Abstract:
A process for producing a circuit component having a double-sided circuit device between a pair of substrates. The process entails depositing a solder material on contact areas on surfaces of the substrates, placing a first of the substrates within a cavity in a receptacle, and then placing a lead member on the substrate so that the lead member is supported by the receptacle and a portion of the lead member is aligned with a portion of the contact area of the substrate. A fixture is then placed on the lead member and over the substrate so that the fixture is supported by the receptacle. After aligning the circuit device with the contact area of the remaining substrate, the substrate-device assembly is placed in an aperture in the fixture so that a surface of the device electrically contacts the contact area of the first substrate and the opposite surface of the device electrically contacts the contact area of the second substrate. The resulting fixtured assembly then undergoes reflow.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to circuit board components, and more particularly to processes for assembling and reflow soldering a double-sided circuit component to a pair of opposing substrates. 
     Various types of circuit board components have been specifically developed for high current and high power applications, such as hybrid and electric vehicles. Such components often comprise a semiconductor device, such as a diode, thyristor, MOSFET (metal oxide semiconductor field effect transistor), IGBT (isolated gate bipolar transistor), resistors, etc., depending on the particular circuit and use desired. Vertical devices are typically formed in a semiconductor (e.g., silicon) die having metallized electrodes on its opposite surfaces, e.g., a MOSFET or IGBT with a drain/collector electrode on one surface and gate and source/emitter electrodes on its opposite surface. The die is mounted on a conductive pad for electrical contact with the drain/collector electrode, with connections to the remaining electrodes on the opposite surface often being made by wire bonding. The pad and wires are electrically connected to a leadframe whose leads project outside a protective housing that is often formed by overmolding the leadframe and die. 
     Components of the type described above include well-known industry standard package outlines, such as the T0220 and T0247 cases, which are prepackaged integrated circuit (IC) components whose leads are adapted for attachment (e.g., by soldering) to a printed circuit board (PCB). The overmolded housings of these packages protect the die, wire bonds, etc., while typically leaving the lower surface of the conductive pad exposed to provide a thermal and/or electrical path out of the package. Such a path allows the package to be connected to an electrical bus for electrical connection to the PCB, or a heat-sinking mass for dissipating heat from the package. If electrical isolation of the path is necessary, a non-electrically conductive heat-sinking pad is provided between the package and heat-sinking mass. In doing so, the heat-sinking pad increases the thermal resistance of the path, typically on the order of 0.1 to 0.5° C./watt. 
     A further drawback of packages of the type described above is their size. As an example, in certain high current hybrid vehicle applications, arrays of packages containing MOSFET&#39;s in a three-phase configuration are utilized, with two or three devices in parallel per switch. The resulting assembled array may contain, for example, sixteen to twenty-four packages, requiring a relatively large area on the PCB. In high current, high voltage (e.g., 150 to 400 V) hybrid vehicle applications, this situation is exacerbated by the need for paired sets of IGBT&#39;s and diodes, with the resulting assembled array twice as many individual packages. 
     As a solution to the above, commonly-assigned U.S. Pat. No. 6,812,553 to Gerbsch et al. and U.S. patent application Ser. No. 10/707,005 (U.S. Patent Publication 2004/0094828) to Campbell et al. disclose double-sided circuit devices that are packaged between a pair of substrates in such a way as to reduce the overall size of the resulting component while also meeting both current and thermal management requirements. There is a need for assembly processes suitable for mass-producing these and other double-sided circuit components. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a process suitable for mass-producing a double-sided circuit component. The process entails assembling and reflow soldering a double-sided circuit device between a pair of substrate members without the use of separate packaging, wire bonds, etc., that would increase the complexity and size of the component and hinder thermal management of the device. 
     The process entails depositing a solder material on first surfaces of first and second substrate members, each of which is formed of an electrically-nonconductive material and comprises at least one electrically-conductive area on the first surface on which the solder material is deposited and a second surface oppositely disposed from the first surface thereof. The first substrate member is then placed within a cavity in a receptacle, and a lead member is placed on the first substrate member so that the lead member is supported by the receptacle and a portion of the lead member is aligned with a portion of the electrically-conductive area of the first substrate member. A fixture is then placed on the lead member and over the first substrate member so that the fixture is supported by the receptacle. After aligning a circuit device with the electrically-conductive area of the second substrate member to yield a preliminary assembly, the preliminary assembly is placed in an aperture in the fixture so that a first surface of the circuit device electrically contacts the electrically-conductive area of the first substrate member and a second surface of the circuit device electrically contacts the electrically-conductive area of the second substrate member. 
     The resulting fixtured assembly, comprising the first and second substrate members, the receptacle, the lead member, the fixture, and the circuit device, is then heated to cause the solder material to melt, flow, and wet the electrically-conductive areas of the first and second substrate members, the portion of the lead member, and the first and second surfaces of the circuit device. Cooling the fixtured assembly yields the double-sided circuit component in which the electrically-conductive area of the first substrate member is solder bonded to the first surface of the circuit device, the electrically-conductive area of the second substrate member is solder bonded to the second surface of the circuit device, and the portion of the lead member is solder bonded to the first and second substrate members so as to be electrically coupled with at least one of the electrically-conductive areas of the first and second substrate members. 
     In view of the above, the process of this invention is capable of producing the double-sided circuit component in a manner that is suitable for use in high-volume manufacturing with high yields. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are top and side views, respectively, of a double-sided component that can be produced with a process in accordance with the present invention. 
         FIG. 3  is an exploded view of the component of  FIG. 2 . 
         FIGS. 4 and 5  are plan views of a fixture and boat configured for use in the process of the present invention. 
         FIG. 6  is an exploded view of the components of  FIGS. 3 ,  4  and  5  as arranged for undergoing the process of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A double-sided circuit component  10  suitable for use in a process according to the present invention is shown in  FIGS. 1 through 3 . The component  10  is shown as comprising a pair of substrates  12  and  14 , a circuit device  18 , and two sets of leads  30  and  32 . From the following it will be appreciated that the component  10  shown in the Figures is intended to be representative of the type of component that can be produced by a process within the scope of the invention, and that the number, configuration, and orientation of the elements of the component  10  can differ from that shown in the Figures, e.g., multiple circuit devices. Examples include the circuit devices disclosed in commonly-assigned U.S. Pat. No. 6,812,553 to Gerbsch et al. and U.S. patent application Ser. No. 10/707,005 (U.S. Patent Publication 2004/0094828) to Campbell et al. 
     If the component  10  is to be used in a high current and high power application, such as a hybrid or electric vehicle, the substrates  12  and  14  are preferably formed of an electrically-nonconductive material, preferably a ceramic material of the type commonly used in electronic systems such as alumina (Al 2 O 3 ), aluminum nitride (AlN), silicon nitride (SiN), beryllium oxide (BeO), or an insulated metal substrate (IMS) material. Ceramic materials vary in thermal performance, such that the selection of a particular ceramic material for the substrates  12  and  14  will depend at least in part on the thermal requirements of the specific application for the component  10 . Consistent with a high current/power application, each of the outward-facing surfaces  22  and  24  of the substrates  12  and  14  is shown as having an outer layer  26  and  28 , respectively, of thermally-conductive material. The outer layers  26  and  28  may be formed of a solderable material, such as copper, copper alloy, plated (e.g., NiAu) aluminum, etc., to permit soldering of the component  10  to heatsinks (not shown) or other suitable structures. If solder is not required for attachment, such as pressure attachment, other materials can be used for the outer layers  26  and  28 . Furthermore, it is foreseeable that the outer layers  26  and  28  could be eliminated, with the benefit of reducing the thermal resistance of the thermal path through these layers  26  and  28 , and therefore reduced component temperature. 
     The device  18  may be, for example, a diode, IGBT, MOSFET, or a combination of these devices could be used if more than one device  18  is included in the component. For convenience, the device  18  will be described as a MOSFET, and as such is preferably formed in a semiconductor die, such as silicon. One or more electrodes (not shown) are formed on the upper and lower surfaces of the device. For example, a drain electrode on the lower surface and gate and source electrodes on the upper surface. The device  18  is configured to be mounted between the substrates  12  and  14  so that the drain electrode electrically contacts a conductive pad  40  on the lower substrate  14  and the gate and source electrodes make electrical contact with conductive pads  36  (only one of which is shown) on the upper substrate  14 . To accommodate multiple MOSFET&#39;s or other combinations of devices, only minor changes to the substrates  12  and  14  and leads  30  and  32  would be required, as will become evident. 
     Electrical connections of the leads  30  and  32  to the electrodes of the device  18  are achieved through electrically-conductive contact areas  36  and  40  defined on the inward-facing surfaces  46  and  48  of the substrates  12  and  14 , respectively. On the substrate  12 , an appropriate number of contact areas  36  are defined to individually register with the electrodes on the upper surface of the device  18 , such as the source and gate electrodes of a MOSFET. Similarly, an appropriate number of contact areas  40  are provided on the substrate  14  for registration with the number of electrodes on the lower surface of the device  18 , such as the drain electrode of a MOSFET. Electrically-conductive bonding between the areas  36  and  40  and their respective electrodes is preferably achieved with solder connections. The contact areas  36  and  40  on the substrates  12  and  14  can each be formed by a single conductive layer, e.g., a copper foil, that is patterned or divided by solder stops. The configurations of the areas  36  and  40  can be modified and solder stops used to match the geometry of a variety of integrated circuit devices incorporated into the component  10 . 
     The leads  30  and  32  are adapted for connecting the component  10  to an electrical bus or other device utilized in the particular application. The leads  30  and  32  can be formed of stamped copper or copper alloy, though other methods of construction are possible. The leads  30  and  32  are depicted as being of a type suitable for use in high current applications (e.g., 200 amperes). For lower current applications, individual lead pins can be used. Each lead  30  and  32  is shown as comprising a plurality of fingers  60  through which physical connection is made to the component  10  and electrical connection is made to the electrodes of the device  18 . In the embodiment shown, the lead  30  is electrically coupled to the electrode(s) on the upper surface of the device  18  through bond pads  56  on the upper substrate  12  and also bonded to the lower substrate  14  through electrically-isolated bond pads  50  on the lower substrate  14 , and the lead  32  is electrically coupled to the electrode(s) on the lower surface of the device  18  through bond pads  52  on the lower substrate  14  and also bonded to the upper substrate  12  through electrically-isolated bond pads  54  on the upper substrate  12 . The pads  50 ,  52 ,  54 , and  56  can be patterned from the same conductive layers as the areas  36  and  40  on the substrates  12  and  14 . The leads  30  and  32  are preferably soldered to their bond pads  50 ,  52 ,  54 , and  56 . 
     In view of the above construction, the component  10  conducts current and uniformly extracts current across its entire face, instead of wire bond connection sites, and therefore has the ability to carry higher currents with less temperature rise than conventional wire bonded and ribbon bonded devices. Also by avoiding wire and ribbon bonding techniques, the component  10  can be readily adapted to enclose various types and configurations of devices. The component  10  also has the advantage of being able to dissipate heat in two directions, namely, up through the upper substrate  12  and/or down through the lower substrate  14 . If both substrates  12  and  14  are used to dissipate heat, the temperature rise of the component  10  can potentially be reduced by about one-half. The solderable outer layers  26  and  28  of the substrates  12  and  14  are isolated from the circuit device  18  by the substrates  12  and  14 . By providing electrically-isolated top and bottom surfaces in this manner, the need for discrete heatsink electrical-isolation pads can be avoided. 
     It can also be seen from the above that the component  10  does not require a plastic overmold, in that the circuit device  18  is protectively enclosed by the substrate  12  and  14 . Avoiding a plastic overmold reduces internal differences in coefficients of thermal expansion (CTE) within the component  10 , as well as CTE mismatches with components and substrates contacting by the component  10 , thereby improving component life during temperature cycling. If desired, a compliant dielectric encapsulating material can be placed around the perimeter of the component package to seal the edges of the substrates  12  and  14  and the gap therebetween, thereby protecting against contaminant intrusion and improving the electrical isolation properties of the package. 
     A process for assembling and soldering the parts of the component  10  is represented in  FIG. 6 , with two tools  62  and  64  for supporting the components  10  during assembly being represented in  FIGS. 4 and 5 . The first tool  62  is a boat specially adapted for the present invention but otherwise generally of the type known for use in reflow processes performed in a belt furnace. For use in the invention, the boat  62  is formed to have any number of sets of three cavities  66 ,  68 , and  70 , with the center cavity  68  provided with support pedestals  72 . The boat  62  is also provided with location pins  74  adjacent the cavities  66  and  70 , and an alignment pin  84  by which the second tool, hereinafter a fixture  64 , is aligned with the boat  62 . The fixture  64  is formed to have sets of three aperture  76 ,  78 , and  80  corresponding in number to the cavities  66 ,  68 , and  70  of the boat  62 , with the center cavity  78  provided with support pedestals  82 . The fixture  64  is also provided with an alignment hole  86  for mating with the alignment pin  84  of the boat  62 . 
     The process of assembling and soldering the double-sided circuit component  10  of  FIGS. 1 through 3  generally entails depositing a solder paste on the contact areas  36  and  40  and bond pads  50 ,  52 ,  54 , and  56  of the substrates  12  and  14 . The substrate  14  is then placed within one of the center cavities  68  in the boat  62  so as to be supported within the cavity  68  by the recessed pedestals  72 . The leads  30  and  32  are then placed on the boat  62  to span the cavities  66  and  70  thereof and so that their fingers  60  are individually aligned with the bond pads  50  and  52  of the substrate  14 , the alignment of which is assured by mating location holes  34  of the leads  30  and  32  with the location pins  74  of the boat  62 . The fixture  64  is then placed on the leads  30  and  32  and over the substrate  14  so that the fixture  64  is supported by the boat  62  and its alignment hole is mated with the alignment pin  84  on the boat  62 . The portions of the fixture  64  separating the center aperture  78  from the other apertures  76  and  80  preferably contact and immobilize the leads  30  and  32  adjacent their fingers  60 , and the rim of the fixture  64  preferably contacts and immobilizes the portions of the leads  30  and  32  in which the location holes  34  are formed. 
     The next step is to position the substrate  12  with its surface  46  facing up, orient the device  18  so that its upper surface (as viewed in  FIGS. 2 ,  3 , and  6 ) is facing down, align the electrode(s) on the upper surface of the device  18  with the contact area  36  of the substrate  12 , and then physically place the device  18  on the substrate  12  to yield what may be termed a preliminary assembly. The preliminary assembly is then inverted to be device-down (as seen in  FIGS. 2 ,  3 , and  6 ) and placed in the center aperture  78  of the fixture  64  so that the substrate  12  is supported within the aperture  78  by the pedestals  82 , the electrode(s) on the lower surface of the device  18  electrically contact the contact area  40  of the substrate  14 , and the bond pads  54  and  56  on the substrate  12  contact the fingers  60  of the leads  30  and  32  so that the fingers  60  of the leads  30  and  32  are between aligned pairs of the bond pads  50 ,  52 ,  54  and  56 . 
     It can be appreciated that the process described above can be performed to simultaneously place parts in the boat  62  and fixture  64  to produce four additional components  10 , and that any number of components  10  could be processed by fabricating the boat  62  and fixture  64  to have the desired number of cavities  66 ,  68 , and  70  and apertures  76 ,  78 , and  80 . The fixtured assembly is then ready for a solder reflow operation, such as by transporting the boat  62  and the parts supported thereby through a belt oven. Notably, the individual parts of the component  10  are supported and held together with the boat  62  and  64  solely under the force of gravity. During reflow, the solder paste that was deposited on the contact areas  36  and  40  and bond pads  50 ,  52 ,  54 , and  56  of the substrates  12  and  14  melts, flows, and wets the contact areas  36  and  40  and the bond pads  50 ,  52 ,  54 , and  56 , the electrodes of the device  18  aligned with the contact areas  36  and  40 , and the fingers  60  of the leads  30  and  32  aligned with the bond pads  50 ,  52 ,  54 , and  56 . Upon cooling the fixtured assembly, the molten solder forms solder connections that solder bond the contact area  36  and  40  of the substrates  12  and  14  to the electrodes of the device  18  and the fingers  60  of the leads  30  and  32  to their respective pairs of bond pads  50 ,  52 ,  54 , and  56 . 
     While the invention has been described in terms of a preferred embodiment, 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.