THERMAL INK JET PRINT HEAD

Abstract: The present invention relates to a thermal ink jet print head comprising an electrically insulating substrate member (10), an array of heater elements (12) formed on the substrate member, a first array of electrical connection members (14) formed on the substrate member and in electrical connection with the array of heater elements, and a second array of electrical connection members (16) formed on the substrate member. A nozzle plate is mounted adjacent to the substrate member and connected to a source of ink (46), and a plurality of nozzles in the nozzle plate each disposed adjacent to a respective one of the heater elements, whereby upon connection of an electrical signal to a selected one of the first array of electrical connection members, a drop of ink is ejected from the corresponding nozzle. According to the invention the print head is characterised in that the array of heater elements and the first array of electrical connection members are all formed on a first surface (11) of said substrate member, and said second array (16) of electrical connection members are formed on a second surface of the substrate member, and the print head further comprises a third array of electrical connection members (15) extending through the substrate member to provide electrical connection between the first and the second arrays of electrical connection members.

Description: This invention relates to an ink jet printing system and more particularly to a print head for a thermal drop-on-demand ink jet printing system. A thermal drop-on-demand ink jet printing system is known in which the print head includes a heater which is selectively energised to form a "bubble" in an adjacent mass of ink. The rapid growth of the bubble causes an ink drop to be ejected from a nearby nozzle. Printing is accomplished by energising the heater each time a drop is required at that nozzle position to produce the desired printed image. One thermal drop-on-demand ink jet printing system is described in US-A-4,520,373. The print head in this system utilises a heater substrate in which the ink drops are ejected in a direction parallel to the plane of the heater element. The system comprises a plurality of chips each having the heater elements, the conductor elements and a control transistor array all on one side of a chip, with a heat sink on the other side of the chip. Another thermal drop-on-demand ink jet printing system is described in US-A-4,601,777 in which the ink drops are ejected in a direction normal to the plane of the heater element. This printing system comprises a chip which includes an array of heating elements and addressing electrodes, a silicon substrate into which an array of grooves is anisotropically etched, and a fixedly mounted electrode board. The silicon substrate is bonded to the heater chip so that one end of each of the grooves is aligned to serve as a nozzle and a second recess serves as the ink manifold. Electrical leads from the heater chip are wire bonded to corresponding conductor pads on the electrode board. The thermal drop-on-demand ink jet printing systems referred to above are unsuitable for a high resolution array having a large number of channels since their design does not permit the required electrical connections to be made in a compactly designed print head. The object of the present invention is to provide a thermal ink jet print head having a large number of channels for a thermal drop-on-demand ink jet printing system capable of printing with high resolution. The present invention relates to a thermal ink jet print head comprising an electrically insulating substrate member, an array of heater elements formed on the substrate member, a first array of electrical connection members formed on the substrate member and in electrical connection with the array of heater elements, and a second array of electrical connection members formed on the substrate member. A nozzle plate is mounted adjacent to the substrate member and connected to a source of ink, and a plurality of nozzles are formed in the nozzle plate, each disposed adjacent to a respective one of the heater elements, whereby upon connection of an electrical signal to a selected one of the first array of electrical connection members, a drop of ink is ejected from the corresponding nozzle. According to the invention the print head is characterised in that the array of heater elements and the first array of electrical connection members are all formed on a first surface of the substrate member, and the second array of electrical connection members are formed on a second surface of the substrate member, and the print head further comprises a third array of electrical connection members extending through the substrate member to provide electrical connection between the first and the second arrays of electrical connection members. In order that the invention may be more readily understood, an embodiment will now be described with reference to the accompanying drawings, in which Fig. 1 is a top view showing a thermal ink jet print head according to the present invention. Fig. 2 is a side view of the print head illustrated in Fig 1 sectioned along the lines 2-2, Fig. 3 is a top view of a multi-nozzle thermal ink jet print head array incorporating the present invention, Fig. 4 is a perspective view of an ink jet print head. Fig. 4a is a partial plan view of the nozzle plate of the print head illustrated in Fig 4 sectioned along the lines a-a, Fig. 5 is a top view of an alternative embodiment of the heater assembly of a thermal ink jet print head designed for multiplexed operation, Fig. 6 is a back view of the heater assembly of Fig. 5, Fig. 7 is a top view of a further embodiment of the heater assembly for a thermal ink jet print head array designed for multiplexed operation, Fig. 8 is a plan view of an intermediate layer interconnect pattern for the heater assembly of Fig. 7, Fig. 9 is a plan view of the contact pads for the back of the heater assembly of Fig. 7, Fig. 10 is a top view of another embodiment of the heater assembly for a thermal ink jet print head array designed for multiplexed operation, and Fig. 11 is a schematic diagram of a circuit for driving a multiplexed thermal ink jet print head. Referring to Figs. 1 and 2, a thermal drop-on-demand ink jet print head, according to the present invention, comprises an electrically insulating substrate member 10, upon one surface 11 of which is formed an array of resistive heater elements 12, only one of which is shown in Figs. 1 and 2 of the drawings. A common electrode 13, and one of an array of control electrodes 14 are provided in electrical contact with each resistive heater element 12. The control electrodes 14 each extend to an electrical contact with a conductive feed through element 15 which passes through the substrate 10 and makes electrical contact with a solder pad 16 on the reverse surface 17 of substrate 10. Substrate member 10 should also have suitable thermal characteristics. These characteristics include forcing heat into the marking fluid such as ink at the beginning of the heat cycle and permitting the heat to dissipate into the substrate later in the heat cycle to prevent heat buildup in the print head. One suitable structure of the substrate member 10 comprises a thermal delay layer, such as a SiO₂ layer 2 to 3 microns thick, on a suitable ceramic substrate material. The top surface 11 of the substrate member 10 for a specific embodiment of an array of resistive heater elements 12 is shown in Fig. 3. The resistive heater elements 12 are aligned in two spaced rows 20, 22, and the heater elements 12 in one row 20 may be staggered with respect to the heater elements 12 in the other row 22, as shown in Fig. 3, if desired. The common electrode 13 makes contact with each of the resistive heater elements 12, and one of a plurality of control electrodes 14 also makes contact with each of the resistive heater elements 12. Feed through elements 15 are provided to make contact between each of the control electrodes and a corresponding solder pad 16 on the reverse surface of the substrate 10. The solder pads 16 on the reverse surface of substrate 10 are shown in dashed lines in Fig. 3. A larger solder pad 18 is provided for the common electrode, and in this case several feed through elements 15 are provided to reduce the current density of each element 15. Note that the solder pads 16 are provided in four spaced rows 24, 25, 26, 27 so that the electrical connections can be provided within the same physical spacing as the resistive heater elements. The large holes 28 between the two rows 20, 22 of resistive heater elements 12 are ink inlets. Fig. 4 is an exploded view of a thermal drop-on-demand ink jet print head which can use a heater chip 30 of the type shown in Fig. 3. The heater chip 30 and a nozzle plate 32 are combined with a chip mount 34 to produce a pluggable unit which has both fluid and electrical connections. As shown in Fig. 4a, the nozzle plate 32 comprises a plurality of nozzles or orifices 36, each of which has a channel 38 which leads to a manifold 37 which is positioned to receive ink from ink supply openings 28. The nozzle plate is bonded in position so that a nozzle is opposite each of the resistive heating elements 12 so that energizing a selected resistive heating element 12 causes a drop of ink to be ejected from the corresponding nozzle 36. Chip mount 34 has an array of electrical connecting pins 40 which are spaced to match corresponding openings in electrical connector 42. In addition, ink connector 44 provides a fluid tight path for ink from ink reservoir 46 to move through openings 28 and channels 38 to each of the nozzles 36. The print head shown in Fig. 4 is symmetrical about the vertical centre line (except for the offset in the nozzles in the two rows 20, 22) and therefore is completely modular. Any number of these modules can be stacked vertically to provide any printer from a low end printer application up to page printer and colour printing applications. As the number of nozzles increases, however, the electrode fan-out and the electrical connections to the supporting electronic circuits become increasingly complex. Furthermore, the cost to drive the printer increases significantly since a large number of parallel electronic driver circuits is also required. It is therefore desirable to reduce the number of electrical connections and electronic drivers. The lower operating frequency, narrow drive pulses and non-linear (threshold) bubble nucleation of the bubble jet permit multiplexing to become an effective way to achieve this reduction in electrical connections and electronic drivers. The embodiment of the invention shown in Fig. 5 shows the front surface of the substrate 47 for a multiplex design print head. The print head comprises four parallel spaced rows 48, 49, 50, 51 of resistive heater elements 12 each having an electrical connection to a common electrode 52 and to one of a set of multiplexing bar electrodes 53. A plurality of feed through elements 15 are provided to make electrical contact from each bar electrode 53 to a secondary multiplexing bar electrode 54 (Fig. 6) on the reverse side of the substrate, and a plurality of openings 55 are provided to distribute ink from the ink reservoir. Each of the secondary multiplexing bar electrodes 54 is electrically connected to two of the feed through elements 15 so this design represents a multiplexing by a factor of four thereby resulting in the printing of the same print data at the same resolution with the use of only one quarter the number of electronic drivers. It is obvious that other multiplexing factors could as well be chosen. In some printing applications the required array of conductor lines cannot be reliably produced within the space confines dictated by the required print resolution. In that case the top, intermediate and bottom surfaces of a multilayer ceramic substrate can be used to produce a network of electrical interconnections. An example of such a print head is shown in Figs. 7, 8 and 9. A view of the pattern on the top surface of the multilayer substrate 60 is shown in Fig. 7. An array of four parallel spaced rows 61, 62, 63, 64 of resistive heater elements 12 is provided, and each of the heater elements is provided with electrical contact to one of the common electrodes 65 and to one of the data electrodes 66, each of which is common to two of the resistive heater elements 12. An array of conductive feed through elements 15 is provided with one of the elements making contact with one of the data electrodes 66 and a conductor pattern on an intermediate layer 67 of substrate 60 (Fig.8). As shown in Fig. 8 each of the conductor patterns 68 is common to two of the feed through elements 15 which are in electrical contact with data electrodes 66. Each of the conductor patterns 68 extends near the edge of substrate 60 and is in electrical contact with a conductive feed through one of the elements 15b which extend through the other intermediate layers, without making electrical contact with the conductor patterns on other intermediate layers, to the back surface of the substrate 60 as shown in Fig. 9. Each of the conductive feed through elements 15b makes electrical contact with one of the contact pads 69 on the back surface of the substrate so that suitable electrical connections can be made to the print head without interference to the front side of the substrate 60 where the resistive heater element 12 array is provided. A further embodiment of a thermal drop-on-demand ink jet print head is shown in Fig. 10. In this embodiment a single opening is built to provide not only the electrical contact but also the opening to distribute ink to the various orifices. In this print head, four parallel spaced rows 70, 71, 72, 73 of resistive heater elements 12 are provided along with an electrical contact with common electrodes 74 and with hollow conductive feed through elements 75 which also serve as conduits to distribute the ink from the ink reservoir at the rear of the substrate to the front of the substrate to the array of nozzles. A multiplexing drive circuit is shown in Fig. 11. In this circuit, the four vertical common bars 80, 81, 82, 83 are supplied with adequate voltage pulses in sequence, i.e. bar 80 is supplied the first pulse at time t1, bar 81 is supplied the second pulse at time t2 and so on. There is no time-overlap of these pulses and the frequency of occurrence is much higher than the repetition rate of the print clock. Each of the data drivers 84a, 84b, 84c --- 84n is turned ON synchronously with the four drive pulses when data signals corresponding to each column are presented to that data driver. In this way, a particular resistive heater element 12 can be turned ON by the full voltage differential and this full voltage differential is present only when a supply voltage pulse (through bars 80, 81, 82 or 83) and a data signal through data drivers 84a, 84b, 84c --- 84n) are concurrently presented to its electrodes. All other resistive heater elements 12 (i.e. the "inactive" ones) also see the drive voltages across the vertical bars 80, 81, 82, 83. However, the voltage differential on each "inactive" element 12 is low enough that no ink vaporisation can be accomplished. The substrate member has been described as having a plurality of layers for circuit and ink supply interconnection. Additional layers can be provided, if desired, to provide cooling for the print head during operation. These layers can take the form of cooling fluid channels to provide thermal cooling for the print head. Cooling fluid is circulated through the channels during operation of the print head to absorb heat from the substrate for disposal at some location external to the substrate.

Claims: A thermal ink jet print head comprising an electrically insulating substrate member (10), an array of heater elements (12) formed on said substrate member, a first array of electrical connection members (14) formed on said substrate member and in electrical connection with said array of heater elements, a second array of electrical connection members (16) formed on said substrate member, a nozzle plate (32) mounted adjacent to said substrate member and connected to a source of ink (46), and a plurality of nozzles (36) in said nozzle plate each disposed adjacent to a respective one of said heater elements (12), whereby upon connection of an electrical signal to a selected one of said first array of electrical connection members, a drop of ink is ejected from the corresponding nozzle, characterised in that, said array of heater elements (12) and said first array of electrical connection members (14) are all formed on a first surface (11) of said substrate member, and said second array (16) of electrical connection members are formed on a second surface of said substrate member, and said print head further comprises a third array of electrical connection members (15) extending through said substrate member (10) to provide electrical connection between said first and said second arrays of electrical connection members (14,15). A thermal ink jet print head as claimed in claim 1 characterised in that each of said third array of electrical connection members is formed with a central opening (75) to convey ink from said source to said nozzle plate. A thermal ink jet print head as claimed in either of the preceding claims characterised in that said array of heater elements is formed in a plurality of rows. A thermal ink jet print head as claimed in claim 3 characterised in that the heater elements in one of said plurality of rows are staggered with respect to the heater elements in another of said plurality of rows.

Inventors: ELDRIDGE, JEROME E., KELLER, GARY S., LEE, FRANCIS C., NELSON, GEORGE N., OLIVE, GRAHAM
Applicants: LEXMARK INTERNATIONAL, INC.
Application year: 1988
Main classification: B41J2
IPC codes: B41J2