Abstract:
A system of mold locks ( 28, 30 ) is formed on a heatsink ( 2 ) of a packaged semiconductor to prevent/mitigate delamination . The mold locks ( 4, 12 ) anchor a plastic mold compound ( 34 ) that forms the protective cover for the packaged semiconductor die. The mold locks ( 4, 12 ) are miniaturized to allow the positioning of them within the flag portion of the heatsink ( 2 ) and leadframe ( 24 ) such that a semiconductor die can be anchored above the mold locks ( 4, 12 ) formed within the flag portion of the heatsink/lead frame ( 2, 24 ). The miniaturized size of the said moldlocks ( 4, 12  do not detract from the purpose of the die attach solder ( 36 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to the field of semiconductor packaging, and more specifically to mold locks formed in a heatsink that anchor the semiconductor package mold compound to the heatsink.  
       BACKGROUND OF THE INVENTION  
       [0002]     Microchips are formed from a variety of dissimilar materials. A semiconductor die, formed from silicon, gallium arsenide, germanium, or some other semiconductor material may be attached to the flag portion of a heatsink. Typically the heatsink is formed out of copper. The semiconductor die is bonded to the copper with a typical die bond soldering compound. The heatsink is attached to a lead frame made from a metal. The lead frame includes a series of lead contacts that serve as the output electrical contacts for the microchip. To electrically couple the semiconductor die to the lead frame, metal wires extend between the semiconductor die and the lead contacts. To protect the semiconductor die, wires, and lead contacts, a plastic mold compound encapsulates the semiconductor package.  
         [0003]     During normal operation, the microchip will go through cycles of heating and cooling. These heating and cooling cycles cause the microchip to expand and contract. Since the microchip is made from dissimilar materials, the expansion and contraction rates within the microchip are not uniform. The different materials forming the microchip expand and contract at different rates. These differing rates of expansion and contraction within the microchip can lead to high internal stresses that can result in device failure. Primarily, the plastic mold compound expands at a sufficiently different rate such that it can separate from the metal heatsink and metal lead frame. This separation is referred to as delamination. When the plastic mold compound delaminates from the heatsink and lead frame, significant stresses are placed on the metal wires that couple the lead contacts to the semiconductor die. These stresses can become high enough such that it breaks the electrical contact between the wires coupling the lead contacts to the semiconductor die. Delamination can also cause microchip failure through moisture and oxidation. If the plastic mold compound delaminates in a manner such that a path from the outside world is created to the semiconductor die and wires, moisture can attack the wires and semiconductor die. Through corrosion, the moisture can cause the microchip to fail. It therefore becomes highly desirable to develop methods to preserve the integrity of the microchip and prevent delimination.  
         [0004]     Chemical bonding is one method of adhering the plastic mold compound to the heatsink. The plastic mold compound is formed from an epoxy resin that has a composition that chemically bonds with the native oxide layer of the copper heatsink. While this chemical bonding does adhere the plastic mold compound to the heatsink, successive heating and cooling cycles can break these chemical bonds causing the plastic mold compound to separate from the heatsink.  
         [0005]     Altering the chemical composition of the plastic mold compound to give it a rate of thermal expansion identical to the metal heatsink is another way of addressing the delamination problem. Minimizing the difference between the expansion and contraction rates of the metal microchip components and the plastic mold compound reduces the problem of delamination. If the metal components of the microchip and the plastic mold compound had identical rates of thermal expansion and contraction, the chemical bonds between the plastic mold compound and the metal heatsink would not experience mechanical stress from the thermal fluctuations. At this time, a reliable plastic compound with such properties remains unknown to the art.  
         [0006]     Another method of addressing the problem of delamination is through locking and bonding the heatsink and plastic mold compound in such a manner that they mechanically interlock. The structures that interlock the plastic mold compound and the heatsink are commonly referred to as mold locks. Mold lock designs currently known to the art have a size and design that require their placement outside of the die attach area of the heatsink. Mold locks currently known to the art are usually formed in a trench-like moat design surrounding the flag portion of the heatsink. Placement of currently known mold locks in the flag portion of the heatsink leads to a variety of problems. If known moat-like mold locks are placed under the semiconductor die, the mold lock acts as a drain for the solder used to bond the semiconductor die to the heatsink. As a result, known mold locks degrade the bond between the semiconductor die and heatsink by draining the solder away. In addition, through draining solder away from the semiconductor die heatsink interface, known mold locks degrade the chemical bond between the plastic mold compound and heatsink when they are placed under the semiconductor die. As discussed above, the plastic mold compound has a composition such that it forms a chemical bond with the oxide layer of the copper heatsink. In contrast, the plastic mold compound does not form a chemical bond with the solder compound used to attach the semiconductor die. When existing mold locks drain solder away from under the semiconductor die heatsink interface, the area of the plastic mold compound heatsink interface is decreased and the area of the plastic mold compound solder interface is increased. Since a chemical bond is not formed between the solder and plastic mold compound, the overall bond between the plastic mold compound and the heatsink is degraded when known mold locks are formed under the semiconductor die. Still further, known mold locks adversely impact the bond line thickness.  
         [0007]     The size of the semiconductor die varies depending upon the application for which the die is designed. At present, the semiconductor industry manufactures heatsinks and lead frames that have a size and design that match the size of each individual semiconductor die. Designing and manufacturing custom heatsinks and lead frames to match each size of semiconductor die is an expensive process. The semiconductor industry is constantly searching for methods to reduce the cost of manufacturing microchips. One method of reducing cost is through designing a “one size fits all” heatsink and lead frame structure that is compatible with a variety of semiconductor die sizes. In designing this “one size fits all” heatsink and lead frame structure, it is highly desirable to develop a mold lock structure that preserves microchip package integrity when used in conjunction with both large and small semiconductor die sizes in order to reduce cost and design time.  
         [0008]     Designing a “one size fits all” heatsink structure presents a variety of challenges for mold lock design and microchip integrity. To view these design challenges, examine the integration of a large semiconductor die and a small semiconductor die with the same heatsink structure. When integrating the large semiconductor die with the heatsink, the plastic mold compound will see the mold locks formed on the exterior edge of the heatsink outside the die attach area and the semidconductor die itself lying in the flag area. However, when a small semiconductor die is placed on the same heatsink, a large bare area of the die attach area is exposed to the plastic mold compound. This large area of the flag has no mold locks to secure the plastic mold compound due to the difficulties known to the art discussed above. Consequently, the semiconductor package having the small semiconductor die and “one size fits all” heatsink is now at risk for delamination. It is therefore highly desirable to develop a mold lock structure that is useable in the flag portion of the heatsink that addresses the problems known to the art. In this manner, it is possible to develop a “one size fits all” heatsink structure that preserves the package integrity of the microchip. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  illustrates a top view of a preferred embodiment of the present invention.  
         [0010]      FIG. 2  illustrates a sectional view of a preferred embodiment of the present invention along section  2 - 2  of  FIG. 1 .  
         [0011]      FIG. 3  illustrates a sectional view of a preferred embodiment of the present invention along section  3 - 3  of  FIG. 1 .  
         [0012]      FIG. 4  illustrates a top view of an alternative embodiment of the present invention.  
         [0013]      FIG. 5  illustrates a sectional view of an alternative embodiment of the present invention along section  5 - 5  of  FIG. 4 .  
         [0014]      FIG. 6  illustrates a sectional view of an alternative embodiment of the present invention along section  6 - 6  of  FIG. 4 .  
         [0015]      FIG. 7  illustrates a mechanical process of forming a preferred embodiment of the invention along section  3 - 3  of  FIG. 1 .  
         [0016]      FIG. 8  illustrates a mechanical process of forming an alternative embodiment of the invention along section  6 - 6  of  FIG. 4 .  
         [0017]      FIG. 9  illustrates a plurality of mold locks formed on a heatsink where the mold locks are coupled to a mold compound.  
         [0018]      FIG. 10  illustrates a plurality of mold locks formed on a heatsink where some mold locks are coupled to a solder used to bond a semiconductor die to the heatsink.  
         [0019]      FIG. 11  illustrates a sectional view of a packaged semiconductor where a mold lock is coupled to a mold compound.  
         [0020]      FIG. 12  illustrates a sectional view of a packaged semiconductor where a mold lock is coupled to a solder bonding a semiconductor die to a heatsink. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]     Referring to the figures by characters of reference,  FIG. 1  illustrates a top view of a preferred embodiment of the present invention. A top surface of a heatsink  2  is illustrated having a mold lock  4  formed in therein. Mold lock  4  includes a primary channel  6  and a secondary channel  8 . Both primary channel  6  and secondary channel  8  are formed in heatsink  2  through conventional metal stamping processes. Alternatively, an etching process could produce channels  6  and  8 . Together, primary channel  6  and secondary channel  8  function to mechanically lock heatsink  2  to the plastic mold compound placed on the top surface of heatsink  2 . Dovetail profiles  10  are formed by the creation of secondary channel  8 . Dovetail profiles  10  acquired their name due to their visual similarity to joints used in wooden furniture making that are referred to as dovetail. In  FIG. 1 , dovetail profiles  10  are a full dovetail profile in that there is a dovetail profile  10  that extends from each side of primary channel  6 .  
         [0022]      FIG. 2  illustrates a sectional view of a preferred embodiment of the present invention along section  2 - 2  of  FIG. 1 . Heatsink  2  is illustrated having primary channel  6  formed therein. Dovetail profiles  10  extend from either side of primary channel  6 . Dovetail profiles  10  are formed from the fabrication of secondary channel  8 . Together, primary channel  6 , secondary channel  8 , and dovetail profiles  10  form mold lock  4 .  
         [0023]      FIG. 3  illustrates a sectional view of a preferred embodiment of the present invention along section  3 - 3  of  FIG. 1 . Dovetail profiles  10  are illustrated protruding from the walls of primary channel  6 . Both primary and secondary channels  6  and  8  are formed from a conventional metal stamping process, with primary channel  6  being stamped first so that upon stamping secondary channels  8 , dovetails  10  are formed. Dovetail profiles  10  are the structure that forms the mechanical lock with the plastic mold compound that forms a protective cover over heatsink  2 . Mold lock  4  has very small dimensions. As an example, one embodiment has secondary channel  8  depth of 0.003 inches, which is 0.0000762 meters. An exemplary depth of primary channel  6  is 0.006 inches, which is 0.0001524. Other depths and profiles for secondary channel  8  and primary channel  6  are used depending upon the desired application.  
         [0024]      FIG. 4  illustrates a top view of an alternative embodiment of the present invention. The top surface of heatsink  2  is illustrated as having a half-dovetail profiles mold lock  12 . Half-dovetail profiles mold lock  12  includes a primary channel  6  and a half-dovetail profile  14 . Half dovetail profile  14  is produced from the formation of depression  16 .  FIG. 5  illustrates a sectional view of an alternative embodiment of the present invention along section  5 - 5  of  FIG. 4 .  FIG. 6  illustrates a sectional view of an alternative embodiment of the present invention along section  6 - 6  of  FIG. 4 . Referring to  FIGS. 5 and 6 , primary channel  6  is formed in heatsink  2 . Half dovetail profile  14  extends into primary channel  6 . Half dovetail profile  14  is the structure that forms the mechanical lock with the plastic mold compound that forms the protective cover over heatsink  2 . Half dovetail profile  14  is created by the formation of depression  16 . Mold lock  12  has very small dimensions including, as an example, a depth for the primary channel  6  of 0.006 inches, which is 0.0001524 meters. Other depths for primary channel  6  are used depending upon the application. This small geometry of mold lock  12  avoids any problems associated with attaching a semiconductor die to heatsink  2  while achieving the desired effect of reducing the delamination between the plastic mold compound and heatsink  2 .  
         [0025]      FIG. 7  illustrates a mechanical process of forming a preferred embodiment of the invention along section  3 - 3  of  FIG. 1 .  FIG. 7  illustrates the two step manufacturing process that forms mold lock  4 . In step  1  of  FIG. 7 , heatsink  2  is positioned in a conventional stamping machine under a first punch  18 . First punch  18  is forced into heatsink  2  to form primary channel  6 , illustrated by dashed lines. In step  2  of  FIG. 7 , heatsink  2  is placed under a second punch  20  that is wider than first punch  18 . Second punch  20  forms secondary channels  8 , illustrated in the dashed lines. In step  2  of  FIG. 7 , primary channel  6  is formed. In step  3  of  FIG. 7 , both primary channel  6  and secondary channel  8  are formed. Dovetail profiles  10  are formed from the fabrication of secondary channel  8  illustrated in step  2  of  FIG. 7 . Dovetail profiles  10  are formed from the displacement of heatsink  2  material shown in dashed lines in step  2  of  FIG. 7 . Through punching secondary channel  8  into heatsink  2 , heatsink  2  material is displaced into primary channel  6  to form dovetail profiles  10 .  
         [0026]      FIG. 8  illustrates a mechanical process of forming an alternative embodiment of the invention along section  6 - 6  of  FIG. 4 .  FIG. 8  illustrates the two step manufacturing process that forms mold lock  12 . In step  1  of  FIG. 8 , heatsink  2  is positioned in a conventional stamping machine under a first punch  18 . First punch  18  is forced into heatsink  2  to form primary channel  6 , illustrated by dashed lines. In step  2  of  FIG. 8 , heatsink  2  is placed under a second punch  22  that forms depression  16 . Forming depression  16  with punch  22  displaces heatsink  2  material into primary channel  6  to form half dovetail profile  14  as illustrated in step  3  of  FIG. 8 .  
         [0027]      FIG. 9  illustrates a plurality of mold locks formed on a heatsink where the mold locks are coupled to a mold compound. A lead frame  24  made of metal is illustrated. Lead frame  24  serves two primary function. Lead frame  24  includes the electrical contacts that electrically couple a semiconductor die  26  to the outside world. In addition, lead frame  24  provides a structure that facilitates the manufacture of the microchip that includes heatsink  2 , lead frame  24 , and semiconductor die  26 . Lead frame  24  is formed from a series of progressive metal stamping process that are performed on a flat sheet of metal. Some lead frames  24  are made with a sufficient thickness such that lead frame  24  also forms heatsink  2 . Alternatively, for thin lead frames  24 , a copper heatsink  2  may be attached to lead frame  24 . Heatsink  2  is illustrated as having a plurality of mold locks  4  formed therein. Alternatively, half-dovetail profiles mold locks  12  could be formed in heatsink  2 . Mold locks  4  are positioned to form three rows,  28  and  30 . Rows  28  lie on the exterior region of heatsink  2 . Row  30  lies in the interior portion of heatsink  2 . This interior portion of heatsink  2  is commonly referred to as the flag because this is an area of the heatsink on which a die may be attached. Illustrated next to row  30  is semiconductor die  26 . Die  26  is a small die for this size of heatsink  2 . Rows  28  of mold locks  4  lock the exterior region of heatsink  2  to the plastic mold compound that molded over the top of heatsink  2 . Row  30  serves the function of locking the plastic mold compound to the heatsink  2  in the interior region of heatsink  2 . In this figure, semiconductor die  26  does not cover any of mold locks  4 . As a result, all mold locks  4  couple with the plastic mold compound that forms the protective package.  
         [0028]     Referring again to  FIG. 9 , note that each mold lock  4  in rows  28  and  30  is formed individually at a distance from the other mold locks  4 . As a result, there is a region of heatsink  2  material that lies between each mold lock  4 .  
         [0029]      FIG. 10  illustrates a plurality of mold locks formed on a heatsink where some mold locks are coupled to a solder used to bond a semiconductor die to the heatsink. Again, heatsink  2  is illustrated as attached to lead frame  24 . Heatsink  2  is illustrated as having a plurality of mold locks  4  formed therein. Alternatively, half-dovetail profiles mold locks  12  could be formed in heatsink  2 . Mold locks  4  are positioned to form three rows,  28  and  30 . Rows  28  lie on the exterior region of heatsink  2 . Row  30  lies in the interior portion of heatsink  2 . Again, this interior portion of heatsink  2  is commonly referred to as the flag.  
         [0030]     In  FIG. 10 , a large semiconductor die  32  is attached to heatsink  2 . With large semiconductor die  32 , a larger area of heatsink  2  is covered by die  32 . As illustrated in  FIG. 10 , die  32  covers row  30  of mold locks  4 . As a result, mold locks  4  in row  30  do not couple with the plastic mold compound. Instead, mold locks  4  in row  30  couple with the soldering compound that is used to secure die  32  to heatsink  2 . In both  FIGS. 9 and 10 , semiconductor dies  26  and  32  are secured to heatsink  2  with a soldering compound.  
         [0031]     Through forming mold locks  4  individually at a distance from each other, mold locks  4  do not form a channel under die  32  that would drain solder out from under die  32  onto the surface of heatsink  2 . Further, due to the small size and cross section of mold locks  4 , it is possible to attach semiconductor die  32  without degrading the bond between semiconductor die  32 , heatsink  2 , and the solder connecting the two.  
         [0032]     Forming individual mold locks  4  into rows enables mold locks  4  to serve the function of anchoring the plastic mold compound to heatsink  2 . Despite their small size, using mold locks  4  in mulitiples provides a desired mechanical lock between the plastic mold compound and heatsink  2 .  
         [0033]     Through the use of these miniature mold locks  4  or  12 , it is possible to locate mold locks  4  or  12  at any position on heatsink  2  without adversely affecting the bond between die  32  and heatsink  2 . As a result, it is possible to manufacture a generic “one size fits all” heatsink  2  and lead frame  24  that can be used with a small die  26  or large die  32  while preserving the overall package integrity and preventing delamination. A plurality of mold locks  4  or  12  are formed on the surface of heatsink  2 . While mold locks  4  are shown formed in rows  28  and  30 , this row configuration is merely exemplary. Other configurations of mold locks are possible such as a grid pattern or a checker pattern.  
         [0034]     When a small semiconductor die such as die  26  is used, a large number, if not all of mold locks  4  are exposed to anchor the plastic mold compound to heatsink  2 . In this manner, mold locks  4  function to prevent delamination. When a larger semiconductor die such as  32  is used, the die will cover many of mold locks  4 . However, due to the fact that mold locks  4  have such a small geometry and are formed individually, it is possible to reliably attach large die  32  with solder over mold locks  4  without degrading the anchoring of die  32  to heatsink  2  or the anchoring of the plastic mold compound to heatsink  2 . As a result, it is possible to design and manufacture a generic “one size fits all” heatsink and lead frame that is useable with varying sizes of semiconductor dies without exposing the overall package to delamination problems.  
         [0035]      FIG. 11  illustrates a sectional view of a packaged semiconductor where a mold lock is coupled to a mold compound. The sectional view in  FIG. 11  corresponds to rows  28  and  30  in  FIG. 9  and row  28  in  FIG. 10 . Referring again to  FIG. 11 , the sectional view illustrates heatsink  2  having a mold lock  4  formed therein. Mold lock  4  includes a primary channel  6  and a secondary channel  8 . Through forming secondary channel  8 , dovetail profiles  10  are created that protrude into primary channel  8 . Plastic mold compound  34  is formed on the top surface of heatsink  2 . Plastic mold compound  34  is made of a thermoset plastic that has a composition such that it chemically bonds to the oxidized copper surface of heatsink  2 . Plastic mold compound  24  forms the protective cover for the overall packaged semiconductor.  
         [0036]      FIG. 11  also illustrates sectional view of semiconductor die  26 / 32 . Die  26 / 32  is anchored to heatsink  2  with a solder  36 . A conventional die attach solder is commonly used for solder  36 . Wires  38  extend from die  26 / 32  to electrically couple die  26 / 32  to lead frame  24  in order to make the overall semiconductor package operational. Plastic mold compound  34  functions to protect wires  38  from damage and to protect the die.  
         [0037]     During normal operation, the microchip, which includes heatsink  2 , die  26 / 32 , and mold lock  4 , will go through cycles of heating and cooling. These heating and cooling cycles cause the microchip to expand and contract. Correspondingly, these heating and cooling cycles causes the plastic mold compound  34 , heatsink  2 , semicondcutor die  26 / 32 , lead frame  24 , and solder  36  to expand and contract. Since the microchip is made from dissimilar materials, the expansion and contraction rates are not uniform across the microchip. The different materials forming the microchip expand and contract at different rates. These differing rates of expansion and contraction of the plastic mold compound  34 , heatsink  2 , semicondcutor die  26 / 32 , lead frame  24 , and solder  36  that form the microchip can lead to high internal stresses that can result in device failure. Primarily, plastic mold compound  34  expands at a sufficiently different rate such that it can separate from metal heatsink  2  and metal lead frame  24 . This separation is referred to as delamination. When plastic mold compound  34  delaminates from heatsink  2  and lead frame  24 , significant stresses are placed on the metal wires  38  that couple lead frame  24  to semiconductor die  26 / 32 . These stresses can become high enough such that it breaks the electrical contact between wires  38  coupling lead frame  24  to semiconductor die  26 / 32 . Delamination can result in the failure of the microchip. It therefore becomes highly desirable to develop methods to preserve the integrity of the microchip and prevent delimination.  
         [0038]     A further problem posed by delamination is exposure of wires  38  and semiconductor die  26 / 32  to moisture. Moisture can lead to corrosion in semiconductor die  26 / 32  causing device failure. If the delamination of the microchip opens up a path from the outside world to the interior of the microchip where semiconductor die  26 / 32  lies, moisture can destroy semiconductor die  26 / 32 .  
         [0039]     Mold lock  4  functions to anchor plastic mold compound  34  to heatsink  2  and prevent/mitigate delamination. Referring again to  FIG. 11 , plastic mold compound  34  is in a liquid state when it initially contacts heatsink  2 . The liquified plastic mold compound  34  flows into the crevaces formed by dovetail profiles  10  in primary channel  6 . Plastic mold compound  34  hardens into a solid state and consequently becomes anchored in place by mold lock  4 . As is visible in  FIG. 11 , plastic mold compound  34  is mechanically held in position by dovetail profiles  10 . As a result, mold lock  4  anchors plastic mold compound  34  to heatsink  2 . Consequently, mold lock  4  functions to prevent plastic mold compound  34  from delaminating from heatsink  2  as the microchip goes through successive cycles of heating and cooling. Through preventing delamination, mold lock  4  reduces the level of internal stress on wires  38 . Reducing the level on stress on wires  38  reduces the chance that wires  38  will break away from die  26 / 32  or lead frame  24 . Consequently, the chance that the microchip will fail is reduced. As a result, mold locks  4  or  12  increase the reliability of the microchip.  
         [0040]      FIG. 12  illustrates a sectional view of a packaged semiconductor where a mold lock  4  is coupled to solder layer  36  that attaches semiconductor die  26 / 32  to heatsink  2 . The sectional view in  FIG. 12  corresponds to row  30  of mold locks  4  in  FIG. 10 . Semiconductor die  32  is mounted to heatsink  2  with a layer of solder  36 . Solder  36 , made of conventional die attach solder, when in a molten state, flows into mold lock  4 . The very small geometry of mold lock  4  minimizes its impact on the integrity of the bond between heatsink  2 , solder  36 , and die  32 . In addition, the small geometry of mold lock  4  ensures that mold lock  4  has a minimal impact on the bond line thickness of solder  36 . As a result, it is possible to manufacture mold locks  4  in the area on heatsink  2  that is occupied by a large die  32  and that is not occupied by a small die  26 . Consequently, mold locks  4  enable the fabrication of one heatsink  2  and lead frame  24  structure that is useable with differing die  26 / 32  sizes.  
         [0041]     The small dimensions of mold locks  4  and  12  have additional benefits. Creating a mold lock  4  feature upsets material due to the stamping into the material body. Stated another way, the stamping process that forms mold locks  4  merely displaces material to another portion of heatsink  2 . Unlike manufacturing processes such as milling or drilling, no material is removed from heatsink  2  in a stamping process. If mold locks  4  had a large geometry, the stamping process would displace large amounts of material. Consequently, creating several large mold lock features on heatsink  2  would make the surface of heatsink  2  uneven and non-flat. It is not possible to reliably anchor semiconductor die  26 / 32  to a uneven and non-flat heatsink. In order to facilitate the bonding of semiconductor die  26 / 32  to heatsink  2 , a coining process is performed to flatten out heatsink  2  and remove the uneven and non-flat created by the stamping process. The preferred mold lock  4  has an overall depth of  0 . 006  inches, which is  0 . 0001524  meters. This very small geometry means that very little heatsink  2  material is displaced in the stamping process. As a result, a minimal coining process is required to flatten heatsink  2  after the stamping process. Consequently, it is possible to locate mold locks  4  anywhere within the flag area of heatsink  2 .  
         [0042]     While the invention has been shown and described with reference to a particular embodiment thereof, it will be understood to those skilled in the art, that various changes in form and details may be made therein without departing from the spirit and scope of the invention.