Abstract:
The invention is a leadframe receiver package comprising a first conductive element, a solar cell electrically coupled to the first conductive element and comprising an active area, and a mold compound disposed on the leadframe and the solar cell. The mold compound defines a first aperture wall over at least a portion of the active area and a second aperture wall over at least a portion of the first conductive element. The mold compound includes a reflective surface to improve heat resistance around an aperture wall receiving solar radiation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 12/046,152 filed on Mar. 11, 2008 entitled “Leadframe Receiver Package For Solar Concentrator,” which claims priority to U.S. Provisional Patent Application Ser. No. 61/016,314, filed on Dec. 21, 2007 and entitled “Leadframe Receiver Package For Solar Concentrator,” the contents of which are incorporated herein by reference for all purposes. 
     
    
     BACKGROUND 
       [0002]    As the demand for solar energy continues to increase as a source of renewable energy, concentrated solar energy collectors must be designed to operate under a wide range of climate conditions with easily manufacturable parts. Many of these parts need to withstand concentrated solar irradiation. 
         [0003]    A solar cell is an integral component of a solar collection system and requires some manner of package for use within a power-generating system. The package must provide protection from exposure to a variety of environmental conditions and concentrated solar irradiation while providing for secure electrical connections. The package may provide heat dissipation, electrical connectivity and/or other functions to the solar cell. A concentrating solar power unit may operate to concentrate incoming light onto a solar cell. This concentrated light, which may exhibit a power per unit area of 500 or more suns, requires a solar cell package which can withstand such intensity over an operational lifetime. The package must also be capable of supporting high power levels generated by systems in which the concentrating solar power unit will typically be implemented. 
         [0004]    Conventional attempts to address the foregoing issues have led to solar cell packages which are expensive due to material costs and/or manufacturing difficulties. What is needed is an improved solar cell package for use in a solar concentrator. Such a system may improve manufacturability, cost, operational lifetime, alignment, power generation efficiency, power dissipation and electrical isolation. 
       SUMMARY 
       [0005]    The invention provides a leadframe package that includes a solar cell with connected conductive elements encased in a mold compound with apertures for exposing the solar cell and conductive elements. The mold compound may be a reflective or heat insensitive material such as a polymer mixed with a ceramic (e.g. silica). The aperture walls surrounding the solar cell may be reflective to serve as a heat shield to surrounding components. The leadframe package may also include an optically transparent material on the active surface of the solar cell. The leadframe package may include an optical element disposed on the active surface of the solar cell. 
         [0006]    The conductive elements may pass through one or more apertures, and there may be an insulating material such as silicone disposed in the apertures. The conductive elements may include electrical connectors. The leadframe package may include a heat spreader or a dielectric layer disposed on a portion of the leadframe package. The leadframe package may be fabricated by electrically coupling a solar cell to a conductive element and molding a mold compound to form an aperture around the solar cell and a separate aperture over at least one portion of a conductive element. The aperture wall around the solar cell may be inherently reflective, or a reflective surface may be deposited onto the aperture wall. The reflective surface can be a separate element mounted onto the leadframe package. An optical element may be co-molded into the leadframe or the mold compound may be used to align an optical element after the mold compound is cured. In some embodiments, the leadframe package may include a dielectric coating. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a perspective top view of a leadframe according to some embodiments. 
           [0008]      FIG. 2A  is a perspective top view of a molded package showing enclosed apertures. 
           [0009]      FIG. 2B  is a similar view of another embodiment showing two apertures open at the sides. 
           [0010]      FIG. 2C  is a similar view of another embodiment of a molded leadframe package. 
           [0011]      FIG. 3A  is a cutaway side view of a molded leadframe receiver package with a dielectric layer according to some embodiments. 
           [0012]      FIG. 3B  is a cutaway side view of another embodiment of a leadframe package with a dielectric layer. 
           [0013]      FIG. 4A  is a cutaway side view of a molded package aligning the optical element according to some embodiments. 
           [0014]      FIG. 4B  is a similar view showing the optical element co-molded in the leadframe package. 
       
    
    
     DESCRIPTION 
       [0015]    The following description is provided to enable any person in the art to make and use the described embodiments and sets forth the best mode contemplated for carrying out some embodiments. Various modifications, however, will remain readily apparent to those in the art. The invention provides for an improved molded leadframe package for housing a solar energy cell. The leadframe package includes an aperture with a reflective surface to assist in heat shielding the components within the leadframe package. The leadframe package provides for improved electrical insulation that may result in better performance during a high potential electrical withstand test (Hi-pot). The improved leadframe package may provide better protection against environmental conditions in a concentrated photovoltaic (CPV) system. The molded leadframe package of this invention may provide reduced manufacturing costs by minimizing the number of parts in the overall receiver design. 
         [0016]      FIG. 1  is a top view of an exemplary conductive panel strip (e.g., copper) for illustration of the fabrication of a leadframe  100  that may be packaged by embodiments of the present invention. Leadframe elements of three devices  105   a - 105   c  are illustrated, but a panel strip may include elements for any number of devices. The leadframe elements  105   a - 105   c  may be etched or stamped from a conductive panel strip using known leadframe manufacturing techniques. In the embodiment of  FIG. 1 , leadframe  100  includes tiebar elements  150  connecting the leadframe devices  105   a - 105   c,  and also includes conductive elements  335   a - 335   c  and  340   a - 340   c  which provide electrical connections in an assembled leadframe package. Lines  110   a - 110   d  indicate cutting lines for separating leadframe components after being assembled into a molded leadframe receiver package of the present invention. 
         [0017]    A dielectric layer may be applied to the leadframe surface at any point during the manufacture of the leadframe device. In a particular embodiment the dielectric layer may be Al 2 O 3 . The dielectric layer may be applied by chemical vapor deposition or any method known in the art for applying material to a leadframe. In a particular embodiment, the dielectric layer may be applied by thermal plasma spraying. The circuit pathway of the leadframe strip may be modified to facilitate one frame testing before the singulation of individual leadframe devices. 
         [0018]    Various embodiments of assembled leadframe receiver packages of the present invention are given in the top views of  FIGS. 2A-2C  and the cross-sectional views of  FIGS. 3A ,  3 B,  4 A, and  4 B. As shall be described subsequently in relation to these figures, solar cells (e.g. solar cell  390  of  FIG. 3A-3B  or solar cell  420  of  FIG. 4A-4B ) are attached to conductive elements  340   a - 340   c  after fabrication of the leadframe  100 . A solar cell may comprise a III-V solar cell, a II-VI solar cell, a silicon solar cell, a thin film solar cell sitting on its own support element or any other type of solar cell that is or becomes known. The solar cell may comprise any number of active, dielectric and metallization layers, and may be fabricated using any suitable methods that are or become known. The solar cell is capable of generating charge carriers (i.e., holes and electrons) in response to received photons. 
         [0019]    A solar cell used in the present invention may have conductive terminals (not shown) on its upper side. Each of the conductive terminals may comprise any suitable metal contact, and may include a thin adhesion layer (e.g., Ni or Cr), an ohmic metal (e.g., Ag), a diffusion barrier layer (e.g., TiW or TiW:N), a solderable metal (e.g., Ni), and a passivation metal (e.g., Au). These conductive terminals may be interconnected to conductive leadframe elements  340   a - 340   c  by methods such as soldering, stud bumping and wirebonding. Alternatively, interconnects may be formed by any method known in the art for attaching cell terminals to cell carriers. 
         [0020]    A further conductive terminal (not shown) may be disposed on a lower side of the solar cell. The lower conductive terminal may exhibit a polarity opposite from the polarity of the upper conductive terminals. This lower conductive terminal may be coupled to conductive leadframe element  335   a - 335   c  using silver die attach epoxy or solder according to some embodiments. By virtue of the foregoing arrangement, current may flow between conductive elements  335  and  340  while a solar cell actively generates charge carriers. If the solar cell is faulty or otherwise fails to generate charge carriers, a bypass diode may electrically couple conductive element  335  to conductive element  340  in response to a received external signal. 
         [0021]      FIG. 2A  is a top view of an assembled leadframe package  200  according to some embodiments. Mold compound  255 , which may comprise any suitable material, may be molded over the leadframe strip  100  shown in  FIG. 1  or any leadframe known in the art followed by singulation of individual leadframe devices. The assembled leadframe package  200  may also include a bottom mold compound or other backing surface, not shown. In  FIG. 2A , apertures  260  and  265  are defined by mold compound  255 . In one embodiment apertures  260  and  265  are fully enclosed by the mold compound  255 . Conductive element  235  and conductive element  240  are respectively exposed by apertures  260  and  265 . The resulting package may contain exposed tiebar  250  and conductive elements  235  and  240  at the sides. The tiebars  250  may be coated with a dielectric material for insulation. Aperture  269  is disposed so as to expose an active area of solar cell  220 , and the wall of the aperture  269  beneficially provides a reflective surface that may withstand exposure to concentrated solar radiation. Any percentage of the active area of solar cell  220 , including 100%, may be visible through aperture  269 . The surface of aperture wall  269  may be reflective to assist in directing incoming light to the active area and to prevent damage to the leadframe package  200  from light or heat. In one embodiment of this invention the material of mold compound  255  is natively reflective such as a polymer mixed with ceramic particles (e.g., silica). In another embodiment of this invention, aperture wall  269  is coated with a reflective material to form a reflective surface. According to some embodiments, the mold compound  255  is light-colored to assist in reflecting solar energy incident thereon. Mold compound  255  may have a high thermal conductivity in some embodiments and may assist in the dispersion of heat from incident solar energy. 
         [0022]      FIG. 2B  shows a top view of an embodiment of a leadframe package  210  of this invention in which apertures  261  and  266  are open at the sides of the leadframe package  210  and conductive elements  235  and  240  are exposed to the edges of the leadframe package  210 . This alternative embodiment provides for a wider variety of access geometries to conductive elements  235  and  240 . The mold compound of this invention may include any aperture arrangement that may cover any arrangement of leadframe and tiebar elements. Similarly, the mold compound may include mold compound covering a portion of the bottom side of the leadframe. In still another embodiment the bottom surface of the leadframe package may be laminated or coated with a dielectric material. 
         [0023]      FIG. 2C  shows another embodiment of a leadframe package  215  of this invention in which a portion  236  and  241  of conductive elements  235  and  240  may protrude into apertures  260  and  265 , respectively. In this embodiment the protruding conductive portions  236  and  241  may provide for improved electrical conductivity for the leadframe package  215 . The shape of the protruding portions  236  and  241  of the conductive elements  235  and  240  may be modified to facilitate electrical connection with external electrical wiring. 
         [0024]      FIG. 3A  is a cross-sectional view of a leadframe package  300  along axis of  FIG. 2A . In this embodiment, the bottom surface of the leadframe package  300  is covered by a dielectric coating  380 . The coating  380  may be any material known in the art to provide a thermally conductive and electrically insulative layer (e.g., Al 2 O 3 , diamond, BN, AlN, or SiN). The coating  380  may be applied by chemical vapor deposition, thermal spraying or any method known in the art for depositing a dielectric material. The coating  380  may be applied after the mold compound  355  has been applied to the leadframe package  300 , resulting in a uniform dielectric layer  380  on the bottom surface of the leadframe package  300  covering the bottom surface of gap  352  between conductive elements  340  and  335 . Also shown are apertures  361 ,  366  and  369 . Apertures  361  and  366  expose conductive elements  335  and  340 , while aperture  369  exposes an active area of solar cell  390 . 
         [0025]    One aspect of a leadframe package of this invention is that geometry of the aperture walls may result in better performance of the leadframe package during testing of safety and conductivity by providing improved insulation for the conductive elements. In another aspect, the material used to form aperture  369  may also improve the performance of the leadframe package. The wall surface  375  of aperture  369  may be reflective, resulting in reduced heating of the leadframe package  300  as concentrated sunlight is directed to the solar cell  390 . The reflective wall surface  375  may be comprised of, for example, aluminum, chromium, or other reflective metals or dielectric layers. The reflective wall surface  375  may be deposited by vapor deposition or any method known in the art for depositing a material onto the surface of a mold compound. Alternatively, the wall surface  375  may be made reflective by inclusion of a separate part such as a separate piece of reflective metal insert made of aluminum or other known reflective material. 
         [0026]    In an alternative embodiment, the reflective wall surface  375  of the aperture wall  369  may be a property of the mold compound  355 . In one embodiment the mold compound  355  may be a polymer (e.g., moldable silicones and epoxies) with added particles (e.g., calcium carbonate, silica or titania). The particles may be on the order of 10&#39;s of micrometers in size. The particles may comprise 50-90% by weight of the polymer compound. In a particular embodiment, the particles may be 90% by weight of the polymer compound. In yet another particular embodiment the mold compound may be silicone with added silica particles. 
         [0027]      FIG. 3B  illustrates another embodiment of a leadframe package  310 . In this embodiment, the bottom surface of only the conductive leadframe elements  335 ,  340  are covered by a dielectric coating  385 . In this embodiment, the mold compound  355  is disposed completely through the bottom layers of the leadframe package  310  including gap  352 . In an alternative embodiment, an optional bottom mold compound (not shown) which may be molded from any moldable material such as silicone may be disposed on the bottom surface of the leadframe package. In a particular embodiment the bottom mold compound is silicone. In yet another embodiment, the moldable material may be configured with variable thickness to provide a thermally conductive heat path for the solar cell  390 . 
         [0028]    In an alternative embodiment not shown, a portion of the conductive elements may be in a recessed position in the center of the leadframe package. This downset configuration may facilitate the dissipation of heat. In other embodiments, the conductive elements may be configured with variable thickness to provide a heat sink for the solar cell. 
         [0029]      FIG. 4A  is a cutaway view of a device  400  according to some embodiments. Device  400  includes conductive elements  435  and  440  which may be coupled to a dielectric layer  480 , which may or may not comprise a bottom mold compound. In one embodiment layer  480  may be coupled to a heat spreader. According to other embodiments, electrical isolation between the heat spreader or other devices and conductive elements  435  and  440  may be further improved by disposing an insulator (not shown) such as silicone or epoxy within apertures  461  and  466 . The aperture  469  may be filled with an optically transparent encapsulant which may provide added protection from the environment. 
         [0030]    In one embodiment shown in  FIG. 4A  of this invention, an optical element  440  may be aligned by the geometry of aperture  469  in the molded compound  455 . Optical element  440  may increase an acceptance angle of the concentrating solar radiation collector, homogenize incoming concentrated light over the surface of solar cell  420 , and/or further concentrate the light. Aperture  469  may assist in retaining optical element  440  in a suitable position. In other embodiments, additional optical elements or optically active layers may be similarly aligned by the geometry of aperture  469  in the mold compound  455  of this invention. The additional optical layers may provide additional protection for the solar cell  420  from the environment. 
         [0031]      FIG. 4B  illustrates alternative embodiments of the present invention. In one embodiment the optical element  440  may be co-molded with the mold compound  455  into the leadframe package  401 . In yet another embodiment, a portion  436 ,  441  of conductive elements  435 ,  440  may protrude into apertures  461  and  466  for improved electrical connectivity. According to other embodiments, electrical isolation of conductive elements  435  and  440  may be further improved by disposing an insulator (e.g., silicone or epoxy) within apertures  461  and  466 . Insulated wires may be coupled to elements  435  and  440  through apertures  461  and  466  prior to such filling. Conductive elements  435  and  440  may comprise electrical connectors to facilitate the electrical connection with an electrical system and thereby improve the manufacturability of the invention. 
         [0032]    While the specification has been described in detail with respect to specific embodiments of the invention, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention. Thus, it is intended that the present subject matter covers such modifications and variations as come within the scope of the appended claims and their equivalents.