Abstract:
An integrated circuit assembly cleaning apparatus and method allow a cleaning solution to completely fill spaces within an integrated circuit assembly. Such spaces include, for example, the thin space between the die and substrate of a flip-chip integrated circuit. The cleaning solution fills the space while the air initially occupying the space escapes. These actions are accomplished by first tilting the integrated circuit assembly from horizontal. The integrated circuit assembly is then immersed in the bath at a controllable rate to allow the cleaning solution to completely fill the space while the air in the space escapes.

Description:
BACKGROUND OF THE INVENTION 
     The continuing increases in the functional capacity of integrated circuits (ICs) over the last few years have been both astounding and beneficial. However, accompanying these increases are attendant technical problems that demand creative solutions. One such problem has been the increase in input/output (I/O) pads that typically result from increases in the amount of circuitry that can be incorporated onto an IC. The number of I/O pads on a traditional wire-bonded IC, which involves bonding wires from the I/O pads of the IC die to the substrate, is generally limited by the length of the IC perimeter because such I/O pads typically reside at the edges of the IC. Thus, reductions in the size of transistors and other electronic devices incorporated on a single die generally create a need for more I/O pads than what traditional wire-bonding technology can offer. 
     To satisfy this need, alternatives to wire-bonding techniques have been devised to increase the overall interconnection density of ICs. One such alternative is the “flip chip,” which utilizes I/O connections across the top surface of the die. Thus, the connections are not restricted to the perimeter of the IC. Typically, solder “bumps” are formed on these connections. The solder bumps are then covered with solder flux, the die is flipped over so that the bumps make contact with the connection points of the IC substrate, and the die-substrate assembly is heated to reflow the solder. Hence, the necessary electrical contacts between the die and substrate are made by way of the solder bumps with the aid of the solder flux. FIG. 1 is a simplified perspective view of a typical flip chip assembly  100 , with a die  110  connected to a substrate  120  by way of solder bumps  130 , with die  110  and substrate  120  defining a narrow, substantially planar space  140  therebetween. 
     Tests on flip chip devices have shown that repeated heating and cooling of the IC during normal use tends to place sufficient thermal stress on the integrated circuit (die-substrate) assembly to cause some of the connections made via solder bumps  130  to break, creating electrical discontinuities between die  110  and substrate  120 . To prevent such breaks, an underfill material (generally an adhesive) is normally employed to fill planar space  140  to maintain the structural integrity of the assembly and prevent the electrical connections from breaking. However, after the solder reflow, some flux residue remains in planar space  140  that must be removed by way of an IC cleaning solution before the underfill can be applied. The cleaning process is vital since leftover residue within planar space  140  prevents the underfill from reaching the entirety of planar space  140 , thus adversely affecting the structural integrity and overall reliability of flip chip assembly  100 . 
     Complete cleaning of the flux residue from planar space  140  of flip chip assembly  100  has proven to be rather difficult. The distance between die  110  and substrate  120  is normally quite narrow, on the order of 70 um or less. Further complicating the process is the fact that several rows of solder bumps  130  may exist in planar space  140 , thus making access to all of planar space  140  even more problematic. 
     Currently, IC assemblies are normally cleaned using commercially available centrifugal cleaners and cleaning solutions. As shown in a simplified manner in FIG. 2, a centrifugal cleaner  200  employs a tank  210  that is filled with an IC cleaning solution  220  during the cleaning process. Centrifugal cleaner  200  usually holds several IC assemblies, such as flip chip assembly  100 , using a cleaning fixture  230  immersed in cleaning solution  220  inside tank  210 . A tank-filling mechanism (not shown) of centrifugal cleaner  200  is used to fill tank  210  with IC cleaning solution  220 . Cleaning fixture  230  is then spun or agitated on a central vertical axis in cleaning solution  220  by way of a motor  240 . Cleaning solution  220  is then drained from tank  210 , and water rinse and spin-drying cycles in centrifugal cleaner  200  then normally follow. Cleaning fixture  230  holds several flip chip assemblies  100 , or other similar IC assemblies, horizontally within IC cleaning solution  220 . 
     Cleaning fixture  230  may be implemented in a variety of ways. For example, cleaning fixture  230  may consist of a central carousel to which one or more cassettes are attached. Each carousel would then be loaded manually with flip chip assemblies  100  prior to the cleaning process. Also, flip chip assemblies  100  may be held in boats  300  (FIG.  3 ), each of which holds several flip chip assemblies  100  throughout a majority of the IC manufacturing process. In that case, a cleaning fixture holds several such boats  300  containing flip chip assemblies  100  to be cleaned. Other methods of implementing cleaning fixture  230  not disclosed herein are also employed in the industry. 
     Unfortunately, as displayed in FIG. 4, which shows a top view of flip chip assembly  100  after being agitated or spun in a bath of cleaning solution  220  in centrifugal cleaner  200 , tests have shown that cleaning solution  220  almost always fails to penetrate the entirety of planar space  140  (not shown explicitly in FIG. 4) between die  110  and substrate  120 , leaving some flux residue behind because an air pocket  400  becomes trapped in planar space  140 . When flip chip assembly  100  is positioned horizontally, cleaning solution  220  encroaches from all sides of planar space  140  simultaneously, trapping air pocket  400  approximately in the center of planar space  140 . Air pocket  400  then acts as a countervailing force against the entry of cleaning solution  220  into planar space  140 . Cleaning solution  220  is thus prevented from reaching all of planar space  140 , allowing some of the flux residue from the solder reflow phase to remain. The remaining flux residue thus prohibits the underfill material subsequently applied from occupying all of planar space  140 . Tests also confirm that no amount of spinning or agitation in cleaning solution  220  will force air pocket  400  from planar space  140  so that cleaning solution  220  may occupy all of planar space  140 . 
     To remedy this problem, the use of a apparatus and method of cleaning the tight spaces in integrated circuit assemblies, such as, for example, between the die and substrate of a flip-chip IC, that would result in the complete removal of the flux residue in the space would be advantageous. Without any flux residue present in the planar space, the underfill material to be applied for purposes of structural integrity may fill all of the space, thus preventing the breakage of the various connections between the substrate and die. The cleaning of other types of integrated circuit assemblies involving similar tight spaces, such as, for example, ball grid arrays (BGAs) and direct chip attach (DCA) assemblies, whereby a die is attached directly to a printed circuit board (PCB), would also benefit from such an apparatus and method. 
     SUMMARY OF THE INVENTION 
     Specific embodiments according to the present invention, to be described herein, provide an effective way of cleaning a space within an integrated circuit assembly without trapping air inside the space. For example, one embodiment of the invention provides a method of cleaning an IC assembly, such as a flip chip IC. To allow the cleaning solution to enter the space without trapping an air pocket inside, the IC assembly is held at an incline from horizontal. The IC assembly is then immersed slowly in the cleaning solution so that the space is completely filled with the cleaning solution prior to the integrated circuit assembly becoming completely submerged within the solution. Since the cleaning solution fills all of the space, all flux residue will be dissolved, allowing the underfill material used later in the IC manufacturing process to fill the entire space, helping to create a structurally reliable IC assembly. 
     Another embodiment of the invention involves an IC cleaning apparatus that holds an IC assembly at an incline from horizontal. The IC assembly is usually retained either directly or indirectly by a cleaning fixture. The cleaning apparatus then slowly immerses the IC assembly in the cleaning solution bath so that the cleaning solution completely fills the space, thereby allowing air in the space to escape prior to total submergence of the IC assembly in the solution. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified perspective view of a typical flip chip IC assembly according to the prior art. 
     FIG. 2 is a simplified cross-sectional side view of an IC centrifugal cleaner according to the prior art. 
     FIG. 3 is a simplified perspective view of a boat that holds flip chip IC assemblies during the IC manufacturing process according to the prior art. 
     FIG. 4 is a simplified plan view of the flip chip IC assembly after being spun or agitated while submerged horizontally according to the prior art in the IC cleaning solution in the centrifugal cleaner of FIG.  2 . 
     FIG. 5 is a simplified perspective view of a cleaning fixture for a centrifugal IC cleaner that employs cassettes that hold IC assemblies at an incline from horizontal according to an embodiment of the invention. 
     FIG. 6 is a simplified perspective view of a cleaning fixture for a centrifugal IC cleaner that employs IC boats containing IC assemblies, with the IC assemblies being held at an incline from horizontal according to an embodiment of the invention. 
     FIG. 7 is a simplified perspective view of a cleaning fixture for a centrifugal IC cleaner that holds the IC assemblies directly at an incline from horizontal according to an embodiment of the invention. 
     FIG. 8 is a flow chart of a method of cleaning IC assemblies according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     An IC assembly cleaning apparatus according to an embodiment of the invention utilizes a version of centrifugal cleaner  200  (of FIG. 2) having a cleaning fixture that holds IC assemblies to be cleaned at an incline from horizontal. (All of the embodiments discussed below involve a centrifugal cleaner, although other type of IC cleaners may also utilize the principles of the invention described herein.) The IC assemblies define a narrow space that contains flux residue to be removed. As described earlier, one such type of IC assembly is flip chip assembly  100  (of FIG.  1 ), in which die  110  and substrate  120  define substantially planar space  140 , which contains flux residue to be removed before space  140  is filled with an adhesive. In one embodiment, depicted in FIG. 5, a first cleaning fixture  500  employs removable cassettes  510  that are attached to a central carousel  520 . Each cassette  510  holds several flip chip assemblies  100  at an incline of approximately 30 degrees from horizontal. In another embodiment, shown in FIG. 6, a second cleaning fixture  600  is capable of holding one or more IC assembly boats  300  (from FIG.  3 ), each of which may hold several flip chip assemblies  100  to be cleaned. Boats  300  are attached to cleaning fixture  600  via slots  610 . In the particular embodiment of FIG. 6, boat  300  is held at an incline of 45 degrees from horizontal. In another embodiment, a third cleaning fixture  700  holds flip chip assemblies  100  directly in a circular fashion in slots (not shown). In the particular embodiment of FIG. 7, flip chip assemblies  100  are maintained at an angle of 90 degrees from horizontal. With respect to any of the embodiments of FIGS. 5,  6 , and  7 , the IC assembly cassettes or boats may be held within the cleaning fixture using methods employed in prior art cleaning fixtures that orient the IC assemblies horizontally. Additionally, other cleaning fixture configurations not specifically mentioned herein may also be utilized, provided that the IC assemblies are held at an angle from horizontal. 
     Flip chip assemblies  100 , while held at an incline from horizontal, are placed in contact with the surface of a bath of IC cleaning solution  220  in tank  210  (from FIG.  2 ). According to one embodiment, cleaning fixture  500 ,  600 , or  700  is lowered into tank  210  that is already filled with cleaning solution  200 . More likely, cleaning fixture  500 ,  600 , or  700  is first lowered into an empty tank  210 , and then tank  210  is filled with cleaning solution  220  until the surface of cleaning solution  220  makes contact with flip chip assemblies  100  such that cleaning solution  220  enters planar space  140 , possibly being drawn into planar space  140  by capillary action. While cleaning solution  220  is filling planar space  140 , at least some of the perimeter of planar space  140  is not submerged in cleaning solution  220 , due to the inclined orientation of planar space  140 . The inclined position of flip chip assemblies  100  allows any air within planar space  140  to escape while cleaning solution  220  continues to enter planar space  140 . 
     To allow air to escape from planar space  140 , flip chip assemblies  100  must be positioned in cleaning solution  220  such that only a portion of the perimeter of planar space  140  is submerged. This positioning is accomplished in one embodiment by controlling the descent of flip chip assemblies  100  into cleaning solution  220 , in the case that tank  210  is already filled with cleaning solution  220 . Alternately, in the case that cleaning fixture  500 ,  600 , or  700  already resides within tank  210 , the filling of tank  210  with cleaning solution  220  is controlled so that the surface of the bath of cleaning solution  220  rises slowly enough to allow planar space  140  to be completely filled with cleaning solution  220  prior to the entire perimeter of planar space  140  becoming submerged, thus allowing all air in planar space  140  to escape prior to submergence. 
     In the embodiment of FIG. 6, second cleaning fixture  600  employs an incline of 45 degrees from vertical. In several embodiments, this angle is thought to be a fair compromise between the needs of a higher angle for purposes of reducing the time to fill solvent tank  210  (or increasing the speed with which the cleaning fixture may be lowered into cleaning solution  220 ) and the desire of a lower angle to facilitate the agitation or centrifugal extraction of cleaning solution  220 , depending on the particular configuration of the cleaning fixture. However, other angles of inclination, ranging from a slight tilt from horizontal to a fully vertical position, will also work well, such as the 30 degrees utilized in first cleaning fixture  500 , or the 90 degrees employed in third cleaning fixture  700 . 
     Both the agitation of flip chip assemblies  100  and the extraction of cleaning solution  220  from flip chip assemblies  100  are affected by the angle of incline and the angle of orientation with respect to the rotational axis of the particular cleaning fixture. For example, third cleaning fixture  700  provides excellent extraction because the parts are held radially with respect to the rotational axis. However, that same fixture provides poor agitation because of that same orientation. Agitation may be improved, however, by reducing the angle of incline, at the possible expense of a reduced rate of filling planar space  140 . 
     Furthermore, each embodiment shown in FIGS. 5,  6 , and  7  is not limited to the angle of incline shown for that particular fixture. For example, second cleaning fixture  600  could have been designed to hold flip chip assemblies  100  at an incline of 60 degrees or any other angle deemed necessary for proper cleaning. 
     Additionally, other embodiments of the present invention take the form of a method of cleaning an IC assembly, such as a flip chip assembly, that allows flux residue to be removed from tight spaces of the IC assembly. FIG. 8 displays the steps involved in a method embodiment of the invention. First, the IC assembly to be cleaned, such as a flip chip assembly with a substantially planar space, for example, is held at an incline from horizontal so that the top surface of a bath of cleaning solution coming in contact with the IC assembly may enter the space without submerging the entire perimeter of the space in the cleaning solution (step  800 ). Next, the IC assembly is immersed in the cleaning solution at a slow enough rate to allow the cleaning solution to fill the space while allowing air within the space to escape or vent without being impeded by the cleaning solution (step  810 ). The IC assembly should not be completely submerged in the cleaning solution until the space has been filled with the solution. Afterward, the steps of moving (via spinning or agitation) the IC assembly in the cleaning solution (step  820 ), rinsing the IC assembly with water to help remove the cleaning solution from the IC assembly (step  830 ), and spin-drying the IC assembly to ensure all cleaning solution and water are extracted from the IC assembly (step  840 ), are customarily employed. 
     From the use of embodiments of the present invention, not only has the ability to clean narrow spaces in flip chip assemblies been enhanced greatly, but manufacturing line throughput has been increased significantly. Although the time required to fill the tank of a centrifugal cleaner has been increased due to the time needed to allow the cleaning solution to completely fill the thin space of the IC assembly, the amount of time required for spinning and agitating the IC assembly within the cleaning solution has been reduced dramatically since the cleaning solution reaches all of the flux residue within the space. Current use of embodiments of the invention described herein have allowed a previous cleaning cycle time of 15 minutes to be reduced to less than 12 minutes, thus boosting cleaning process throughput by about 25%. Since the IC cleaning process is a significant limiting factor in overall IC manufacturing throughput, such a reduction in the cleaning process cycle time increases the capacity of the entire IC manufacturing line substantially.