Abstract:
A system for underfilling in a chip package includes an underfill mixture that ameliorates the CTE mismatch that typically exists between a packaged die and a resin-impregnated fiberglass mounting substrate. In one embodiment, the system includes an underfill mixture that alone exhibits a CTE that is characteristic of an inorganic-filled underfill composite previously known. An embodiment is also directed to the assembly of a flip-chip package that uses an underfill mixture.

Description:
TECHNICAL FIELD 
     Embodiments of the present invention relate to a packaged semiconductive die with integrated circuitry. More particularly, an embodiment relates to bonding a packaged die to a board with an underfill mixture that includes properties similar to a particulate-filled underfill composite. 
     BACKGROUND INFORMATION 
     Flip-chip technology is well known in the art for electrically connecting a die (hereinafter, a “chip” or “flip-chip”) to a mounting substrate such as a printed wiring board. 
     The active surface of the die is subject to numerous electrical couplings that are usually brought to the edge of the chip. Heat generation is significant at the active surface of the die, and consequently at the active surface of the chip. Electrical connections, referred variously to as balls, bumps, and others, are deposited as terminals on the active surface of a flip-chip. The bumps include solders and/or plastics that make mechanical connections and electrical couplings to a substrate. The chip is inverted (hence, “flip-chip”) onto a mounting substrate with the bumps aligned to bonding pads on the mounting substrate. If the bumps are solder bumps, the solder bumps on the flip-chip are soldered to the bonding pads on the substrate. A gap exists between the flip-chip active surface and the mounting substrate. 
     One electronic device includes a flip-chip and mounting substrate, among other things. One characteristic of flip-chip technology is shear stress on the solder joints during temperature cycling of the device. This shear stress is partially a result of a difference in the coefficients of thermal expansion (“CTE”) of the flip-chip and the mounting substrate. Die materials such as silicon, germanium, and gallium arsenide, along with their packaging materials, may have CTEs in a range from about 3 ppm/° C. to about 6 ppm/° C. Mounting substrates are usually composites of organic-impregnated fiberglass dielectrics and metallic circuitry. These substrates may have CTEs in a range from about 15 ppm/° C. to about 25 ppm/° C. Consequently, a mismatch in the CTEs exists between the flip-chip and the mounting substrate. 
     To reduce solder joint failures due to stress during thermal cycling, the solder joints are reinforced by filling the space between the flip-chip and the mounting substrate, and around the solder joints, with an underfill composite. The two main processes that are commonly used to underfill the flip-chip include the capillary underfill process and the no-flow underfill process. 
     A capillary underfill process typically proceeds by first aligning the solder bumps on a flip-chip with the pads on a substrate and the solder is reflowed to form the solder joints. After forming the interconnect, the underfill is flowed between the flip-chip and the mounting substrate. Thereafter, the underfill composite is cured. Capillary underfilling can be assisted by pumping the underfill composite between the flip-chip and the mounting substrate, or by vacuum-assisted drawing the underfill composite between the flip-chip and the mounting substrate. 
     The effectiveness of an underfill composite depends on its chemical, physical, and mechanical properties. Properties that make an underfill composite desirable include low CTE, low moisture uptake, high adhesion, high toughness, high glass transition (Tg) temperature, high heat distortion temperature, and others. The underfill composite includes particulate filler inorganics such as silica or the like, and metal flakes or the like. The particulate filler increases the modulus and acts as a CTE intermediary for the mismatched CTEs of flip-chip and the mounting substrate. An example of a silica-filled composite is silica-filled, epoxy-based organics. However, the capillary underfill process that includes silica-filled composites or the like has technical challenges that are tedious, expensive, and sensitive to process rules and to chip size and shape. For example, particulate fillers in the underfill composite cause flowability to decrease between the flip-chip active surface and the mounting substrate upper surface. Further, flow around the electrical bumps is also hindered by the increasingly smaller pitch and the increasingly smaller spacing between the flip-chip and the mounting substrate. 
     The no-flow underfill process avoids some challenges in the capillary flow underfill processes. In a no-flow underfill process, the underfill composite is dispensed on the mounting substrate or the flip-chip, and the flip-chip and the mounting substrate are brought into contact. The solder bumps on the chip and the pads on the substrate are aligned. Next, the underfill composite is cured prior to or substantially simultaneously with reflowing the solder to create the solder joints. 
     The no-flow underfill process also has technical challenges. The no-flow underfill material also must be made into a composite with the addition of a filler of silica or the like. Silica filler reduces the bump interconnection yield, because the filler gets deposited between the bumps and the pads such that electrical connections are not achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the manner in which embodiments of the present invention are obtained, a more particular description of various embodiments of the invention briefly described above will be rendered by reference to the appended drawings. Understanding that these drawings depict only typical embodiments of the invention that are not necessarily drawn to scale and are not therefore to be considered to be limiting of its scope, the embodiments of the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1A is a cross-section of a package during an underfill process according to an embodiment; 
     FIG. 1B is a cross-section of the package depicted in FIG. 1A after further processing; 
     FIG. 1C is a cross-section of the package depicted in FIG. 1B after further processing; 
     FIG. 1D is a cross-section of the package depicted in FIG. 1C after further processing; 
     FIG. 2A is a cross-section of a package during a no-flow underfill process according to an embodiment; 
     FIG. 2B is a cross-section of the package depicted in FIG. 2A after further processing; 
     FIG. 2C is a cross-section of the package depicted in FIG. 2B after further processing; 
     FIG. 2D is a cross-section of the package depicted in FIG. 2C after further processing; 
     FIG. 3A is a process flow diagram that depicts a packaging process embodiment; and 
     FIG. 3B is a process flow diagram that depicts an alternative packaging process embodiment. 
    
    
     DETAILED DESCRIPTION 
     One embodiment relates to a system that includes a flip-chip and a mounting substrate with an underfill mixture that exhibits a coefficient of thermal expansion (CTE) that is characteristic of inorganic-filled underfill composites previously known, among other properties. The underfill mixture includes a principal underfill composition selected from silesquioxanes, thermosetting liquid crystal monomers, combinations thereof, and others as will be further delineated according to various embodiments. 
     One embodiment includes a method of assembling the flip-chip and the mounting substrate. 
     The following description includes terms, such as “upper”, “lower”, “first”, “second”, etc. that are used for descriptive purposes only and are not to be construed as limiting. The embodiments of a device or article of the present invention described herein can be manufactured, used, or shipped in a number of positions and orientations. The terms “die” and “processor” generally refer to the physical object that is the basic workpiece that is transformed by various process operations into the desired integrated circuit device. A die is usually singulated from a wafer, and wafers may be made of semiconducting, non-semiconducting, or combinations of semiconducting and non-semiconducting materials. The term “chip” as used herein refers to a die that has been encapsulated in an organic, an inorganic, or a combination organic and inorganic housing. A “board” is typically a resin-impregnated fiberglass structure that acts as a mounting substrate for the chip. 
     FIG. 1A is a cross-section of a package during an underfill process according to an embodiment. In FIG. 1, a package  100  is depicted during a capillary underfill process. The package  100  includes a solder bump  110  that is attached to a flip-chip  112 . Electrical coupling for the flip-chip  112  is accomplished through a bond pad  114  that is landed on a mounting substrate  116 . 
     FIG. 1B is a cross-section of the package depicted in FIG. 1A after further processing. In FIG. 1B, the flip-chip  112  and the mounting substrate  116  are aligned and brought together. Next, reflow of the solder bump  118  is carried out. A reflowed solder bump  118  is depicted, and a flip-chip assembly  120  includes the flip-chip  112  and the mounting substrate  116  as bonded. 
     FIG. 1C is a cross-section of the package depicted in FIG. 1B after further processing. An underfill mixture  122  according to an embodiment is applied to the flip-chip assembly  120  under conditions that cause a wicking action of the underfill mixture between the flip-chip  112  and the mounting substrate  116 . Underfill mixture  122  has unique formulations, according to various embodiments, and will be described below in greater detail. 
     FIG. 1D is a cross-section of the package depicted in FIG. 1C after further processing. A curing process is carried out to achieve a chip package  124 . The curing process is carried out according to specific embodiments. In one embodiment, curing the mixture is done by an autocatalytic process. The autocatalytic process is carried out in one embodiment by providing a reactive diluent in the underfill mixture  122 . In another embodiment, the curing process is carried out by an additive catalytic curing process. The additive catalytic curing process includes an additive such as a metal catalyst powder that causes the underfill mixture  122  to cure. In another embodiment, a cross-linking/hardening process is carried out to cure the underfill mixture  122 . Examples of specific cross-linker/hardener composition are set forth herein. In another embodiment, a thermoset curing process is carried out. Typically, several curing process embodiments are assisted by thermal treatment. However, in some embodiments, such as the use of a liquid crystal thermoset monomer, thermoset processing may be done without other curing agent processes 
     FIG. 2A is a cross-section of a package  200  during a no-flow underfill process according to an embodiment. The package  200  includes a solder bump  210  that is attached to a flip-chip  212 . Electrical coupling for the flip-chip  212  is accomplished through a bond pad  214  that is landed on a mounting substrate  216 . 
     FIG. 2B is a cross-section of the package  200  depicted in FIG. 2A after further processing. An underfill mixture  222  according to an embodiment is applied, either to the flip-chip  212  or to the mounting substrate  216 . In FIG. 2B, the underfill mixture  222  is depicted as being deposited upon the mounting substrate  116 , although it may alternatively deposited upon the flip-chip  212  (FIG.  2 A). Underfill mixture  222  has unique formulations, according to various embodiments, and will be described below in greater detail. 
     FIG. 2C is a cross-section of the package  200  depicted in FIG. 2B after further processing. Next, the flip-chip  212  and the mounting substrate  216  are aligned and brought together to form a flip-chip assembly  220  that includes the uncured underfill mixture  222  and the not yet reflowed solder bump  210 . 
     FIG. 2D is an elevational cross-section of the flip-chip assembly  220  depicted in FIG. 2C after further processing. A curing process is carried out to achieve a chip package  224 . The curing process is carried out according to specific embodiments. In one embodiment, curing is followed by reflowing of the solder bump  210  (FIG. 2C) onto the bond pad  214  to form a reflowed solder bump  218 . In another embodiment, curing and reflowing are carried out substantially simultaneously. As in the embodiment depicted in FIGS. 1C and 1D, curing may be carried out by various processing paths including autocatalytic, additive catalytic, diluent cross-linking/hardening, thermoset, and a combination thereof. 
     According to an embodiment, the solder bumps  110  and  210  include substantially Pb-free solder technology that is being developed in the art. In another embodiment, the solder bumps  110  and  210  are Pb-containing solder. By “substantially Pb-free solder”, it is meant that the solder is not designed with Pb content according to industry trends. 
     One example of a Pb-containing solder includes a tin-lead solder. In selected embodiments, Pb-containing solder is a tin-lead solder composition such as from Sn97Pb. A tin-lead solder composition that may be used or a Sn37Pb composition. In any event, the Pb-containing solder may be a tin-lead solder comprising Sn x Pb y , wherein x+y total 1, and wherein x is in a range from about 0.3 to about 0.99. In one embodiment, the Pb-containing solder is a tin-lead solder composition of Sn97Pb. In another embodiment, the Pb-containing solder is a tin-lead solder composition of Sn37Pb. 
     A substantially Pb-free solder in one embodiment includes an Sn90-98Ag10-2Cu2-0 solder as is known in the art. Specifically one such composition is Sn 97 Ag 2.5 Cu 0.5    
     Underfill Formulations 
     One embodiment relates to achieving an underfill mixture that includes physical properties of the previously known underfill composites that included inorganic particles. The previously known underfill composites had coefficients of thermal expansion (CTEs) that were between the CTEs of the chip and the board. Accordingly, some embodiments include underfill mixtures that have a range of compositions and combinations, so long as they have minimal overall physical properties of the previously known underfill composites. 
     Some desirable properties of underfill mixtures include a high modulus, low viscosity at the time of dispensing onto a chip and/or substrate, low CTE and good adhesion to the interfaces post cure, so that no delamination at the interface occurs during device testing and field use. Some other desirable properties of underfill mixtures include a high glass-transition (Tg) temperature, and a low moisture uptake. 
     According to various embodiments, the principal underfill compositions include at least one of silesquioxanes, thermosetting liquid crystal monomers, and mixtures thereof. 
     Additive Materials 
     Additive materials are included with the principal underfill compositions. The additive materials and the principal underfill compositions constitute “underfill mixtures” according to embodiments set forth herein. 
     One additive material according to an embodiment is an elastomer for imparting flexibility to the principal underfill composition. Another additive material according to an embodiment is a hardener/crosslinker. The specific hardener/crosslinker that is employed will depend upon compatibility with the principal underfill composition. Hardeners/crosslinkers can be both aromatic and aliphatic in nature. The hardener/crosslinker in one embodiment is an anhydride composition. In another embodiment, the hardener/crosslinker is an amine. 
     Another additive material according to an embodiment is a catalyst. The specific catalyst that is employed will depend upon compatibility with the principal underfill composition. Some catalysts for the above principal underfill compounds according to embodiments include imidizole and its derivatives, amines, and triphenylphosphine. Suitable anhydride hardners include nadic anhydride, methyl cyclohexy anhydride, pthalic anhydride, or the like. Other suitable examples include, methylene diamine, di-aminodiphenyl sulfone, or the like. 
     Another additive material according to an embodiment is a reactive diluent. The specific reactive diluent that is employed will depend upon compatibility with the principal underfill composition. Because of the bonding and sealing nature of the process embodiments, the reactive diluent reacts and/or dissolves into the final underfill mixture instead of volatilizes. Reactive diluents for the above principal underfill compounds according to embodiments of the present invention include other low viscosity epoxy monomers such as Bi-phenyl epoxy, Bis-Phenol A epoxy, Bis-Phenol F epoxy, or the like. Other epoxies include phenyl glycidyl ethers, nonyl phenyl glycidyl ethers, p-tbutylphenyl glycidyl ethers, alkyl C8-C14 glycidyl ethers, cyclo aliphatic epoxies and the like. 
     Another additive material according to an embodiment is an adhesion promoter. The specific adhesion promoter that is employed will depend upon compatibility with the principal underfill composition. Adhesion promoters that can be added to the above underfill compositions include organic and inorganic combinations. In one embodiment, a silane coupling agent is used as an adhesion promoter. In another embodiment, an organo-ziconate composition is used as an adhesion promoter. In one embodiment, an organo-titanate composition is used as an adhesion promoter. 
     Another additive material according to an embodiment is a flow modifier such as a surfactant. The specific flow modifier that is employed will depend upon compatibility with the principal underfill composition. The surfactant requires properties such as compatibility with the principal underfill composition. In one embodiment, the surfactant is anionic such as long chain alkyl carboxylic acids, such as lauric acids, steric acids, and the like. In another embodiment, the surfactant is nonioninc. Examples of nonionic surfactants are polyethtlene oxides, poly propylene oxides, and the like. In yet another embodiment, the surfactant is cationic such as alkyl ammonium salts such as terta butyl ammonium chlorides, or hydroxides. 
     Another additive material according to an embodiment is a deforming agent. The specific deforming agent that is employed will depend upon compatibility with the principal underfill composition. The deforming agent is also referred to as a plasticizer. 
     Another additive material according to an embodiment is a fluxing agent. Fluxing agents are mainly useful in no-flow underfill materials applications according to embodiments, including acids that can be dissolved in mixtures of the base resins and the cross-linking agents. One fluxing agent type includes organic carboxylic acids and the like. Another fluxing agent type includes polymeric fluxing agents and the like. The examples of fluxing agents are any chemicals containing hydroxyl (—OH) group or carboxylic (—COOH) group or both, such as glycerin, ethylene glycol, tartaric acid, adipic acid, citric acid, malic acid, meilic acid, and glutaric acid. 
     Another additive material according to an embodiment is a toughening agent. A toughening agent causes the underfill mixture to resist crack propagation. In one embodiment, an elastomer is used as the toughening agent. The specific elastomer that is employed to toughen the matrix will depend upon compatibility with the principal underfill composition. For example, an elastomer that is used with epoxy-functionalized compositions is carboxy-terminated polybutadiene-acrylonitrile (CTBN). CTBN is the generic name for a family of elastomer additives for epoxies, with the primary elastomer being functionalized butadine-acrylonitrile copolymer. These elastomers are available as epoxy, carboxy, amino and vinyl terminal functionalities. Other elastomers may be used that are compatible with a given underfill composition. 
     Another additive material according to an embodiment is an inorganic filler. Inorganic filler particles that optionally are added to the underfill mixtures include oxides of various elements such as silica, alumina, and others. Other inorganic filler particles include nitrides such as silicon nitride and the like. Other inorganic filler particles include conductive materials such as graphite, diamond, and the like. When added, the underfill mixture is more appropriately referred to as an “underfill composite”, in that it has inorganic filler particles as existing technology does, but it includes a principal underfill composition according to various embodiments. The underfill composite embodiments, unlike most other embodiments, include a more-than-one-phase substance. 
     Principal Underfill Compositions and Underfill Mixtures 
     The principal underfill compositions, as set forth above, include various embodiments and their equivalents. The following non-limiting discussion of various principal underfill compositions relates to silesquioxanes, thermosetting liquid crystal monomers, mixtures thereof, and others that contribute to a low CTE underfill mixture or composite. Other compositions may be selected to impart similar final properties of underfill mixtures or underfill composites. 
     Silesquioxanes 
     In a first general embodiment relating to underfill mixtures, a principal underfill composition is a silesquioxane compound. In one embodiment, the silesquioxane is a functionalized, polyhedral silesquioxane. The silesquioxane includes a polyhedral, oligomeric silesquioxane (POSS). The silesquioxane is functionalized by compositions such as an epoxy, a phenol, or an aldehyde, among others. A first epoxy functionalized silesquioxane is depicted below.                           
     A second epoxy-functionalized silesquioxane composition is depicted below.                           
     A third epoxy-functionalized silesquioxane composition is depicted below.                           
     In one embodiment, the number of functional groups on functionalized silesquioxanes (that is, the functionality) useful in accordance with embodiments set forth herein varies between 1 and 10 inclusive. According to this embodiment, the functionalized silesquioxane may be oligomeric or polymeric. The degree of oligomerization or polymerization that is chosen depends upon the specific application. In one embodiment, the functionality is in a range from about 1 to about 10. In another embodiment, the functionality is from about 3 to about 6. In another embodiment, the functionality is about 5. 
     In some embodiments, functionalized silesquioxanes having epoxy functional groups are utilized. In alternative embodiments the functionalized silesquioxanes are functionalized by other functional groups. 
     In one embodiment, a silesquioxane is mixed with other monomers before functionalizing. For example, the silesquioxane is prepared as an epoxy monomer, and it is mixed with a monomer, such as Bi-phenyl epoxy, or the like. In another example, the silesquioxane is prepared as an epoxy monomer, and it is mixed with a monomer, such as Bis-Phenol A epoxy, Bis-Phenol F epoxy, mixtures thereof, or the like. In another example, the silesquioxane is prepared as an epoxy monomer, and it is mixed with a monomer, such as cyclo aliphatic epoxy, or the like. Other mixtures of the silesquioxane monomer include the above mixture combinations, while functionalization of the silesquioxane is done by non-epoxy functional groups including amines, hydroxides, phenols, and others set forth above. 
     The functionalized silesquioxanes are also mixed with other compositions and/or with particles in order to achieve specific mixture embodiments and/or composite embodiments. These additive materials are set forth above. 
     The following are epoxy functionalized silesquioxane mixture embodiments that are processed according to process embodiments. 
     EXAMPLE 1 
     In Example 1, reference is made to FIGS. 1A-1D. The flip-chip  112  is aligned with the mounting substrate  116  and the solder bump  110  is reflowed according to known technique. The underfill mixture  122  is fed to an edge of the flip-chip  112 , and the underfill mixture  122  flows by capillary action to fill interstitially between the flip-chip  112  and the mounting substrate  116 . Alternatively, the underfill mixture is flow-assisted by a positive or negative pressure. Thereafter, the underfill mixture  122  is cured by thermal processing. The underfill mixture  122  in this example includes the first silesquioxane depicted above, and it is prepared as an epoxy monomer. It is mixed with the Bis F epoxy monomer and a hardener such as an anhydride at a mole ratio of 1:1 between the epoxy and anhydride function groups. Additionally, the functionalized polysilesquioxane set forth above is blended with CTBN elastomer, about 5-10 wt % of the mixture and a catalyst, about 1 wt % of the mixture. 
     EXAMPLE 2 
     In a first alternative to Example 1, the same silesquioxane is prepared, but an amine hardener/crosslinker at epoxy to amine mole ratio of 0.5 to 1.5 of the mixture is used in place of the anhydride hardener/crosslinker. 
     EXAMPLE 3 
     In a second alternative to Example 1, the same silesquioxane is prepared, and a reactive diluent, about &lt;10 wt % of the mixture is added, along with an adhesion promoter, about &lt;5% of the mixture and a flow modifier, about &lt;5 wt % of the mixture. 
     EXAMPLE 4 
     In a third alternative to Example 1, the same silesquioxane is prepared, and an inorganic particulate of silica is included in the mixture, about 10-70 wt % of the mixture, to comprise an underfill composite. 
     Once cured, the silesquioxane-containing underfill mixture  122  or  222  exhibits a combination of properties that make the materials an improvement over prior art underfill composites. These properties include low CTE (20-50 ppm/° C.), high Tg (50 to 175/° C.), high modulus, low moisture uptake (&lt;2 wt % ), high heat distortion temperature, high elongation, and low volume shrinkage(&lt;2% ). 
     Thermosetting Liquid Crystal Monomers 
     In a second general embodiment, certain liquid crystal monomers are thermoset cured and provided as the underfill mixture. The monomers that are useful in this embodiment of the present invention are those that can form liquid crystalline thermosets (“LCTs”) upon curing of the monomers. Accordingly, these monomers are referred to herein as LCT monomers, although after cure they have taken on morphologies such as isotropic, nematic, and smectic. LCT monomers, according to various embodiments, exhibit improved thermal stability, good orientation stability, low shrinkage upon cure, low CTE, and enhanced reaction rates due to close proximity of the reactive groups. LCT monomers, according to various embodiments, are also self-reinforcing due to improved impact resistance, higher fracture toughness, and high modulus. 
     Useful monomers are those that are functionalized with various functional groups. In some embodiments, functionalized LCP monomers have epoxy functional groups. In one alternative embodiment the LCP monomers are functionalized by other functional groups. In one embodiment, the LCP monomers are functionalized by alcohols or phenols. In one embodiment, the LCP monomers are functionalized by amines. In one embodiment, the LCP monomers are functionalized by chlorosilanes. In one embodiment, the LCP monomers are functionalized by epoxides. In one embodiment, the LCP monomers are functionalized by esters. In one embodiment, the LCP monomers are functionalized by fluoroalkyls. In one embodiment, the LCP monomers are functionalized by aromatic and aliphatic halides. In one embodiment, the LCP monomers are functionalized by isocyanates. In one embodiment, the LCP monomers are functionalized by methacrylates or acrylates. In one embodiment, the LCP monomers are functionalized by cyclo-aliphatic compounds. In one embodiment, the LCP monomers are functionalized by nitriles. In one embodiment, the LCP monomers are functionalized by norbomenyls. In one embodiment, the LCP monomers are functionalized by olefins. In one embodiment, the LCP monomers are functionalized by phosphenes. In one embodiment, the LCP monomers are functionalized by polymers. In one embodiment, the LCP monomers are functionalized by silanes. In one embodiment, the LCP monomers are functionalized by silanols. In one embodiment, the LCP monomers are functionalized by styrenes. In one embodiment, the LCP monomers are functionalized by aldehydes. In one embodiment, the LCP monomers are functionalized by hydroxides. In one embodiment, the LCP monomers are functionalized by a combination of functional groups. 
     In one embodiment, the functionality of epoxy-functionalized monomers varies from 1 to about 10. In one embodiment, the functionality of epoxy-functionalized monomers varies from about 3 to about 7. In one embodiment, the functionality of epoxy-functionalized monomers is about 5. One such monomer includes the diglycidyl ether of dihydroxy alpha methylsilbene (DHAMS). In one embodiment, the functionalized monomers are crosslinked in the presence of one or more hardeners and in the presence of heat to form a microscopic liquid crystalline structure. The microscopic liquid crystalline structures exhibit very low CTE and high modulus. 
     The following are epoxy functionalized LCP mixture embodiments that are processed according to process embodiments. 
     EXAMPLE 5 
     In Example 5, reference is made to FIGS. 1A-1D. The flip-chip  112  is aligned with the mounting substrate  116 , and the solder bump  110  is reflowed according to known technique. The LCT monomer underfill mixture  122  is fed to an edge of the flip-chip  112 , and the LCT monomer underfill mixture  122  flows by capillary action to fill interstitially between the flip-chip  112  and the mounting substrate  116 . Thereafter, the LCT monomer underfill mixture  122  is cured by thermal processing. The LCT monomer underfill mixture  122  in this example includes the DHAMS monomer and is prepared as an epoxy mixture. It is mixed with the Bi-phenyl epoxy monomer and functionalized thereby. Additionally, the functionalized DHAMS monomer is blended with CTBN elastomer, an anhydride hardener/crosslinker, at a mole ratio 1:1 for epoxy to anhydride and a catalyst, about 1-2 wt % of the mixture. 
     EXAMPLE 7 
     In a first alternative to Example 5, the same DHAMS mixture is prepared, but an amine hardener/crosslinker, at a mole ratio of 0.5 to 1.5 for epoxy to amine of the mixture] is used in place of the anhydride hardener/crosslinker. 
     EXAMPLE 8 
     In a second alternative to Example 5, the same DHAMS mixture is prepared, and a reactive diluent, about &lt;5 wt % of the mixture is added, along with an adhesion promoter, about &lt;5 wt % of the mixture]. 
     EXAMPLE 9 
     In a third alternative to Example 5, the same DHAMS monomer is prepared, and an inorganic particulate of silica is included in the mixture, about 10 to 60 wt % of the mixture, to comprise an underfill composite. 
     Once cured, the thermoset LCP-containing underfill mixture exhibits a combination of properties that make the materials an improvement over prior art underfill composites. These properties include low CTE, high Tg, high modulus, low moisture uptake, high heat distortion temperature, high elongation, and low volume shrinkage 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Siol- 
                 Liquid 
               
               
                   
                 esquioxane 
                 Thermoset Monomer 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Tensile strength, KSI 
                   
                   
               
               
                 CTE, ppm/° C. 
                 20-50 
                 20-30 
               
               
                 Tg, ° C. 
                 50-175 
                 75--150° C. 
               
               
                 Elongation, % 
                 3-10 
               
               
                 Heat Distortion  1 Temperature, ° C. 
               
               
                   2 Moisture absorption, % 
                 &lt;1 wt % 
                 &lt;2 wt % 
               
               
                 Compression Strength, KSI 
               
               
                 Modulus 
                 2-8 GPa 
                 1-8 GPa 
               
               
                   
                   
                 (unfilled to filled) 
               
               
                   
               
               
                   1 264 psi fiber stress.  
               
               
                   2 200 hour water boil.  
               
             
          
         
       
     
     Package Assembly 
     FIG. 3A is a process flow diagram that depicts a packaging process embodiment. Reference may also be made to FIGS. 1A-1D. In  310 , a flip-chip and a substrate are aligned and brought together. In  320 , the solder that is usually attached to the flip-chip is reflowed and adhered to the bonding pad on the mounting substrate. The solder is a Pb-free solder in one embodiment, and a Pb-containing solder in another embodiment. In  330 , an underfill mixture or an underfill composite as set forth herein is deposited onto the mounting substrate and the flip-chip, and it flows by at least one of capillary action, positive pressure expulsion, or negative pressure (vacuum) draw. In  340 , the underfill mixture or underfill composite is cured. In one embodiment, the cure is a thermal process. In one embodiment, the cure is an autocatalytic process. In one embodiment, the cure is a catalytic process. 
     FIG. 3B is a process flow diagram that depicts an alternative packaging process embodiment. Reference may also be made to FIGS. 2A-2D. In  350 , a no-flow underfill mixture is deposited onto a mounting substrate. Alternatively, the no-flow underfill mixture is deposited onto a flip-chip. In  360 , the flip-chip and the substrate are aligned and brought together. In  370 , the solder is reflowed and adhered to the bonding pad on the mounting substrate. The solder is a Pb-free solder in one embodiment, and a Pb-containing solder in another embodiment. In  380 , the underfill mixture is cured. Alternatively, the processes depicted by  370  and  380  are reversed. In yet another alternative, the processes depicted by  370  and  380  are carried out substantially simultaneously. This means that cure of the underfill mixture starts when heat is applied to reflow the solder, but the cure is not completed before the reflow of the solder is completed. In one embodiment, the cure is a thermal process. In one embodiment, the cure is an autocatalytic process. In one embodiment, the cure is a additive catalytic process. In one embodiment, the cure is a combination of at least two of the above cure processes. 
     In some embodiments, the underfill mixture is a single-phase liquid. Single-phase liquid underfill mixtures are useful in some semiconductor packaging wherein prior art underfill composites have failed. For example, in semiconductor packages that have tight pitch bumps (that is, the bumps are dense due to their small size) or a small gap height between the chip and the substrate, prior art underfill composites have proven complicated to use because of the increased viscosity of the base resin, the difficulty in flow especially at tight bump pitches, and their poor mechanical properties. 
     It is emphasized that the Abstract is provided to comply with 37 C.F.R. §1.72(b) requiring an Abstract that will allow the reader to quickly ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 
     In the foregoing Detailed Description, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description of Embodiments of the Invention, with each claim standing on its own as a separate preferred embodiment. 
     It will be readily understood to those skilled in the art that various other changes in the details, material, and arrangements of the parts and method stages which have been described and illustrated in order to explain the nature of this invention may be made without departing from the principles and scope of the invention as expressed in the subjoined claims.