Abstract:
A method and apparatus for desoldering electronic components from a substrate. A vacuum is used, to enhance the flow of a hot gas under an electronic component to reflow the solder connections attaching the electronic component to a substrate. Water vapor is added to the hot gas to increase the heat capacity of the hot gas. A system for periodically changing the direction of flow of the hot gas and vacuum under the electronic component is used to uniformly heat the solder connections.  
     A method and apparatus for depositing underfill material between an electronic component and the substrate on which the electronic component is mounted. A vacuum is applied to enhance the flow of underfill material into the space between the electronic component and the substrate.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is in the field of integrated circuits. More particularly, the present invention provides a method and apparatus for desoldering electronic components from a substrate. Another embodiment of the present invention provides a method and apparatus for depositing underfill material between an electronic component and the substrate on which the electronic component is mounted.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electronic components, such as integrated circuit chips, are commonly attached to a substrate (e.g., a printed circuit board (PCB) or printed circuit card (PCC)) with solder connections using a ball grid array (BGA), chip scale package (CSP), or direct chip attach (DCA) technique. Occasionally, an electronic component may be found to be defective, and will therefore have to be removed and replaced with a functional electronic component using a rework process. In a conventional rework process, the defective electronic component is removed by first heating the solder material, used to connect the component&#39;s solder connectors to corresponding contact pads on the substrate, to its melting, or “reflow” temperature. Then, the defective electronic component is pulled off the substrate and replaced.  
           [0003]    In a conventional rework process, a stream of hot gas is typically directed toward the top of the electronic component. This method works well if the solder connections are only located around or near the periphery of the electronic component, or when there is a relatively large gap between the bottom of the electronic component and the substrate. Unfortunately, using currently available reflow methods, heat from the stream of hot gas is not effectively or evenly transmitted to solder connections located away from the periphery of the electronic component (e.g., near the center of the electronic component). This is especially problematic if the space between the electronic component and the substrate is small, thereby restricting the flow of hot gas from the periphery to the center of the electronic component.  
           [0004]    Electronic components mounted on a substrate commonly require underfill to increase reliability, mechanical integrity, and to ensure adequate operational life. For example, an underfill material such as epoxy is commonly inserted between an electronic component and a substrate to cover the solder connections, thereby protecting the solder connections from corrosion causing fluids or gases, and mechanically strengthening the connection between the electronic component and the substrate. Further, the use of underfill reduces failure of the solder connections due to cycling stresses caused by differences in the coefficients of thermal expansion of the electronic component and the substrate. Thus, underfill provides a robust mechanical connection preventing damaging relative motion between the electronic component and the substrate.  
           [0005]    Commonly, the underfilling is accomplished by depositing a bead of underfill material along one or more sides of the electronic component and allowing capillary action to pull the underfill material under the electronic component. Unfortunately, not only is the process relatively slow and may leave voids in the underfill, but also requires the underfill material to be very fluid in nature. Thus, restrictions are placed on the composition of the underfill material.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention avoids the disadvantages of the prior art by providing an improved method and apparatus for removing an electronic component from a substrate. Also, the current invention provides an improved method and apparatus for applying underfill between the electronic component and the substrate.  
           [0007]    In accordance with the present invention, a rework nozzle apparatus is used to remove an electronic component from a substrate. The rework nozzle apparatus includes an outer tube, an inner shaft, baffles, a vacuum source, a hot gas source, and a water vapor port. The outer tube has a cross-sectional shape slightly larger than that of the electronic component. A first end of the outer tube contacts the substrate surface, encloses the electronic component, and provides an essentially gas tight seal. The inner shaft has a cross-sectional shape similar to the top surface of the electronic component. A first end of the inner shaft contacts, and essentially provides a gas tight seal against, the top surface of the electronic component. The first end of the inner shaft may include projections for locating the electronic component in the horizontal direction. Baffles are attached between the inner shaft and the outer tube to direct a flow of hot gas beneath the electronic component, and to provide a seal against the substrate adjacent two sides of the electronic component. The outer tube, inner shaft, and the baffles form two ducts. The first duct is used to carry and direct a stream of hot gas to a region under a first side of the electronic component. The second duct is used to apply a vacuum to a region under a second side of the electronic component to increase the flow of hot gas under the electronic component. The vacuum is provided to the second duct by a vacuum source such as a vacuum pump. Solder connections under the electronic component are heated to a reflow temperature allowing the electronic component to be removed from the substrate. In order to increase the heat capacity of the hot gas, thereby enhancing thermal transfer to the solder connections, water vapor, or other suitable substance, is added to the hot gas through a water vapor port.  
           [0008]    The rework nozzle apparatus may additionally include a vertical positioning apparatus, a heating element, and a reversing valve. The vertical positioning apparatus provides vertical positioning relative to the inner shaft, by means of a drive system such as a linear motor or stepper motor. The vertical positioning apparatus is slidably attached to the inner shaft. Heat is applied by the heating element to the inner shaft, preventing the inner shaft from drawing heat away from the electronic component during the rework process. The reversing valve periodically switches the vacuum from the second duct to the first duct, and simultaneously switches the hot gas from the first duct to the second duct, effectively reversing the direction of flow of the stream of hot gas. At the same time, the water vapor is switched from a water vapor port on the first duct to a water vapor port on the second duct. Advantageously, the use of the reversing valve provides a more uniform heating of the solder connections.  
           [0009]    In accordance with the present invention, an underfill nozzle apparatus is used to insert underfill material under the electronic component. Preferably, underfill material is deposited along three sides of the electronic component, and a vacuum is applied under the fourth side of the electronic component to draw the underfill material under the electronic component.  
           [0010]    The underfill nozzle apparatus includes a vacuum tube and a vacuum source. A first end of the vacuum tube contacts the substrate surface and provides an essentially gas tight seal. A side of the vacuum tube contacting the electronic component has an opening sized according to the cross-sectional open area under the electronic component. A vacuum is drawn through this opening promoting the flow of the underfill material under the electronic component.  
           [0011]    Another embodiment of the underfill nozzle apparatus includes a vacuum tube, a vacuum source, an underfill tube, an underfill material source, baffles, a heat generating apparatus, and a control system. A first side of the vacuum tube contacts a first side of the electronic component. A first side of the underfill tube contacts a second, opposing side of the electronic component. The first side of the vacuum tube and the first side of the underfill tube each include an opening sized according to the cross-sectional open area under the electronic component.  
           [0012]    The vacuum tube includes a first end that contacts the substrate surface, and a second end that is connected to a vacuum source. The underfill tube includes a first end that contacts the substrate surface, and a second end that is connected to a source of underfill material. A series of baffles are used to couple the vacuum tube to the underfill tube, and to seal the openings under the remaining open sides of the electronic component.  
           [0013]    The heat generating apparatus provides means for heating the electronic component and the underfill material in the underfill tube to reduce the effective viscosity of the underfill material. The reduced viscosity of the underfill material results in a faster flow rate of underfill material beneath the electronic component.  
           [0014]    Another embodiment of an underfill nozzle apparatus in accordance with the present invention includes a vacuum tube. A through hole is provided in the substrate at a location under the electronic component. Underfill material is deposited along the periphery of the electronic component and a first end of the vacuum tube is placed over the substrate through hole. The first end of the vacuum tube contacts the substrate surface on the side opposite from the electronic component, forming an essentially gas tight seal. A vacuum source is connected to a second end of the vacuum tube to generate a vacuum in the vacuum tube, the substrate hole, and the space underneath the electronic component. This vacuum rapidly pulls the underfill material under the electronic component.  
           [0015]    In another embodiment of the underfill nozzle apparatus, a vacuum tube surrounds the electronic component on a first side of the substrate. A through hole is provided in the substrate at a location under the electronic component. A first end of an underfill tube contacts the substrate surface on the side opposite the electronic component, and encloses the through hole. A second end of the underfill tube is connected to an underfill supply source that provides underfill material to the area under the electronic component via the underfill tube and through hole. The vacuum surrounding the electronic component causes the underfill to rapidly fill the space under the electronic component.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The features of the present invention will best be understood from a detailed description of the invention and a preferred embodiment thereof selected for the purposes of illustration and shown in the accompanying drawings in which:  
         [0017]    [0017]FIG. 1 illustrates a plan cross-sectional view of a rework nozzle apparatus in accordance with a first embodiment of the present invention;  
         [0018]    [0018]FIG. 2 illustrates a cross-sectional view taken along line  2 - 2  of the rework nozzle apparatus of FIG. 1;  
         [0019]    [0019]FIG. 3 illustrates a cross-sectional view taken along line  3 - 3  of the rework nozzle apparatus of FIG. 1;  
         [0020]    [0020]FIG. 4 illustrates a plan cross-sectional view of a rework nozzle apparatus in accordance with a second embodiment of the present invention;  
         [0021]    [0021]FIG. 5 illustrates a cross-sectional view taken along line  5 - 5  of the rework nozzle apparatus of FIG. 4;  
         [0022]    [0022]FIG. 6 illustrates a cross-sectional view taken along line  6 - 6  of the rework nozzle apparatus of FIG. 4;  
         [0023]    [0023]FIG. 7 illustrates a plan cross-sectional view of a first embodiment of an underfill nozzle apparatus in accordance with the present invention;  
         [0024]    [0024]FIG. 8 illustrates a cross-sectional view taken along line  7 - 7  of the underfill nozzle apparatus of FIG. 7;  
         [0025]    [0025]FIG. 9 illustrates a plan cross-sectional view of a second embodiment of an underfill nozzle apparatus in accordance with the present invention;  
         [0026]    [0026]FIG. 10 illustrates a cross-sectional view taken along line  9 - 9  of the underfill nozzle apparatus of FIG. 9;  
         [0027]    [0027]FIG. 11 illustrates a plan view of another embodiment of an underfill nozzle apparatus in accordance with the present invention;  
         [0028]    [0028]FIG. 12 illustrates a cross-sectional view taken along line  12 - 12  of the underfill nozzle apparatus of FIG. 11;  
         [0029]    [0029]FIG. 13 illustrates a plan cross-sectional view of another embodiment of an underfill nozzle apparatus in accordance with the present invention; and  
         [0030]    [0030]FIG. 14 illustrates a cross-sectional view taken along line  14 - 14  of the underfill nozzle apparatus of FIG. 13.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    The features and advantages of the present invention are illustrated in detail in the accompanying drawings, wherein like reference numerals refer to like elements throughout the drawings.  
         [0032]    A rework nozzle apparatus  10  in accordance with a first embodiment of the present invention is illustrated in FIGS. 1, 2 and  3 . The rework nozzle apparatus  10  is used to remove an electronic component  26  from a substrate  30  by heating the solder connections  46  that attach the electronic component  26  to the substrate  30  to a suitable reflow temperature. The rework nozzle apparatus  10  includes an outer tube  12 , an inner shaft  14 , baffles  16 ,  18 , a vacuum source  20 , a hot gas source  22 , and a water vapor port  24 . The outer tube  12  has a cross-sectional shape slightly larger than that of the electronic component  26 . A first end  28  of the outer tube  12  contacts the surface of the substrate  30 , and provides an essentially gas tight seal around the electronic component  26 . The first end  28  of the outer tube  12  can also include curving bottom walls  29  as shown by the phantom lines in FIG. 2. The curving bottom walls  29  assist the flow of gas or liquid under the electronic component  26 . Although described in conjunction with the embodiment illustrated in FIG. 2, it should be clear that the curving bottom walls  29  can also be used in the additional embodiments of the present invention described below. The inner shaft  14  has a cross-sectional shape similar to the top surface  32  of the electronic component  26 . A first end  34  of the inner shaft  14  contacts and provides a gas tight seal against the top surface  32  of the electronic component  26 .  
         [0033]    The inner shaft  14  is attached to the outer tube  12  by the baffles  16 ,  18 . The baffles  16 ,  18  also provide a seal against the surface of the substrate  30  on opposing sides  36 ,  39  of the electronic component  26 . The outer tube  12 , the inner shaft  14 , and the baffles  16 ,  18 , form two ducts  38 ,  40  as shown in FIGS. 1 and 2. Duct  38  is used to carry and direct a supply of hot gas from the hot gas source  22  to the space  42  under the electronic component  26 . Also, to increase the heat capacity of the hot gas, water vapor  44  is added through a water vapor port  24 . Duct  40  is used to apply a vacuum generated by the vacuum source  20  to the space  42  under the electronic component. The use of the vacuum increases the flow of hot gas  22  passing over the solder connections  46 , thereby decreasing the time required to heat the solder connections  46  to a required reflow temperature. After the solder connections  46  are heated to the reflow temperature, the electronic component  26  can be removed from the substrate  30 .  
         [0034]    A second embodiment of a rework nozzle apparatus  13  in accordance with the present invention is illustrated in FIGS. 4, 5 and  6 . The rework nozzle apparatus  13  includes an outer tube  52 , an inner shaft  54 , a vertical positioning apparatus  72 , a heating element  70 , baffles  56 ,  58 , a vacuum source  20 , a hot gas source  22 , a water vapor source  44 , water vapor ports  24 ,  25 , and a reversing valve  80 .  
         [0035]    The outer tube  52  has a cross-sectional shape slightly larger than that of the electronic component  26 . A first end  41  of the outer tube  52  contacts the surface of the substrate  30 , and provides an essentially gas tight seal around the electronic component  26 . The inner shaft  54  has a cross-sectional shape similar to the top surface  32  of the electronic component  26 , and has projections  74  for locating the electronic component  26  in the horizontal direction.  
         [0036]    As illustrated in FIGS. 4 and 6, the baffles  56 ,  58  are attached to the inner shaft  54  and the outer tube  52 . As further illustrated in FIG. 6, the baffles  56 ,  58  extend to the surface of the substrate  30 , thereby providing a seal against the surface of the substrate  30  on opposing sides  36 ,  39  of the electronic component  26 . The outer tube  52 , the inner shaft  54 , and the baffles  56 ,  58  form two ducts  82  and  84  as illustrated in FIGS. 4 and 5.  
         [0037]    The vertical positioning apparatus  72  is slidably attached  76  to the inner shaft  54 . A vacuum is applied to a vacuum port  78  to hold the top surface  32  of the electronic component  26  against the vertical positioning apparatus  72 . The vertical positioning apparatus  72  provides vertical positioning of the electronic component  26  relative to the substrate  30 . The vertical positioning apparatus  72  preferably includes a linear motor or stepper motor (not shown) or other suitable drive system.  
         [0038]    The heating element  70  preheats the inner shaft  54  to prevent heat from being drawn away from the electronic component  32  during the rework process. The inner shaft  54  may be preheated using a resistive heating element, a source of hot gas, or other suitable heating system.  
         [0039]    As in the first embodiment of the rework nozzle apparatus  10 , a stream of hot gas, a vacuum, and a supply of water vapor are used to facilitate and accelerate the rework process. In the second embodiment of the rework nozzle  13 , however, a reversing valve  80  is additionally used to provide a more uniform heating of the solder connections  46  of the electronic component  26 .  
         [0040]    As illustrated in FIG. 5, the hot gas supply  22  and the water vapor supply  44  are provided to the reversing valve  80 . In addition, the vacuum source  20  is coupled to the reversing valve  80 . A first set of conduits  92 ,  94  are provided to selectively direct a stream of hot gas or a vacuum from the reversing valve  80  to the ducts  82 ,  84  through ports  86 ,  88 , respectively. A second set of conduits  90 ,  96  are provided to selectively direct water vapor from the reversing valve  80  to the water vapor ports  24 ,  25  located in ducts  82 ,  84 , respectively.  
         [0041]    A first operating position of the reversing valve  80  causes the vacuum source  20  to generate a vacuum in duct  84  via conduit  92  and port  88 , and simultaneously causes a stream of hot gas provided by the hot gas source  22  to be supplied through conduit  94  and port  86  to the duct  82 . Thus, hot air passes in a first direction through duct  82 , under the electronic component  26 , and into duct  84 . The hot air is subsequently drawn out of duct  84  through port  88  and conduit  92 . In addition, the first operating position of the reversing valve  80  causes water vapor from the water vapor source  44  to be supplied through conduit  96  and water vapor port  24  to the duct  82 .  
         [0042]    With the reversing valve  80  in the first operating position, hot gas and water vapor rapidly flow from duct  82 , under electronic component  26 , to duct  84 , thereby rapidly heating the solder connections  46  that attach the electronic component  26  to the substrate  30 . This flow direction causes a solder connection  46 ′ on the end of the electrical component  26  adjacent the duct  82  to heat faster than a solder connection  46 ″ located on the opposite side of the electrical component  26  (i.e., near duct  84 ). This uneven heating occurs because the hot gas releases energy as it travels from duct  82 , under the electrical component  26 , into duct  84 . The release of energy is accompanied by a corresponding decrease in the temperature of the gas.  
         [0043]    In order to provide a rapid uniform heating of all of the solder connections  46 , thereby avoiding the uneven heating described above, the reversing valve  80  is switched to a second operating position. In the second operating position, the reversing valve  80  causes the vacuum source  20  to generate a vacuum in duct  82  via conduit  94  and port  86 , and simultaneously causes a stream of hot gas provided by the hot gas source  22  to be supplied through conduit  92  and port  88  to the duct  84 . Thus, hot air passes in a second, opposite direction through duct  84 , under the electronic component  26 , and into duct  82 . The hot air is subsequently drawn out of duct  82  through port  86  and conduit  94 . In addition, the second operating position of the reversing valve  80  causes water vapor from the water vapor source  44  to be supplied through conduit  90  and water vapor port  25  to the duct  84 . Thus, when the reversing valve  80  is in the second position, hot gas and water vapor rapidly flow from duct  84  to the vacuum in duct  82 .  
         [0044]    By periodically switching the reversing valve from the first to the second operating position, rapid, essentially uniform heating of the solder connections  46  occurs. That is, hot air flows past each of the solder connections  46  in two different directions, thereby providing a substantially uniform heating of the solder connections  46 . This minimizes the time that is required to heat all the solder connections  46  to a required reflow temperature.  
         [0045]    The operation of the reversing valve and the configuration of associated conduits and ports may be expanded to periodically direct a flow of hot air under the electronic component  26  from more than two different directions. This would provide an even more uniform heating of the solder connections  46 . In addition, a heating medium other than a hot gas may be used. For example, a liquid heated to a sufficient temperature could be directed under the electronic component  26  in a single direction, or in multiple directions, to reflow the solder connections  46 .  
         [0046]    [0046]FIGS. 7 and 8 illustrate a first embodiment of an underfill nozzle apparatus  100  in accordance with the present invention. Underfill material  102  is deposited along three sides  104 ,  106 , and  108 , of the electronic component  26 . Solder connections  46  attach, and create a space  122  between, the electronic component  26  and the surface of the substrate  30 . The underfill nozzle apparatus  100  includes a vacuum tube  110  and a vacuum source  112 . A first end  114  of the vacuum tube  110  contacts the surface of the substrate  30 , and provides an essentially gas tight seal. A second end  113  of the vacuum tube  110  is connected to the vacuum source  112 . A side  116  of the vacuum tube  110  is positioned in contact with a side  118  of the electronic component  26 . A vacuum is drawn through the opening  120  between the electrical component  26  and the substrate  30  on side  118  of the electronic component  26 . The vacuum rapidly draws the underfill material  102  previously deposited along the three remaining sides  104 ,  106 , and  108  of the electronic component  26  into the space  122  under the electronic component  26 , thereby covering the solder connections  46 .  
         [0047]    [0047]FIGS. 9 and 10 illustrates a second embodiment of an underfill nozzle apparatus  124  in accordance with the present invention. Solder connections  46  attach, and create a space  160  between, the electronic component  26  and the surface of the substrate  30 . The underfill nozzle apparatus  124  includes a vacuum tube  126 , a vacuum source  128 , an underfill material source  130 , an underfill tube  132 , baffles  134 ,  136 , a heat generating apparatus  138 , and a control system  140 .  
         [0048]    A first end  142  of the vacuum tube  126  contacts the surface of the substrate  30 , and provides an essentially gas tight seal. A second end  144  of the vacuum tube  126  is connected to the vacuum source  128 . A first end  146  of the underfill tube  132  contacts the substrate surface  30 , and forms a seal against the surface of the substrate  30 . A second end  149  of the underfill tube is connected to the underfill material source  130 . A side wall  148  of the vacuum tube  126  extends partially toward the substrate  30 , thereby providing an opening  150  that allows access to the area under the electronic component  26 . Similarly, a side wall  152  of the underfill tube  132  extends partially toward the substrate  30 , and provides an opening  154  that allows access to the area under the electronic component  26 .  
         [0049]    The heat generating apparatus  138  supplies heat to the underfill material  102  using heating coils  156  or other suitable means. The heat supplied by the heating coils  156  reduces the viscosity of the underfill material  102 , thereby increasing the flow rate of the underfill material  102  as the underfill material is drawn into the space  160  under the electronic component  26 . Heat  158  may also be supplied to the electronic component  26  by the heat generating apparatus  138  to prevent the electronic component  26  from acting as a heat sink and causing a reduction in the temperature of the underfill material  102 . Any reduction in the temperature of the underfill material  102  would result in an increase in viscosity and a decrease in the flow rate. The heat  158  may be in the form of radiant heat, a stream of hot gas, a hot liquid, or the like.  
         [0050]    Baffles  134 ,  136  join the vacuum tube  126  to the underfill tube  132 . The baffles  134 ,  136  also form a seal against the surface of the substrate  30  on opposing sides  164 ,  162  of the electronic component  26 .  
         [0051]    A control system  140  may be provided to control the operation of the underfill material source  130  and the vacuum source  128 . The control system  140  may include a timer and a transducer  165 . The transducer  165  is utilized to sense when the region  160  under the electronic component  26  has been filled to a predetermined level with the underfill material  102 . The transducer  165  may comprise, for example, a proximity sensor, a light beam switch, or the like.  
         [0052]    The control system  140  is configured to activate the vacuum source  128 , the underfill material source  130 , and the heat generating apparatus  138  to draw a supply of underfill material  102  beneath the electronic component  26 . The underfill material  102  rapidly flows from the underfill tube  132 , under the electronic component  26 , toward the vacuum tube  126 , due to the vacuum generated by the vacuum source  128 , and the heat supplied by the heat generating apparatus  138 . The control system  140  deactivates the vacuum source  128 , the underfill material source  130 , and the heat generating apparatus  138 , upon receipt of a signal from the transducer  165  indicating that the space  160  under the electronic component  26  has been filled to a predetermined level with underfill material  102 . Alternately, a timer may be used by the control system  140  to activate/deactivate the various components of the underfill rework apparatus  124 .  
         [0053]    [0053]FIGS. 11 and 12 illustrate another embodiment of an underfill nozzle apparatus  170  in accordance with the present invention. Underfill material  102  is deposited along the periphery of the electronic component  26 . Solder connections  46  create a space  178  between the electronic component  26  and the surface of the substrate  30 . A vacuum source  172  supplies a vacuum via a through hole  174  in the substrate  30 . As a vacuum is produced by the vacuum source  172 , the underfill material  102  is rapidly drawn toward the through hole  174  from the periphery of the electronic component  26 , thereby rapidly filling the space  178  under the electronic component  26 .  
         [0054]    [0054]FIGS. 13 and 14 illustrate yet another embodiment of an underfill nozzle apparatus  180  in accordance with the present invention. The underfill nozzle apparatus  180  includes a vacuum tube  182  and an underfill tube  186 . A through hole  184  is located under the electronic component  26  and passes through the substrate  30 . Solder connections  46  create a space  200  between the electronic component  26  and the surface of the substrate  30 .  
         [0055]    A first end  190  of the vacuum tube  182  has a cross-sectional area slightly larger than that of the electronic component  26 . The first end  190  of the vacuum tube  182  contacts the surface of the substrate  30  and provides an essentially gas tight seal around the electronic component  26 . A second end  194  of the vacuum tube  182  is connected to a vacuum source  192 .  
         [0056]    A first end  196  of the underfill tube  186  surrounds the through hole  184 . The second end  198  of the underfill tube  186  is connected to a underfill material source  199 . Underfill material  102  is injected via the through hole  184  into the space  200  under the electronic component  26 . The vacuum applied in the vacuum tube  182  assists the flow of underfill material  102  into the space  200  under the electronic component  26 . Specifically, the underfill material  102  is drawn by the vacuum from the center toward the periphery of the space  200  under the electronic component  26 .  
         [0057]    The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.