Abstract:
A method and structure for controlling solder spread in a predefined/designed area during flip chip assembly build is disclosed. Using conventional processes used in the art blind holes or dimples are incorporated onto the lead frame which then act as containers or wells trapping the solder and thereby preventing it from spreading wider.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a high density semiconductor flip chip memory package and more specifically to the fabrication of a lead frame assembly in which a method for solder spread and solder bump thickness control is disclosed.  
         BACKGROUND OF THE INVENTION  
         [0002]    As current and future microelectronic packaging requirements trend towards the application of large size dies with high-density bumps, and the demand for denser, lighter, smaller, thinner and faster electronic products better control of solder spread during reflow, as well as, thicker solder thickness has become imperative. The present invention provides a method for fabricating a high density fine-pitch lead frame flip chip assembly using a dimple feature build onto the lead frame.  
           [0003]    U.S. Pat. No. 6,386,436 to Hembree describes a method of forming solder bump interconnections for flip chip assemblies.  
           [0004]    U.S. Pat. No. 6,386,433 to Razon et al. discloses a solder ball delivery and reflow method and apparatus.  
           [0005]    U.S. Pat. No. 6,056,191 to Brouillette et al. shows a method and apparatus for forming solder bumps.  
           [0006]    U.S. Pat. No. 6,045,032 to Longgood et al. describes a method of preventing solder reflow of electrical components during wave soldering.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, it is an object of the present invention to provide a lead frame structure and method for the fabrication of said structure to control the solder spread during the reflow process in the construction of a fine-pitch microelectronic flip chip package.  
           [0008]    It is another object of the invention to provide a means for better control solder spread.  
           [0009]    In yet another objective of the present invention is to assure thicker solder thickness after reflow with improved reliability performance.  
           [0010]    In order to accomplish these and other objectives of the invention, a method is provided for constraining the spread of solder by means of a dimple build into the substrate at solder bump locations of a semiconductor IC chip in a lead frame flip chip package.  
           [0011]    In accordance with the present invention the lead frame may comprise any of the following four categories of materials: nickel-iron, clad strip, copper and copper based alloys. The lead frame is personalized using photolithography patterning technology with a dimple built directly into the substrate at the solder bump location.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The foregoing and other objects, aspects and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. The drawings illustrate like reference numerals designating similar or corresponding elements, regions and portions and in which:  
         [0013]    [0013]FIG. 1 is a cross-sectional representation of the invention, showing a lead frame structure without dimple illustrating solder overflow.  
         [0014]    [0014]FIG. 2 is a cross-sectional representation of a preferred embodiment of the present invention showing the lead frame structure with dimple built in acting as a solder well or trap.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    Problems Discovered by the Inventors  
         [0016]    The inventors have discovered the following problems and disadvantages with the current practice:  
         [0017]    1. For large size devices thermomechanical stress buildup leads to fails at the interconnect joint between bump/device resulting from CTE mismatch between chip and substrate which is exacerbated when a means for solder thickness control during reflow is not implemented.  
         [0018]    2. Flip chip structures utilizing spherical and pillar bump interconnects have been limited in meeting fine pitch wiring requirements, because of failure generated on a device from the stress imposed on the silicon by the interconnect solder bumps, leading to the need for solder bump size thickness control and solder spread which if not contained results in reduced stand-off causing underfill reliability issues.  
         [0019]    3. Fine pitch wiring requirements have introduced process complexity, and reduced yields where thicker solder thickness after reflow cannot be efficiently controlled in during the packaging build process.  
         [0020]    4. The process as practiced in the current art using conventional methods lead to limited solder bump thickness which lends to exposure to reduced yields, and short/long term reliability performance issues in large die, high density applications.  
         [0021]    Initial Structure  
         [0022]    Referring to the drawings, and more particularly to FIG. 1, there is shown a cross-sectional view of a lead frame without dimple. Structure  1  is preferably a chip attach substrate and is also understood to possibly include a dam that prevents plastic from rushing out between leads during the molding operation as well as electrical and thermal conductor from chip to board. FIG. 1 may also be a cross-sectional representation of a substrate having a base layer  1  which can be composed of pre-plated palladium, or a material selected from three categories of materials, such as, nickel-iron, clad strip, and copper-based alloys. Furthermore, FIG. 1 illustrates copper terminal pad  2  (of which flip chip reflowed solder balls are subsequently deposited) as well as solder overflow  3  and solder thickness  4 .  
         [0023]    [0023]FIG. 2 is a cross-sectional view of the preferred structure of the invention illustrating lead frame with dimple. In a key feature of the invention FIG. 2 illustrates a dimple  4  which acts as a trap or well/blind hole built onto the lead frame  1 . Thus, showing a means for controlling solder spread from predefined as designed area during the flip chip assembly process.  
         [0024]    Key Steps of the Invention  
         [0025]    The process of the invention may be best understood with respect to FIG. 2. FIG. 2 is comprised of a lead frame metal rolled strip stock substrate  1  typically of 0.20 mm strip thickness on which patterned layers are formed by chemical milling using photolithography and metal dissolving chemicals are used to etch a pattern in the metal substrate. Lead frame substrates may also be fabricated by a stamping process in which metal is mechanically removed from the strip stock using tungsten carbide progressive dies. The lead frame metal substrate  1  is either bare (not plated) or pre-plated with palladium and chromium/copper (Cr/Cu) or titanium/copper (Ti/Cu) conductors are patterned for by standard deposition methods, for example, by a combination of plating and as practiced in the art sputtering and conventional photolithography methods. A prebuilt dimple or well solder trap  4  is selectively patterned at the location of the solder bumps etched on the substrate followed by deposition of flip chip solder bumps  3 . The dimple  4  opening diameter and depth is dependent on the diameter of the solder ball. Typically, solder ball diameter ranges from 100 um to 300 um. Next chip attach to the copper terminal pad  2  of the device using the flip chip solder bump  3  interconnect reflow process is performed. In the final step the total assembly comprising, chip electrical interconnects and lead frame is covered with a polymeric encapsulant. The advantages of the invention is described below.  
         [0026]    Advantages of the Present Invention  
         [0027]    The advantages of the present invention include:  
         [0028]    1. Allows for control of solder spread during reflow.  
         [0029]    2. Provides for achievement of enhanced solder thickness control after reflow and reduction of thermal and mechanical stress under solder bumps resulting in improved assembly process yields.  
         [0030]    3. Demonstrates improved reliability performance with solder thickness control.  
         [0031]    4. Formation of a robust flip chip structure/package meeting the requirements of fine pitch, high pin count and large size high density devices.  
         [0032]    While the present invention has been described and illustrated with respect to preferred embodiments, it is not intended to limit the invention, except as defined by the following claims. Furthermore, numerous modifications, changes, and improvements will occur to those skilled in the art without departing from the spirit and scope of the invention.