Abstract:
The present invention concerns a package for an integrated circuit ( 3 ) of the type comprising a cavity ( 2 ) in which the integrated circuit ( 3 ) is mounted, the active surface ( 10 ) of the integrated circuit ( 3 ) being electrically connected to the package on the level of connection (Nc) of an array of balls ( 13   i ) to the package, providing a mechanical and electrical link between the integrated circuit ( 3 ) and a printed circuit card to which the package must be assembled. It is characterized in that it comprises an additional layer ( 14 ) that is rigid and electrically neutral, mounted on the level of connection (Nc) of the integrated circuit ( 3 ) and the balls ( 13   i ) and containing the balls ( 13   i ). 
     It particularly applies to the connections of BGA and PBGA packages.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of connections in BGA, or “Ball Grid Array” packages in printed circuit cards, and more particularly concerns PGBA, or “Plastic BGA” packages. 
     BACKGROUND OF THE INVENTION 
     The standard “cavity down” type of PBGA package comprises a cavity in which an integrated circuit, also called a “chip,” is housed. 
     In this type of package, the areas for connecting the chip to the package and the areas for connecting an array of solder balls to the package are disposed on the same level of the package, generally corresponding to the bottom surface of the package, i.e. the surface that faces the printed circuit card when the package is mounted on the card. 
     The ball array connects the chip to the printed circuit card both electrically and mechanically. 
     In order to protect the active surface of the chip constituted by a semiconductor layer, and the connections, for example bonding wires, both mechanically and from the environment, an encapsulating material is used, for example a thermosetting and electrically neutral resin, which coats the outside of the chip and its connections. 
     Such a method is specifically described in U.S. Pat. No. 5,397,921A. 
     This method, applied to the BGA package described above, has the drawback that the height of the wires thus coated by the resin must be compatible with the height of the balls, which is typically on the order of 0.6 mm for a pitch between the balls on the order of 1.27 mm. 
     This imposes the near exclusive use of the “gold”, or “ball bonding” technique, and the package must include an “electrolytic gold” metallization, requiring the use of “feeders,” which after metallization leave remaining wire ends that are equivalent to antennas for high frequencies, particularly for clock signals. 
     In addition to controlling the wiring loops of the wires, it is also necessary to control the height of the coating so that the wires are completely covered. 
     In general, a line of highly viscous adhesive is deposited around the areas for connecting the wires to the package in order to serve as a wall for retaining the coating resin, which by contrast is very fluid in nature, so that it properly coats the wires and the chip. 
     This wall also makes it possible to prevent an overflow of the coating resin in the areas for connecting the balls to the package. 
     Another drawback relates to the phenomenon of the “collapse” of the balls when the package is assembled to the printed circuit card. 
     In essence, during assembly, the superficial tensile forces of the alloy constituting the balls, typically remelted tin-lead, causes the balls to collapse onto the card. 
     Thus, a ball typically having a height of 0.6 mm for a pitch of 1.27 mm and an initial diameter before assembly of 0.76 mm will decrease in height by about 0.2 mm. This phenomenon is amplified, particularly when an integrated heat sink covers the top surface of the package; this can result in short-circuiting between the balls. 
     SUMMARY OF THE INVENTION 
     The specific object of the invention is to eliminate the aforementioned drawbacks. 
     To this end, the subject of the invention is a package for an integrated circuit of the type comprising a cavity in which the integrated circuit is mounted, the active surface of the integrated circuit being electrically connected to the package on the level of connection of a ball array to the package, providing a mechanical and electrical link between the integrated circuit and a printed circuit card to which the package must be assembled. The package is characterized in that it comprises an additional layer that is rigid and electrically neutral, attached to the level of connection of the integrated circuit and the balls and containing the balls. 
     The invention has the particular advantage of simultaneously facilitating the mounting of the balls onto the package, also called the “ball bonding” of the package, and of protecting the connections and the card. 
     It also makes it possible to guarantee a predefined distance of the package from the card, thus playing the role of a spacer between the package and the card. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and characteristics of the present invention will emerge through the reading of the following description, given in reference to the attached figures, which represent: 
     FIG. 1, a top view of a PBGA package equipped with an ball array; 
     FIG. 2, a partial cross-sectional view of the package illustrated in FIG. 1; and 
     FIG. 3, a partial cross-sectional view of a package according to the invention. 
    
    
     In these figures, the same elements are designated by the same references, and the drawings are not to scale. 
     Moreover, the printed circuit card to which the package is assembled is not represented. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The methods for mounting the balls on the PBGA package, mounting the chip on the card, connecting the chip to the package, and assembling the package to the card are assumed to be known and will not be described. 
     FIG. 1 illustrates the bottom surface of a PBGA package. 
     FIG. 2 illustrates the package of FIG. 1 in partial cross-section along line AA of FIG.  1 . 
     Conventionally, a “cavity down” PBGA package is constituted by a rigid copper substrate  1 , generally in the shape of a parallelepiped, comprising a cavity  2  that is generally central, enclosing a chip  3  that is attached to the bottom  4  of the cavity  2  by means of a thermally conductive adhesive  5 . 
     The substrate  1  also supports a dielectric substrate  6  that is generally flexible, disposed on the periphery of the cavity  2 , and hence of the chip  3 . 
     The substrate  6  supports, on one of its surfaces, conductive traces  7   i  disposed according to a given design, which include the connection areas for the bonding wires and the balls. 
     On its other surface, it supports a conductive plane  8  that is attached to the substrate  1 , for example using the same adhesive  9  that attaches the chip  3  to the bottom  4  of the cavity  2 . 
     The active surface  10  of the chip  3  opposite the bottom  4  of the cavity  2  is electrically connected to the traces  7   i  by means of connections  11   i,  in this case connecting wires, also called “bonding” wires. 
     The conductive traces  7   i  extend from the cavity  2  and radially relative to the cavity  2 , and respectively end in conductive pads  12   i  disposed on the periphery of the package according to a given design, as illustrated in FIG.  1 . 
     The pads  12   i  are intended to receive the respective solder balls  13   i,  as represented in FIGS. 2 and 3. 
     These balls  13   i,  which form a ball array, provide the mechanical and electrical link between the chip  3  and the card once the package is assembled to the card. 
     The wiring of the wires  11   i  and the balls  13   i  to the package takes place on the same level Nc, which corresponds to the plane of the traces  7   i  and the conductive pads  12   i.    
     FIG. 3 schematically illustrates, also in partial cross-section along the line AA of FIG. 1, a PBGA package according to the invention. 
     It has the same basic structure as the package illustrated in FIG.  2  and also comprises, at a level lower than the level of connection Nc of the wires  11   i  and the balls  13   i  to the package, an additional layer  14  containing the balls  13   i  that runs between the balls  13   i  while remaining offset relative to the cavity  2  so as not to cover the area of connection Zc of the wires  11   i  to the package. 
     The layer  14  has a given mechanical rigidity and is electrically neutral. 
     This layer  14  defines a grid comprising a plurality of holes  15   i  in which the balls  13   i  are respectively housed. The holes are chosen so as to be large enough to contain the balls  13   i  with a slight clearance on the order of 0.2 mm. 
     The grid  14  can be attached by an adhesive in the form of a resin or in the form of a preform, when the package is assembled to the card, or can be integrated into the structure of the package during its production. 
     In the embodiment described, the grid  14  is attached by means of an adhesive. 
     A layer of adhesive material  16  is applied so that it does not cover the connection pads  12   i  and so that it cannot penetrate into the holes  15   i  of the grid  14 , for example using a serigraphic screen or a preform. 
     A coating material  17  fills in the space left open by the grid  14 , which forms a second cavity  18  larger than the first cavity  2  containing the chip  3 . 
     It is therefore understood that the grid  14  plays the dual role of a grid for positioning the balls  13   i  and an obstacle for the coating material  17  that covers the active surface  10  of the chip  3  and its bonding wires  11   i.  The internal periphery of the grid  14  is attached to the connection level Nc, forming a retaining wall between the balls  13   i  and the wires  11   i.    
     Moreover, in accordance with a given thickness, this additional layer or grid  14  also plays the role of a spacer in the case of packages with very high ball weights, typically more than 50 mg per ball. This is especially true of packages having an integrated heat sink that covers the entire surface of the package. 
     This spacer role makes it possible to guarantee a predefined distance of the package from the card and prevents any uncontrolled collapse of the balls. 
     Finally, this layer makes it possible to guarantee electrical insulation between the balls when the package is assembled to the card. 
     It has the added advantage of making it possible to increase the solder volume by using balls of larger diameter, for example 0.96 mm instead of 0.76 mm for the same pitch between balls of 1.27 mm. 
     The reliability under thermal cycle of the assembled package is thereby increased. 
     This characteristic is particularly important in applications with high temperature constraints, including automotive applications. 
     The height of the soldered joints on the card, or “ring,” can be higher, typically 0.7 mm instead of 0.5 mm. A soldered joint typically corresponds to a copper deposit to which the components on the periphery of the package are soldered. 
     This results in a relaxation of the wiring constraints, and makes it possible to use aluminum wires in accordance with the so-called “wedge” technique. 
     The package can then be given an “electroless gold” finish, which is less expensive than an “electrolytic gold” finish, without the presence of antennas (metallization flex). 
     The dimensions of a package according to the invention, as well as the materials used for each of the layers constituting the package and their respective thicknesses, are given below as a non-limiting example. 
     This example, given only as an illustration, does not take dimensional tolerances into account. 
     The package defines a parallelepiped of 35 mm per side. 
     The central cavity  2  defines a square of 15 mm per side, and is 40 mm deep. It is designed to receive a chip  3 , generally in the shape of a parallelepiped, of 13 mm per side and 0.40 mm thick. The chip  3  is attached to the bottom  4  of the cavity  2  by means of an adhesive with a thickness of 0.10 mm, which is a high-silver epoxy resin. 
     The sides of the cavity  2  are respectively parallel to the sides of the package. 
     The package is constituted by a substrate  1  with an overall thickness of 0.7 mm and the thickness of the bottom of the substrate  1  corresponding to the bottom  4  of the cavity  2  is equal to 0.3 mm. 
     The substrate  1  is made of copper. 
     The substrate  1  receives on its bottom surface a stack of several layers, which are successively described in ascending order of the thickness of the package. 
     The first layer  9  is a 0.10 mm-thick layer of epoxy. It is used to attach the flexible substrate  6  to the substrate  1 . 
     The second layer  8  corresponds to the conductive plane, which is made of copper and is 17·m thick. 
     The third layer  6  corresponds to the flexible substrate itself, which is made of polyimide and is 50·m thick. 
     The fourth layer  7   i  corresponds to the conductive traces, which are made of copper  17  and are·m thick. 
     The balls  13   i  and the connecting wires  11   i  are attached to this fourth layer  7   i.    
     The fifth layer  16  is a layer of 0.10 mm-thick adhesive for attaching the sixth layer  14 , called the “additional layer,” to the fourth layer  7   i  of the stack, i.e. to the connecting layer Nc. 
     The additional layer  14  is made of 0.5 mm-thick epoxy resin. It defines a grid with a pitch of 1.27 mm comprising a plurality of holes  15   i  for containing the plurality of balls  13   i,  which in the example chosen have a diameter of 0.96 mm. The holes  15   i  have a generally cylindrical shape, with a circular cross section. 
     The diameter of the holes  15   i  of the grid  14  is substantially larger than the diameter of the balls  13   i  and is on the order of 1.05 mm. 
     The diameter of the balls and the maximum acceptable height after remelting define the maximum thickness of the grid  14 , which plays its role as a spacer and for positioning the balls  13   i  on the package. 
     A coating material  17 , for example an epoxy adhesive, covers the active surface  10  of the chip  3  as well as the connecting wires  11   i.  It is prevented from expanding by the fifth and sixth layers of the stack, respectively  16  and  14 , and is contained in a second cavity  18  larger than the first one and defined by the inner periphery of the grid  14  bonded to the flexible substrate  6 . 
     While this invention has been described in conjuction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein, are intended to be illustrative, not limiting. Various changes may be made without departing from the true spirit and full scope of the invention as set forth herein and defined in the claims.