Abstract:
A process for fabricating a BGA flip chip package containing a stiffener or heat spreader monitors edges of the adhesive that attaches the stiffener or heat spreader. The monitoring ensures that the adhesive extends beyond the centers of the outermost solder balls in the BGA. Stress at the edge of the adhesive thus does not cause warping or variations within the BGA.

Description:
BACKGROUND OF THE INVENTION 
     Warping is a significant problem for flip chip ball grid array (BGA) packages. Generally, warping reduces the planarity of the package and makes external and internal connections of the package more difficult and less dependable. In particular, the stress that causes the warping typically varies with the temperature so that thermal cycling of a package can make connections unreliable in the area of the warping. 
     A common operation in the manufacture of a flip chip BGA package is the attachment of a stiffener or heat spreader around the perimeter of the substrate in the package. The same structure typically acts as both a stiffener and a heat spreader to improve the mechanical and thermal performance of the package. To simplify terminology, the term stiffener is used herein to designate such a structure whether the primary purpose of the structure is to improve mechanical or thermal performance of a package. 
     A typical process for attaching a stiffener applies a thermally activated adhesive to the substrate and/or the stiffener, places the stiffener and substrate in contact with the adhesive sandwiched between the stiffener and substrate, and applies pressure and heat. Control or monitoring of the process ensures that the resulting product is flat and uniform in appearance and/or ensures that the separation between the substrate and stiffener is uniform. 
     Adhesive generally must be kept from escaping or protruding from the edge of the package to avoid changing the package outline from the required design standard. A key control of the attachment process keeps the adhesive well back from the edge of the package to prevent the escape of adhesive. However, edge effects of the adhesive can cause stress that warps the substrate and can affect the outer solder balls in the BGA. Accordingly, methods for manufacturing a flip chip BGA package and attaching a stiffener without creating stress and warping that affects the BGA are desired. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the invention, an attachment process controls the minimum extent of the adhesive that attaches a stiffener to a substrate in a BGA flip-chip package. In particular, the edge of the adhesive is controlled to be beyond the centers of the outermost balls in the ball grid array. Accordingly, edge effects that may warp the substrate at the edge of the adhesive do not affect the ball grid array. 
     One specific embodiment of the invention is a process for fabricating an integrated circuit package. The process includes: creating a composite structure including the substrate, an adhesive layer, and a stiffener; and controlling an edge of the adhesive layer to ensure that a distance from an edge of the integrated circuit package to the edge of the adhesive is less than a distance from the edge of the integrated circuit package to a center of an outermost solder ball on the substrate. The stiffener can change the tensile characteristics of the package and/or act as a heat spreader to improve the thermal characteristics of the package. Conventional techniques for controlling the amount and placement of the adhesive between the stiffener and the substrate and the pressure applied to the composite structure to force the adhesive toward the edge of the package can place the edge of the adhesive outside the outermost solder balls in the ball grid array. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a cross-sectional view of a portion of a BGA flip-chip package in accordance with an embodiment of the invention. 
     FIG. 2 is a flow diagram of a process for attaching a stiffener for a BGA flip-chip package in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     A manufacturing process for a ball grid array (BGA) flip-chip package controls the minimum extent of adhesive that attaches a stiffener to a substrate in the package. Prior packaging processes, which have not controlled minimum extent of the adhesive, have allowed the fillet of adhesive to creep inside a boundary corresponding to the centers of outer solder balls in the BGA. If the fillet of adhesive is within that boundary, stress at the edge of the adhesive can warp a portion of the substrate on which the outer solder balls reside. The warping primarily results from the turning moment of the substrate at the edge of the adhesive and the shrinkage of the adhesive during curing. Accordingly, the stiffener does not ensure uniform performance inside the final BGA. 
     The action of laminating the stiffener to the substrate creates a modified tensile characteristic or elasticity of the substrate in the substrate-adhesive-stiffener sandwich. For optimum performance, variations in the elasticity or stiffness inside any BGA pitch must be avoided. An underfill material maintains uniform performance for the portion of the substrate near the chip and an inner boundary of the stiffener, but at the outer edge, variations in the extent of the adhesive can cause variation in performance. In accordance with an aspect of the invention, a fabrication process controls the adhesive so that the variations are outside the ball grid array. 
     FIG. 1 shows a cross-sectional view of a BGA flip-chip package  100  having an adhesive fillet  160  controlled in accordance with an embodiment of the invention. Package  100  includes an integrated circuit chip  110  having an array of contacts or solder balls  120  attached to a substrate  130 . Chip  110  can generally be any type of semiconductor device but generally is an integrated circuit having a large array of contacts to take advantage of the large number of electrical connections that BGA packages provide. 
     Substrate  130  is typically made of circuit board material such as polyimide, polyimide alloy, non alloy general polymer such as a BT epoxy and metal composites or ceramic, or metal composites and glass or similar material, and substrate  130  typically has a thickness of 0.5 mm or greater in current flip-chip BGA packages. Metal traces on and through substrate  130  electrically connect solder balls  120  to solder balls  140 . 
     Solder balls  140  form a BGA on the under side of substrate  130 . The BGA serves to electrically connect chip  110  to an external circuit (not shown) in conventional manner well known in the art. In the cross-section illustrated in FIG. 1, the center of an outermost ball  140  is a distance A, typically about 1.0 mm, from the edge of package  100 . 
     Adhesive fillet  160  is on a top surface of substrate  130  and attaches substrate  140  to stiffener  150 . Stiffener  150  is a one-piece structure with a central opening in which chip  110  resides when stiffener  150  is attached to substrate  130 . Stiffener  150  serves to improve the mechanical or thermal performance of package  100 . In a typical BGA flip-chip package, stiffener  150  is made of a material such as filled organic resin or epoxy, a polymer which is filled with property modifying materials such as quartz, ceramic, metal spheres or fibers or pieces, modified silicones, a variety of metals or metal alloys and is typically about 0.5 mm thick or greater. 
     Adhesive fillet  160  can be made of a material such as filled organic resin or epoxy, polyimide, polymer with property modifying materials added, a dispensed organic isolative material, modified silicone, thermo setting organic mold compound, or thermo plastic organic mold compound and has a typical thickness of less than 2.0 mil. The outer edge of adhesive fillet  160  is a distance X, typically less than 0.5 mm, from the outer edge of package  100 . In accordance with an aspect of the invention, manufacture of package  100  is controlled so that distance X is greater than or equal to zero and less than distance A. Accordingly, the edge of adhesive  160  is outside the perimeter of the BGA, and any stress at the edge of adhesive  160  does not affect the BGA. Additionally, the tensile and elasticity characteristics of the BGA and package  100  are uniform across the entire area of the BGA including the outermost solder balls  140  in the BGA. 
     FIG. 2 is a flow diagram of a process  200  for forming flip-chip package  100 . Process  200  begins with a step  210  of attaching chip  110  to substrate  130  using conventional techniques that are well known in the art and not critical to the invention. An adhesive such as one of those previously described is applied to substrate  130  and/or stiffener  150  in step  220 . The amount and placement of the uncured adhesive when applied is controlled according to the target position for the outer edge of adhesive  160 . 
     Once the uncured adhesive is applied, stiffener  150  is placed on substrate  130  in step  230 , and step  240  applies heat and pressure to stiffener  150  and substrate  230  to cure adhesive  160 . The heat and pressure force adhesive  160  to its furthest extent, and step  240  may include monitoring the location of the edge of adhesive  160  to ensure distance X from the edge of adhesive  160  to the edge of package  100  is in the desired range (0&lt;X&lt;A). More particularly, if monitoring indicates the edge is further than distance A the pressure can be increased to force adhesive  160  closer to the edge of package  100 . Monitoring  240  preferably checks the location of the edge of adhesive  160  around the entire perimeter of package  100 . 
     After adhesive  160  has the desired edge position and has cured, packaging process step  250  finishes flip-chip package  100 . Package process step  250  can include conventional processing techniques known in the art. However, in accordance with an aspect of the invention, an underfill  170  can be added to fill a volume under chip  110  and between chip  110  and stiffener 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.