Patent Publication Number: US-6667547-B2

Title: High current capacity semiconductor device package and lead frame with large area connection posts and modified outline

Description:
RELATED APPLICATION 
     This application is a division of U.S. patent application Ser. No. 09/103,035, filed Jun. 23, 1998 entitled HIGH CURRENT CAPACITY SEMICONDUCTOR DEVICE PACKAGE AND LEAD FRAME WITH LARGE AREA CONNECTION POSTS AND MODIFIED OUTLINE which is a non-provisional of U.S. Provisional Application No. 60/084,224 filed May 5, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a high power semiconductor device lead frame and package containing enlarged connection posts and conductors with a novel lead sequence and having an increased dielectric creepage distance between adjacent leads and having a modified outline. 
     Semiconductor devices such as diodes, thyristors, MOSgated devices such as MOSFETs, IGBTs and the like are commonly formed in a silicon semiconductor die containing the device junctions. The die have metallized bottom drain or other power electrodes and have source and gate electrodes or other power electrodes on their upper surface. The die are mounted on enlarged conductive lead frame pads and the power electrodes on the upper die surface have connection wires which are wire bonded by plural wires from the conductive electrode area of the die to flat connection post areas which are in turn connected to the exterior lead conductors of the lead frame. These exterior lead conductors extend through a molded housing which overmolds the lead frame and die. The lead frame will contain a plurality of identical sections, for example, 20 or more sections which are simultaneously processed to receive separate die and wire bonds and overmolding. The individual devices are then separated after the molding process. The final device may have well known industry standard package outlines, for example, the well known TO220 or TO247 package outlines. 
     Known package structures have a current capacity which is limited by the number of parallel bonding wires which can connect the die power electrode, for example, the source electrode of a power MOSFET or the cathode of a diode to a corresponding lead frame post. It would be desirable to arrange the lead frame so that an increased number of parallel bonding wires can be used to reduce package resistance without increasing the size of the package. 
     Known package structures, particularly for MOSgated devices such as power MOSFETs also conventionally have parallel external lead conductors in a sequence of gate, drain and source. This causes an added spacing between gate and source leads. It would be very useful to have the gate and source leads adjacent to one another, while maximizing the source post area. It would be further desirable to increase the conduction cross-sectional area of the source or other elongated external leads. 
     In conventional molded housing packages the lead frame conductors extend from the interior of the high dielectric housing to the area exterior of the package. The “creepage” distance along the surface of the package is thus related to the external spacing of the external conductors, and limits the maximum voltage which can be applied between these leads. It would be desirable to increase the creepage distance along the package surface at which the lead frame leads extend out of the package without increasing the size of the package. 
     Known package structures have lead conductors extending outwardly from the lead frame and through the plastic housing surface. These lead conductors are contentionally rectangular, or V-shaped and are designed to fit into a metallized opening in a printed circuit board. The cross-section of these conductors must be large enough to carry the device current without excessive heating. However, the diameter of the holes in the board is limited, because their spacing is determined by the spacing of the device lead conductors and their conductive bushings. It would be desirable to increase the cross-sectional area of these lead conductors, without excessively increasing the diameter of the holes in the circuit board which receives the conductor. 
     The present package outline has a relatively thick plastic volume which is joined to a thinner volume by a vertical rise. The thicker region extends from the lead conductor edge of the package to the vertical rise which is located above a central region of the die. The thinner volume extends to the end of the package which is opposite to its output lead conductor side. the vertical rise to the thicker region of the package forms a 90° angle to the top surface of the thicker region. The material within this sharp angle tends to accumulate bubbles in the plastic during molding which leads to device rejections and failure. 
     Further, when devices of the above outline are to be surface mounted on a support board and held in place by a cantilevered spring, the spring presses atop the surface of the thick plastic region. Consequently, spring pressure is applied at a location which is removed from over the center of the die. It would be desirable to have the point of application of the spring pressure located over the center of the die which is mounted within the package. It would be further desirable to simplify the mounting of the package under a cantilevered spring without requiring special tools. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with a first feature of the invention it is recognized that the area of the drain post and gate post of the lead frame can be smaller than that of the source post and the area then made available can be used to increase the source post area. This makes it possible to use an increased number of bonding wires to bond the die source electrode to the source post of the lead frame, thus increasing the current carrying capacity of the package. 
     In accordance with another feature of the present invention, a novel lead frame structure is provided in which, for a MOSFET type package for example, the lead frame external lead sequence is changed from the prior art gate, drain, source, to a novel sequence of gate, source, drain. This new sequence improves the application of the device by reducing the spacing between gate and source connections, thus reducing the leakage inductance of the gate circuit. The novel new sequence further makes it possible to increase the area of the source post and to decrease the area of the drain post (which has a very wide area bottom die connection area), and makes it possible to use an increased number of bonding wires from the die source to the lead frame source pad to reduce package resistance without increasing the package size. 
     A further feature of the invention permits an outward (from the lead frame center) reentrant bend of the two outermost leads from the centermost lead frame lead where the lead conductors exit from the package, without reducing lead conduction cross-section. This outward reentrant bend increases the creepage distance between the outer leads and center lead along the surface of the package insulation to increase the breakdown voltage of the device. By an outward reentrant bend is meant a bend which redirects an elongated conductor to a generally perpendicular path away from the center of the package, and then again redirects the elongated conductor to a path which is parallel, but spaced from its original path. 
     A further feature of the invention comprises the increase in cross-sectional area of the lead frame external conductors without requiring a significant increase in board hole diameter. Thus, in a first embodiment it was found that making the normally rectangular conductor more square in shape, that a larger area of copper conductor can fit into the same diameter opening. Further, it was found that the use of a slight chamfer of the edges of the rectangular conductor will increase the total cross-section of the final conductor. 
     As a still further feature of the invention, the plastic package outline is modified so that it has a uniform thickness and flat top exterior surface extending from the lead conductor edge and atop substantially the full area of the interior die. The end wall of the package opposite to the lead conductors is then tapered down toward the package bottom and edge at an angle of about 45° to the vertical. 
     As a result of this novel structure, when the package is mounted by a cantilevered spring, the center of the spring force against the top of the package can be centered over the center of the die, which is the most efficient location for application of force to the surface mounted device. 
     Further, the use of the tapered end surface increases the angle between the end surface and top surface to about 135°, thus making it easier to exclude bubbles from this top edge area of the die during molding. 
     Finally, the tapered end surface of the package, makes it possible to use the package as a wedge, to press the package under the raised lip of a cantilevered spring and under the spring without needing special tools. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a novel package containing the novel lead frame of the invention. 
     FIG. 2 is a side view of the package of FIG.  1 . 
     FIG. 3 is an end view of FIG.  1 . 
     FIG. 4 is a top view of a single section of a lead frame which can be encapsulated in the package of FIGS. 1,  2  and  3 , using the novel increased outwardly bent lead conductors to increase creepage distance along the plastic housing surface, and using the known sequence of gate, drain and source lead conductors. 
     FIG. 5 is a top view of a second embodiment of a lead frame like that of FIG. 4, with the novel lead conductor sequence of gate, source and drain and an increased area source post. 
     FIG. 5 a  is a cross-section of FIG. 5 taken across the section lines  5   a — 5   a  in FIG.  5 . 
     FIG. 6 shows a third embodiment of the invention in which the lead frame section of FIG. 5 is modified for a diode die. 
     FIG. 7 shows a fourth embodiment of the invention and is a modification of FIG. 4 to increase the source post area. 
     FIG. 8 is a cross-section of FIG. 7, taken across section line  8 — 8  of FIG.  7 . 
     FIG. 9 is a top view of a further embodiment of the invention, modified to be a surface mount type package. 
     FIG. 10 is a side view of FIG.  9 . 
     FIG. 11 is an end view of FIG.  9 . 
     FIG. 12 is a cross-section of a lead frame conductor of a standard TO220 package for a 50 ampere rating. 
     FIGS. 13 and 14 are cross sections of improved lead frame conductors made in accordance with the invention and having a higher current capacity. 
     FIG. 15 is a cross-section of an improved lead frame conductor made in accordance with the invention in which the edges of the rectangular conductors are chamfered to enable the use of a larger conductor cross-section without an excessive increase in the circuit board hole diameter. 
     FIG. 16 is a side view of the package outline of a prior art device with mounting spring. 
     FIG. 17 is a side view of a novel package outline in accordance with the invention with a mounting spring. 
     FIG. 18 is a top view of the package of FIG.  17 . 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring first to FIGS. 1,  2  and  3 , there is shown a TO220 type semiconductor device package  20  containing certain features of the present invention. Package  20  consists of a conventional plastic molded housing  21 , which may be transfer molded, and which encapsulates a semiconductor die  22 , shown in dotted lines in FIG. 2. A bottom main conductive lead frame paddle  23  receives the bottom drain electrode of die  22 . Three lead conductors  25 ,  26  and  27  extend through the front wall  28  of plastic housing  21 . Leads  25 ,  26  and  27  can be gate, drain and source contacts respectively for die  22  (if it is a power MOSFET die), or, in accordance with an aspect of the invention to be described, can be gate, source and drain contacts respectively. 
     In accordance with an important feature of the invention, conductors  25  and  27  are bent away from the center of lead  26  and the package  20  at areas adjacent the front surface  28  by reentrantly shaped bends  30  and  31  respectively. This causes added creepage distances  32  and  33  along surface  28  to permit the use of higher voltages between leads  25  and  27 , and center lead  26 . 
     FIG. 4 shows one section  35  of a multisection lead frame which can be used in package  20  of FIGS. 1,  2  and  3 . The shaded area  36  shows the sections of the lead frame which are conventionally trimmed away after the overmolding of housing  21  as shown in FIGS. 1 to  3 . The paddle section  23  may be of any suitable copper or copper alloy and may have a thickness, for example, of 1.27 mm. The top surface of paddle  23  which receives semiconductor die  22  may be unplated to improve the bonding of the bottom electrode of die  22  (such as the bottom drain of a MOSFET die) to the paddle  23  as by soldering or the like. The remainder of lead frame  35  may be nickel plated. 
     The portion of the frame  35  containing external lead conductors may also be nickel platted copper and may have a thickness of 0.8 mm. External lead conductors or fingers  25  and  27  terminate in solder posts  40  and  42  respectively, corresponding to gate source connectors respectively. Note that solder posts  40  and  42  are coplanar and are in a plane which is above the plane of the upper surface of paddle  23 . Paddle  23  also has an extension  41  which is aligned with and is continuous with conductor  26 . Thus, die  22  is provided at its top with a source electrode  43  and a gate electrode  44 . Gate electrode  44  is wire bonded to post  40  (which is suspended above and insulated from pad  23 ), and source electrode  43  is wire bonded to post  42  which is also above and insulated from pad  23  (see FIG.  2 ). Because of the relatively small area of post  42 , only two 20 mil bond wires  45  and  46  can be used to bond source electrode  43  to post  42  (and thus external lead conductor or finger  27  when the excess metal, shown shaded, is stripped away). 
     FIGS. 5 and 5 a  show another aspect of the invention which permits the use of additional bonding wires to the source electrode  43 . Thus, the conductors  25 ,  26  and  27  are gate, source and drain electrodes respectively, bringing the source and gate conductors closer together as shown in FIG.  5 . Further, the source post  50  is now centrally located and is much wider than post  42  in FIG.  4 . Significantly, post  50  may be wider than about one-half the width of the lead frame. This enables the use of 4 20 mil bonding wires  51 ,  52 ,  53  and  54 , each identical to wires  45  and  46  in FIG. 4, from source electrode  43  to post  50  which extends from conductor  26 . The use of 4 bond wires instead of 2 permits a significant reduction in package resistance and thus an increase in the current capacity of the package without changing the package size or die size. 
     The bonding wires  45 ,  46 ; and  51  to  54  are preferably pure aluminum wires (0.9999 purity) and 20 mils in diameter. A single wire has a resistance of almost 1 milliohm; two parallel wires have a resistance of about 0.5 milliohms, and 4 wires have a resistance of about 0.25 milliohms. Thus, the use of added wires causes a substantial reduction in package resistance. 
     The gate post  56 , which receives only a thin gate bonding wire is reduced in area, and the pad extension  41  from pad  23  is moved to align with and to be connected to lead connector  27 . Note that drain connector  27  is at the potential of pad  23 . 
     FIG. 6 shows the novel lead frame for a diode die  60 , rather than a MOSFET die  22 . Thus, in FIG. 6, a diode has a bottom electrode bonded to pad  23 . The wire bond posts are modified to merge together posts  50  and  56  of FIG. 5 into a single large area post  61 . Post  61  enables the use of  5  bonding wires  62 ,  63 ,  64 ,  65  and  66  from the diode top electrode  67 . The metal of the central conductor is stripped away during the metal stripping operation following plastic encapsulation. 
     FIGS. 7 and 8 show a further embodiment of a lead frame (two sections are shown), using the lead sequence of FIG. 4, in which the post  42  of FIG. 4 is modified (enlarged) to allow 3 bonding wires to bond the upper electrode of a die on pad  23  to the post  70 . Significantly, post  70  has a width which is close to about one-half the width of the lead frame. Note that pad  23  has a narrowed and curved extension  71  connected to exterior conductor  26  (a drain conductor). 
     FIGS. 9,  10  and  11  show a modification of package  20  of FIGS. 1,  2  and  3  to create a surface mount type package. Thus, plastic package  80  is provided with a lead frame which, like those of the preceding figures, has a main die support pad  23  and three output extending lead conductors  81 ,  82  and  83 . Conductors  81 ,  82  and  83  may be gate, drain and source conductors respectively, and may have the basic structure of that of FIGS. 4 or  7 . However, the leads  81  and  83  are reentrantly bent downward as shown in FIGS. 10 and 11 to lie in a plane parallel to the plane of the exposed bottom surface of pad  23 . Thus, the device of FIGS. 9 to  11  is adapted for surface mounting, and otherwise has the various advantages previously described for FIGS. 4 and 7. 
     FIG. 12 shows a cross-section of any of the lead conductors  25  to  27  of FIGS. 1 to  3 . In the prior art TO220 package, the conductor  25  had a height of about 0.8 mm and a width of about 0.46 mm giving a cross-section area of about 0.388 mm 2 . This was used for devices rated at about 50 amperes RMS and required a printed circuit board hole of about 0.92 mm diameter. 
     It was found that the use of added bond wires within the package, as in FIG. 5, enabled an increase in device current. Thus, as shown in FIG. 13, the cross-section of contact  25  can be made 0.8 mm×0.8 mm for a cross-section of 0.64 mm 2 . This requires only a minor increase in board hole diameter to 1.15 mm, but permits an increased current capacity to 65 amperes RMS without added temperature rise or increased package resistance. 
     Further, as shown in FIG. 14, an increase of board hole diameter to 1.27 mm enable a contact cross-section of 0.8 mm×1.0 mm, increasing its current capacity to about 80 amperes RMS. 
     FIG. 15 shows the use of chamfered edges  90  on rectangular conductor  25 . This permits the use of an increased copper cross-section of conductor  25  without a change in the thru-opening  91  in a circuit board. 
     FIG. 16 shows the outline used for the plastic housing, such as that of a TO 220 outline or the like. As in the case of the package of FIGS. 1 to  10 , the package  20  (shown with the lead frame of the invention) has a relatively thick region with a flat shelf  200 , a relatively thin region with a flat shelf  201  and vertical or near-vertical walls  202  and  203 . The wall  202  intersects top surface  200  at almost 90°. this has created a molding problem in which air bubbles become entrapped in edge region  204 . Furthermore, the package  20  is frequently surface mounted atop a circuit board  210  or other surface by a cantilevered spring  211  which is fixed at one of its ends to the support surface  210 . Spring  211  may have an upturned end edge  212 , which also defines a pressure point to apply pressure to the top surface  200  to press package  20  against the support surface. 
     In the prior art package design, the pressure point  215  is removed toward an edge of die  22 . It is preferable, in order to reduce contact resistance between frame  23  and surface  210  and to better distribute stress over the area of die  22 , to move the point of pressure application toward the center of die  22 . 
     A further problem with the package of FIG. 16 is that the edge of the package where wall  202  intersects surface  201  is low, and reduces the tolerance in the bend of lead wire  46 . 
     In accordance with the present invention, the package outline is changed as shown particularly in FIGS. 17 and 18. Thus, in FIGS. 17 and 18, the flat top surface  220  is extended well beyond the center of die  22 , and end wall  221  forms an angle of about 45° to the vertical. Consequently, the pressure point of spring  211  is applied over the center of die  22  to improve the mounting of package  20  to surface  210 , both electrically and mechanically. 
     Furthermore, the novel wedge shape of the end of package  20  of FIGS. 17 and 18 enable its mounting in spring  211 , simply by pressing the package  20  to the right in the figures, thus coming up the end  212  of spring  211  and moving the package  20  under spring  211  to its desired position. A notch, or positioning posts not shown, can be formed in the package  20  to locate the package  20  in its desired end position. 
     A further advantage of the package outline of FIGS. 17 and 18 is that the edge  230  will not entrap air during molding. Furthermore, additional room is provided for lead  46  and other similar leads without fear of their coming too close to a shallow shelf as in FIG.  16 . Note that in the package of FIGS. 1 to  10 , that a short chamfer is found on the edge equivalent to edge  202  in FIG. 16, to prevent the formation of bubbles in that edge during molding. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.