1. Field of the Invention
This invention relates to a thermal printing head. More specifically, the present invention relates to a line-type thermal printing head which is used in a facsimile machine for example.
2. Description of the Prior Art
As is well-known, line-type thermal printing heads are widely used in facsimile machines for printing out transmitted information on thermosensitive paper. For the convenience of explanation, a typical prior art line-type thermal printing head is shown in FIG. 10 of the accompanying drawings.
As shown in FIG. 10, the prior art line-type thermal printing head comprises a metallic support plate 1 working also as a heat sink, and an insulating head substrate 2 mounted on the support plate. The upper surface of the head substrate 2 carries a heating resistor line 3 extending longitudinally of the substrate, and a common electrode 4 extending in parallel to the resistor line 3. The upper surface of the head substrate further carries an array of drive IC's 5.
The respective drive IC's 5 are electrically connected to the resistor line 3 through individual electrodes 6, whereas the common electrode 4 is electrically connected to the resistor line through comb-like teeth 7 in staggered relation to the individual electrodes 6. The common electrode 4 has a pair of power supply terminals 8 at both ends of the substrate for electrical connection to an external power source (not shown).
In operation, power is supplied at the connection terminals 8 to pass current through the common electrode 4, the respective comb-like teeth 7, the resistor line 3, and the individual electrodes 6. Thus, the resistor line 3 provides separate current paths divided by the individual electrodes 6 and the comb-like teeth 7, thereby forming a line of heating dots. The drive IC's 5 selectively actuate the heating dots to perform intended printing.
The common electrode 4, though made of a good conductive material, still exhibits its inherent resistivity. Therefore, the voltage applied across the respective heating dots of the resistor line 3 reduces toward the center of the resistor line due to an unavoidable voltage drop along the common electrode 4. Thus, the heating capability (namely the printing quality) becomes uneven along the length of the heating line 3. This problem is particularly serious if the overall length L' of the thermal head is large.
A solution to the above problem is to reduce the voltage drop by applying a wider metallic film or conductive layer over the common electrode, as disclosed in Japanese Patent Application Laid-open No. 57-24273, Japanese Utility Model Application Laid-open No. 61-183652 or Japanese Patent Application Laid-open No. 64-18649 for example. However, this solution may give rise to a new problem that the overlying film or layer, which is located above the common electrode, may come into interfering contact with the platen which is used for pressing the thermosensitive paper against the resistor line. Further, since the overlying film or layer is wider than the common electrode, the overall width W' (see FIG. 10) of the thermal head must be correspondingly increased.