This invention relates to thermal printing and more particularly to means for assembling and making connections to a thermal printhead.
Thermal printing presents many advantages over mechanical means, particularly when implemented with electronic calculators. These advantages include lower cost of equipment and fewer moving parts, resulting in fewer mechanical failures. When a printed tape showing keyboard entries and calculated results is desired, a thermal printing system may be utilized to produce such an output. Using thermally sensitive paper which may be advanced as in conventional printers, a thermal printing system may accept signals from the calculator, decode those signals, use the decoded signals to selectively heat portions of the printhead, and thereby produce characters or numerals on heat sensitive paper.
With reference to FIG. 1, thermal printheads are generally produced with a plurality of heating elements or mesas 16 which may be selectively energized to produce a pattern of lines or dots. When the proper signals are provided to these elements or mesas 16 by means of metallized leads combined with proper advancement of the thermally sensitive paper 17 by means of a platen-advancing mechanism 18, characters or numerals are formed on the thermally sensitive paper 17. The heating elements 16 must be in very close contact with the paper 17 for satisfactory printing. These elements 16, as part of a thermal printhead assembly 14, are typically held in contact with the thermally sensitive paper 17 by a spring-loaded pivot arrangement 15.
The heating elements or mesas 16 are located on the one side of a substrate 10 which is usually ceramic or a similar material. The leads are generally brought through the ceramic substrate to the other side where pins 11 are attached. This combination of the ceramic substrate 10, the heating elements or mesas 16, and the leads 15 hereinafter described generally as a printer ceramic.
A standard lead frame 12 may be provided in which to insert the pins 11. From the lead frame 12, individual wire conductors 20 may be used to make connections to the printer logic 21. Because there is usually very little clearance between the substrate 10 and the platen 18 which advances the thermally sensitive paper 17 (not enough to locate a standard lead frame or other conventional connector), the leads from the heating elements 16 must be brought through the ceramic substrate 10. The lead frame 12 not only allows connection to the heating elements or mesas 16 but may also operate to hold the printer ceramic in the proper position. The printhead may also be held in place by attaching the ceramic substrate 10 to a pivoting bracket 15. A number of printer ceramic-lead frame assemblies may be installed adjacent to each other, in a printhead assembly 14, thereby providing for the printing of as many digits or characters as are desired.
There are a number of difficulties with such an arrangement. The process of bringing metal leads from the heating elements 16 through the ceramic substrate 10 and attaching pins 11 to them is more expensive than merely providing metallized leads on a single side of the substrate 10. In addition, the lead frames 12 are bulky and require a good amount of space in an area where space is at a premium. Further, the use of printer ceramic-lead frame assemblies often makes replacement of printer ceramics a difficult task since a housing 13 may first have to be disassembled. The attachment of the ceramic substrate 10 to a pivoting bracket 15 may require removal of screws, bolts, or other fasteners 19. Finally, because so much space on the ceramic substrate 10 is required by the pins 11 and lead frame 12, there is little room for thermal contact between the substrate 10 and metal parts of the thermal printer system which might act as heat sinks. The thermal contact that is available is some distance away from the heating elements or mesas 16 themselves, and the ceramic is not a good conductor of heat. The heating elements 16 of the thermal printer chip must cool quickly so that, as the thermally sensitive paper 17 is advanced, there will be no trailing effect. Unless proper heat sinking is available, the elements 16 will still be hot enough when the paper is advanced to leave a streak on the thermally sensitive paper 17. To avoid this problem, printing must be slowed down to allow time for the elements 16 to cool before the paper 17 is advanced, or heat sinking must be provided. Thus, proper heat sinks will allow faster operation.