Abstract:
The present invention is a method and apparatus for a very low profile ball grid array package. A substrate is provided with an aperture. A thin sheet material is secured to the substrate, covering the aperture, so as to form a cavity. A semiconductor die is mounted in the formed cavity on the thin sheet material. The semiconductor die is encapsulated with the thin sheet material supporting it during encapsulation. The use of the thin sheet material to form the cavity is a cost effective way to construct a ball grid array package having a very low profile.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a semiconductor package and more particularly to a low cost cavity type ball grid array (BGA) semiconductor package with a very low profile and to a method for its fabrication. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices are widely used in various types of electronic products, consumer products, printed circuit cards, and the like. In an integrated circuit, a number of active semiconductor devices are formed on a chip of silicon and interconnected in place by leads to form a complete circuit. The size and cost of the semiconductor devices are important features in many of these applications. Any reduction in the cost of producing the package or reduction in the size or thickness of the package can provide a significant commercial advantage. 
     Ball grid array semiconductor packages are well known in the electronics industry. Currently available prior types include the Plastic Ball Grid Array (PGBA), the Ceramic Ball Grid Array (CBGA), and the Tape Ball Grid Array (TBGA). A BGA package typically comprises a substrate, such as a printed circuit board, with a series of metal traces on the top side. This series of metal traces is connected to a second series of metal traces on the bottom side of the substrate through a series of wire channels located around the periphery of the substrate. A semiconductor die, having a plurality of bond pads, each associated with an input or output of the semiconductor die, is mounted to the top side of the substrate. The bond pads are connected to the series of metal traces on the top side of the substrate by wire bonds. Typically, the semiconductor die and wire bonds are encapsulated with a molding compound. The second series of metal traces located on the bottom side of the substrate each terminate with a contact pad where a conductive solder ball is attached. The conductive solder balls are arranged in an array pattern, and are connected to the next level assembly or a printed wiring board in the final application. 
     Alternatively, the substrate may be provided with a series of metal traces on only the bottom side, and the semiconductor die is attached to the bottom of the substrate. The bond pads of the semiconductor die are attached to the series of metal traces on the bottom side of the substrate. The series of metal traces located on the bottom side of the substrate terminate with a contact pad where a conductive solder ball is attached. The conductive solder balls are arranged in an array pattern, and are connected to the next level assembly or a printed wiring board in the final application. 
     FIG. 1A illustrates a cross-sectional view of a typical prior art perimeter BGA integrated circuit package  10 . BGA package  10  comprises a substrate  11  having top conductive traces  12  formed on an upper surface of substrate  11 . Substrate  11  typically is formed from an organic epoxy-glass resin based material, such as bismaleimide-triazin (BT) resin or FR-4 board. The thickness of substrate  11  is generally on the order of 0.35 mm. Bottom conductive traces  13  are formed on a lower surface of substrate  11  and are electrically connected to top conductive traces  12  through vias or plated through-holes  14 . Vias  14  extend from the upper surface of substrate  11  to the lower surface. Vias  14  contain a conductive material such as copper. Top conductive traces  12  terminate with bond posts or pads  21 . Bottom conductive traces  13  terminate with ball or terminal pads  16 . Top conductive traces  12 , bottom conductive traces  13 , ball pads  16 , and bond posts  21  comprise an electrically conductive material such as copper or copper plated with gold. Not all top conductive traces  12 , bottom conductive traces  13 , and vias  14  are shown to avoid overcrowding of the drawing. 
     BGA package  10  further comprises a semiconductor element or semiconductor die  18  attached to a die attach pad  23  on the upper surface of substrate  11 . Semiconductor die  18  is attached to die attach pad  23  using an epoxy. Semiconductor die  18  has a plurality of bonding or bond pads  22  formed on an upper surface. Each of the plurality of bond pads  22  is electrically connected to top conductive traces  12  with a wire bond  19 . Typically, semiconductor die  18 , wire bonds  19 , and a portion of substrate  11  are covered by an encapsulating enclosure  24 , such as an epoxy enclosure. 
     Conductive solder balls  26  are each attached to a ball pad  16 . Conductive solder balls  26  are metallurgically wetted to ball pads  16  during a reflow process. The inner-most conductive solder balls  26  are typically underneath or adjacent to semiconductor die  18 . Conductive solder balls  26  are later connected to a next level of assembly or printed circuit board  28  using a standard reflow process. Conductive solder balls  26  connect to contact pads  29  to form solder joints  25 . After the mounting process, solder joints  25  take a flattened spherical shape defined by solder volume and wetting areas. The number and arrangement of conductive solder balls  26  on the lower surface of substrate  11  depends on circuit requirements including input/output (I/O), power and ground connections. 
     FIG. 1B illustrates a cross-sectional view of another typical prior art perimeter BGA integrated circuit package  30 . BGA package  30  comprises a substrate  31  and a support or base substrate  32  attached to substrate  31 . Substrate  31  and support substrate  32  typically are formed from an organic epoxy-glass resin based material, such as bismaleimide-triazin (BT) resin or FR-4 board. The thickness of substrate  31  and support substrate  32  is generally on the order of 0.35 mm each. Substrate  31  has an opening or aperture  33 , which forms a cavity with support substrate  32  as the lower cavity surface. The dimensions (length and width) of support substrate  32  are greater than the dimensions of opening  33  and less than the dimensions of substrate  31 . Substrate  31  has top conductive traces  34  formed on the upper surface, and bottom conductive traces  35  formed on the lower surface electrically connected to top conductive traces  34  through vias or plated through holes  36 . Top conductive traces  34  terminate at one end with a bond post or pad  38 . Bottom conductive traces  35  terminate with a conductive ball pad or contact  39 . A plurality of conductive solder balls or contacts  40  are each coupled to a conductive ball pad  39 . 
     BGA package  30  also contains a semiconductor element or semiconductor die  48  attached to a die attach pad  43  on the upper surface of support substrate  32 . Support substrate  32  and opening  33  provide a cavity for semiconductor die  48 , which minimizes the effect of die thickness on the overall package height. Bond pads  42  are electrically connected to top conductive traces  34  with a wire bond  49 . Typically, semiconductor die  48 , wire bonds  49  and a portion of substrate  31  are covered by an encapsulating enclosure  50 , such as an epoxy enclosure. Conductive solder balls  40  are later connected to a next level of assembly or a printed circuit board  52  using a standard reflow process. 
     BGA packages  10 ,  30  have several disadvantages, including a high profile. Height  17  of BGA package  10  is typically on the order of 2.4 mm, while height  54  of BGA package  30  is typically on the order of 0.9 to 1.46 mm. It is often desirable to minimize the thickness of a packaged semiconductor device since they are widely used in various types of electronic products, portable consumer products, telephones, pagers, automobiles, integrated circuit cards, and the like, in order to make the final products as thin as possible. Thus, there exists a need in the electronics industry for a BGA package that has a very low profile. 
     Another disadvantage of BGA packages  10 ,  30  is the cost of production. The use of substantial amounts of substrate in the manufacturing of BGA packages increases the overall cost of production. Thus, there exists a need in the electronics industry for a BGA package that is cost effective. 
     The present invention has been designed to address the needs of the electronics industry and to overcome some of the limitations associated with a low cost, low profile BGA package. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously provides a low cost semiconductor device having a very low profile on the order of approximately 0.7 mm, and a method for making the same. 
     In one embodiment, a single or multi-layered substrate, having conductive traces on at least the top and bottom sides, is provided with an opening. A layer of very thin material, such as polyimide or metal foil based tape, is secured on the bottom side of the substrate to cover the opening in the substrate. A semiconductor die is inserted into the cavity formed by the opening in the substrate and the tape. The semiconductor die has a plurality of input/output terminals on its top surface, which are electrically connected to the conductive traces on the top of the substrate by bonding wires. The top conductive traces of the laminate are connected to the bottom conductive traces of the substrate by vias. Contacts or solder balls are connected to the bottom conductive traces of the substrate for connection to a next level of assembly or a printed wiring board. The semiconductor die, bond wires and part of the substrate are typically encapsulated with an encapsulating material. 
     In a second embodiment, a single or multi-layered substrate having conductive traces on at least the bottom side is provided with an opening. A layer of very thin material, such as polyimide or metal foil based tape, is secured to the top side of the substrate to cover the opening. A semiconductor die is mounted upside down in the downward facing cavity formed by the opening in the substrate and tape. The semiconductor die has a plurality of input/output terminals on its top surface, which is now facing downward, which are electrically connected to the conductive traces on the bottom of the substrate by wire bonds. Contacts or solder balls are connected to the bottom conductive traces of the substrate for connection to a next level of assembly or a printed wiring board. The semiconductor die, bond wires and part of the substrate are typically encapsulated with an encapsulating material. 
     Thus, in one aspect the invention provides a semiconductor device which has a very low profile on the order of 0.7 mm. 
     In yet another aspect the invention provides a semiconductor device with a very low profile which can be manufactured at a very low cost. 
     In yet another aspect the invention provides a method for manufacturing a low cost, very low profile semiconductor device. 
     The above and other objects, advantages, and features of the invention will become more readily apparent from the following detailed description of the invention which is provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B illustrate diagrammatic cross-sectional views of prior art perimeter BGA packages; 
     FIG. 2 illustrates a diagrammatic cross-sectional view of a ball grid array package according to the present invention; 
     FIG. 3 illustrates a diagrammatic cross-sectional view of an alternate ball grid array package according to the present invention; 
     FIG. 4 illustrates a top view of an integrated circuit containing multiple ball grid array packages according to the present invention; and 
     FIG. 5 is a block diagram of a typical processor controlled system in which the present invention would be used. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be described as set forth in the preferred embodiments illustrated in FIGS.  2 - 5 . Other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. 
     A ball grid array (BGA) package according to the present invention is illustrated generally at  100  in FIG. 2 in diagram form. BGA package  100  comprises a substrate  102  having top conductive traces  104  formed on an upper surface of substrate  102 . Substrate  102  can be of either a single or multi-layered construction as is commonly known in the art, and typically is formed from an organic epoxy-glass resin based material, such as bismaleimide-triazin (BT) resin or FR-4 board as is commonly known in the art. The thickness of substrate  102  is typically on the order of 0.35 mm. Bottom conductive traces  106  are formed on a lower surface of substrate  102  and are electrically connected to top conductive traces  104  through vias or plated through-holes  108 . Vias  108  contain a conductive material such as copper. Top conductive traces  104  terminate with bond posts or pads  110 . Bottom conductive traces  106  terminate with ball or terminal pads  112 . Top conductive traces  104 , bottom conductive traces  106 , ball pads  112 , and bond posts  110  comprise an electrically conductive material such as copper or copper plated with gold, as is known in the art. Not all top conductive traces  104 , bottom conductive traces  106 , and vias  108  are shown. 
     Substrate  102  has an opening or aperture  114 , extending from the top surface of substrate  102  to the bottom surface of substrate  102 . An upward facing cavity is formed by securing a support base such as thin sheet material  116  to the bottom of substrate  102  to cover aperture  114 . Thin sheet material  116  is typically any type of polyimide or metal foil based or backed material, such as copper or aluminum, on the order of approximately 0.025 to 0.1 mm thick and preferably 0.05 mm thick, and must be able to withstand temperatures involved in typical solder reflow processes without degradation. An adhesive may be used to secure the thin sheet material  116  to substrate  102 . The adhesive could be a thermoplastic, thermoset, or pressure sensitive type. The dimensions (length and width) of the thin sheet material  116  are greater than the dimensions (length and width) of aperture  114  so as to completely cover aperture  114 , but typically less than the dimensions (length and width) of substrate  102 . 
     BGA package  100  further comprises a semiconductor element or die  120  mounted in the cavity formed by the aperture  114  and thin sheet material  116 , which minimizes the effect of die thickness on the overall package height. Semiconductor element  120  has a plurality of bonding pads  122  formed on an upper surface. Each of the plurality of bond pads  122  is electrically connected to top conductive traces  104  with a wire bond  124 . Typically, a solder mask material (not shown) with openings over the bond posts  110  and ball pads  112  is applied to the outer surfaces of the substrate  102 . Typically, semiconductor element  120 , wire bonds  124 , and a portion of substrate  102  are covered by an encapsulating compound  126 , such as epoxy. 
     Conductive solder balls  128  are each attached to a ball pad  112 . Conductive solder balls  128  are later connected to a next level of assembly or printed circuit board  302  (FIG. 4) using a standard reflow process. The number and arrangement of conductive solder balls  128  on the lower surface of substrate  102  depends on circuit requirements including input/output, power and ground connections. 
     FIG. 3 illustrates a portion of a cross-sectional view of a further embodiment of a BGA package  200  according to the present invention. BGA package  200  comprises a substrate  202  having bottom conductive traces  204  formed on a lower surface of substrate  202 . Substrate  202  can be of either a single or multi-layered construction as is commonly known in the art, and typically is formed from an organic epoxy-glass resin based material, such as bismaleimide-triazin (BT) resin or FR-4 board as is commonly known in the art. The thickness of substrate  202  is typically on the order of 0.35 mm. Bottom conductive traces  204  terminate with ball or terminal pads  212 . Bottom conductive traces  204  and ball pads  212  comprise an electrically conductive material such as copper or copper plated with gold, as is known in the art. Not all bottom conductive traces  204  are shown. 
     Substrate  202  has an opening or aperture  214  extending from the top surface of substrate  202  to the bottom surface of substrate  202 . A downward facing cavity is formed by securing a support material such as thin sheet material  216  to the top surface of substrate  202  to cover aperture  214 . Thin sheet material  216  is typically any type of polyimide or metal foil based or backed material, such as copper or aluminum, on the order of approximately 0.025 to 0.1 mm thick and preferably 0.05 mm thick, and must be able to withstand temperatures involved in typical solder reflow processes without degradation. An adhesive may be used to secure the thin sheet material  216  to substrate  202 . The adhesive could be a thermoplastic, thermoset, or pressure sensitive type. The dimensions (length and width) of the thin sheet material  216  are greater than the dimensions (length and width) of aperture  214  so as to completely cover aperture  214 , but typically less than the dimensions (length and width) of substrate  202 . 
     BGA package  200  further comprises a semiconductor element or die  220  inverted and mounted in the cavity formed by the aperture  214  and thin sheet material  216 , which minimizes the effect of die thickness on the overall package height. Semiconductor element  220  has a plurality of bonding pads  222  formed on its upper surface, which is now facing downwards. Each of the plurality of bond pads  222  is electrically connected to bottom conductive traces  204  with a wire bond  224 . Typically, a solder mask material (not shown) with openings over the bond pads  222  and ball pads  212  is applied to the outer surfaces of the substrate  202 . Typically, semiconductor element  220 , wire bonds  224 , and a portion of substrate  202  are covered by an encapsulating compound  226 . 
     Conductive solder balls  228  are each attached to a ball pad  212 . Conductive solder balls  228  are later connected to a next level of assembly or printed circuit board  302  (FIG. 4) using a standard reflow process. The number and arrangement of conductive solder balls  228  on the lower surface of substrate  202  depends on circuit requirements including input/output, power and ground connections. 
     FIG. 4 illustrates an integrated circuit  300 , such as a SDRAM or SLDRAM memory module or the like, which utilizes multiple ball grid array packages according to the present invention. Integrated circuit  300  is comprised of printed circuit board  302 . Printed circuit board  302  contains a plurality of top conductive traces  304  on the top surface, and may or may not contain conductive traces on the bottom surface or intermediate layers. Mounted on printed circuit board  302  are various electronic components  304 , as necessary for operation of the integrated circuit  300 , and low profile ball grid array packages  308  as hereinbefore described with reference to FIGS. 2 and 3. 
     Printed wiring board  302  is provided with input/output connectors  310  for connection in an end product system (FIG.  5 ). The use of the low profile ball grid array packages  308  minimizes the overall height of the integrated circuit  300  and allows for smaller end-product packaging. 
     A typical processor system which includes integrated circuits, such as memory devices, that contain low profile ball grid array packages according to the present invention, is illustrated generally at  400  in FIG. 5 in block diagram form. A computer system is exemplary of a device having integrated circuits such as memory devices. Most conventional computers include memory devices permitting the storage of significant amounts of data. The data is accessed during operation of the computers. Other types of dedicated processing systems, e.g. radio systems, television systems, GPS receiver systems, telephones and telephone systems also contain integrated circuit devices which can utilize the present invention. 
     A processor system, such as a computer system, generally comprises a memory device  402 , such as a SDRAM or SLDRAM memory module, a memory device controller  403 , a central processing unit (CPU)  404 , input devices  406 , display devices  408 , and/or peripheral devices  410 . It should be noted that a system may or may not include some or all of the aforementioned devices, and may or may not include multiple devices of the same type. 
     Memory device  402  and CPU  404  include integrated circuits which contain ball grid array packages according to the present invention hereinbefore described with reference to FIGS. 2 and 3. The use of low profile ball grid array packages according to the present invention reduces the size and cost of the integrated circuits, effectively reducing the size and cost of the end product processor system. 
     Reference has been made to preferred embodiments in describing the invention. However, additions, deletions, substitutions, or other modifications which would fall within the scope of the invention defined in the claims may be found by those skilled in the art and familiar with the disclosure of the invention. Any modifications coming within the spirit and scope of the following claims are to be considered part of the present invention.