Abstract:
Methods for applying an underfill with vacuum assistance. The method may include dispensing the underfill onto a substrate proximate to at least one exterior edge of an electronic device attached to the substrate. A space between the electronic device and the substrate is evacuated through at least one gap in the underfill. The method further includes heating the underfill to cause the underfill to flow into the space. Because a vacuum condition is supplied in the open portion of the space before flow is initiated, the incidence of underfill voiding is lowered.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/548,965, filed Jul. 13, 2012, which is a continuation-in part application of U.S. patent application Ser. No. 13/004,198, filed Jan. 11, 2011, the disclosure of each of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The invention relates generally to methods for applying an underfill between an electronic device and a substrate. 
     It is typical for an electronic device, such as a flip chip, chip scale package (CSP), ball grid array (BGA) or package on package assembly (PoP) to include a pattern of solder bumps that, during mounting, are registered with pads on a substrate, or joined using another type of interconnect technology such as copper pillars or other types of thermal compression bonding interconnects. The substrate can be a printed circuit board, electronic chip or wafer, for example. The solder is reflowed by heating and, following solidification, solder joints connect the electronic device and the substrate. Underfill, which may be an epoxy, may be used to fill the open space between the electronic device and the substrate that remains between the reflowed solder balls. The underfill protects the solder joints against various adverse environmental factors, redistributes mechanical stresses due to shock, and prevents the solder joints from moving under strain during thermal cycles. 
     In the process of underfilling, voids may be formed due to the following reasons, but not limited to, uneven surface topography in the gap between the electronic device and substrate, fluid flow rate race conditions as underfill flows around a solder connection, different wetability conditions on the substrate, air in the underfill, or air induced from the dispensing process. Because the voids are unfilled by underfill, unsupported solder joints adjacent to voids may not be adequately protected against cold flow when exposed to strain from thermal expansion during operation or to mechanical shock caused by dropping the assembled end product, such as a cell phone, that includes the underfilled electronic device. Voids at solder joints prevent the solder bump from being in held in a state of hydrostatic compression and strain restraint, which may increase solder joint fatigue and thereby increase the probability of solder joint cracking. 
     Therefore, improved methods are needed for applying an underfill that reduces the probability of forming voids in the underfill. 
     SUMMARY 
     In one embodiment, a method is provided for distributing an underfill into the space between the reflowed solder balls which connect an electronic device to a substrate. The method includes providing the underfill onto the substrate near to at least one exterior edge of the electronic device with at least one gap in the underfill, providing an air path to the space between the electronic device and the substrate and then evacuating that space through the gap, or gaps, to provide a vacuum condition in the space. After evacuating the space, the underfill is heated above room temperature to cause capillary flow of the underfill to the exterior edge, or edges, and into the space between the electronic device and substrate and around the reflowed solder balls. The underfill can be provided as a material which is solid at room temperature and is positioned by pick and place equipment onto the substrate, and thereafter becomes liquid at elevated temperatures, or as a liquid material that can be dispensed onto the substrate by, for example, a valve or dispenser. 
     Another embodiment of the invention is directed to a method of providing an underfill on a substrate upon which electronic device is mounted by electrically conductive joints and is separated from the substrate by a space. The space has an open portion that is unoccupied by the conductive joints. The method includes providing the underfill onto the substrate proximate to at least one exterior edge of the electronic device, and evacuating the space to provide a vacuum condition in the open portion of the space between the underfill and external edges of the electronic device. After evacuating the space to a vacuum condition, the underfill is heated to a temperature above room temperature to cause flow of the underfill to at least one exterior edge and into the open portion of the space, thereby allowing any air trapped under the underfill itself to vent before reaching the external edge of the electrical device and the gap between the electrical device and substrate 
     Other embodiments of the invention are directed to methods of blocking air that has been trapped under the underfill from flowing under the electronic device. In one such method, an obstacle is placed between the edge of the electronic device and the underfill prior to applying the vacuum. After applying the vacuum condition, the underfill is heated to a temperature above room temperature to cause flow of the underfill over the obstacle and from at least one exterior edge into the open portion of the space. Forcing the underfill to flow over an obstacle, helps block the air trapped under the underfill from flowing under the electronic device and allows the trapped air to vent prior to reaching the gap under the electrical device 
     Yet another embodiment of the invention is directed to a method of exposing a surface of the substrate to a plasma so as to change the wettability of the substrate prior to providing the underfill onto the substrate proximate to at least one exterior edge of the electronic device. This plasma treatment reduces the opportunity for air to be trapped under the underfill. The method further includes evacuating the space to provide a vacuum condition, in the open portion of the space. After evacuating the space to a vacuum condition, the underfill is heated to cause flow of the underfill toward at least one exterior edge and into the open portion of the space. Since the plasma treatment of the substrate reduces the entrapment of air under the underfill, an amount of air trapped under the electronic device during the underfill operation may also be reduced. 
     Similar to the plasma treatment method, a glass-like film may be deposited on the substrate so as to provide a more perfectly smooth and flat surface. This flat surface has fewer depressions or imperfections in which air can be trapped when the underfill is positioned on top of the glass-like film. As entrapment of air under the underfill is reduced, an amount of air trapped under the electronic device during the underfill operation may also be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and, together with a general description of embodiments of the invention given above, and the detailed description given below, serve to explain the principles of the embodiments of the invention. 
         FIG. 1  is a side view of an electronic device mounted to a substrate by an array of solder balls and with underfill provided along a side edge of the electronic device. 
         FIG. 1A  is a side view similar to  FIG. 1  in which the underfill has moved into the open space between the electronic device and substrate that is unoccupied by the solder balls. 
         FIG. 2  is a flow chart of a procedure for vacuum underfilling in accordance with an embodiment of the invention. 
         FIGS. 3A-C  are diagrammatic top views illustrating a sequence for vacuum underfilling beneath an electronic device mounted on a substrate in accordance with an embodiment of the invention. 
         FIGS. 4A-C  are diagrammatic top views similar to  FIGS. 3A-C  in accordance with another embodiment of the invention. 
         FIGS. 5A-C  are diagrammatic top views similar to  FIGS. 3A-C  in accordance with yet another embodiment of the invention. 
         FIGS. 5D, 5E and 5F  are diagrammatic top views similar to  FIG. 5A  in which the underfill is provided on the substrate with, respectively, an L pattern, a U pattern, and an I pattern. 
         FIG. 5G  is a diagrammatic top view similar to  FIG. 5A  in which the underfill is provided on the substrate with no gaps. 
         FIG. 6A  is a diagrammatic top view similar to  FIG. 5A  in which a dam is positioned between at least one side edge of the electronic device and the underfill. 
         FIG. 6B  is a side view of the dam positioned between at least one side edge of the electronic device and the underfill. 
         FIGS. 7A and 7B  are side views of the underfill flowing over the dam at different time sequences. 
         FIG. 8A  is a diagrammatic top view similar to  FIG. 5A  in which a channel is positioned between at least one the side edge of the electronic device and the underfill. 
         FIG. 8B  is a side view of the channel positioned between the side edge of the electronic device and the underfill. 
         FIGS. 9A and 9B  are side views of the underfill flowing over the channel at different time sequences. 
         FIG. 10  is a cross-sectional view of a plasma-treated substrate according to an embodiment of the invention. 
         FIG. 11  is a cross-sectional view of a plasma-treated substrate according to another embodiment of the invention. 
         FIG. 12  is a schematic representation of a vacuum underfilling system in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Generally, the embodiments of the invention are directed to a vacuum-assisted process for underfilling an electronic device mounted on a substrate by an array of solder balls. Underfill is dispensed or otherwise provided (e.g., in either a liquid or solid form) in one or more lines around the edges of an unheated electronic device, which is mounted to an unheated substrate by means of an array of reflowed solder balls. Preferably, at least one gap is left in the one or more lines of underfill and, preferably, if the space between the electronic device and substrate is very small, there is a space between the underfill and the exterior edges of the electrical device. The substrate is transported into a vacuum chamber, before significant capillary underfilling (and air or gas entrapment) occurs, and a vacuum is applied to evacuate the space. While the vacuum is being applied, the gap, or gaps, in the one or more lines of underfill allows air to flow out from under the device through the gap(s), to establish a vacuum condition (i.e., a pressure less than atmospheric pressure) under the electronic device between the electronic device and the substrate. An alternative, less preferred process, is to provide no gap in the underfill and to rely upon the air trapped under the device to bubble through the underfill when the device is placed under vacuum. Under either process, while the vacuum condition is being maintained, the electronic device and substrate are heated to cause the underfill to completely flow under the electronic device into the spaces between the reflowed solder balls. Underfilling in the presence of the vacuum condition means any void entrapped in the underfill will be partially evacuated of gases commensurate with the level of the applied vacuum. The vacuum pressure applied must not be lower than the vapor pressure of the underfill, otherwise the underfill will boil and the process will become less stable. The vacuum chamber is then vented. Any voids present in the underfill will now collapse because of the evacuated condition and become filled with underfill. The underfilled electronic device and the substrate are then moved out of the vacuum chamber. 
     The embodiments of the invention also apply to other interconnect technologies, in addition to solder bumps, for creating conductive joints between the electronic device and the substrate, such as copper pillars and other thermal compression bonding interconnect technologies. 
     With reference to  FIG. 1 , an assembly  10  includes a substrate  12 , such as a printed circuit board, and an electronic device  14  that is mounted to a surface  16  of the substrate  12 . In representative embodiments, the electronic device  14  may be a flip chip, chip scale package (CSP), ball grid array (BGA) or package on package assembly (PoP), for example. Likewise, the substrate  12  may be a printed circuit board (PCB), electronic chip or wafer, for example, or any substrate or interposer used in semiconductor packaging of electronic devices 
     With reference to  FIGS. 1, 1A, and 3A , the electronic device  14  has a footprint on the substrate  12  such that the substrate  12  is exposed adjacent to each of the side or exterior edges  18 ,  20 ,  22 ,  24  of the electronic device  14 . Solder joints  26  mechanically and electrically connect the electronic device  14  with the substrate  12 . A space  28  is defined between the electronic device  14  and substrate  12  and a portion of the space  28  is open (i.e., unoccupied) and unfilled by the solder joints  26  that may have a representative form of solder balls. At each of the exterior edges  18 ,  20 ,  22 ,  24 , a gap  27  is defined between the electronic device  14  and the substrate  12 . The gap  27  communicates with the space  28 . Preferably, for small gaps  24  (e.g., less than 200 microns), a space  43  on the surface of substrate  12  exists between the underfill  30  and corresponding device edges  18 ,  20 ,  22  and  24 . 
     An underfill  30  is used to fill the space  28  between the electronic device  14  and the substrate  12 , as shown in  FIG. 1A . In one example, the underfill  30  is a curable non-conductive silicon dioxide particle filled epoxy that is fluid when applied to the substrate  12  and flows by capillary action. Other types of underfill can be used including those that are solid at room temperature or are frozen. Underfills are typically filled with small particles of glass, for example to provide the desired properties in the cured underfill. When cured and hardened, the underfill forms a strongly bonded, cohesive mass. 
     With reference to  FIG. 2 , a procedure for vacuum underfilling in accordance with an embodiment of the invention is described. In the  FIG. 2  embodiment, a liquid underfill is dispensed onto the substrate. Instead of dispensing the underfill  30  in a liquid form, the underfill  30  could be applied in a solid form in position to buy a pick and place machine, for example, as mentioned above. In block  52 , liquid underfill  30  is dispensed onto the substrate  12 . The underfill  30  may be applied as one or more continuous lines ( FIG. 3A ) proximate to one or more exterior edges  18 ,  20 ,  22 ,  24  of the electronic device  14 . Preferably, underfill  30  does not touch edges  18 ,  20 ,  22 , or  24  so that surface  43  is not covered by the underfill  30  until full vacuum is applied. Typically, the dispensed amount of underfill  30  is equal to the volume of the open space  28  under the electronic device  14  plus the fillet  31  ( FIG. 1A ) that forms along the perimeter of the device  14  after the underfill operation has been completed. The substrate  12  is unheated when the underfill  30  is applied and a gap  42  ( FIG. 3A ) is preferably present in the underfill  30  so that an air path to the open portion of space  28  through the gap  42  is maintained. As discussed above, the less preferred method is not to leave a gap  42  or open space  43 , and to rely on air trapped under the electronic device  14  to bubble through the underfill  30 . 
     The underfill  30  may be applied to the substrate  12  using multiple different types of dispensers and in multiple different ways. For example and although the invention is not so limited, a series of droplets of underfill  30  may be dispensed onto the surface  16  of the substrate  12  from a moving jetting dispenser that is flying above the surface  16 . 
     In block  54 , the underfill  30  is cooled when dispensed onto the substrate  12 . In one embodiment, the substrate  12  is cooled, for example, by one or more thermoelectric coolers to a temperature below room temperature and the underfill  30  cools shortly after application to approximately the temperature of the substrate  12 . Alternatively or in addition to cooling the substrate  12 , the underfill  30  may be cooled in the dispenser before being dispensed onto the substrate  12 . In one embodiment, the underfill  30  is cooled to a temperature in the range of 0° C. to 10° C. Cooling increases the viscosity of the underfill  30 , which further prevents or reduces capillary flow into the open portion of the space  28  between the electronic device  14  and the substrate  12  before vacuum is applied 
     In block  56 , the unfilled portion of space  28  is evacuated to a sub-atmospheric pressure through the gap  42  in the underfill  30  or space  43  to establish a vacuum condition (i.e., a pressure less than atmospheric pressure) in space  28 . Or, if no gap has been provided, or if the open space  43  is not maintained, the gas will bubble through the underfill  30 . To create the vacuum, in one embodiment, the substrate  12 , which carries the electronic device  14  and the underfill  30 , is moved into a vacuum chamber, sealed inside the chamber, and the vacuum chamber is evacuated to a sub-atmospheric pressure. In one embodiment, a suitable sub-atmospheric pressure for the vacuum is greater than or equal to 25 inches of Hg (about 95 Torr) to 26 inches of Hg (about 100 Torr). In any event, the sub-atmospheric pressure is limited such that the physical properties of the underfill are not significantly or detrimentally modified. 
     Any suitable technique may be used for moving the substrate  12  into and out of the vacuum chamber, and conventional vacuum systems are familiar to a person having ordinary skill in the art. The substrate  12  is preferably transferred into the vacuum chamber before the occurrence of capillary underfilling (and air or gas entrapment) or before the underfill  30  is allowed to touch any of surfaces  18 ,  20 ,  22 ,  24  thereby maintaining surface  43  be uncovered with underfill  30 . 
     In block  58 , after the vacuum chamber is evacuated, and while the vacuum condition is being maintained, the underfill  30  is heated to a temperature in excess of room temperature, for example to a temperature in a range of 30° C. to 120° C. The underfill  30  may be heated by heating the substrate  12 , the electronic device  14  or both and in any desired sequence to direct flow. In response to the heating, the underfill  30  flows by capillary action through the narrow gap  27  from each of the exterior edges  18 ,  20 ,  22 ,  24  into the space  28  and around the reflowed solder balls. Because the open portion of the space  28  is evacuated, the underfill  30  can flow across the space  28  such that any void entrapped in the underfill  30  will be evacuated of gases to the vacuum level. 
     In block  60 , after sufficient time has been provided for complete capillary flow to have occurred, then the vacuum condition is removed and atmospheric pressure is restored. For example, the vacuum chamber may be vented to provide the atmospheric pressure condition. Under the influence of atmospheric pressure, any voids present in the underfill  30  will collapse because of their evacuated state of sub-atmospheric pressure and become filled with underfill  30  ( FIG. 3C ). The substrate  12  is then transferred from the vacuum chamber to a curing oven and the underfill  30  is cured. 
     With reference to  FIGS. 4A-4C  and in alternative embodiments, the underfill  30  may be applied proximate to the exterior edges  18 ,  20 ,  22 ,  24  of the electronic device  14  as a series of disconnected regions ( FIG. 4A ) with multiple gaps  61 . In  FIG. 4B , the gaps  61  disappear as the underfill  30  is heated after evacuating the open portion of spaces  28  and  43  to a vacuum condition. In  FIG. 4C , the underfill  30  flows beneath the device  14 . 
     With reference to  FIGS. 5A-5E  and in alternative embodiments, the underfill  30  may be applied proximate to one or more of the exterior edges  18 ,  20 ,  22 ,  24  of the electronic device  14  in one or more passes. In this case,  FIG. 5A  shows a line of underfill applied along each of the four edges of the device, with a gap  62  and space  43  present at each corner between each pair of exterior edges  18 ,  20 ,  22 ,  24 . In  FIG. 5B , the underfill  30  is heated after evacuating the space  28  through the gaps  62  to a vacuum condition. In  FIG. 5C , the underfill  30 , in the heated state, flows beneath the device  14 . 
     In an alternative embodiment and as shown in  FIG. 5D , the underfill  30  could be provided as lines using an L pass along exterior edges  18  and  24  of the electronic device  14 , preferably providing space  43 . In this case, a gap is present along the exterior edges  20  and  22 . In another alternative embodiment and as shown in  FIG. 5E , the underfill  30  could be provided as lines using a U pass along exterior edges  18 ,  20 ,  22  of the electronic device  14  preferably providing space  43 , but not along exterior edge  24  of the electronic device  14 . In another alternative embodiment and as shown in  FIG. 5F , the underfill  30  could be provided as a line using an I pass along exterior edge  20  of the electronic device  14 , preferably providing space  43 , but not along exterior edges  18 ,  22 , and  24 . As probably the least preferred alternative embodiment and as shown in  FIG. 5G , the underfill  30  could be applied as lines along all four edges  18 ,  20 ,  22  and  24  and in an overlapping manner with no gaps defined at the corners. In this case, the air, or gas, trapped under the electronic device  14  will bubble through the underfill  30  when the vacuum is applied. 
     The lines of underfill, in addition to being applied in the preferred method from a non-contact jetting valve, such as the DJ 9000 sold by Nordson ASYMTEK of Carlsbad, Calif., could alternatively be applied as solid pre-forms of epoxy. The solid pre-forms are placed on the substrate  12  and then melted upon the application of heat. The solid pre-forms could be placed into position by a pick and place machine or mechanism. 
     Gas or air  66  can be trapped under the underfill  30  when the underfill is provided on the substrate. Air that is trapped under the underfill  30  when the underfill  30  is applied or laid along the edge of the electronic device  14  may vent underneath the electronic device  14  after the vacuum is applied and the underfill  30  is heated it induce capillary flow. The vented air may become trapped under the electronic device  14  as air pockets, which may lead to the formation of voids in the underfill  30 . Ensuring space  43  is maintained until the full vacuum is applied mitigates this trapped air from venting under the electronic device  14 . 
     In accordance with alternative embodiments of the invention, the substrate  12  may include an obstacle positioned on the surface  16  proximate to at least one exterior edge  18 ,  20 ,  22 ,  24  of the electronic device  14 . In a representative embodiment, the obstacle may be formed as a linear body. The obstacle is located between the location of the dispensed underfill  30  and the adjacent exterior edge  18 ,  20 ,  22 ,  24  of the electronic device  14 . 
     The obstacle serves as an impediment over which the underfill  30  must flow before flowing toward the exterior edge  18 ,  20 ,  22 ,  24  of the electronic device  14  and into the open portion of the space  28 , during the procedure for vacuum underfilling shown in  FIG. 2  thereby maintaining space  43 . The liquid underfill  30  (or a majority thereof) is able to flow over the obstacle, and the obstacle has only a negligible or minor effect on the flow and flow rate of the underfill liquid. However, air or gas pockets are generally incapable of surmounting the obstacle or are vented as the underfill  30  flows over space  43  before reaching the gap  27 . As such, the obstacle removes air or gas pockets from the underfill. Therefore, this embodiment helps reduce or eliminate trapped gas under the electronic device  14  during the vacuum-assisted underfilling operation. 
     As the distance between the dispensed underfill  30  and the exterior edge  18 ,  20 ,  22 ,  24  of the electronic device  14  increases, the ability of trapped gas  66  under the underfill  30  to reach the gap  27  decreases. If air  66  is trapped under the underfill  30  and the underfill  30  is laid adjacent to the exterior edges  18 ,  20 ,  22 ,  24  of the electronic device  14  (i.e., in contact with the electronic device  14 ), the air  66  trapped under the underfill  30  may be vented under the electronic device  14  when the vacuum is applied and the underfill  30  is heated. Air that vents under the electronic device  14  may become trapped under the electronic device  14 . Therefore, the underfill  30  should be positioned on the substrate  12  far enough away from the exterior edge  18 ,  20 ,  22 ,  24  of the electronic device  14  so as to avoid venting under the electronic device  14 . When the underfill  30  is positioned far away from the exterior edge  18 ,  20 ,  22 ,  24  of the electronic device  14 , the substrate  12  may be tilted so as to help induce the underfill  30  to flow toward the exterior edge  18 ,  20 ,  22 ,  24  and under the electronic device  14  when the underfill  30  is heated. The overall purpose is to prevent air  66  trapped under the underfill  30  from venting under the electronic device  14 , with the underfill  30  then flowing around the air so as to form a bubble under the electronic device  14 . The use of the obstacle, as in the present embodiment, effectively achieves the same result, as the underfill  30  is spaced apart from the exterior edges  18 ,  20 ,  22 ,  24  of the electronic device  14  by a distance required for the placement of the obstacle. 
     With reference to  FIGS. 6A-7B  in which like reference numerals refer to like features in  FIGS. 1-5G  and in accordance with an alternative embodiment, the obstacle may be a dam  68  formed on the surface  16  of the substrate  12 . The dam  68  may have a top wall  72  raised above the surface  16  of the substrate  12  and side walls  70  ascending from the surface  16  to the top wall  72 . As discussed above, the surface  16  receives the dispensed underfill  30 . Consequently, the dam  68  is located on the same surface  16  that receives the dispensed underfill  30  and on which the electronic device  14  is mounted and between underfill  30  and the exterior edges of  18 ,  20 ,  22 ,  24 . A height of the dam  68  is sufficiently low so that the underfill  30  may flow over the dam  68  when the assembly  10  is heated to a given temperature. The height of the dam  68  is low enough not to impede the underfill flow after heating. Although the underfill  30  may flow over the dam  68 , the air  66  is unable to surmount the wall  70  or the air vents through the underfill as it flows over space  32  toward external edges  18 ,  20 ,  22 , 24  and, therefore, the air does not flow under the electronic device  14 . 
     The dam  68  may be formed of a legend ink, such as that which is typically used on PC boards for visible markings or letters. Alternatively, a damming material such as that which is typically used for dam and fill operations could be employed. More generally, the damming material could be any thixotropic material, meaning any material that does not flow once it is deposited on the surface  16  of the substrate  12 . 
     Although the side walls  70  and the top wall  72  of the dam  68  form two right angles in the representative embodiment, the side walls  70  and/or the top wall  72  may be inclined, contoured, and/or curved. Alternatively, the two side walls  70  may converge at an angle, such that the top of the dam  68  forms a peak or a crest rather than a wall that is parallel to the surface  16  of the substrate  12 . Moreover, a width of the dam  68 , including the dimensions of the side walls  70  or the top wall  72 , may vary. 
     With reference to  FIGS. 8A-9B  in which like reference numerals refer to like features in  FIGS. 6A-7B  and in accordance with an alternative embodiment, the obstacle may be a channel  74  formed in the substrate  12  and recessed below the surface  16  of the substrate  12 . The channel  74  may be formed by a router, for example. As discussed above, the surface  16  receives the dispensed underfill  30 . Consequently, the channel  74  is located on the same surface  16  that receives the dispensed underfill  30  and on which the electronic device  14  is mounted. The channel  74  may have a base  78  positioned at a distance below a level of the surface  16  and side walls  76  descending from the surface  16  to the base  78 . The channel  74  may obstruct or impede the underfill  30  from flowing to external edges  18 ,  20 ,  22 ,  24 , prior to heating the underfill. As shown in  FIGS. 9A and 9B , after vacuum is applied and the underfill  30  is heated, the underfill  30  flows into and/or over the channel  74  before flowing toward the at least one exterior edge  18 ,  20 ,  22 ,  24  of the electronic device  14  and into the open portion of the space  28 . However, the air  66  trapped under the underfill  30  is trapped in the channel  64 ; once the air  66  flows into the channel  64 , it is unable to surmount the sidewalls  76 . Any remaining air vents through the underfill  30  before the underfill  30  reaches the gap  27 . In this way, the channel  74  helps to prevent the air  66  from flowing under the electronic device  14 . The depth of the channel  74  should be sufficiently shallow so that substantially all of the liquid underfill  30  may flow through or over the channel  74 . However, the depth of the channel  74 , and thus the heights of the side walls  76 , may vary. 
     Although the side walls  76  and the base  78  of the channel  74  form two right angles in the representative embodiment, the walls  76  and/or base  78  may be inclined, contoured, and/or curved. Alternatively, the two side walls  76  may converge at an angle, such that the channel  74  lacks a planar base. Moreover, a width of the channel  74 , including the dimensions of the side walls  76  or the base  78 , may vary. 
     In an alternative embodiment, the obstacle may include combined features of the dam  68  and the channel  74 . For example, the dam  68  may be immediately followed by the channel  74  on the substrate  12 , such that the underfill  30  flows over the dam  68  and through the channel  74  before flowing toward the exterior edge  18 ,  20 ,  22 ,  24  of the electronic device  14 . 
     In the representative embodiment, a single obstacle is shown extending around an entire periphery of the electronic device  14 . However, in alternative embodiments, one or more obstacles may extend along any combination of the one or more exterior edges  18 ,  20 ,  22 ,  24 . Moreover, the obstacles may be longer or shorter than the lengths of the one or more exterior edges  18 ,  20 ,  22 ,  24 . Preferably, a positioning of the one or more obstacles will correspond to the positioning of the dispensed underfill  30 , such that all of the underfill  30  must flow over the obstacles in order to reach the exterior edges  18 ,  20 ,  22 ,  24  of the electronic device  14 . 
     With reference to  FIGS. 10 and 11  and in accordance with an alternative embodiment, the surface  16  of the substrate  12  having an original composition and wettability may be modified to mitigate the trapping of air under the underfill  30  is originally dispensed. In this embodiment, the substrate  12  may be plasma treated so as to change the wettability of the surface on which the underfill  30  is dispensed. The plasma treatment process may be activated by methods known to those of ordinary skill. 
     With particular reference to  FIG. 10 , the substrate  12  may also be plasma treated so as to activate a surface layer  94  of the substrate  12 . Such activation may alter a chemical composition, and, thus, physical characteristics of the surface layer  94  of the substrate  12  so as to change its wettability. The surface layer  94  of the substrate  12  has a thickness t 1 . The plasma activation does not add a layer to the substrate  12 ; rather, it modifies the layer  94  with thickness t1 of the preexisting substrate  12 . 
     In an embodiment, the plasma treatment decreases the wettability of the layer  94  of the substrate  12 . By rendering the surface layer  94  of the substrate  12  less wettable, less air may be trapped and air that is trapped under the underfill  30 , when the underfill  30  is positioned on the substrate  12 , may more easily escape from beneath the underfill  30  when the vacuum is applied. The surface layer  94  with decreased wettability may have more surface imperfections through which the air may escape than the original surface  16  of the substrate  14 . In this way, the trapping of air under the electronic device  14  during the vacuum-assisted underfill operation may be reduced by the reduction of trapped air  66  under the underfill  30 . 
     In another embodiment, the plasma treatment increases the wettability of the layer  94  of the substrate  12 . Less air is entrapped under underfill  30  deposited on the plasma-treated surface having an increased wettability than on a non-plasma treated surface because air may be more easily displaced as the underfill  30  is applied. By reducing the initial trapping of air under the underfill  30 , the trapping of air under the electronic device  14  during the vacuum-assisted underfill operation may also be reduced. 
     With particular reference to  FIG. 11 , plasma deposition may be used to deposit a very thin, glass-like layer  90  or film on the surface  16  of the substrate  12 . The layer  90  has a thickness t2, and, thus, a height of the plasma-treated substrate is increased (as compared to a height of the original substrate  12 ) by height t 2 . The plasma-treated surface may be so smooth and flat that there are fewer surface imperfections, such as depressions, in which air can be trapped. As such, conducting vacuum-assisted underfilling on the plasma deposited layer  90  helps prevent air or gas from being trapped under the underfill  30 . By reducing the initial trapping of air under the underfill  30 , the trapping of air under the electronic device  14  during the vacuum-assisted underfill operation may also be reduced. In an embodiment, a combined plasma treatment method may be employed, in which the glass-like layer  90  is deposited on the substrate  12  and then is activated so as to further increase wettability. 
     In an embodiment, a combination of the methods provided above may be employed to help prevent the entrapment of air bubbles under the electronic device  14 . For example, the top surface  16  of the substrate  12  may be plasma treated so as to increase the wettability of the top surface  16  and/or to deposit a glass-like layer  90  on the substrate  12 . Such plasma treatment will help prevent air from being trapped under the underfill  30  when it is provided on the substrate  12 . In addition, an obstacle, such as a dam  68  or a channel  74 , may be provided on the plasma-treated substrate  12  so as to block any air  66  that may have been trapped under the underfill  30  from flowing under the electronic device  14  during the vacuum-assisted underfill operation or to prevent the underfill  30  from flowing over space  43  prior to heating the underfill  30  in the vacuum 
     With reference to  FIG. 12 , a system  110  for use in vacuum underfilling is configured to dispense amounts of the underfill  30  on the substrate  12  upon which the electronic device  14  is mounted by reflowed solder balls, or another interconnect technology, and is separated from the substrate  12  by the space  28 . The space  28  has an open portion that is not occupied by the conductive joints  26 , which in this case are in the form of reflowed solder balls. 
     A controller  120 , which is electrically coupled with a motion controller  118  and a dispenser controller  116 , coordinates the overall control for the system  110 . Each of the controllers  116 ,  118 ,  120  may include a programmable logic controller (PLC), a digital signal processor (DSP), or another microprocessor-based controller with a central processing unit capable of executing software stored in a memory and carrying out the functions described herein, as will be understood by those of ordinary skill in the art. 
     The system  110  preferably includes a cooling device  133  and a cooling device  135  that is coupled with the dispenser  132 . The cooling device  133  is configured to cool the substrate  12  such that the underfill  30  cools when dispensed onto the substrate  12 . The cooling device  135  is configured to cool the underfill  30  such that the underfill  30  is cooled before dispensing onto the substrate  12 . The cooling devices  133 ,  135  are preferred, and optional, and may be respectively operated by a temperature controller  139  under the control of controller  120  to reduce the temperature of the substrate  12  to below room temperature and/or to reduce the temperature of a portion of the dispenser  132  to below room temperature. 
     The system  110  includes a dispenser  132 , which may be a jetting dispenser, used to dispense the amounts of the underfill. Downstream from the dispenser  132 , the system  110  further includes a vacuum chamber  154  configured to permit access for inserting and removing each assembly  10  and configured to provide a sealed condition in which an interior space of the vacuum chamber  154  is isolated from the surrounding atmospheric-pressure environment. A vacuum pump  160  is coupled with the interior space of the vacuum chamber and is configured to evacuate the interior space as operated by the controller  120 . A vent  174  is used under the control of the controller  120  to admit gas to the interior space to raise the chamber pressure. The controller  120  supplies motion instructions to the motion controller  118  to operate a transfer device  122  used to move the substrate  12 , which is carrying the underfill  30 , into the vacuum chamber  154 . 
     A heater  166  is disposed inside the vacuum chamber  154  and is configured to be powered by a temperature controller  169  linked with the controller  120 . Heat is transferred from the heater  166  to each substrate  12 . In one embodiment, the temperature of the substrate  12  and underfill on the substrate ranges from 30° C. to 120° C. 
     In use, the substrate  10  is moved to a location beneath the dispenser  132  and underfill is dispensed or otherwise applied. In the representative embodiment, the controller  120  sends commands to the motion controller  118  to cause the transfer device  122  to move the dispenser  32  and the controller  120  sends commands to the dispenser controller  116  to cause the dispenser  32  to dispense the underfill in one or more lines around the exterior edges  18 ,  20 ,  22 ,  24  of the electronic device  14 . The substrate  12  is not heated during the dispensing operation. Preferably, at least one gap is left in the one or more lines of underfill  32  and preferably the underfill  30  is not in contact with the exterior edge  18 ,  20 ,  22   24 . For a jetting dispenser  132 , the dispenser controller  16  triggers the jetting of droplets at appropriate times during the movement such that the droplets will impact at a desired location on the substrate  12 . Each dispensed droplet contains a small volume of the underfill, which is typically controlled with high precision by the dispenser controller  16 . 
     In one embodiment, the cooling device  133  may be used to cool the substrate  12  so that the underfill  30  cools to a temperature below room temperature upon contact with the substrate  12 . Alternatively, the cooling device  135  coupled with the dispenser  132  may be used to cool the underfill  30  before dispensing. 
     After the dispensing operation is completed and before significant capillary underfilling (and air or gas entrapment) occurs, the controller  120  sends commands to the motion controller  118  to cause the transfer device  122  to transport the assembly  10  and dispensed underfill  30  on the substrate  12  into the vacuum chamber  54 . Once the assembly  10  and dispensed underfill  30  on the substrate  12  are isolated inside the vacuum chamber  54  from the ambient environment, the controller  120  causes the vacuum pump  160  to evacuate the interior space inside the vacuum chamber  154 . While the vacuum is being applied, each gap allows a vacuum condition (i.e., a pressure less than atmospheric pressure) to be established under the electronic device  14  between the electronic device  14  and the substrate  12  or, if there is no gap, then the gas bubbles through the underfill to create a vacuum condition under the electronic device  14 . 
     When a suitable vacuum pressure exists inside the vacuum chamber  154  and with the vacuum condition being maintained, the controller  120  causes the temperature controller  169  to operate the heater  166 , which heats the substrate  12 , electronic device  14 , and the underfill  30 . The elevated temperature encourages the underfill  30  to flow over the substrate space  43  and into the open portion of the space beneath the electronic device  14 . The underfill  30  completely flows under the electronic device  14  and into the spaces between the reflowed solder balls. Underfilling in the presence of the vacuum condition means any void entrapped in the underfill will be partially evacuated of gases. After flow ends, the controller  120  sends commands to the motion controller  118  to cause the vent  174  to admit gas to the vacuum chamber  154  so that the pressure inside the vacuum chamber  154  is returned to atmospheric pressure. Any voids present in the underfill  30  collapse because of the evacuated condition and become filled with underfill  30 . The substrate  12  with the underfilled electronic device  14  is transferred out of the vacuum chamber  154  to, for example, a curing oven (not shown). 
     While the invention has been illustrated by the description of one or more embodiments thereof, and while the embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of Applicant&#39;s general inventive concept.