Abstract:
A method of making a thermal printhead including a primary substrate and an auxiliary substrate adjacent to the primary substrate. The method comprises the following steps. First, at least one positioning cutout is formed in either one or both of the primary and auxiliary substrates. The positioning cutout is formed at an edge of the selected substrate. Then, the primary and auxiliary substrates are positioned to each other by using a position-adjusting device provided with an upright pin fitted into the positioning cutout. Then, the electrical connection is established between the first and the second substrates via clip pins.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates to a thermal printhead and also a method of making the same. 
     2. Description of the Related Art 
     As is well known, a thermal printhead is provided with an elongated heating resistor divided into a multiplicity of heating dots. In operation, the heating dots are selectively energized, so that heat is selectively supplied to transfer ink ribbon or thermosensitive paper for printing required images on recording medium. 
     Referring to FIG. 11 of the accompanying drawings which shows a related art (not prior art), a thermal printhead may include a heating resistor unit  31 A, a signal relay unit  34 A and a heat sink  38  supporting these two units. The heating resistor unit  31 A includes a primary substrate  31  which is provided with a heating resistor  32  extending longitudinally of the substrate  31 . A plurality of drive ICs  33  are mounted on the substrate  31  for controlling the operation of the heating resistor  32  based on externally supplied print data. Though not illustrated, a wiring pattern is formed on the substrate  31  for connecting the drive ICs  33  to the heating resistor  32 . 
     The signal relay unit  34 A includes an auxiliary substrate  34  formed with a predetermined wiring pattern (not shown). This non-illustrated wiring pattern is connected to terminal pads  36  formed on the obverse surface of the substrate  34 . The terminal pads  36  are spaced from each other in the longitudinal direction of the substrate  34 . A connector  37  is attached to the reverse surface of the substrate  34  for making electrical connection between the non-illustrated wiring pattern and an external device or circuit (not shown). 
     The illustrated thermal printhead also includes a plurality of conductive clip pins  35  for electrically connecting the heating resistor unit  31 A to the signal relay unit  34 A. Each pin  35  is formed with a generally U-shaped portion and a straight lead portion integral with the U-shaped portion. The U-shaped portion is clipped onto a connection terminal (not shown) formed on the primary substrate  31 , while the lead portion is soldered to the relevant one of the terminal pads  36  corresponding to the non-illustrated connection terminal on the substrate  31 . 
     The thermal printhead having the above arrangement is fabricated in the following manner. First, the heating resistor unit  31 A and the signal relay unit  34 A are prepared. At this stage, the clip pins  35  are attached to the primary substrate  31  of the heating resistor unit  31 A but not connected to the signal relay unit  34 A yet. 
     Then, as shown in FIG. 12, the signal relay unit  34 A is mounted onto a first chuck member  41 . Thereafter, the heating resistor unit  31 A is mounted onto a second chuck member  42 . The first chuck member  41  may be stationary, while the second chuck member  42  may be movable in sliding engagement with the first chuck member  41  in the directions shown by the double head arrow A in the figure. 
     For positioning the heating resistor unit  31 A to the second chuck member  42 , a plurality of upright pins  43  are provided on the second chuck member  42 . In use, the substrate  31  of the unit  31 A is brought into engagement with the respective pins  33 , as shown in FIG.  12 . The first chuck member  41 , on the other hand, is provided with a positioning piece  46  having an L-shaped cross section, as shown in FIG.  13 . (The positioning piece  46  is omitted in FIG. 12 for convenience of illustration.) The positioning piece  46  includes an upright contact surface  46   a,  which is brought into engagement with a contact edge  34   a  of the auxiliary substrate  34 . With such an arrangement, the signal relay unit  34 A is positioned relative to the first chuck member  41 . 
     After the signal relay unit  34 A and the heating resistor unit  31 A are positioned on the first chuck member  41  and the second chuck member  42 , respectively, the second chuck member  42  is moved relative to the first chuck member  41  as shown by the above-mentioned arrow A. In this manner, the lead portion of each clip pin  35  can be aligned with a relevant one of the terminal pads  36  of the signal relay unit  34 A. Then, the lead portion of the clip pin  35  is soldered to the terminal pad  36 . 
     Finally, the two units  31 A,  34 A are removed from the chuck members to be mounted on the heat sink  38  (FIG.  11 ). At this stage, the connector  37  is attached to the auxiliary substrate  34 . 
     The use of the positioning piece  46  (FIG. 13) may suffer from the following drawback. 
     Specifically, for improving the production efficiency, the substrate  34  and many other identical substrates may be collectively obtained by cutting a large mother board into small pieces. In this case, the contact edge  34   a  of the resulting substrate  34  (and the other three edges as well) may often be formed with burrs due to the cutting operation. As readily understood, such burrs will prevent the substrate  34  from coming into proper contact with the contact surface  46   a  of the positioning piece  46 . This means that the substrate  34  will fail to be positioned accurately relative to the first chuck member  41 , which may result in inaccurate positioning of the clip pins  35  to the terminal pads  36  on the substrate  34 . 
     Instead of using the illustrated positioning piece  46 , at least two positioning holes  50  may be formed in the auxiliary substrate  34 , as shown in FIG. 14, by drilling for example. In this case, the first chuck member  41  may be provided with upright protrusions to be fitted into the positioning holes  50 . With such an arrangement, the auxiliary substrate  34  can be properly positioned on the first chuck member  41  since fewer burrs are formed on the surface of the drilled positioning holes  50  as compared to the contact edge  34   a.    
     However, as shown in FIG. 14, the positioning holes  50  are formed in the wiring pattern-forming region S of the substrate  34 . With such an arrangement, the room for providing the wiring pattern is disadvantageously restricted since the wiring pattern should avoid the location of the positioning holes  50 . 
     SUMMARY OF THE INVENTION 
     The present invention has been proposed under the circumstances described above, and its object is to provide a method of making a thermal printhead, whereby the positioning of the heating resistor unit to the signal relay unit can be performed accurately. 
     According to a first aspect of the present invention, there is provided a method of making a thermal printhead including first and second substrates spaced from each other, the first substrate being provided with a heating resistor. The method comprises the steps of: forming at least one positioning cutout in at least one of the first and the second substrates; positioning the first and the second substrates relative to each other; and establishing electrical connection between the first and the second substrates. The positioning cutout is to be formed at an edge of said one of the first and the second substrates. 
     Preferably, the positioning cutout may comprise a semi-cylindrical groove. Instead, the positioning cutout may have a triangular or rectangular cross section. 
     For facilitating the positioning of the substrates, said one of the first and the second substrates may be formed with two positioning cutouts each of which is arranged at an edge of said one of the first and the second substrates. 
     Preferably, said one of the first and the second substrates may be provided with two longer edges and two shorter edges, in which the two positioning cutouts may be arranged at the shorter edges. 
     Preferably, the positioning cutout may be formed in the second substrate. 
     The method of the present invention may further comprise the step of mounting said one of the first and the second substrates onto a position adjusting chuck member provided with a positioning pin to be fitted into the positioning cutout. 
     Preferably, the electrical connection between the first and the second substrates may be established via linear conductive members bridging between the two substrates. 
     According to a second aspect of the present invention, there is provided a thermal printhead comprising: a first substrate provided with a heating resistor; a second substrate associated with the first substrate; and connecting means bridging between the first and the second substrates, wherein at least one positioning cutout is formed at an edge of at least one of the first and the second substrates. 
     Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a thermal printhead according to the present invention; 
     FIG. 2 is a side view showing the same thermal printhead; 
     FIG. 3 is a plan view showing the same thermal printhead; 
     FIGS. 4 and 5 illustrate how the heating resistor unit of the above thermal printhead is aligned with the signal relay unit; 
     FIGS. 6A-6C illustrate how the signal relay unit of the above thermal printhead is obtained from a mother board; 
     FIGS. 7 and 8 show a different example of positioning grooves and pins used for positioning the signal relay unit; 
     FIGS. 9 and 10 show another example of positioning grooves and pins used for positioning the signal relay unit; 
     FIG. 11 is a perspective view showing an example of thermal printhead for better understanding of the present invention; 
     FIGS. 12 and 13 illustrate a step for fabricating the thermal printhead of FIG. 11; and 
     FIG. 14 is a plan view showing another example of thermal printhead for better understanding of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
     Reference is first made to FIGS. 1-3 showing a thermal printhead embodying the present invention. The illustrated printhead is provided with a heating resistor unit  1 A and a signal relay unit  2 A which is connected to the heating resistor unit  1 A via a plurality of clip pins  3 . 
     The heating resistor unit  1 A includes an insulating substrate  1 , an elongated heating resistor  6  and drive ICs  7 . The heating resistor  6  and the drive ICs  7  are arranged on the obverse surface of the substrate  1 . The substrate  1  is made of an insulating material such as alumina ceramic, and has a generally rectangular configuration. 
     The heating resistor  6  is divided into a multiplicity of small sections or “heating dots”, as well known in the art. The heating dots are selectively heated up under the control of the drive ICs  7 . As shown in FIG. 2, the drive ICs  7  are enclosed by a hard resin coating (C) formed on the substrate  1 . 
     A predetermined wiring pattern (not shown) is formed on the substrate  1  for connecting the drive ICs  7  to the heating resistor  6 . A plurality of connection terminals  8  (only one is shown in FIG. 2) are formed at a longitudinal edge  1   a  of the substrate  1 . The connection terminals  8  are arranged at predetermined intervals along the edge  1   a.  Each of the connection terminals  8  is connected to a relevant one of the drive ICs  7 . 
     The signal relay unit  2 A includes an insulating substrate  2  which may be made of GFR (glass fiber reinforced) epoxy resin and has a generally rectangular configuration. As shown in FIG. 3, the length L of the substrate  2  (called “auxiliary substrate” below) is substantially equal to that of the substrate  1  (called “primary substrate”). The width W 1  of the auxiliary substrate  2  is smaller than the width W 2  of the primary substrate  1 . 
     The signal relay unit  2 A also includes a plurality of terminal pads  9  formed on the obverse surface of the auxiliary substrate  2 . The terminal pads  9  are arranged at predetermined intervals along a longitudinal edge  2   a  of the substrate  2 . The signal relay unit  2 A further includes a connector  10  attached to the reverse surface of the auxiliary substrate  2 . As shown in FIG. 2, the connector  10  protrudes beyond another longitudinal edge  2   b  of substrate  2 . 
     A wiring pattern (not shown) is formed on the auxiliary substrate  2  for connecting the connector  10  to the terminal pads  9 . A flexible cable (not shown) may be inserted into the connector  10  for establishing the electrical connection between the thermal printhead and an external device or circuit (not shown). 
     According to the present invention, the auxiliary substrate  2  is formed, at its shorter edges  2   c  and  2   d,  with grooves  11  extending through the thickness of the substrate  2 . The grooves  11  are used for performing accurate positioning of the substrate  2  relative to a movable member, as will be described later. It should be appreciated here that the grooves  11  are disposed at the edges  2   c  and  2   d  of the substrate  2 , and therefore do not interfere with the wiring pattern-forming portion of the substrate  2 . 
     The heating resistor unit  1 A and the signal relay unit  2 A described above are mounted on a heat sink  4 , so that unfavorable heat generated at these units will be dissipated through the heat sink  4 . The heat sink  4  has a generally rectangular configuration and is substantially equal in length to the primary substrate  1  or auxiliary substrate  2  (see FIG.  3 ). The heat sink  4  is formed with a groove  4   a  (FIG. 2) extending in its upper surface, and this groove divides the upper surface of the heat sink  4  into two parts: a broader first portion  4   b  and a narrower second portion  4   c.  The first portion  4   b  is attached to the reverse surface of the primary substrate  1 , while the second portion  4   c  is attached to the reverse surface of the auxiliary substrate  2 . As best shown in FIG. 3, the primary substrate  1  is horizontally offset from the auxiliary substrate  1 . For securing the heat sink  4  to the primary and auxiliary substrates  1  and  2 , use may be made of an adhesive such as glue or double-sided tape. 
     As stated above, the electrical connection between the heating resistor unit  1 A and the signal relay unit  2 A is established by the clip pins  3 . Each of the clip pins  3  may be made of phosphor bronze and is arranged to connect one terminal  8  on the substrate  1  to a relevant one of the terminal pads  9  on the substrate  2 . 
     More specifically, as shown in FIG. 2, each clip pin  3  is provided with a linear lead portion  3   a  and a clip portion  3   b  integral with the lead portion  3   a.  The elastic clip portion  3   b,  having a generally C-shaped configuration, is clipped onto the connection terminal  8  on the primary substrate  1 . In the clipping state, the clip portion  3   b  is held in close contact with the connection terminal  8 . Thus, the clip portion  3   b  is properly connected to the terminal  8 . The lead portion  3   b,  on the other hand, is soldered to the relevant one of the terminal pads  9  on the auxiliary substrate  2 . Thus, the paired connection terminal  8  and terminal pad  9  are electrically connected to each other. Though not illustrated, the clip portion  3   b  and the relevant connection terminal  8  are enclosed by a resin material, whereby the clip pin  3  is secured to the primary substrate  1 . 
     With the arrangement described above, external electric signals supplied to the connector  10  are sent to the primary substrate  1  via the non-illustrated wiring pattern of the auxiliary substrate  2  and the clip pins  3 . Based on the thus supplied signals, the drive ICs  7  of the heating resistor unit  1 A are caused to supply electric currents selectively to the heating dots in the heating resistor  6 . Consequently, the selected heating dots are heated, whereby an desired image is formed on e.g. thermosensitive paper coming into contact with the heating resistor  6 . 
     According to the present invention, the alignment of the heating resistor unit  1 A with the signal relay unit  2 A (except for the connector  10 ) may be performed in the following manner. 
     Specifically, referring to FIG. 4, the alignment of the two units  1 A,  2 A may be performed with the use of a chuck device  20  consisting of a first chuck member  21  and a second chuck member  22 . 
     The first chuck member  21  has a rectangular configuration, and is provided with a sufficiently large upper surface for supporting the signal relay unit  2 A. The first chuck member  21  is provided with two positioning pins  23  extending upright from the upper surface of the first chuck member. The positioning pins  23  are spaced from each other by a predetermined distance corresponding to the distance between the two grooves  11  formed on the substrate  2  of the unit  2 A. 
     The second chuck member  22  has a generally rectangular configuration, and is provided with a step portion  22   a  on the upper side. The upper surface  22   b  of the second chuck member  22  is sufficiently large for supporting the heating resistor unit  1 A. The second chuck member  22  is provided with three positioning pins or stoppers  24  extending upright from the upper surface  22   b.  The stoppers  24  are arranged at predetermined locations for positioning the heating resistor unit  1 A relative to the second chuck member  22 . 
     For performing the alignment of the two units  1 A and  2 A, the signal relay unit  2 A is mounted onto the first chuck member  21  before the heating resistor unit  1 A is mounted onto the second chuck member  22 . As shown in FIG. 4, each of the positioning pins  23  is fitted into the relevant one of the grooves  11  of the substrate  2  in mounting the unit  2 A on the first chuck member  21 . The distance between the two positioning pins  23  is determined so that each pin  23  comes into contact with the concave surface of the relevant groove  11 . With such an arrangement, the signal relay unit  2 A is properly positioned relative to the first chuck member  21 . 
     After the mounting of the unit  2 A onto the first chuck member  21  has been completed, the heating resistor unit  1 A is mounted onto the second chuck member  22 . The positioning of the unit  1 A relative to the second chuck member  22  is performed by bringing the primary substrate  1  of the unit  1 A into simultaneous engagement with the three stoppers  24 . 
     Then, referring to FIG. 5, the second chuck member  22  is moved, as shown by the double head arrow A, relative to the first chuck member  21  in sliding contact therewith. When the respective clip pins  3  are aligned with the relevant terminal pads  9  (one pin for one pad), the movement of the second chuck member  22  is stopped. In this state, the clip pins  3  are soldered to the relevant terminal pads  9 , respectively. 
     Thereafter, the assembly of the two units  1 A,  2 A is detached from the chuck device to be mounted on the heat sink  4  (see FIG. 1 for example). Finally, the connector  10  is attached to the bottom surface of the substrate  2  of the signal relay unit  2 A. 
     According to the present invention, the signal relay unit  2 A and many other identical units may be collectively obtained in the following manner. 
     First, referring to FIG. 6A, a mother board  16  made of GFR epoxy resin is prepared. The mother board  16  is large enough to provide a predetermined number of rectangular substrates identical to the auxiliary substrate  2  shown in FIGS. 1-3. The mother board  16  is provided, on its obverse and reverse surfaces, with predetermined wiring patterns (not shown) which may be formed by photolithography. Together with these wiring patterns, a plurality of rows of terminal pads  9  are also formed on the obverse surface of the mother board  16 . In FIG. 6A, only eight rows  9   a - 9   h  of terminal pads are depicted for convenience of illustration. 
     Then, as shown in FIG. 6B, a plurality of through-holes  18  are bored in the mother board  16  by drilling for example. As illustrated, the through-holes  18  are arranged in a plurality of columns (five columns  18   a - 18   e  are shown). These columns are equally spaced from each other. In each column, the through-holes  18  are arranged at regular intervals. 
     The above-described through-holes  18  are formed after the locations of the non-illustrated wiring patterns and terminal pads  9  are detected by an optical sensing device (not shown). The detected locations of these elements are used as reference data for determining where to bore the through-holes  18  in the mother board  16 . In this manner, the through-holes  18  are to be formed at the desired locations relative to the terminal pads  9  and the non-illustrated wiring patterns. 
     Finally, referring to FIG. 6C, the mother board  16  is cut along cut lines L 1  and cut lines L 2  perpendicular to the cut lines L 1 . Preferably, the cutting may be first performed along the cut lines L 1 , and then along the cut lines L 2 . The cutting may be performed using a cutting blade. 
     As illustrated in FIG. 6C, the cut lines L 1  and L 2  define a plurality of rectangular regions  17 , which correspond to the auxiliary substrate  2  and other identical substrates. Each cut line L 1  extends through the center of the relevant through-holes  18  arranged in a column. The distance between the adjacent cut lines L 1  is d 1 , while the distance between the adjacent cut lines L 2  is d 2 . The distance d 1  is equal to the length L shown in FIG. 3, while the width W 1  is equal to the width W 1  shown in the same figure. 
     In the above manner, advantageously, a plurality of rectangular substrates identical to the auxiliary substrate  2  shown in FIGS. 1-3 are collectively obtained from a single mother board. 
     Though not illustrated in FIG. 6B, different kinds of through-holes other than the through-holes  18  may be collectively formed in the mother board  18 . These additional through-holes may be used for connecting the wiring pattern on the obverse surface of the mother board to the wiring pattern on the reverse surface of the mother board. The collective forming of the through-holes  18  and the additional through-holes is advantageous to improving the production efficiency since no additional step is required for forming the non-illustrated additional through-holes. 
     Differing from the process shown in FIGS. 6A-6C, the rectangular regions  17  defined by the cut lines L 1  and L 2  may be determined before the terminal pads  9 , the non-illustrated wiring pattern and the through-holes  18  are formed. After such regions  17  are determined, the terminal pads  9 , the non-illustrated wiring pattern and the through-holes  18  are formed, in each region  17 , with reference to the locations of the cut lines L 1  and L 2 . In this case, the center of each through-hole  18  should be located on the relevant one of the cut lines L 1 , as shown in FIG.  6 C. 
     The heating resistor unit  1 A shown in FIGS. 1-3 and many other identical units may also be collectively obtained from a single mother board in the following manner. First, a rectangular mother board (not shown) made of alumina ceramic is prepared. Then, the mother board is subjected to photolithography to form predetermined wiring patterns on its obverse and reverse surfaces. As in the mother board  16  shown in FIG. 6C, a plurality of regions are defined in the alumina ceramic mother board by a predetermined number of cut lines. Each of these identical regions is formed with the same wiring pattern. 
     Then, an elongated heating resistor may be formed in each of the above-mentioned regions of the mother board. Specifically, resistive paste is applied in each rectangular region by screen-printing, and then the applied paste is baked. Thus, the respective rectangular regions of the mother board are provided with a heating resistor. 
     Then, a protection coat may be formed on the mother board for covering the heating resistors and the wiring patterns. 
     Then, the mother board is divided along the predetermined cut lines to separate one rectangular region from another. 
     Then, required electronic components such as drive ICs are mounted on each separated region, while wire-bonding is performed for e.g. connecting the drive ICs to the wiring pattern on each separated region. A plurality of connection terminals (reference numeral  8  in FIG. 2) are formed a longitudinal edge of each separated region. 
     Finally, a resin coating made of e.g. epoxy resin may be formed on each separated region to cover the drive ICs and the bonding-wires. Thus, heating resistor units as shown in FIGS. 1-3 (reference numeral  1 A) are obtained. 
     Thereafter, a predetermined number of clip pins  3  are clipped onto each of the heating resistor units 
     The present invention is not limited to the above-described examples. For instance, each of the positioning grooves  11  of the substrate  2  may have a triangular cross section, as shown in FIGS. 7 and 8. In this case, the positioning pins  23  may also have a triangular cross section. Further, the positioning grooves  11  and the positioning pins  23  may have a rectangular cross section, as shown in FIGS. 9 and 10. 
     According to the present invention, the positioning grooves  11  may be arranged at the longitudinal edges of the substrate  2  other than at the illustrated shorter edges. The number of the positioning grooves  11  (and hence the positioning pins  23 ) may be larger than two. 
     In the illustrated preferred embodiment of the present invention, the grooves  11  and pins  23  are provided for positioning the signal relay unit  2 A to the first chuck member  21  (FIGS.  4  and  5 ). However, the same kind of positioning means may be used for positioning the heating resistor unit  1 A to the second chuck member  22 . 
     The present invention being thus described, it is obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to those skilled in the art are intended to be included within the scope of the following claims.