Patent Publication Number: US-2007123066-A1

Title: Interconnection element for BGA housings and method for producing the same

Description:
This application is a divisional of patent application serial number 10/944,684, entitled “Interconnection Element for BGA Housings and Method for Producing the Same,” filed on Sep. 17, 2004, which claims priority to German Patent Application 103 43 255.8, which was filed Sep. 17, 2003, both of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD  
      The invention relates to an interconnection element for BGA housings and comparable housings having an interposer substrate.  
     BACKGROUND  
      In accordance with the customary prior art for FPBGA housings (FPBGA: Fine Pitch Ball Grid Array), caBGA housings (caBGA: Chip Array Ball Grid Array) or TFBGA housings (TFBGA: Thin Fine Pitch Ball Grid Array Package), the packages are provided with an interposer substrate, for example made of a bismaleimide triazine resin. The BGA housings are characterized by the fact that component terminals are applied on their underside area in the form of solder balls.  
      The solder balls are applied by means of a so-called ball placement. During this method, the solder balls are introduced into a flux in the envisaged contact locations in the form of a ball grid array situated on the interposer. In its conventional form, this ball placement is very complicated and very demanding in terms of process engineering.  
      The conventional ball placement for an FPBGA package construction is characterized by the fact that firstly a water-soluble, rosin-containing flux is applied to the interposer substrate. This flux has the task of breaking up the oxide layers on the conductive tracks on the interposer substrate and on the solder balls and of making the surfaces wettable. The solder balls placed into the flux are subsequently held at the introduction location by adhesion forces.  
      In the subsequent reflow soldering process in a soldering furnace, the solder balls are then fused onto the interposer substrate in a multizone cycle at temperatures of 110° C. to 225° C. In this case, the soldering temperature is dependent on the solder materials used.  
      In order that the flux residues are removed again after the fusing operation, the interposer is subsequently cleaned, preferably by means of spray washing.  
      The solder balls mounted on the interposer in this way then have to take up in part considerable shear forces after the mounting of the BGA assembly on a printed circuit board, which arise as a result of the thermally governed different coefficients of expansion of the materials involved.  
      DE 198 33 131 A1 describes a method for the contact-connection of electronic components on flexible substrates, in which semiconductor chips provided with FBGA contact terminals are pressed in a force-locking or positively-locking manner onto the flexible substrate provided with a corresponding contact structure. Utilizing the elasticity of the flexible substrate for contact production ensures that a reliable contact-connection is affected even when the microballs have small size deviations.  
      What is disadvantageous in this case is that, on the one hand, the placement of the semiconductor chips has to be effected with high accuracy for lack of floating effects and that, on the other hand, quite large-area contacts with elevated contact resistance arise.  
      Finally, WO 97/00598 (U.S. Pat. No. 6,321,443) describes a connection substrate comprising a dielectric, in the case of which a real connection structures are provided on both sides. Metallized vias as the connection elements are connected to one another. However, this connection element is not suitable for the contact-connection of BGA housings.  
     SUMMARY OF THE INVENTION  
      In one aspect, the invention provides an interconnect element for BGA housings that can be produced cost-effectively and can take up thermally governed forces (shear forces) without any problems, and also of specifying a method for producing the same.  
      In the preferred embodiment, the interconnection element comprises the interposer substrate and a multiplicity of connection elements which are formed in hollow-cylindrical or rod-shaped fashion and are fixedly connected to the interposer substrate, in a manner penetrating through the latter, and also the rewiring.  
      The interconnection element according to embodiments of the invention can be produced simply, on the one hand, and, on the other hand, enables a substantially simplified mounting technology compared with the solder balls used heretofore. Thus, the interposer substrate not only functions for receiving the rewiring and as a chip-carrying element, but also serves for forming and receiving the new hollow-cylindrical or pin-shaped connection elements that completely replace the solder balls used heretofore.  
      A further advantage of embodiments of the invention is to be seen in the fact that it is possible to completely prefabricate the interposer substrate with the rewiring and the connection elements embedded in it.  
      In order that the FBGA package completed by means of chip mounting, wire bonding and molding can be mounted on a printed circuit board, a development of the invention provides for the connection elements to at least partly project from the surface of the interposer substrate. The connection elements may comprise copper or gold and have a diameter of approximately 25 μm.  
      For the case where the connection elements comprise copper, it is advantageous if they have a surface refinement, e.g., made of nickel and/or gold.  
      In another aspect, the invention provides a method, which is characterized by the fact that firstly microvias formed in the pitch of ball grid array contact terminals are introduced into an interposer substrate. An electrically conductive material is subsequently introduced into the microvias in a manner forming the connection elements. At the same time, an electrical connection to rewiring of the interposer substrate being produced. A part of the material of the interposer substrate is then removed areally, so that the connection elements protrude from the surface of the interposer substrate.  
      A further advantage of embodiments of the invention is to be seen in the fact that the electrical connection between the rewiring and the connection elements can be produced in a manner completely free of solder, in contrast to the conventional FBGA technology. At the same time, this also results in a gain in reliability.  
      In this case, the areal removal (thinning) of the material of the interposer substrate may be effected before or after the chip mounting/molding. It is particularly advantageous if the thinning of the interposer substrate is performed after the chip bonding or before the mounting of the FBGA package on a printed circuit board, because damage to the connection elements during the handling of the package is then virtually precluded.  
      A development of the invention provides for the microvias to be completely filled with an electrically conductive material, in a manner forming the connection elements.  
      In a variant of the invention, only the walls of the microvias are coated with an electrically conductive material, in a manner forming hollow-cylindrical connection elements.  
      This variant of the invention results in a significantly higher thermal/mechanical than in the case of conventional solder ball connections.  
      The coating of the walls of the vias may be performed by means of electrolytic direct metalization.  
      In a further method step the connection elements may be provided with a surface refinement by means of nickel and/or gold application.  
      Finally, the microvias may be produced in a simple manner by means of plasma through a mask situated on the interposer substrate.  
      A further possibility for producing the microvias consists in the fact that the latter are ed into the interposer substrate by mechanical processing, e.g., by stamping.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention is explained in more detail below using an exemplary embodiment. In the associated drawings:  
      Figure  1  shows an interposer substrate provided with microvias;  
      Figure  2  shows the interconnection element according to the invention, comprising the interposer substrate according to  FIG. 1 , in the case of which the microvias are filled with a conductive material;  
      Figure  3  shows an FBGA package equipped with the interconnection element according to the invention;  
      Figure  4  shows the FBGA package according to  FIG. 3 , in the case of which the interposer substrate is areally thinned, so that the interconnection elements project from the surface of the interposer substrate; and  
      Figure  5  shows the FBGA package after mounting on a printed circuit board.  
      List of reference symbols 1 Rewiring 8 Wire bridge 2 Interposer substrate 9 Potting composition 3 Microvia 10 Molding cap 4 Interconnection element 11 FBGA module 5 Connection element 12 Printed circuit board 6 Chip 13 Soldering connection 7 Tape 14 Bond channel  
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS  
       FIG. 1  firstly shows an interposer substrate  2  provided with a rewiring  1 . In  FIG. 1 , the rewiring  1  is shown as extending across the entire bottom surface of the substrate  2 . This view is schematic in nature and it is understood that rewiring  1  can be patterned on the bottom surface, top surface or within the substrate  2 . The substrate  2  has been provided with microvias  3  (holes) with the aid of plasma etching through an etching mask (not illustrated) applied beforehand by means of known photolithography. The microvias  3 , as well as bond channel  14 , are shown as clear rectangles to indicate that these are recesses formed in the substrate  2 .  
      In order to configure the interposer substrate  2  to form an interconnection element  4  according to embodiments of the invention, during the production of the interposer substrate  2 , the microvias  3 , which have been introduced in a manner formed in the pitch of a ball grid array of a BGA housing by means of plasma etching or mechanical processing, are filled with an electrically conductive material, as shown in  FIG. 2 . In one embodiment, the diameter of the microvias  3  is approximately 25 μm. The filling with the electrically conductive material results in connection elements  5  that are electrically and mechanically connected to the rewiring  1 .  
      Referring now to  FIG. 3 , a chip  6  can now be mounted onto the now completed interconnection element  4  by means of chip bonding, for example with the interposition of a tape  7 . The chip  6  is electrically connected to the rewiring  1  of the interposer substrate  2  by means of wire bridges  8  using the customary wire bonding technology. For the sake of simplicity, the actual connections are not shown. Finally, the FBGA component can then be completed by closing off the bonding channel with a potting composition  9  and subsequently molding with a molding cap  10 . The potting composition  9  can be situated to cover the wire bridges  8  regardless of which surface of the substrate  2  they are bonded to. The rewiring  1  is not shown in  FIG. 3 , but it is understood that it will remain as necessary to form the proper electrical connections.  
      In a further etching operation, preferably a plasma etching operation, which may be effected before or after the chip mounting/contact connection, a part of the material of the interposer substrate  2  is areally removed, so that the connection elements  5  project from the surface of the interposer substrate  2  in a matrix-type terminal grid, as shown in  FIG. 4 . Preferably, a uniform thickness of substrate material is removed from the surface of substrate  2 . In one embodiment, the potting composition  9  protects the portions of substrate  2  adjacent the wire bridges  8 .  
      In order to achieve a high contact stability of the connection elements  5 , it is expedient to perform a surface refinement thereof. This may be affected, for example, by means of a nickel/gold coating.  
      In a particular refinement of the invention, the filling of the microvias  3  is dispensed with and the latter are metalized only on the hole walls. This may be affected by means of an electrolytic direct metalization.  
      In this way, the connection elements  5  can be configured as hollow cylinders, which leads to a significantly higher thermal/mechanical stability than when using solder balls.  
       FIG. 5  shows an FBGA module  11  equipped with an interconnection element  5  according to the invention after the module has been mounted on a conventional printed circuit board  12  by means of soldering. In this case, the connection elements  5  of the interconnection element  4  are electrically and mechanically connected to the contact islands of the conductive tracks of the printed circuit board  12  by means of soldering connections  13 .  
      The invention revolutionizes the mounting technology of BGA modules because the solder balls (microballs) used heretofore have thus become completely superfluous and have been replaced by connection elements  4  in the form of contact pins or hollow cylinders, which can be produced particularly simply.  
      In particular, the hollow-cylindrical connection elements  4  guarantee a substantially better electrical connection and durability compared with the solder balls used heretofore. Moreover, the complicated step of ball placement, i.e., the fixing of the solder balls on the interposer substrate  2 , is obviated.  
      The term FBGA module or the like used hitherto in the description correspond to the previous practice for the designation of modules in which solder balls (microballs) are used for the electrical contact-connection. The invention renders the solder balls superfluous and replaces them by pins or hollow cylinders, so that the designations used hitherto for such modules are no longer completely accurate.