Patent Abstract:
This disclosure describes a method for the solderless electrical connection of two contact elements by using a laser light beam attached to a fiber optic system which directs the light to the spot to be bonded. By using a fiber optic system the laser beam is optimally converted into thermal energy and bad connections due to underheating or destruction of the contacts due to overheating does not occur. The method and apparatus provides rapid, reproducible bonding even for the smallest of contact geometries. For example, the method of the invention results in solderless gold to gold compression bonding of conductive leads contained in a polymer flex circuit tape, such as a polyimide, without damaging the tape. A strong solderless gold to gold bond can be formed between the gold plated copper lead on the flex circuit tape and a gold plated pad on a semiconductor chip without the need for a window in the flex circuit and without any damage to the tape. In the application of the present invention to the bonding of conductive leads on a TAB circuit to the silicon substrate of an inkjet printhead the need for a window in the TAB circuits is eliminated. The elimination of the window results in elimination of the need for an encapsulation material to cover the conductive leads in the TAB circuit. This in turn results in die size reduction, or increased number of nozzles with the same die size, ease of assembly, higher yields, improved reliability, ease of nozzle serviceability, and overall material and manufacturing cost reduction.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application relates to the subject matter disclosed in the following U.S. Patents and co-pending U.S. Applications:  
         [0002]    U.S. Pat. No. 5,442,384, entitled “Integrated Nozzle Member and TAB Circuit for Inkjet Printhead;” and  
         [0003]    U.S. Pat. No. 5,278,584, entitled “Ink Delivery System for an Inkjet Printhead; ” 
         [0004]    The above patent and co-pending applications are assigned to the present assignee and are incorporated herein by reference.  
     
    
     
       FIELD OF THE INVENTION  
         [0005]    The present invention generally relates to the electrical connection of two elements and, more particularly, to the solderless connection of two elements using an optical fiber that holds the electrical elements in contact while directing a laser emission to the location to be bonded.  
         BACKGROUND OF THE INVENTION  
         [0006]    Thermal inkjet print cartridges operate by rapidly heating a small volume of ink to cause the ink to vaporize and be ejected through one of a plurality of orifices so as to print a dot of ink on a recording medium, such as a sheet of paper. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the paper as the printhead is moved relative to the paper.  
           [0007]    An inkjet printhead generally includes: (1) ink channels to supply ink from an ink reservoir to each vaporization chamber proximate to an orifice; (2) a metal orifice plate or nozzle member in which the orifices are formed in the required pattern; and (3) a silicon substrate containing a series of thin film resistors, one resistor per vaporization chamber.  
           [0008]    To print a single dot of ink, an electrical current from an external power supply is passed through a selected thin film resistor. The resistor is then heated, in turn superheating a thin layer of the adjacent ink within a vaporization chamber, causing explosive vaporization, and, consequently, causing a droplet of ink to be ejected through an associated orifice onto the paper.  
           [0009]    In U.S. application Ser. No. 07/862,668, filed Apr. 2, 1992, entitled “Integrated Nozzle Member and TAB Circuit for Inkjet Printhead,” a novel nozzle member for an inkjet print cartridge and method of forming the nozzle member are disclosed. This integrated nozzle and tab circuit design is superior to the orifice plates for inkjet printheads formed of nickel and fabricated by lithographic electroforming processes. A barrier layer includes vaporization chambers, surrounding each orifice, and ink flow channels which provide fluid communication between a ink reservoir and the vaporization chambers. A flexible tape having conductive traces formed thereon has formed in it nozzles or orifices by Excimer laser ablation. By providing the orifices in the flexible circuit itself, the shortcomings of conventional electroformed orifice plates are overcome. The resulting nozzle member having orifices and conductive traces may then have mounted on it a substrate containing heating elements associated with each of the orifices. Additionally, the orifices may be formed aligned with the conductive traces on the nozzle member so that alignment of electrodes on a substrate with respect to ends of the conductive traces also aligns the heating elements with the orifices. The leads at the end of the conductive traces formed on the back surface of the nozzle member are then connected to the electrodes on the substrate and provide energization signals for the heating elements. The above procedure is known as Tape Automated Bonding (“TAB”) of an inkjet printhead assembly, or TAB Head Assembly, (hereinafter referred to as a “THA”)  
           [0010]    An existing solution for connecting the conductive traces formed on the back surface of the nozzle member to the electrodes on the substrate for a THA requires a flexible TAB circuit with a window in the Kapton tape. This window provides an opening for the bonder head, which permits direct contact of the thermode (single point or gang) with the TAB leads. Therefore, the attach process is performed without direct contact between the thermode and Kapton tape. A TAB bonder thermode comes in direct contact with the flex circuit copper TAB leads through this window. The thermode provides the thermal compression force required to connect the TAB conductive leads to the printhead substrate electrode. Alternatively, ultrasonic method may be used to connect the TAB conductive leads to the printhead substrate electrode. This window is then filled with an encapsulation material to minimize damage to the conductive leads, shorting, and current leakage. This encapsulation material may flow into the nozzles and cause blockages. Therefore, the Tab Head Assembly (“THA”) is designed in a manner that allows a 0.50 to 0.75 mm gap between the edge of the encapsulation and the nozzles. This increases the substrate size by 1 to 1.5 mm. The encapsulation material also creates an impression that is not desirable for serviceability, and creates co-planarity, and reliability problems.  
           [0011]    Accordingly, it would be advantageous to have a process that eliminates the need for a window in the TAB circuits. The elimination of the window results in elimination of the need for an encapsulation material to cover the conductive leads in the TAB circuit. This in turn would result in die size reduction, or increased number of nozzles with the same die size, ease of assembly, higher yields, improved reliability, ease of surface serviceability, and overall material and manufacturing cost reduction.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides a method for the solderless electrical connection of two contact elements by using a laser light beam attached to a fiber optic system which directs the light to the spot to be bonded. By using a fiber optic system the laser beam is optimally converted into thermal energy and bad connections due to underheating or destruction of the contacts due to overheating does not occur. The method and apparatus provides rapid, reproducible bonding even for the smallest of contact geometries. For example, the method of the invention results in solderless gold to gold compression bonding of conductive leads contained in a polymer flex circuit tape, such as a polyimide, without damaging the tape. A strong solderless gold to gold bond can be formed between the gold plated copper lead on the flex circuit tape and a gold plated pad on a semiconductor chip without the need for a window in the flex circuit and without any damage to the tape.  
           [0013]    In the application of the present invention to the bonding of conductive leads on a TAB circuit to the silicon substrate of an inkjet printhead the need for a window in the TAB circuits is eliminated. The elimination of the window results in elimination of the need for an encapsulation material to cover the conductive leads in the TAB circuit. This in turn results in die size reduction, or increased number of nozzles with the same die size, ease of assembly, higher yields, improved reliability, ease of nozzle serviceability, and overall material and manufacturing cost reduction.  
           [0014]    While the present invention will be described, for purposes of illustration only, in conjunction with the bonding of conductive leads on a TAB circuit to the silicon substrate of an inkjet printhead, the present method and apparatus for the solderless electrical connection of two contact elements by using a laser light beam attached to a fiber optic system is applicable to bonding any electrical members to each other.  
           [0015]    Other advantages will become apparent after reading the disclosure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The present invention can be further understood by reference to the following description and attached drawings which illustrate the preferred embodiment.  
         [0017]    Other features and advantages will be apparent from the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.  
         [0018]    [0018]FIG. 1 is a perspective view of an inkjet print cartridge according to one embodiment of the present invention.  
         [0019]    [0019]FIG. 2 is a perspective view of the front surface of the Tape Automated Bonding (TAB) printhead assembly (hereinafter “TAB head assembly”) removed from the print cartridge of FIG. 1.  
         [0020]    [0020]FIG. 3 is a perspective view of an simplified schematic of the inkjet print cartridge of FIG. 1. for illustrative purposes.  
         [0021]    [0021]FIG. 4 is a perspective view of the front surface of the Tape Automated Bonding (TAB) printhead assembly (hereinafter “TAB head assembly”) removed from the print cartridge of FIG. 3.  
         [0022]    [0022]FIG. 5 is a perspective view of the back surface of the TAB head assembly of FIG. 4 with a silicon substrate mounted thereon and the conductive leads attached to the substrate.  
         [0023]    [0023]FIG. 6 is a side elevational view in cross-section taken along line A-A in FIG. 5 illustrating the attachment of conductive leads to electrodes on the silicon substrate.  
         [0024]    [0024]FIG. 7 is a top perspective view of a substrate structure containing heater resistors, ink channels, and vaporization chambers, which is mounted on the back of the TAB head assembly of FIG. 4.  
         [0025]    [0025]FIG. 8 illustrates one process which may be used to form the preferred TAB head assembly.  
         [0026]    [0026]FIG. 9 is a schematic diagram for a fiber push connect laser system as used in the present invention.  
         [0027]    [0027]FIG. 10 shows in detail the flex circuit, the contact bond point, the TAB lead and die pad.  
         [0028]    [0028]FIG. 11 shows the temperature profile of the flex circuit, TAB lead, bond location and die pad during the bonding process with the FPC laser.  
         [0029]    [0029]FIG. 12 shows the absorption property versus wavelength for various metals.  
         [0030]    [0030]FIG. 13 ilustrates the optical transmission results for five samples of the Kapton tape sputtered with 2, 5, 10, 15, and 25 nm of chromium.  
         [0031]    [0031]FIG. 14 illustrates the temperature rise in flex circuits with Ti/W seed layers.  
         [0032]    [0032]FIG. 15 illustrates the temperature rise in flex circuits with a chromium seed layer.  
         [0033]    [0033]FIG. 16 illustrates temperature increase versus time in a 3-layer tape with different thickness chromium seed layers.  
         [0034]    [0034]FIG. 17 shows the results of a laser bonding experiment to evaluate the laser bondability of a flex circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    While the present invention will be described, for purposes of illustration only, in conjunction with the bonding of conductive leads on a TAB circuit to the silicon substrate of an inkjet printhead, the present method and apparatus for the solderless electrical connection of two contact elements by using a laser light beam attached to a fiber optic system is applicable to bonding other types of electrical members to each other.  
         [0036]    Referring to FIG. 1, reference numeral  10  generally indicates an inkjet print cartridge incorporating a printhead according to one embodiment of the present invention simplified for illustrative purposes. The inkjet print cartridge  10  includes an ink reservoir  12  and a printhead  14 , where the printhead  14  is formed using Tape Automated Bonding (TAB). The printhead  14  (hereinafter “TAB head assembly  14 ”) includes a nozzle member  16  comprising two parallel columns of offset holes or orifices  17  formed in a flexible polymer flexible circuit  18  by, for example, laser ablation.  
         [0037]    A back surface of the flexible circuit  18  includes conductive traces  36  formed thereon using a conventional photolithographic etching and/or plating process. These conductive traces  36  are terminated by large contact pads  20  designed to interconnect with a printer. The print cartridge  10  is designed to be installed in a printer so that the contact pads  20 , on the front surface of the flexible circuit  18 , contact printer electrodes providing externally generated energization signals to the printhead. Bonding areas  22  and  24  in the flexible circuit  18  are where the bonding of the conductive traces  36  to electrodes on a silicon substrate containing heater resistors occurs.  
         [0038]    In the print cartridge  10  of FIG. 1, the flexible circuit  18  is bent over the back edge of the print cartridge “snout” and extends approximately one half the length of the back wall  25  of the snout. This flap portion of the flexible circuit  18  is needed for the routing of conductive traces  36  which are connected to the substrate electrodes through the far end window  22 . The contact pads  20  are located on the flexible circuit  18  which is secured to this wall and the conductive traces  36  are routed over the bend and are connected to the substrate electrodes through the windows  22 ,  24  in the flexible circuit  18 .  
         [0039]    [0039]FIG. 2 shows a front view of the TAB head assembly  14  of FIG. 1 removed from the print cartridge  10  and prior to windows  22  and  24  in the TAB head assembly  14  being filled with an encapsulant. TAB head assembly  14  has affixed to the back of the flexible circuit  18  a silicon substrate  28  (not shown) containing a plurality of individually energizable thin film resistors. Each resistor is located generally behind a single orifice  17  and acts as an ohmic heater when selectively energized by one or more pulses applied sequentially or simultaneously to one or more of the contact pads  20 .  
         [0040]    The orifices  17  and conductive traces  36  may be of any size, test, and pattern, and the various figures are designed to simply and clearly show the features of the invention. The relative dimensions of the various features have been greatly adjusted for the sake of clarity.  
         [0041]    The orifice  17  pattern on the flexible circuit  18  shown in FIG. 2 may be formed by a masking process in combination with a laser or other etching means in a step-and-repeat process, which would be readily understood by one of ordinary skilled in the art after reading this disclosure. FIG. 14, to be described in detail later, provides additional details of this process. Further details regarding TAB head assembly  14  and flexible circuit  18  are provided below.  
         [0042]    [0042]FIG. 3 is a perspective view of a simplified schematic of the inkjet print cartridge of FIG. 1 for illustrative purposes. FIG. 4 is a perspective view of the front surface of the Tape Automated Bonding (TAB) printhead assembly (hereinafter “TAB head assembly”) removed from the simplified schematic print cartridge of FIG. 3.  
         [0043]    [0043]FIG. 5 shows the back surface of the TAB head assembly  14  of FIG. 4 showing the silicon die or substrate  28  mounted to the back of the flexible circuit  18  and also showing one edge of the barrier layer  30  formed on the substrate  28  containing ink channels and vaporization chambers. FIG. 7 shows greater detail of this barrier layer  30  and will be discussed later. Shown along the edge of the barrier layer  30  are the entrances to the ink channels  32  which receive ink from the ink reservoir  12 . The conductive traces  36  formed on the back of the flexible circuit  18  terminate in contact pads  20  (shown in FIG. 4) on the opposite side of the flexible circuit  18  at location  38 . The bonding areas  22  and  24  locate where the conductive traces  36  and the substrate electrodes  40  (shown in FIG. 6) are bonded by using a laser light beam attached to a fiber optic system which directs the light to the location to be bonded in accordance with the present invention.  
         [0044]    [0044]FIG. 6 shows a side view cross-section taken along line A-A in FIG. 5 illustrating the connection of the ends of the conductive traces  36  to the electrodes  40  formed on the substrate  28 . As seen in FIG. 6, a portion  42  of the barrier layer  30  is used to insulate the ends of the conductive traces  36  from the substrate  28 . Also shown in FIG. 6 is a side view of the flexible circuit  18 , the barrier layer  30 , the bonding areas  22  and  24 , and the entrances of the various ink channels  32 . Droplets of ink  46  are shown being ejected from orifice holes associated with each of the ink channels  32 .  
         [0045]    [0045]FIG. 7 is a front perspective view of the silicon substrate  28  which is affixed to the back of the flexible circuit  18  in FIG. 5 to form the TAB head assembly  14 . Silicon substrate  28  has formed on it, using conventional photolithographic techniques, two rows or columns of thin film resistors  70 , shown in FIG. 7 exposed through the vaporization chambers  72  formed in the barrier layer  30 .  
         [0046]    In one embodiment, the substrate  28  is approximately one-half inch long and contains 300 heater resistors  70 , thus enabling a resolution of 600 dots per inch. Heater resistors  70  may instead be any other type of ink ejection element, such as a piezoelectric pump-type element or any other conventional element. Thus, element  70  in all the various figures may be considered to be piezoelectric elements in an alternative embodiment without affecting the operation of the printhead. Also formed on the substrate  28  are electrodes  74  for connection to the conductive traces  36  (shown by dashed lines) formed on the back of the flexible circuit  18 .  
         [0047]    A demultiplexer  78 , shown by a dashed outline in FIG. 7, is also formed on the substrate  28  for demultiplexing the incoming multiplexed signals applied to the electrodes  74  and distributing the signals to the various thin film resistors  70 . The demultiplexer  78  enables the use of much fewer electrodes  74  than thin film resistors  70 . Having fewer electrodes allows all connections to the substrate to be made from the short end portions of the substrate, as shown in FIG. 4, so that these connections will not interfere with the ink flow around the long sides of the substrate. The demultiplexer  78  may be any decoder for decoding encoded signals applied to the electrodes  74 . The demultiplexer has input leads (not shown for simplicity) connected to the electrodes  74  and has output leads (not shown) connected to the various resistors  70 . The demultiplexer  78  circuity is discussed in further detail below.  
         [0048]    Also formed on the surface of the substrate  28  using conventional photolithographic techniques is the barrier layer  30 , which may be a layer of photoresist or some other polymer, in which is formed the vaporization chambers  72  and ink channels  80 . A portion  42  of the barrier layer  30  insulates the conductive traces  36  from the underlying substrate  28 , as previously discussed with respect to FIG. 4.  
         [0049]    In order to adhesively affix the top surface of the barrier layer  30  to the back surface of the flexible circuit  18  shown in FIG. 5, a thin adhesive layer  84  (not shown), such as an uncured layer of poly-isoprene photoresist, is applied to the top surface of the barrier layer  30 . A separate adhesive layer may not be necessary if the top of the barrier layer  30  can be otherwise made adhesive. The resulting substrate structure is then positioned with respect to the back surface of the flexible circuit  18  so as to align the resistors  70  with the orifices formed in the flexible circuit  18 . This alignment step also inherently aligns the electrodes  74  with the ends of the conductive traces  36 . The traces  36  are then bonded to the electrodes  74 . This alignment and bonding process is described in more detail later with respect to FIG. 8. The aligned and bonded substrate/flexible circuit structure is then heated while applying pressure to cure the adhesive layer  84  and firmly affix the substrate structure to the back surface of the flexible circuit  18 .  
         [0050]    [0050]FIG. 8 illustrates one method for forming the TAB head assembly  14 . The starting material is a Kapton or Upilex type polymer tape  104 , although the tape  104  can be any suitable polymer film which is acceptable for use in the below-described procedure. Some such films may comprise teflon, polyamide, polymethylmethacrylate, polycarbonate, polyester, polyamide polyethylene-terephthalate or mixtures thereof.  
         [0051]    The tape  104  is typically provided in long strips on a reel  105 . Sprocket holes  106  along the sides of the tape  104  are used to accurately and securely transport the tape  104 . Alternately, the sprocket holes  106  may be omitted and the tape may be transported with other types of fixtures.  
         [0052]    In the preferred embodiment, the tape  104  is already provided with conductive copper traces  36 , such as shown in FIGS. 2, 4 and  5 , formed thereon using conventional metal deposition and photolithographic processes. The particular pattern of conductive traces depends on the manner in which it is desired to distribute electrical signals to the electrodes formed on silicon dies, which are subsequently mounted on the tape  104 .  
         [0053]    In the preferred process, the tape  104  is transported to a laser processing chamber and laser-ablated in a pattern defined by one or more masks  108  using laser radiation  110 , such as that generated by an Excimer laser  112 . The masked laser radiation is designated by arrows  114 .  
         [0054]    In a preferred embodiment, such masks  108  define all of the ablated features for an extended area of the tape  104 , for example encompassing multiple orifices in the case of an orifice pattern mask  108 , and multiple vaporization chambers in the case of a vaporization chamber pattern mask  108 .  
         [0055]    The laser system for this process generally includes beam delivery optics, alignment optics, a high precision and high speed mask shuttle system, and a processing chamber including a mechanism for handling and positioning the tape  104 . In the preferred embodiment, the laser system uses a projection mask configuration wherein a precision lens  115  interposed between the mask  108  and the tape  104  projects the Excimer laser light onto the tape  104  in the image of the pattern defined on the mask  108 . The masked laser radiation exiting from lens  115  is represented by arrows  116 . Such a projection mask configuration is advantageous for high precision orifice dimensions, because the mask is physically remote from the nozzle member. After the step of laser-ablation, the polymer tape  104  is stepped, and the process is repeated.  
         [0056]    A next step in the process is a cleaning step wherein the laser ablated portion of the tape  104  is positioned under a cleaning station  117 . At the cleaning station  117 , debris from the laser ablation is removed according to standard industry practice.  
         [0057]    The tape  104  is then stepped to the next station, which is an optical alignment station  118  incorporated in a conventional automatic TAB bonder, such as an inner lead bonder commercially available from Shinkawa Corporation, Model No. ILT-75. The bonder is preprogrammed with an alignment (target) pattern on the nozzle member, created in the same manner and/or step as used to created the orifices, and a target pattern on the substrate, created in the same manner and/or step used to create the resistors. In the preferred embodiment, the nozzle member material is semi-transparent so that the target pattern on the substrate may be viewed through the nozzle member. The bonder then automatically positions the silicon dies  120  with respect to the nozzle members so as to align the two target patterns. Such an alignment feature exists in the Shinkawa TAB bonder. This automatic alignment of the nozzle member target pattern with the substrate target pattern not only precisely aligns the orifices with the resistors but also inherently aligns the electrodes on the dies  120  with the ends of the conductive traces formed in the tape  104 , since the traces and the orifices are aligned in the tape  104 , and the substrate electrodes and the heating resistors are aligned on the substrate. Therefore, all patterns on the tape  104  and on the silicon dies  120  will be aligned with respect to one another once the two target patterns are aligned.  
         [0058]    Thus, the alignment of the silicon dies  120  with respect to the tape  104  is performed automatically using only commercially available equipment. By integrating the conductive traces with the nozzle member, such an alignment feature is possible. Such integration not only reduces the assembly cost of the printhead but reduces the printhead material cost as well.  
         [0059]    The automatic TAB bonder then uses a gang bonding method to bond the conductive traces down onto the associated substrate electrodes. Higher bond temperatures are generally preferred to decrease the bond time, but higher bond temperatures will soften the flex circuit and cause more deformation of the Kapton tape. It is extremely preferred to have higher temperature at the contact point and lower temperature at the Kapton tape layer. This optimum contact temperature profile may be achieved by utilizing a Fiber Push Connect (FPC) single point laser bonding process FPC in conjunction with a windowless TAB circuit provides an ideal solution for a TAB head assembly for an inkjet printer printhead.  
         [0060]    A schematic for a FPC laser system  200  is illustrated in FIG. 9. This system consists of an Nd YANG or Diode laser  202 , equipped with a glass (SiO 2 ) optical fiber  204 . The system guides the laser beam to the contact or attach point  206  via the optical glass fiber  204 . An optimum thermal coupling is achieved by pressing two parts together by means of the fiber  204  which creates a zero contact gap between the TAB lead  208  and die pad  210  and thus improved thermal efficiency. FIG. 10 shows in greater detail the flex circuit  18 , the contact point  206 , the TAB lead  208  and die pad  210 .  
         [0061]    Referring to FIG. 9, a feedback temperature loop is achieved by means of an infrared detector  212  through the glass fiber. The temperature or absorption behavior response of the IR-radiation reflected by the contact elements  208 ,  210  at the contact point  206  is gathered. The outgoing laser beam  220  from the laser source  202  goes through a half-transmission mirror or beam splitter  214  and through a focussing lens  216  into the glass fiber optic  204 . The reflected light  218  from the fiber optic shown with dashed lines is reflected by the half mirror  21  and arrives via focussing lens  222  at an IR detector  212  that is connected to a PC Controller  224 . The graph shown on the monitor  226  of PC controller  224  is meant to show that the PC Controller  224  can store definite expected plots for the temperature variation of the bonding process with which the actual temperature variation can be compared. The PC Controller  224  is connected with the laser source  202  so that the laser parameters can be controlled if necessary.  
         [0062]    The reproducibility of a FPC laser bond depends both on a high degree of thermal coupling between the two connectors  208 ,  210  and high absorption of the laser energy by conductive leads  208 ,  210 . To optimize the bonding process, minimum absorption is desired in the Kapton tape and maximum absorption is desired in the flex circuit  18  metal layer. Metals with higher absorption rate will transform a higher share of the laser energy into heat. This will result in a shorter attach process which in turn will result in a higher quality bond.  
         [0063]    The laser utilized is a YAG laser with a wavelength of 1064 nm. FIG. 12 illustrates the absorption property versus wavelength for several metals. As can be observed from FIG. 12, chromium and molybdenum have the highest absorption characteristics at this wavelength. chromium was selected as the base metal due to the fact that most flex circuit manufacturers are using chromium as the seed layer. The penetration depth of the laser into chromium is about 10 nm with a spot size of 5 nm, thus requiring a minimum chromium thickness of 15 nm. The laser beam creates a localized heated zone causing the metals (or solder material), to melt and create a bond between two joining surfaces without increasing the temperature of the Kapton tape. However, any gap between two mating metal parts will cause over heating of the metal surface exposed to the laser beam. This will cause deformation of the TAB leads with no bond between metal surfaces. Also, an increased temperature in the flex will cause damage to the flex circuit.  
         [0064]    [0064]FIG. 11 illustrates a typical temperature profile of the flex circuit  18  during bonding process with the FPC laser. As it can be observed from FIG. 11, the temperature at the attach area  206  is considerably higher than the Kapton tape  18  temperature. This is achieved due to the high degree of the transparency of the Kapton tape at different wavelengths.  
         [0065]    The Kapton polyimide tape is transparent to the YAG laser beam and the laser beam passes through the 2 mil thick layer of polyimide without any absorption. Chromium is a conventional seed layer that is used extensively to provide an adhesion layer between the copper trace and Kapton polyimide in a two-layer flex circuit manufacturing process. A chromium layer with a minimum thickness of 10 nm (or 20 nm nominal) is required to provide a media which absorbs the laser energy. The thickness of the chromium layer varies depending upon the flex circuit manufacturer, with reported thicknesses between 2 and 30 nm. A typical flex circuit manufacturing process utilizes a thin layer (20 nm) of sputtered chromium as a seed (adhesion) layer between the copper traces and Kapton polyimide.  
         [0066]    Five samples of the Kapton tape were sputtered with 2, 5, 10, 15, and 25 nm of chromium, and optical transmission was measured for these samples. FIG. 13 illustrates the optical transmission results for these samples. It can be seen that optical transmission initially drops rapidly with increased chromium thickness (from 65% for 2 nm of chromium, to 12% for 15 nm of chromium), but optical transmission changes very slowly when chromium thickness increases from 15 to 25 nm.  
         [0067]    Laser bonding process requires a fast temperature rise in the conductive trace to minimize the temperature rise in the Kapton and therefore minimize damage to the Kapton tape. FIGS. 14 and 15 illustrate temperature rise in several flex circuits with different constructions. FIG. 14 illustrates temperature rise in flex circuits with thicker seed layers. It is important to notice that flex circuits with 10 nm or less of Ti/W did not reach the temperature that is required for gold/gold bonding, but the flex circuit with 20 nm of Ti/W did reach the bonding temperature. Also, it should be noted that the rise time in the flex circuit with thicker Ti/W is faster, minimizing the potential of damage due to high localized temperatures in the Kapton tape.  
         [0068]    The temperature (IR-Signal) fluctuation in the flex circuit with 20 nm of Ti/W is indicative of the fact that this flex circuit reached the maximum preset temperature required for gold/gold bonding and then the laser feed-back loop temporarily dropped the laser energy so that increase in the TAB bond temperature did not damage the Kapton tape. As soon as the temperature of the Kapton tape dropped (by a preset amount), the laser energy automatically increased to fill power to increase the TAB lead temperature, and created a reliable gold/gold bond.  
         [0069]    [0069]FIG. 15 illustrates similar results for different flex circuits with a chromium seed layer as opposed to Ti/W seed layer. It can be observed that flex circuit with 10 nm of chromium did reach the preset temperature required for gold/gold bonding. Therefore, chromium seed layer has higher absorption characteristics compared to Ti/W seed layer for a YAG laser.  
         [0070]    [0070]FIG. 16 illustrates temperature increase versus time in a 3-layer tape with a 20 nm chromium layer, a tape with a 5 nm chromium layer, and a tape with no chromium layer. As can be seen in FIG. 16, only the flex circuit with a 20 nm chromium layer indicated a rapid temperature rise.  
         [0071]    Since it was established that chromium thickness is essential to the integrity of the gold/gold laser bond, when a YAG laser is used, an optimum chromium thickness was selected as a base line. Referring to FIG. 13, a chromium thickness over 15 nm does not decrease transmission drastically. Based on FIG. 15, a chromium thickness of manometers is the absolute minimum required thickness to provide a successful laser bond. FIG. 15 also illustrates that a flex circuit with 15 nm of chromium exhibit a much faster temperature rise in the copper trace, resulting in less or no damage to the Kapton tape. Therefore, 15 nm of chromium is optimum to provide a reliable and repeatable laser bond.  
         [0072]    Some chromium diffusion into the copper is expected during the subsequent sputtering of chromium as a seed layer and plating processes during manufacture of the flex circuits. Diffusion of the chromium into the copper is a time and temperature dependent process, and it is difficult to determine the amount of chromium that will be diffused into the copper during these processes. Normally, it is estimated that maximum amount of diffused chromium is under 5 nm. Based on these factors, a minimum chromium thickness after the sputtering process was established as 20 nm. This thickness should gurantee a minimum chromium thickness of 15 nm after the completed manufacture of the flex circuit.  
         [0073]    [0073]FIG. 17 shows the results of a laser bonding experiment to evaluate the laser bondability of a flex circuit having about 5 nm of chromium as a seed layer. In this experiment the bond force was varied from 20 to 140 grams (20, 40, 60, 80, 100 and 140 grams), and the laser pulse length was varied from 2 to 40 milliseconds (2, 7, 10, 20, 30 and 40 milliseconds). The fixed factors in this experiment are die nest temperature, laser current, maximum feed back temperature and temperature rise time. By varying the laser energy no bond was formed between the TAB lead  208  and the die pad  210 . This is due to low laser energy absorption of the flex circuit due to insufficient thickness of chromium seed layer.  
         [0074]    Table I indicates the test conditions and test results for several experiments. These tests covered a large cross-section of operating conditions, covering from no visible effect on the bond to full Kapton damage. Based on the results illustrated in Table I, it was concluded that the existing YAG laser is not capable of bonding existing flex circuits with low chromium thickness.  
         [0075]    A 3-layer flex circuit with 20 nm of chromium with an adhesive layer between the Kapton, and copper trace was tested. A successful gold/gold laser bonding was achieved with a laser power set at 10 W, pulse length set at 20 ms, bond force set at 140 grams, and die nest temperature set at 100 degrees C. No mechanical damage was observed in the die pad area. This is an indication that neither the laser energy or the force caused any mechanical damage to the die pad area.  
         [0076]    Table II indicates the test conditions and test results for seven experiments. For grading the laser bond results; an “X” quality bond is defined as a bond that has a cross section similar to the thermal compression bonded die, with the same or better peel strength. A “B” quality bond is a bond that still has an acceptable bond strength, but the Kapton joint has been degraded due to higher temperatures (a “B” quality bond may still be acceptable). A “C” quality bond is when the bond is formed, but the bond strength is lower than that of thermal compression bonded parts. A “F” quality bond is defined as a situation that a bond was not formed between the copper trace, and the die pad (in most cases Kapton burned due to increased localized temperature).  
         [0077]    By increasing the pulse length from 5 to 10 milliseconds in Test No. 2, bond quality improved drastically, but in this case Kapton did burn in one die site. By reducing the pulse length again to 5 milliseconds, and increasing the laser power (by means of increasing the laser current), the bonds became weak again, but burned Kapton was not observed any more. To further improve the bonding, the laser power was increased a second time by increasing the current. In Test No. 4, good, clean bonds were formed and no damage to the Kapton was observed. A peel test of parts built with these set of parameters indicated a good peel strength also. Joint strength was further improved by increasing the laser power. In Test No. 5 the power was increased by increasing the pulse length from 5 to 10 milliseconds. In this case the joint strength improved drastically, but some burned Kapton was also observed. In the case of Tests No. 2 and 5 the burned Kapton was on the copper lead side, and there were no openings exposing the copper lead. Therefore, it is suspected that the adhesive layer between the Kapton and copper lead has burned. In Test No. 6, the laser current was maintained at 19 amps, but pulse length was increased from 10 to 15 milliseconds. This resulted in a laser over energy which burned several holes all the way through the Kapton, without causing any connection between the TAB lead and the die pad.  
         [0078]    Test No. 7 is a repeat of the Test No. 5, with a smaller probe force. In Test No. 7 probe force was reduced from 140 grams to 100 grams. In this case, very much similar to Test No. five, excellent bonds were observed, with high joint strength. However, a possible tape damage was observed in one die site. In this case also, there was no exposed copper trace or TAB lead.  
                                                             TABLE I                           Force   Laser Current   Pulse Length   Max Temp.           Item   Grams   Amp   milli-sec   Setting   Observation                                1   140   17   5   0.4   No bond/No damage to flex       2   140   17   30   0.4   No bond/No damage to flex       3   140   19   5   0.4   No bond/No damage to flex       4   140   19   30   0.4   No bond/No damage to flex       5   100   17   5   0.4   No bond/No damage to flex       6   100   17   30   0.4   No bond/No damage to flex       7   100   19   5   0.4   No bond/No damage to flex       8   100   19   30   0.4   No bond/No damage to flex       9   140   19   30   0.6   No bond/Flex started to burn       10   140   19   30   0.8   No bond/Some flex damage observed       11   140   19   30   1   No bond/Flex damage clearly observed       12   140   19   50   1   No bond/Some flex damage observed       13   140   19   50   2   No bond/Flex damage clearly observed       14   140   19   30   5   No bond/Flex damage clearly observed       15   140   19   30   9   No bond/Excessive flex damage       16   140   19   50   9   No bond/Excessive flex damage                  
 
         [0079]    [0079]                                                                             TABLE II                           Force   Laser   Pulse   Max Temp.   Bond               Test   Grams   Amp   milli-sec   Setting   Quality   FIG.   Observation                                1   140   17   5   0.6   C   18   Weak bond formed at most bond sites       2   140   17   10   0.6   B   19   Acceptable bond formed, but burned kapton in one site       3   140   17.5   5   0.8   C   20   Weak bond formed at most bond sites       4   140   19   5   0.8   A   21   Good bond formed, no damage to kapton       5   140   19   10   0.8   B   22   Excellent bond formed, but kapton burned in some sites       6   140   19   15   0.8   F   23   Burned kapton, no bonds were formed       7   100   19   10   0.8   B   24   Excellent bond formed, but kapton burned in one site                    
         [0080]    Based on the results stated in Table II, a bondability window for 3-layer tape may be defined as follows:  
                                                       Bond Force:   100-140 grams           Laser Current:    17-20 Amps           Pulse Length:    5-10 milliseconds           Maximum Set Temperature:       0.6-0.8                      
 
         [0081]    Experiments were also performed utilizing a 2-layer tape with 20 manometers of sputtered chromium. An experimental design was set-up to evaluate effects of force, pulse length, and laser power on the quality of the bond. This experiment was set-up with the variables force, pulse length, and laser power tested at three levels, resulting in 27 individual tests and 27 bonded parts utilizing the FPC laser. All 27 parts passed visual inspection, indicating no damage to Kapton or barrier. The Kapton was then etched to expose the TAB lead. A shear test and a pull test were performed on the 27 parts to evaluate the bond strength. The shear and pull tests indicated a bond strength of well over 200 grams for higher laser powers. Table III indicates the test conditions and the bond strength results for the 27 experiments.  
                                                             TABLE III                           Bond       Laser   Shear           Test   Force   Bond Time   Power   Strength   Push Strength       Number   Grams   Milliseconds   Watts   Grams   Grams                                1   310   20   5.0   0   0       2   310   20   6.2   82   106       3   310   20   8.5   176   177       4   310   40   5.0   0   0       5   310   40   6.2   90   137       6   310   40   8.5   182   169       7   310   60   5.0   0   0       8   310   60   6.2   131   132       9   310   60   8.5   186   191       10   360   20   5.0   0   0       11   360   20   6.2   139   112       12   360   20   8.5   189   165       13   360   40   5.0   0   0       14   360   40   6.2   146   154       15   360   40   8.5   205   201       16   360   60   5.0   0   0       17   360   60   6.2   105   177       18   360   60   8.5   225   224       19   412   20   5.0   0   0       20   412   20   6.2   88   165       21   412   20   8.5   211   207       22   412   40   5.0   0   0       23   412   40   6.2   178   198       24   412   40   8.5   222   195       25   412   60   5.0   0   0       26   412   60   6.2   148   177       27   412   60   8.5   210   193                  
 
         [0082]    The experiments established that gold to gold windowless TAB bonding is feasible. Shear strengths of well over 200 grams can be achieved easily and repeatedly. No Kapton or barrier damage was observed due to the laser bonding process. Based on the results stated in Table III, a bondability window for 2-layer tape may be defined as follows:  
                                                                                     Low   Medium   High                                    Bond Force   310   grams   360   grams   420   grams       Pulse Time   20   msec   40   msec   60   msec       Laser Power   5   watts   6.2   watts   8.5   watts                  
 
         [0083]    The present invention eliminates the need for the TAB window and the associated encapsulation of the prior art and results in a planar TAB connect process. This in turn results in lower cost, higher reliability and ease of serviceability.  
         [0084]    The tape  104  is then stepped to a heat and pressure station  122 . As previously discussed with respect to FIGS. 9 and 10, an adhesive layer  84  exists on the top surface of the barrier layer  30  formed on the silicon substrate. After the above-described bonding step, the silicon dies  120  are then pressed down against the tape  104 , and heat is applied to cure the adhesive layer  84  and physically bond the dies  120  to the tape  104 .  
         [0085]    Thereafter the tape  104  steps and is optionally taken up on the take-up reel  124 . The tape  104  may then later be cut to separate the individual TAB head assemblies from one another.  
         [0086]    The resulting TAB head assembly is then positioned on the print cartridge  10 , and the previously described adhesive seal  90  is formed to firmly secure the nozzle member to the print cartridge, provide an ink-proof seal around the substrate between the nozzle member and the ink reservoir, and encapsulate the traces in the vicinity of the headland so as to isolate the traces from the ink.  
         [0087]    Peripheral points on the flexible TAB head assembly are then secured to the plastic print cartridge  10  by a conventional melt-through type bonding process to cause the polymer flexible circuit  18  to remain relatively flush with the surface of the print cartridge  10 , as shown in FIG. 1.  
         [0088]    The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. As an example, while the present invention was described in conjunction with the bonding of conductive traces on a TAB circuit to the silicon substrate of an inkjet printhead, the present method and apparatus for the solderless electrical connection of two contact elements by using a laser light beam attached to a fiber optic system is applicable to bonding other types of electrical members to each other. Likewise, while the present invention was described in conjunction with solderless gold to gold bonding of electrical members to each other, the present method could be used for the solderless bonding of other conductive metals. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.

Technology Classification (CPC): 1