Abstract:
The present disclosure relates to a bonding system ( 100 ) comprising an adhesive ( 200 ), comprising thermoplastic material, in contact with a first contact surface ( 115 ) and a second contact surface ( 125 ), and a plurality of solder particles ( 300 ) positioned in the adhesive ( 200 ) in contact with the first contact surface ( 115 ). Also, the present disclosure relates to a bonding method to produce a solder-reinforced adhesive bond joining a first substrate ( 110 ) and a second substrate ( 120 ), comprising applying, on a first contact surface ( 115 ) of the first substrate ( 110 ), an adhesive ( 200 ) comprising thermoplastic material, positioning, at least partially into the adhesive ( 200 ), each of a plurality of particles ( 300 ), such that each of the plurality of solder balls ( 300 ) contacts the first contact surface ( 115 ), connecting, to a portion of the adhesive ( 200 ) opposite the first contact surface ( 115 ), a second contact surface ( 125 ) of the second substrate ( 120 ), and applying heat to the first contact surface ( 115 ) such that each of the plurality of solder particles ( 300 ) reaches a solder bonding temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to International application No. PCT/CN2014/074945, filed Apr. 9, 2014, and titled “Systems and Methods for Reinforced Adhesive Bonding,” which application in its entirety is incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present technology relates to adhesive bonding for substrate materials. 
         [0003]    More specifically, the technology provides reinforced adhesive bonding in various ways through the use of solder material. 
       BACKGROUND 
       [0004]    Structural adhesives replace welds and mechanical fasteners in many applications because structural adhesives reduce fatigue and failure commonly found around welds and fasteners. Structural adhesives can also be preferable to welds and mechanical fasteners where resistance to flex and vibration is desired. 
         [0005]    Adhesive bonding uses structural adhesives to connect a substrate surface of one material to another substrate surface of the same material or a different material. Adhesive bonding is widely used in applications in which joining of dissimilar materials are required or in applications requiring the absence of electric voltage and current. Additionally, adhesive bonding may help improve corrosion resistance through eliminating substrate material contact with fasteners and other corrosive elements. 
         [0006]    When structural adhesives are applied to substrate surfaces, a bond line forms at the meeting of the substrate surfaces. Uniformity within the bond line is an important factor for optimal adhesive performance, thus dictating that bond line thickness is critical in designing a bond joint. 
         [0007]    When substantial force exists, structural adhesives used in adhesive bonding may be loaded (1) normal to the bond line, which creates a peeling effect causing substrate materials to be on different planes (i.e., peel fracture), or (2) perpendicular to the leading edge of a fracture, whether in-plane or out-of-plane, which creates a shearing effect where substrate materials remain on the same plane (i.e., shear fracture). While fracturing is typically avoided, if there is to be fracturing, shear fracture is preferred over peel fracture because shear fracture requires an external loading that is greater than that of peel fracture to produce failure. 
         [0008]    Glass beads may be added to some structural adhesives to ensure bond line uniformity for adequate bond line thickness control. However, the use of glass beads may cause strength issues within the structural adhesive because glass beads do not bond to substrate materials. 
       SUMMARY 
       [0009]    A need exists for a structural adhesive that creates bond line uniformity and promotes fracture propagation along a fracture path requiring the greatest amount of fracture energy. The present disclosure relates to systems and methods for establishing a structural adhesive that creates bond line uniformity and improves adhesive joint strength by facilitating a fracture path with the greatest amount of fracture energy. 
         [0010]    In one aspect, the present technology includes a bonding system, comprising an adhesive ( 200 ) comprising thermoplastic material, in contact with a first contact surface ( 115 ) of a first substrate ( 110 ), and a second contact surface ( 125 ) of a second substrate; and a plurality of solder particles ( 300 ) positioned at least partially in the adhesive ( 200 ) and at least one solder particle ( 300 ) in contact with the first contact surface ( 115 ). 
         [0011]    In some embodiments, the plurality of solder particles are positioned in a distribution (i) arresting crack propagation or (ii) promoting crack propagation along a path requiring, in at least one section of the system, the greatest amount of fracture energy. 
         [0012]    In some embodiments, the adhesive has a composition that promotes de-bonding of the first substrate and the second substrate during disassembly. The embodiments additionally promote re-bonding during multiple assembly-disassembly cycles. 
         [0013]    In some embodiments, one or more of the plurality of solder particles are further positioned in contact with the second contact surface. 
         [0014]    In a further aspect, the present technology includes a method, to produce a solder-reinforced adhesive bond joining a first substrate and a second substrate, comprising (i) applying, on a first contact surface of the first substrate, an adhesive, consisting at least partially of thermoplastic material, (ii) positioning, at least partially into the adhesive, each of a plurality of solder particles, such that each of the plurality of solder particles contacts the first contact surface, (iii) connecting, to a portion of the adhesive opposite the first contact surface, a second contact surface of the second substrate, and (iv) applying heat to the first contact surface such that each of the plurality of solder particles reaches a solder bonding temperature. 
         [0015]    In some embodiments, the plurality of solder particles are positioned in a distribution (i) arresting crack propagation or (ii) promoting crack propagation along a path requiring, in at least one section of the bond, the greatest amount of fracture energy. 
         [0016]    In some embodiments, the adhesive has a composition that promotes de-bonding of the first substrate and the second substrate during disassembly. 
         [0017]    In some embodiments, positioning the plurality of solder particles further comprises positioning the solder particles so that at least one of the solder particles contacts the second contact surface. 
         [0018]    Some embodiments, further comprise applying heat to the second contact surface such that the at least one solder particles reaches the solder bonding temperature. 
         [0019]    In a further aspect, the present technology includes a method, to produce a solder-reinforced adhesive bond joining a first substrate and a second substrate, comprising (i) applying, on a first contact surface, a composite including an adhesive, consisting at least partially of thermoplastic material, and a plurality of solder particles, such that at least one of the plurality of solder particles is in contact with the first contact surface, (ii) connecting, to a portion of the composite opposite the first contact surface, a second contact surface of the second substrate, and (iii) applying heat to the first contact surface such that each of the plurality of solder particles reaches a solder bonding temperature. 
         [0020]    In some embodiments, the plurality of solder particles are positioned in a distribution (i) arresting crack propagation or (ii) promoting crack propagation along a path requiring, in at least one section of the bond, the greatest amount of fracture energy. 
         [0021]    In some embodiments, the adhesive has a composition that promotes de-bonding of the first substrate and the second substrate during disassembly. 
         [0022]    In some embodiments, one or more of the plurality of solder particles are further positioned in contact with the second contact surface. 
         [0023]    Some embodiments, further comprise applying heat to the second contact surface such that the at least one solder particles reaches the solder bonding temperature. 
         [0024]    Other aspects of the present technology will be in part apparent and in part pointed out hereinafter. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  illustrates a side view of an exemplary embodiment of a bonding system. 
           [0026]      FIG. 2  illustrates a side view of an alternate embodiment of the bonding system of  FIG. 1 . 
           [0027]      FIG. 3  illustrates a side view of an alternative embodiment of the bonding system of  FIG. 1 . 
           [0028]      FIG. 4  is a graph illustrating load and displacement of adhesives with (i) no solder balls, (ii) solder balls in contact with one substrate surface of  FIG. 2 , and (iii) solder balls in contact with both substrate surfaces of  FIG. 1 . 
           [0029]      FIG. 5  illustrates an exploded perspective view of an exemplary embodiment of the bonding system containing solder balls with a gathered distribution and a reduced adhesive volume. 
           [0030]      FIG. 6  is a graph illustrating energy absorption of adhesives containing (i) no solder balls, (ii) the solder ball configuration of  FIGS. 1 and 2 , and (iii) the solder ball configuration with a reduced adhesive bond line thickness of  FIG. 4 . 
           [0031]      FIG. 7  illustrates an exploded perspective view of the exemplary embodiment of the bonding system containing solder balls with a random distribution and a solder ball coating. 
           [0032]      FIG. 8  is a graph illustrating load and displacement of (i) the embodiment of  FIG. 1  containing solder balls without coating and (ii) the embodiment of  FIG. 6  containing solder balls with coating. 
           [0033]      FIG. 9  illustrates top view of an embodiment of the bonding system containing solder balls with a linear distribution. 
           [0034]      FIG. 10  illustrates an alternate embodiment of the bonding system of containing solder balls with a meandering distribution. 
           [0035]      FIG. 11  illustrates load and displacement of adhesives with (i) no solder balls, (ii) solder balls containing a random distribution of  FIG. 6 , (iii) solder balls containing a linear distribution of  FIG. 8 , and (iv) solder balls containing a meandering distribution of  FIG. 9 . 
           [0036]      FIG. 12  is a graph illustrating strength of adhesives containing (i) thermoplastic material and (ii) thermoplastic material including the solder material. 
           [0037]      FIG. 13  illustrates an exploded perspective view of the exemplary embodiment of the bonding system containing a solder mesh. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    As required, detailed embodiments of the present disclosure are disclosed herein. The disclosed embodiments are merely examples that may be embodied in various and alternative forms, and combinations thereof. As used herein, for example, exemplary, illustrative, and similar terms, refer expansively to embodiments that serve as an illustration, specimen, model or pattern. 
         [0039]    Descriptions are to be considered broadly, within the spirit of the description. For example, references to connections between any two parts herein are intended to encompass the two parts being connected directly or indirectly to each other. As another example, a single component described herein, such as in connection with one or more functions, is to be interpreted to cover embodiments in which more than one component is used instead to perform the function(s). And vice versa—i.e., descriptions of multiple components described herein in connection with one or more functions are to be interpreted to cover embodiments in which a single component performs the function(s). 
         [0040]    In some instances, well-known components, systems, materials, or methods have not been described in detail in order to avoid obscuring the present disclosure. Specific structural and functional details disclosed herein are therefore not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present disclosure. 
         [0041]    While the present technology is described primarily in connection with manufacturing components of a vehicle in the form of an automobile, it is contemplated that the technology can be implemented in connection with manufacturing components of other vehicles, such as marine craft and air craft, and non-vehicle apparatus. 
       I. BONDING SYSTEM 
       [0042]    Now turning to the figures, and specifically to the first figure,  FIG. 1  illustrates the cross sectional view of a bonding system identified by reference numeral  100 . The bonding system  100  includes a structural adhesive  200  and solder material  300  which are used to join a first substrate  110  to a second substrate  120 . 
         [0043]    The substrates  110 ,  120  are the materials that require bonding to one another. The substrates  110 ,  120  may be composed of the same or differing material compositions. Typical substrate materials include aluminum, steel, magnesium, composite, or the like. 
         [0044]    The adhesive  200  is a structural material used to bond a contact surface  115  of the first substrate  110  to a contact surface  125  of the second substrate  120 . The adhesive  200  forms a bonded area (more often referred to a bond line  210 , seen in a cross sectional view) between the contact surfaces  115 ,  125 . In  FIGS. 1 and 2 , the bond line  210  extends laterally between the substrates  110 ,  120  and has a thickness  212 . 
         [0045]    The bond line  210  forms when heat and/or pressure are applied to the at least one of the substrates  110 ,  120 . Alternately, the bondline  210  can form by the system  100  being heated without bringing a heating element into direct contact with either substrate, e.g., placing the system  100  in an oven. 
         [0046]    As stated above, bond line uniformity is critical in designing a bond joint since uniformity within the bond line is important for optimal to the performance of an adhesive. Some literature contemplates that thin bond lines are preferred over thick bond lines, because the stress concentration at a joint corner is smaller in thin bond lines. Additionally, air cavity concentration is reduced in thin bond lines as compared to thick bond lines because the volume of the adhesive in thin bond lines leaves less room for air cavities to form. 
         [0047]    In the present disclosure, the thickness  212  approximately between about 0.05 to about 0.3 millimeters (mm). As an example, if the contact surfaces  105 ,  115  are relatively flat, the bond line  210  may have a thickness  212  of approximately 0.2 mm to allow for optimal shear and tensile strength. 
         [0048]    In some embodiments, it is desirable for bonded substrates  110   120  to be separable after assembly. To allow for reversibility in the bond, the composition of adhesive  200  may include materials whose strength decreases upon the application of a triggering energy, such as heat. 
         [0049]    Adhesives based on thermoplastic polymers (referred to as thermoplastic adhesive hereafter) are useful in scenarios where disassembly of bonded components, e.g., system  100 , is desired since thermoplastic adhesive softens when heated above a threshold temperature. Separable thermoplastic adhesive eliminates traditional mechanical fasteners (e.g., bolts, nuts) and simplifies design in hermetically sealed assemblies that require subsequent access to components after assembly. 
         [0050]    Thermoplastic adhesives soften when heated to allow for ease of disassembly. Heating the same thermoplastic adhesive after disassembly allows the thermoplastic adhesive to re-bond, thus allowing the same adhesive to be reused during multiple assembly-disassembly cycles. However, thermoplastic adhesives, due to the easy malleability, decrease in strength at moderately high temperatures. 
         [0051]    In some embodiments, the adhesive  200  includes the thermoplastic material, to allow disassembly and reassembly of the system  100 , as well as the solder material  300 , to increase the strength of the thermoplastic adhesive under ambient temperature and elevated temperature, for example. In thermoplastic material embodiments, the solders balls  300  may melt and solidify in a temperature range similar to where the thermoplastic material softens and solidifies. For example, the system  100 , when bonded with the separable thermoplastic adhesive, can be disassembled, reworked, and reassembled on a manufacturing line in scenarios where repair is needed. 
         [0052]    Further embodiments and arrangements of the adhesive  200  are described below, in association with  FIGS. 1-4  and  FIG. 12 . 
         [0053]    The solder material  300  are used in conjunction with the adhesive  200  to form a bridge between the substrates  110 ,  120 . Unlike prior art, which may incorporate glass beads within structural adhesives, the present technology promotes bonding of the substrates  110 , 120  using the adhesive  200  with the solder material  300 . 
         [0054]    The solder material  300  have the ability to bond to at least one of the substrates  110 ,  120  during manufacturing process (e.g., a curing process). Using solder material  300  enables a crack  220  to propagate along a fracture path  222 ,  224 , or  226 , described below, which requires more fracture energy for crack propagation in the adhesive  200  and increases energy-absorption capability of the system  100 . 
         [0055]    The solder material  300  may be covered by a layer of adhesive  200  such that the solder material  300  is not in direct contact with either or both of the contact surfaces  115 ,  125  upon assembly. However, during subsequent manufacturing processes (e.g., heating and pressing), the adhesive  200  becomes malleable and displaces to allow the solder material  300  to establish contact with one or both of the contact surfaces  115 ,  125 . 
         [0056]    Incorporating solder material  300  within the adhesive  200  also improves fracture resistance of a bond joining the substrates  110 ,  120 . As an example, a fracture threshold in an adhesive without solder material may occur approximately near 1.8 newtons per millimeter (N/mm), whereas the same fracture in adhesive containing solder material may occur at approximately near 11.5 N/mm. 
         [0057]    The embodiments and the examples provided herein illustrate and describe the solder material  300  as spherical in shape (e.g., solder balls), which promotes uniform distribution of the solder material  300  from adjacent solder material  300  throughout the adhesive  200 . However, the solder material  300  may include other shapes such as, but not limited, to cylinders, rectangles, and the like. Using shaped solder material  300  may be beneficial in applications, for example, (1) where desired contact of the solder material  300  is only to one of the contact surfaces  115 ,  125 , (2) the solder material  300  are specifically placed on the substrates  110 ,  120  (e.g., through a manufacturing process—e.g., hot/cold spray), or (3) the solder material  300  is strategically placed within the adhesive  200  (e.g., through a manufacturing process—e.g., hot/cold spray). 
         [0058]    In some embodiments, the solder material  300  can be combined with the adhesive  200  prior to bonding the second contact surface  125 . For example, the adhesive  200  and the solder material  300  may create an adhesive-solder mixture, which can be stored until the mixture is dispensed onto the first contact surface  115 . As another example, the adhesive  200  and the solder material  300  may create an adhesive-solder tape, where the adhesive  200  is in the form of a double-sided tape with the solder material  300  embedded within tape. To facilitate bonding, the solder-adhesive tape can be laid onto the first contact surface  115  before bonding the second contact surface  125 . 
         [0059]    The solder material  300  should be of a dimension that allows contact to at least one of the substrates  110 ,  120 . If contact to both of the substrates  110 ,  120  is desired, the solder material  300  can be configured to have a dimension slightly larger than the bond line  210 . 
         [0060]    For example, when the solder material  300 , if the bond line  210  has a thickness  212  of 0.2 mm, the solder material  300  may be a solder ball having a dimension of approximately near 0.2 mm or larger, to ensure compression of the solder material  300  during bonding, which will ensure adequate joining to contact surfaces  115 ,  125 . 
         [0061]    If contact is desired only on one of the substrates  110  or  120 , it may be desirable to have the solder material  300  of a dimension slightly smaller than the bond line  210 . As an example, if the bond line thickness  212  is approximately 0.2 mm, the solder material  300  may be a solder ball with a dimension of approximately 0.1 mm, to ensure that the solder material  300  are not large enough do not contact both surfaces  115 ,  125  during bonding. For example, the solder material  300  may be secured to the second contact surface  125  (seen in  FIG. 2 ) during a manufacturing process such that when the adhesive  200  is applied, the solder material  300  are only in contact with the contact surface  125 , and only the adhesive  200  is in contact with the first contact surface  115 . 
         [0062]    The solder material  300  may be composed of any commercially available material or a custom composition. When at least one of the substrates  110 , 120  is at least partially composed of metal and/or metal composites, composition materials of the solder material  300  may include materials such as tin (Sn), lead (Pb), silver (Au), copper (Cu), zinc (Zn), bismuth (Bi), and/or the like. If at least one of the substrates  110 , 120  is at least partially composed of polymer and/or polymer composites, the solder material  300  composition may also include polymer materials such as polycarbonate (PC), polyethylene (PE), polypropylene (PP), divinylbenzene (DVB), and/or the like. 
         [0063]    Desirable characteristics of the solder material  300  include, but are not limited to (1) a density conducive for bonding, (2) a temperature conducive for bonding, and (3) increased tensile strength over prior art. 
         [0064]    The density should be such that the solder material  300  maintain its structure when incorporated into the adhesive  200  prior to bonding. The solder material  300  density can be approximately between about 2.50 and about 15.00 g/cm 3 . For example, a solder ball containing tin-lead (Sn—Pb) or tin-silver-copper (Sb—Ag—Cu or SAC) may have a density approximately near 7.5 g/cm 3 , which may provide adequate density for bonding when at least one of the substrates  110 , 120  is at least partially composed of metal and/or metal composites. As another example, a solder ball containing ethenylbenzene or divinylbenzene (DVB) may have a density approximately near 0.9 g/cm 3 . 
         [0065]    The temperature should be such that the solder material  300  bond without affecting (e.g., deforming) composition materials of the substrate  110 ,  120 . The solder material  300  bonding temperatures can be approximately between about 0.7 and 1.0 of a melting temperature of the solder material  300 . In some embodiments it is desirable to include solder material that has a melting point of less than 200° C. to prevent de-bonding (e.g., fracture) of the solder material  300  from the contact surfaces  115 ,  125 . 
         [0066]    Tensile strength should increase strength of the system  100  under tension forces when compared to an adhesive without filler material or an adhesive containing non-bonding filler material. For example, when solder material  300  are used in conjunction with the adhesive  200 , the overall system  100  may have a tensile strength of approximately between about 50 MPa and 150 MPa, whereas an automotive adhesive alone may have a tensile strength of approximately between about 15 MPa and 35 MPa, and an automotive adhesive with glass beads may have a tensile strength of approximately between about 15 MPa and 35 MPa. 
         [0067]    In some embodiments, the solder material  300  may be in the form of a mesh  310  of solder material (as seen in  FIG. 13 ). Using the mesh  310  may be beneficial in applications, for example, where the adhesive  200  contains thermoplastic polymer materials. 
         [0068]    The mesh  310  may be composed partially of any combination of solder material such as, but not limited to, solder wire, solder balls, solder disks, and the like. The mesh  310  can be a continuous mesh or web of solder material, for example, a solder wire mesh include solder balls or alternate shape. In some embodiments, the mesh  310  can include non-solder materials such as polymers or thermoplastics to assist in the structure of the mesh or to facilitate in the bonding of the mesh  310  and adhesive with the substrates  110 ,  120 . 
         [0069]    The mesh  310  should be of a thickness  315  that facilitates bonding of the combination of the adhesive  200  and the mesh  310  to at least one of the substrates  110 ,  120 . The thickness  315  may increase to a point that the mesh  310  can facilitate bonding between both of the substrates  110 ,  120 . 
         [0070]    The solder material  300  may be configured and arranged according to any of various embodiments described herein, including below in association with  FIGS. 6-10  and  FIG. 13 . 
       II. STRUCTURAL ADHESIVE EMBODIMENTS—FIG.  1  THROUGH  6   
       [0071]    In some embodiments, the bond line thickness  212  is such that the solder material  300  may join to both of the contact surfaces  115 ,  125  (seen in  FIG. 1 ). Joining the solder material  300  to both contact surfaces  115 ,  125  has benefits including promoting a crack  220  that propagates in the adhesive  200  approximately near solder material  300  according to a fracture path that requires the greatest amount fracture energy (i.e, the amount of energy required to commence a crack—e.g., crack  220 ). The crack  220  may (i) propagate along a pre-identified fracture path  222  (depicted as a series of short solid arrows in  FIG. 1 ), (ii) propagate along a pre-identified fracture path  224  (depicted as a series of dashed arrows in  FIG. 1 ), (iii) propagate along a pre-identified fracture path  226  (depicted as a series of long solid arrows in  FIG. 1 ), or (iv) arrest at the interface of the adhesive  200  and the solder material  300 . 
         [0072]    The fracture paths  222 ,  224 ,  226  correlate generally to a path of greatest resistance for any fracture. Because the adhesive  200  is generally weaker than the substrates  110 ,  120  and the solder material  300 , the fracture paths may extend through the adhesive  200  as illustrated by the fracture paths  222 ,  224  or along one of the contact surfaces as illustrated by the fracture path  226 . 
         [0073]    When the crack  220  propagates around each solder material  300 , the fracture path  222  is formed along one of contact surfaces  115 ,  125 , as shown in  FIG. 1 . Although  FIG. 1  depicts the fracture path  222  extending around each solder material  300  toward the first contact surface  115 , alternatively, the fracture path  222  could extend around any one or more of pieces of solder material  300  toward the second contact surface  125 . Although  FIG. 1  depicts the fracture path as continuing around each subsequent solder material  300 , in actuality, when the fracture path  222  approaches each subsequent solder material  300 , the fracture path  222  may (i) travel around the solder material  300 , (ii) travel through the solder material  300 , (iii) travel along one of the contact surface  115 ,  125 , or (iv) arrest at the interface of the adhesive  200  and the solder material  300 . 
         [0074]    The fracture path  224  is formed when the crack  220  propagates through the solder material  300  and then propagates into the adhesive  200  prior to reaching a subsequent solder material  300 . Similar to the fracture path  222 , when the fracture path  224 , reaches each subsequent solder material  300 , the fracture path  224  may (i) travel around the solder material  300 , (ii) travel through the solder material  300 , or (iii) travel along one of the contact surface  115 ,  125 , or (iv) arrest at the interface of the adhesive  200  and the solder material  300 . 
         [0075]    The fracture path  226  is formed when the crack  220  propagates around the solder material  300  and along one of the contact surfaces  115 , 125 . Unlike the fracture paths  222 ,  224 , when the fracture path  226  is formed, the crack  220  continues to propagate along the contact surface  115 ,  125  where the crack  220  commenced. 
         [0076]    Alternately, the crack  220  may arrest at any interface of the adhesive  200  and the solder material  300  along the paths  222 ,  224 ,  226 . Arresting of the crack  220  may be highly desired within the system  100  because reduced or eliminated propagation of the crack  220  may prevent failure of the system  100  due to fracture. 
         [0077]    In some embodiments, the bond line thickness  212  is such that the solder material  300  join to only one of the contact surfaces  115 ,  125  (seen in  FIG. 2 ). A benefit of restricting solder material  300  contact to one contact surface  115  or  125  is the ability to join dissimilar substrate materials (e.g., metal material joining with a composite material—e.g., polymer composite) without compromising the integrity of either substrates  110 ,  120 . 
         [0078]    Additionally, joining the solder material  300  to one of the contact surfaces  115 ,  125  propagates a crack  230  within the adhesive  200  approximately near solder material  300 , along a fracture path that requires the most fracture energy (e.g, the amount of energy required to commence the crack  230 ). The crack  230  may (i) propagate along a pre-identified fracture path  232  (depicted as a series of solid arrows in  FIG. 2 ), (ii) propagate along a pre-identified fracture path  234  (depicted as a series of dashed arrows in  FIG. 2 ), or (iii) arrest at the interface of the adhesive  200  and the solder material  300 , described below. 
         [0079]    In some embodiments, it is desirable to reduce the volume of structural adhesive  200  used in the bonding process. Reducing the volume of the adhesive  200  can be beneficial by leading to a thinner bond line  210 . Additionally, reducing the volume of the adhesive  200  results in adhesive material savings. Other benefits of using less adhesive can include streamlining manufacturing processes and allowing the adhesive to be used on a greater amount surface area. 
         [0080]    In some embodiments, the amount of adhesive  200  used may be reduced by the presence of a substrate surface adaptation—e.g., a protrusion, projection, bump, or protuberance  130  (shown in  FIG. 2 ). The protuberance  130  may be positioned on at least one of the contact surfaces  115 ,  125  to reduce the amount of the adhesive  200  applied. The protuberance  130  illustrated in  FIG. 2  may be adhered to the substrates  110 ,  120  during a manufacturing process or, in the case of sheet metal, the protuberance  130  may be thermally pressed, or otherwise formed, into the protuberance  130  during a sheet forming process. 
         [0081]    The protuberance  130  promotes shear loading, generally in a direction included to the substrates  110 ,  120 , for the adhesive  200  in a transition zone  235 , described below, to arrest crack propagation in the adhesive  200  and increase energy-absorption capability of the system  100 . Fracture path propagation due to presence of protuberances  130  is also described below. 
         [0082]    Where the first substrate  110  has a different composition than the second substrate  120 , bonding the substrates  110 ,  120  according to the present technology may have an added benefit of enhanced strength at the bond line  210  compared to prior art. Specifically, e.g., the bond line  210  is stronger with the incorporation of solder material  300  because the energy required to initiate fracture path propagation around the solder material  300  is higher than the energy required for fracture path propagation in the adhesive alone or along an adhesive/metal interface. 
         [0083]    As mentioned above, the crack  230  may propagate along the fracture path  232 . The fracture path  232  may propagate around each solder material  300  as well as any protuberances  130  along one of the contact surfaces  115 ,  125 . Forcing the fracture path  232  to change directions along the contact surface  115  forms a transition zone  235 , being an area between the top surface of protuberances  130 /solder material  300  and the opposite contact surface (i.e., the first contact surface  115  in the example of  FIG. 2 ). This transition zone  235  forces fracture propagation in the form of shear fracture because the path of least resistance for any fracture ends up being around the solder material  300  and through the adhesive instead of through the solder material  300 . Although  FIG. 2  depicts the fracture path  232  as continuing around each subsequent solder material  300  or protuberance  130 , in actuality, when the fracture path  232  approaches each subsequent solder material  300 , the fracture path  232  may (i) travel around the solder material  300 , (ii) travel through the solder material  300 , or (iii) arrest at the interface of the adhesive  200  and the solder material  300 . 
         [0084]    The crack  230  may alternately propagate along the fracture path  234 , where propagation occurs through the solder material  300  but around the protuberances  130 . As the crack  230  propagates through a solder material  300 , the fracture path  234  the transition zone  235  is not created as with fracture path  232 . However, the transition zone  235  is created when the fracture path  234  encounters the protuberance  130 , and must change direction along the contact surface  115 . 
         [0085]    Alternately, the crack  230  may arrest at any interface of the adhesive  200  and the solder material  300  along the paths  232 ,  234 . Arresting of the crack  230  may be highly desired within the system  100  because reduced or eliminated propagation of the crack  230  may prevent failure of the system  100  due to fracture. 
         [0086]    In some embodiments, it may be desirable to reduce distortion deformation during bonding. Distortion deformation may occur, e.g., when substrates  110 ,  120  have different coefficients of thermal expansion. The difference in thermal expansion rate can cause distortion internal to each of the substrates  110 ,  120 , which can lead to de-bonding (e.g., fracture) of the bondline  210 . 
         [0087]    In some embodiments, the surface adaptation may include a groove  140  (shown in  FIG. 3 ). The groove  140  may be embossed into each of the substrates  110 ,  120  during a manufacturing process. Or, in the case of sheet metal, the groove  140  may be thermally pressed, or otherwise formed, into the substrates  110 ,  120  during a sheet forming process. 
         [0088]    Similar to the protuberance  130 , the groove  140  changes the loading condition of the assembly  100 , between the first substrate  110  and the second substrate  120 , from a peel fracture condition into a shear fracture condition, as a crack propagates along the bondline  210 . However, the combination of the groove  140  and the solder material  300  may be enough prevent a crack from forming and/or propagating through the adhesive  200 , since the solder material  300  are more ductile than the adhesive  200 . 
         [0089]    The groove  140  may be defined generally as by a shape on one or both of the substrates  110 ,  120 . The groove  140  may be square or round (as seen in  FIG. 3 ) or another geometric shape, and have an associated depth  145  therewith to reduce distortion within the substrates  110 ,  120 . 
         [0090]    When the groove  140  is rounded, the shape defines a concave groove generally, as depicted in  FIG. 3 . However, it should be appreciated that the groove  140  may also define a generally convex groove. The depth  145  associated with a rounded groove may be a value such that the substrates  110 ,  120  are not distorted during bonding. An acceptable depth  145  for a rounded groove is in some embodiments a fractional value of the substrate  110 ,  120  thickness up to a value multiple times the substrate  110 ,  120  thickness. For example, the groove  140  may be between approximately 0.05 mm and approximately 10 mm, measured from the base of the groove  140 . 
         [0091]    When the groove  140  is square, the shape generally defines a square groove with a rounded edge, as depicted in  FIG. 3 . However, it should be appreciated that the groove  140  may also define a square groove with other transition edges—e.g., square, linear, or the like. The depth  145  associated with a square groove may be a value such that the substrates  110 ,  120  are not distorted during bonding. An acceptable depth  145  for a square groove is in some embodiments a fractional value of the substrate  110 ,  120  thickness up to a value multiple times the substrate  110 ,  120  thickness. For example, the groove  140  may be between approximately 0.05 mm and approximately 10 mm, measured from the base of the groove  140 . 
         [0092]    It should be appreciated that one or both of the substrates  110 ,  120  may include several surface adaptations (e.g., protuberance  130  and groove  140 ) at intermittent intervals (e.g., distance  147  seen in  FIG. 3 ) along a longitudinal axis. An intermittent interval, such as distance  147 , should be such that one groove  140  is adequately spaced from a subsequent groove  140 . An acceptable distance  147  may be a value between approximately 10 mm and approximately 100 mm. 
         [0093]    Although the grooves  140  are designed to prevent deformation and facilitate secure bonding of the substrates  110 ,  120  to prevent fracture, when fracture does occur a crack may propagate along fracture paths described above. Specifically, when the solder material  300  are in contact with both the substrates  110 ,  120 , as seen in  FIG. 1 , the fracture paths would be similar to fracture paths  222 ,  224 , and/or  226 , described in association with  FIG. 1 . However, when the solder material  300  are in contact with only one of the substrates  110 ,  120 , as seen in  FIG. 2 , the fracture paths would be similar to fracture paths  232  and/or  234 , described in association with  FIG. 2 . 
         [0094]      FIG. 4  illustrates load, γ (N/mm) [y axis], versus displacement, δ (mm) [x axis], of (i) an adhesive with no solder balls (represented by a first data line  312 ), (ii) an adhesive containing solder balls in contact with one substrate surface (represented by a second data line  314 ), and (iii) an adhesive containing solder balls in contact with both substrate surfaces (represented by a third data line  316 ). As seen, generally, the first data line  312  has a surface tension that is below that of the second and third data lines  314  and  316 , thus making the adhesive prone to fracture when compared with the adhesives containing solder balls. The surface tension of the second and third data lines  314  and  316  vary depending on displacement of the adhesive, thus making the choice of single contact solder balls or double contact solder balls a preference derived from the application and use of the adhesive. 
         [0095]    In some embodiments, reduction in the amount of adhesive  200  used may also be occasioned by creating voids, such as cavities  240  (shown in  FIG. 5 ). Each cavity  240  may be a void, within adhesive  200 , of any number of shapes or sizes.  FIG. 5  also illustrates an embodiment of the system  100  containing solder material  300  arranged according to a gathered distribution. 
         [0096]    The gathered distribution of solder material  300  may be beneficial in applications where the adhesive  200  is reduced in surface area (and thus volume) due to the existence of a void within the adhesive  200 , such as the cavity  240  mentioned above. The volume of the adhesive  200  is decreased due to a reduction in a bond line width  214  in pre-identified areas within of the adhesive  200 . The distribution density of the solder material  300  increases where the width  214  is the narrowest (e.g., between the cavities  240 ). 
         [0097]    Distribution density may be accomplished by, for example, a dispensing device that controls distribution of the solder material  300 . Such a dispensing device may expand a distribution nozzle to generate higher solder material  300  distribution density in areas were the width  214  is narrow and retract the distribution nozzle to generate lower solder material  300  distribution density in remaining areas. The dispensing device may also include a self-control function to open or close the device nozzle. To expand and retract the distribution nozzle, the dispensing device may include items such as but not limited to, electromagnetic device(s), valves, and other mechanical components. 
         [0098]    Increasing distribution density, reinforces vulnerable of areas of fracture (e.g., near the cavities  240 ). By strategically distributing a greater number of the solder material  300  in areas of the reduced bond line width  214 , the gathered distribution reduces the volume of the adhesive  200  while promoting shear fracture along a path which requires the greatest amount of fracture energy. 
         [0099]      FIG. 6  illustrates levels of energy absorption for apparatus having (i) an adhesive with no solder balls (prior art; represented by a first data block  252 ), (ii) an adhesive containing solder balls (represented by a second data block  254 ), and (iii) an adhesive containing solder balls with a reduced adhesive bond line width  214  (represented by a third data block  256 ). 
         [0100]    Each of the data blocks  252 ,  254 ,  256  measure the energy absorption, in Joules (J), of each adhesive covering a surface area of 100*25 mm2. The y-axis is marked in increments of 5 J. 
         [0101]    As shown, the first data block  252  absorbs energy of approximately near 15 J per the surface area. When solder balls are added to an adhesive (second data block  254 ), the energy absorption is much higher, approximately near 24 J for the same surface area, an increase of nearly 60%. 
         [0102]    When solder balls are added and the bond line width  214  is reduced at least in some areas (e.g., around the cavities  240 ), the energy absorption is generally the same as the adhesive without solder balls, i.e., data block  252 . However, the volume of adhesive used in this latter case is reduced by about 40%. Benefits of using less material are described above. 
         [0103]      FIG. 12  is a graph illustrating a strength, measured in megapascals (MPa) versus temperature, measured in ° C., of an adhesive containing (i) thermoplastic material (represented by a first data line  392 ), and (ii) thermoplastic material including the solder material  300  (represented by a first data line  394 ). 
         [0104]    As seen in  FIG. 12 , the thermoplastic adhesive seen in data line  392  begins to increasingly lose strength near 80° C., and near 90° C., the thermoplastic adhesive loses a substantially majority of strength. The thermoplastic adhesive containing the solder material  300 , seen in data line  394 , also begins to lose strength near 80° C. However, the solder material  300  keep the strength of the adhesive bond from degrading until temperatures of approximately 120° C. The strength of the adhesive bond could degrade up to 300° C., depending on the melting temperature of the composition of the solder alloy. 
       III. SOLDER MATERIAL EMBODIMENTS—FIG.  7  THROUGH  11  AND FIG.  13   
       [0105]    In some embodiments, the outer surface of the solder material  300  contain a partial or full coating  320 , shown in  FIG. 7 , such as a flux. The coating  320  is selected and applied to improve the bonding and/or the controlled fracture characteristics of the system. The coating  320  in some cases does this by enhanced bonding of the interface between the solder material  300  and the contact surfaces  115 ,  125 , the enhanced bonding forcing the cracks  220 ,  230  to either alter the path of fracture or arrest propagation, as described above. 
         [0106]    Where the solder material  300  is in the form of the mesh  310 , and possesses a top surface, intended to be in contact with the first contact surface  115 , and a bottom surface, intended to be in contact with the second contact surface  125 , different coatings may be applied to the solder material  300  on one or both of the continuous surfaces of the mesh  310 . Coatings may be beneficial where material of the substrates  110 ,  120  are dissimilar or both substrates  110 ,  120  benefit from the interface created between the solder balls  300  and the coatings. 
         [0107]    The coating  320  may also be utilized arrest (i.e., stop) fracture propagation through the adhesive  20 . Alternately, the coating  320  may deflect fracture propagation to another feature contained within the adhesive  200  (e.g., solder material  300  or protuberance  130 ) to promote failure in shear mode through the adhesive  200  adjacent the solder material  300 . 
         [0108]    In some embodiments, the coating  320  improves the interface between the solder material  300  and the substrates  110 ,  120  through removing impurities at the site of the bond (e.g., dirt, oil or oxidation). The improved interface promotes fracture propagation around solder material  300  in addition to the promotion of the fracture paths already occasioned by the general design (e.g., fracture paths  222 ,  224 ,  226  in  FIG. 1  and fracture paths  232 ,  234  in  FIG. 2 ). 
         [0109]    The coating  320  may be a cleaning agent that promotes soldering, brazing, or welding by removing oxidation from the metals to be joined. Materials suitable for include but are not limited to ammonium chloride, rosin (natural or chemically modified), hydrochloric acid, zinc chloride, and borax. 
         [0110]      FIG. 8  illustrates load, γ (N/mm) [y axis], versus displacement, δ (mm) [x axis], of (i) an adhesive containing solder balls without flux (represented by a first data line  332 ), and (ii) an adhesive containing solder balls with flux (represented by a second data line  334 ). As seen, generally, the first data line  332  has a surface tension that is below that of the second data line  334 , showing that a bond may withstand greater force prior to fracture when a coating such as coating  320  is used prior to bonding. 
         [0111]    In some embodiments, the solder material  300 , whether coated, may be distributed in patterns and designs, which may function to strengthen the bonding of the substrates  110 ,  120  by reducing stress concentrations within the bonding system  100 . Stress concentrations may be formed where solder material  300  cluster in the same area of the adhesive  200 . Creating patterns with the solder material  300  may prevent clusters of solder material  300  from forming though intentional placement of each solder material  300 . 
         [0112]    Distribution of the solder material  300  may occur in conjunction with new or existing manufacturing or assembly processes, which spray adhesives, coatings, waxes, or the like. Spray processes such as hot/cold and the like may be used to distribute the solder material  300  into patterns on substrates  110 ,  120  or within the adhesive  200 . Additionally, the solder material  300  that contain patterns may also contain the coating  320  discussed above to facilitate removal of impurities. 
         [0113]      FIG. 9  illustrates a top view of an embodiment of the system  100  containing solder material  300  with a linear distribution. The balls  300  may be coated as described above in connection with  FIG. 7 , through such coating is not shown in detail in  FIG. 9 . 
         [0114]    In the linear distribution of  FIG. 9 , each of the solder material  300  is separated by a horizontal distance  340  (distance between two solder material  300  on the same column) and a vertical distance  350  along the bond line width  214  (distance between two solder material  300  on the same row). As provided, references to direction (e.g., horizontal, vertical) are provided to aid in the present descriptions and not necessarily to limit application of the present technology or orientation of constituent parts before, during, or after the bonding process. 
         [0115]    Positioning the solder material  300  with a linear distribution generates a fracture path  260  (depicted as a series of arrows in  FIG. 9 ) to propagate in a way that propagates a crack along a fracture path requiring the greatest amount of fracture energy. Similar to the fracture paths  222 ,  224 ,  226  (seen in  FIG. 1 ), the fracture path  260  may propagates around each solder material  300 , forcing the fracture path  260  along at least one of the contact surfaces  115 ,  125 . The fracture path  260  can alternatively propagate along any row of the solder material  300  to allow the shear fracture to occur. 
         [0116]      FIG. 10  illustrates an alternate embodiment of the system  100  containing solder material  300  with a meandering distribution. The meandering distribution is formed the solder material  300  forming two meandering patterns, oriented in opposite directions. 
         [0117]    As with the linear distribution, the solder material  300  within the meandering distribution are separated by a horizontal distance  370  and a vertical distance  360 . The horizontal distance  360  is the distance between each meandering wave revolution about a centerline (not shown) of the adhesive width  214 . The vertical distance  370  is the distance between the centerline of the adhesive width  214  and the outermost solder material  300  of the sine formation. 
         [0118]    Positioning the solder material  300  with a meandering distribution generates a fracture path  270  (depicted as a series of arrows in  FIG. 10 ) to propagate in a way that facilitates a shear fracture instead of a peel fracture. The fracture path  270  propagates around each solder material  300  within a single sine within the meandering distribution. The fracture path  270  can alternatively propagate along the second sine within the meandering distribution to allow the shear fracture to occur. Due to the pattern formed by the meandering distribution the fracture path  270  is longer than the fracture path when compared to the fracture paths  222 ,  224 ,  226  (shown in  FIG. 1 ) and fracture paths  232 ,  234  (shown in  FIG. 2 ) formed by the random distribution and the fracture path  260  (shown in  FIG. 9 ) formed by the linear distribution. 
         [0119]    To withstand the maximum joint stress without creating stress concentrations, there exists a correlation between the horizontal distance  340  and the vertical distance  350  within the linear distribution. A similar correlation is also true for the horizontal distance  360  and the vertical distance  370  within the meandering distribution. For example, in the linear distribution, the correlation may have a ratio approximately a 1:1, whereas in the meandering distribution, the correlation may have a ratio approximately near 1:4. 
         [0120]      FIG. 11  illustrates load, γ (N/mm) [y axis], versus displacement, δ (mm) [x axis], of (i) an adhesive with no solder balls (represented by data line  382 ), (ii) an adhesive containing a random distribution of solder balls (represented by data line  384 ), (iii) an adhesive containing a linear distribution of solder balls (represented by data line  386 ), and (iv) an adhesive containing a meandering distribution of solder balls (represented by data line  388 ). 
         [0121]    As seen, generally, the data line  382  has a surface tension that is below that of the data lines  384 ,  386 ,  388 . The surface tension of the data line  384  has a surface tension that gradual increases and decreases with displacement, whereas the data lines  386  and  388  have surface tension that gradually decrease with displacement, thus making the linear distribution and the meandering distribution suitable for some applications such as bonds where the substrates  110 ,  120  are different materials. 
       IV. SELECTED FEATURES 
       [0122]    Many features of the present technology are described herein above. The present section presents in summary some selected features of the present technology. It is to be understood that the present section highlights only a few of the many features of the technology and the following paragraphs are not meant to be limiting. 
         [0123]    The technology allows bond line uniformity to be accomplished within the structural adhesive. Bond line uniformity can achieve optimal tensile and shear strength as well as regulate the thickness of the bond line, which reduces the volume of adhesive required in applications. Reducing the volume of the adhesive can be beneficial to form a thinner bond line. Additionally, reducing the volume of the adhesive, can result in a material savings. 
         [0124]    The technology allows enhanced contact of the structural adhesive with the substrate material. Enhancing contact of the structural adhesive allows the substrate materials to bond more effectively the adhesive creating a more secure bond, which can withstand a greater force prior to fracture. The addition of solder to thermoplastics adhesive retains the reversibility allowing for repeated assembly and disassembly while enhances the joint strength under ambient temperature and under elevated temperature. 
         [0125]    The technology allows fracture to propagate along a path that requires the greatest amount of fracture energy. Unlike glass beads, which facilitate fracture perpendicular to the substrate materials, fractures that occur in a direction generally inclined toward substrate materials facilitate a shearing effect where substrate materials remain on the same plane. 
       V. CONCLUSION 
       [0126]    Various embodiments of the present disclosure are disclosed herein. The disclosed embodiments are merely examples that may be embodied in various and alternative forms, and combinations thereof. 
         [0127]    The above-described embodiments are merely exemplary illustrations of implementations set forth for a clear understanding of the principles of the disclosure. 
         [0128]    Variations, modifications, and combinations may be made to the above-described embodiments without departing from the scope of the claims. All such variations, modifications, and combinations are included herein by the scope of this disclosure and the following claims.