Patent Publication Number: US-8120154-B2

Title: Interconnection of lead frame to die utilizing flip chip process

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. patent application Ser. No. 12/172,117, filed Jul. 11, 2008, which is a nonprovisional patent application claiming priority to U.S. Provisional Patent Application No. 61/126,243, filed May 1, 2008 and incorporated by reference herein for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
       FIG. 1  shows a simplified perspective view of a conventional flip chip package  100 .  FIG. 1A  shows a cross-sectional view of the package of  FIG. 1  taken along line  1 A- 1 A′. 
     This conventional flip chip package  100  comprises die  102  enclosed within plastic package body  104 . Electrical contacts  106  on the top surface of die  102  are in electrical communication with the leads  108  projecting from the package body, through electrically conducting solder connections  110 . 
     As indicated in  FIG. 1A , it is required that the leads  108  maintain a minimum height Z over the die. However, as shown  FIG. 1B , during the step of attaching the leads to the die, heating of the solder connection may result in solder reflow, causing the shape of the solder connection to deform and possibly resulting in the height of the lead Z′ being less than the required height Z. 
     Accordingly, there is a need in the art for improved techniques for fabricating a flip chip die which avoids the problems resulting from deformation in a solder contact resulting from reflow. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments in accordance with the present invention relate to techniques which avoid the problems of deformation in the shape of a solder connection in a flip chip package, resulting from solder reflow. In one embodiment, a solder-repellent surface is created adjacent to the solder to constrain the reflow and thereby maintain the vertical profile of the solder. Examples of such a solder-repellent surface include an oxide (such as Brown Oxide) of the lead frame, or a tape (such as Kapton) which is used as a clam bar to control/constrain the solder flow on the leads prior to the encapsulation step. In another embodiment, the solder connection may be formed from at least two components. The first component may reflow at high temperatures to provide the necessary adhesion between solder ball and the die, with the second component reflowing at a lower temperature to provide the necessary adhesion between the solder ball and the leads. An example of such multi-component connections include a first high temperature reflow solder ball paired with a second low temperature reflow solder. Another example includes a solder ball with a hard core (such as Cu, stainless steel, or a plastic material that is stable at high temperatures) that is coated with a lower temperature reflow material. 
     These and other embodiments of the present invention, as well as its features and some potential advantages are described in more detail in conjunction with the text below and attached figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a simplified perspective view of a conventional flip chip package. 
         FIGS. 1A-B  shows simplified perspective views a conventional flip chip package. 
         FIG. 2  shows a simplified shows a simplified cross-sectional view of one embodiment of the present invention, which utilizes a non-wettable oxide surface on a lead finger, to constrain solder reflow. 
         FIG. 3A  shows a plan view of an embodiment of the present invention, which utilizes an adhesive, non-wettable tape material that is stable at high temperatures, on a lead linger, to constrain solder reflow. 
         FIG. 3B  shows a simplified shows a simplified cross-sectional view of the embodiment of the present invention shown in  FIG. 3A . 
         FIG. 3C  shows a plan view of an alternative embodiment of the present invention utilizing tape material to constrain solder reflow. 
         FIG. 4  shows a simplified shows a simplified cross-sectional view of one embodiment of the present invention, which utilizes a solder contact comprising a second material having a reflow temperature lower than a first material. 
         FIG. 5  shows a simplified shows a simplified cross-sectional view of one embodiment of the present invention, which utilizes a solder contact comprising a hard core material having a reflow temperature that is higher than that of an outside coating. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments in accordance with the present invention relate to techniques, employed alone or in combination, which avoid the problems of deformation in the shape of a solder connection in a flip chip package, resulting from solder reflow. 
     In certain embodiments, a solder-repellent surface is formed on the surface of the lead adjacent to the surface expected to be in contact with the solder connection. This solder-repellent surface constrains reflow of the solder and thereby maintains the requisite vertical spacing between the lead and the die. 
       FIG. 2  shows a simplified cross-sectional view of one embodiment of the present invention. In  FIG. 2 , lead  208  is provided having portion  208   a  of its surface oxidized (for example Brown Oxide), and other portions  208   b  substantially free of oxide. Such oxidation of only portions of the lead may be accomplished by masking selected portions, followed by exposure of unmasked portions to an oxidizing ambient. 
     Solder hall  210  is provided attached to pad  206  present on the top surface of the die. As particularly shown in  FIG. 2 , the solder ball is supported on an electroless nickel immersion gold (ENIG) material  220 , that is present within an opening of passivation  222  covering the top surface of the die. Alternatively or in conjunction with ENIG, the solder ball may be supported on a wettable surface provided by sputtering. 
     In the step of attaching the lead  208  to the die via the solder contact, the solder ball is heated to above its reflow temperature. As shown in  FIG. 2 , however, the presence of the oxidized lead portions  208   a  adjacent to the oxide-free lead portion  208   a  in contact with the solder, serve to restrain the flow of heated solder. In particular, the non-wettable character of the oxide inhibits the spread of the molten solder. Thus, while the portions of the solder distal from the lead may bulge, the solder proximate to the lead is constrained from flowing, and thereby allows the solder to substantially maintain its vertical profile. 
     On the other side of the solder contact, the combination of the ENIG material  220  within the opening in the passivation, similarly serves to constrain the flow of molten solder, thereby preserving the vertical profile. 
     It is noted that obtaining a completely oxide-free surface to receive the solder may be difficult to achieve, given the prevalence of oxygen in the environment. Accordingly, prior to bringing the lead into contact with the molten solder, the solder may be dipped into a flux material that serves to remove any native oxide. 
     While the particular embodiment just described utilizes a bare Cu surface to receive the solder, this is not required by the present invention. In accordance with alternative embodiments, the surface that is configured to receive the solder may comprise a plated metal such as silver, or a stack of plated stack of metal such as Ni/Au or Ni/Pd/Au. In such embodiments, the wettability of the plated metal/metal stack may allow only a native oxide layer (rather than an intentionally grown Brown Oxide layer) to contain the flow of solder in the desired manner. 
     And while the particular embodiment just described utilizes a solder-repellent, non-wettable surface in the form of an oxide, this is not required by the present invention. In accordance with alternative embodiments, other materials could be employed to limit the spread of solder and resulting deformation in the shape of the solder contact. For example in alternative embodiments portions of the lead could be coated with a non-wettable polymer material configured to withstand temperatures in excess of the solder reflow temperature. 
     In yet another embodiment, the flow of molten solder could be constrained by the presence of a tape. For example, during fabrication of a flip chip package, it may be necessary to maintain the orientation and spacing of a number of leads to allow those leads to match with the corresponding locations of contacts on the top of the die. In order to ensure this lead orientation and spacing is maintained, the leads may be secured to an adhesive tape such as Kapton. This is shown in the underside view of  FIG. 3A , wherein the bottoms of a plurality of leads  308  are maintained in place by tape  330 . 
     In accordance with an embodiment of the present invention, this tape  330  may be utilized to constrain the reflow of solder.  FIG. 3B  shows a simplified cross-sectional view, wherein tape  330  is present proximate to the edge of lead  308 , leaving an end portion  308   a  of the lead  308  uncovered. 
     Solder ball  310  is provided attached to pad  306  present on the top surface of the die. As particularly shown in  FIG. 3 , the solder ball is supported on ENIG material  320  present within an opening or passivation  322  covering the top surface of the die. Alternatively or in conjunction with ENIG, the solder ball may be supported on a wettable surface provided by sputtering. 
     In the step of attaching the lead  308  to the die via the solder contact, the solder ball is heated to above its reflow temperature. As shown in  FIG. 3B , however, the presence of tape  330  adjacent to lead portion  308   a  in contact with the solder, restrains the flow of heated solder. In particular, the non-wettable character of the tape inhibits the spread of the molten solder. In addition, surfaces of the lead such as the vertical edge of the lead, may also bear a non-wettable oxide that further inhibits the flowing of the molten solder. 
     Thus as shown in  FIG. 3B , while the portions of the solder distal from the lead may bulge, the solder proximate to the lead is constrained from flowing, and thereby allows the solder to substantially maintain its vertical profile. 
     While the above embodiment illustrates an arrangement of a tape in continuous rectangle configuration, this is not required by the present invention. In accordance with alternative embodiments, strips of tape could be used as shown in  FIG. 3C . Further alternatively, tape in the form of adjacent pairs of strips or pairs of rectangles could be utilized to constrain the reflow of solder between them. 
     In accordance with certain embodiments, the thickness of the tape may act a physical impediment to the flow of solder. Moreover, in accordance with certain embodiments, use of tape of a predetermined thickness may serve to maintain the desired spacing between the lead and the die. In such an embodiment, the tape serves not only as a non-wettable surface, but its bulk physically supports the lead over the die. 
     Moreover, while the above embodiment is described in conjunction with the use of a Kapton tape, this is also not required by the present invention. A variety of tape materials exhibiting stability at high temperatures and the desired adhesive and non-wettable property, could be employed to constrain molten solder flow. Any organic material that can withstand the high reflow temperature, with pressure sensitive adhesive on one side or the film to adhere/attach to the leads, can be used. 
     While the above embodiment illustrates an arrangement of a tape in rectangle, this is not required by the present invention. In accordance with alternative embodiments, strips of tape could be used as shown in  FIG. 3C . 
     In accordance with alternative embodiments, a vertical profile of a solder connection may be maintained despite reflow, utilizing a solder connection comprising at least two components. Specifically, a first component of the solder may reflow at a higher temperatures to provide the necessary adhesion to the die, while remaining as a solid at a lower reflow temperature to provide the desired vertical spacing. The second component of the solder may reflow at the second, lower temperature, to provide adhesion between the first solder component and the lead finger. 
       FIG. 4  shows a simplified cross-sectional view of one example of such an embodiment in accordance with the present invention. In particular, solder ball  410  is provided attached to pad  406  present on the top surface of the die  402 . As particularly shown in  FIG. 4 , the solder ball is supported on ENIG material  420 , that is present within an opening of passivation  422  covering the top surface of the die. Alternatively or in conjunction with ENIG, the solder ball may be supported on a wettable surface provided by sputtering. 
       FIG. 4  shows that lead  408  is provided with an electrically conducting adhesive material  440  exhibiting a lower reflow temperature than the material of the solder ball  410 . Upon heating, material  440  reflows, allowing adhesive attachment with the solder ball  410 . The material comprising solder ball  410 , however, remains solid at this temperature, and its bulk ensures that the desired height Z between the lead and the die, is maintained. 
     The following TABLE provides a listing of electrically conducting adhesive materials which can be used according to embodiments of the present invention together with their respective reflow temperatures: 
     
       
         
           
               
               
               
             
               
                 TABLE 
               
               
                   
               
               
                   
                   
                 REFLOW 
               
               
                   
                   
                 TEMPERATURE 
               
               
                 MATERIAL 
                 COMPOSITION (%) 
                 (° C.) 
               
               
                   
               
             
            
               
                 Solder SAC 101 
                 Sn 98.9, Ag 1.0, Cu 0.1 
                 217 
               
               
                 Solder SAC 105 
                 Sn 98.5, Ag 1.0, Cu 0.5 
                 217 
               
               
                 Solder SAC 405 
                 Sn 95.5, Ag 4.0, Cu 0.5 
                 217 
               
               
                   
                 Sn 96.5/Ag 3.5 
                 221 
               
               
                 lead-based solder 
                 Sn 10/Pb 90, 
                 275 
               
               
                   
                 Sn 62/Pb 36/Ag 2, 
                 179 
               
               
                   
                 Sn 63/Pb 37 
                 183 
               
               
                   
               
            
           
         
       
     
     During operation of the die, a certain amount of heat would be expected to be generated. Accordingly, a requirement of the solder materials used in a package according to an embodiment of the present invention, is that they remain solid at such expected operating temperatures. 
     The solder contact serves not only to allow for electrical communication between the die and the lead finger, but also desirably serves as a conduit for thermal energy generated by the die, to be transported outside the package through the lead finger for dissipation in the environment. Accordingly, the various materials comprising the solder contact should exhibit a thermal conductivity necessary to allow the requisite flow of heat from the die. 
     Another example of an approach in accordance with an embodiment of the present invention involves the use of a solder contact comprising a solder ball having a hard core made from a first material having a relatively high reflow temperature, and having a coating made from a material having a lower reflow temperature. 
       FIG. 5  illustrates a simplified cross-sectional view of such an embodiment. In particular, solder contact  510  comprising an outer material  510   a  (coating) and an inner material (hard core)  510   b , is provided attached to pad  506  present on the top surface of the die  502 . The solder contact is provided with the die  502 , supported on ENIG material  520 , that is present within an opening of passivation  522  covering the top surface of the die. Alternatively or in conjunction with ENIG, the solder ball may be supported on a wettable surface provided by sputtering. 
       FIG. 5  shows that upon heating of the solder contact to a reflow temperature of the outer coating material  510   a , this material reflows, allowing adhesion to the lead finger  508 . The core material  510   b  comprising the core of the solder contact, remains solid at this temperature. In an example, the core of the solder contact may comprise a solder ball, and the coating may comprise an electrically conducting material that reflows at a temperature lower than the melting point of the solder ball. The presence of this solid core maintains the spacing Z between the lead and the die. Examples of electrically and thermally conducting materials exhibiting relatively high reflow temperatures which could be used as the core material for a solder contact, include but are not limited to copper, stainless steel, and plastic materials exhibiting a sufficiently high reflow temperature. 
     While not specifically mentioned above, in certain embodiments more than one of the techniques specifically described may be employed in combination. For example, in accordance with particular embodiments the vertical profile offered by a solder contact may be substantially maintained above a reflow temperature utilizing both a solder-repellent surface (such as oxide or tape), and a solder contact structure having a first component exhibiting lower reflow temperature. 
     While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims.