Abstract:
An IC power package particularly suitable for use as a rectifier for an automotive ac generator, as well as an IC packaging method. The power package comprises an electrically-conductive base, a MOSFET die mounted to the base so that the drain region of the MOSFET is electrically connected to the base, an electrically-conductive pin mounted to the die and electrically connected to the source region of the MOSFET, and an electrically-conductive member electrically connected to the gate region of the MOSFET. The conductive member can take several forms, including a second pin or a leadframe mounted to the die, or an annular-shaped member mounted to the base by an electrically-insulative member. In the latter embodiment, the annular-shaped member may be electrically connected to the gate region with a bond wire. The die is preferably encapsulated on the base, so that the base, pin and conductive member provide three connections for a rugged IC package.

Description:
TECHNICAL FIELD 
     The present invention generally relates to integrated circuit (IC) packaging. More particularly, this invention relates to an IC power package and packaging method for a generator rectifier bridge, in which the package employs a metal-oxide-semiconductor field effect transistor (MOSFET). 
     BACKGROUND OF THE INVENTION 
     Diodes are conventionally used in rectifier bridges for ac generators used in automotive applications. The diodes are often formed as press-fit packages with two connections that enable the packages to be readily installed in the rectifier bridge. One connection is made with a conductive base connected to one terminal of the diode, while the second connection is made with a pin connected to the opposite terminal of the diode. The package is installed by pressing the base into a bridge bracket, and then brazing the pin to the bridge circuit. 
     Automotive generators equipped with conventional diode-based rectifier bridges can be challenged as the number and power requirements of automobile accessories and systems increase, particularly as a result of low generator output at idle conditions. In certain applications, a larger generator or a battery with a higher rated output must be used, incurring both a size and weight penalty for the automobile. From a system standpoint, it would be advantageous to replace all of the rectifier diodes of an automotive ac generator with field effect transistors (FET) to allow for more precise control of the generator DC output. AC-DC conversion through FETs would allow phase shifting and power factor control, enabling higher charging system efficiency especially at low engine rpms. Such a capability would be especially desirable for automobiles equipped with a large number of power-hungry systems. However, FETs require three connections—one each for the source, drain and gate. Accordingly, the replacement of a diode-based rectifier bridge package with an FET-based package would appear to require a redesigned rectifier bridge and potentially additional system modifications, all of which are undesirable from a system and assembly standpoint. 
     SUMMARY OF THE INVENTION 
     The present invention provides an IC power package particularly suitable for use as a rectifier for an automotive ac generator, as well as an IC packaging method. The power package comprises an electrically-conductive base, a MOSFET die mounted to the base so that the drain region of the MOSFET is electrically connected to the base, an electrically-conductive pin mounted to the die and electrically connected to the source region of the MOSFET, and an electrically-conductive member electrically connected to the gate region of the MOSFET. The conductive member can take several forms, including a second pin or a leadframe mounted to the die, or an annular-shaped member mounted to the base by an electrically-insulative member. In the latter embodiment, the annular-shaped member may be electrically connected to the gate region with a bond wire. The die is preferably encapsulated on the base, yielding a rugged IC package with three connections provided by the base, pin and conductive member. 
     In view of the above, it can be appreciated that the base and pin of the IC package of this invention can be substantially similar to that of a conventional diode-based rectifier package. Therefore, the IC package of this invention is compatible with existing generator rectifier bridge designs, so that only minimal modifications are required to use the package with conventional generator designs. The packaging method of this invention is also uncomplicated, and amenable to fixturing and conventional processing to yield a package whose base, pin and conductive member are precisely aligned for installation. 
     With the IC package of this invention, more precise control of an ac generator output can be achieved than with conventional diode-based rectifier bridges. Also possible are phase shifting and power factor control, enabling higher charging system efficiency especially at low engine rpms. The IC package of this invention achieves these advantages without requiring a redesigned rectifier bridge or other significant system modifications, even though the package has one more connection than that required for a diode-based rectifier bridge. Finally, while particularly suitable for use with ac generators, the present invention and its advantages should find use in a variety of applications, including high wattage audio/video equipment, electric motor controllers, etc. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 through 3 illustrate process steps for assembling an IC power package in accordance with a first embodiment of this invention. 
     FIG. 4 is a side view illustrating a multi-pin unit that may be used with the package of the first embodiment. 
     FIGS. 5,  6  and  7  are side views of packages in accordance with alternative embodiments of this invention, and illustrate the packages at a process stage corresponding to FIG. 2 of the first embodiment. 
     FIG. 8 is a cross sectional view of the final package for the embodiment illustrated in FIG.  7 . 
     FIGS. 9 and 10 are side and end views, respectively, of a package in accordance with a preferred embodiment of this invention, and illustrate the package prior to encapsulation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Represented in FIGS. 1 through 3 are three process steps in the assembly of an IC power package  10  in accordance with a first embodiment of this invention. The package  10 , as well as alternative package designs represented in FIGS. 5 through 10, is configured to replace a conventional diode-based rectifier for an automotive ac generator. Consequently, while similar in appearance to a diode-based package, the package  10  employs a FET transistor instead of a diode, necessitating the inclusion of a third electrical lead in the package  10 . The package  10  has an electrically-conductive base  12 , a MOSFET die  14  mounted to the base  12 , and a pair of pins  16  and  18  mounted to the die  14 . The pin  16  is centrally-disposed along the axis of the disk-shaped base  12 , while the pin  18  is offset and parallel to the pin  16 . The die  14  has source, drain and gate contacts  20 ,  22  and  24 , respectively, for the source, drain and gate regions of the MOSFET. The source and gate contacts  20  and  24  are shown as being on an upper surface of the die  14 , with the pins  16  and  18  electrically connected to the source and gate contacts  20  and  24 , respectively. The drain contact  22  is shown as being on the lower surface of the die  14  to electrically contact an axial surface  26  of the base  12 . 
     In FIG. 1, the die  14  has been positioned on the base  12 , and the pin  16  registered with the source contact  20  of the die  14 . The pin  18  is shown as having been registered with the gate contact  24  in FIG.  2 . Connections between the base  12  and pins  16  and  18  with the contacts  20 ,  22  and  24  are preferably by soldering, though other attachment methods could be used. To promote solder adhesion, the surfaces of the base  12  and pins  16  and  18  to be soldered are preferably grooved (not shown). At the process level represented by FIG. 2, solder (not shown) has been provided at the contacts  20 ,  22  and  24 , such as by depositing a solder paste on the contacts  20 ,  22  and  24  or the appropriate surfaces of the base  12  and pins  16  and  18 . Preferred solder compositions for this purpose are high-temperature alloys known and used in the industry. Alternatively, solder bumps could be formed on the contacts  20 ,  22  and  24 . Fixturing (not shown) is preferably used to precisely locate and align the die  14  and pins  16  and  18  on the base  12 , allowing all internal solder connections to be performed with a single pass through an oven. 
     FIG. 3 represents a third process step, in which a nonconductive sleeve  28  has been placed on the base  12  to surround the die  14  and the lower ends of the pins  16  and  18 . The sleeve  28  is preferably secured to the base  12  by an interference fit between the inner diameter of the sleeve  28  and a boss  30  on the base  12 . A suitable material for the sleeve  28  is a phenolic available under the name PHENOLKRAFT® from Accurate Plastics, Inc., though other nonconductive materials could be used. A final preferred assembly step is to seal the package  10  by filling the cavity defined by the sleeve  28  with a suitable encapsulating material. While various materials could be used, a preferred material is an epoxy, such as epoxies available under the names EO1072, FP4450 and FP4457 from Dexter Electronic Materials Division. As a result of the encapsulation step, the upper surface  26  of the base  12 , the die  14 , and the lower ends of the pins  16  and  18  are all protectively encased, and the physical robustness of the solder connections between the pins  16  and  18  and the die  14  is promoted. 
     As seen from FIGS. 1 through 3, the base  12  is sized and provided with ribbing  32  to allow the package  10 , via the base  12 , to be press-fit into an appropriate opening in a rectifier bridge bracket. The pins  16  and  18  are preferably brazed to the system level source and gate connections, respectively, of the bridge circuit. The base  12  and pins  16  and  18  are preferably formed of a highly conductive material, such as copper plated with an electroless nickel. To facilitate brazing, the pins  16  and  18  are preferably plated with an electroless nickel that has a high phosphorous content, for example, about  12  weight percent. The pin  16  is shown as being much larger than the pin  18 , both in terms of diameter and length. The larger diameter of the pin  16  is due to the much greater current carried by the pin  16  as compared to the pin  18 . The height difference between the pins  16  and  18  is to facilitate system level connections. The pin  18  is effectively the third lead of the package  10 , and therefore can be seen to readily distinguish the package  10  from prior art diode-based packages. While FIGS. 1 and 2 represent the pins  16  and  18  as being discrete components that must be individually aligned with their respective contacts  20  and  24  on the die  14 , FIG. 4 represents an alternative approach in which the pins  16  and  18  are formed as part of a single pin unit  34 . The pin unit  34  has a U-shaped connector region  36  between the pins  16  and  18 . After placement of the unit  34  on the die  14 , by which the pins  16  and  18  are simultaneously aligned with their contacts  20  and  24 , the connector region  36  is removed by clipping or any other suitable technique to yield the two discrete pins  16  and  18 . 
     In the second embodiment of the invention represented in FIG. 5, a package  110  is shown in which the pin  18  shown in FIGS. 1 through 4 has been replaced with a leadframe  118 . The other components of the package  110  can otherwise be identical to those in FIGS. 1 through 3, and therefore are identified by the same reference numbers. FIG. 5 represents the package  110  at the same process level as that represented in FIG. 2, i.e., prior to installation of the sleeve  28  and encapsulation. The leadframe  118  is shown as having four blade-like fingers  134  extending from a central web  136  so as to be substantially parallel to the pin  16 . Electrical contact between the leadframe  118  and the gate region of the MOSFET die  14  is preferably through soldering the web  136  to four gate contacts  124  provided at the four corners of the die  14 . For this purpose, if the leadframe  118  is formed of a nonsolderable material such as copper, the surface regions of the web  136  to be soldered to the contacts  124  are preferably rendered solderable by spot plating. Solder bumps or another suitable bump structure can be formed on the gate contacts  124  or the mating surface regions of the web  136  to allow joining by a solder reflow operation. The fingers  134  of the leadframe  118  can be connected to the rectifier circuit by brazing, soldering or staking. 
     In FIG. 6, a package  210  configured in accordance with a third embodiment of the invention is represented. As with FIG. 5, FIG. 6 represents the package  210  at the same process level as that represented in FIG. 2, i.e., prior to installation of the sleeve  28  and encapsulation. The package  210  differs from that shown in FIGS. 1 through 3 primarily by the orientation of the MOSFET die  214 , which is shown as being attached on-edge to the base  12 . An advantage of the package  210  is its ease of assembly due to the resulting coplanar orientation of the pins  216  and  218 . As before, the base  12  is electrically connected to a drain contact  222  on the die  214 , while pins  216  and  218  are electrically connected to the source and gate contacts  220  and  224 , respectively. The connections between the die  214  and the pins  216  and  218  are again preferably made by soldering. As a result of the orientation of the die  14  to the base  12 , the pins  216  and  218  are substantially parallel to the surface of the die  14  on which the contacts  220  and  224  are formed. Consequently, the lower ends of the pins  216  and  218  preferably have a blade-like shape to provide greater surface contact with the contacts  220  and  224 . Other than the above-noted differences, the embodiment of FIG. 6 is substantially identical to that of FIGS. 1 through 3. 
     FIGS. 7 through 10 represent IC power packages in which the additional “third” lead necessitated by the MOSFET die  14  is not a pin or blade, but instead is a ring  318 / 418  secured to the sleeve  28  opposite the base  12 . Furthermore, the ring  318 / 418  is electrically connected to the gate contact  324 / 424  of the die  14  by wire bonding instead of soldering. 
     In FIGS. 7 and 8, a package  310  is represented at the same process levels as those represented in FIGS. 2 and 3, respectively. As before, the base  12  is electrically connected to the drain contact  22  on the die  14 , while the pin  16  is electrically connected to the source contact  20 . The connections between the base  12 , die  14  and pin  16  are again preferably made by soldering. The pin  18  shown in FIGS. 1 through 4 has been replaced with an electrically-conductive ring  318  shown as being mounted to the sleeve  28  in FIG.  8 . The ring  318  is shown as having a boss  332  by which the ring  318  is secured to the sleeve  28  with an interference fit. Because the sleeve  28  is formed of a nonconductive material (e.g., a phenolic), the sleeve  28  serves to electrically insulate the ring  318  from the base  12 . The electrical connection between the ring  318  and the gate region  324  of the die  14  is made through bond wires  330  that are bonded in accordance with known wire-bonding techniques to gate contacts  324  provided at the corners of the die  14 . Electrical connection to the ring  318  is made by forming the wires  330  so that their ends  334  are secured by the interference fit between the ring  318  and the sleeve  28 . 
     Once the ring  318  is in place, the interior cavity  336  defined by the sleeve  28  and the ring  318  can be filled with a suitable encapsulant (not shown). As with the previous embodiments, the base  12 , pin  16  and ring  318  are all preferably formed of electroless nickel-plated copper, with electroless nickel having a high phosphorous content being preferred for the pin  16  to promote brazeability. Electrical connection of the ring  318  to the rectifier circuit is preferably made by press fitting a bracket or clip (not shown) onto the ring  318 . 
     In a final and preferred embodiment of this invention, a package  410  is represented in FIGS. 9 and 10 at the same process level as that represented in FIG. 3, i.e., prior to encapsulation. FIG. 10 is an end view looking toward the pin  16 . As before, the base  12  and pin  16  are electrically connected to the drain and source contacts (not shown) on the die  14 , again preferably by soldering. In this embodiment, the pin  18  shown in FIGS. 1 through 4 has been replaced with an electrically-conductive ring  418  mounted to the sleeve  28  in the same manner described in reference to FIG.  8 . In contrast to the embodiment of FIGS. 7 and 8, the ring  418  is electrically connected to two gate contacts  424  on the die  14  through bond wires  430  that are ball bonded to the gate contacts  424  and wedge bonded to flanges  432  defined on the interior of the ring  418 . A suitable wire bonding process entails ultrasonic welding gold wires having diameters on the order of about 0.001 to 0.0015 inch (about 25 to 38 micrometers), and employs commercially available equipment such as a process and equipment available from Palomar Technologies, Inc. The sleeve  28  and ring  418  are assembled to the base  12  prior to wire bonding, after which the interior cavity  436  defined by the sleeve  28  and the ring  418  can be filled with a suitable encapsulant (not shown). The package  410  of FIGS. 9 and 10 is a particularly preferred embodiment of the invention because of the symmetry provided by its three electrical connections  12 ,  16  and  418 , which simplifies the assembly and use of the package  410 . 
     As with the embodiment of FIGS. 7 and 8, the base  12 , pin  16  and ring  418  are all preferably formed of electroless nickel-plated copper, with the electroless nickel plating of the pin  16  having a high phosphorous content to promote brazeability. Also according to the previous embodiment, the electrical connection of the ring  418  to the rectifier circuit can be made by press fitting a bracket or clip (not shown) onto the ring  418 . 
     In view of the above, it can be appreciated that the base  12  and pin  16  of each IC package of this invention can be substantially similar to that of a conventional diode-based rectifier package. Therefore, the IC packages of this invention are compatible with existing generator rectifier bridge designs, so that only minimal modifications are required to use the package with conventional generator designs. The packaging methods entailed by this invention are also uncomplicated, and amenable to fixturing and conventional processing to yield a package whose base  12 , pin  16  and “third” lead  18 ,  118 ,  218 ,  318  or  418  are precisely aligned for installation. While the invention has been described in terms of particular and preferred embodiments, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.