Abstract:
An apparatus and method for underfilling a silicon chip ( 16 ) to a substrate ( 12 ) by depositing an underfill dam ( 18 ) on the surface ( 20 ) of the substrate ( 12 ) prior to addition of the underfill material ( 14 ), is disclosed. A bead of underfill material ( 14 ) is provided on the substrate ( 12 ) about the periphery of the silicon chip ( 16 ), within the underfill dam ( 18 ). The underfill material ( 14 ) fills the gap ( 22 ) between the electrical contacts, the substrate ( 12 ) and the silicon chip ( 16 ) by capillary action and differential pressure created by a vacuum system ( 40 ).

Description:
This is a Division of application Ser. No. 08/977,078, filed Nov. 24, 1997, currently U.S. Pat. No. 5,942,798. 
    
    
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, its background is described in connection with the packaging of integrated circuits, as an example. 
     Bumped silicon chips, also known as flip-chips, use a solder bump or ball to solder the electrical connections that interconnect integrated circuits. Flip-chip arrangements avoid the problems of breakage and lack of planarity encountered with conventional integrated circuit packages that are wire-bonded. Solder bumps or balls allow direct coupling between the pads on the silicon chip and matching contacts on the substrate. The flip-chip is aligned to the substrate and all connections are made simultaneously by reflowing the solder. 
     The use of solder bumps on the underside of silicon chips has led to the need to underfill the gap between the silicon chip and the substrate following reflow of the solder bumps. The underfill increases the mechanical integrity and reliability of integrated circuit packages, but requires a time consuming series of passes in which underfill polymer in liquid form is added to the periphery of the silicon chip. With each addition of underfill polymer, the underfill is built-up over the solder bumps in the gap by capillary action. The entire sequence can take up to 3 minutes per integrated circuit package. 
     Typically, an underfill polymer is dispensed onto each of the four adjacent sides of bonded flip-chip integrated circuits (ICs). The liquid underfill polymer is drawn into the gap by capillary action. The duration of each flow cycle can last as long as twenty minutes depending on the number of passes and the size of the gap. Furthermore, the sides on which the underfill polymer out-flow are substantially less homogeneous than the in-flow side. A non-symmetric underfill geometry causes stress gradients leading to premature failures of the integrated circuit package. 
     The inability to provide a uniform underfill has frustrated attempts to provide flip-chip mounting techniques that are reliable in a mass production environment. The types of flip-chip automation that would benefit from a reliable means of dispersing underfill polymer include: in-line automation, repeatable automatic process control, high throughput productions of flip-chips on substrates such as printed circuit boards (PCB), flip-chip multi-chip modules, or carriers to form flip-chip chip carriers, and high volume production of flip-chip chip carriers on PC boards and ceramic substrates. 
     Yet another solution to the problem of improving the speed of automation of underfilling the gap between the silicon chip and the substrate in flip-chip configuration has been the use of alternating cycles of vacuum and pressure. U.S. Pat. No. 5,203,076 issued to Banjeri, et al. discloses such a method. The Banjeri patent, however, fails to address the need to increase the efficiency in the automation process, as liquid underfill polymer must still be added in repeated cycles. 
     U.S. Pat. No. 5,073,816 (INMOS Limited) discloses an apparatus and method for packaging semiconductor devices aimed at reducing package size and providing improved heat dissipation over the packages in the prior art. While the specification describes an apparatus and method for flip-chip packaging, it fails to address the need to automate the underfill process. 
     A need has arisen for a simple, effective apparatus and method for providing a uniform liquid underfill polymer in a manner that avoids the difficulties of prior infiltration methods. A need has also arisen for increasing the speed and automation of the underfill process. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to a provide a simple, effective apparatus and method for dispensing a liquid underfill polymer under an integrated circuit. The present invention also increases the speed and automation of the underfill process. More particularly, the present invention comprises an integrated circuit package having a substrate and a silicon chip mounted on the substrate. The space between the substrate surface and the silicon chip defines a gap which is underfilled with a liquid underfill polymer. A liquid underfill dam is disposed on the surface of the substrate. The underfill dam serves to create a reservoir for the liquid underfill polymer. 
     The liquid underfill polymer can be, for example, an epoxy or a plastic polymer. In another embodiment of the present invention, the underfill dam is made of, for example, an epoxy or a plastic that is molded or extruded onto the substrate surface. 
     The present invention, disclosed herein, also comprises a method of attaching ah integrated circuit package to a substrate comprising the steps of, disposing an underfill dam on a substrate and electrically interconnecting a silicon chip on the substrate. The distance between the substrate and the silicon chip define a gap, which is filled by placing a bead of underfill material on the substrate about the periphery of the silicon chip and within the underfill dam. 
     In one embodiment of the present invention, the method of the present invention may further comprise the step of applying at least a partial vacuum to the integrated circuit package. Applying a vacuum removes air entrapped in the gap between the silicon chip and the substrate by bubbling the entrapped air through the underfill material. In yet another embodiment of the present invention, the step of providing a bead of underfill material comprises providing a sufficient amount of underfill material to completely fill the gap in one step. Once the gap has been filled with the liquid underfill material, the integrated circuit package having an underfill dam may be stored, handled or further processed by curing the liquid underfill polymer into a solid underfill. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
     FIG. 1 is a cross-sectional view of an underfill retaining apparatus for a solder-bumped flipped integrated circuit and substrate in accordance with the present invention; 
     FIG. 2 is a top view of the solder-bumped flipped integrated circuit and substrate of FIG. 1; 
     FIG. 3 is a cross-sectional view of the solder-bumped flipped integrated circuits and substrate; 
     FIG. 4 is a top view of the solder-bumped flipped integrated circuit and substrate of FIG. 3; 
     FIG. 5 is a cross sectional view of another embodiment of the underfill retaining apparatus for a solder-bumped flipped integrated circuit and substrate; 
     FIG. 6 is a top view of the underfill retaining apparatus for a solder-bumped flipped integrated circuit and substrate; and 
     FIG. 7 is a cross-sectional. view of a vacuum application step for the solder-bump flipped integrated circuit and substrate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     Referring to FIG. 1, an integrated circuit package  10  is shown mounted on a substrate  12  using solder bumps  24 . An example of a substrate  12  is a printed circuit board (PCB), as described in U.S. Pat. No. 4,940,181, incorporated herein by reference. 
     In a flip-chip package, such as integrated circuit package  10 , a plurality of solder bumps  24  are disposed on the substrate  12  and make electrical contact with pads on the substrate  12 . The array of solder bumps  24  is normally arranged in a square or other geometric pattern suitable to receive the corresponding integrated circuit package  10 , or the like. As can be seen in FIG. 1, with this mounting arrangement, a gap  22  is formed between the top surface  20  of the substrate  12  and the bottom surface  26  of the silicon chip  16 , which varies from 2-6 mils. 
     To fill the gap  22 , an infiltrant or underfill polymer is dispensed between at least one of the edges of the silicon chip  16  and an underfill dam  18  of the integrated circuit package  10  in space  15 . The underfill polymer comprises a liquid polymer of suitable viscosity. The underfill dam  18  allows the user to deposit, in one step or pass, sufficient underfill polymer to fill the gap  22  in an automated assembly line rather than in multiple passes. The underfill dam  18  prevents the liquid underfill polymer from spilling over the sides of the substrate  12  while the integrated circuit package awaits curing. 
     By providing an underfill dam  18 , the user can process more integrated circuits using the same equipment that is currently used. In operation, the underfill dam  18  may be a polymer, such as an epoxy, that is deposited on the substrate  12  prior to, concurrent with or after the silicon chip  16  has been soldered onto the substrate  12 . Liquid underfill polymer is deposited into the gap  22  and capillary action draws the liquid underfill polymer into the gap  22 . During the fill stage, the integrated circuit package  10  can be handled or stored prior to curing. Furthermore, a large number of integrated circuit packages  10  can be processed. using the one-step fill method using the apparatus of the present invention while others are stored or cured. 
     FIG. 2 is a top view of the integrated circuit package  10 , in which a silicon chip  16  has been soldered onto a substrate  12 . In this embodiment, an underfill dam  18  is depicted as a generally square-shaped dam surrounding the space  15  and the silicon chip  16 . The underfill dam  18  serves to restrain the liquid underfill polymer in a liquid state during storage and processing prior to curing. The actual height of the underfill dam  18  above the substrate  12  will depend on the size of the solder bumps  24 , the height of the gap  22  and the amount of liquid underfill polymer that is applied to fill gap  22 . Furthermore, the distance between the edge of the silicon chip  16  and the underfill dam  18  will depend on like considerations. As will be appreciated by those of skill in the art in light of the present disclosure, the actual shape of the underfill dam  18  may vary, e.g., depending on the specific requirements of the integrated circuit package. The space  15  has a residual coating of the polymer. 
     FIG. 3 shows an alternative embodiment of the integrated circuit package  10  of the present invention in which the underfill dam  18  is a molded cover that creates a space  15  that extends from the underfill  14  to the edge of the substrate  12 . Using an underfill dam  18  that is molded onto the substrate  12  requires less processing than the underfill dam  18  depicted in FIG. 1, and can be molded or extruded onto the substrate  12  prior to reflow of the solder bumps  24  between silicon chip  16  and pads on the substrate  12 . The molded underfill dam  18  also has the advantage that is can be provided with thermal properties that improve heat dissipation, as known to those of skill in the art. 
     FIG. 4 is a top view of the integrate circuit package  10  depicted in FIG. 3, having a substrate  12 , silicon chip  16 , underfill  14 , space  15  and underfill dam  18 . The underfill dam  18  is shown molded from the edge of the underfill  14  to the edge of the substrate  12 . 
     FIG. 5 shows yet another embodiment of the underfill dam  18  of the present invention in which the underfill dam  18  is molded onto the substrate  12 , and is trapezoidal in shape. The trapezoidal underfill dam  18  shares the advantages of the extruded rounded underfill dam  18  depicted in FIG.  1  and the molded underfill dam of FIG. 3 as it relates to retaining the liquid underfill polymer prior to curing. It also has the advantage provided by the rounded underfill dam  18  of FIG. 1 in that is requires small amounts of polymer or plastic, saving resources and the environment. At the same time it has the advantage of the molded underfill dam  18  depicted in FIG. 3, because it can be extruded or molded using standard single-step molding techniques. 
     FIG. 6 is a top view of the integrated circuit package  10  of the present invention in which the underfill  14  is still in its liquid state and the liquid underfill polymer is filling the gap  22  by capillary action. Within space  15  can be seen the excess liquid underfill polymer that forms a residual polymer coating once the polymer has been cured. The liquid underfill dam  18  serves as a reservoir of liquid underfill polymer while capillary action draws the liquid underfill polymer into the gap  22 . At this stage the integrated circuit package  10  can be handled and stored pending curing of the liquid underfill polymer. 
     In order to insure that the underfill  14  fills the gap  22  more completely further processing may be required. As depicted in FIG. 7, a vacuum system  40  has a vacuum shield  30  that is positioned over the integrated circuit package  10  on, for example, a table  36 . A seal  38  is positioned between the table  36  and the vacuum shield  30 . Vacuum  32  is continuously applied as controlled by a vacuum valve  34  and entrapped air that was in the gap  22 , is evacuated by bubbling past the underfill  14  prior to curing the liquid underfill polymer into a solid underfill  14 . 
     In operation, an underfill dam  18  is used to retain the liquid underfill polymer in the gap  22  during processing. To reduce the possibility of bubbles forming or remaining in the underfill  14 , vacuum  32  is applied to evacuate bubbles from the gap  22  under the silicon chip  16  of the integrated circuit package  10 . A vacuum of  50  mTorr is generally required to evacuate bubbles. This process enables the gap  22  to be completely underfilled, resulting in a reduced infiltration cycle time and eliminating any voids in the underfill  14  by providing a uniform underfill  14 . The position of the vacuum  32  can be anywhere where the vacuum does not draw the liquid underfill polymer into the vacuum pump  34 . 
     Additionally, the one-step addition of liquid underfill polymer followed by, or prior to, the application of vacuum  32  uses existing in-line automation technology, repeatable automatic process control, and high through-put production of integrated circuit packages  10  on any suitable substrate. It also allows the handling and storage of the integrated circuit packages  10  during processing. 
     While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.