Abstract:
A single-lid flash memory card and methods of manufacturing same are disclosed. The single-sided lid flash memory card may be formed from a semiconductor package having two or more tapered, stepped or otherwise shaped edges capable of securing a single-sided lid thereon. The taper, step or other shape may be fabricated by various methods, including during the molding step or during the singulation step. A semiconductor package having shaped edges may be enclosed within an external lid to form a finished flash memory card. The lid may be applied to a single side of the semiconductor package by various processes, including over-molding, or by pre-forming the lid with interior edges to match the exterior edges of the semiconductor package, and then sliding the lid over the package to form a tight fit therebetween. The shaped edge of the semiconductor package effectively holds the lid securely on the memory card without any adhesives and prevents the lid from dislodging from the semiconductor package.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     Embodiments of the present invention relate to methods of manufacturing a semiconductor package.  
         [0003]     2. Description of the Related Art  
         [0004]     The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones.  
         [0005]     While a wide variety of packaging configurations are known, flash memory storage cards may in general be fabricated as system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted on a substrate. The substrate may in general include a rigid base having a conductive layer etched on one or both sides. Electrical connections are formed between the die and the conductive layer(s), and the conductive layer(s) provide an electric lead structure for integration of the die into an electronic system. Once electrical connections between the die and substrate are made, the assembly is then typically encased in a molding compound to provide a protective package.  
         [0006]     In view of the small form factor requirements, as well as the fact that flash memory cards need to be removable and not permanently attached to a printed circuit board, such cards are often built of a land grid array (LGA) package. In an LGA package, the semiconductor die are electrically connected to exposed contact fingers formed on a lower surface of the package. External electrical connection with other electronic components on a host printed circuit board (PCB) is accomplished by bringing the contact fingers into pressure contact with complementary electrical pads on the PCB. LGA packages are ideal for flash memory cards in that they have a smaller profile and lower inductance than pin grid array (PGA) and ball grid array (BGA) packages.  
         [0007]     Semiconductor die are typically batch processed on a panel and then singulated into individual packages upon completion of the fabrication process. Several methods are known for singulating the semiconductor packages including, for example, sawing, water jet cutting, laser cutting, water guided laser cutting, dry media cutting and diamond coated wire cutting.  
         [0008]     While laser cutting is an efficient and economical method of singulating semiconductor packages from the panel, laser singulation has certain drawbacks. First, the large amount of heat produced by the laser to cut through the molding compound causes an uncontrolled burning/melting of the various materials in the semiconductor package along the cut lines. For example, the laser cuts through the epoxy of the molding compound, the dielectric material of the substrate and the metal conductive layers on the substrate. These materials may melt and burn differently from each other when cut by the laser. The result of the cuts therefore are rough, non-uniform edges along the cut lines.  
         [0009]     Additionally, while steps are taken to minimize moisture within the package during fabrication, it is possible that some moisture or other gasses remain between layers within a package after the molding compound is applied, for example between the substrate and the molding compound. During the laser cut, this trapped moisture/gas heats up rapidly and expands, resulting in pressure build-up between the layers of the package. This pressure build-up may be sufficient to separate package layers. Moreover, during laser singulation, the laser burns and vaporizes the molding compound at the kerf (i.e., cut width). These vapors may adhere to and contaminate the cut edges and/or other surface of the packages.  
       SUMMARY OF THE INVENTION  
       [0010]     Embodiments of the present invention relate in general to a method of laser singulation of semiconductor packages from a panel. Singulation occurs while the panel is at least partially submerged within a liquid bath filled with water or other liquid capable of rapidly carrying away heat generated by the laser during singulation. The liquid bath may include a fixture for supporting the panel within a fixed, repeatable and registered position.  
         [0011]     In embodiments, the fixture supports the panel so that the panel is immersed in the liquid with the bottom surface of the panel being below the surface of the liquid, and the top surface of the panel being approximately even with the upper surface of liquid. It is understood that the upper surface of the panel may be above or below the surface of the liquid in alternative embodiments.  
         [0012]     The panel may be cut by a laser to singulate the panel into a plurality of semiconductor packages. The cut may form rectangular, square, irregular-shaped and/or curvilinear-shaped semiconductor packages in embodiments of the invention. During the cut, the liquid may cover the upper surface of the panel to further facilitate cooling of the semiconductor package. After cutting, the singulated semiconductor packages may be removed from the liquid bath.  
         [0013]     Immersing the panel within liquid during laser singulation provides several advantages. First, while the laser burns and/or vaporizes the material at the kerf, the edges of the package adjacent the kerf are rapidly cooled and uneven burning/vaporization of the materials at the edges of the package is largely or entirely avoided. Second, moisture and/or other gasses trapped between the layers during the fabrication process are largely prevented from expanding, and the possibility of layers separating under the pressure of the expanding moisture/gasses is minimized. Third, the liquid against the surfaces of the semiconductor package largely prevents the material vaporized by the laser from redepositing on the package, thus greatly reducing contamination due to such vapors.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a flowchart of the method of fabricating a flash memory card according to embodiments of the present invention.  
         [0015]      FIG. 2  is a flowchart of the method of singulating a panel into semiconductor packages according to embodiments of the present invention.  
         [0016]      FIG. 3  is a top view of a portion of a panel of integrated circuits during the fabrication process according to the present invention.  
         [0017]      FIG. 4  is a cross-sectional view through line  4 - 4  in  FIG. 3 .  
         [0018]      FIG. 5  is a top view of a panel of molded integrated circuits according to embodiments of the present invention prior to being cut into individual integrated circuit packages.  
         [0019]      FIGS. 6 and 7  are perspective and top views, respectively of a liquid bath in which the singulation process may be performed according to embodiments of the present invention.  
         [0020]      FIG. 8  is a side view of an integrated circuit panel and liquid bath in which the singulation process may be performed according to embodiments of the present invention.  
         [0021]      FIG. 9  is a side view of an integrated circuit panel within a liquid bath in which the singulation process may be performed according to embodiments of the present invention.  
         [0022]      FIG. 10  is a top view of a liquid bath in which the singulation process may be performed according to an alternative embodiment of the present invention.  
         [0023]      FIG. 11  is a side view of an integrated circuit panel and liquid bath in which the singulation process may be performed according to an alternative embodiment of the present invention.  
         [0024]      FIG. 12  is a side view of an integrated circuit panel within a liquid bath in which the singulation process may be performed according to an alternative embodiment of the present invention.  
         [0025]      FIG. 13  is a perspective view of a sample flash memory card formed according to embodiments of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0026]     Embodiments of the invention will now be described with reference to  FIGS. 1 through 13  which relate to a method of laser singulation of semiconductor packages from a panel while the panel is at least partially submerged within a liquid. It is understood that the present invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the invention to those skilled in the art. Indeed, the invention is intended to cover alternatives, modifications and equivalents of these embodiments, which are included within the scope and spirit of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be clear to those of ordinary skill in the art that the present invention may be practiced without such specific details.  
         [0027]     In general, semiconductor packages according to the present invention are formed in a process described with respect to  FIG. 1 . The fabrication process begins in step  50  with a panel  100 , shown partially for example in  FIGS. 3 and 4 . The type of panel  100  used in the present invention may for example be a leadframe, printed circuit board (“PCB”), a tape used in tape automated bonding (“TAB”) processes, or other known substrates on which integrated circuits may be assembled and encapsulated.  
         [0028]     In embodiments where panel  100  is a PCB, the substrate may be formed of a core, having a top conductive layer formed on a top surface of the core, and a bottom conductive layer formed on the bottom surface of the core. The core may be formed of various dielectric materials such as for example, polyimide laminates, epoxy resins including FR4 and FR5, bismaleimide triazine (BT), and the like. The conductive layers may be formed of copper or copper alloys, plated copper or plated copper alloys, Alloy 42 (42 Fe/58 Ni), copper plated steel, or other metals and materials known for use on substrates.  
         [0029]     The metal layers of panel  100  may be etched with a conductance pattern in a known process for communicating signals between one or more semiconductor die and an external device (step  52 ). Once patterned, the substrate may be laminated with a solder mask in a step  54 . In embodiments where substrate  100  is used for example as an LGA package, one or more gold layers may be formed on portions of the bottom conductive layer in step  56  to define contact fingers on the bottom surface of the semiconductor package as is known in the art for communication with external devices. The one or more gold layers may be applied in a known electroplating process. It is understood that the semiconductor package according to the present invention need not be an LGA package, and may be a variety of other packages in alternative embodiments including for example BGA packages.  
         [0030]     A plurality of discrete integrated circuits  102  may be formed on panel  100  in a batch process to achieve economies of scale. The fabrication of integrated circuits  102  on panel  100  may include the steps  58  and  60  of mounting one or more semiconductor die  104  and passive components  106  on panel  100  for each integrated circuit  102 .  
         [0031]     The one or more semiconductor die  104  may be mounted in step  58  in a known adhesive or eutectic die bond process, using a known die-attach compound. The number and type of semiconductor die  104  are not critical to the present invention and may vary greatly. In one embodiment, the one or more die  104  may include a flash memory array (e.g., NOR, NAND or other), S-RAM or DDT, and/or a controller chip such as an ASIC. Other semiconductor die are contemplated. The one or more die  114  may be electrically connected to panel  100  by wire bonds  108  in step  62  in a known wire-bond process. The die may be stacked in an SiP arrangement, mounted side-by-side in an MCM arrangement, or affixed in another packaging configuration.  
         [0032]     Although not specifically called out on the flowchart of  FIG. 1 , various visual and automated inspections may be made during the above-described fabrication of the plurality of integrated circuits  102  on panel  100 .  
         [0033]     Once the plurality of integrated circuits  102  have been formed on panel  100 , each of the integrated circuits  102  may be encapsulated with a molding compound  120  in step  64  and as shown in  FIG. 5 . Molding compound  120  may be an epoxy such as for example available from Sumitomo Corp. and Nitto Denko Corp., both having headquarters in Japan. Other molding compounds from other manufacturers are contemplated. The molding compound may be applied according to various processes, including by transfer molding or injection molding techniques, to encapsulate each of the integrated circuits  102 . As shown in  FIG. 5 , contact fingers  122  may be left exposed.  
         [0034]     Although shown with a generic rectangular shape in  FIG. 5 , the molded integrated circuits may have irregular and/or curvilinear shapes in embodiments. A method for forming irregular shaped semiconductor packages is disclosed for example in U.S. patent application Ser. No. 11/265,337, entitled “Method of Manufacturing Flash Memory Cards,” which application is assigned to the owner of the present application and which application is incorporated by reference herein in its entirety.  
         [0035]     After molding step  64 , a marking can be applied to the molding compound  120  in step  66 . The marking may for example be a logo or other information printed on the surface of the molding compound  120  for each integrated circuit  102 . The marking may for example indicate manufacturer and/or type of device. Marking step  66  may be omitted in alternative embodiments of the present invention.  
         [0036]     Each of the integrated circuits  102  may next be singulated in step  68 . Singulation step  68  involves cutting integrated circuits  102  on panel  100  into a plurality of individual semiconductor packages. The singulation step  58  is explained in greater detail with respect to the flowchart of  FIG. 2 , the perspective view of  FIG. 6 , the top view of  FIG. 7  and the side views of  FIGS. 8-9 . The panel  100  may be at least partially immersed in a liquid bath  170  in a step  70 . Bath  170  may include a liquid  172  for rapidly cooling the edges of the semiconductor packages after laser singulation as explained hereinafter. The liquid  172  may be water, but may be a variety of other liquids in alternative embodiments. Where liquid  172  is water, the temperature of the water may be between room temperature and 0° C. It is understood that the liquid may be warmer than room temperature in embodiments, and may be cooler than 0° C. in embodiments where the liquid  172  used has a freezing point lower than 0°C.  
         [0037]     In embodiments, the liquid  172  may be a solution of more than one liquid. Moreover, solid objects such as ice or other objects may be included within bath  170  to maintain the liquid at a desired temperature. Furthermore, liquid may be added to and/or taken out of bath  170  to maintain the liquid within bath  170  at a desired temperature.  
         [0038]     The bath  170  may include a fixture  174  for positioning the panel  100  within the liquid  172 . The fixture may be adapted to support the panel  100  in a fixed, repeatable and registered position within the liquid bath  170 . The fixture may be formed of various non-oxidizing materials, such as for example stainless steel, and may be supported within the bath  170  on posts  176  which extend to the bottom of bath  170  (as shown), or connect to the vertical sides of bath  170 . While the fixture is shown as having four contiguous sides, it is understood that fixture  174  may have other configurations in alternative embodiments. For example, the fixture may be formed of two opposed rails. Alternatively, the fixture may support the panel only at the corners of the panel.  
         [0039]     A plurality of support posts  178  may further be provided for supporting the panel  100  within liquid bath  170 , and for supporting the individual semiconductor packages within liquid bath  170  after they are singulated from the panel  100 . Each support post  178  may include a stainless steel support extending from the bottom of the bath  170  and a rubber suction cup at its upper end on which the panel  100  is supported. In embodiments, the support posts  178  are aligned so that there is one support post positioned beneath each singulated semiconductor package. Thus, after the packages are singulated from the panel, each package remains supported on the suction cup of its respective support post. It is understood that the number of support posts  178  are by way of example only, and there may be more or less support posts than shown in alternative embodiments. Moreover, it is understood that the panel  100  and the individual semiconductor packages may be supported within the bath  170  by other positioning and support structures in alternative embodiments.  
         [0040]     In embodiments, the fixture  174  and support posts  178  support the panel  100  so that the panel is immersed in the liquid  172  with the bottom surface of the panel  100  being below the surface of the liquid, and the top surface of the panel  100  being approximately even with the upper surface of liquid  172 . It is understood that the upper surface of the panel  100  may be immersed below the upper surface of the liquid, so long as the distance by which the panel is immersed is not so great that the liquid prevents the laser from making an effective cut through the panel. Similarly, it is understood that the upper surface of the panel  100  may be slightly above the surface of the liquid in embodiments of the invention.  
         [0041]     The liquid  172  in bath  170  may be a few inches deep (with the fixture  174  and support posts  178  spaced from the bottom surface of the bath accordingly). There may be more or less liquid  172  than that in alternative embodiments. In a further embodiment, there may only be enough liquid  172  in bath  170  to cover the panel while the panel is supported directly on the bottom surface of the bath  170 . In such an embodiment, the support posts  178  may be omitted.  
         [0042]     As seen in  FIG. 9 , the panel may be cut by a laser  180  to singulate the panel  100  into a plurality of semiconductor packages (step  72 ,  FIG. 2 ). As indicated above, the cut may form rectangular, square, irregular-shaped and/or curvilinear-shaped semiconductor packages in embodiments of the invention. A variety of lasers are known for singulating panel  100 , including for example CO 2  lasers, YBO 4  lasers, Argon lasers, etc. Such lasers are manufactured for example by Rofin-Sinar Technologies of Hamburg, Germany. During the cut, the liquid  172  may cover the upper surface of the panel  100  due to capillary action, turbulence in the liquid and/or other factors.  
         [0043]     After cutting, the singulated semiconductor packages may be removed from the liquid bath  170  in step  74 . In an embodiment, a matrix handler of known construction having suction cups may be used to retrieve each singulated package from the bath  170 . The suction cups may be provided on the matrix handler so as to align with each of the respective singulated packages. Thus, the matrix handler may retrieve each of the singulated packages from bath  170  in a single retrieving process. It is understood that other mechanisms may be used to retrieve the singulated packages in alternative embodiments.  
         [0044]     Immersing the panel  100  within liquid  172  during laser singulation provides several advantages. First, upon cutting through the panel, the liquid  172  rapidly draws heat away from the cut edge. Thus, while the material at the kerf is burned away and/or vaporized, the layers of the package (i.e., the epoxy molding compound, the dielectric substrate and the metal conductive layers) cool down together and relatively quickly. Thus, the uneven melting/burning of the respective materials of the package found in conventional laser singulation is largely or entirely prevented and the cut edge is smooth and even. Second, as heat is drawn away by the liquid  172  so quickly, moisture and/or other gasses trapped between the layers during the fabrication process are largely prevented from expanding, and the possibility of layers separating under the pressure of the expanding moisture/gasses is minimized. Third, the liquid against the surfaces of the semiconductor package largely prevents the material vaporized by the laser from redepositing on the package, thus greatly reducing contamination due to such vapors. Fourth, it is known that laser cutting offer significant advantages over other cutting methods as far as yield rates. For example, singulating semiconductor packages with non-linear cuts using water jet cutting or routing yields about 500 to 1000 units per hour (UPH). Singulating semiconductor packages with a laser as described above yields about 2500 to 4000 UPH.  
         [0045]     While there are advantages to singulating a panel at least partially immersed in liquid bath  170  using a laser, it is understood that the panel may be singulated into semiconductor packages while immersed at least partially within bath  170  using other cutting methods.  
         [0046]     It is understood that a variety of other fixtures and structures may be used to position the panel  100  within the liquid bath  100  for singulation. The top view of  FIG. 10  and the side views of  FIGS. 11 and 12  show one such alternative. In  FIGS. 10 through 12 , posts  182  include ends capable of mating within registration holes conventionally formed in panel  100  to fix the panel in a repeatable and registered position for singulation. The posts  182  may position the panel  100  at or near the surface of the liquid  172  as described above. The embodiment of  FIGS. 10 through 12  may include support posts  178  as described above.  
         [0047]     Referring again to the flowchart of  FIG. 1  and the bottom view of  FIG. 13 , a semiconductor package  186  formed as described above may further be enclosed within an external lid  190  in step  80  to form a finished flash memory card  200 . Such a lid  190  would provide an external covering for the semiconductor package and establish external product features (for example any notches, chamfers, etc. to aid in proper insertion of the card  200  in a host device). Lid  190  may be applied on one or two sides of the semiconductor package by various processes, for example by over-molding. In such a process, a semiconductor package may be put into an injection molding press and positioned in a fixed and registered position (such as for example by guide pins). Molten plastic or the like may then flow into the mold around the semiconductor package.  
         [0048]     The memory card  200  may be formed according to any of a variety of standard card configurations, including for example a Pico card, xD card, an MMC card, an RS-MMC card, an SD Card, a Compact Flash, a Smart Media Card, a Mini SD Card, a Transflash memory card or a Memory Stick. Other devices are contemplated.  
         [0049]     The foregoing detailed description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The described embodiments were chosen in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.