Abstract:
In accordance with the present invention, there is provided a CPV module wherein a solder paste is used as an alternative to wire bonds or braided ribbon/mesh connectors to facilitate the electrical connectivity between the concentrated photovoltaic receiver cell or die of the CPV module and the conductive pattern of the underlying substrate thereof. In accordance with the present invention, the possibility of accidentally shorting the top of the receiver die with the other metal parts of the CPV module is avoided by molding at least the periphery of the receiver die with a mold body, and then dispensing or printing the conductive paste between the top of the receiver die and the substrate, the mold body defining a reservoir which facilities the flow of the conductive paste in a prescribed pattern.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to semiconductor devices and, more particularly, to a concentrated photovoltaic (CPV) receiver package or module wherein a conductive paste is used as an alternative to wire bonds or braided ribbon/mesh connectors to facilitate the electrical connectivity between the photovoltaic die of the module and the conductive pattern of the underlying substrate thereof. 
     2. Description of the Related Art 
     Photovoltaic cells or dies are a well known means for producing electrical current from electromagnetic radiation. Traditional photovoltaic cells comprise junction diodes fabricated from appropriately doped semiconductor materials. Such devices are typically fabricated as thin, flat wafers with the junction formed parallel to and near one of the flat surfaces. Photovoltaic cells are intended to be illuminated through their so-called “front” surface. Electromagnetic radiation absorbed by the semiconductor produces electron-hole pairs in the semiconductor. These electron-hole pairs may be separated by the electric field of the junction, thereby producing a photocurrent. Currently known photovoltaic cells typically have a generally quadrangular (e.g., square) configuration defining four peripheral side edges, and include a pair of bus bars which are disposed on the top or front surface and extend along respective ones of an opposed pair of the side edges. The bus bars are used to facilitate the electrical connection of the photovoltaic cell to another structure, as described in more detail below. 
     There is currently known in the electrical arts semiconductor devices known as CPV receiver die packages or modules. Currently known CPV modules typically comprise a ceramic substrate having a conductive pattern disposed on one side or face thereof. Attached to the substrate and electrically connected to the conductive pattern are electrical components, including a pair of preformed wire connectors and a packaged diode. Also attached to the substrate and electrically connected to the conductive pattern thereof is a CPV receiver cell or die. The electrical connection between the receiver die and the conductive pattern is often facilitated by a pair of punched thin metal foil or braided ribbon/mesh connectors which extend along and are welded or soldered to respective ones of opposed sides of the receiver die, which typically has a quadrangular or square configuration as indicated above. More particularly, the pair of punched thin metal foils or braided ribbon/mesh connectors are welded or soldered to respective ones of the bus bars on the top or front surface of the receiver die. In certain existing CPV modules, the electrical connection of the receiver die to the conductive pattern is facilitated by the use of multiple wires bonded to the bus bars on the front surface of the receiver die and the bond pads of the conductive pattern of the substrate, the wires being used as an alternative to the aforementioned braided ribbon or mesh interconnects. These wire bonds are often fabricated from gold, and are provided in differing numbers and/or diameters depending on the design of the CPV module. The CPV module may further include a light concentration means which is adapted to concentrate solar radiation onto the front surface of the receiver die. 
     Current CPV receiver die packages or modules typically generate up to ten amps of electrical current. In order to carry such high current, the above-described ribbons made of metal foil or braided wire mesh, or the above-described multiple bond wire bonds are used to form the interconnection between the bus bars on the front surface of the receiver die and the bond pads of the conductive pattern on the substrate. However, the use of the ribbon/mesh type interconnects or, alternatively, the bond wires give rise to certain deficiencies in currently known CPV modules which detract from their overall utility. More particularly, the ribbon/mesh type interconnects do not have good shape control for automatic pick up, and require the use of specialized welding equipment for the fabrication of the CPV module using the same. Stated another way, it is often difficult to control the shape of the ribbon/mesh type interconnects for automatic pick up and placement, with the fabrication process being mostly done through the use of special welding equipment or manual soldering which is more labor intensive and thus more costly. When bond wires are used as an alternative to the ribbon/mesh type interconnects, problems may arise in relation to current crowding if too few wires are used. Moreover, the use of bond wires (as well as the use of the soldered or welded ribbon/mesh interconnects) often gives rise to concerns regarding the electrical current carrying capability of the CPV module including the same. 
     The present invention addresses these and other shortcomings of prior art CPV modules by providing a CPV module wherein a conductive paste is used as an alternative to wire bonds or braided ribbon/mesh connectors to facilitate the electrical connectivity between the concentrated photovoltaic receiver cell or die of the CPV module and the conductive pattern of the underlying substrate thereof. In accordance with the present invention, the possibility of accidentally shorting the top of the receiver die with the other metal parts of the CPV module is avoided by molding at least the periphery of the receiver die with a mold body, and then dispensing or printing the conductive paste between the top of the receiver die and the substrate, the mold body defining a reservoir which facilities the flow of the conductive paste in a prescribed pattern. These and other features of the present invention will be described in more detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein: 
         FIG. 1  is a top plan view of a CPV module constructed in accordance with the present invention; 
         FIG. 2  is a cross-sectional view taken along line  2 - 2  of  FIG. 1 ; 
         FIG. 3  is a top plan view of the CPV module of the present invention in an initial state of fabrication prior to the formation of the mold body thereof; 
         FIG. 4  is a cross-sectional view taken along line  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a top plan view of the CPV module of the present invention in an intermediate state of fabrication subsequent to the formation of the mold body thereof, but prior to the dispensation or printing of the conductive paste included therein; 
         FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 5 ; 
         FIG. 7  is a top plan view of the CPV module of the present invention in an intermediate state of fabrication subsequent to the dispensation or printing of the conductive paste included therein but prior to the dispensation of the protective encapsulant thereof; and 
         FIG. 8  is a cross-sectional view taken along line  8 - 8  of  FIG. 7 . 
     
    
    
     Common reference numerals are used throughout the drawings and detailed description to indicate like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein the showings are for purposes of illustrating a preferred embodiment of the present invention only, and not for purposes of limiting the same,  FIGS. 1-8  depict a concentrated photovoltaic (CPV) package or module  10  constructed in accordance with the present invention. In  FIGS. 1 and 2 , the CPV module  10  is depicted in its fully fabricated state. In  FIGS. 3-8 , the CPV module  10  is depicted in partially completed states of fabrication, with an initial fabrication stage being shown in  FIGS. 3 and 4 , an intermediate fabrication stage being shown in  FIGS. 5 and 6 , and an intermediate fabrication stage just prior to completion being shown in  FIGS. 7 and 8 . 
     As is most easily seen in  FIGS. 3 and 4 , the CPV module  10  comprises a substrate  12  which has a generally quadrangular (e.g., square) configuration. When viewed from the perspective shown in  FIG. 4 , the substrate  12  comprises a first (top) layer  14  which is preferably fabricated from copper, a second (middle) layer  16  which is preferably fabricated from a ceramic material, and a third (bottom) layer  18  which, like the first layer  14 , is also preferably fabricated from copper. As is apparent from  FIGS. 3 and 4 , the second layer  16  is sized relative to the first and third layers  14 ,  18  such that the peripheral edge of the second layer  18  protrudes beyond those defined by each of the first and third layers  14 ,  18 . 
     As best seen in  FIG. 3 , the first layer  14  of the substrate  12  is not a continuous, unitary structure. Rather, the first layer  14  includes a first section  20  and a separate second section  22 . The first section  20  includes a generally rectangular base portion  24  which defines opposed pairs of longitudinally and laterally extending side edges. In addition to the base portion  24 , the first section  20  includes a generally square die pad portion  26  which protrudes from one of the longitudinally extending side edges of the base portion  24 . 
     The second section  22  of the first layer  14  includes a generally rectangular base portion  28  defining opposed pairs of longitudinally and laterally extending side edges. In addition to the base portion  28 , the second section  22  includes an identically configured pair of prong portions  30  which each have a quadrangular configuration, and extend in spaced, generally parallel relation to each other from a common longitudinally extending side edge of the base portion  28 . As is further seen in  FIG. 3 , the base and prong portions  28 ,  30  of the second section  22  collectively define a generally square gap or recess  32  which is sized and configured to accommodate the die pad portion  26  of the first section  20 . More particularly, in the substrate  12 , the first and second sections  20 ,  22  of the first layer  14  are oriented relative to each other such that the die pad portion  26  is disposed within the recess  32 . However, the first and second sections  20 ,  22  are maintained in spaced relation to each other, with a continuous gap or void being defined therebetween as also shown in  FIG. 3 . 
     The CPV module  10  constructed in accordance with the present invention further comprises a photovoltaic receiver cell or die  34 . The receiver die  34  has a generally quadrangular (e.g., square) configuration, and defines a generally planar top or front surface  36 , and an opposed, generally planar bottom or back surface  38 . In addition, the receiver die  34  defines four (4) generally straight peripheral side surface segments  40 . The receiver die  34  is fabricated from a number of layers, including an active layer  42  which is applied to one side or face of an underlying substrate, and defines the front surface  36  of the receiver die  34 . As seen in  FIG. 3 , the active layer  42  itself includes a pair of elongate contact strips of bus bars  44  which extend in spaced, generally parallel relation to each other along respective ones of an opposed pair of the peripheral side surface segments  40  of the receiver die  34 . More particularly, the bus bars  44  extend along those peripheral side surface segments  40  which in turn extend along respective ones of the prong portions  30  of the second section  22  of the first layer  14 . As seen in  FIG. 4 , in the CPV module  10 , the back surface  38  of the receiver die  34  is mounted and electrically connected to the top surface of the die pad portion  26  of the first section  20  of the first layer  14 . Such mounting and electrical connection is preferably facilitated by the use of a layer  46  of a conductive epoxy which is interposed between the back surface  38  and the die pad portion  26 . Thought not shown, the CPV module  10  may further include an electronic device such as a packaged diode or rectifier which is attached to the top surface of the base portion  24  of the first section  20  and electrically connected to one of the prong portions  30  of the second section  22 . 
     In the CPV module  10 , the side surface segments  40  and portions of the first and second sections  20 ,  22  of the first layer  14  of the substrate  12  are covered by an encapsulant material which, upon hardening, defines a mold body  48  of the CPV module  10 . The mold body  48  has a generally quadrangular (e.g., square) configuration and, as shown in  FIGS. 1 ,  2  and  5 - 8 , is formed so as to cover the entirety of the top surface of the base portion  24  of the first section  20 , and that area of the top surface of the die pad portion  26  of the first section  20  which is not covered by the receiver die  34 . In addition, the mold body  48  covers the entirety of the top surface of the base portion  28  of the second section  22 , and the majority of the top surfaces of the prong portions  30  of the second section  22 . However, as will be described in more detail below, the mold body  48  is formed such that, in an initial stage of the fabrication of the CPV module  10 , an elongate, strip-like segment of each of the prong portions  30  is not covered thereby, and thus exposed therein. 
     As is most easily seen in  FIGS. 2 ,  6  and  8 , the mold body  48  is not of uniform thickness. In this regard, the mold body  48  includes an inner portion  50  which is of a first thickness, and a peripheral outer portion  52  which is of a second thickness exceeding the first thickness. More particularly, the inner portion  50  comprises two elongate segments  50   a,    50   b  of the mold body  48  which extend along and cover respective ones of those side surface segments  40  of the receiver die  34  which in turn extend along respective ones of the prong portions  30  of the second section  22 . As viewed from the perspective shown in  FIG. 6 , each segment  50   a ,  50   b  of the inner portion  50  defines a top surface which extends in generally co-planar relation to the front surface  36  of the receiver die  34 . The inner portion  50  also covers those regions of the top surface of the die pad portion  26  which are not covered by the receiver die  34  and extend along those side surface segments  40  of the receiver die  34  also covered by the inner portion  50 . As further shown in  FIG. 6 , the inner portion  50  of the mold body  40  also extends through those gaps defined between the prong portions  30  and the corresponding, adjacent peripheral edge segments of the die pad portion  26 , into direct contact with the top surface of the second layer  16  of the substrate  12 . 
     As is also apparent from the perspective shown in  FIG. 6 , the outer portion  52  of the mold body  48  defines a generally planar top surface which is elevated above that defined by the inner portion  50 , i.e., the top surfaces of the inner and outer portions  50 ,  52  extend along respective ones of a spaced, generally parallel pair of planes. Additionally, as shown in  FIGS. 5 and 6 , the outer portion  52  circumvents the receiver die  34 , though the outer portion  52  only contacts and covers that opposed pair of the side surface segments  40  of the receiver die  34  which are not in contact with the segments  50   a,    50   b  of the mold body  48  collectively defining the inner portion  50  thereof. In this regard, the outer portion  52  is separated from each of the two elongate segments  50   a,    50   b  of the mold body  48  defining the inner portion  50  by respective ones of a pair of elongate slots or channels  54 . As further seen in  FIGS. 5 and 6 , each of the channels  54  extends generally perpendicularly between the top surface of one of the segments  50   a,    50   b  of the inner portion  50  and a prescribed section or segment of the top surface of a corresponding one of the prong portions  30 . Thus, the aforementioned elongate segments of the top surfaces of the prong portions  30  which are not covered by the mold body  48  are exposed within respective ones of the channels  54  defined by the mold body  48 . 
     Though not apparent from  FIGS. 2 ,  6  and  8 , the outer portion  52  of the mold body  48  extends through the gap defined between the base portion  28  of the second section  22  and the adjacent peripheral edge segment of the die pad portion  26  defined by the distal end thereof, and into contact with the top surface of the second layer  16  of the substrate  12 . The outer portion  52  of the mold body  48  also extends into and through the gaps defined between the distal ends of the prong portions  30  of the second section  22  and the base portion  24  of the first section  20 , into contact with the top surface of the second layer  16  of the substrate  12 . 
     Based on the structural features of the mold body  48  described above, such mold body  48  defines a centrally oriented, generally quadrangular reservoir  56 . The reservoir  56  is circumvented by the top surface of the outer portion  52  of the mold body  48 . Additionally, when viewed from the perspective shown in  FIGS. 2 ,  6  and  8 , the bottom surface of the reservoir  56  is collectively defined by the front surface  36  of the receiver die  34  and the top surface of the inner portion  50  of the mold body  48  (i.e., the top surfaces of the segments  50   a,    50   b ). The side surfaces or side walls of the reservoir  56  are defined by inner surface segments of the outer portion  52  of the mold body  48 . The aforementioned channels  54  thus extend from the bottom surface of the reservoir  56  to prescribed segments of the top surfaces of respective ones of the prong portions  30  in the above-described manner. 
     Referring now to  FIGS. 7 and 8 , in the CPV module  10 , a conductive paste  58  is dispensed or printed into each of the channels  54  so as to completely fill the channels  54 , the conductive paste  58  thus coming into direct contact with the elongate segments of the prong portions  30  of the second section  22  originally exposed in the channels  54 . The conductive paste  58  is also dispensed or printed so as to flow over and thus cover the top surface of the inner portion  50  of the mold body  48 , and more particularly the top surfaces of the two elongate segments  50   a,    50   b  of the mold body  48  defining the inner portion  50 . As further seen in  FIG. 8 , the conductive paste  58  is also dispensed or printed so as to flow over and make electrical contact with the bus bars  44  of the receiver die  34 . 
     More particularly, the conductive paste  58  is segregated into two separate and distinct segments  58   a,    58   b.  In this regard, each segment  58   a,    58   b  of the conductive paste  58  extends between and is electrically connected to one of the bus bars  44  of the receiver die  34 , that prong portion  30  disposed closest to such bus bar  44 , and the top surface of that segment  50   a,    50   b  of the inner portion  50  which extends between the bus bar  44  and corresponding prong portion  30 , as shown in  FIG. 8 . As will be recognized, in the CPV module  10 , the separate segments  58   a ,  58   b  of the conductive paste  58  are used as an alternative to the bond wires or braided ribbon/mesh connectors described above to facilitate the electrical connection of each of the bus bars  44  of the receiver die  34  to respective ones of the prong portions  30  of the second section  22  of the first layer  14  included in the substrate  12 . Though being of relatively low viscosity, the conductive paste  58 , when initially dispensed or printed into the reservoir  56 , is prevented from outflow over the mold body  48  by the outer portion  52  thereof, and in particular the aforementioned inner surface segments of the outer portion  52 . Those of ordinary skill in the art will recognize that other conductive materials, such as a metallic paste (e.g., solder or copper paste), may be substituted for the conductive paste  58  without departing from the spirit and scope of the present invention. 
     Referring now to  FIGS. 1 and 2 , of the CPV module  10  of the present invention further comprises an encapsulant layer  60  which is applied to those portions of each of the two segments  58   a,    58   b  of the conductive paste  58  which are exposed in the reservoir  56  prior to the formation of the encapsulant layer  60 . In the CPV module  10 , the encapsulant layer  60  is preferably segregated into two elongate strips  60   a,    60   b,  each such strip  60   a,    60   b  of the encapsulant layer  60  being applied to and covering the exposed portion of a respective one of the segments  58   a,    58   b  of the conductive paste  58 . Thus, as seen in  FIG. 2 , small areas of each segment  60   a,    60   b  of the encapsulant layer  60  come into contact with the front surface  36  of the receiver die  34 . Additionally, each of the strips  60   a,    60   b  extends between an opposed pair of the inner surface segments of the outer portion  52  which partially define the reservoir  56 . 
     In the CPV module  10 , it is contemplated that reservoir  56  of the mold body  48 , and in particular the inner surface segments of the outer portion  52  which partially define the same, may be mechanically adapted to facilitate the alignment of a light concentrating device such as an optical light guide or prism with the front surface  36  of the receiver die  34  exposed in the reservoir  56 . The alignment function may be assisted by one or both of the strips  60   a,    60   b  residing within the reservoir  56 . 
     An exemplary sequence of steps to facilitate the fabrication of the CPV module  10  may comprise the initial step of providing the substrate  12  having the above-described structural attributes, and thereafter attaching the receiver die  34  thereto in the above-described manner, and in accordance with the showings in  FIGS. 3 and 4 . In the next step of the fabrication method, the mold body  48  having the aforementioned structural attributes may be formed on the substrate  12  and receiver die  34  in the above-described manner, and in accordance with the showings in  FIGS. 5 and 6 . The step of forming the mold body  48  may be followed by the dispensation or printing of the conductive paste  58  in the manner also described above and in accordance with the showings in  FIGS. 7 and 8 . The last step of the fabrication process for the CPV module  10  may comprise forming the encapsulant layer  60  to cover the exposed portions of the conductive paste  58  in the above-described manner and in accordance with the showings in  FIGS. 1 and 2 , the formation of the encapsulant layer  60  completing the fabrication of the CPV module  10 . 
     Thus, in the CPV module  10  of the present invention, the conductive paste  58  completes the electrical connection of the bus bars  44  of the receiver die  34  to the substrate  12 , and in particular the prong portions  30  of the second section  22  thereof. The use of the conductive paste  58  (or other conductive material) as an alternative to bond wires or braided ribbon/mesh connectors ensures low resistivity and hence low voltage drop in the CPV module  10  due to the uniformity and thickness of each segment  58   a,    58   b  of the conductive paste  58 . This enhanced electrical performance of the CPV module  10  is realized even though the conductive paste  58  or other conductive material may not be as conductive as gold, copper or other materials typically used to make wire bonds or braided ribbon/connectors. Additionally, the formation of the mold body  48  in the CPV module  10  may be accomplished using readily available molding techniques, thus not excessively increasing the complexity of the manufacturing process for the CPV module  10 , or resulting in a substantial increase in the cost associated therewith. 
     This disclosure provides exemplary embodiments of the present invention. The scope of the present invention is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification, such as variations in structure, dimension, type of material and manufacturing process may be implemented by one of skill in the art in view of this disclosure.