Abstract:
A method for bonding a concentrating photovoltaic receiver module to a heat sink using a reactive multilayer foil as a local heat source, together with layers of solder, to provide a high thermal conductivity interface with long term reliability and ease of assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is related to, and claims priority from, U.S. Provisional Patent Application Ser. No. 61/144,876 filed on Jan. 15, 2009, which is herein incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention is related generally to methods for bonding concentrating photovoltaic (CPV) receiver modules to heat sinks, and in particular, to a method for bonding a CPV receiver module to a heat sink with a reactive composite foil and solder at the bond interface. 
         [0004]    Concentrating photovoltaic (CPV) modules are used to concentrate sunlight onto high-efficiency solar cells for the purpose of electrical power production. The solar cells are typically mounted onto substrates called receivers, and groups of the receiver modules are mounted onto heat sinks to maintain low solar cell junction temperatures and to achieve correspondingly high electrical conversion efficiencies. 
         [0005]    Current CPV systems have developed power levels up to 2000 suns. The systems require highly efficient cooling methods to maintain low temperatures in the solar cells. The thermal interface between the CPV and its heat sink is a critical aspect in the transfer of heat generated by the CPV cells into heat sinks. The materials and bonding methods employed when forming the receiver modules have a direct impact on the cell performance, efficiency, and operational life. Typically thermal adhesives and pastes are used at the interface between CPV receiver modules and heat sinks. Both of these materials and bonding methods have disadvantages which fail to meet the thermal requirements of a CPV system rated for a power level at or above 2000 suns. 
         [0006]    Thermal adhesives and pastes typically create an interface with thermal resistance of 20 Kmm 2 /W. At rated power levels equal to or exceeding 2000 suns, the waste heat which needs to be transferred from the cell to the heat sink through the interface can reach or exceed 140 W. A large thermal resistance for the interface will generate large temperature differences across the interface and will make it difficult to keep the solar cells running at temperatures below those that are required to avoid thermal destruction of the cell. 
         [0007]    These adhesives and pastes are normally based on silicone materials, which require about 0.5-1.0 hours at elevated temperatures to cure. The curing process increases the production time and reduces the production output. The materials remain soft after curing and are not desirable for long term reliability and longevity of photovoltaic systems. 
         [0008]    Adhesive or grease bonds degrade due to exposure to environment; the resulting degradation will increase the cell junction temperature and therefore will reduce the cell electrical conversion efficiency and cell longevity. 
         [0009]    Given the limitations of the current interface material and bonding methods, there is a need for a novel material that can provide a high thermal conductivity interface with long term reliability and easy assembly process. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Briefly stated, the present disclosure provides a method for bonding a CPVB receiver module to a heat sink using a reactive multilayer foil as a local heat source, together with a solder, to provide a high thermal conductivity interface with long term reliability and ease of assembly. 
         [0011]    In alternate embodiments, the present disclosure further provides a method for of bonding polymers or composites, as well as dissimilar materials that cannot be easily bonded by welding, brazing, or diffusion bonding. The present invention can result in reduction in machining time and costs either before or after bonding, and will result in lower thermal resistances for a given interface, compared to conventional thermal interface materials and methods. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]      FIG. 1  is a sectional illustration of a CPV receiver module prior to bonding to a heat sink; 
           [0013]      FIG. 2  is a sectional illustration of the CPV receiver module and heat sink of  FIG. 1 , arranged with a reactive foil and solder layers for bonding; and 
           [0014]      FIG. 3  is a sectional illustration of the CPV receiver module and heat sink of  FIGS. 1 and 2  after bonding. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The following detailed description illustrates the invention by way of example and not by way of limitation. The description enables one skilled in the art to make and use the present disclosure, and describes several embodiments, adaptations, variations, alternatives, and uses of the present disclosure, including what is presently believed to be the best mode of carrying out the present disclosure. 
         [0016]    In a first embodiment, shown schematically in  FIGS. 1-3 , a receiver module (solar cell substrate—Cu/ceramic/Cu board—PCB, Al, etc.)  12  with a CPV cell (solar cell die(s) Si, Ge, compound semiconductor)  11  mounted on the top, is positioned to be bonded to a heat sink (Al, Cu or composite)  13  using a reactive composite joining process with a reactive multilayer foil  18  and solder layers  16  and  17  to form a bond  19 . Reactive multilayer foils  18  and their related composite joining processes have been described in several patents including U.S. Patent Application Publication No. 2008/0063889 A1 to Duckham, et al., filed Sep. 3, 2007 as U.S. patent application Ser. No. 11/851,003, which is incorporated herein by reference. 
         [0017]    As seen best in  FIG. 2 , the faying surface  14  of the receiver module  12  and the faying surface  15  of heat sink are pre-wet with layers  16  and  17  of solder alloy by suitable means known in the art, such as, but not limited to, application of solder with a hot plate or the screen printing of solder. Methods of solder application are described in U.S. patent application Ser. No. 11/851,003, which is herein incorporated by reference. The surfaces of the solder alloy  16  on the receiver module and the solder alloy  17  on the heat sink are aligned parallel to each other to within one part in 1000 by machining or other suitable alignment means known in the art. 
         [0018]    Once the solder layers  16  and  17  are disposed and aligned, one or more pieces of a reactive multilayer foil  18  are placed between the layer  16  of solder alloy and layer  17  of solder alloy, and a pressure is applied perpendicular to the aligned components to hold the faying surfaces  14  and  15  against the reactive multilayer foil pieces  18 , as shown in  FIG. 2 . The foil pieces  18  are then ignited by a suitable application of initiation energy and the resulting exothermic reaction in the reactive multilayer foil  18  melts a quantity of the solder alloy layers  16  and  17  sufficient to cause wetting and bonding between the faying surfaces. When the solder alloy solidifies, the receiver module  12  and heat sink  13  are bonded together by a bond layer  19  of solder material infused with the remnants of the reactive multilayer foil, as shown in  FIG. 3 . 
         [0019]    The reactive multilayer foils  18  utilized in the reactive composite joining methods of the present disclosure are typically formed by magnetron sputtering and consist of thousands of alternating nanoscale layers of materials, such as nickel and aluminum. The layers react exothermically when atomic diffusion between the layers is initiated by an external energy pulse, and release a rapid burst of heat in a self-propagating reaction. If the reactive multilayer foils  18  are sandwiched between layers of a bonding material or fusible material, such as the solder alloy layers  16  and  17 , the heat released by the exothermic reaction of the reactive multilayer foils  18  can be harnessed to melt these layers of bonding material. The resulting bonding layer  19  comprises a solder layer that includes the reaction products of the reactive multilayer foil. By controlling the properties of the reactive multilayer foils  18 , the amount of heat released by the reactive multilayer foils  18  during the exothermic reaction can be tuned to ensure there is sufficient heat to melt the fusible material layers  16  and  17 , but at the same time maintain the bulk of the adjacent components  11 ,  12 , and  13  at or close to room temperature. Further details concerning reactive multilayer foils  18 , joining with them, and their reaction products can be found in U.S. Pat. No. 6,736,942, which is incorporated herein by reference. 
         [0020]    In related embodiments, the solder alloy may be applied to the faying surfaces  14  and  15  of one or both components via a thermal spray method. Any of a variety of thermal spray methods known in the art may be used, including flame spraying, arc spraying, plasma spraying, detonation spraying, high velocity oxy-fuel (HVOF) spraying, laser spraying and cold spraying. The advantage of thermally spraying a layer of solder  16  or  17  is that the component onto which the solder is deposited is not heated as much as in conventional pre-tinning, pre-soldering or pre-brazing methods that require the component to be heated above the melting temperature of the solder or braze. These thermal spray methods work best for metal components which can be grit blasted prior to spraying to improve the adhesion between the solder layer and the component surface. Thermal spray methods may also be used to apply a fusible layer to a component made of a ceramic or a polymer matrix composite. 
         [0021]    In another embodiment a solder alloy is applied to the faying surfaces  14  and  15  using a screen printing method. Such a method is commonly used in microelectronics manufacturing and can enable the deposition of  50  microns or more of solder paste onto a solar cell substrate without damaging the solar cell  12  that is attached to the substrate. It can also be used to apply a solder paste to a heat sink  13 . 
         [0022]    As an alternative to pre-wetting the components with a solder layer  16  or  17 , the faying surfaces  14  and  15  of the components  12  and  13  may be metallized by methods known in the art, such as physical vapor deposition. The object of the metallization process is to produce a faying surface  14  or  15  that may be easily wet by molten solder during the instant that the solder is molten in the reactive composite joining process. The metallization layer may be a noble metal such as gold or silver or a very thin layer of solder such as tin, or a thin layer of braze such as Incusil®. Metallization may also be carried out via electroplating or chemical (electroless) plating, or immersion (chemical) plated, for instance with tin, nickel and gold. 
         [0023]    If more solder is present in the resulting bond layer  19 , the thickness of the layers  16  and  17  that are pre-adhered on each component may be as thick as 100 μm. The maximum thickness of any pre-wet layer is dictated by the constraints of the application method or the desired properties of the resulting bond. 
         [0024]    Solder thickness at the interface requires optimization to meet both the thermal performance and reliability performance requirements. As the solder thickness in the resulting bond layer  19  increases, thermal performance of the interface decreases as the thermal resistance increases but reliability performance such as temperature cycling performance is improved. Thus, there is a tradeoff between the thermal performance and reliability performance. In one embodiment of the present disclosure, the bond layer  19  of the receiver module  12  to heat sink  13  with a layer of multilayer foil  18  and 50 μm thick solder at the bond layer interface showed good bonding quality and thermal performance, however, the bond cracked after 100 cycles of temperature range −40 C to 125 C. With thicker solder layers  19  at the bonding interface, to accommodate the thermal stress caused by CTE mismatch between two components during temperature cycling, the bonds could survive up to 500 cycles without obvious degradation at the interfaces. Tests show the solder thickness of 200 μm to 500 μm provides good thermal performance with positive temperature cycling results for applications involving bonding a CPVB receiver module  12  to a heat sink  13 . 
         [0025]    In another embodiment, a freestanding solder preform such as tin solder may be applied to the faying surfaces  14  and  15  of one or both components  12 ,  13 . 
         [0026]    In another embodiment, the two surfaces of the reactive multilayer foil  18  which are facing the components  12 ,  13  are electroplated or coated by other means known in the art with a layer of tin or other fusible alloy, replacing the need to apply layers of solder onto the faying surfaces  14  and  15 . The maximum tin layer thickness is limited by the heat produced by the reactive multilayer foil and the thermal characteristics of the bond and components. The layer must be thin enough so that all the tin melts during the joining reaction. For a Ni—Al reactive multilayer foil 60 μm thick, the tin on each surface may be up to about 25 μm thick if the components are thermally conductive metals. 
         [0027]    The following examples are illustrative of the use of the methods of the present disclosure, but are not intended to limit the present disclosure in any way. Those of ordinary skill in the art will recognize the wider application so of the methods of the present disclosure beyond the specific examples set forth herein. 
       Example 1 
       [0028]    A heat sink  13  is placed on a hot plate, and a layer of tin solder  17  is applied on the faying (joining) surface  15 . The heat sink  13  is then cooled and the tin solder  17  is machined flat to a thickness of 200 μm. The faying surface of receiver module  12  is electroplated with tin to a thickness of 100 μm. A single piece  18  of Ni—Al reactive multilayer foil 60 μm thick is cut to the shape of the bond area ( 14 ,  15 ) and placed between the faying surfaces  14 ,  15  of the receiver module  12  and heat sink  13 . A compliant layer and an aluminum spacer 1.25″ (3.2 cm) thick are placed above the receiver module. A pressure of 200 psi (1.4 MPa) is applied to urge the faying surfaces  14 ,  15  together. The reactive multilayer foil piece  18  is ignited at an edge and reacts across the bond area to melt a fraction of the solder layers  16  and  17 . When the solder  16  and  17  solidify, the receiver module  12  and heat sink  13  are bonded together. The reactive multilayer foil piece  18  may consist of more than one piece of foil, laterally adjacently arranged to cover the surface of the entire bond area. 
       Example 2 
       [0029]    In a second example, both the faying surfaces  14  and  15  of the receiver substrate  12  and heat sink  13  are grit-blasted to a surface finish of between 120 and 800 μin (3-20 μm). The faying surfaces  14 ,  15  are then coated with a layer of tin 500 μm thick using wire arc spray. The tin layer is subsequently machined to a thickness of 150 μm on each component. A single piece  18  of Ni—Al reactive multilayer reactive foil 60 μm thick is cut to the shape of the bond area and placed between the faying surfaces  14 ,  15  of the receiver module  12  and heat sink  13 . A pressure of 200 psi (1.4 MPa) is applied to urge the faying surfaces  14 ,  15  together. The reactive multilayer foil piece  18  is ignited at an edge and reacts across the bond area to melt a fraction of the solder. When the solder solidifies, the receiver module and heat sink are bonded together. 
         [0030]    Example 3 
         [0031]    In a third example, the faying surface  15  of heat sink  13  is grit-blasted to a surface finish of between 120 and 800 μin (3-20 μm). The faying surface  15  is then coated with a layer of tin 500 μm thick using wire arc spray. The tin layer is then machined to a thickness of 250 μm. The faying surface  14  of the receiver module  12  is electroplated with tin to a thickness of 25 μm. A single tin solder perform  16 , 25 μm thick, and a single Ni—Al reactive multilayer foil  18  which is 80 μm thick are cut to the shape of the bond area and placed between the faying surfaces  14  and  15  of the receiver module  12  and heat sink  13  with tin solder perform  16  adjacent the faying surface  14  of the receiver module  12 . A pressure of 600 psi (4.1 MPa) is applied to urge the faying surfaces  14  and  15  together. The reactive multilayer foil piece  18  is ignited at an edge and reacts across the bond area to melt a fraction of the solder. When the solder solidifies, the receiver module  12  and heat sink  13  are bonded together. 
         [0032]    It can now be seen that in one aspect the present disclosure sets forth an improved method for bonding a concentrating photovoltaic receiver module  12  to a heat sink  13  utilizing a reactive multilayer foil  18 . The resulting bond layer  19  is highly thermal conductive and durable. The assembly process is simplified and allows multiple receiver modules  12  to be assembled at one time. With the thermally conductive interface between receiver module  12  and heat sink  13 , the heat transfer between solar cell  12  and heat sink  13  is more efficient, which allows the manufacturer to reduce the size of receiver module  12  without increasing the solar cell junction temperature and reducing the corresponding electrical conversion efficiency. 
         [0033]    In an alternate embodiment, the present novel bonding method using a reactive multilayer foil  18  can be used to bond a solar cell die  11  to receiver module  12 , or another electronic package to a substrate. In this case the solar cell die  11  is metalized on its backside and a solder perform is used. The receiver module  12  can be metalized or pre-tinned with a layer of solder, prior to bonding. 
         [0034]    As various changes could be made in the above constructions without departing from the scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.