Abstract:
High voltage circuitry often requires assembling an array of high voltage devices that are electrically isolated from one another. A lead frame that fits within conventional plastic packaging has been designed that will allow electrical isolation of the substrates of two or more transistors mounted together in a single package.

Description:
BACKGROUND 
     This invention relates generally to packaging high voltage devices and more particularly concerns assembling an array of high voltage devices that are electrically isolated from one another on a lead frame that fits within conventional plastic packaging. 
     High voltage transistors have maximum voltage limitations. When a high voltage circuit is needed for a voltage greater than that sustainable by an individual high voltage transistor, several transistors are used to share the voltage between them. For instance, a circuit capable of sustaining 3000 volts may be made from a collection of transistors capable of each sustaining 1500 volts. This is done by having the substrates of some transistors at ground voltage and the substrates of some transistors at 1500 volts so that any one individual transistor only sees the 1500 volt drop it is capable of sustaining. 
     Packaging these high voltage transistors has been problematic because of the different substrate voltages needed. The substrates of the individual transistors must be kept electrically isolated from each other so that each transistor can maintain its appropriate voltage drop over the correct range of voltages. Current single tab lead frame designs do not provide the necessary electrical isolation of the substrates of the transistors, therefore the only way to accomplish this has been to package each transistor separately and then mount and connect the packages on a circuit board. This requires a great deal of board space. If a lead frame were available that provided electrical isolation of the substrates of two or more transistors then multiple transistors could be mounted together in a single package saving considerable board space and complexity. 
     Accordingly, it is the primary aim of the invention to provide a single lead frame that will allow for the mounting of two or more transistors with electrically isolated substrates within a single package. 
     SUMMARY OF THE INVENTION 
     Briefly stated and in accordance with the present invention, there is provided a method of packaging high voltage chips in low voltage packaging by using a unique lead frame that electrically isolates the substrate of the individual chips from one another. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a top view of a conventional prior an packaging of a high voltage device in a plastic eight, dual in line pin package. 
     FIG. 2 shows a top view of a conventional multi die mount tab lead frame. 
     FIG. 3 shows a top view of electrically isolated multi die mount tab lead frame. 
     FIG. 4 shows a top view of an alternative embodiment of an electrically isolated multi die mount tab lead frame. 
    
    
     While the present invention will be described in connection with a preferred embodiment and method of use, it will be understood that it is not intended to limit the invention to either that embodiment or procedure. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
     Alpha-Numeric List of Elements 
     breakaway line B1 
     breakaway line B2 
     packaging line P 
     lead frame 10 
     die mount tab 12 
     die mount tab support arm 14 
     die mount tab support arm 16 
     bonding pad 18 
     bonding pad 20 
     bonding pad 22 
     bonding pad 24 
     bonding pad 26 
     bonding pad 28 
     bonding pad 30 
     bonding pad 32 
     bonding pad support arm 34 
     bonding pad support arm 36 
     bonding pad support arm 38 
     bonding pad support arm 40 
     bonding pad support arm 42 
     bonding pad support arm 44 
     bonding pad support arm 46 
     bonding pad support arm 48 
     lead frame 50 
     die mount tab area 52 
     die mount tab support ann 54 
     die mount tab support arm 56 
     bonding pad 58 
     bonding pad 60 
     bonding pad 62 
     bonding pad 64 
     bonding pad 66 
     bonding pad 68 
     bonding pad 70 
     bonding pad 72 
     bonding pad support arm 74 
     bonding pad support arm 76 
     bonding pad support arm 78 
     bonding pad support arm 80 
     bonding pad support arm 82 
     bonding pad support arm 84 
     bonding pad support arm 86 
     bonding pad support arm 88 
     die mount tab 90 
     die mount tab 92 
     support arm 94 
     lead frame 100 
     die mount tab 102 
     die mount tab 104 
     die mount tab support arm 106 
     die mount tab support arm 108 
     die mount tab support arm 110 
     die mount tab support arm 112 
     bonding pad 114 
     bonding pad 116 
     bonding pad 118 
     bonding pad 120 
     bonding pad support arm 122 
     bonding pad support arm 124 
     bonding pad support arm 126 
     bonding pad support arm 128 
     lead frame edge 129 
     lead frame 130 
     die mount tab 132 
     die mount tab 134 
     die mount tab support arm 136 
     die mount tab support arm 138 
     die mount tab support arm 140 
     die mount tab support arm 142 
     bonding pad 144 
     bonding pad 146 
     bonding pad 148 
     bonding pad 150 
     bonding pad 152 
     bonding pad 154 
     bonding pad support arm 156 
     bonding pad support arm 158 
     bonding pad support ann 160 
     bonding pad support ann 162 
     bonding pad support arm 164 
     bonding pad support arm 166 
     lead frame edge 168 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a conventional lead frame 10. The lead frame 10 has a die mount tab 12 supported by two die mount tab support arms 14, 16. The lead frame 10 also has 8 bonding pads 18, 20, 22, 24, 26, 28, 30, 32 supported by eight bonding pad support arms 34, 36, 38, 40, 42, 44, 46, 48. 
     FIG. 2 shows a conventional lead frame 50 for mounting two dies on a single lead frame within a single package. The lead frame 50 has a die mount tab area 52 supported by two die mount tab support arms 54, 56. The lead frame 50 also has 8 bonding pads 58, 60, 62, 64, 66, 68, 70, 72 supported by eight bonding pad support arms 74, 76, 78, 80, 82, 84, 86, 88. The die mount tab area 52 is modified from the die mount tab 12 shown in FIG. 1 to mount two dies. The die mount tab area 52 has two die mount tabs 90, 92 connected by a support arm 94. The lead frame 50 is made out of metal, typically a nickel plated copper, so die mount tab 90 and die mount tab 92 are electrically connected through support arm 94. 
     FIG. 3 shows a top view of electrically isolated multi die mount tab lead frame 100. There are two separate die mount tabs 102, 104. Die mount tab 102 is supported by two die mount tab support arms 106, 110 and die mount tab 104 is supported by two separate die mount tab support arms 108, 112. Unlike the two die mount tabs 90, 92 in die mount tab area 52 shown in FIG. 2, the die mount tabs 102, 104 shown in FIG. 3 are completely separated from each other and supported only by each die mount lab&#39;s respective die mount tab support arms. 
     To avoid confusion, although the die mount tab support arms 106, 108, 110, 112 and the bonding pad support arms appear to be connected to each other 122, 124, 126, 128 through lead frame edge 129 around the perimeter of lead frame 100, when the lead frame 100 is mounted or molded into a package the lead frame edge 129 will be completely removed along breakaway lines B1 and B2 and the die mount tab support arms 106, 108, 110, 112 and the bonding pad support arms 122, 124, 126, 128 will be completely isolated from one another. 
     The additional space within lead frame 100 needed for the extra die mount tab support arms to provide completely separate support for the second die mount tab was gained at the cost of four bonding pads. Therefore, in contrast to the lead frame 10 shown in FIG. 1 or lead frame 50 shown in FIG. 2, lead frame 100 shown in FIG. 3 only has 4 bonding pads 114, 116, 118, 120 instead of 8. The reduced number of bonding pads 114, 116, 118, 120 requires a reduced number of bonding pad support arms 122, 124, 126, 128. The die mount tab support arms 106, 108, 110, 112 use the space in the lead frame 100 that would have been used by the four eliminated bonding pad support arms. Putting the die mount tab support arms 106, 108, 110, 112 in the space previously used for bonding pad support arms allows each of the die mount tabs 102, 104 to have two die mount tab support arms 106, 108, 110, 112 that are attached to the die mount tabs on opposing sides of the die mount tab 102, 104. This configuration provides the greatest amount of support and stability to the die mount tabs 102, 104. 
     FIG. 4 shows a top view of an alternative embodiment of an electrically isolated multi die mount tab lead frame 130. Like lead frame 100 shown in FIG. 3, lead frame 130 has two die mount tabs 132, 134. Die mount tab 132 is supported by two die mount tab support arms 136, 138 and die mount tab 134 is supported by two die mount tab support arms 140, 142. However, a different configuration of the die mount tab support arms 136, 138, 140, 142 is used from the configuration used in lead frame 100 shown in FIG. 3. In lead frame 130, shown in FIG. 4 the die mount tab support arms 136, 138, 140, 142 are attached to adjacent sides, rather than opposing sides of the die mount tabs 132, 134 respectively. This configuration provides less stability and support for the die mount tabs 132, 134 but allows room for the lead frame 130 to have two additional bonding pads, when compared with lead frame 100 shown in FIG. 3. 
     Lead frame 130, shown in FIG. 4, has 6 bonding pads 144, 146, 148, 150, 152, 154; each one supported by its respective bonding pad support arm 156, 158, 160, 162, 164, 166. By moving the die mount tab support arms 136, 140 from the opposing side of the die mount tabs 132, 134 from the die mount tab support arms, 138, 142 space is regained for two bonding pads 154, 148 and their respective bonding pad support arms 166, 160 at the cost of some stability and support for the die mount tabs 132, 134. 
     As in the example of lead frame 100 shown in FIG. 3, complete isolation of the die mount tab support arms 106, 108, 110, 112 and bonding pad support arms 122, 124, 126, 128 will be accomplished when the lead frame 130 is mounted or molded inside a package and lead frame edge 168 around the perimeter of lead frame 130 is removed along breakaway lines B1, B2. 
     Further isolation of the die also occurs when either lead frame 100 or lead frame 130 is molded into a plastic package. During the conventional packaging process, plastic or an isolating epoxy fills in the area between breakaway lines B1 and packaging lines P. The plastic or isolating epoxy encapsulates the die mounted on the die mount tabs further isolating them from each other. This encapsulation provides further isolation because the dielectric constant of the encapsulant is much greater than air.