Abstract:
A flip-chip packaging method for a semiconductor device treats a portion of an interconnect substrate so that a fill material when liquid beads on the treated portion of the interconnect substrate. When the fill material is dispensed on the interconnect substrate to fill a gap under the semiconductor device, the beading of the fill material prevents formation of fillets that might otherwise create a variation in the thermal coefficient of expansion of fill material and/or warp the interconnect substrate. The treated portion of the interconnect substrate can be roughened or coated with a material that differs from other portions of the interconnect substrate and thereby causes beading.

Description:
BACKGROUND OF INVENTION 
     Flip-chip packaging can generally provide a small footprint package with a large number of electric connections to an integrated circuit die. 
     FIG. 1A illustrates a packaged device  100  using flip-chip packaging of an integrated circuit die  110 . Die  110  is an integrated circuit chip formed from a semiconductor wafer and having solder bumps  115  on an active surface. Solder bumps  115  are electrically connected to circuit elements formed in and on die  110 . In packaged device  100 , die  110  is flipped so that bumps  115  contact a substrate  130 . 
     Substrate  130  is typically a printed circuit made of a material such as polyimide, polyimide alloy or compound or non alloy general polymer and metal composites; ceramic, silicon, or glass and metal composites; or similar materials forming a flexible or rigid carrier having conductive traces (not shown), which are generally made of copper or another metal. Solder bumps  115  on die  110  contact the conductive traces on the top surface of substrate  130 , and the conductive traces, which extend through substrate  130 , electrically connect solder bumps  115  to solder balls  135  on the bottom surface of substrate  130 . Solder balls  135 , which can be arranged in a ball grid array, form the terminals of packaged device  100  and can be attached to a printed circuit board or other circuitry in a product containing packaged device  100 . 
     One concern in flip-chip packages is the difference between the coefficients of thermal expansion of semiconductor die  110  and substrate  130 . This difference creates mechanical displacement stress on the connections between die  110  and substrate  130 . In packaged device  100 , underfill  120  between die  110  and substrate  130  strengthens the attachment of die  110  to substrate  130  to help prevent the thermal stresses from breaking the connections between die  110  and substrate  130 . 
     FIG. 1B illustrates an edge of underfill  120 . Underfill  120  contains filler particles  122  suspended in an organic resin  124 . Filler particles  122  generally have a size selected according to a gap between die  110  and substrate  130 , e.g., the filler particles have a diameter about one third the size of the gap. Generally, the composition and concentration of filler particles  122  are selected to control the coefficient of thermal expansion and the shrinkage of underfill  120 . 
     Organic resin  124  that when initially applied in device  100  is a liquid that flows into the gap between die  110  and substrate  130 . Accordingly, the edge of underfill  120  has a concave shape that depends on the viscosity of liquid organic resin  124  and the adhesion of organic resin  124  to die  110  and substrate  130 . Organic resin  124  subsequently cures, and the presence of filler particles  122  helps control the shrinkage that occurs in underfill  120  during curing. 
     As shown in FIG. 1B, the distribution of filler particles  122  is relatively uniform where underfill  120  is significantly thicker than the diameter of filler particles  122 . However, in fillet regions  126  and  128  where the thickness of underfill  120  approaches or is less than the diameter of a filler particle, the density of filler particles  122  falls or is reduced. The lack of filler particles  122  causes more shrinkage in fillet regions  126  and  128  during curing. This shrinkage can warp substrate  130  and disrupt electrical connections between substrate  130  and die  110  or between substrate  130  and an external circuit. In particular, shrinkage and surface tension in underfill  120  causes stress S on substrate  130  near the edge of die  110 . This stress S is along a direction that depends on the wetting angle α of underfill  120  at the edge of die  110 . 
     The lack of filler particles  122  in region  126  also makes the coefficient of thermal expansion of in regions  126  and  128  differ from the coefficient of thermal expansion in thicker regions of underfill  120 . Accordingly, temperature changes can induce further stress in fillet regions  126  and  128  if the composition of underfill  120  is selected to minimize stress created by thermal expansion in thick regions of underfill  120 . 
     To improve reliability and yield of good packages, methods and structures are sought that avoid increased shrinkage, stress that warps the substrate, and/or change in coefficient of thermal expansion that occurs at the edges of the underfill. 
     SUMMARY OF INVENTION 
     In accordance with an aspect of the invention, a dam, barrier, or other damming feature or discontinuity changes the shape or accumulation of the underfill material to reduce stress resulting from edge effects. In particular, the dam controls the wetting angle of the underfill material to provide a much smaller stress component perpendicular to the surface of the underlying substrate, and the underfill as shaped by the dam lacks underfill fillet regions that shrink significantly and cause stress on the substrate. The dammed underfill additionally avoids or reduces the size of areas having low filler particle concentration and thus avoids thermal coefficients of expansion that differ from the optimal coefficients. The resulting package has superior performance as defined by co-planarity, reliability, and mechanical improvement when compared to conventional overall flip-chip packages. 
     One specific embodiment of the invention is a packaged device that includes a substrate, a die, and a dam. The die has contacts placed as in a conventional flip-chip package so that the contacts electrically contact conductive traces of the substrate. The dam attaches to the substrate and surrounds the die to confine the edges of underfill that fills a gap between the die and the substrate. The dam controls the shape of the underfill so that wetting angles at the die and at the dam are less than 45° or so that the underfill lacks fillet regions. 
     Generally, the device has a ball grid array on a side of the substrate opposite to the die. In an exemplary embodiment, the ball grid array has a pitch that is less than or about equal to one half a separation between the dam and an edge of the die. The width of the dam is typically between one and two times the pitch of the ball grid array, and the height of the dam is chosen to provide a wetting angle for the underfill that avoids stress on the substrate or areas of underfill having a low filler concentration. 
     Another embodiment of the invention is a method for packaging an integrated circuit die. The method includes: attaching the die to a substrate so that conductive traces on the substrate electrically contact contacts on the die; forming a dam on the substrate around the die; and filling a volume between the die and the substrate and between the die and the dam with an underfill material. The dam can be constructed before applying the underfill by placing, depositing, growing, or otherwise accumulating material on the substrate to form the dam. Alternatively, the dam can be preformed to the desired shape and attached to the substrate. The underfill is applied after the dam is in place so that the dam controls the shape and location of the edge of the underfill. Suitable materials for such dams include but are not limited to a material such as a metal layer or feature and a polymer which is filled with property modifying materials such as spheres, fibers or pieces of quartz, ceramic, or metal. 
     In an alternative embodiment, removing material from the substrate (e.g., by machining or etching) before a die is attached can leave a dam surrounding a die attachment area on the substrate. 
     In yet another alternative embodiment, treatment of the substrate increases adhesion or stiction between the underfill and the substrate in specific areas on the substrate. The underfill accumulates and can be shaped and cured to form the dam in the treated area of the substrate. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIGS. 1A and 1B show cross-sectional views of a conventional flip-chip packaged device. 
     FIGS. 2A and 2B show cross-sectional views of a flip-chip packaged device in accordance with an embodiment of the invention that has an underfill dam surrounding a die on a substrate. 
     FIG. 3 shows a cross-sectional view of a flip-chip packaged device in accordance with an embodiment of the invention in which a substrate includes a treated region on which an underfill forms a dam. 
     FIG. 4 shows a cross-sectional view of a flip-chip packaged device in accordance with an embodiment of the invention in which a substrate includes a depression for a die and has a dam surrounding the die. 
     Use of the same reference symbols in different figures indicates similar or identical items. 
    
    
     DETAILED DESCRIPTION 
     In accordance with an aspect of the invention, a flip-chip package uses a dam surrounding a die to eliminate underfill fillets and control of underfill wetting angles. The dam thereby reduces warping of a substrate in the flip-chip packaged device. 
     FIG. 2A illustrates a flip-chip packaged device  200  in accordance with an embodiment of the invention. Packaged device  200  contains an integrated circuit die  110  having contacts  115  connected to conductive traces (not shown) in and on a substrate  130 . The conductive traces connect contacts  115  to external terminals (solder balls)  135  on substrate  130 . Die  110 , contacts  115 , substrate  130 , and external terminals  135  are generally conventional structures such as well known in the art and described above. 
     In accordance with an aspect of the invention, flip-chip packaged device  200  includes a dam  240  that surrounds die  110  and controls the shape of the edge of an underfill  220 . Dam  240  can be formed of a variety of materials including but not limited to a dispensed organic isolative material such as benzotriazole (BT) or modified silicone, a thermo setting mold compound such as epoxy creasol novolac (ECN) or a modified BT, or a thermo plastic compound such as polyethyl sulfone (PES) polycarbonate or polysulfone, that is deposited and formed into the desired shape on substrate  130 . 
     Dam  240  can be formed on or attached to substrate  230  using a variety of techniques. For example, suitable dam forming techniques include but are not limited to liquid dispense methods, injection transfer molding, and thermocompression transfer molding. Alternatively, dam  240  can be a preformed organic or metallic structure that is formed into the desired shape and then attached to substrate  240  by gluing, staking, or riveting. In one particular embodiment, dam  240  doubles as a stiffener or heat spreader that attaches to substrate  130  to improve the mechanical or thermal properties of packaged device  200 . Co-filed patent application Ser. No. 09/683,304, entitled “Adhesive Control During Stiffener Attachment To Provide Co-Planarity In Flip Chip Packages”, further describes attachment of a stiffener and is hereby incorporated by reference in its entirety. 
     After formation or attachment of dam  240  on substrate  130 , a measured amount of underfill is applied to flow under die  110  and fill a volume that dam  240  defines. Ideally, the volume of underfill and the height H, width W, and shape of dam  240  and the separation D between dam  240  and die  110  are set according to the natural flow of the organic underfill  220  and the cure schedule during fabrication of device  200 . In particular, the volume of underfill and dam&#39;s height H, width W, and separation D should provide total filling of the volume under die  110 , and the shape of underfill  220  in the area in and around the periphery of die  110  should lack fillet regions or steep wetting angles. In particular, to prevent the creation of stress concentrations, the height H of dam  240  is selected to prevent the creation of any sharp angles or areas of high shrinkage such as those resulting from the formation of underfill fillets. 
     Generally, the volume of underfill and the height and shape of dam  240  should be selected to ensure that a wetting angle α′ of underfill  220  is less than 45° (maximum) from the top surface of die  110  as shown in FIG.  2 B. The underfill wetting angle α″ to dam  240  should also be less than 45°, thereby ensuring a complete and balanced stress spreader of underfill. Additionally, each region of underfill  220  should be thick enough to ensure minimum shrinkage and maximum retention of the bulk fill allowing creation of the best case material performance and easiest methodology of underfill process across the space between the edge of die  110  and dam  240 . 
     The shape for dam  240  will depend on the particular underfill used since commercially available underfills have different flows, viscosities, and curing schedules. However, for any particular underfill, empirical or analytic techniques can find a height and width of dam  240  that provides the desired performance. Table 1 illustrates some exemplary dam and fill configurations and the wetting angles achieved. Each of the examples of Table 1 uses 82 mg of underfill and 50 mg of dam material. A die back temperature of 90° is used with Hysol 4549 as underfill, and a die back temperature of is 130° is used with Namics 8444-3 or CRP4152R-2 as underfill. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Dam &amp; Fill Structure and Underfill Wetting Angle 
               
             
          
           
               
                 Dis- 
                   
                   
                   
                   
                 Dam 
                   
               
               
                 pen- 
                   
                   
                   
                   
                 to Die 
               
               
                 sing 
                   
                   
                 Dam 
                 Dam 
                 Dis- 
                 Wet- 
               
               
                 Gap 
                 Underfill 
                 Dam 
                 Height 
                 Width 
                 tance 
                 ting 
               
               
                 (mil) 
                 Material 
                 Material 
                 (mm) 
                 (mm) 
                 (mm) 
                 Angle  
               
               
                   
               
             
          
           
               
                 17 
                 Hysol 
                 Hysol 
                 0.26899 
                 1.00601 
                 1.53801 
                 32° 
               
               
                   
                 4549 
                 4155 
                   
                   
                   
                   
               
               
                 14 
                 Hysol 
                 Hysol 
                 0.28899 
                 0.94801 
                 1.57101 
                 30° 
               
               
                   
                 4549 
                 4155 
                   
                   
                   
                   
               
               
                 10 
                 Hysol 
                 Hysol 
                 0.27699 
                 1.12001 
                 1.61701 
                 29° 
               
               
                   
                 4549 
                 4155 
                   
                   
                   
                   
               
               
                 17 
                 Namics 
                 CRP3600H 
                 0.36598 
                 0.74200 
                 1.71901 
                  6° 
               
               
                   
                 8444-3 
                   
                   
                   
                   
                   
               
               
                 14 
                 Namics 
                 CRP3600H 
                 0.34798 
                 0.78700 
                 1.72101 
                  4° 
               
               
                   
                 8444-3 
                   
                   
                   
                   
                   
               
               
                 10 
                 Namics 
                 CRP3600H 
                 0.27399 
                 0.88990 
                 1.74001 
                  9° 
               
               
                   
                 8444-3 
                   
                   
                   
                   
                   
               
               
                 17 
                 CRP4152-R 
                 CRP3600H 
                 0.41900 
                 0.8500 
                 1.58801 
                 14° 
               
               
                 14 
                 CRP4152-R 
                 CRP3600H 
                 0.31199 
                 0.94401 
                 1.55101 
                 16° 
               
               
                 10 
                 CRP4152-R 
                 CRP3600H 
                 0.24699 
                 1.00901 
                 1.60801 
                 16° 
               
               
                   
               
             
          
         
       
     
     In an exemplary embodiment of the invention, the distance D of dam  240  from the die edge of die  110  is at least of twice the pitch of the ball grid array (BGA) containing solder balls  135 . A separation greater than twice the BGA pitch ensures that stress in underfill  220  will be spread over multiple solder balls  135 , and no stress concentration is within one BGA pitch. 
     The natural performance of dam  240  is maximized when the width W of dam  240  is no less than one BGA ball pitches width. Following these rules, a high aspect ratio of silicon to package preferably keeps dam  240  a distance of at least 2 mm from the edge of die  110  and does not allow dam  240  or underfill  220  to overflow or exceed the edge of body package outline. 
     FIG. 3 illustrates a flip-chip packaged device  400  in accordance with an embodiment of the invention in which substrate  230  has a treated region  340 . Treated region  340  has a high affinity to or stiction with underfill and can be made of a material such as a polymer, metal, ceramic, or combination thereof or can be formed by a surface roughening or preparation technique designed to increase surface area contact, which may hinder or control the flow characteristics of the underfill due to increased surface tension. When dispensing liquid underfill inside the perimeter of treated region  340 , the outward flow of the liquid underfill forms a bead  325  over treated regions  340 . Treated region  340  thus acts as a dam to limit the flow of underfill  320  and shape underfill  320  to avoid thin fillet regions that cause stress and warping in substrate  130 . Curing underfill  320  preserves the shape of underfill  320  as controlled by treated region  340 . 
     FIG. 4 illustrates a flip-chip packaged device  400  in accordance with yet another alternative embodiment of the invention. Device  400  includes a substrate  430  having a depression in which die  110  resides. Machining, etching, or another material removal process can form the depression in substrate  430  before die  110  is attached. After attaching die  110  to metal traces in the depression of substrate  430 , a surrounding portion  435  of substrate  430  forms a dam that shapes and contains underfill  220  to avoid stress and warping that edge effects in underfill  220  could otherwise cause. 
     As noted herein, a dam shapes the edge of an underfill structure in a flip-chip package to reduce stress concentrated around the edge of the die. The resulting flip-chip package has superior planarity of the substrate for better connections of the BGA, superior reliability by avoiding inhomogeneity in the coefficient of thermal expansion and associated stress during thermal cycling, and better mechanical attachment of the die and substrate when compared to conventional flip-chip packages. 
     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.