Abstract:
A method of manufacturing a fluid ejection device includes attaching a plurality of resistors to a portion of the substrate for heating fluid; attaching a plurality of solderable interconnect pads to another portion of the substrate; soldering at least one chip onto the interconnect pads; and electrically connecting the resistors with the chip by attaching a layer having circuit traces that run from the interconnect pads to the resistors, whereby the chip is operable to control the resistors.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This is a divisional of copending application Ser. No. 08/694,964 filed on Aug. 9, 1996. 
    
    
     TECHNICAL FIELD 
     This invention relates to a wide array thermal ink-jet print head for a printer. 
     BACKGROUND OF THE INVENTION 
     Thermal ink-jet printers have become widely popular as inexpensive printing devices. An essential feature of a thermal ink-jet printer is a print head that is controlled to selectively eject tiny droplets of ink onto a printing surface, such as a piece of paper, to form desired images and characters. 
     The print head generally has an architecture plate with multiple tiny nozzles through which ink droplets are ejected. Adjacent to the nozzles are ink chambers, where ink is stored prior to ejection through the nozzles. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. 
     The print head usually is formed of a sandwich construction, having a substrate at its base. Attached to the substrate is a layer of circuit traces and a layer of the resistors. The resistors are overlaid with a protective, passivation layer. The architecture plate is bonded to the substrate and substantially covers the other layers. 
     The resistors are lined up beneath the chambers in the architecture plate. Electrical signal inputs to the resistors “fire” the resistors, heating the resistors and thereby a volume of ink within the adjacent ink chamber. The heating generates a vapor bubble in the ink to force an ink droplet out of the nozzle. 
     Usually, remote bus lines provide signal inputs from an external signal source to the resistors on the print head. Oftentimes, the signals are delivered through multiplexed circuitry on the substrate. The print head is generally connected to these bus lines by a thin flat electrical cable, such as a tape automated bond (“TAB”) circuit. A TAB circuit generally has copper leads supported on a copper-coated tape. The tape is usually bonded onto the print heads with gold bump contacts. Conventional TAB circuit bonding cannot be done over live silicon circuitry without damaging the circuitry and requires use of an encapsulant to protect the leads from the ink, which adds a process step and decreases the robustness of the bond. Nevertheless, TAB circuit bonding is generally used because it is space-efficient, allowing the contact to be made in a tiny area. 
     In most ink-jet printers, the print head is mounted on an ink pen that is mounted to a carriage that traverses the printing surface to move the print head back and forth over the printing surface. Thus, the print head can be made relatively small in comparison to the width of the printing surface because the ink pen traverses the width of the printing surface. However, it takes the carriage a certain amount of time to traverse the paper, which slows down the speed of printing. 
     One way to increase the printing speed is to increase the number of nozzles on the print head, which necessitates an increase in the size of the print head. However, increasing the size of the print head requires a larger architecture plate, and a large architecture plate increases the likelihood of failure of the bonding of the interface between the architecture plate and the substrate. One reason for such failure is that the materials for the substrate and the architecture plate usually have considerably different coefficients of thermal expansion. Thus, the sandwich construction may bow or delaminate after assembly as the print head is heated and cooled during operation. 
     Sometimes, components within ink-jet printers are attached together by flip-chip processing. Flip-chip processing is the term used to describe the method of attaching two parts, such as a die and a substrate, by providing both parts with solderable pads, depositing a solder ball on the solderable pad on the substrate, then placing the solderable pad of the die on top of the solder ball, and heating and pressing the die and substrate to form a solder joint. Often, the solder ball is formed by depositing solder paste on the solderable pad on the substrate and heating the paste and pad to reflow the solder paste into a solder ball. 
     Oftentimes, after the two parts are attached by flip chip processing, an underfill made of liquid epoxy will be shot between the parts and allowed to wick therebetween, in a process separate from flip-chip processing. The underfill layer comprising the cured epoxy fills gaps between the parts and relieves some of the stress on the solder joint. 
     Page wide array printheads have been disclosed in U.S. Pat. No. 6,135,586, filed on behalf of Paul H. McClelland et al. on Oct. 31, 1995, titled “Large Area InkJet Printhead” and U.S. Pat. No. 6,017,117, filed on behalf of Paul H. McClelland et al. on Oct. 31, 1995, titled “Printhead With Pump Driven Ink Circulation”. These applications are assigned to the assignee of the present invention. 
     SUMMARY OF THE INVENTION 
     The invention generally includes a method of manufacturing a fluid ejection device by providing a substrate; attaching a plurality of resistors to a portion of the substrate for heating fluid; attaching a plurality of solderable interconnect pads to another portion of the substrate; soldering at least one chip onto the interconnect pads; and electrically connecting the resistors with the chip by attaching a layer having circuit traces that run from the interconnect pads to the resistors, whereby the chip is operable to control the resistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial top schematic view of the print head of the present invention. 
     FIG. 2 is a partial top view of the print head of the present invention, with the driver chip removed. 
     FIG. 3 is a sectional view of an interconnect area of the present invention, taken along line  3 — 3  of FIG.  2 . 
     FIG. 4 is a sectional view taken along line  4 — 4  of FIG. 2 of the present invention, showing the process of applying solder paste. 
     FIG. 5 is a sectional view, like FIG. 4, after reflowing the solder paste and removing any flux residues. 
     FIG. 6 is a sectional view, like FIG. 4, but including the driver chip. 
     FIG. 7 is a sectional view, like FIG. 4, showing the driver chip bonded to the interconnect area at the print head. 
     FIG. 8 is a sectional view, like FIG. 4, but showing the use of a removable stencil. 
     FIG. 9 is a sectional view, like FIG. 8, but with the stencil removed. 
     FIG. 10 is a partial side view of an ink ejection area of the print head of FIG.  2 . 
     FIG. 11 is a partial side view, like FIG.  10 . but of an alternative embodiment of the present invention in which an underfill layer defines ink chambers. 
     FIG. 12 is a partial side view, like FIG.  10 . but of an alternative embodiment of the present invention having an edge-firing architecture plate. 
     FIG. 13 is a partial sectional view, similar to FIG. 10, but taken through an alternative embodiment of the present invention having a face-firing architecture plate. 
     FIG. 14 is a sectional view of the architecture plate, taken along line  14 — 14  of FIG.  2 . 
     FIG. 15 is a partial side view of the face-firing architecture plate of FIG. 13 being cast on a mandrel. 
     FIG. 16 is a partial side view of the architecture plate detaching from the mandrel of FIG.  13 . 
     FIG. 17 is a partial side view of the architecture plate of FIG. 13 after chem-lapping. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A print head  20  in accordance with the present invention is illustrated in FIG.  2 . The print head  20  is mounted on a printer (not shown) and selectively ejects ink droplets onto a printing surface (not shown), such as a piece of paper, which is advanced through the printer. 
     As shown in FIGS. 1 and 2, the print head  20  of the present invention has two main areas: an ink-ejection area  22 , from which ink is ejected, or “fired”, onto paper adjacent the ink-ejection area  22 , and an interconnect area  24  that includes a driver chip  26 , or multiple driver chips, for sending signals to the ink-ejection area  22  to eject the ink from the ink-ejection area  22 , as will be described in more detail below. 
     The ink-ejection area  22  has an architecture plate  30  having chambers  32  for containing small amounts of ink, as best shown in FIG.  10 . Beneath the chambers  32  are resistors  34  that are heated upon receiving a signal from the driver chip  26 . The heat from the resistor heats the ink in the adjacent chamber  32 , which expands the ink, forcing the ink from the chamber  32  onto the paper. 
     Both the ink-ejection area  22  and the interconnect area  24  are fabricated on a common substrate  36 , as shown in FIG.  2 . The illustrated substrate  36  is an elongated, rectangular block of amorphous silicon with a thickness of 25-50 micrometers (about 1-2 mils). Silicon is particularly well-suited because it is flexible out-of-the-plane and yet is very stiff in the plane, as well as being chemically unreactive at temperatures near room temperature. The stiffness in the plane allows good registration with the architecture plate  30 , which is particularly important when the substrate  36  and plate  30  are several inches long. Nevertheless, the substrate  36  could be made of other materials, such as glass, ceramic, or a metal substrate with a ceramic coating. 
     As partially indicated in FIG. 3, a circuit trace layer, comprising a plurality of discrete, conductive circuit traces  40 , extends across substrate  36 . The circuit traces  40  connect the ink ejection area  22  with the interconnect area  24  to deliver signals from the driver chip  26  to the resistors  34 , as described in more detail below. The circuit traces  40  are deposited on the substrate  36  by sputtering or, alternatively, by evaporation. The traces  40  preferably are made of tantalum-aluminum/aluminum, approximately 0.6 micrometers thick. 
     A passivation layer  46 , shown in FIG. 3, is deposited over the traces  40 . The passivation layer  46  preferably is made of nitride/carbide and is 0.25 micrometers (approximately 0.01 mils) thick. 
     In the interconnect area  24 , vias  50  are created through the passivation layer  46 , such as by etching, to expose portions of the circuit traces  40  to provide an electrical path between the traces  40  and interconnect pads  52  that are sputtered onto the vias  50  and passivation layer  46 . The interconnect pads  52  are deposited by sputter-coating, electroless plating, or a comparable process. The interconnect pads  52  should be solderable. In other words, the interconnect pads  52  should be able to be wetted by solder. The interconnect pads  52  preferably are made of nickel-vanadium/gold and in the illustrated embodiment are disk-shaped with a diameter of 125 micrometers (approximately 5 mils) and a thickness of approximately 1500 angstroms. 
     As shown in FIG. 2, the interconnect pads  52  are arranged in parallel rows, extending longitudinally along the portion of the substrate  36  shown. Each interconnect pad  52  is connected to a single resistor  34 , except a ground interconnect pad  54  in each row is reserved for ground, as shown in FIG.  3 . Preferably, the pad  54  closest to the resistors  34  is the pad for the common line of a set of resistors  34 , as illustrated in FIG. 2, so as to decrease the line resistance for the line that will carry the most current between the chip  26  and the resistor  34 . The illustrated embodiment shows a print head  20  with a resolution of 600 dots per inch having eight interconnect pads  52  per row on a 250 micrometer pitch, meaning that the centers of the illustrated pads are spaced apart 250 micrometers. 
     The driver chip  26 , which sends firing signals to the resistors, is attached to the interconnect pads  52  by a combination soldering and polymer bonding technique of the present invention. The technique involves spinning on a thin underfill layer  60 , of approximately 25-125 micrometers (about 1-5 mils) thick, over the circuit traces and interconnect pads  52  (FIG.  3 ). The underfill layer  60  could also be deposited by thick-film lamination. Preferably the underfill layer  60  is made of a photoimageable polyimide. 
     Openings  62 , preferably circular, are created in the underfill layer  60  to expose the pads  52  and to define a cavity  64  around each pad, as shown in FIG.  3 . The openings  62  could be created by patterning the layer by photoimaging and developing or by chemical etching. The openings  62  could also be created by laser drilling, for example. 
     Solder paste  66  is deposited in the cavities  64 , as shown in FIG.  4 . The solder paste  66  is reflowed to form a dome-shaped solder ball  68  on top of each interconnect pad  52 , as illustrated in FIG.  5 . The solder paste  66  is approximately 50% solder alloy (containing, for example, tin, lead, bismuth, silver, or indium) and 50% flux by volume. 
     Preferably, the cavity  64  volume is appropriate for the amount of solder paste  66  necessary to create a solder ball  68  of a sufficient size to attach to a solder pad on one of the driver chips  26  as explained below. In this way, the underfill layer  60 , which defines the cavity volume, acts as an in situ stencil to measure and contain the solder paste  66 , allowing the appropriate amounts of solder paste  66  to be applied quickly. Preferably, the solder paste  66  is applied in and around the openings  62 , and a squeegee  72  is pushed across the surface  74  of the underfill layer  60  to torce the paste  66  into the cavities  64 , to remove any excess solder paste  66  from the surface  74  of the underfill layer  60 , and to level off the solder paste  66  in the cavities  64 , as indicated in FIG.  4 . 
     If a volume of solder paste  66  larger than the volume of the cavity  64  in the underfill layer  60  is needed, a removable, auxiliary stencil  76  could be placed on top of the underfill layer  60  to create second layer openings  78  to enlarge the volume of the cavity (FIG.  8 ). Solder paste  66  could be deposited while the stencil  76  is in place. The stencil could then be removed before or after the solder ball is formed. The former is indicated in FIG.  9 . 
     To reflow the solder paste  66 , the paste  66  is heated, preferably to 220 degrees celsius in an inert environment, such as nitrogen. Other inert environments, such as argon and helium, could also be used. The flux residues may be removed, such as by washing away, before assembling the driver chip  26  on the substrate  36  to provide better adhesion of the underfill layer  60  to the driver chip  26 . Eliminating the flux residues is beneficial because the flux residues promote corrosion in high-humidity environments. 
     A volatile flux could be applied to the solder ball  68  or the interconnect pad  52  on the substrate  36  to promote solder wetting to the interconnect pad  52  on the substrate  36 . The flux is useful when the surface of the solder ball  68  is too oxidized to permit fluxless soldering. 
     Alternatively, if flux is not used, the surface oxide film that will probably form on the surface of the solder ball  68  could be cracked, as will be described in greater detail below. 
     The driver chip  26  is provided with solderable pads  86  (FIG.  6 ), similar to the interconnect pads  52  on the substrate  36 . The pads  86  are spaced to correspond with the spacing of the interconnect pads  52 . The driver chip  26  is placed on the solder balls  68  with the solderable pads  86  contacting the tops of the solder balls  68 . 
     If flux is not used, the surface oxide film on the solder ball  68  may be cracked by pressing the solderable pads  86  on the driver chip  26  against the solder ball  68 , which will allow the liquid solder to wet the solderable pads  86 . 
     The assembly of the interconnect pads  52 , the solder balls  68 , and the driver chip  26  with the solderable pads  86  is heated to melt the solder and is pressed together. The heat and pressure collapse the solder balls  68  and bond the underfill layer  60  to the driver chip  26 , as shown in FIG.  7 . Heating to a temperature of 220 degrees celsius in nitrogen is effective to melt the solder. 
     Preferably, the pressure is applied to the assembly in a manner that permits some minute, lateral shifting of the driver chip relative to the substrate so that the surface tension forces on the liquid solder balls  68  and solderable pads  86  tend to pull the driver chip  26  into lateral alignment with the substrate  36 . Alternatively, the parts  26 ,  36  could be manually aligned. 
     After the chip  26  is pressed with heat to collapse the solder balls  68 , solder joints  90  (FIG. 7) or metallic connections between the driver chip  26  and the substrate  36  are formed, and the underfill layer  60  is polymerically bonded to the driver chip  26 , forming a sandwich construction. 
     Alternatively, the underfill layer  60  could be bonded after the solderjoints  90  are formed. In such a case, the underfill layer  60  would be heated again after the solder joints  90  are formed, and the driver chip  26  would be pressed against the underfill layer  60  to effect the bonding. 
     The process of the present invention provides a fairly inexpensive way to electrically connect parts, and the configuration of this invention, in particular the rows of interconnect pads  52  extending perpendicularly from the row of resistors  34 , allows the interconnect pads  52  to be spaced apart further than past configurations, which allows this inexpensive connection method to be used. 
     The process of the present invention could also be used to attach various other microelectronic parts together, such as flexible circuits or wafers of integrated circuits. 
     The underfill layer  60  functions both as an in situ stencil and as a pre-placed underfill, which supports the driver chip on the substrate to relieve stress from the solder joint between the interconnect pad and the driver chip to thereby increase the fatigue life of the solderjoint. A pre-placed underfill expedites the attachment of the driver chip  26  to the interconnect pads  52  by reducing the number of fabrication steps required and because the underfill layer  60  does not require a long curing time, as does liquid epoxy. The underfill layer  60  is also advantageous because it obstructs moisture and chemicals from entering between the parts, which inhibits corrosion. 
     Although this description discussed applying the underfill layer to and forming solder balls  68  on the substrate  36 , the underfill layer  60  and solder balls  68  could be deposited on the driver chip  26  instead. 
     The ink-ejection area  22  on the substrate  36  has a layer of resistors  34  extending across the top of the substrate  36 , near a side 100 of the substrate  36 , as shown in FIG.  2 . The resistors  34  are made from tantalum aluminum, having a thickness of about 950 angstroms, and are sputtered on top of the substrate  36 , as is common in ink-jet technology. As best seen in FIG. 1, the resistors  34  are electrically connected to the driver chip  26  by the circuit traces  40 . The illustrated resistors  34  are square and are sized between about 3 microns by 3 microns and 75 microns by 75 microns, although other shapes and sizes could be used. 
     The resistors  34  are grouped in sets  102  of, for example, seven resistors, with each set corresponding to a row of interconnect pads  52 . Each row of interconnect pads  52  extends generally perpendicularly from each set of resistors  102 . The illustrated print head  20  has a resolution of 600 dots per inch and has approximately 680 sets of seven resistors each, spaced so that the overall length of the resistor sets is sufficient to cover the printing area on a standard piece of paper. The 680 sets of resistors are driven by the driver chip  26 , or possibly multiple driver chips, through 680 rows of eight interconnect pads  52 . It is envisioned that the resistors  34  could extend to sixty inches to accommodate larger widths of paper. 
     A cavitation barrier (not shown), preferably of nitride and carbide and a passivation layer (not shown), preferably of tantalum, is deposited over the resistors. Such barriers are commonly used in ink-jet technology to shield the resistors  34  from the ink in the chambers  32 , which is highly corrosive, and from cavitation erosion. Other types of barriers could also be used. 
     FIG. 10 shows an edge-firing architecture plate  104  positioned over the resistors  34  so that each resistor  34  is centered within one of the chambers  32  in the architecture plate  104 . The edge-firing architecture plate  104  is an elongate, flat, solid, rectangular piece and has small cut-outs  108  in the longitudinal edge  106  of the plate  104 , which form the ink chambers  32 , in which ink is stored until a resistor  34  is heated to eject the ink from the chamber  32 . The cut-outs  108  are longitudinally aligned in sets having the same spacing as the resistors  34 . Partitions  110  are left between the chambers  32  to segregate the ink in adjacent chambers  32 . The ink chambers  32  are fluidically connected with ink channels (not shown), through which ink is delivered from an ink supply (not shown) to the ink chambers. 
     The illustrated plate  104  also has rectangularly shaped solder wells  112  to provide an area in which to attach the plate  104  to the substrate  36 , as will be explained in greater detail below. The solder wells  112  (one of which is shown in FIG. 10) are positioned at the ends  105  of the plate  104 , and several additional solder wells  113  are positioned laterally adjacent the chambers  32  toward the interconnect area  24 . One well  113  is shown in FIG.  14 . 
     The architecture plate  104  is preferably made from a material with a coefficient of thermal expansion similar to that of the substrate  36 . Etched or molded glass or amorphous silicon are particularly suitable. Ceramic is also a possibility. Amorphous silicon is preferable if the substrate is also made of amorphous silicon. A glass plate on a glass substrate would also work well. Similar coefficients of thermal expansion will help the plate and substrate maintain alignment over a wide range of temperatures, will not stress the joints unreasonably, and will not tend to warp the assembly. Silicon is especially desirable because it will be somewhat flexible and therefore more resistant to handling damage than a comparable glass part. 
     The architecture plate  104  is attached to the substrate  36  using the soldering technique described above, which has the added advantage of aligning the architecture plate  104  with the substrate  36 . Specifically, the technique comprises: depositing solderable pads  116 ,  117  on the bottom surface  118  of each of the solder wells  112  in the architecture plate  104  and on the circuit traces  40  on the substrate  36  at corresponding locations; depositing solder paste around each of the solderable pads  117  on the circuit trace layer  40 ; heating the solder paste to form a solder ball  166  (shown in dashed lines, FIG. 10) on each of the solderable pads  117  on the circuit trace layer  40 ; positioning the plate  104  adjacent the substrate  36  so that the solderable pads  116  on the plate  104  are aligned with the solder balls  166  on the substrate  36 ; and heating the solderable pads  116  on the plate  104  and the solder balls  166  on the substrate  36  to join (as shown at  168 ) the plate  104  and substrate  36 . The heating is preferably done while at least one of the substrate  36  and plate  104  are unconstrained so that the plate  104  and substrate  36  may self-align. The solder balls  166  should be of a sufficient size to ensure that the plate  104  does not drag on the substrate  36  and prevent alignment. It should be understood that the solder balls  166  could be formed on the solderable pads  116  on the plate  104  instead of the substrate  36 . 
     After joining the plate  104  and substrate  36 , preferably heat and pressure are applied to the plate  104  and substrate  36  to close any gaps that may exist between the chambers  32  in the plate  104  so that no crosstalk occurs through this path. 
     It should be evident that the illustrated solder joints formed within the solder wells  112 ,  113  only serve to mechanically align the architecture plate  104  and the substrate  36 ; no electrical connections are made. 
     This invention would also be suitable for other configurations of architecture plates. FIG. 11 shows an alternative edge-firing architecture plate  180  in which the architecture plate  180  is a solid, dielectric block  181  (without cut-outs) and the ink chambers  182  are defined by depositing, preferably by spinning on, a dielectric layer  184 , such as polyimide, over the resistors  34 . The openings for the chambers  182  are created by photoimaging, chemical etching, laser drilling, or the like. 
     FIG. 12 shows an alternative edge-firing architecture plate  200  having chambers  232  cut into the bottom surface  233  of the architecture plate  200 . Unlike the previously mentioned edge-firing plate  104 , the chambers  232  in the alternatively configured plate  200  do not extend all the way to the longitudinal edge  206  of the plate. Rather, the alternatively configured plate has a nozzle  238  extending from approximately the center of an interior wall (not shown) nearest the edge  200  of each chamber  232  to the longitudinal edge  206  of the plate  200 . For optimal ink flow and directional stability of the ejected ink, the nozzle  238  should taper from the interior wall of the chamber  232  to the edge  206  of the plate. 
     FIG. 13 shows yet another alternative architecture plate, designated a face-firing plate  300 . The face-firing plate  300  has frustrum-shaped ink chambers  332  extending from the bottom surface  333  of the plate  300  approximately two-thirds of the way through the plate  300 . The chambers  332  are aligned longitudinally along the plate  300  and are spaced in sets to correspond with the spacing of the sets of resistors  34  on the substrate  36 . Frustum-shaped nozzles  342  extend from the chambers  332  to the top surface  343  of the plate  300 . Ink is fired by the resistors  34  from the chambers  332 , through the nozzles  342 , and onto paper being fed along the top surface  343  of the architecture plate  300 . 
     Alternatively, ink chambers for the alternative plates  200 ,  300  could be defined using a dielectric layer, as described in conjunction with plate  180 . 
     FIG. 15 illustrates the fabrication of the face-firing architecture plate  300  on a mandrel  120 . Any of the illustrated architecture plates  104 ,  200 ,  300  may be fabricated using a mandrel. The mandrel  120  has the negative of the features of the desired architecture plate and is preferably made of alumina or another material having a higher enthalpy of formation than the material of the architecture plate (silicon dioxide, for instance), so the mandrel is not affected chemically during fabrication, and a higher coefficient of thermal expansion than the material of the architecture plate. Thus, as long as the mandrel  120  is designed with the proper draft angles the plate  300  will “pop off the mandrel during cooling, as indicated in FIG.  16 . The architecture plate is then “chem-lapped” to the desired flatness and thickness and to create the nozzles  342 , as shown in FIG.  17 . Chem-lapping involves subjecting the plate to chemical mechanical planarization, in which the plate is abraded by a combination of mechanical disturbance and etching chemicals. 
     An encapsulant (not shown), such as polyimide, could be applied to the periphery of the joint (not shown) between the plates  104 ,  200 ,  300  and the substrate  36  to prevent ink leakage therefrom. 
     The solder balls for attaching any of the architecture plates  104 ,  200 ,  300  to the substrate  36  could be formed at the same time the solder balls  68  for attaching the driver chip  26  to the substrate  36  are formed. 
     This description illustrates various embodiments of the present invention and should not be construed to limit the scope thereof in any way. Other modifications and variations may be made to the method and assembly described without departing from the invention as defined by the appended claims and their equivalents.