Abstract:
Various embodiments disclose a molding compound comprising a resin, a filler, and a carbon nano-tube dispersion and methods of forming a package using the molding compound is disclosed. The carbon non-tube dispersion has a number of carbon nano-tubes with surfaces that are chemically modified by a functional group to chemically bridge the surfaces of the carbon nano-tubes and the resin, improving adhesion between the carbon nano-tubes and the resin and reducing agglomeration between various ones of the carbon nano-tubes. The carbon nano-tube dispersion achieves a low average agglomeration size in the molding compound thereby providing desirable electro-mechanical properties and laser marking compatibility. A shallow laser mark may be formed in a mold cap with a maximum depth of less than about 10 microns. Other apparatuses and methods are disclosed.

Description:
BACKGROUND 
       [0001]    Embodiments of the invention relate to a molding compound including a carbon nano-tube dispersion. In particular, the present description refers to the manufacture of a molding compound for molded packages. 
         [0002]    Epoxy resin molding compounds are widely used in microelectronic molded packages due to their inexpensive cost. For example, a lead frame with die bonding and wire bonding can be positioned in a mold cavity corresponding to a desired shape of a package to be produced, and an epoxy molding compound is charged into the mold cavity and a molding is then carried out. After molding, the molded package can be trimmed and further shaped for the outer leads. 
         [0003]    The mold cap is then commonly laser marked with an alpha-numeric code for identification purposes. During laser marking an intense beam of light is scanned over the mold cap surface to write out the desired markings, or is projected onto the mold cap surface through a shadow mask containing an image of the desired markings. The intense beam of light burns, melts, ablates, or otherwise alters the surface of the mold cap to leave a visible imprint. The result is generally a color or texture change, or both. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is side view illustration of a molded chip package in accordance with an embodiment. 
           [0005]      FIG. 2  is a side view illustration of a laser marking system in accordance with an embodiment. 
           [0006]      FIG. 3  is an illustration of a system in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]    Embodiments of the invention relate to a molding compound including a carbon nano-tube dispersion. In particular, the present description refers to the manufacture of a molding compound for molded packages. 
         [0008]    Various embodiments described herein are described with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, configuration, composition, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “in one embodiment” or “an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, configurations, compositions, or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0009]    Embodiments disclose a molding compound comprising: a resin, a filler, and a carbon nano-tube dispersion. In an embodiment, the carbon nano-tube dispersion includes a plurality of carbon nano-tubes with a 1-10 nm diameter and up to 100 nm length dispersed in the molding compound with an average agglomeration size less than one micron. In an embodiment, the carbon nano-tubes are functionalized to assist in inhibiting agglomeration by enhancing the compatibility between the carbon nano-tubes and surrounding resin. The molding compound composition provides desirable electro-mechanical properties such as low coefficient of thermal expansion (CTE), moderate glass transition temperature (Tg), low elastic modulus, low moisture absorption and high thermal conductivity. The molding compound may be formed into a mold cap for chip packaging. The molding compound allows the formation of a shallow laser mark in the mold cap with a maximum depth of less than 10 microns. 
         [0010]      FIG. 1  is a side view illustration of a molded chip package in accordance with an embodiment. The molded chip package illustrated in  FIG. 1  is a wire-bonded Plastic Ball Grid Array (PBGA) package, though embodiments of the present invention are not limited to such, and may be applied to other package designs such as, but not limited to, Molded Matrix Array Package (MMAP), Package on Package (PoP) and Thin Small Outline Package (TSOP), etc. As shown, the molded chip package  100  includes a substrate  110  upon which a chip  120  may be adhesively bonded with a die attach film  130  such as a filled epoxy, an unfilled epoxy, an acrylic, or a polyimide material. 
         [0011]    The substrate  110  can be a printed wiring substrate with wire bonding pads  114  on a top surface and wiring  112  extending to a lower surface where solder balls  116  are mounted on ball bonding pads  118 . In an embodiment, the substrate  110  comprises an electrically insulating material such as an organic polymer resin reinforced with glass fibers. The wire bonding pads  114  can be plated with metals such as nickel and gold to facilitate the wire bonding process. The ball bonding pads  118  can also be plated with a solder flux to facilitate attachment of solder balls  116 . 
         [0012]    Chip  120  may include a variety of devices. In an embodiment, chip  120  includes a volatile and/or nonvolatile memory device. For example, chip  120  may include any type of random access memory, a volatile memory, and/or a non-volatile memory such as a flash memory, phase change memory (PCM), phase-change random access memory (PRAM or PCRAM), ovonic unified memory (OUM) and chalcogenide random access memory (C-RAM). Chip  120  includes wire bonding pads  124 . Wire bonding pads  124  can be plated with metal such as nickel and gold to facilitate the wire bonding process. A wire bond loop  140 , such as a gold wire, connects a wire bonding pad  124  to wire bonding pad  114 . 
         [0013]    The molding compound may be formed into a mold cap  150  covering chip  120 . The mold cap  150  may be formed from a variety of commercially available techniques such as injection molding. After formation of the mold cap  150 , a top surface of the mold cap  150  may be laser marked to identify product data, for example. In an embodiment, mold cap  150  includes a laser mark  160  on a top surface having a maximum depth of less than 10 microns into the mold cap  150 . 
         [0014]    The mold cap  150  may be formed of a molding compound including a resin, a filler, and a carbon nano-tube dispersion. Suitable molding compound resins can be epoxies, silicones, and polyimides, for example. In an embodiment, the resin comprises approximately 10-30 wt % of the molding compound. Filler materials commonly used in molding compounds include silicon oxide in the form of powdered vitreous silica such as quartz, alumina and mixtures thereof. In an embodiment, the filler may comprise approximately 60-89 wt % of the molding compound. 
         [0015]    The carbon nano-tube dispersion comprises a plurality of carbon nano-tubes. In an embodiment, the carbon nano-tubes have a diameter of 1-10 nm and a length of up to 100 nm. In an embodiment, the carbon nano-tube dispersion comprises 0.001-0.05 wt % of the molding compound. The substantially low concentration of carbon nano-tubes allows the ability to add a greater amount of filler if needed to control other properties of the molding compound, such as CTE. 
         [0016]    In an embodiment, the carbon nano-tubes are functionalized to assist in inhibiting agglomeration. This can be done by chemical modification of the carbon nano-tube surfaces to chemically bridge the carbon surface and resin matrix. In an embodiment, the resin is an epoxy resin and the carbon nano-tubes are functionalized with epoxy group. For example a silane coupling agent is anchored to the surface of carbon nano-tubes with C—O—Si covalent bonding at the carbon nano-tube surface. A functional group such as an epoxy group can be bonded to the other end of the silane group to react with an epoxy curing agent in the molding compound. This surface functionalization improves the adhesion between individual carbon nano-tubes and the epoxy resin system and therefore improves the ultra-fine dispersion of carbon nano-tubes by inhibiting agglomeration of carbon nano-tubes. 
         [0017]    The carbon nano-tube dispersions in accordance with embodiments of the present invention represent a significant improvement over state of the art molding compounds. A state of the art molding compound may typically comprise 0.1-5 wt % of a micron-sized carbon black powder or other masking pigment as a colorant and laser energy absorbing additive that decomposes, vaporizes or becomes colorless when exposed to laser energy. However, as shown in  FIG. 1 , these particles form agglomerations greater than 20 microns which introduces several problems. As a foremost problem, the large agglomerations require laser markings of at least 15-23 microns in depth. Carbon ablates during laser marking, and the poorly dispersed large agglomerates tend to explode which creates laser mark defects and poor overall resolution of laser marks. The low resolution, and necessary laser mark depth of 15-23 microns in turn represents a lower limit on mold cap height above the chip. In addition, large agglomerations increase the risk of shorting through the mold cap. 
         [0018]    Embodiments of the present invention allow the formation of laser markings  160  with greater resolution, lower defects, and a maximum depth of less than 10 microns into the mold cap  150 . The shallower depth of the laser markings  160  can additionally guard against unintentional exposure of a wire bond loop  140 , and allows for the formation of a thinner mold cap  150  height above chip  120 . In an embodiment, the height is within the range of approximately 50-150 microns. Embodiments of the present invention also provide the additional benefit of being able to provide and increased electrical resistance thereby lowering the risk of shorting. This is realized because of the lower overall wt %, nano-sized nano-tube, and agglomeration size of the carbon nano-tube dispersion. In an embodiment, functionalization of the carbon nano-tubes assists in dispersion of the carbon nano-tubes for reducing agglomeration size. Additional benefits include good colorization, good process compatibility, mechanical reliability including good warpage performance, electro-chemical reliability to low moisture absorption, higher thermal dissipation property when used for processor or logic device packaging, and good adhesion properties. 
         [0019]    The molding compound according to embodiments of the invention can be formulated utilizing commercially available techniques. For example, carbon nano-tubes, filler, and epoxy curing agents can be mechanically mixed by ball milling or other manners for mixing known in the art. Typical epoxy curing agents include amine and imidazol curing agents. Additional additives such as adhesion promoters, coupling agents, dispersion agents, defoaming agents, and/or flame retardants may be included as is known in the art. The mixture may then be exposed to a low level partial cure and compressed into pellets. The pellets may then be fed into an injection molding apparatus for formation of the mold cap as is known in the art. 
         [0020]    In an embodiment, functionalization of the carbon nano-tubes is done by chemical modification of carbon nano-tube surfaces to chemically bridge the carbon surface and resin matrix. Suitable chemical compounds include a silane coupling agent and 3-glycidoxypropyltrimethoxysilane (3-GPTMS; Aldrich, 99% purity). The silane process is done by purified and UV surface treated carbon nano-tubes immersed in silane solution during 6 hours at approximately 60-80 degree C. Then silane coupling agent is anchored to the surface of carbon nano-tubes and hydrolysis of silane coupling agent is performed resulting in C—O—Si covalent bonding at the carbon nano-tube surface. The other functional group, epoxy group for 3-GPTMS, is the other end, ready to react with epoxy curing agent in the compound. This surface functionalization improves the adhesion between individual carbon nano-tube and epoxy resin system and therefore improves the ultra-fine dispersion of carbon nano-tubes by inhibiting agglomeration of carbon nano-tubes. In an embodiment, the carbon nano-tubes are randomly dispersed in the epoxy resin system by mechanical mixing with the functionalized carbon nano-tubes in epoxy resin and epoxy curing agent. 
         [0021]    In an embodiment, a mold cap comprising a molding compound in accordance with embodiments of the present invention is fabricated. An internal assembly is provided. The internal assembly may comprise a chip  120  adhesively bonded to a substrate  110 . The internal assembly is then placed in a mold cavity. A molding compound in accordance with embodiments of the present invention is then provided to the molding cavity. In an embodiment, the molding compound includes a resin, filler, and a plurality of carbon nano-tubes. In an embodiment, the carbon nano-tubes are functionalized. The molding compound is then cured. In an embodiment, the cured molding compound covers or encapsulates the chip  120 . 
         [0022]    Curing of the molding compound may be performed with a thermal cure process, though is not so limited. In an embodiment, a microwave cure process may be used in place of, or in combination with, a typical thermal oven cure. In such an embodiment, the carbon nano-tube dispersion with an average agglomeration size below one micron can be more effective in heat absorption than a typical molding compound with micron sized carbon black having agglomerations of 20 microns or larger. During microwave heating the nano-sized carbon nano-tubes can serve as a uniform heating source, which provides for more uniform curing in less time than thermal oven curing, which is approximately 175 degree C. for more than 4 hours. This benefit is realized by the improved dispersion of carbon nano-tubes which provides uniform heating throughout the mold cap, and allows for the application of a microwave cure process. As a result, utilizing embodiments of the invention the molding process time can be reduced. 
         [0023]    In an embodiment, a molding compound in accordance with embodiments of the present invention is laser marked. The molding compound can be in the form of a mold cap. As shown in  FIG. 2 , a package  100  such as that illustrated in  FIG. 1  having a portion formed from a molding compound is provided to a laser marking system  200 . The laser marking system  200  may include a laser source  210  and lens  220  to direct a laser beam  230  to the top surface of package  100  having a portion formed from a molding compound. In an embodiment, the molding compound includes a resin, filler, and a plurality of carbon nano-tubes. In an embodiment, the carbon nano-tubes are functionalized. A portion of the molding compound is then irradiated with a radiation source to produce a laser mark  160 , as illustrated in  FIG. 1 . In an embodiment, the radiation ablates the resin, the filler and the carbon nano-tube dispersion comprising the molding compound to form a laser mark having a maximum depth of less than 10 microns. 
         [0024]    Turning to  FIG. 3 , a portion of a system  300  in accordance with an embodiment of the present invention is described. System  300  may be used in wireless devices such as, for example, a personal digital assistant (PDA), a laptop or portable computer with wireless capability, a web tablet, a wireless telephone, a pager, an instant messaging device, a digital music player, a digital camera, or other devices that may be adapted to transmit and/or receive information wirelessly. System  300  may be used in any of the following systems: a wireless local area network (WLAN) system, a wireless personal area network (WPAN) system, a cellular network, although the scope of the present invention is not limited in this respect. 
         [0025]    System  300  may include a controller  310 , an input/output (I/O) device  320  (e.g. a keypad, display), static random access memory (SRAM)  360 , a memory  330 , and a wireless interface  340  coupled to each other via a bus  350 . A battery  380  may be used in some embodiments. It should be noted that the scope of the present invention is not limited to embodiments having any or all of these components. 
         [0026]    Controller  310  may comprise, for example, one or more microprocessors, digital signal processors, microcontrollers, or the like. I/O device  320  may be used by a user to generate a message. System  300  may use wireless interface  340  to transmit and receive messages to and from a wireless communication network with a radio frequency (RF) signal. Examples of wireless interface  340  may include an antenna or a wireless transceiver, although the scope of the present invention is not limited in this respect. 
         [0027]    Memory  330  may be used to store messages transmitted to or by system  300 . Memory  330  may also optionally be used to store instructions that are executed by controller  310  during the operation of system  300 , and may be used to store user data. Memory  330  may be provided by one or more different types of memory. For example, memory  330  may comprise any type of random access memory, a volatile memory, and/or a non-volatile memory such as a flash memory, phase change memory (PCM), phase-change random access memory (PRAM or PCRAM), ovonic unified memory (OUM) chalcogenide random access memory (C-RAM). 
         [0028]    In an embodiment, a chip including a nonvolatile memory array is packaged within any of the molding compounds described herein. Embodiments of the present invention are applicable to all molded packages such as, but not limited to, MMAP, PoP and TSOP etc. Volatile and nonvolatile memories may also be combined within a single chip or separate chips. For example, volatile and nonvolatile memories can be combined in a stacking process to reduce the footprint on a board, packaged separately, or placed in a muli-chip package with the memory component placed on top of the processor. 
         [0029]    In the foregoing specification, various embodiments of the invention have been described. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.