Abstract:
The present inventors have observed that in some applications of reactive composite joining there is escape of a portion of the molten joining material through the edges of the joining regions. Such escape is not only a waste of expensive material (e.g. gold or indium) but also a reduction from the optimal thickness of the joining regions. In some applications, such escape also presents risk of short circuits or even fire. In this invention, two approaches are taken toward preventing damage to surroundings by the escape of molten joining material. First, escape may be prevented by trapping or containing the molten material near the joint, using barriers, dams, or similar means. Second, escape may be reduced by adjusting parameters within the joint, such as solder composition, joining pressure, or RCM thickness.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/795,534 of same title filed Apr. 27, 2006 herein incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002]     The United States government has certain rights in this invention pursuant to NSF Award DMI-0349727 and NSF Award DMI-0321500. 
     
    
     FIELD OF THE INVENTION  
       [0003]     This invention relates to the joining of components with joining material such as solder or braze by reactive composite materials such as reactive multilayer foils. In particular, it relates to methods for such joining adapted for minimal escape of joining material from the joint.  
       BACKGROUND OF THE INVENTION  
       [0004]     Joining of components with joining materials melted by reactive composite materials is advantageous for many important applications. Many conventional methods of joining two components use a heat source external to the joint to melt or cure joining material disposed between the component surfaces to be joined. Such external heat sources are typically ovens or torches. They are relatively expensive and burdensome to transport. Moreover, external heating typically heats the components far from the joining region with the potential of damaging temperature sensitive components, e.g., integrated circuits, and causing stresses due to differential thermal contraction on cooling.  
         [0005]     Reactive composite materials (RCMs) such as reactive multilayer foils can provide a source of portable, highly localized heat that melts or cures the joining material with minimal heating of component regions outside the joint. An RCM is typically composed of multiple alternating thin layers of materials that, upon ignition, will react with one another in an exothermic and self-propagating reaction. One advantageous RCM is comprised of many alternating nanoscale (&lt;1 micrometer) thickness layers of nickel and aluminum. Further details concerning the structure and fabrication of RCMs may be found in U.S. Pat. No. 6,863,992 issued to T. Weihs et al. on Mar. 8, 2005, which is incorporated herein by reference.  
         [0006]      FIG. 1 , which is prior art, illustrates a typical method of forming a joint using an RCM. In the exemplary joint, one component  11  is covered with a layer of solder  12  pre-applied as by conventional reflow and then machined to the desired thickness, typically 100-500 μm. A sheet of RCM  13  is placed against this pre-wet solder layer, and a piece of sheet solder  14 , typically 25-50 μm thick, is placed against the RCM. The joining surface of a second component  15 , which is typically gold metallized, is pressed against the sheet solder; the assembly is placed under pressure  16 ; and the RCM is ignited, as symbolized by match  17 . The RCM reacts, giving off heat, melting the solder sheet completely, partially melting the pre-wet solder, and ejecting some solder from the joint. When the solder remaining in the joint cools and re-solidifies, the two components are joined together. The present invention is directed to improvements in this process of joining components.  
       SUMMARY OF THE INVENTION  
       [0007]     The present inventors have observed that in some applications of reactive composite joining there is escape of a portion of the molten joining material through the edges of the joining regions. Such escape is not only a waste of expensive material (e.g. gold or indium) but also a reduction from the optimal thickness of the joining regions. In some applications, such escape also presents risk of short circuits or even fire. In this invention, two approaches are taken toward preventing damage to surroundings by the escape of molten joining material. First, escape may be prevented by trapping or containing the molten material near the joint, using barriers, dams, or similar means. Second, escape may be reduced by adjusting parameters within the joint, such as solder composition, joining pressure, or RCM thickness. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0008]     The advantages, nature, and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings:  
         [0009]      FIG. 1  illustrates reactive composite joining of components with joining materials melted by reactive composite materials;  
         [0010]      FIG. 2  illustrates a window frame package assembly for containing molten joining material;  
         [0011]      FIG. 3  illustrates a window frame of double-stick tape to contain molten joining material;  
         [0012]      FIG. 4  illustrates a cut-away view of a joint surrounded by a space-filling barrier layer;  
         [0013]      FIG. 5  illustrates a cut-away view of a joint surrounded by a thin barrier layer;  
         [0014]      FIG. 6  illustrates a thick frame around an RCM sheet and a sheet of joining material;  
         [0015]      FIG. 7  illustrates a conductive strip folded around the edges of an RCM sheet and a sheet of joining material;  
         [0016]      FIG. 8  illustrates a joint wherein one component has an integral ridge to trap molten joining material;  
         [0017]      FIG. 9  illustrates a joint wherein the RCM is smaller in area than the layers of joining material surrounding it;  
         [0018]      FIG. 10  illustrates a layer of joining material with a non-melting mesh embedded in it; and  
         [0019]      FIG. 11  illustrates a joint wherein the joining surfaces of the components are concave. 
     
    
       [0020]     It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]     This discussion is divided into two parts: Part I describes reactive composite joining including steps for containing molten joining material and Part II is directed to joining with steps for reducing the amount of molten joining material. Depending on the specific application, an improved joining process can include one or more of these approaches.  
         [0000]     I. Joining With Containment  
         [0022]     Referring to the drawings,  FIGS. 2 through 6  illustrate various ways of containing molten joining material in the reactive composite joining process.  
         [0023]     In a first embodiment ( FIG. 2 ), an RCM sheet  21  and a solder sheet  22  are adhered together, as by cladding or pressure. A window frame  23  of high-temperature resistant material with adhesive, such as Kapton® or aluminum tape, is placed against the solder sheet, extending beyond the sheet&#39;s edges, and adhered to component  24  and the pre-wet layer  25 . The second component  26  fits within the window frame  23 , thus pressing against the solder sheet  22 . Ignition may be through the window frame  23  or via an electric lead or RCM lead  27  extending under the frame. The window frame  23  catches solder escaping from the pre-wet layer  25  and most of the solder escaping from solder sheet  22 .  
         [0024]     In variations on the first embodiment, the RCM piece  21  may be smaller than the second component  26 , or the RCM  21  may extend under the window frame  23  for easier ignition. The RCM  21  may extend past the solder sheet  22  or be smaller than the solder sheet  22 . The window frame  23  can be Kapton® or another polymer, or it can be metal, such as aluminum tape or copper tape. The frame  23  may provide protection from electrostatic discharge (ESD) before the packet is attached to component  24 . The area of the pre-wet solder  25  on component  24  may be much larger than the area of the RCM  21 .  
         [0025]     For example, a gold-metallized piece of silicon wafer  26  ( FIG. 2 ) with area approximately 1″×1″ was joined to a copper heat sink  24  with area approximately 3″×3″. The copper was pre-wet with indium solder  25  that was then machined back to a thickness of 150 μm. A sheet of indium ( 22 ) 25 μm thick and approximately 1.1″×1.1″ was pressed against a piece of RCM  21  approximately 1.1″×1.1″ in area, 60 μm thick with 40 nm bilayers (one Al layer+one Ni layer thickness) and 1 μm Incusil® on both surfaces. A window frame  23  of Kapton® tape 0.002″ (50 μm) thick was pressed against the indium solder sheet  22 . The opening in the Kapton® tape  23  was just slightly larger than the silicon piece  26 . A strip of RCM  27  was affixed to the window frame with one end against the RCM sheet  21 . First, the copper heat sink  24  was placed on a flat surface. Next, the packet comprising window frame  23 , solder sheet  22 , RCM  21 , and igniter  27  was affixed to the copper heat sink  24 . The silicon piece  26  was placed against the solder sheet  22 , inside the window frame  23 . A spacer block was placed on the silicon piece and a round-end spring plunger was pressed against the spacer block to provide 50 PSI of pressure during joining. A small electric current was used to ignite the RCM igniter tab  27 , which in turn ignited the joining foil  21 . Some fraction of the solder melted, and upon resolidification, a joint was formed. Almost no spray was observed outside the window frame  23 .  
         [0026]     In a second embodiment ( FIG. 3 ), the packet of the first embodiment is used in conjunction with a second window frame  32  of high-temperature tape, preferably Kapton®, with adhesive on both sides. In this case, component  31  can be a silicon chip or die attached to a circuit board  33 . The window frame  32  is placed around the die  31 , covering some area of the circuit board  33  nearest the die  31 . The exposed adhesive on the window frame  32  traps any escaping molten solder while the tape  32  protects capacitors or other components located near the die  31 . Advantageously, the window frame  32  may be left in place after joining.  
         [0027]     In a variation on the second embodiment, solder wicking braid (usually tinned copper, used to remove solder during rework of a joint) may be affixed to the free surface of the double-sided tape. This braid will act as a dam and absorb escaping solder, containing it for easy removal. Alternately, the solder wicking braid may be placed around the joint without the aid of the tape to hold it in place, similar to the space-filling material described below.  
         [0028]     In a third embodiment, shown in  FIG. 4 , a space-filling material  41  may be placed around component (die)  42  between a circuit board  43  and the other component (heat sink)  44 . This material  41  serves to trap molten solder and to prevent its contact with the circuit board. The space-filling material may be foam, caulk, rubber, mesh, or any other compliant material that is compatible with the electronics on the board. Advantageously, the space-filling material may be left in place after joining. If the space-filling material  41  can be removed after joining, it may be reusable. If desired, the space-filling material  41  may be positioned some distance away from the joint, to avoid containing the escaped solder and any vapors right next to the joint. For example, a joint as shown in  FIG. 2  was assembled, but an additional window frame of very compliant open-cell polyurethane foam was placed around component  42  (or  26 ). A spacer block larger than the area of the foam window frame  41  was placed above the silicon piece  42  (in place of the circuit board  43 ) to apply pressure to the joint and to the foam. The RCM was ignited and the joint formed. After joining, no solder was observed to have escaped the foam  41 .  
         [0029]     In a variation on this third embodiment ( FIG. 5 ), the space-filling material may be a simple barrier  51 , such as a polymer or metal tape, wrapped around the two components. It may be a reusable shield that is put in place for the joining event, then removed and reused. If the barrier is porous, such as a mesh, air and gases may escape without entraining molten solder, preventing solder escape and preventing pressure buildup inside the barrier.  
         [0030]     In a fourth embodiment illustrated in  FIG. 6 , the RCM  63  and solder sheet  62  are attached to a thick frame  61  that may provide support to the foil and solder sheet during transport and handling. It can also provide ESD protection, and it can block solder escape. The frame  61  can be coplanar with the RCM  63  and solder sheet  62  on one face, or the RCM  63  and solder sheet  62  can be suspended within the frame  61 , with the frame  61  extending above and below the surface of the RCM  63  and the solder sheet  62 , as shown in  FIG. 6 . There may be a layer of solder sheet  62  on each side of the RCM  63 , and the solder sheet or sheets can be clad to the RCM by cold or warm pressing or by other methods known in the art. The frame  61  advantageously resides outside the joining area. The embodiment may be implemented similarly to  FIG. 4 .  
         [0031]     In a variation of this embodiment shown in  FIG. 7 , a thin conductive strip of metal foil  73  folded over the edges of the RCM  71  and solder sheet or sheets  72  may replace the thick frame of  FIG. 6 . The conductive strip permits electrical ignition through it. This embodiment may be combined with others: e.g. a conductive strip wrapped over a portion of an RCM-solder sheet package and an insulating tape frame covering the rest of the joint surroundings.  
         [0032]     In another embodiment ( FIG. 8 ), one of the parts may itself have a structure (e.g., a ridge  85 ) around the joining area to trap solder that might otherwise escape.  
         [0000]     II. Joining With Reduction of Molten Material  
         [0033]     Various changes in the solder and RCM configuration may reduce solder escape. In one embodiment, reducing the area of the RCM compared to the bond region reduces solder escape.  FIG. 9  illustrates this geometry. Component  91  is pre-wet with solder layer  92 . Component  95  and solder sheet  94  are arranged as shown. The area of RCM  93  is smaller than the areas of pre-wet layer  92  and solder sheet  94 . The joining surfaces of components  91  and  95  may be larger in area than solder layers  92  and  94 . RCM  93  may be ignited with RCM tab  96 , one end of which touches or overlaps RCM  93  while the other end extends past components  91  and  95  to permit ignition by a heat source. For example, a block of copper  91  (in  FIG. 9 ) was pre-wet with indium solder  92  before bonding to silicon  95  with an aluminum-nickel RCM ( 93 ) 60 μm thick and a 25 μm thick sheet of indium solder  94 . The RCM  93  was 10 mm×10 mm, which was smaller than the joint dimensions of 15 mm×15 mm. The RCM  93  was ignited with tab  96  to form a bond. Seven percent of the volume of the solder and RCM originally present in the bonding region was expelled during bonding. Compared to a joint wherein the RCM dimensions were 15 mm×15 mm, eighty-five percent (85%) less solder was lost.  
         [0034]     In another embodiment, shown in  FIG. 10 , a wire mesh  101  is incorporated either between the RCM and solder sheet or within the solder sheet  102 , to provide small barriers to solder loss and prevent complete compression of the joint. A convenient way to implement this is to include the mesh  101  in the pre-wet solder layer  102  on one component  103 . For example, a block of copper  103  was pre-wet with indium solder  102 . A thin Monel® mesh  101  was placed in the indium layer while it was still molten during the pre-wet process. The copper block was then bonded to silicon with an aluminum-nickel RCM 60 μm thick and a 25 μm thick sheet of indium solder. Five percent of the volume of the solder and RCM originally present in the bonding region was expelled during bonding. Compared to a joint without the Monel® mesh, eighty-one percent (81%) less solder was lost. In a variation on this embodiment, a wire spiral or short lengths of wire would also prevent complete compression of the joint but would still allow the solder to flow within the joint.  
         [0035]     In another embodiment, a high-viscosity solder is used in the bond, reducing escape due to the solder&#39;s resistance to pressure. Off-eutectic solders exhibit a so-called “mushy zone” upon heating: they do not melt completely at one fixed temperature. If the temperature of the solder can be raised into the mushy zone but not beyond, the solder will be viscous and resist spray. Similarly, a two-component solder in which the two components are not thoroughly mixed but are layered in the solder sheet can impede melting and increase viscosity.  
         [0036]     In another embodiment, a solder with a high melting point is pre-wet to the first component and a low-melting point solder sheet is placed against the second component. During joining the pre-wet layer will melt only partially, reducing escape, while the solder sheet will still melt completely to permit wetting of the second component.  
         [0037]     In another embodiment, the geometry of the joint is chosen to reduce solder escape. If one or both joining surfaces were concave, as shown in  FIG. 11 , solder would tend to flow toward the center rather than the edges of the joint.  
         [0038]     In another embodiment, the volume or thickness of the RCM is reduced to provide the minimum heat required to bond the surfaces. Excessive heat can cause excessive solder flow and escape.  
         [0039]     It is to be understood that the above-described embodiments are illustrative of only a few of the many embodiments that can represent applications of the invention. Numerous and varied other arrangements can be devised by those skilled in the art without departing from the spirit and scope of the invention.