Abstract:
An improved package for a semiconductor device. The semiconductor device includes an apparatus having at least two access leads to facilitate electrical connection of the apparatus within an electrical circuit. The package has generally a closed polyhedral shape presenting a plurality of faces and substantially insulatingly surrounding the apparatus in a manner leaving the access leads exposed for effecting electrical connection. The access leads extend a distance from exit loci from the package. The exit loci are situated on an exit face of the package, adjacent pairs of exit loci being generally in a common plane. An intraplanar distance within the common plane is established intermediate each adjacent pair of the exit loci. The improvement comprises configuring the exit face to establish an on-surface path greater than the intraplanar distance intermediate selected adjacent pairs of the exit loci.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to packaging for electronic devices, and is particularly directed to an improved package for semiconductor devices. Some commonly available semiconductor devices, such as the TO-220 are ill suited for high voltage circuit applications, such as a switching transistor in the primary side of a switch-mode power supply. A principal shortcoming is in the lead-to-lead spacing. In the TO-220 device, the lead spacing is not sufficiently great to allow adequate creepage on the package of the device itself. Creepage is defined as the shortest distance between two conductive parts as it is measured along an insulated surface. Thus, creepage in a device having two leads extending from an insulating package is the shortest distance between the two leads, as measured along the face of the package. Failures due to insufficient creepage may be caused by the presence of environmental contaminants, such as dust. Such conditions may lead to a short circuit between leads. Such high impedance shorts may cause catastrophic failure of the device and, indeed, may cause failure of the entire circuit in which the device is employed. Generally, shorting is a function of several factors: the working voltage employed in the circuit, the dielectric constant of the insulating medium, and the presence or absence of environmental contaminants. 
     Insufficient creepage and the potential for high impedance shorts are known. Previously, the solution to the problem has simply been to incorporate a physically larger part in the circuit design for higher voltage or essential circuits. Such larger parts have wider lead-to-lead spacing and, therefore, greater creepage. In today&#39;s market the pressure is for ever smaller, more compact products. In view of this pressure to produce smaller products requiring smaller component circuitry, the previous expedient solution of simply specifying and employing a larger part to obviate creepage problems is no longer a good solution. 
     Another attempt to solve the creepage problem has been to encase the leads of devices in silicone materials. However, this is not a reliable solution because of the difficulty of handling silicone during manufacturing and assembly operations. Adequate coverage of the leads cannot be guaranteed as silicone has a tendency to migrate. This use of silicone can be deleterious, as silicone tends to contaminate other operations in a manufacturing plant. For example, the printing of labels on components for a product may be seriously negatively impacted by silicone contamination. Further, once silicone contamination has occurred across processes in a manufacturing plant, it is difficult to eliminate. 
     There is a need for a package which will establish acceptable creepage for high-voltage applications without requiring larger components occupying excessive board area in circuit implementations. 
     SUMMARY OF THE INVENTION 
     The preferred embodiment of the present invention is an improved package for an electronic device, and especially for a semiconductor device. The semiconductor device includes an apparatus having at least two access leads to facilitate electrical connection of the apparatus within an electrical circuit. The package has generally a closed polyhedral shape presenting a plurality of faces and substantially insulatingly surrounding the apparatus in a manner leaving the at least two access leads uninsulatedly exposed for effecting electrical connection with the apparatus. The at least two access leads extend a distance from at least two exit loci from the package. The at least two exit loci are situated on an exit face of the package and lie generally in a common plane. An intraplanar distance within a common plane is established intermediate each adjacent pair of the at least two exit loci. The improvement comprises configuring the exit face to establish an on-surface path greater than the intraplanar distance intermediate selected adjacent pairs of the exit loci. 
     The invention provides an improved high-voltage power semiconductor package. By including structure on the face of the package intermediate selected electrical leads of the semiconductor device to increase the on-surface distance between the selected leads, creepage is increased. Increasing creepage increases the potential necessary to short the selected leads. The result is a higher voltage capacity for the semiconductor device without having to use a larger, bulkier device to achieve greater creepage. 
     The preferred embodiment of the present invention involves providing walls intermediate leads. The provision of walls intermediate leads is an especially attractive embodiment since it would not require adding material to an existing package mold for a part to effect the change. To add the desired walls between leads, one would merely need to appropriately remove some material from the package mold. The removed area would then fill with packaging material during molding, and the result would be manifested in the form of the desired walls. 
     Alternate embodiments of the present invention may be configured using depressions, or trenches, or grooves intermediate leads. Such alternate embodiments require that material be added to an existing package mold to construct the desired depressions in a finished molded package. Thus, this alternate construction would require the construction of a wholly new mold. 
     Other alternate embodiments of the present invention may be configured by forming insulating wraps on selected leads during the molding of the package by over-molding package material on the selected leads. The overmolding may be effected on the two outboard leads adjacent the center lead on a three-lead semiconductor device, such as a power transistor. Another construction would have the overmolding being effected only on the center lead of a three-lead device. 
     It is, therefore, an object of the present invention to provide an improved package for an electronic device which increases creepage in the finished packaged device. 
     Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective drawing illustrating one embodiment of a prior art semiconductor device. 
     FIG. 2 is a perspective drawing illustrating a second embodiment of a prior art semiconductor device. 
     FIG. 3 is a perspective drawing illustrating a first embodiment of the present invention. 
     FIG. 4 is a front plan view of the first embodiment of the present invention illustrated in FIG.  3 . 
     FIG. 5 is a perspective drawing illustrating a second embodiment of the present invention. 
     FIG. 6 is a perspective drawing illustrating the preferred embodiment of the present invention. 
     FIG. 7 is an elevation view of face  94  of the preferred embodiment of the present invention. 
     FIG. 8 is a perspective drawing illustrating another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a perspective drawing illustrating one embodiment of a prior art semiconductor device. In FIG. 1, a prior art semiconductor device  10  has an insulating package  12 , generally in the shape of a closed polyhedron having a plurality of faces  14 ,  16 ,  18 ,  20 ,  22 ,  24 . Package  12  substantially encloses, or surrounds, an interior electronic apparatus (not shown in FIG.  1 ), except a tab  26 , which may or may not be insulated, and electrical leads  28 ,  30 ,  32 , which are not insulated. Free access to electrical leads  28 ,  30 ,  32  and, when not insulated, to tab  26 , facilitates connecting semiconductor device  10  in electronic circuitry (not shown in FIG. 1) for employment in a product. Electrical leads  28 ,  30 ,  32  exit package  12  from a common exit face  24  at exit loci  34 ,  36 ,  38 . The distance between selected adjacent electrical leads, for example electrical leads  28 ,  30  in FIG. 1, is important. Such separation contributes to the prevention of high impedance shorts of semiconductor device  10  between the selected electrical leads  28 ,  30  which may operate at a high potential difference relative to one another. The displacement “d” between selected electrical leads  28 ,  30 , when measured through the air separating electrical leads  28 ,  30 , is called the clearance between leads  28 ,  30 . When the separation between selected leads  28 ,  30  is measured along the surface of exit face  24  intermediate exit loci  34 ,  36  (that is, between the closest proximate conductive points on adjacent leads  28 ,  30 ), the displacement is called creepage. 
     FIG. 2 is a perspective drawing illustrating a second embodiment of a prior art semiconductor device. In FIG. 2, a prior art semiconductor device  40  has an insulating package  42 , generally in the shape of a closed polyhedron having a plurality of faces  44 ,  46 ,  48 ,  50 ,  52 ,  54 . Package  42  substantially encloses, or surrounds, an interior electronic apparatus (not shown in FIG.  2 ), except a tab  56 , which may or may not be insulated, and electrical leads  58 ,  60 ,  62 , which are not insulated. Tab  56  is substantially, though not entirely, enclosed at faces  44 ,  48 ,  50 ,  52 ,  54  of package  42 ; not shown in detail in FIG. 2 is the construction that allows substantially free access to tab  56  at lower face  46  of package  42 . That is, tab  56  substantially overlays lower face  46  of package  42  and is configured to rest in abutting relationship with a printed wiring board or heat sink (not shown in FIG. 2) when semiconductor device  40  is installed in an electrical circuit. Electrical access to tab  56  and other electrical leads  58 ,  60 ,  62  facilitates connecting semiconductor  40  in electronic circuitry (not shown in FIG. 2) for employment in a product. Electrical leads  58 ,  60 ,  62  exit package  42  from a common exit face  54  at exit loci  64 ,  66 ,  68 . The separation distance between selected adjacent electrical leads, for example displacement “s” between electrical leads  58 ,  60  in FIG. 2, when measured through the air separating electrical leads  58 ,  60 , is the clearance between leads  58 ,  60 . When the separation between selected leads  58 ,  60  is measured along the surface of exit face  54  intermediate the most proximate conductive points  70 ,  72  of exit loci  64 ,  66 , the distance is called creepage. 
     FIG. 3 is a perspective drawing illustrating a first embodiment of the present invention. In FIG. 3, a semiconductor device  80  has an insulating package  82 , generally in the shape of a closed polyhedron having a plurality of faces  84 ,  86 ,  88 ,  90 ,  92 ,  94 . Package  82  substantially encloses, or surrounds, an interior electronic apparatus (not shown in FIG.  3 ), except a tab  96 , which may or may not be insulated, and electrical leads  98 ,  100 ,  102 , which are not insulated. Electrical access to tab  96  and other electrical leads  98 ,  100 ,  102  facilitates connecting semiconductor  80  in electronic circuitry (not shown in FIG. 3) for employment in a product. Electrical leads  98 ,  100 ,  102  exit package  82  from a common exit face  94  at exit loci  104 ,  106 ,  108 . 
     Electrical leads  98 ,  102  each have overmolded thereupon an insulating wrap, or shroud,  114 ,  116 . Preferably insulating wraps  114 ,  116  are integrally formed with package  82 . Thus, with insulating wraps  114 ,  116  incorporated into exit face  94 , there is established an effective exit locus  115  where lead  98  exits package  82 , and there is established an effective exit locus  121  where lead  102  exits package  82 . In the embodiment of the invention illustrated in FIG. 3, the clearance between leads  98 ,  100  is measured within a plane containing effective exit locus  115  and point  112 . Those two points  115 ,  112  are the closest proximate conductive points on adjacent leads  98 ,  100 . The creepage between leads  98 ,  100  in the embodiment illustrated in FIG. 3 is measured along the surface of insulating wrap  114  from point  115 , to point  113 , and thence to point  112 . Similarly, the clearance between leads  100 ,  102  is measured in a plane containing effective exit locus  121  and point  117 . Those two points  121 ,  117  are the closest proximate conductive points on adjacent leads  100 ,  102 . The creepage between leads  100 ,  102  in the embodiment illustrated in FIG. 3 is measured along the surface of insulating wrap  116  from point  121 , to point  119 , and thence to point  117 . Thus, insulating wrap  114  increases creepage between electrical leads  98 ,  100  and insulating wrap  116  increases creepage between electrical leads  100 ,  102 . 
     FIG. 4 is a front plan view of the first embodiment of the present invention illustrated in FIG.  3 . Elements appearing in both FIGS. 3 and 4 are identified with like reference numerals to aid in understanding the invention. In FIG. 4, the separation distance, “x”, between adjacent electrical leads  98 ,  100  when measured through the air separating electrical leads  98 ,  100 , is the clearance between leads  98 ,  100 . The distance “x” is substantially the same as the distance between points  112 ,  113 , which is substantially what the creepage between leads  98 ,  100  would be without insulating wrap  114 . Similarly, the separation distance, “x”, between adjacent electrical leads  100 ,  102  measured through the air separating electrical leads  100 ,  102 , is the clearance between leads  100 ,  102 , and is substantially the same as the distance between points  117 ,  119 . The distance between points  117 ,  119  is substantially what the creepage between leads  100 ,  102  would be without insulating wrap  116 . It is not required that the clearance between leads  98 ,  100  must be equal to the clearance between leads  100 ,  102 . Such equal clearances are illustrated here merely for convenience and to simplify the description of the invention. 
     As mentioned earlier, the measurement of the separation of adjacent leads along the surface of exit face  94  intermediate the most proximate conductive points of the adjacent leads is the creepage between the selected leads. Thus, creepage between leads  98 ,  100  is the distance from point  115 , to point  113 , to point  112 . If insulating wraps  114 ,  116  extend a distance “e” from exit face  94  of package  82 , inspection of FIG. 4 reveals that distance “e” is substantially equal to the distance from point  115  to point  113 , and substantially equal to the distance from point  121  to point  119 . For purposes of simplifying the explanation of the invention, assume, for example, that the width of insulating wrap  114  is relatively small compared with the distance “e+x”. Then creepage between leads  98 ,  100  is substantially equal to “e+x”. In the embodiment of the present invention illustrated in FIGS. 3 and 4, distance “x” is substantially what the creepage between leads  98 ,  100  and between leads  100 ,  102  would be without insulating wraps  114 ,  116 . That is, the distance “x” in FIGS. 3 and 4 would be the creepage of semiconductor device  80  between the pairs of leads  98 ,  100  and  100 ,  102  if semiconductor  80  were configured according to the prior art construction illustrated in FIG.  1 . Thus, by the structure employed in the embodiment of the present invention illustrated in FIGS. 3 and 4, creepage between pairs of leads  98 ,  100  and  100 ,  102  is increased from “x” to “x+e”. 
     FIG. 5 is a perspective drawing illustrating a second embodiment of the present invention. For purposes of facilitating understanding alternate embodiments of the present invention illustrated in FIGS. 5-7, and in order to avoid unnecessary prolixity, like elements will be identified using like reference numerals in the various FIGS. 4-7, and descriptions of similar structural features will not be repeated. In FIG. 5, a semiconductor device  80  has a package  82  with electrical leads  98 ,  100 ,  102  extending from an exit face  94 . Lead  100  only has an insulating wrap  124 , preferably integrally formed with package  82 . Insulating wrap  124  extends a distance “e” from exit face  94 , between points  119 ,  121 . Creepage between leads  98 ,  100  is measured between the closest proximate conductive points  120 ,  121  on leads  98 ,  100 . Leads  98 ,  100  have a clearance of “x”. Discounting the thickness of insulating wrap  124  as significantly less than the distance “e+x”, the embodiment of the present invention illustrated in FIG. 5 has creepage between leads  98 ,  100  substantially equal to the distance “e+x”. 
     FIG. 6 is a perspective drawing illustrating the preferred embodiment of the present invention. In FIG. 6, a semiconductor device  80  has a package  82  with electrical leads  98 ,  100 ,  102  extending from an exit face  94 . An insulating elevation, or wall  130  is established intermediate leads  98 ,  100 , and an insulating elevation, or wall  132  is established intermediate leads  100 ,  102 . Insulating walls  130 ,  132  are preferably integrally formed with package  82  and each wall  130 ,  132  extends substantially from rear face  86  to front face  92  of package  82 . Insulating walls  130 ,  132  extend a distance “e” from exit face  94 , and each insulating wall  130 ,  132  has a width “w”. It is not necessary that insulating walls  130 ,  132  have equal width “w” or equal height “e”, only convenient to facilitate simple illustration. Thus, in the embodiment of the present invention illustrated in FIG. 6, creepage between leads  98 ,  100  is measured between the closest proximate conductive points  120 ,  119  on leads  98 ,  100 . Leads  98 ,  100  have a clearance of “x”. Creepage, measured on the surfaces between proximate conductive points  119 ,  120 , is equal to “x” plus twice the height “e” of wall  130 . The width “w” of wall  130  is merely equal to the expanse of exit face  94  which would be traversed by the described path if wall  130  were not present. Thus creepage between leads  98 ,  100 , in the embodiment of the present invention illustrated in FIG. 6, is equal to the distance “x+b  2 e”. 
     FIG. 7 is an elevation view of face  94  of the preferred embodiment of the present invention. Alternate creepage paths exist in “end around” traversals of walls  130 ,  132 . Thus, by way of example in FIG. 7, an alternate creepage path exists between conductive points  119 ,  120  that departs from conductive point  119 , progresses to the intersection  93  of wall  130  with face  92 , proceeds along the integral juncture line  97  of face  92  and wall  130  to the intersection  95  of wall  130  with face  92 , and continues onward from intersection  95  to conductive point  120 . If the distance from conductive point  119  to juncture  93  is a distance “a”, and if wall  130  is equidistant from conductive points  119 ,  120 , then the distance from conductive point  120  to the juncture  95  is also a distance “a”. Integral juncture line  97  is equal to the width “w” of wall  130 . Thus, the exemplary alternate creepage path illustrated in FIG. 7 has a length equal to the distance “2a+w”. Wall  130  need not be equidistant from conductive points  119 ,  120 ; it is illustrated in that configuration in FIG. 7 solely to simplify explanation of the invention. Thus, a creepage path over wall  130  will be greater than a creepage path around wall  130 , only when (x+2e)&gt;(2a+w). This relationship is useful to determine what height a wall, such as wall  130 , must be in order to provide an advantage of longer creepage path than would be established in an “end around” creepage path, such as the exemplary creepage path illustrated in FIG.  7 . 
     FIG. 8 is a perspective drawing illustrating another embodiment of the present invention. In FIG. 8, a semiconductor device  80  has a package  82  with electrical leads  98 ,  100 ,  102  extending from an exit face  94 . A depression  134  is established intermediate leads  98 ,  100 ; a depression  136  is established intermediate leads  100 ,  102 . Depressions  134 ,  136  are preferably in the form of grooves, or trenches, extending substantially completely from rear face  86  to front face  92  of package  82 . Each depression  134 ,  136  extends a depth “e” from exit face  94  and has a width “w”. It is not necessary that depressions  134 ,  136  have equal depth “e” or equal width “w”; it is only convenient here to facilitate simple illustration. Thus, in the embodiment of the present invention illustrated in FIG. 8, creepage between leads  98 ,  100  is measured between the closest proximate conductive points  120 ,  119  on leads  98 ,  100 . Leads  98 ,  100  have a clearance of “x”. Creepage, measured on the surfaces of package  82  between proximate conductive points  119 ,  120 , is equal to “x” plus twice the depth “e” of depression  134 , plus the width “w” of depression  134 . Thus creepage between leads  98 ,  100  in the embodiment of the present invention illustrated in FIG. 8 is equal to the distance “x+2e”. 
     It is to be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for the purpose of illustration only, that the apparatus and method of the invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims: