Pulsed current resistive heating for bonding temperature critical components
This is a continuation of co-pending application Ser. No. 07/796 489, filed on Nov. 22 1991, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to an apparatus and method for bonding a small, heat sensitive component onto another surface and, more particularly, for bonding heat sensitive chips to a base or substrate utilizing a current-pulsed, shaped resistive heating element to provide localized heating to a thermally setting thin film agent at the bond interface. 
     In magnetic recording technology, as well as in other technologies generally related to integrated circuit chips and other electronic components, it is often desired to accurately align and bond small components in a controlled and repeatable fashion both in research and manufacturing environments. Often, while one or more critical components may be severely damaged by the high temperature required by most conventional bonding methods, a lower temperature bond may not provide sufficient strength and stability and thus be unacceptable due to creep and delamination generally associated with such low temperature bonds. 
     It is known in the art to provide localized heating of a heat sensitive bonding agent by direct resistance heating utilizing an interface heater layer or element of a conductive material such as a thin metallic tape or a thick-film conductive paste. For example, published United Kingdom patent application no. 2127740A to Wolfson discloses a method for hermetically sealing a ceramic cover to a ceramic package or housing utilizing a high temperature vitreous sealant without excessively heating the electronic circuits and components therein. A heater layer of electrically conductive material is bonded to the sealing area of the ceramic package, for example. A layer of sealant material is applied over the heater layer and the package cover, for example, is then pressed against the package aligning the sealing area of the cover with the heater and sealant layers while an electric current is applied to the heater layer, causing the sealant material to bond to the heater layer thus bonding the cover to the package. To concentrate the thermal energy at the bonding area and minimize heating of components mounted within the package, portions of the package are comprised of ceramic material having a relatively low degree of thermal conductivity. Since it is also a necessary function of the package to conduct heat away from the electronic components therein, other portions of the package are necessarily comprised of a ceramic material having a high degree of thermal conductivity. While Wolfson effectively minimizes adverse heating of components mounted therein, at least two different ceramic materials having differing degrees of thermal conductivity are required for the package, thus increasing the complexity and expense of the package fabrication. Additionally, while most components within the package of Wolfson are mounted relatively large distances from the package sealing areas, critical components mounted at or in close proximity to the bonding site will not be thermally protected. 
     SUMMARY OF THE INVENTION 
     In accordance with the principles of the present invention, a method of bonding small, temperature critical components to one another or to larger base components comprises forming a shaped heating element on a surface of a first component or substrate, coating said shaped heating element with a layer of a heat sensitive, thermally setting bonding agent, placing a bonding surface of a second component over said bonding agent layer and shaped heating element and in contact with the surface of said first component, and applying an electric current pulse to said shaped heating element for heating and thermally actuating said bonding agent thus bonding the two components together or bonding the second component to said substrate. 
     Additionally, the present invention provides shaped heating element means to effect localized heating at a bonding interface for use in a manufacturing environment. 
     In accordance with the present invention, a resistive heating element having contact pins or pads at each end is formed or patterned, such as by a well-known screen printing process, on a bonding surface of a base or substrate component. The bonding surface and the heating element are coated with a layer of a suitable thermally setting thin film bonding agent. The bonding surface may be made non-wetting such that only the heating element is coated with the bonding agent. The component to be bonded is retained in a desired position against the bonding surface over the heating element and bonding agent while one or more current pulses are passed through the heating element via the contact pads. The pulse magnitude and duration are controlled to provide sufficient heat and temperature at the bonding interface to affect the desired bonding without excessive heating of any critical components. Proper tailoring of the current pulse and shaping of the heating element can provide temperatures in the range of between 400° C. to 600° C. at the bond interface while maintaining a temperature less than 200° C. at a critical component as close as 250-500 micrometers (um) from the bond interface. Tailoring the pulse shape also provides an optimum heating and cooling profile to minimize the thermal shock to the bond, thus minimizing delamination and cracking of the bond. 
     The shape of the resistive heating element is designed to provide even heating over the area of the bonding interface between the bonding components. The shaped heating element provides a similarly shaped bond line having a width of much smaller dimension than the bonding interface to greatly reduce shear stress in the bond. Additionally, the shape of the heating element greatly reduces bond creep and the probability of bond delamination. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a component having a heater element formed on a bonding surface thereof in accordance with the principles of the present invention; 
     FIG. 2 is a perspective view of a component bonded to a substrate illustrating the method of the present invention; 
     FIGS. 3a-3d are plan views of different embodiments of the shaped heating element shown in FIGS. 1 and 2; 
     FIG. 4 is a perspective view of a substrate illustrating a coated heating element formed thereon in accordance with the principles of the present invention; 
     FIG. 5 is a diagram illustrating the time and spatial temperature profile for two components being bonded together at a bonding interface in accordance with the principles of the present invention; and 
     FIG. 6 is a perspective view illustrating a piezoelectric alignment device for use as shown in FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a base or substrate 11 having a shaped heating element 15 formed on a bonding surface 12 is shown. The heating element 15 and surrounding portions of the surface 12 are coated with a layer 16 of a suitable thermally setting thin film bonding agent. One or more desired components 13 are then positioned on the surface 12 over the heating element 15 for bonding thereto. The heating element 15 is provided with plural contacts or connection pads 21 to allow the heating element to be pulsed electrically to achieve controlled local heating. 
     Referring now also to FIG. 2, an electric pulse generator 17 is temporarily connected to the heating element via contact pads 21 during an assembly process to provide controlled heating at the bonding interface between electronic component 13 and bonding surface 12. The output of the pulse generator 17 comprises a series of current pulses in which the pulse magnitude, both current and voltage, the pulse width and the pulse repetition rate are controlled to provide a predetermined amount of power having a desired time distribution to the bonding interface. 
     In one preferred embodiment wherein a ceramic or silicon chip 13 having a thin film magnetic recording head or read/write transducer formed on an outwardly facing surface 26 thereof is bonded to a larger magnetic recording slider 11, for example, the bond interface is spaced approximately 250-500 um from the magnetic recording head element formed on surface 26 parallel to the bond interface. In this example, utilizing pulses of less than one millisecond at an appropriate current and voltage, the bond interface is heated to approximately 400°-600° C. peak temperature while maintaining the magnetic recording head element on surface 26 at a peak temperature of less than 200° C. (as shown in FIG. 5). In another preferred embodiment, a magnetic read/write transducer fabricated on a ceramic or silicon chip forming an integral transducer/slider component is bonded to a suspension arm for use in magnetic disk storage device, for example. 
     An alignment device 23 is temporarily attached to the component surface 26 to adjust angle and displacement and thus provide accurate positioning of component 13 during the bonding process. Appropriate control signals are coupled to alignment device 23 via leads 25 to facilitate desired alignment of the component on the surface 12 during the bonding process. Additionally, a separate bender device (not shown) may be utilized to apply a force normal to the bonding interface thus pressing component 13 against the bonding surface 12 during the bonding process. Alignment and bender devices will be discussed in greater detail herein below with reference to FIG. 6. 
     Referring now to FIGS. 3a-3d, the shape and dimensions of the heating element 15 are important not only to achieve an even heating pattern and desired temperature profiles, but also to minimize thermally induced stress in the components being bonded. A serpentine-shaped resistor 41 or a uniform flat spiral-shaped resistor (not shown) provides a generally even heat pattern at a bond interface satisfactory for most applications. However, a uniform serpentine-shaped resistor 41 (as shown in FIG. 3a) tends to generate higher temperatures towards the center and lower temperatures towards the edges of the heat pattern. A non-uniform serpentine-shaped resistor 45 having widely spaced legs at the center portion 46 and less widely separated legs at the periphery of the serpentine-shaped resistor 45 minimizes hot spots in the heat pattern and provides a flatter temperature distribution at the bond interface. Applications requiring non-uniform heat patterns and temperature distributions at the bond interface may be achieved utilizing different shaped heating elements such as the triangular-shaped serpentine resistor 49 shown in FIG. 3c. In certain applications, an application wherein a possible central hot spot would not be a potential problem, for example, a straight line-shaped resistor 51 having contact pads 53 as shown in FIG. 3d is preferred. 
     The heating element resistor is fabricated from any suitable electrical conductive material. In a preferred embodiment the serpentine resistor 41 is fabricated of hafnium diboride. Such shaped resistors having line widths of less than 5 um and contact pins or pads 43 are easily achieved utilizing well-known photolithography or screen printing or other suitable deposition techniques. Similarly, sheet resistors may be formed on a bonding surface utilizing well-known deposition techniques. 
     Referring now also to FIG. 4, the shape and dimensions of the bonding material trace or layer is a critical factor for achieving a bond of sufficient strength and having minimum stress induced bond creep and probability of bond delamination. Since a low bond aspect ratio, that is the ratio of the bond width to the bond thickness, greatly improves the bond quality (considering bond creep and stress, for example), line or trace patterns or dot matrix patterns of bonding material are utilized rather than continuous layers of bonding material. For example, spaced droplets of bonding material are applied along the length of the line resistor 51 to reduce bond stress and provide resilience minimizing the potential of bond delamination. The bonding agent can be a metal, that of the resistor or a thin film layer deposited over the resistor, glass, spun on epoxy, or any suitable thermally setting thin film bonding agent. In a preferred embodiment, the substrate or component material 11 is selected such that the material forming the bonding surface 12 is non-wetting for the selected bonding material, thus only the serpentine resistor film 15 is coated. A layer of bonding material 31 is applied to the bonding surface 12 over the heating element 15, providing a bonding line having the same dimensions as the heating resistor lines. Because the width of the bond line is reduced to essentially the width of the resistor line, the bond aspect ratio is low and the residual shear stress in such a bond is greatly reduced. 
     Referring now to FIG. 5, a diagram illustrating the time and spatial relationship between a current pulse applied to the heating element at the bond interface 24 between two components 11,13 being bonded together and the temperature profile at the interface 24 and the outwardly facing surface 26 for the chip component 13 is shown. Sufficient energy must be provided to the heating element at interface 24 to heat the bonding agent to relatively high temperatures for a sufficient period of time to effect the desired bond without heating components mounted on surface 26, a thin film magnetic read/write transducer, for example, beyond a critical temperature. The current pulse 28 is tailored to have step increase or relatively steep ramp increase with a quick drop, such as a square pulse or sawtooth pulse, having the appropriate magnitude and duration to minimize excessive energy provided to the interface 24. If multiple current pulses are required, the pulse repetition rate is chosen to allow sufficient spatial dissipation of heat from the surface 26 to maintain the temperature at surface 26 below the critical value. For some applications, the base or substrate 11 may be comprised of a material having a relatively high degree of heat conductivity to serve as a heat sink for the bond interface 24. 
     Referring now to FIG. 6, a three-degree of freedom piezoelectric bender device is shown. Essentially, such a piezoelectric bender device comprises a pair of elongated rectangular flat substrates 27,29 bonded together at a grounded interface, each substrate having a layer of piezoelectric material deposited in parallel, lengthwise strips on the substrate surface opposite the grounded interface. Each of the piezoelectric strips A,B,C and D, has a width slightly less than half the width of the substrate and is connected to a power source via leads 25a,25b,25c and 25d, respectively. When the appropriate voltages and polarities are applied to the respective piezoelectric strips, a free end of the bender device 23 will rotate up or down about a transverse axis in the plane of the bender device or twist about a longitudinal axis in the plane of the bender device. Alternatively, the bender device 23 can be comprised of two flat sheets of piezoelectric material bonded together at a grounded interface having electrode subsections A,B,C and D formed with arrows 22 indicating the pole direction of each electrode subsection A,B,C and D, respectively. 
     Utilizing one or more such bender devices 23 (as shown in FIG. 2) the position and orientation of a component 13 on the surface 12 may be adjusted as required. Additionally, pressure may be applied normal to the bonding interface to provide heat sinking at the component outer surface 26 and to ensure a thin bonding interface and total contact of the bonding material with the bonding surfaces of both components 11,13. A suitable ultrasonic signal may be applied to one or more piezoelectric bender devices to ultrasonically excite the bonding interface during the bonding process. Such ultrasonic excitation during the bonding process can provide pressure, localized heating and sheer action, thus enhancing the strength and stability of the resulting bond. 
     While the present invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in the form and details may be made therein without departing from the spirit and scope of the appended claims.