface, and a plurality of protruding portions  702  to  704  and a plurality of recessed portions  705  and  706  are alternately located as illustrated in  FIG.  5   . The individual electrode  19  does not conform to the uneven main surface  7   e  in a case of where the individual electrode  19  is formed by printing and firing the electrode material, for example, and is located while supported by the protruding portions  702  to  704  of the main surface  7   e . For this reason, the gap  20  is located between the substrate  7  and the individual electrode  19 . 
     Glass  21  is located in an inner portion of the gap  20 . Since the glass  21  is located in the inner portion of the gap  20 , the contact area between the substrate  7  and the individual electrode  19  increases via the glass  21 , compared to a case where the glass  21  is not located. For this reason, the individual electrode  19  is less likely to peel or disconnect from the substrate  7 . As a result, in the embodiment, the thermal head X 1  has improved durability. 
     Here, the “inner portion of the gap  20 ” refers to the portion located on the recessed portion  705  side in a gap  20 A from the line segment  707  connecting the protruding portion  702  and the protruding portion  703 , for example, when the substrate  7  is viewed in a cross section as illustrated in  FIG.  5   . For example, also in a case of a gap  20 B with the protruding portion  704  having different dimensions in the thickness direction of the substrate  7  from those of the protruding portions  702  and  703 , the “inner portion of the gap  20 B” refers to the portion located on the recessed portion  706  side from the line segment  708  connecting the adjacent protruding portion  703  and protruding portion  704 . 
     The glass  21  located in the inner portion of the gap  20  may protrude from the individual electrode  19  (see, e.g., a gap  20   e ) as illustrated in  FIG.  4   . When the glass  21  protrudes from the individual electrode  19  and is located in the inner portion of the gap  20  as described above, the contact area between the substrate  7  and the individual electrode  19  increases. For this reason, the individual electrode  19  is less likely to peel or disconnect from the substrate  7 . As a result, in the embodiment, the thermal head X 1  has improved durability. 
     The gap  20  may be filled with the glass  21  (see, e.g., a gap  20   c ). Here, “the gap  20  is filled with” refers to, for example, a case where the gap  20 A is filled with the glass  21  in the area of 80% or greater of the portion on the recessed portion  705  side from the line segment  707  connecting the protruding portion  702  and the protruding portion  703 , when the substrate  7  is viewed in a cross section as illustrated in  FIG.  5   . When the gap  20  is filled with the glass  21  as described above, the contact area between the substrate  7  and the individual electrode  19  further increases. For this reason, the individual electrode  19  is less likely to peel or disconnect from the substrate  7 . As a result, in the embodiment, the thermal head X 1  has improved durability. 
     The glass  21  may connect the individual electrode  19  and the substrate  7  via the gap  20  (e.g., see a gap  20   b ). When the glass  21  connects the individual electrode  19  and the substrate  7  via the gap  20  as described above, the contact area of the substrate  7  and the individual electrode  19  and the glass  21  increases. For this reason, the individual electrode  19  is less likely to peel or disconnect from the substrate  7 . As a result, in the embodiment, the thermal head X 1  has improved durability. 
     The glass  21  may be located only in the inner portion of the gap  20  (see, e.g., a gap  20   f ). Even when the glass  21  is located only in the inner portion of the gap  20  without coming into contact with the individual electrode  19  as described above, the glass  21  is in contact with the individual electrode  19  in the depth direction from the illustrated surface. For this reason, the individual electrode  19  is less likely to peel or disconnect from the substrate  7 , compared to the case in which no glass  21  is located in the inner portion of the gap  20 . As a result, in the embodiment, the thermal head X 1  has improved durability. 
     A plurality of pieces of glass  21  may be located in one gap  20  (see, e.g., a gap  20   d ). Even when a plurality of pieces of glass  21  is located in the inner portion of one gap  20 , the contact area between the substrate  7  and the individual electrode  19  increases. For this reason, the individual electrode  19  is less likely to peel or disconnect from the substrate  7 , compared to the case in which no glass  21  is located in the inner portion of the gap  20 . As a result, in the embodiment, the thermal head X 1  has improved durability. 
     A conductive component  190  may be located in the inner portion of the gap  20  together with the glass  21  (see, e.g., a gap  20   a ). The conductive component  190  may be, for example, a metal such as aluminum, nickel, gold, silver, platinum, palladium, or copper, and an alloy of these metals. The individual electrode  19  that is an electrode contains the conductive component  190  and a glass component  191 . A part of the glass component  191  turns into the glass  21  located in the inner portion of the gap  20  through a firing process. In this case, the individual electrode  19  is less likely to peel or disconnect from the substrate  7  even when a part of the conductive component  190  included in the individual electrode  19  is located in the inner portion of the gap  20 , compared to the case in which no glass  21  is located in the inner portion of the gap  20 . As a result, in the embodiment, the thermal head X 1  has improved durability. Note that the conductive component  190  located in the inner portion of the gap  20  may have a different composition from the conductive component  190  included in the individual electrode  19 . 
     Glass  21   a  may be located inside the substrate  7 . The glass  21   a  is located inside a hole  7   f  open to the main surface  7   e  of the substrate  7 . Since the glass  21   a  is located inside the hole  7   f , the substrate  7  improves in insulating properties. Since the glass  21   a  is located inside the hole  7   f , improvement in the heat storage properties can be expected. 
     The protective layer  25  is located on the individual electrode  19 . For example, when the protective layer  25  contains a glass component, and the protective layer  25  covers the individual electrode  19  containing the glass component  191 , this improves the adhesiveness between the individual electrode  19  and the protective layer  25 . In particular, when the glass component  191  is located in the upper layer portion of the individual electrode  19  facing the protective layer  25 , the adhesiveness between the individual electrode  19  and the protective layer  25  is further improved. As a result, in the embodiment, the thermal head X 1  has improved durability. 
     The substrate  7  may contain a glass component. For example, an underlying portion of the substrate  7  contains a glass component. When the individual electrode  19  is located on the substrate  7  containing the glass component, the adhesiveness between the individual electrode  19  and the substrate  7  is further improved. As a result, in the embodiment, the thermal head X 1  has improved durability. 
     More description will be given next using  FIGS.  6  and  7   .  FIG.  6    is an enlarged cross-sectional view of a region B illustrated in  FIG.  2   .  FIG.  7    is an enlarged cross-sectional view of a region C illustrated in  FIG.  2   . 
     The substrate  7 , the individual electrode  19 , and the covering layer  27  are located in the region B as illustrated in  FIG.  6   . The region B has a configuration the same as and/or similar to that of the region A illustrated in  FIG.  2    except that the protective layer  25  is not located on the individual electrode  19 . 
     The covering layer  27  is located on the individual electrode  19  as illustrated in  FIG.  6   . For example, the surface roughness of an upper surface  19   e  of the individual electrode  19  facing the covering layer  27  is less than the surface roughness of the main surface  7   e  of the substrate  7 . For this reason, a film defect of the covering layer  27  is less likely to occur. As a result, in the embodiment, the thermal head X 1  has improved durability. 
     The heat storage layer  13 , the individual electrode  19 , the heat generating part  9 , and the covering layer  27  are located in the region C as illustrated in  FIG.  7   . 
     The individual electrode  19  is located on the heat storage layer  13  as illustrated in  FIG.  7   . The gap  20  is located between the heat storage layer  13  and the individual electrode  19 . 
     Glass  21  is located in an inner portion of the gap  20 . When the glass  21  is located in the inner portion of the gap  20 , the contact area between the heat storage layer  13  and the individual electrode  19  increases via the glass  21  compared to when no glass  21  is located. For this reason, the individual electrode  19  is less likely to peel or disconnect from the heat storage layer  13 . As a result, in the embodiment, the thermal head X 1  has improved durability. 
     The heat storage layer  13  contains a glass component as described above. Thus, when the individual electrode  19  is located on the heat storage layer  13 , the adhesiveness between the individual electrode  19  and the heat storage layer  13  is improved. As a result, in the embodiment, the thermal head X 1  has improved durability. 
     The heat generating resistor  15  (the heat generating part  9 ) is located on the individual electrode  19 . When the heat generating resistor  15  is located on the individual electrode  19  containing the glass component  191 , the adhesiveness between the individual electrode  19  and the heat generating resistor  15  is further improved. In particular, when the glass component  191  is located in the upper layer portion of the individual electrode  19  facing the heat generating resistor  15 , the adhesiveness between the individual electrode  19  and the heat generating resistor  15  is further improved. As a result, in the embodiment, the thermal head X 1  has improved durability. 
     Variation 
       FIG.  8    is a plan view illustrating the main portion of a thermal head according to a variation of the embodiment.  FIG.  9    is a cross-sectional view taken along line E-E illustrated in  FIG.  8   .  FIG.  10    is a cross-sectional view taken along line F-F illustrated in  FIG.  8   . Note that illustration of some configurations illustrated in  FIG.  10    is omitted in  FIGS.  8  and  9   . 
       FIG.  8    illustrates the individual electrode  19  in plan view. The individual electrode  19  is located in a non-disposition area of the heat storage layer  13  where no heat storage layer  13  is located on the main surface  7   e  of the substrate  7 . The non-disposition area of the heat storage layer  13  may include a bonding layer  777  located between the substrate  7  and the individual electrode  19  as illustrated in  FIGS.  8  to  10   . The protective layer  25  and the covering layer  27  may be located on the individual electrode  19  in this order. 
     The bonding layer  777  is a portion protruding from the main surface  7   e  in the thickness direction of the substrate  7 , and is located between the substrate  7  and the individual electrode  19 . The individual electrode  19  is located on the bonding layer  777 . The gap  20  is located between the substrate  7  and the bonding layer  777  as illustrated in  FIG.  10   . 
     The bonding layer  777  contains, for example, a glass component. Glass  21  from the bonding layer  777  is located in an inner portion of the gap  20 . When the glass  21  is located in the inner portion of the gap  20 , the contact area between the bonding layer  777  and the substrate  7  increases via the glass  21  compared to when no glass  21  is located. 
     Since the bonding layer  777  contains a glass component, when the individual electrode  19  is located on the bonding layer  777 , the adhesiveness between the individual electrode  19  and the bonding layer  777  is improved. As a result, in the embodiment, the thermal head X 1  has improved durability. 
     The bonding layer  777  is made by, for example, applying a predetermined glass paste obtained by mixing glass powder with an appropriate organic solvent onto the main surface  7   e  of the substrate  7  using a known screen printing method or the like, and firing the main surface. 
     Note that, in the non-disposition area of the heat storage layer  13 , the bonding layer  777  includes a non-disposition area  999  in a non-disposition area  888  of the individual electrode  19 . A width w 1  of the non-disposition area  999  may be greater than, less than, or equal to a width w 2  of the non-disposition area  888 . When the non-disposition area  999  is located in the non-disposition area  888  of the individual electrode  19 , the occurrence of migration caused by diffusion of the electrode material of the individual electrode  19  via the bonding layer  777  can be reduced. As a result, in the embodiment, the thermal head X 1  has improved durability. 
     A thermal printer Z 1  including the thermal head X 1  will be described with reference to  FIG.  11   .  FIG.  11    is a schematic view of a thermal printer according to an embodiment. 
     In the present embodiment, the thermal printer Z 1  includes the above-described thermal head X 1 , a transport mechanism  40 , a platen roller  50 , a power supply device  60 , and a control device  70 . The thermal head X 1  is attached to a mounting surface  80   a  of a mounting member  80  disposed in a housing (not illustrated) of the thermal printer Z 1 . Note that the thermal head X 1  is attached to the mounting member  80  such that the thermal head is aligned in the main scanning direction orthogonal to a transport direction S. 
     The transport mechanism  40  includes a drive unit (not illustrated) and transport rollers  43 ,  45 ,  47 , and  49 . The transport mechanism  40  transports a recording medium P, such as heat-sensitive paper or image-receiving paper to which ink is to be transferred, on the protective layer  25  located on a plurality of heat generating parts  9  of the thermal head X 1  in the transport direction S indicated by an arrow. The drive unit has a function of driving the transport rollers  43 ,  45 ,  47 , and  49 , and a motor can be used for the drive unit, for example. The transport rollers  43 ,  45 ,  47 , and  49  may be configured by, for example, covering cylindrical shaft bodies  43   a ,  45   a ,  47   a , and  49   a  made of a metal such as stainless steel, with elastic members  43   b ,  45   b ,  47   b , and  49   b  made of butadiene rubber or the like. Note that, if the recording medium P is an image-receiving paper or the like to which ink is to be transferred, an ink film (not illustrated) is transported between the recording medium P and the heat generating part  9  of the thermal head X 1  together with the recording medium P. 
     The platen roller  50  has a function of pressing the recording medium P onto the protective layer  25  located on the heat generating part  9  of the thermal head X 1 . The platen roller  50  is disposed extending in a direction orthogonal to the transport direction S, and both end portions thereof are supported and fixed such that the platen roller  50  is rotatable while pressing the recording medium P onto the heat generating part  9 . The platen roller  50  includes a cylindrical shaft body  50   a  made of a metal such as stainless steel and an elastic member  50   b  made of butadiene rubber or the like. The shaft body  50   a  is covered with the elastic member  50   b . 
     As described above, the power supply device  60  has a function of supplying a current for causing the heat generating part  9  of the thermal head X 1  to generate heat and a current for operating the drive IC  11 . The control device  70  has a function of supplying a control signal for controlling operation of the drive IC  11 , to the drive IC  11  in order to selectively cause the heat generating parts  9  of the thermal head X 1  to generate heat as described above. 
     The thermal printer Z 1  performs predetermined printing on the recording medium P by selectively causing the heat generating parts  9  to generate heat with the power supply device  60  and the control device  70 , while the platen roller  50  presses the recording medium P onto the heat generating parts  9  of the thermal head X 1  and the transport mechanism  40  transports the recording medium P on the heat generating parts  9 . Note that, if the recording medium P is image-receiving paper or the like, printing is performed onto the recording medium P by thermally transferring, to the recording medium P, an ink of the ink film (not illustrated) transported together with the recording medium P. 
     Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the embodiments described above, and various modifications can be made without departing from the spirit thereof. For example, although an example in which the heat generating part  9 , the heat storage layer  13 , the common electrode  17 , the individual electrode  19 , the bonding layer  777 , and the like are located on the main surface  7   e  of the substrate  7 , they may be located on a surface other than the main surface  7   e  of the substrate  7 . 
     Although description has been made using a so-called thick film head including the heat generating resistor  15  formed by printing, the present disclosure is not limited to a thick film head. A thin film head including the heat generating resistor  15  formed by sputtering may be used. 
     The connector  31  may be electrically connected to the head base  3  directly without providing the FPC  5 . In this case, a connector pin (not illustrated) of the connector  31  may be electrically connected to the electrode pad  10 . 
     Although the thermal head X 1  including the covering layer  27  is exemplified, the covering layer  27  may not be necessarily provided. In this case, the protective layer  25  may be extended to the region in which the covering layer  27  could be provided. 
     Further effects and variations can be readily derived by those skilled in the art. Thus, a wide variety of aspects of the present disclosure are not limited to the specific details and representative embodiments represented and described above. Therefore, various changes can be made without departing from the spirit or scope of the general inventive concepts defined by the appended claims and their equivalents.