FIG. 7 is a schematic plan view of a prior art thin-film thermal printhead. The thermal printhead 51 includes an elongated rectangular substrate 52 having longitudinal sides 52a and 52b. The substrate 52 has a surface formed with a linear resistor layer 53 extending longitudinally adjacent one longitudinal side 52a. A band-like region between the resistor layer 53 and the longitudinal side 52a of the substrate 52 is provided with a common wiring pattern 54. The common wiring pattern 54 has opposite ends extending to the other longitudinal side 52b of the substrate 52. One of the opposite ends of the common wiring pattern 54 is connected to a common terminal 55.
FIG. 8 is an enlarged plan view showing a principal portion of the thermal printhead 51. The common wiring pattern 54 includes a plurality of comb-tooth electrodes 54a extending therefrom. Individual electrodes 56 have respective one end extending between two adjacent comb-tooth electrodes 54a. The other end of each individual electrode 56 extends adjacent to a drive IC 57 mounted on the substrate 52 and is connected, via a non-illustrated wire-bonding pad, to an output terminal of the drive IC 57.
As indicated by the chain lines in FIG. 8, the resistor layer 53 is laid over the comb-tooth electrodes 54a and the individual electrodes 56 alternate therewith, thereby defining a heating element 53a between each two adjacent comb-tooth electrodes 54a. Thus, when power is applied to any individual electrode 56, current passes through a portion of the resistor layer 53 defined between two comb-tooth electrodes 54a sandwiching this individual electrode 56, consequently working as a heating element 53a.
FIG. 9 is an enlarged sectional view showing a principal portion of the thermal printhead 51. The substrate 52 formed of an insulating material such as alumina ceramic material is provided, on a surface thereof, with a glaze layer 61 extending longitudinally at a portion adjacent to the longitudinal side 52a. The glaze layer 61 is formed with a resistor layer 53 in the form of a thin film for covering the glaze layer. Conductor layers 62a, 62b are formed on the resistor layer 53 in such a manner as to expose the resistor layer 53 at a portion at the top of the glaze layer 61. The exposed portion of the resistor layer 53 serves as heating elements 53a. The conductor layer 62b extending rightward in FIG. 9 serves as the individual electrodes 56, whereas the conductor layer 62a extending leftward in FIG. 9 serves as comb-tooth electrodes 54a. Further, an anti-oxidation film 63 and a protective film 64 are formed to cover the heating elements 53a and the conductor layers 62a, 62b while exposing the wire-bonding pad of each individual electrode 56.
An aggregate board divisible into a plurality of substrates 52 may be used for forming the glaze layer 61, the resistor layer 53, the conductor layers 62a, 62b and the anti-oxidation film 63A. A protective film 64 is further formed on the aggregate board thus formed with the anti-oxidation film 63. Specifically, the protective film 64 may be formed in the following manner for example. First, a resist layer 65 is formed to cover the region, including the wire-bonding pads, which is not to be covered with the protective film 64. Then, a Ta.sub.2 O.sub.5 film for example may be formed by chemical vapor deposition or spattering. Subsequently, the resist layer 65 is etched away. The aggregate board thus formed with the protective film 64 is then divided into a plurality of individual substrates 52 to each of which drive ICs 57 are mounted. The drive ICs 57 and the individual electrodes 56 are connected by wire-bonding for example to provide a thermal printhead 51.
However, according to the method described above, the thermal printhead 51 is prepared by dividing the aggregate board after forming the protective film 64. Accordingly, the divisional surface 66, namely the side surface of the substrate 52 along the longitudinal side 52a at longitudinal edge of each layer 61, 53, 62a, 63, are not formed with the protective film 64. Thus, the divisional surface 66 is exposed. Generally, the division of the aggregate board is performed, for example, by providing a nick along a scribing line and then applying stress therealong. This results in irregularities at the divisional surface 66, which is, therefore, in poor condition. In this way, the divisional surface 66 of the thermal printhead 51 is not only in a poor condition but also is exposed. Accordingly, during handling of the thermal printhead 51 such as incorporation into a casing, the edge of the substrate 52 along the longitudinal side 52a or the edges of the layers 61, 53, 62a, 63 may chip or break if the side surface of the substrate 52 along the longitudinal side 52a comes into contact with the casing or any other object.
Further, since the protective film 64 is formed by first forming the resist layer 65 and then removing the resist layer 65 after the growth of the protective film, an edge 64a of the protective film 64 results in a step which is equivalent in height to the thickness to the protective layer 64. When the thermal printhead 51 having such a step at the edge 64a of the protective film 64 is incorporated in an image forming apparatus, an edge of a recording paper 67 transferred in contact with the heating elements 53a may get caught at the step. In such a case, the image forming apparatus recognizes a paper jam because the recording paper 67 does not reach the heating elements 53a, thus resulting in stoppage of the apparatus.
Another method of making a thermal printhead is also proposed wherein a plurality of substrates 71 are laminated in such a manner as to expose a film-forming portion of each substrate 71 which is subsequently formed with a protective film by spattering (See e.g. JP-A-5-92596), as shown in FIG. 10. At the time of forming a glaze layer on the substrate 71, a plurality of projections 72 each made of the same material as the glaze layer are formed in a row on the surface of the substrate. These projections 72 are provided to prevent the laminated substrates 71 from rubbing against each other to avoid damaging of the individual electrodes or other elements due to such rubbing.
However, such a method makes it necessary to provide a mounting space on the surface of the substrate 71 for mounting the plurality of projections, which may bar increasing the density of the wiring pattern. Moreover, depending on the location of the projections 72, the edge of the recording paper may get caught at the projections 72 during the recording operation of the thermal printhead.