Abstract:
A semiconductor package includes a semiconductor die attached to a support having electrically conductive paths, the semiconductor die having a bond-pad electrically connected to the electrically a conductive path on the support by a bond-wire of a first metallic composition, the bond-wire and the bond-pad being coated with a protection layer of a second metallic composition.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of PCT patent application number PCT/IB2009/053463, filed on Mar. 31, 2009, entitled “Semiconductor Package,” which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor packages, in particular to those using wire-bonding. 
     2. Background of the Invention 
     The technology of wire-bonding has been used for a long time in the manufacture of semiconductor packages. 
       FIG. 1  represents a cross-section of part of a semiconductor package  1 . The semiconductor package has a support or substrate  2  which has on an upper surface attached a semiconductor die  3 . On a top surface of the semiconductor die  3 , there is arranged a pad  4 , known as a bond-pad and of conductive material. Common materials for bond-pads  4  are copper and aluminum. A part of the upper surface of the bond-pad is exposed and this area is delimited by an opening in a passivation layer  5  covering most of the top surface of the semiconductor die. 
     There is a metal pad  6 , commonly called a bond-finger, arranged on the top surface of the substrate. 
     A metal wire  7 , called a bond-wire, connects the bond pad  4  and the bond finger  6 . At the end of the bond-wire  7  connected to the bond-pad  4 , there is a structure  8  resembling a squashed ball and known as a ball. The method of connecting the bond-wire  7  to the bond-pad  4  involves melting a free end of the wire so that a ball is formed. This ball is then forced down onto the pad while heat is applied. This results in the ball being crushed and a weld being formed at the interface of the ball  8  and the bond pad  4 . 
     The bonding machine (not shown) then forms a loop of the bond-wire  7  down to the bond-finger  6 . The bond-wire  7  is then crushed into the bond-finger  6  and a weld is formed at the interface between bond-wire  7  and bond-finger  6 . The bonding machine then breaks the bond-wire  7  and the resulting structure is known as a stitch  9 . 
     Hitherto the materiel mostly used for the bond-wires  7  is a high purity alloy of gold. This choice was dictated by following reasons. Firstly, gold is a very good electrical conductor which is useful because this allows bond-wires  7  which are relatively fine diameter (of the order of 20μ or less). Secondly, gold is a soft material which makes it well adapted to the techniques described above. Finally, gold suffers from practically no corrosion, making the wires very chemically stable. 
     Unfortunately, however, because gold is a very expensive material there is a strong financial incentive to use a cheaper metal. Therefore, recently, attention has been turned to copper and alloys thereof. 
     It has been possible to accomplish the above described steps using wires based on copper. Copper also fulfils the requirements of electrical conductivity. However copper presents a significant disadvantage in that it does not resist corrosion well. Its oxide is permeable and therefore does not protect the metal from corrosion to the extent that, as long as moisture is available, the corrosion continues. 
     Furthermore, most semiconductor packages are manufactured using a plastic resin which does protect the semiconductor die and the bond-wire from mechanical damage. However the plastic resin is, itself, also permeable to moisture. The result is that over sufficient time, the corrosion of the copper wires will continue until some of them fail and the device is rendered inoperative. It is found that the lifetime of the device may thus be unacceptably shortened. 
     This problem can be mitigated by the use of thicker copper wires but this is inconvenient where the devices have a large number of pins. 
     Attempts have also been made to address this problem by using copper alloy wires which have been previously coated in a protective metal. However, the mechanical action of the bonding process has a tendency to break this protective layer at the points shown by the arrows  7 A in  FIG. 1 . Furthermore, the melting of the end of the wire in order to form the ball has a tendency to consume the coating and leave the ball unprotected. 
     Since the wires have been subjected to mechanical stress, there are residual stresses remaining in the areas indicated by the arrows mentioned previously. These residual stresses can accelerate the corrosion precisely at those areas where the protective coating has been inadvertently removed. 
     It is therefore desirable to provide a means that permits the use of bond-wires of inexpensive materials which do not have intrinsically high corrosion resistance yet will be stable over time. 
     SUMMARY OF THE INVENTION 
     Embodiments described herein address this need by providing a semiconductor package comprising a semiconductor die attached to a support having electrically conductive paths, the said semiconductor die having a bond-pad electrically connected to the electrically conductive path on said support by a bond-wire of a first metallic composition, wherein said bond-wire and said bond-pad are coated with a protection layer of a second metallic composition. 
     According to an embodiment of the present invention, said bond-wire is made of an alloy of copper. 
     According to an embodiment of the present invention, said protection layer is made of an alloy of nickel. 
     According to an embodiment of the present invention, the protection layer also partially covers said electrically conductive path on the support. 
     According to an embodiment of the present invention, said support is a substrate for a ball-grid array or a land-grid array package. 
     According to an embodiment of the present invention, said protective layer on said bond-wire is at least partially contiguous with said protective layer on said bond-pad. 
     It is also provided a process for manufacturing a semiconductor package comprising the steps of: 
     providing a support; 
     providing a semiconductor die having bond pads; 
     then attaching said semiconductor die to a support; 
     then electrically connecting a bonds pad of the semiconductor die to an electrically conductive path by a bond-wire of a first metallic composition; and 
     then electroplating a layer of a second metallic composition onto the bond wire and bond pad. 
     According to an embodiment of the present invention, the step of providing the support involves providing a plurality of supports arranged in a matrix on a larger support, each support having electrically conductive paths, wherein the electrically conductive paths of said plurality of supports are all electrically connected together. 
     According to an embodiment of the present invention, the process further comprises the step of attaching a connector for electroplating to a connection point on said larger support. 
     According to an embodiment of the present invention, the process further comprises the steps of: 
     before the step of electroplating, placing a protective layer on a face of said support not having said semiconductor die attached; and, 
     after the step of electroplating, removing said protective layer. 
     According to an embodiment of the present invention, the process further comprises the steps of: 
     after the step of electroplating, placing a block of molding resin on said larger support such that said semiconductor die and said bond-wire are completely covered; and then 
     sawing said larger support so that said supports are rendered separate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a cross-section view of a semiconductor packages with a wire-bond made using a known technique. 
         FIG. 2  represents a plan view of package substrates in an array. 
         FIG. 3  represents a process flow according to an embodiment. 
         FIG. 4  represents a cross-section view of a semiconductor packages with a wire-bond according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, the same references designate the same elements. The figures are intended for representation only and are not drawn to scale. 
       FIG. 2  represents a plan view of a matrix  10  of the substrates  2 , as they are arranged before the packaging process begins. In the case of a ball or land-grid array package, this matrix  10  is in the form of a substantially rectangular piece of printed circuit board material. Conductive tracks  11  are arranged around each of the individual substrates  2  so that the bond-fingers  6  are all electrically connected together. Furthermore the conductive tracks  11  are connected to contact points  12  which allow electrical connections between the conductive tracks  11  and electrical equipment (not shown). 
       FIG. 3  represents a process flow according to an embodiment in terms of cross-section views of the work in progress. 
     At step  20 , a strip  10  containing substrates  2  is provided with a plurality of semiconductor dice  3 , attached to a top surface. 
     At step  21 , bond-wires  7  are attached to bond-pads  4  on the semiconductor dice  3  and to bond-fingers  6  on the substrates  2  in the strip  10 . A protective layer  211  may be placed on a lower surface of the strip  10  in order to protect exposed electrical contact areas thereon (not shown) from subsequent process steps. 
     At step  22 , suitable electrical cables  221  are attached to the contact points on the strip. Then the strip is placed into a bath  222  for electroplating containing a suitable solution. A layer of a metal is deposited, as shown by the arrows  223 , by electroplating onto all metal surfaces electrically connected to the contact points. Thus the layer of metal envelopes the bond wires and coats the exposed areas of the bond-pads  4  and bond-fingers  6 . 
     One of ordinary skill will be able to choose the aqueous solution to be used and the accompanying conditions, for example in the case of copper bond-wires  7  and nickel coating. 
     The protective layer  221  serves to stop the electroplating coating exposed electrical contact areas on the lower surface. This may be desirable depending on the method that will be used to attach the semiconductor package to the printed circuit board for which it is intended. 
     At step  23 , the strip  10  with its semiconductor dice  3  is removed from the bath  222  and cleaned by suitable means. A block of molding resin  231  is then placed on the top surface of the substrate  10  so as to cover the semiconductor dice  3  and bond-wires  7 . The strip is then cut into individual semiconductor packages  1 . The action of cutting severs the connection between the bond fingers  6  and the conductive tracks  11 . Where the protective layer  211  has been used, it is removed at an appropriate time during this step, the choice of which moment being within the scope of one of ordinary skill. 
     The choice of the alloy for the wire is governed by the ease of bonding onto the particular composition of bond pads  4 . Copper at 99% purity has been shown to give acceptable results for some common structures of the bond pads  4 . 
     The choice of the coating is governed by its own resistance to corrosion, its compatibility with the wire metal, the ease with which it can be successfully electroplated and the overall cost of the process. Also a low permeability to moisture of the resultant electroplated layer is desirable. In the case of wires of 99% copper, either a two layer coating of first titanium and then nickel or a single layer coating of silver may be used. The titanium provides a barrier to diffusion between the copper and nickel and so reduces the risk of excessive dissolution of the nickel by the copper. It also offers a good adhesion to the copper. The nickel provides a good resistance to corrosion. Silver adheres well to copper and has a relatively high resistance to corrosion. It is however a more expensive material. 
     There are however many other possible choices and these are within the realm of one of ordinary skill. 
       FIG. 4  represents a cross section of a semiconductor package with a bond-wire, the whole according to an embodiment. The elements already discussed in relation to  FIG. 1  will not be discussed further. 
     There is present a coating  40  which coats, in a conformal manner all the surfaces of the bond-wire  7 , including the ball  8  and the end of the stitch  9 . The coating  40  also coats the exposed surface of the bond-pad  4  and the exposed area of the bond-finger  6 . 
     As can be seen the metal coating  40  covers the whole structure of the bond-wire  7 , particularly the zones around the ball  8  and the stitch  9 . Thus the wire is protected from subsequent corrosion, even in these areas. The presence of the coating  40  on the bond-pad  4  and the bond-finger  6  should not be of any consequence. It may, depending on the materials used on the surfaces of these, be also beneficial. 
     The foregoing, with its features, aspects and purposes is given by way of illustration and not limitation. Indeed, it is not intended that the embodiments described be considered the only ones concerned by the present invention. 
     Indeed the structures discussed concern ball and land-grid array packages. However this technique could be adapted to other package types such as lead-frame packages, so long as the bond-fingers are all connected together when the lead-frames are in the form of the strip. This is very often the case. 
     Furthermore, copper bond-wires  7  and coating with them with alloys of titanium, nickel or silver have been discussed. However, other materials could be envisaged both for the wires and for the coating material, for example palladium, chromium or tin. Preferable, the material chosen should adhere well to the metal of the bond-wire  7 . Also, preferably, it should resist corrosion well, which can be achieved by it being relatively inert or by it having an adhering oxide of low permeability to moisture. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalent thereto.