Abstract:
A cascaded die mounting device and method using spring contacts for die attachment, with or without metallic bonds between the contacts and the dies, is disclosed. One embodiment is for the direct refrigerant cooling of an inverter/converter carrying higher power levels than most of the low power circuits previously taught, and does not require using a heat sink.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/738,746 filed on Dec. 17, 2003. 
     
    
     STATEMENT REGARDING FEDERAL SPONSORSHIP  
       [0002]     This invention was made with Government support under contract no. DE-AC05-00OR22725 to UT-Battelle, LLC, awarded by the United States Department of Energy. The Government has certain rights in the invention. 
     
    
     TECHNICAL FIELD  
       [0003]     The present invention relates to methods and apparatus for interconnecting and packaging miniature electronic components, such as integrated circuit chips and other devices to form larger systems. This invention recognizes that cooling of power electronic dies by a liquid refrigerant no longer restricts packaging arrangements. These packaging innovations do not require flat cooling surfaces previously necessary for achieving a low thermal resistance connection to the heat sink.  
       DESCRIPTION OF THE BACKGROUND ART  
       [0004]     Methods of cascading semiconductor die have been in use in the semiconductor industry since the 1990s. This technology has been primarily used for memory chips in computer systems. With increasing trends towards systems on chips this packaging methodology is rapidly expanding into other applications. Stacking of semiconductor die is ideal for low current, low power systems. Thermal considerations have prevented these packaging methods from being used for higher power devices.  
         [0005]     Typical IGBT (Insulated Gate Bipolar Transistor) module die arrangements have the dies attached to copper which is bonded to a ceramic backplate. The heat generated by the dies is transferred through the ceramic to the heat sink located under the backplate. Bonded wires are used for electrical connection between the dies and electrodes. The connection wires are encased in a podding gel for insulation, thermal transfer, and mechanical stability purposes. Thermal resistance along the heat transfer path makes the junction temperature higher than it would be for a directly liquid cooled arrangement.  
         [0006]     With dies stacked, the small die resides atop the larger and wire bonds are brought out to either the substrate or a lower level die. This method can be expanded to larger numbers of layers. Using several layers of die, the topmost layer has two separate dies sitting on the layer beneath it. Cascading die in this manner results in increases in packing density, reductions in cost, less inductance, and faster signal transmissions because the dies are closely stacked. Most stacking methods still rely on wire bonds for bringing the I/O from the outside world to the silicon.  
         [0007]     Within the semiconductor packaging industry there is a growing desire to move away from the wire bonding of the die to the substrate or output pins. Size, performance, and cost considerations are driving new packaging methods. This is true for low as well as high power devices.  
         [0008]     Silicon Power Corporation is developing a new packaging method for power devices. The wire-bondless package is a soldered assembly of a semiconductor power device, such as an IGBT, and a thin ceramic lid. The lid is metalized on the bottom side and designed to mate to large and small device electrodes, which are connected by metalized vias to a more rugged and convenient pattern of top-side metal. If the lid material is a good conductor and/or if the lid vias are very dense, the ThinPak lidded device can be cooled from both sides or treated as a flip chip device, but without the usual limitations in achievable breakdown voltage. The low impedance and small size and weight of the device, as well as the rather large mechanical tolerances of the lid, make it convenient for module applications.  
         [0009]     The thin-film power overlay (POL) technology developed at General Electric is an approach to reducing the cost of mass production and improving the reliability and efficiency of power electronics packages. Power semiconductor devices are soldered to a Direct Bonded Copper (DBC) substrate from the backside. Differences in device thickness are compensated by copper shims. A thin layer of polyamide sheet is laminated over the die after vias are laser machined or mechanically punched through the film. These vias provide openings for the power interconnect to the top layer. The whole top surface is then metallized (electro-plated) with copper. Circuit patterns are achieved by the application of photo resist and chemical etching processes. More layers can be built up repeatedly to realize a multilayered interconnect structure. Low-profile passive components can be embedded into the overlay flex.  
         [0010]     Dimple Array Interconnect (DAI) packaging involves the use of a copper sheet with arrays of dimples preformed serving as electrical interconnections. The Dimple Array Interconnect structure has similar shape as the hourglass-shaped flip chip interconnect.  
         [0011]     International Rectifier&#39;s DirectFET is a surface-mount package that improves MOSFET performance by lowering both the package&#39;s electrical and thermal resistance. It does so with a design that permits direct attachment of the die to the pc board via solderable pads on the chip and through attachment to a copper drain clip that allows double-sided cooling. The DirectFET package consists of a passivated die attached to a copper clip. Solderable metal contacts on the bottom of the die provide gate and source contacts to the pc board, while the copper clip provides an electrical connection to the drain and permits topside cooling. Although the package has the same outline as an SO-8, the DirectFET&#39;s height is 60% less.  
         [0012]     In Vishay Siliconix&#39;s PowerConnect technology, traditional bondwires are replaced with a direct connection between the MOSFET die and the copper leadframe to lower a package&#39;s contribution to the device&#39;s on resistance, R DS(on)  in low-voltage power MOSFETs. To accomplish this direct connection, the top surface of the MOSFET die had to be made solderable. Toward that end, the company developed a nickel-based metallization process on top of the aluminum. The result is that the leadframe can be attached to both the bottom and the top surface of the die.  
         [0013]     Semikron is making baseless IGBT power modules based on pressure contact packaging technology through the use of a spring pad and a pressure plate. The elimination of the base plate and thus the solder joint between the base plate and substrate, together with the use of spring contacts to establish connections between the built-in gate drive board and the substrate, leads to improved reliability and enables a very cost effective module and power electronic system assembly. Semikron packaging is designed for traditional heat-sink cooling and is not applicable to direct refrigerant cooling. The chips are cooled by a heat sink.  
         [0014]     Virginia Polytechnic Institute and State University disclosed a three-dimensional packaging technique developed for power electronics building blocks using direct copper bonding to interconnect power devices. The parallel-plate structure provides the potential for double-sided cooling, direct liquid cooling of the power devices between the plates, and integration of passive components in the module.  
         [0015]     FlipChip technology uses a ball grid array package resulting in a wire bondless system. The interconnection between the die and carrier in flip chip packaging is made through a conductive “bump” that is placed directly on the die surface. The bumped die is then “flipped over” and placed face down, with the bumps connecting to the carrier directly. After the die is soldered, underfill is added between the die and the substrate. Underfill is a specially engineered epoxy that fills the area between the die and the carrier, surrounding the solder bumps. It is designed to control the stress in the solder joints caused by the difference in thermal expansion between the silicon die and the carrier. Once cured, the underfill absorbs the stress, reducing the strain on the solder bumps, greatly increasing the life of the finished package. The chip attach and underfill steps are the basics of flip chip interconnect.  
         [0016]     A flex-circuit interconnection system is being developed at the Center for Power Electronics Systems at Rensselaer Polytechnic Institute. This method offers extra layout design freedom in the vertical dimension of the package. Compared with the conventional power module, the power terminals in the flex-circuit package are distributed on the DBC and the flex substrate with close proximity and planar configuration. As a result, the package parasitic inductance can be reduced significantly.  
       SUMMARY OF THE INVENTION  
       [0017]     A cascaded die mounting device and method using spring contacts for die attachment, with or without metallic bonds between the contacts and the dies, is disclosed. One embodiment is for the direct refrigerant cooling of an inverter/converter carrying higher power levels than most of the low power circuits previously taught, and does not require using a heat sink. The invention is an interconnect and mounting device comprising at least three cascaded layers of electronic components, a means for electrically interconnecting said layers at contact points, a means for connecting input power to said device, a means for connecting output power to said device, wherein said components modify said input power to produce said output power, and wherein said components are cooled by direct refrigerant contact.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a diagram of a basic cascaded mounting arrangement with example wirebonds.  
         [0019]      FIG. 2  is a diagram of a basic cascaded mounting arrangement with example spring-loaded contacts with or without metallic bonds.  
         [0020]      FIG. 3  is a diagram of example spring contacts arrangement on a layer.  
         [0021]      FIG. 4  is a diagram of example spring-loaded isolated contact-bond arrangement  
         [0022]      FIG. 5  is a diagram of a cascade silicon die mounting to a solid conducting plate.  
         [0023]      FIG. 6  is a diagram for a slotted geometry cascade silicon die mounting.  
         [0024]      FIG. 7  is a diagram of a cascaded mounting arrangement having a triangular inner structure and a hexagonal outer structure.  
         [0025]      FIG. 8  is a diagram of a cascaded mounting arrangement having four legs. 
     
    
     DETAILED DESCRIPTION  
       [0026]      FIG. 1  shows a basic cascaded mounting embodiment. Three layers are shown but the invention is not limited to a specific number of layers. Beginning with the first layer  1 , a metal such as copper alloy with other appropriate thermal expansion controlling material such as a piece of ceramics, carbon foam, or low expansion alloy to form a direct metal bond substrate is used as a conducting plate  11 . Optional features such as modifying the conducting plates  11 ,  12 ,  13  with an array of small holes that enable an array of thin wires going through the thermal expansion controlling material, resulting in another option of matching thermal expansion with the dies. The semiconductor switches  6  and diodes  4  dies are metallically bonded to a surface of the conducting plate  11  to form the first layer  1 . The semiconductor switches can have built-in diodes. Wirebonds  7  are used at interconnecting wiring contact points. The second layer  2  consists of three separate small conducting plates  12 . One surface of each small conducting plate  12  has a set of a switch  6 , and a diode  4  metallically bonded to it. Each three-phase power leg  8 ,  9 , and  10  is connected to a separate small conducting plate  12 . The third layer is a conducting plate  13 . The three plates are mounted in a cascaded form with sufficient spacing between the layers for liquid refrigerant to flow through and for bubbles formed by refrigerant nucleate boiling to be rapidly expelled by the moving liquid refrigerant.  
         [0027]      FIG. 2  shows the basic cascaded mounting embodiment with spring-loaded contacts  29 . There are three cascaded layers  21 ,  22 ,  23 . On the first layer  21 , the dies of switches  30  and diodes  32  are placed on a thermal expansion controlled conducting plate  33  to form the first layer  21 . Bonding material can be distributed on the conducting plate  33  prior to component placement. The second layer  22  consists of three separate small conducting plates  34 . On the lower surface of each small conducting plate  34 , a sufficient number of fine spring-loaded contacts  29  are disposed between each small conducting plate  34  and a switch  30 /diode  32  on the upper surface of the first layer  21 . There can be bonding material distributed on the small conducting plates  34  prior to assembly. The number of spring-loaded contacts  29  is determined by the permissible current density of the contacts and the required distribution on the die  30 . The upper surface of each small conducting plate  34  has a switch  30 /diode  32  disposed on the upper surface with or without use of a bonding material. The third conducting plate  35  has spring-loaded contacts  29  disposed between the lower surface of the third conducting plate  35  and each switch  30 /diode  32  on the second layer  22 . The spring-loaded contacts  29  supply contact pressure between each switch  30 /diode  32  and its respective conductive plate mounting. The three layers are assembled together under a predetermined spring load.  
         [0028]     Optionally, metallic bonding of the sufficient number of fine spring-loaded contacts  29  to its respective switch  30 /diode  32  is used. The reasons for sufficient number of fine contacts are to carry sufficiently high current and not to post thermal expansion stress on the dies. There must be sufficient clearance to perform metallic bonding once the layers are assembled. For example, metallic bonding could be performed using a multiple-finger ultrasonic bonding head to create metallic bonds between the spring-loaded contacts  29  and switches  30  or diodes  32 . Alternatively, a laser could be utilized for the multiple-finger bonding. Another option would be placing the assembly in an oven under proper environment control to bond the switches  30  and diodes  32  to the fine spring-loaded contacts. It would be necessary to ensure the oven&#39;s operating temperature did not exceed the temperature specifications of the switches  30  and diodes  32 .  
         [0029]     There are many possible methods for assembling the conducting plates. The distance between the layers is determined by (1) the required spring loads for the proper operation of the contacts, (2) the clearance for a self adjustment of the spring loads, (3) the sufficient clearance available between the layers for the liquid refrigerant to flow and for the bubbles to be rapidly expelled by the moving liquid refrigerant, (4) the mechanical integrity of the mounting structure, and (5) the tolerable maximum stray inductance of the spring contacts for specific applications. As an example, the mounting can be a structure consisting of two or more insulation bolts  25  per small conducting plate. Each bolt penetrates the three layers with given insulation spacers situated between the layers for a predetermined spring load. Nuts at an end of the insulation bolt  25  can be used to draw the conducting plates together. Other mounting parts determined by specific application can be used for assembly.  
         [0030]      FIG. 3  illustrates three views of a small conducting plate  34  using spring-loaded contacts  29  with or without metallic bonds. The spring-loaded contacts  29  are extended out from the copper alloy of the conducting plate  45 . Bonding material can be distributed on the conducting plate  45  prior to assembly. Only if needed the edges  41  of the copper alloy plate may be folded to increase the rigidity during manufacturing process. Each individual spring of the spring-loaded contact  29  can be self adjusted by the bending of their spring arms  42 . An assembly guide  44  is included to allow the small conducting plate  34  sufficient freedom of movement for the self-adjustment of the spring load in the spring-loaded contacts  29 . A three-layer assembly similar to that shown in  FIG. 2  can be fabricated using this technique. The fine spring-loaded contacts  29  can be constructed with metallic bonds using (a) multiple-head ultrasonic bonding techniques, (b) laser bonding, or (c) oven bonding.  
         [0031]      FIG. 4  shows an insulated holder  57  attached to a stamped tab  56  to extend a spring contact  51  to the gate  31  of a switch  30  or other isolated points. A conductive lead  55  for bringing out the gate control can be attached to the spring contact  51 . Since all spring contacts  51  are precisely located after the cascaded mounting is assembled, it is possible to incorporate a reasonably simple step to bond the spring contacts  51  to a gate  31  or other isolated points with a bonding material  52 . Bonding will prevent fatigue of the springs.  
         [0032]      FIG. 5  shows the silicon switch  30  and diode  32  dies in position to mount directly onto the conducting plate  45  with the thermal expansion controlling material  58  bonded to the back side of the conducting plate  45  to thermally stabilize the plate and decrease thermal stress on the silicon die. The thermal expansion controlling material  58  can be a ceramic, low expansion metal, graphite foam, or other suitable material.  
         [0033]      FIG. 6  shows the silicon switch  30  and diode  32  dies in position to mount onto a slotted conducting plate  59  with the thermal expansion controlling material  58  bonded to the back side of the slotted conducting plate  59  to thermally stabilize the plate and decrease thermal stress on the silicon die. Various slot geometries  60  can be used to match thermal growth of the silicon dies  30 ,  32  to the slotted conducting plate  59 . The thermal expansion controlling material  58  can be a ceramic, low expansion metal, graphite foam, or other suitable material.  
         [0034]      FIG. 7  shows a three-phase, three-leg embodiment of the invention having semiconductor switches  62  cascade-mounted onto a triangular inner structure  63 . The triangular inner structure  63  supports lower switches of the phase legs and also serves as a positive DC link terminal. Spring-loaded and soldered contacts  64  can be used in this embodiment. A hexagonal outer structure  61  also serves as a negative DC link terminal.  
         [0035]      FIG. 8  is a diagram of a four leg cascade inverter embodiment having four (4) copper conductor bars  71  for output connections supported, for example, on an insulated base behind the cascade mounts. The cascade segment  73  shows a two layer silicon die stack with the outer cylinder contacts not shown. The outer DC link cylinder  72  and the inner DC link  74  feed DC power to the silicon dies. The DC power is then modified by the silicon dies to produce AC power on the four output copper conductor bars  71 . An insulating sleeve  75  is provided between the phases and DC link.  
         [0036]     The embodiments shown are suitable for generating up to four-phase output power however two-phase or other number of phases output power is generated by providing additional legs in the device.  
         [0037]     While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope.