Abstract:
A terminal case formed by integrally molding a lead frame and a case that has internally an inner face on which the lead frame is mounted and has externally a step portion fixed to a circuit block having an insulating substrate and semiconductor chips formed on the insulating substrate. An opening portion is formed between the step portion and the inner face so as to extend through them, and the opening portion is filled with an adhesive to bond the insulating substrate to the step portion. Since a connecting area to which a bonding wire of the lead frame is ultrasonically bonded is fixed, it is possible to reduce the bonding failures of the lead frames.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application of International Application PCT/JP2015/055066 filed on Feb. 23, 2015 which designated the U.S., which claims priority to Japanese Patent Application No. 2014-093422, filed on Apr. 30, 2014, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments discussed herein relate to a semiconductor module and a method for manufacturing the same. In particular, they relate to a semiconductor module used in applications including an inverter device, such as a motor drive device, and a power conversion device, such as a switching power supply device, and to the method for manufacturing the same. 
         [0004]    2. Background of the Related Art 
         [0005]    In a semiconductor module for power conversion, a plurality of semiconductor chips for power, such as power transistors or diodes for power conversion, is integrated into one package. In such a semiconductor module, circuit wiring suitable for a desired application is performed in advance inside a package, thereby contributing to miniaturization of the whole application device. In the semiconductor module for power conversion, a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) or an IGBT (Insulated-Gate Bipolar Transistor) is generally used as a power transistor. 
         [0006]    An element for power conversion is configured so as to be exposed to the outside of a semiconductor module via a copper foil surface on an insulating substrate and via the insulating substrate, in order to reduce by heat radiation the heat generated by the power loss of the element. Here, the heat radiation is performed from the exposed surface using a radiator (e.g., see Japanese Laid-open Patent Publication No. 2013-258321 (Paragraph [0043], FIG. 2)). That is, in this Japanese Laid-open Patent Publication No. 2013-258321, an insulating layer is arranged in the surface of a thermally-conductive metal plate, a main circuit wiring pattern is formed on the insulating layer, and a semiconductor chip is bonded onto the main circuit wiring pattern. The heat generated by this semiconductor chip is transferred to the metal plate via the main circuit wiring pattern and the insulating layer, and is radiated by a heat sink bonded to this metal plate. 
         [0007]    Moreover, in the element for power conversion, a desired circuit is formed from a lead frame, a conductive wire, and/or a wiring pattern formed on a substrate, and the circuit is directly connected or indirectly connected via a wire or the like, to a terminal which is the means for electrically connecting to the outside. 
         [0008]    Furthermore, a module called IPM (Intelligent Power Module) which is a semiconductor module having a control IC (integrated circuit) mounted thereon has been commercialized and widely used. The control IC has a driver function to drive the element for power conversion and a function to detect abnormal states of the element, such as overcurrent, and protect the element. Since the control IC generates a small amount of heat, and in order to avoid the noise of the element for power conversion, the control IC is mounted at a position away from the substrate on which the element for power conversion is mounted. In Japanese Laid-open Patent Publication No. 2013-258321, the control IC is mounted on a wiring pattern formed inside a case. 
         [0009]    Moreover, in the IPM, there is also a module having mounted therein a current detection element, a temperature detection element, a snubber element, and/or passive components, such as a capacitor connected to supply a stable electric power to the control IC. The semiconductor module having these elements mounted therein and having a desired electric connection made therein is resin-sealed to complete the IPM. 
         [0010]    For example, in the IPM disclosed in Japanese Laid-open Patent Publication No. 2013-258321, elements for power conversion, such as an IGBT and an FWD (Free Wheeling Diode), are mounted, by soldering, onto an insulating substrate in which a wiring pattern is formed in advance. In a terminal case holding this insulating substrate, a lead frame and a PPS resin (Polyphenylene Sulfide Resin) are integrated by insert molding. The terminal case is molded by injecting the PPS resin into a mold which is fixed and set by a fixing pin that is placed so that the lead frame may neither displace nor deform due to a resin pressure during molding, or a fixing pin such as an ejector pin, but an opening portion formed at that time (hole by the fixing pin) is sealed with the resin by moving the fixing pin during molding. In the terminal case, elements, such as a control IC, are mounted on a lead terminal with an adhesion means, such as an Ag (gold) paste. Subsequently, the insulating substrate is bonded to the terminal case with an adhesive, electrical connection (ultrasonic bonding) is made using an aluminum wire so as to form a desired circuit across the element, the wiring pattern of the insulating substrate, and the lead frame, and then the terminal case is sealed with a cast resin so as to constitute a semiconductor module. 
         [0011]    It is known that a bonding defect may be caused in ultrasonically bonding a bonding wire to a lead terminal (e.g., see Japanese Laid-open Patent Publication No. 2004-134518 (Paragraphs [0048]-[0056], FIGS. 4 to 7)). According to this Japanese Laid-open Patent Publication No. 2004-134518, in manufacturing a terminal case, due to a difference in the linear expansion coefficient between a resin and a metal lead terminal, a lead terminal closely attached to the terminal case may depart from the case when a high-temperature resin gets cold. Moreover, because there is no adhesive force between the lead terminal and the PPS resin, a very small gap will be produced between the both even if the both are formed so as to be closely attached to each other. In a state where a lead terminal is floating from the case, in attempting to ultrasonically bond a bonding wire to the lead terminal, the ultrasonic vibration energy will be absorbed by the lead terminal, which causes a bonding defect. 
         [0012]    In contrast, in Japanese Laid-open Patent Publication No. 2004-134518, a through-hole is formed in a case directly under an ultrasonic bonding portion of a terminal plate serving as a lead terminal, and a support member insertion port is formed, also in a heat sink bonded to the case, at the position corresponding to the through-hole. In ultrasonically bonding a bonding wire, the bonding wire is ultrasonically bonded to the upper surface of the terminal plate using a rod-like tool while the lower surface of the ultrasonic bonding portion of the terminal plate is supported with the fixing pins arranged in the support member insertion port and the through-hole. Note that, because the terminal plate faces the through-hole, the lower surface of the terminal plate is exposed to the air to improve the radiation performance. 
         [0013]    Moreover, in Japanese Laid-open Patent Publication No. 2004-134518, in the lower surface of a frame-shaped case made of an insulating resin and having an opening formed in the center portion thereof, a plate-like heat sink formed from a thermally conductive metal is arranged so as to plug the opening with the heat sink. Although an adhesive is used in bonding such a resin and metal, if the adhesive is excessively applied more than needed, then a high bonding strength may be obtained but the adhesive may protrude from the bonding area. The adhesive flowing out to the outside of the case will deteriorate the appearance of the case, and furthermore the adhesive flowing into the case will contaminate the bonding portion formed by ultrasonic bonding and reduce the bonding strength. 
         [0014]    As a method for suppressing such protrusion of the adhesive, there is known a method in which a plurality of grooves is formed in the surface facing the heat sink of a case, among the plurality of grooves, a groove in the vicinity of the center is used as the adhesive application groove, and the outer and inner grooves are used as the adhesive outflow prevention grooves (e.g., see Japanese Laid-open Patent Publication No. 2012-15349 (FIG. 1)). Thus, even if the adhesive protrudes from the adhesive application groove, the adhesive is accommodated into the adhesive outflow prevention groove, thus preventing the adhesive from flowing out beyond the adhesive outflow prevention groove. 
         [0015]    The conventional semiconductor module has a problem that the equipment cost increases because when a bonding wire is ultrasonically bonded to a lead frame, a pin-like pressing jig is needed for pressing the face opposite to an ultrasonic bonding face and in addition the pressing jig is complicated. Moreover, there is also a problem that the manufacturing throughput decreases because the pressing jig needs to be attached and detached in ultrasonic bonding and thus the attachment and detachment take a time. 
       SUMMARY OF THE INVENTION 
       [0016]    According to one aspect of the embodiments, there is provided a semiconductor module including: a lead frame with a connecting area to which a bonding wire is ultrasonically bonded; and a case having internally a mounting face on which the lead frame is mounted, having externally a fixing face to which a circuit block having a semiconductor chip formed on an insulating substrate is fixed, and having an opening portion which is formed so as to extend through between the mounting face and the fixing face, wherein the opening portion is filled with an adhesive to bond the circuit block and a portion near the connecting area of the lead frame. 
         [0017]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0018]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is an exploded perspective view of a semiconductor module according to an embodiment discussed herein when seen from the bottom-face side of the semiconductor module; 
           [0020]      FIG. 2  is a cross sectional view illustrating the semiconductor module in an upside down state before mounting of an insulating substrate; 
           [0021]      FIG. 3  is a cross sectional view illustrating the semiconductor module at mounting of the insulating substrate; 
           [0022]      FIG. 4  is a cross sectional view illustrating the semiconductor module when wiring with a bonding wire is performed; 
           [0023]      FIG. 5  is a cross sectional view illustrating the semiconductor module after resin sealing; 
           [0024]      FIG. 6  is a flow chart illustrating a method for manufacturing the semiconductor module; and 
           [0025]      FIG. 7  illustrates a specific example of the semiconductor module. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Hereinafter, embodiments of a semiconductor module according one aspect will be described in detail with reference to the accompanying drawings. Note that, in the drawing used for description of the following embodiments, the same constituent element is given the same reference numeral to omit the duplicated description. 
         [0027]      FIG. 1  is an exploded perspective view of a semiconductor module according to an embodiment discussed herein when seen from the bottom-face side of the semiconductor module,  FIG. 2  is a cross sectional view illustrating the semiconductor module in an upside down state before mounting of an insulating substrate,  FIG. 3  is a cross sectional view illustrating the semiconductor module at mounting of the insulating substrate,  FIG. 4  is a cross sectional view illustrating the semiconductor module when wiring with a bonding wire is performed, and  FIG. 5  is a cross sectional view illustrating the semiconductor module after resin sealing. 
         [0028]    The semiconductor module according to the embodiment discussed herein includes a terminal case  1  as seen in  FIG. 1  that illustrates the outline before assembling of the semiconductor module. The terminal case  1  is formed by integrally molding a lead frame  2  and a case  3  (by insert-molding a lead frame). The terminal case  1  includes a pair of opposed parallel sidewall portions  4   a  having the lead frame  2  inserted thereinto and a pair of opposed parallel sidewall portions  4   b  connected to the both ends in the longer direction of the sidewall portion  4   a,  and is formed in a rectangular frame shape in a plan view. The sidewall portions  4   a  and  4   b  of the terminal case  1  each have an L-shaped cross section. In the bottom face (the upper surface in  FIG. 1 ) of the terminal case  1 , a step portion  6   a  is formed along the periphery of a center opening portion  5 . The step portion  6   a  of the sidewall portions  4   a  and  4   b  is a fixing face to which an insulating substrate  7  is fixed, and has a depth thinner than the thickness of the insulating substrate  7 . 
         [0029]    In the terminal case  1 , also as illustrated in  FIG. 2 , the inner side of the lead frame  2  is mounted on an inner face  6   b  which is the mounting face located on the opposite side of the step portion  6   a  and the outer side of the lead frame  2  extends outward through the sidewall portion  4   a.  In the lead frame  2 , the opposite side of a face mounted on the inner face  6   b  of the case  3  serves as the connecting area by ultrasonic bonding. 
         [0030]    Here, in the inner face  6   b  having the lead frame  2  mounted thereon of the terminal case  1 , an opening portion  8  extending through to the step portion  6   a  is formed. That is, in a case portion in which the step portion  6   a  is formed, the lead frame  2  and a connecting area of the insulating substrate  7  are located on the upper side and lower side of the opening portion  8 , respectively. The opening portion  8  is formed in a case portion directly under the connecting area of the lead frame  2 , i.e., in a case portion near a portion where wire bonding is performed by ultrasonic bonding. Note that, the portion where wire bonding is performed by ultrasonic bonding is also a portion through which a main current of the lead frame  2  flows. Furthermore, the opening portion  8  may be, as needed, formed also in a portion where the lead frame  2  is arranged but ultrasonic bonding is not performed, for the purpose of fixing the lead frame  2  to the insulating substrate  7 . 
         [0031]    The opening portion  8  is formed during insert molding of the lead frame. That is, in setting the pre-molded lead frame  2  to a mold, an ejector pin is installed in advance so as to support the lead frame  2 . In this state, a PPS resin, for example, is injected into the mold to mold the terminal case  1 . Here, because the resin filling the mold will be solidified without moving of the ejector pin, the resin will not turn around to the ejector pin. Subsequently, the terminal case  1  is pushed out from the mold by means of the ejector pin, and furthermore the ejector pin is pulled out from the terminal case  1  so as to form the opening portion  8 . 
         [0032]    The opening portion  8  may be also formed by a so-called pressing pin which is used to prevent twisting and/or displacement of the lead frame  2  during insert molding, or may be formed using both the ejector pin and the pressing pin. 
         [0033]    Next, a procedure for constructing a semiconductor module using the terminal case  1  that is formed in this manner will be described. First, in the terminal case  1 , depending on the application and/or the purpose, a control IC, and passive elements, such as a capacitor and a resistor, are placed on the lead frame  2  which serves also as a circuit.  FIG. 2  illustrates an example in which a control IC  9  is placed on the lead frame  2 . 
         [0034]    The terminal case  1  having the control IC  9  placed on the lead frame  2  is turned upside down as illustrated in  FIG. 2 , and next as illustrated in  FIG. 3  an adhesive  10  is applied to the step portion  6   a  to which the insulating substrate  7  is fixed. As the adhesive  10 , a resin having a thermal conductivity (e.g., 0.5 (W/mK)) higher than the thermal conductivity (e.g., 0.3 (W/mK)) of a PPS resin constituting the case  3  is used. The adhesive  10  is not only applied to the surface of the step portion  6   a  but fills the opening portion  8 . Thus, in the lead frame  2 , the portions facing the opening portion  8  will be bonded with the adhesive  10 . 
         [0035]    Next, in the terminal case  1 , as illustrated in  FIG. 3 , the insulating substrate  7  is placed onto the step portion  6   a  applied with the adhesive  10 . The insulating substrate  7  may be an Al (aluminum) insulating substrate or a DCB (Direct Copper Bonding) substrate, in which semiconductor chips  13  and  14  for power are mounted on a conductive wiring pattern  12 , which is formed in advance in the surface, to constitute a circuit block  11 . These semiconductor chips  13  and  14  are assumed to be an IGBT and an FWD, here. 
         [0036]    The circuit block  11  is placed onto the step portion  6   a,  with the mounting faces of the semiconductor chips  13  and  14  set downward, so that the outer peripheral portion of the insulating substrate  7  is not only bonded to the terminal case  1  with the adhesive  10  but also is bonded to the lead frame  2  with the adhesive  10  of the opening portion  8 . Thus, the lead frame  2  will be not only secured to the insulating substrate  7  with the adhesive  10  that fills the opening portion  8  directly thereunder but will be thermally coupled with the insulating substrate  7  via the adhesive  10 . 
         [0037]    Because the lead frame  2  is secured to the insulating substrate  7  via the opening portion  8  and a vicinity of a portion where wire bonding is performed is fixed, a complicated pressing jig is not needed during ultrasonic bonding and in addition a bonding defect will not be caused. 
         [0038]    Moreover, because the lead frame  2  is thermally coupled with the insulating substrate  7  via the opening portion  8 , the lead frame  2  may transfer the heat generated by the main current flowing, to the insulating substrate  7 , allowing for heat radiation of the lead terminal. Thus, the radiation performance of the lead frame  2  may be further improved as compared with the conventional example in which a lead frame is exposed to the air whose thermal conductivity is 0.0241 (W/mK) through a support member insertion port and an opening portion serving as a through-hole. 
         [0039]    Here, when the control IC  9  placed on the lead frame  2  has a temperature protection function and/or a temperature output function, the control IC  9  may precisely protect the semiconductor chips  13  and  14 . That is, the thermal resistance between the insulating substrate  7  to which the semiconductor chips  13  and  14  are connected with a low thermal resistance and the lead frame  2  on which the control IC  9  is placed is further reduced as compared with the conventional example in which the lead frame  2  is exposed to the air. Therefore, the thermal resistance between the semiconductor chips  13  and  14  and the control IC  9  decreases and accordingly the control IC  9  may more accurately detect the temperature, allowing for accurate protection of the semiconductor chips  13  and  14 . 
         [0040]    Next, the terminal case  1  to which the circuit block  11  is secured is rotated by 180 degrees as illustrated in  FIG. 4 , and the inner face  6   b  of the terminal case  1  and the mounting faces of the semiconductor chips  13  and  14  of the circuit block  11  are faced up. Here, the terminal case  1  is ready to be wire-bonded as the upper end face thereof is pressed in the direction indicated by an arrow  15 . Thus, as compared with a conventional example in which during ultrasonic bonding a lead frame floating from a case needs to be pressed down for each portion where wire bonding is performed, a pressing jig with simple configuration may be used. 
         [0041]    In this state, in the semiconductor module, wire bonding is performed using a bonding wire  16  of a main circuit and a bonding wire  17  of the control circuit as illustrated in  FIG. 5 . Here, the interface between the inner face  6   b  of the terminal case  1  and the lead frame  2  is cohesive, and therefore in ultrasonically bonding the bonding wire  16  to the lead frame  2 , an ultrasonic vibration energy may be reliably transferred without being absorbed. Moreover, wire bonding of the bonding wire  16  to the semiconductor chips  13  and  14  and wire bonding of the bonding wire  17  to the semiconductor chip  13 , control IC  9 , and lead frame  2  are also performed. 
         [0042]    Then, in the semiconductor module, as illustrated in  FIG. 5 , the terminal case  1  is filled with a liquid cast resin  18  to resin-seal the semiconductor chips  13  and  14  and control IC  9 . The cast resin  18  may be the same resin as the adhesive  10  for bonding the insulating substrate  7  of the circuit block  11  to the terminal case  1 . Use of the same material for the cast resin  18  and adhesive  10  achieves common use of a member. 
         [0043]    Next, a specific example of the process of manufacturing the semiconductor module will be described. 
         [0044]      FIG. 6  is a flow chart illustrating a method for manufacturing the semiconductor module, and  FIG. 7  illustrates a specific example of the semiconductor module. 
         [0045]    In manufacturing the semiconductor module, the circuit block  11  and the terminal case  1  are prepared in separate steps in advance, respectively, as illustrated in  FIG. 6 . First, the insulating substrate  7  is prepared (step S 1 ) for the circuit block  11 , a solder is applied to one of the faces of the insulating substrate  7  (step S 2 ). Next, the semiconductor chips  13  and  14  serving as an IGBT and an FWD are prepared (step S 3 ). These semiconductor chips  13  and  14  are mounted onto the solder applied to the insulating substrate  7  (step S 4 ), and then put into a reflow furnace and soldered to form the circuit block  11  (step S 5 ). 
         [0046]    On the other hand, with regard to the terminal case  1 , first the pre-molded lead frame  2  is prepared (step S 6 ), the lead frame  2  is set to a mold of an injection molding machine, and then a PPS resin is injected into the mold to mold the terminal case  1  (step S 7 ). Here, in the terminal case  1 , the opening portion  8  is formed using an ejector pin or pressing pin. Next, a thermoset Ag paste, for example, is applied to a control IC mounting position of the lead frame  2  in the terminal case  1  (step S 8 ). Next, the control IC  9  is prepared (step S 9 ), and the control IC  9  is mounted on the Ag paste and fixed to the lead frame  2  (step S 10 ). 
         [0047]    Next, the terminal case  1  having the control IC  9  mounted on the lead frame  2  is turned upside down (see  FIG. 2 ), and the adhesive  10  is applied to the step portion  6   a  formed around the center opening portion  5  of the terminal case  1 . Next, the insulating substrate  7  with the mounting faces of the semiconductor chips  13  and  14  set downward is mounted on the step portion  6   a,  thereby bonding the terminal case  1  and the insulating substrate  7  (step S 11 ). In the bonding step, because the same operations/equipment as the conventional operations/equipment used in bonding the terminal case and the insulating substrate may be used, no additional equipment or cycle time is needed, and therefore no extra cost incurs. 
         [0048]    Next, the terminal case  1  is turned upside down to face up the mounting faces of the semiconductor chips  13  and  14  of the insulating substrate  7  and the control IC mounting face of the lead frame  2 , and then wire bonding with the bonding wires  16  and  17  is performed (step S 12 ). 
         [0049]    Next, the terminal case  1  is filled with the cast resin  18  being a PPS resin to resin-seal the terminal case  1  (step S 13 ). 
         [0050]    A specific example of the semiconductor module manufactured through the above steps will be described. A semiconductor module  20  illustrated in  FIG. 7  is the one when seen from the upper surface of a case  3  before resin sealing. In the semiconductor module  20 , six wiring patterns  12  are formed in the insulating substrate  7  of the circuit block  11 , and the semiconductor chip  13  as an IGBT and the semiconductor chip  14  as an FWD are mounted on each of the wiring patterns  12 . Moreover, three control ICs  9  are mounted on a lead frame  2   a  serving as an internal interconnection. 
         [0051]    Here, the wiring pattern  12  or semiconductor chips and  14  and the lead frame  2  serving as the lead terminal are connected by the bonding wire  16 , and the opening portion  8  is formed directly under a portion near a portion of the lead frame  2  to which the bonding wire  16  is bonded. Thus, the lead frame  2  is fixed to the insulating substrate  7  with the adhesive  10  near a portion to which the bonding wire  16  is bonded. Accordingly, in ultrasonically bonding the bonding wire  16  to the connecting area of the lead frame  2 , a bonding failure due to flapping of the lead frame  2  will not be caused. Moreover, because the thermal conductivity of the adhesive  10  is higher than the thermal conductivity of the case  3 , the heat generated by the main current flowing into the lead frame  2  is transferred to the insulating substrate  7  via the adhesive  10 , the radiation performance of the lead frame  2  will be improved. 
         [0052]    On the other hand, the opening portion  8  formed directly under the lead frame  2   a  is to be filled with the adhesive  10  to fix the lead frame  2   a  to the insulating substrate  7 . While the both ends of the lead frame  2   a  are fixed by the sidewall portions  4   a  and  4   b  of the case  3 , the intermediate portion of the lead frame  2   a  is fixed to the insulating substrate  7  by the adhesive  10 , so that the lead frame  2   a  is more firmly fixed to the case  3 . 
         [0053]    Moreover, the adhesive  10  having a higher adhesion to the lead frame  2  rather than to the case  3  is more preferable. Thus, when a gap is produced between the case and the lead frame  2  due to a difference in linear expansion coefficient, the lead frame  2  and the insulating substrates  7  may be firmly and closely attached to each other and thus the radiation performance will be improved. 
         [0054]    Note that, the embodiments are not limited to the above-described configurations but may be appropriately combined and configured as needed. 
         [0055]    The embodiments described above simply illustrate the principle of the present invention. Furthermore, many variants and modifications are possible for those skilled in the art, the present invention is not limited to the precise configurations and exemplary applications illustrated and described above, and all the corresponding variants and equivalents are construed to be within the scope of the appended claims and their equivalents. 
         [0056]    The semiconductor module having the above-described configuration and method for manufacturing the same have an advantage that the productivity (wire bondability) may be improved because a portion near the connecting area of the lead frame may be fixed without changing the conventional step and/or adding any step to the conventional step. 
         [0057]    Because the lead frame is bonded to the insulating substrate with an adhesive having a high thermal conductivity, the thermal resistance between the insulating substrate and the lead frame is reduced and thus the radiation performance of the lead frame may be further improved. 
         [0058]    Furthermore, if a control IC placed on the lead frame has a temperature protection function and/or a temperature output function, the thermal resistance between the insulating substrate to which a semiconductor chip is connected with a low thermal resistance and the lead frame on which the control IC is placed will also decrease. Therefore, the thermal resistance between the semiconductor chip and the control IC will also decrease, and thus the control IC may more accurately detect the temperature, allowing for accurate protection of the semiconductor chip. 
         [0059]    All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.