Thermal head and its fabrication method

Abstract: There is realised a structure and a fabrication method of a very inexpensive thermal head in which a both face tape can be pasted or an adhering agent can be coated on a radiating plate (5) simply and conveniently and a flexible circuit board (4) or a radiating plate (5) with increased cost is avoided. Faces of the radiating plate (5) for fixing the heat generating resistor substrate (1) and the flexible circuit board (4) are constituted by a flat flush face and the flexible circuit board (4) is fixed to the radiating plate (5) by flexing the circuit board (4).

Description: The present invention relates to the structure of a thermal head mounted on a facsimile, a printer or the like. As shown by Fig. 3 and Fig. 4, in a conventional thermal head, a driver IC 3 for selectively generating heat at a heat generating resistor 2 is mounted on an insulative heat generating resistor substrate 1 where a heat generating register row and electrodes for feeding electricity thereto, are formed. A flexible circuit board 4 is electrically connected to the driver IC 3 for transmitting electric signals for operational control and supplying power source from an external circuit. The connection is generally carried out by electrically connecting the flexible circuit board 4 to electrode terminals of the heat generating resistor substrate 1 by soldering or the like. Further, the heat generating resistor substrate 1 and the flexible circuit board 4 are fixed to a radiating plate 5 for excellently radiating heat generated from the heat generating resistor 2. Fig. 3 and Fig. 4 show an example of the structure of a conventional thermal head. The heat generating resistor substrate 1 and the flexible circuit board 4 are fixed to the radiating plate 5 by using a both face tape or an adhering agent. However, when the radiating plate 5 is provided with a structure having a stepped portion, in the case where fixing operation is carried out by using a both face tape or an adhering agent, it is difficult to paste or coat the both face tape or the adhering agent uniformly over an entire face of the radiating plate in a single operation. Particularly, in the case of the structure having a stepped portion, an adhesive agent cannot be coated by adopting a printing system. As mentioned above, it is ideal that a face of the radiating plate 5 for fixing the heat generating resistor substrate 1 and the flexible circuit board 4 is a plane flush face having no stepped portion and accordingly, as shown by Fig. 3, a stepped difference between the flexible circuit board 4 and the radiating plate 5 caused by the thickness variation of the heat generating resistor substrate 1, has been dealt with by pasting a thickness substrate 6 in correspondence with the stepped difference to the flexible circuit board 4. However, according to this method, the fabrication cost of the flexible circuit board 4 is significantly increased and therefore, the method has hardly been adopted. Fig. 4 shows a structure where the radiating plate 5 is provided with a stepped portion. According to the structure, the radiating plate 5 is provided with the stepped difference and accordingly, when the fixing operation is carried out by using a both face tape, operation of pasting the both face tape at a portion of the radiating plate for pasting with the heat generating resistor substrate 1 and operation of pasting the both face tape at a portion of the radiating plate 5 for pasting with the flexible circuit board 4 must be performed separately. Further, in the case of using an adhering agent, as mentioned above, a printing system cannot be adopted and only a coating system by using a brush or the like or an injection system by using a dispenser or the like can be adopted and therefore, not only the operation is troublesome but also uniform coating is difficult to perform. According to the example of Fig. 4, there is provided a method in which in respect of the portion of the flexible circuit board 4 for pasting with the radiating plate 5, a both face tape is pasted previously on the side of the flexible circuit board 4. In this case, the cost is not as high as that of the structure where the thickness plate 6 in correspondence with the stepped difference is pasted as described above, however, an increase in the cost is unavoidable also in view of a number of steps of operation where operation of detaching exfoliation paper of the both face tape is needed and so on. Accordingly, in either of examples of Fig. 3 and Fig.4, factors of increasing the fabrication cost of the thermal head are resulted since the flexible circuit substrate 4 becomes expensive or the step of pasting a both face tape or the step of coating an adhesive agent becomes complicated. It is an object of the present invention to realize a structure and a fabrication method of a very inexpensive thermal head in which a both face tape can be pasted or an adhering agent can be coated on a radiating plate simply and conveniently without using a radiating plate with increased cost. The thermal head and its fabrication method are defined in claims 1 and 4 respectively. According to the present invention, a face of a radiating plate for fixing a heat generating resistor substrate and a flexible circuit board is made a flat flush face and the flexible circuit board is fixed to flex onto the radiating plate. By adopting the structure described above, a substrate for adjusting a stepped difference needs not to be fixed on the flexible circuit board and a both face tape needs not to be pasted thereon. Further, the both face tape or the adhering agent is coated on a flat face in a single operation. An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Fig. 1 is an outline sectional view of a structure according to the present invention; Fig. 2 is a partially-cut sectional view of the structure according to the present invention; Fig. 3 is a sectional view of a conventional example; and Fig. 4 is a sectional view of a conventional example. An explanation will be given of an embodiment of the present invention as follows. First, an explanation will be given of an example when a both face tape is used. A face of the radiating plate 5 for pasting with the heat generating resistor substrate 1 and the flexible circuit board 4 is a flush face with no stepped difference and accordingly, one sheet of a both face tape is pasted at a region of the face in conformity with a width for pasting with two of the substrates. In this case, only exfoliating paper of the both face tape need to exfoliate and for that purpose, the both face tape must be pressed to the radiating plate by a force to some degree and therefore, the adhering face is swept by squeezee or the like. At this occasion, according to the conventional structure where the face of the radiating plate is provided with a stepped portion, not only a squeezee having a special shape must be used to press the both face tape uniformly over an entire face of the pasted portion but also it has been difficult to adhere the tape to the radiating plate at a stepped portion with no gap. When an adhering agent is used, the adhering agent is coated at a portion of the radiating plate the same as that of the region for the both tape face. This operation is preferably carried out by the screen printing where the coating can be performed most simply, conveniently and uniformly. As described above, coating by the screen system cannot be carried out in the case of the structure where the face of the radiating plate is provided with a stepped portion and the screen system is applicable only when the radiating plate is flat. The heat generating resistor substrate 1 and the flexible circuit board 4 are previously connected to each other by soldering or the like. The heat generating resistor substrate 1 and the flexible circuit board 4 are mounted on the radiating plate 5 pasted with the both face tape described above and both of the heat generating resistor substrate 1 and the flexible circuit board 4 are brought into press contact to the radiating plate 5. In this case, although the flexible circuit board 4 is flexed as shown by Fig. 1, normally, disconnection of wire or the like does not constitute a problem at all in respect of the flexing of the substrate to such a degree. Meanwhile, the flexibility of the flexed portion can further be promoted by using a flexible circuit board having a structure as shown by Fig. 2. A substrate of both face wiring comprising a first conductor 7, an insulating layer 8 and a second conductor layer 9 is used normally as a circuit board of a thermal head. In this example, a flexed portion is constituted only by the first conductor layer 1. Both of operational performance and reliability are enhanced by using such a flexible circuit board. Further, the main stream of material of the conventional radiating plate is an extruded material of metallic aluminum and the fabrication is comparatively facilitated even with a radiating plate having a shape with a stepped portion. However, in recent years, an iron plate which is more inexpensive than aluminum is being used as a material therefor. The iron plate is of a plate shape both faces of which are flat and therefore, when a stepped portion as in the radiating plate 5 of Fig. 4 as mentioned above is added to a portion of the radiating plate 5 for pasting with the flexible circuit board 4, bending is needed. In such a case, even when an inexpensive material is used, the cost is increased and accordingly, it is important that a shape of a radiating plate is of a flat flush face with no stepped difference. According to the present invention, a both face tape can be pasted or an adhering agent can be coated on a radiating plate simply and conveniently and a flexible circuit board or a radiating plate with increased cost needs not to use and therefore, a very inexpensive thermal head can be fabricated.

Claims: A thermal head comprising: a heat generating resistor substrate constituted by a plurality of heat generating resistors and a driver IC for selectively generating heat at the heat generating resistors on an insulating substrate; a flexible circuit board connected to the heat generating resistor substrate for feeding electricity to the driver IC; a radiating plate to which the heat generating resistor substrate and the flexible circuit board are fixed; and wherein a face of the radiating plate for fixing with the heat generating resistor substrate and a face of the radiating plate for fixing with the flexible circuit board constitute a flat flush face and the flexible circuit board is fixed to the radiating plate by flexing the flexible circuit board. The thermal head according to claim 1, wherein the flexible circuit board includes a plurality of conductor layers interposing insulating layers and the conductor layer at a flexed portion of the flexible circuit board is the conductor layer electrically connected to the heat generating resistor substrate. The thermal head according to claim 1, wherein the radiating plate is formed by an iron plate material. A method of fabricating the thermal head according to claim 1, said method comprising : a step of forming of an adhering layer simultaneously on a fixing face of the radiating plate to which the heat generating resistor substrate is attached and on a fixing face of the radiating plate to which the flexible circuit board is attached; a step of fixing the heat generating resistor substrate to the radiating plate via the adhering layer ; and a step of fixing the flexible circuit board to the radiating plate via the adhering layer by flexing the flexible circuit board. The method of fabricating a thermal head according to claim 4, wherein the adhering layer comprises an adhering agent, and the adhering layer is formed by a screen printing.

Inventors: SHOJI NORIYOSHI, TAKIZAWA OSAMU, SHOJI, NORIYOSHI, TAKIZAWA, OSAMU
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1998
Main classification: B41J2
IPC codes: B41J2, H05K3