Abstract:
A method and a device for introducing solder onto a solar cell is provided. The method and device employ a solder wire introduced in the molten state onto the solar cell under the action of ultrasonic vibrations applied by a sonotrode. Solder is introduced very precisely onto the solar cell, without subjecting the solar cell to undesirably high temperatures, by introducing the solder wire into a gap running between a heating device and the sonotrode, which applies ultrasonic vibrations and melts and flows through the gap onto the solar cell.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit under 35 U.S.C. §119(a) of German Patent Application No. 10 2010 016 814.9, filed May 5, 2010, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a method for introducing solder onto a workpiece, preferably onto a semiconductor device such as a solar cell, wherein solder is applied in the molten state onto the workpiece under ultrasonic action with the use of a solder wire. Further, the invention relates to a device for introducing solder onto a workpiece, in particular onto a semiconductor device such as a solar cell, comprising a solder-wire introduction means, a heating means for the solder wire, an ultrasonic sonotrode as well as a transport means for the transport of the workpiece relative to both the heating means as well as the ultrasonic sonotrode, wherein the solder wire is melted in a heating zone associated with the heating means. 
         [0004]    2. Description of Related Art 
         [0005]    A method for introducing a connecting conductor onto a solar cell, in which a solder is applied onto the solar cell by means of ultrasonic soldering, is known from WO-A-2008/014900 (DE-A-10 2006 035 626). In this case, the solder in the form of a solder wire or solder preforms is soldered on at the soldering temperature by means of an ultrasonic sonotrode. 
         [0006]    The soldering of solder onto solar cells, in particular by means of ultrasound has the advantage that a fluxing agent does not need to be used, whereby if it had to be used, the danger of damage to the solar cell would increase. Due to the ultrasonic action, oxide layers present on the solar cells are broken up, in order to assure a mechanically solid and electrically well-conducting joint between the solder and the corresponding metal layer of the solar cell. This is particular of advantage when the metal layer involves an aluminum layer such as a back-surface contact composed of aluminum. 
         [0007]    Corresponding ultrasound soldering methods are also taken from, e.g., U.S. Pat. No. 6,357,649 or the reference Mardesich et al.: “A Low-Cost Photovoltaic Cell Process Based on Thick Film Techniques”; 14th IEEE PV, Sp. Conf. Proc., 1980, pp. 943-947. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The problem of the present invention is to enhance a method and a device of the type named initially, so that the solder is introduced very precisely onto the workpiece without subjecting the workpiece to undesirably high temperatures. 
         [0009]    In order to solve the problem, it is essentially proposed according to the method that the solder wire is introduced in a gap running between a heating means and a sonotrode producing ultrasonic vibrations, is melted in the gap, and flows through the gap onto the workpiece. 
         [0010]    Unlike the prior art, the solder material is not guided through a heating means and then delivered through an opening in order to solder the workpiece via ultrasonic action, which can also be designated “soldering on”. Rather, the solder wire is introduced into a free gap on the side, whereby the width of the gap is predetermined by the distance between heating means and sonotrode. Here, the gap between the sonotrode and the heating means is preferably designed in such a way that it has a width B that is preferably approximately ½D≦B≦D with D=the diameter of the solder wire. 
         [0011]    According to the invention, not only is the heating means heated to a temperature above the melting point of the solder wire, but the sonotrode is also, the temperature adjustment being made independently in each case. 
         [0012]    Since the gap is bounded by the sonotrode excited to longitudinal vibration, despite the high surface tension that the molten solder possesses, the solder is pulled through the gap. In this respect, the gap exercises roughly a capillary effect. 
         [0013]    Due to the provision provided in this respect, there particularly also results the advantage that conventional solder materials can be successfully used as the basis for the soldering of workpieces, in particular solar cells, materials such as those based on tin-zinc, tin-silver, or even of pure tin. 
         [0014]    It is provided in an enhancement that operation is conducted at a low sonotrode frequency, preferably in the range between 10 kHz and 40 kHz, in order to avoid a shunting of the surface layer, such as an SiN x  layer, running on the top surface of a solar cell. Further, it is particularly provided that the lengthwise axis of the sonotrode encloses an angle of &lt;90° relative to the normal line proceeding from the surface of the workpiece such as a solar cell, so that a completely horizontal alignment, thus a horizontal coupling is possible. 
         [0015]    Further, there exists the possibility of allowing the sonotrode vibration to deviate from the resonance frequency in a targeted manner. This can be carried out by off-tuning the sonotrode or by employing a sonotrode length that deviates from a whole-number λ/2, λ being the amplitude of the ultrasonic vibration. 
         [0016]    The rate of application of the solder should lie in the range between 0.1 millimeter per second (mm/sec) and 200 mm/sec, in particular between 20 mm/sec and 80 mm/sec. 
         [0017]    In addition, there is the possibility of introducing solder strips that exercise the function of busbars, which are commonly applied onto solar cells, by means of the method according to the invention. Thus, there is the possibility of introducing, e.g., two or more busbars, preferably of tin, onto the front side of a solar cell. The solder tracks exercising the function of busbars should thus have widths between 0.5 millimeters (mm) and 15 mm, preferably in the range of 2 mm. 
         [0018]    In order to achieve a sufficient process stability, it is further provided that the ultrasound system used is continuously active, thus the sonotrode constantly vibrates, in order to avoid deviations in resonance that may occur in actuating the ultrasound system. A continuous vibrational excitation additionally has the advantage that solder-caused wetting properties are stabilized in the heating zone between the heating means and the sonotrode. 
         [0019]    In addition, it is particularly advantageous that an encrustation can be eliminated without problem in the region where the molten solder is introduced by cleaning the gap by providing a blow-off pulse after soldering one workpiece or a predetermined number of workpieces. This cleaning is then carried out when the device comprising the heating means and the sonotrode is raised, so as to position a new workpiece in the region of the sonotrode. 
         [0020]    The blowing off can be particularly carried out by employing a gas such as N 2 , air, argon or other suitable inert gases. However, a rinsing with a liquid may also be carried out. 
         [0021]    Alternatively or in addition, a cleaning means can be provided, in order to free the gap between the sonotrode and the heating means from e.g., solder-caused encrustrations, for example, by employing rotating brushes or other suitable elements. There is also the possibility of setting up a suction means in order to remove contaminants. 
         [0022]    In another embodiment of the invention, a block-shaped unit having a high heating capacity is used as the heating means, which assures that a desired, uniform temperature prevails in the region of the gap. In this case, the block-shaped or cuboid-shaped heating unit can be aligned relative to the sonotrode in such a way that the longitudinal axis of the heating unit encloses an acute angle of up to 20°, preferably in the range of 0° to 5°, relative to the surface of the workpiece, proceeding from the end near the sonotrode. In this way, it is assured that an undesired heating of the workpiece is absent, since the bottom of the heating unit is inclined relative to the surface of the workpiece, in order to achieve a sufficient distance. 
         [0023]    The gap is preferably bounded by a heating-unit projection that is shaped like a cylinder segment, wherein the gap may have the same crosswise dimension over its height—crosswise to the direction of application of the solder—and in the direction of the workpiece may expand conically, thus in the direction of application of the solder. A constant width over half of the gap is likewise possible. 
         [0024]    A device of the type named initially is characterized in that the heating zone comprises a gap that is bounded by the heating means and the ultrasonic sonotrode, the gap being the introduction for the solder to the workpiece. It is particularly provided that the gap between the sonotrode and the heating means preferably has a width B with ½D≦B≦D, wherein D=the diameter of the solder wire. 
         [0025]    The gap may have a constant width over its height. There is also the possibility, however, that the gap can be conically expanded in the direction of the workpiece. 
         [0026]    The heating means in particular involves a cuboid-shaped unit which can be equipped with heating cartridges and which has a sonotrode-side projection that particularly possesses a cylindrical as well as a semi-cylindrical geometry. 
         [0027]    In addition, the block-shaped or cuboid-shaped heating unit is preferably aligned relative to the workpiece such that, proceeding from the sonotrode, the bottom of the heating means relative to the workpiece encloses an acute angle, which can amount to approximately 20°. Preferably, the angle should lie between 0° and 5°. In this way, there is a distancing between the heating means and the workpiece with the consequence that the latter is not heated in an undesired manner. 
         [0028]    The sonotrode, which also is heated to a temperature above the melting point of the solder material, is additionally surrounded by a thermally insulating tube, such as a ceramic tube, above the heating means. 
         [0029]    In addition, a cleaning means is provided, by means of which a blow-off pulse is provided for cleaning the heating zone, thus the gap region. Liquid or gas such as N 2 , air, argon or another inert gas can be utilized as the cleaning medium. 
         [0030]    Alternatively or additionally, there is the possibility of assigning a cleaning means to the sonotrode and to the heating means in order to clean the gap or the heating zone. Thus, solder-caused encrustations between the heating means or the heating unit and the sonotrode can be removed without problem. 
         [0031]    Solder-caused contaminations may also be removed by means of a suction device. 
         [0032]    Other details, advantages and features of the invention result not only from the specific embodiments described herein, but also from the features to be derived therefrom—taken alone and/or in combination—but also from the following description of a preferred example of embodiment to be taken from the drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0033]      FIG. 1  shows a section of a device for introducing a solder material onto a workpiece. 
           [0034]      FIG. 2  shows the section of the device according to  FIG. 1  rotated by 90°. 
           [0035]      FIG. 3  shows the section of the device according to  FIGS. 1 and 2  in a bottom view. 
           [0036]      FIG. 4  shows an excerpt from  FIG. 3  in an enlarged representation. 
           [0037]      FIG. 5  shows representations of the principle in question with solder material of a wetted and a non-wetted gap. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    A section or details of a device by means of which a preferably strip-shaped solder strip is introduced onto a workpiece  10  can be taken from the figures. Workpiece  10  particularly involves a semiconductor device such as a solar cell, in order to solder electrical contacts, for example, onto it after introducing the solder material. In this case, cell connectors may be involved. 
         [0039]    The solder material can be introduced in such a way that solder strips are formed, which exercise the function of busbars that are introduced onto the solar cell and are, in particular, connected to current collectors (grid fingers). 
         [0040]    In order to introduce the solder onto workpiece  10 , solder wire  12  is introduced into a heating zone  14  by a solder-wire introduction means (not shown), the heating zone running between a heating means  16  and a sonotrode  18  of an ultrasound device. 
         [0041]    In the figures, sonotrode  18 , which has a rod geometry, is surrounded by a ceramic tube  20  having a heating coil, above heating means  16 , in order to heat the sonotrode to a temperature above the melting point of the solder material. Sonotrode  18  further proceeds in the known way from an ultrasonic transducer  22 , by means of which sonotrode  18  is excited to ultrasonic vibrations in the range between 10 kHz and 80 kHz, particularly between 10 kHz and 40 kHz, just to name numbers by way of example. 
         [0042]    In the example of embodiment, heating means  16  involves a cuboid or block-type heating unit  17 , which is equipped with heating cartridges. Thus, heating unit  17  has a high heat capacity in order to assure the desired constant temperatures in the region of heating zone  14 . 
         [0043]    Solder wire  12  is introduced into heating zone  14 , and, in fact, is introduced obliquely from the side in the example of embodiment, as is illustrated in principle by comparing the figures. However, the teaching according to the invention will not be limited hereby. 
         [0044]    According to the invention, heating zone  14  has a gap  24 , which is bounded on one side by sonotrode  18  and on the other side by a preferably semicylindrically-shaped projection  26  of heating means  16  or of heating unit  17 , as results from  FIGS. 3 and 4 . Of course, it is not absolutely necessary that the heating unit  17  has a corresponding projection. Rather, on the sonotrode side, the heating unit  17  may have a flat surface, which likewise bounds a corresponding gap. 
         [0045]    Preferably, however, projection  26 , which may also possess a geometry having a circular section that differs, however, from that derived from the drawing, extends from the surface of heating unit  17  facing sonotrode  18 . 
         [0046]    As is illustrated in  FIG. 4 , the gap width B, thus the clearance between sonotrode  18  and projection  26  is selected in such a way that it preferably lies between ½D and D with D=diameter of the solder wire. The width B extends in the direction of application of solder  10  onto the workpiece, thus in the plane of the drawing in  FIG. 1  and parallel to the segment of workpiece  10  that is shown. The width B may be constant or may expand conically in the direction of the workpiece. 
         [0047]    Both heating unit  17  as well as sonotrode  18  are adjusted to a temperature that lies above the melting point of solder wire  12 . Preferably, solder material is used which melts in the range between 100 degrees Celsius (° C.) and 350° C., although the temperature range may lie between 80° C. and 600° C. The temperature adjustment of heating unit  17  is thus made in the region of its projection  26 , independently of the adjustment of the temperature of sonotrode  18 . 
         [0048]    According to the invention, solder wire  12  is introduced into gap  24  in heating zone  14 . The solder wire melts in this way. Despite the high surface tension, a wetting of the boundary of gap  24  is produced (see the representation on the right in  FIG. 5 ) due to the sonotrode  18  excited to ultrasonic vibration, with the consequence that the molten solder flows through gap  24  onto the surface of workpiece  10  and then the solder introduced onto workpiece  10  is loaded with ultrasound [vibration] via the flat front surface  28  of sonotrode  18 , this surface facing workpiece  10 , in order to assure the soldering of workpiece  10 . The vibration antinode of the excited sonotrode  18  runs in the region of the front surface  28 . 
         [0049]    The molten solder is shown for sonotrode  18  that is not placed in vibration in the representation on the left in  FIG. 5 . 
         [0050]    As results from  FIGS. 1 and 2 , the cuboid-shaped or block-type heating unit  17  is aligned to the workpiece surface by its bottom  30  in such a way that, proceeding from sonotrode  18 , an acute angle α results, which should lie between 0° and 20°, in particular between 0° and 5°. In this way, it is additionally assured that workpiece  10  is not heated in an undesired manner via heating unit  17 . 
         [0051]    The gap  24  may have a constant width over its height. There is also the possibility, however, that the gap is increased in the direction of workpiece  10 , in particular, if the region of gap  24 , which is bounded by heating unit  17 , extends perpendicular to bottom  30  of heating unit  17 . 
         [0052]    In addition, it is provided according to the invention that with the use of a cleaning means (not shown), after removing the ultrasound device with heating means  16 , a blow-off pulse is delivered for cleaning the gap region. In this case, a gas or a liquid may be used, which loads the gap region in pulse-like manner, so that solder-caused encrustations are removed. 
         [0053]    Alternatively or in addition, a mechanical cleaning means can be provided, in order to remove solder-caused encrustations. In this case, these involve rotating brushes or other equally acting elements that make possible a removal of encrustations. 
         [0054]    Further, a suction device may be provided in order to be able to collect or suction off solder-caused contaminations. 
         [0055]    In order to achieve process stability, the sonotrode may remain continuously excited, even when a solder wire is not introduced. In this way, it is assured that resonance deviations will be avoided. Also, solder-caused wetting properties are stabilized in the gap. 
         [0056]    The rate of application of the solder onto workpiece  10  should lie in the range between 2 mm and 200 mm/sec, preferably between 20 mm and 80 mm/sec. 
         [0057]    Although the longitudinal direction of the sonotrode is aligned along the normal line proceeding from workpiece  10  in the example of embodiment, other angles are also possible. In particular, an oblique coupling of the ultrasound is possible. This means that the longitudinal axis of sonotrode  18  relative to the normal line proceeding from workpiece  10  encloses an angle of &gt;0°. In this case, an alignment parallel to the surface of workpiece  10  may result optionally. The sonotrode tip or the associated surface of heating unit  17  must be designed correspondingly, in order to make available the necessary gap for the solder wire. 
         [0058]    The invention also includes the circumstance when a targeted deviation of the resonance frequency is provided, e.g., by off-tuning sonotrode  18  or by employing a sonotrode  18  of a length that deviates from a whole-number λ/2 with λ=amplitude. 
         [0059]    In the region of the gap, sonotrode  18  should have a cylindrical geometry with a diameter that lies between 0.5 mm and 4 mm, preferably between 1 mm and 2 mm.