Abstract:
An apparatus and method is provided for forming a board-on-chip (BOC) package. An adhesive material including a carrier and microcapsules distributed in the carrier is used to bond a semiconductor component to a mounting surface in a BOC package. The microcapsules contain a hardener and/or a catalyst that, when combined with the carrier, initiate a bonding reaction. The contents of the microcapsules are released via application of an external influence, such as pressure or heat, when the bonding reaction is desired to begin. The use of microcapsules permits the formulation of adhesive blends with a substantially increased pot life, increased stability and reliability at high temperatures, and favorable low temperature reaction and bonding characteristics.

Description:
This is a divisional of U.S. patent application Ser. No. 09/790,527, filed Feb. 23, 2001, now U.S. Pat. No. 6,531,340, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to packages for semiconductor devices, and in particular to using an adhesive material containing microcapsules to attach semiconductor components in a board-on-chip (BOC) package. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices, for example dynamic random access memory (DRAM) devices, are shrinking in the sense that smaller devices are being manufactured that are able to handle larger volumes of data at faster data transfer rates. As a result, semiconductor manufacturers are moving toward chip-scale packages (CSP) for semiconductor components which have a small size and fine pitch wiring. 
     One exemplary CSP is shown in FIG.  1 . The package  20  has a board-on-chip (BOC) construction whereby a semiconductor component, such as an integrated circuit (IC) chip  202 , is attached to a base material, such as a substrate  206 , with an adhesive material  204 . The adhesive material  204  is interposed between and bonds the IC chip  202  and the substrate  206 . The package  20  also includes electrical connectors, e.g. solder balls  208  and metal wires  210 , for connection of the IC chip  202  to external components. 
     Conventional adhesives used in BOC packages are designed to exhibit certain reliability characteristics due to the adverse environments encountered during subsequent manufacturing and assembly processes. For example, adhesives used in BOC packages must be stable at high temperatures (e.g., greater than about 150 degrees Celsius) in order to ensure reliability of the package during subsequent processing such as exposure to a high-temperature solder bath. 
     In addition, health-related issues present other design factors for conventional adhesives used in BOC packaging. For example, to avoid excessive PCB outgassing, adhesives used in BOC packages must permit attachment at low temperatures (e.g., lower than about 150 degrees Celsius). 
     A problem exists with conventional adhesives used in BOC packaging in that these materials often represent a compromise between the low temperature attachment and high temperature stability requirements. These adhesives usually comprise a hybrid material or mixture composed of two materials that differ in glass transition temperature (Tg), often combined in an epoxy blend. The glass transition temperature, Tg, is the temperature at which a rigid glass becomes rubbery. A conventional hybrid adhesive may combine a low Tg material, which exhibits the desired low temperature attachment characteristic, with a high Tg material, which exhibits the desired high temperature stability characteristic. The resulting blend usually exhibits attachment and stability characteristics that fall between the two materials, but possess the desired attachment and/or stability qualities of neither material. 
     Therefore, there is a strong need and desire for an adhesive material for use in semiconductor packages that exhibits the desired attachment characteristics of available low Tg materials and the stability qualities of available high Tg materials. 
     SUMMARY OF THE INVENTION 
     An apparatus and method for forming a board-on-chip (BOC) package is provided. An adhesive material including a carrier and microcapsules distributed in the carrier is provided which may be used to bond a semiconductor component to a mounting surface in a BOC package. The microcapsules contain a hardener and/or a catalyst that, when combined with the carrier, initiate a bonding reaction. The contents of the microcapsules are released via application of an external influence, such as pressure or heat, when the bonding reaction is desired to begin. The use of microcapsules permits the use of adhesive blends with increased stability and reliability at high temperatures, as well as favorable low temperature reaction and bonding characteristics. In addition, the microencapsulated adhesive blend exhibits substantially increased pot life. 
     In another aspect of the invention, the use of microcapsules permits formulation of adhesive materials suitable for use in semiconductor packages allowing room-temperature attachment. 
     The adhesive material may also be formulated in an adhesive tape for ease of use in BOC packaging applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other advantages and features of the invention will be more clearly understood from the following detailed description of the invention which is provided in connection with the accompanying drawings in which: 
     FIG. 1 illustrates a cross-sectional view of a conventional board-on-chip (BOC) semiconductor package; 
     FIG. 2 illustrates a simplified block diagram of a conventional method of using an adhesive blend; 
     FIG. 3 illustrates a simplified block diagram of an exemplary embodiment of the method of the invention; 
     FIG. 4 illustrates a portion of an adhesive tape constructed in accordance with an exemplary embodiment of the invention; 
     FIG. 5 illustrates the adhesive tape portion of FIG. 2 under pressure; 
     FIG. 6 illustrates the adhesive tape portion of FIG. 3 after cross-linking; 
     FIG. 7 illustrates a portion of a board-on-chip (BOC) package undergoing construction in accordance with an exemplary embodiment of the invention; 
     FIG. 8 illustrates the BOC package portion of FIG. 5 at a later stage of construction; 
     FIG. 9 illustrates the BOC package portion of FIG. 6 at a later stage of construction; and 
     FIG. 10 illustrates a processor system including a semiconductor package which employs adhesive structures and bonding process methodologies in accordance with an exemplary embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Because the conventional method of using an adhesive material in the formation of a semiconductor package is instructive to an understanding of the methodologies of the present invention, a typical adhesive formulation process will be described with reference to FIG.  2 . FIG. 2 shows a simplified block diagram of a method conventionally used in constructing a board-on-chip (BOC) package. The method of FIG. 2 includes blending an epoxy resin and hardener to form an epoxy compound. Such an epoxy compound is often used as the adhesive  204  (FIG. 1) for BOC packages. The resin and hardener are removed from their separate packaging in method segment  24 , and mixed in method segment  26 . The resin and hardener usually must be mixed in specific proportions so that an appropriate amount of resin is activated by an appropriately proportional amount of hardener. In addition, thorough mixing is typically required to ensure that the hardener is distributed evenly throughout the resin. 
     As the resin and hardener are mixed, a chemical reaction begins that can last for a variable time period (e.g., ranging from minutes to days), depending on several factors including the particular resin or hardener used, temperature, and mass of the reactants. The resin-hardener mixture hardens during this chemical reaction (method segment  28 ), and the hardening time is called the “pot life,” also known as the “working life” or “gel time.” During the pot life, the adhesive material may be applied to or positioned on the surfaces to be bonded (e.g., between the IC chip and substrate of the BOC package). The chemical reaction between the resin and hardener is usually exothermic (heat generating), and thus to avoid excessive instantaneous generation of heat, a longer pot life is often desired. 
     The pot life is only the first phase of the curing cycle. During the remainder of the curing cycle (method segment  30 ), the molecules of the mixture continue to crosslink, and the chemical, thermal, and structural properties of the mixture continue to develop until they reach the levels of the final epoxy compound at the end of the curing cycle. In addition, an optional “post-cure” process may be used to enhance the chemical, thermal, and/or structural properties of the epoxy compound (method segment  32 ). A typical postcure process includes heating the epoxy compound to an elevated temperature (i.e., above room temperature) for a specified period. 
     In contrast to the conventional process illustrated in FIG. 2, FIG. 3 shows a simplified block diagram of a method of applying an adhesive material in accordance with an exemplary embodiment of the invention. This method may be used in constructing a board-on-chip (BOC) package. In accordance with the invention, one or both of the resin and hardener is provided in microcapsules suspended in a carrier, in a single package. 
     The microcapsules include a permeable exterior that, when triggered (e.g., by heat and/or pressure), releases the contents of microcapsule into the carrier. The carrier and microcapsules are packaged for use as an adhesive, e.g. adhesive material  204  (FIG. 1) for BOC packages. Referring to FIG. 3, one exemplary embodiment of the adhesive material includes the carrier and microcapsules together in a single package that may be positioned as desired (method segment  301 ). When the adhesive material is appropriately positioned at a location where bonding is desired, an external influence such as heat and/or pressure may be applied to activate the adhesive material, i.e., release the contents of the microcapsules into the carrier (method segment  303 ). The microcapsules are distributed throughout the carrier, and thus the contents of the microcapsules when released mix with the carrier material. The chemical reaction of the epoxy compound begins, and during the pot life (method segment  305 ), the adhesive material hardens. The curing cycle of epoxy compound continues as noted through crosslinking during the remaining curing cycle (method segment  307 ) and optional postcure (method segment  309 ). 
     An exemplary embodiment of an adhesive material  40  in accordance with the invention is illustrated undergoing activation and crosslinking in FIGS. 4-6. Referring to FIG. 4, an adhesive material  40  is shown including a carrier  42  and microcapsules  44  distributed throughout the carrier  42 . The adhesive material  40  includes both a resin and a hardener of an epoxy, although at least one of these two components is encapsulated in the microcapsules  44 . For example, the carrier  42  may comprise a resin, and the contents of the microcapsules  44  may comprise a hardener. Alternatively, the carrier  42  may comprise a hardener, and the contents of the microcapsules  44  may comprise a resin. Also, the carrier  42  may comprise neither a resin nor a hardener, and two types of microcapsules  44  may be present in the carrier  42 ; the contents of a first type including a resin and the contents of a second type including a hardener. 
     Exemplary resin materials that may be used in accordance with the invention include epoxides containing polyimide, silicone epoxy, or another polymeric resin containing epoxied functional groups. Exemplary hardener materials that may be used in accordance with the invention include anhydride, phenolic, imine and amide. 
     In addition, the adhesive material  40  may include a catalyst material. A catalyst material may be included in an epoxy to stimulate or expedite the chemical reaction of the epoxy components (the resin and hardener). For example, a reactive catalyst may be included in order to stimulate the chemical reaction of the epoxy resin and hardener at a low temperature, such as room temperature. The catalyst material may be included in the carrier  42  and/or the microcapsules  44 . For example, the carrier  42  may comprise a resin, and the contents of the microcapsules  44  may include both a hardener and a catalyst. Thus, when the contents of the microcapsules  44  are released, both the hardener and the catalyst are released and mixed into the carrier  42 , commencing the epoxy chemical reaction. Exemplary catalyst materials that may be used in accordance with the invention include Lewis acid and imidazole. 
     When both a hardener and catalyst are used, separate microcapsules may be used for each of the hardener and catalyst. Because hardeners are generally relatively heavier (about 10-30% of the weight of the adhesive) than catalysts (about 0.01-5% of the weight of the adhesive), it may be desirable to use an encapsulated catalyst for typical applications. 
     Referring to FIG. 5, the microcapsules  44  are activated via the application of an external influence such as heat and/or pressure. Several types of microcapsules  44  may be used in accordance with the invention, and the different types of microcapsules  44  may have a variety of different release mechanisms. For example, the contents of one type of microcapsule may be released upon application of a compressive force that breaks open the microcapsules  44 . The compressive force may be generated by specialized machinery used in the BOC packaging process such the ESC 9200 manufactured by ESC Manufacturing, Inc. For another example, another type of microcapsule may be broken open in a shear mode by a mechanical blender or mixer. For a further example, the permeable exterior of another type of microcapsule may be dissolved, releasing the contents, when exposed to an elevated temperature. 
     Exemplary microcapsule shell materials that may be used in accordance with the invention include those that are pressure sensitive and/or thermal sensitive such as polymethyl methacrylate (PMMA). 
     Referring to FIG. 6, after the adhesive material  40  is activated and the contents of the microcapsules  44  released, a chemical reaction between the resin and hardener (and optionally a catalyst) occurs, eventually resulting in a crosslinked epoxy compound  46 . 
     The carrier  42  may include a polymeric resin formulation with a low glass transition temperature (Tg) (e.g., less than 150 degrees Celsius), and the microcapsules  44  may include hardener and/or catalyst materials which react with the carrier  42  to form a high Tg material (e.g., greater than 150 degrees Celsius). Prior to release of the microcapsules  44 , the adhesive material  40  may exhibit the low-temperature attachment characteristics of the low-Tg resin in the carrier  42 . This quality may permit desirable room-temperature attachment of the semiconductor components of a BOC package. After the contents of the microcapsules  44  are released, the hardener and/or catalyst may cause the resulting epoxy compound to exhibit the increased stability and reliability characteristics of a high-Tg polymeric resin. Also, postcure heat treatment may be performed to further develop the stability and reliability characteristics of the epoxy compound. 
     The microcapsules  44  permit use of adhesive materials considered unusable in conventional semiconductor packaging applications. Combinations of resins, catalysts and hardeners that exhibited a short pot life and/or fast cure time may have been unusable for semiconductor packaging due to the time expended in mixing and applying a conventional adhesive. In accordance with the invention, the adhesive material  40  may be applied and positioned as desired prior to activation of the epoxy chemical reaction. Thus the need is eliminated for a minimum pot life during which the adhesive is applied to semiconductor components. 
     FIGS. 7-9 illustrate the use of an adhesive material  40  in a BOC semiconductor packaging process in accordance with an exemplary embodiment of the invention. Referring to FIG. 7, the adhesive material  40  may be positioned as desired on a component of a BOC package  50 , for example on the substrate  54 . The microcapsules  44  may be distributed in a variety of types of carriers  42 , such as an adhesive tape as shown in FIG.  7 . Alternatively, the adhesive material  40  may also be a liquid, paste, or other soft solid suitable for use in a semiconductor manufacturing process. After the adhesive material  40  is positioned as desired on the substrate  54 , the other component to be bonded, for example an integrated circuit (IC) chip  52 , may be placed in contact with the adhesive material  40 . 
     The described sequence of positioning is not required, and instead the adhesive material  40  may be first positioned on the IC chip  52 , and the substrate  54  later brought in contact with the adhesive material  40 . Alternatively, the substrate  54  and IC chip  52  may be simultaneously brought into contact with the adhesive material  40 . 
     As shown in FIG. 8, the adhesive material  40  is positioned between the components of the BOC package  50  to be bonded. The adhesive material  40  may be activated by applying an external influence, such as heat and/or pressure. Upon application of the external influence, the contents of the microcapsules  44  are released into the carrier  42 , beginning the chemical reaction of the resin and hardener. A catalyst may also be included in the adhesive material  40  to facilitate this chemical reaction. 
     Referring to FIG. 9, as the chemical reaction progresses, the epoxy compound  46  is produced, bonding the substrate  54  to the IC chip  52 . During the curing cycle and an optional postcure process, the epoxy compound continues to crosslink and develop its thermal, chemical and structural properties. The final structure is shown in FIG. 9, with the substrate  54  and the IC chip  52  bonded together by the epoxy compound  46 . 
     FIG. 10 illustrates an exemplary processor system that may include semiconductor components produced using the BOC packaging methodologies of the invention. Referring to FIG. 10, the processor system, which may be a computer system  100 , for example, generally comprises a central processing unit (CPU)  102 , for example, a microprocessor, that communicates with one or more input/output (I/O) devices  112 ,  114 ,  116  over a system bus  122 . The computer system  100  also includes random access memory (RAM)  118 , a read only memory (ROM)  120  and may also include peripheral devices such as a floppy disk drive  104 , a hard drive  106 , a display  108  and a compact disk (CD) ROM drive  110  which also communicate with the processor  102  over the bus  122 . Any or all of the elements of the processor system  100 , for example processor  102 , RAM  118 , ROM  120  or a controller or other IC chips contained within the other components shown in FIG. 10 may include semiconductor packages  50  formed using the adhesive materials and processing methodologies described above with reference to FIGS. 3-9. It should be noted that FIG. 10 is merely representative of one of many different types of architectures of a processor system  100  which may employ the invention. It may also be desirable to integrate the CPU  102  and the RAM  118  on a single chip. 
     Although the invention has been described with reference to a board-on-chip (BOC) package process, this is not required and other packaging processes may be used in accordance with the invention. For example, the invention may also be used with chip-on-board (COB), lead-on-chip (LOC) and wafer backside lamination (WBL) processes. 
     The adhesive structures and processes of the invention provide an adhesive material suitable for use in semiconductor packaging. Because the adhesive material is provided with the microcapsules already distributed in the carrier, the conventionally-required steps of monitoring the proportions and thoroughly mixing the resin and hardener may not be required. In addition, resin, hardener and catalyst materials conventionally unsuitable for semiconductor processing may be used in accordance with the invention, permitting combination of epoxy materials exhibiting room-temperature attachment, fast curing, and extended pot life characteristics as desired. 
     While the invention has been described and illustrated with reference to specific exemplary embodiments, it should be understood that many modifications and substitutions can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.