Abstract:
A method of power electronic packaging includes a practicable and reliable method of fabricating power circuit modules and associated connections that are compatible with the standard top layer metalization of commercially available power devices. A planar single- or multi-layer membrane structure is attached to a carrier frame, and a via pattern is formed in the membrane. Power devices are aligned and attached to the planar membrane structure; a top layer interconnect structure is formed by metalizing the vias and the film; and a circuit is formed by patterning a deposited metal layer. The carrier frame is removed, and upper and lower thermal base plate sub-assemblies are attached to the power device-on-membrane structure. The planar device-on-membrane structure accommodates different types of power devices having variations in thickness. The thermal base plate sub-assemblies may include integral, high-performance heat exchangers for providing a low thermal resistance path to the ambient.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to electronic packaging technology and, more particularly, to electronic packaging technology applicable to power modules, such as ac-ac, ac-dc and dc-ac converters and electronic switches. 
     Typical power module designs employ wire bonds for connecting power devices to power busses and control terminals. The power devices are commonly soldered onto a metalized insulating ceramic substrate, often beryllium oxide or aluminum nitride metalized with a conductive metal such as copper or aluminum, and subsequently bonded to a massive copper heat spreader. Typically, an injection molded polymer shell covers the module, exposing only the input/output and control terminals and the copper heat spreader. The heat spreader is attached to a heat sink, such as extruded aluminum fins or a cold plate, for example. Thermal contact between the heat spreader and the heat sink is achieved through a thermal paste or a thermally conductive polymer. 
     Disadvantages of presently available power module designs such as those described hereinabove include relatively high parasitic impedance, high volume and weight, high thermal resistance, and limited reliability primarily due to wire bonds. Accordingly, it would be desirable to provide power electronic module packaging technology that overcomes such disadvantages. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with preferred embodiments of the present invention, a method of power electronic packaging comprises a practicable and reliable method of fabricating power circuit modules and associated connections that are compatible with the standard top layer metalization of commercially available power devices. The method comprises attaching a planar single- or multi-layer membrane structure to a carrier frame and forming a via pattern in the membrane. The single-layer structure comprises a flexible organic dielectric film; and the multi-layer structure comprises a flexible organic dielectric film bonded to a metal sheet. Either the multi-layer or single-layer structure is coated with a glue layer that is protected by a release sheet. Power devices are aligned and attached to the planar membrane structure; a top layer interconnect structure is formed by metalizing the vias and the film; and a circuit is formed by patterning a deposited metal layer. The carrier frame is removed, and upper and lower thermal base plate sub-assemblies are attached to the power device-on-membrane structure. The planar device-on-membrane structure may comprise different types of power devices having variations in thickness. The thermal base plate sub-assemblies may include integral, high-performance heat exchangers for providing a low thermal resistance path to the ambient. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates in cross section a planar membrane structure on a carrier frame; 
     FIG. 2 illustrates application of a via pattern in the structure of FIG. 1; 
     FIG. 3 illustrates application of a bond layer to the structure of FIG. 2; 
     FIG. 4 illustrates application of power devices to the structure of FIG.  3  and thus the resulting planar device-on-membrane structure; 
     FIG. 5 illustrates removal of the excess bond layer, sputter cleaning of the metal contacts, sputtering adhesion and seed layers, and electroplating of the structure of FIG. 4 to form a desired circuit pattern; 
     FIG. 6 illustrates removal of the bond layer, sputter cleaning of the metal contacts, and ion vapor deposition of the structure of FIG. 4; 
     FIG. 7 illustrates fabrication of thermal base sub-assemblies; 
     FIG. 8 illustrates application of upper and lower thermal base sub-assemblies of FIG. 7 to the device-on-membrane structure of FIG. 4; 
     FIG. 9 illustrates attachment of heat exchangers to the thermal base sub-assemblies of FIG. 7; 
     FIG. 10 illustrates in cross section a non-hermetic double-sided-cooled power module; 
     FIG. 11 is a top view of the module of FIG. 10; 
     FIG. 12 illustrates in cross section upper and lower base plate sub-assemblies for a single-sided hermetic power module; 
     FIG. 13 illustrates a device-on-membrane structure for the module of FIG. 12; 
     FIG. 14 illustrates assembly of the single-sided hermetic power module components of FIGS. 12 and 13; and 
     FIG. 15 illustrates cross sectional and top views of an assembled single-sided hermetic power module including the components of FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exemplary electronic power module package is described in terms of the following process steps with reference to the drawing figures. Any dimensional and component values given herein are exemplary only for purposes of illustration and are not intended to limit the scope of the invention described herein. Similarly, specific materials are exemplary only for purposes of illustration. 
     As illustrated in FIG. 1, a single-layer ( 14 ) or a multi-layer ( 12  and  14 ) structure is attached to a carrier frame  16 . The multi-layer structure comprises a thin metal layer (preferably copper)  12  and a layer of organic dielectric material (e.g., a polymer film)  14  attached to the carrier frame  16 . Alternatively, the single-layer structure comprises the organic dielectric (e.g., polymer film)  14  attached to the frame  16 . The single-layer ( 14 ) or multi-layer structure ( 14  and  12 ) is also referred to herein as comprising a membrane  18 . 
     An exemplary carrier frame  16  comprises a circular or square-shaped metal ring with a bonding layer  17 . Carrier frame  16  comprises a suitable dielectric material, such as a polyimide film, e.g., Kapton polyrmide film sold by E.I. duPont de Nemours and Company. The polymer film layer  14  is in a flat and stretched condition for processing. In particular, polymer film layer  14  is attached to carrier frame  16  in a lamination process during which bond layer  17  is cured at high temperature (e.g., up to 300° C.) and pressure (e.g., several hundreds of psi). During a cool down cycle in the lamination process, the polymer layer stretches. And since carrier frame  16  has a lower coefficient of thermal expansion (CTE) than that of polymer film layer  14 , the polymer layer remains in a stretched state after the cool down to room temperature. The carrier frame provides a convenient way for transport, ease of handling and dimensional stability for the power device-on-membrane structure fabricated thereon, as described below. 
     Organic dielectric (or polymer film) layer  14  is chosen to have particular thermal, structural and electrical properties, depending on the particular application. The laminated polymer layer  14  preferably has a low modulus (high compliance), low x, y and z-axis CTE, and a high glass transition temperature T g  or melting temperature T m , thereby improving the thermal/structural reliability of the resulting power module interconnect. Polymer film layer  14  may comprise a ceramic-filled composite polymer such as, for example, aluminum nitride (AlN)- or aluminum oxide (Al 2 O 3 )-filled poly-tetrafluoro-ethylene (PTFE); or polyimide cyanide ester- or bismaleimide triazine (BT)-epoxy-infiltrated PTFE resin matrix-based laminate materials from Gore Associates. 
     FIG. 2 illustrates stepping and punching a via ( 20 ) pattern in the single or multi-layer membrane  18  for device power and control connections. Forming the via pattern can be accomplished by mechanical punching or laser processing. Both the frame  16  and the laminated copper sheet  12  (FIG. 1) improve the dimensional stability of the membrane  18 , allowing tighter spacing for punched vias  20 . Increased via ( 20 ) density advantageously reduces resistive losses and current crowding. An exemplary configuration comprises 0.020 inch diameter vias on 0.050 inch staggered centers. 
     FIG. 3 illustrates applying a partially cured polymer resin (e.g., acrylic or epoxy) over the dielectric side  14  (FIG. 1) of the membrane  18  as a bond, or glue, layer  22 . An exemplary bond layer  22  is approximately 0.0005 inch thick. Application of the bond layer may take place before or after the via (or hole)  20  formation process. In addition, a protective release layer (not shown) may be applied over the bond layer to keep the bond layer clean during the via formation process. 
     FIG. 4 illustrates attaching power devices to the bond layer  22 . Two devices  24  and  26  are provided by way of example only. In order to bond the devices, the structure is cured in a vacuum oven under low pressure. During the curing process, some resin from the bond layer may be extruded into the punched holes (or vias)  20 , resulting in a ring ( 28 , FIG. 5) of cured bonding polymer covering the device metalization about each hole. The diameter of the punched holes, the temperature and pressure of the curing process, and the thickness of the glue layer determine the width of the extruded polymer ring. For example, for the case of a 0.0005 inch laser-drilled hole, the entire power device (i.e., die) surface may be covered with the polymer resin. For example, if the thickness of the bond layer and the diameter of the hole were of the same order of magnitude, then a polymer resin layer having a thickness on the order of tenths of an inch would likely cover the die surface. 
     For an exemplary polymer resin surface on the order of 0.0005 inch, reactive ion etching or sputter cleaning of the resin off the die surface may be difficult, expensive and impracticable. However, if the diameter of the punched vias were, for example, orders of magnitude bigger than the thickness of the bond layer, then only a small fraction of the via area would be covered by the extruded polymer layer. In such case, cleaning the polymer resin off the metalized contact surfaces of the power devices may not be necessary. 
     As illustrated in FIG. 5, the residual bond layer  22  and a thin layer of aluminum oxide (not shown) are sputter-cleaned from the top layer metalization  27  (e.g., aluminum) of the power devices. (A non-conductive oxide layer results from normal oxidation of aluminum at standard atmospheric conditions; such an oxide layer needs to be removed prior to metalization.) The cleaning process is followed by blanket sputtering of a layer of adhesion metal  30  and a layer of conductive seed metal ( 32 ) over the aluminum metalization. Suitable adhesion and conductive metal layers comprise a couple of thousands of angstroms thick titanium and copper, respectively. Subsequently, a conductive copper layer  34  (e.g., approximately 0.005 inch to 0.010 inch thick) is electroplated over the sputtered seed copper layer. The plated blanket copper layer is then subtractively patterned to form the power and control circuits and their input/output pads. (The power and control circuits are designated generally in FIG. 7 by the numeral  35 .) 
     Another alternative is to form the power circuit semi-additively where selective electroplating to the desired copper thickness is accomplished through an exposed and developed photoresist. The thin adhesion layer  30  and seed layer  32  are then removed by blanket etching the circuit. The patterned copper land and traces are subsequently plated with an electroless nickel layer (e.g., approximately 200 μinch thick) followed by electroless gold layers (e.g., approximately 10 μinch thick). (The electroless nickel and gold layers are not shown.) 
     Another alternative approach involves sputter-cleaning the excess bond layer and the oxide layer with argon plasma followed by ion vapor deposition (IVD) of an aluminum layer  36  (e.g., approximately 0.005 inch to 0.010 inch thick) through a metal mask that will form both the power and control circuits and the input/output pads, as shown in FIG.  6 . The shadowed aluminum metal is cleaned off by a blanket etching process that reduces the metal thickness over the entire membrane. Subsequently, the assembly is plated over with an electroless nickel layer (e.g., approximately 200 μinch thick) and an electroless gold layer (e.g., approximately 10 μinch thick) for corrosion protection and solderability. (The electroless nickel and gold layers are not shown.) 
     After completion of the processing steps illustrated in FIGS. 1-6, carrier frame  16  (FIG. 1) is removed from the power device-on-membrane structure  38  such that the power circuit module is ready for packaging as described in the ensuing process steps. 
     FIG. 7 illustrates fabrication of electrically-insulating, thermally-conducting base plate sub-assemblies  40  and  41  for packaging the power circuit module. An exemplary base is fabricated by metalizing a ceramic plate  42  (e.g., comprised of a ceramic aluminum nitride, beryllia or alumina) with copper  44 . In one embodiment, an aluminum nitride plate  42  (e.g., approximately 0.040 inch thick) is metalized with copper  44  and a copper molybdenum (CuMo30) plate  46 . (CuMo30 is a composite material made by infiltrating a porous molybdenum base with copper.) Active braze preforms  48  are used to attach the copper and CuMo30 plates to the aluminum nitride plates, thereby forming the lower and upper base plate subassemblies  40  and  41 , respectively. In particular, the power circuit module is fabricated by directly active-brazing one or more physically separated layers of copper sheets  44 , each having a different thickness with respect to the aluminum nitride thermal plate. The copper sheets are preferably over-sized with respect to the underlying aluminum nitride base. The over-sized copper sheets are etched to provide a design-specific pattern having different levels of thickness, e.g., three level including zero thickness, the full thickness of the original copper sheet  12  (FIG.  1 ), and a fractional thickness of the original copper sheet. The selective etching process is preferably performed in multiple steps in order to provide the desired circuit pattern and desired thickness variations. Such thickness variations advantageously accommodate variations in thickness of different types of power devices. For example, thickness variations of 0.015 inch can be accommodated in a step-wise fashion. The screened and re-flowed solder thickness (approximately 0.003 inch, for example) helps to accommodate statistical variations in thickness of the power devices and etched layers, each of which is usually less than +/−0.001 inch. 
     As illustrated in FIG. 8, the backside metalization  29  of the power devices  24  and  26  of the power device-on-membrane structure  38  is soldered via solder layers  52  to the copper metalization  44  of the lower and upper base plate sub-assemblies  40  and  41 , respectively. Exemplary soldering processes involve a single- or double-step, fluxless soldering process in a reducing atmosphere. 
     Advantageously, the planarity of the structure provides for a double-sided cooled module design. FIG. 9 shows attachment of compact, integral heat exchangers  60  to the lower and upper base plate sub-assemblies  40  and  41 , respectively. In particular, the heat exchangers are illustrated as being soldered to the base plate sub-assemblies through solder layers  62 . 
     For an exemplary non-hermetic power module  70 , as illustrated in FIGS. 10-11, the copper-metalized aluminum nitride thermal plates  42  are attached to infiltrated copper-molybdenum plates  46 , as described above with reference to FIG.  9 . An exemplary thickness of copper-molybdenum plates  46  is in a range from 0.050 inch to 0.100 inch, depending on the module size and stiffness requirements. In a preferred embodiment, the copper-molybdenum sheets  46  and integral-reticulated, metal-based, compact, high performance heat exchangers  60  are bonded to the aluminum-nitride thermal plates  42  prior to the solder-attach process, as illustrated in FIG.  9 . As illustrated in FIG. 10, liquid or air coolant enclosures  72  and  74  are sealed against the copper-molybdenum plates using module clamping screws  76 . Also, the lower coolant enclosure  72  is illustrated as being embedded in the upper coolant enclosure  74 , and sealed with a cover plate  77  using seal rings  79 . The upper and lower base plate sub-assemblies  41  and  40 , respectively, are separated from each other by a distance  78  to provide the spacing required by the thickness of the device-on-membrane  38  and the bonding solder layers. The separation distance  78  is controlled by expansion match spacers  80 . Interfaces between the solder joints and semiconductor devices of the device-on-membrane structure  38  are protected against shear and normal loads by the rigid structure formed by the copper-molybdenum plates and the spacers. Furthermore, the lower base plate sub-assembly  40  (with integral heat exchanger) is contained in enclosure (or cavity)  72 , which provides damping and stress isolation against loads. A metalized and patterned extension  82  of the dielectric film supporting the device-on-membrane structure  38  is formed to provide stress-relieved connections to gate control circuits  84  and external connectors  86 , which are integrated into the upper base assembly enclosure  74 . The upper cavity  74  has slots  88  formed therein through which power electrodes extend and form external power bus connections  90 , as shown in FIG.  11 . Capture nuts  92  embedded in the upper cavity help to form secured and reliable connections to the external power bus. 
     FIG. 12 illustrates an exemplary single-sided hermetic power module configuration  100 . Although the embodiment of FIG. 12 is single-sided and does not employ integral heat exchangers in the base plate sub-assemblies, those of ordinary skill in the art will understand that double-sided connections and single- or double-sided integral heat exchangers may readily be incorporated into any such module in accordance with the description given hereinabove. 
     In one embodiment of the single-sided hermetic power module  100 , power vias  120  for high current applications are constructed from CTE-matched CuMo30 disks  102  contained loosely within vias  120 , thereby avoiding any stress resulting from a CTE mismatch which could otherwise lead to hoop cracks for particularly large vias, for example. CuMo30 disks  102  offer good electrical and thermal conductivity and essentially stress-free, high-reliability power and control via connections. Copper sheets  104  actively brazed in a high vacuum hermetically seals the CuMo30 disks  102  and form vertical metal-to-metal connections for both thermal and electrical functions. A frame  106  (e.g., comprising molybdenum or alumina, for example) bonded to the upper base plate sub-assembly  41  functions as a substantially stress-free CTE-matched spacer while hermetically sealing the module cavity. 
     FIG. 13 illustrates a device-on-membrane structure  138  suitable for incorporation into the hermetic power module  100  of FIG.  9 . Structure  138  includes CTE-matched slugs  110  for routing the power connections from the lower base sub-assembly  140  to the upper base sub-assembly  141  (FIG.  12 ). 
     Final assembly of the power module may comprise either a single- or double-step sealing process. A two-step process involves a hierarchy of melting points. In particular, a high-melting-point solder (SH)  150  and a low-melting-point solder (SL)  152  would be reflown over the power circuits of the upper and lower base plate sub-assemblies  141  and  140 , respectively, as shown in FIG.  14 . The first soldering step would attach the backsides of the slugs and power devices to the lower base plate sub-assembly. Prior to the second soldering step, the module is preferably tested for circuit connections and functions. The second soldering step would extend the power and control circuit connections from the device-on-membrane structure  138  to the exterior of the module. As illustrated in FIG. 15, a housing  160  (e.g., of plastic) with external power connectors  162  and control connectors  164  would then be attached to the copper-molybdenum base. Such a plastic shell would cover and protect the finished module assembly. 
     Use of a hermetic or non-hermetic module depends on the application. The hermetic module of FIG. 15 has all the power and control connections on the topside of the device-on-membrane structure  38 . To this end, the non-hermetic module of FIG. 8 has electrically- and thermally-conducting, CTE-matched metal slugs incorporated to the device-on-membrane in order to bring the backside power connections to the top. Advantages of the single-sided hermetic module of FIGS. 12-15 include avoiding the need for sealed power vias for the lower base plate subassembly, avoiding the need for an additional aluminum nitride plate, and improved thermal resistance. But, an advantage of double-sided power connections is the ability to use strip-lined power electrodes, which improve power loop inductance. (Strip-lined power electrodes comprise parallel overlapping conductor strips with a well-defined spacing in a dielectric medium.) 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.