Abstract:
A circuit package for a microwave signal comprises a substrate defining a MMIC surface of the substrate and an opposing non-MMIC surface of the substrate. The substrate is devoid of signal carrying vias. A waveguide is disposed on the MMIC surface of the substrate. A MMIC is disposed on the MMIC surface of the substrate, and the MMIC is in electrical communication with the waveguide. An I/O port is in electrical communication with the waveguide wherein a transmission path for the signal is provided from the I/O port, through the waveguide and to the MMIC. In an alternative exemplary embodiment of the invention, the I/O port of the circuit package is electrically connectable to a PC board. The MMIC surface of the substrate faces the PC board when the I/O port is electrically connected to the PC board.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is related to the field of packages for Microwave Circuits, especially in the frequency band from 20 GHz and higher. 
     2. Prior Art 
     Microwave Monolithic Integrated Circuits (MMIC) and other microwave integrated circuit (IC) devices for microwave signals have been packaged using complex, high reliability, high cost packages, the designs of which are not conducive to high volume assembly. These MMIC packages (packages) have been used in such specialty microwave applications as Radio Frequency (RF) telescopes and other speciality applications. 
     However, in the growing markets of more mainstream commercial applications, e.g., Personal Communication Services, there is the need for low cost, wideband, surface mounted, reliable, user friendly MMIC packages. One such example of a growing commercial microwave application is the development of Line Microwave Distribution Systems (LMDS), two point radios and obstacle detection radar for automotive vehicles. Moreover other applications have been identified at 23, 28, 32, 38, 60 and 70 Ghz and higher, and the industry is approaching production of these products. 
     The MMIC packages are intended primarily for MMICs constructed substantially of semiconductor materials suitable for high frequency operation, e.g, Gallium Arsenide. The MMIC is generally bonded to a package substrate constructed of a ceramic material, e.g., alumina, having thin films of metals, e.g., nickel or titanium, disposed on its surface in the form of an arrangement of wave guides. The waveguides comprise a patterned metalization which includes a plurality of ground planes, signal traces and unmetalized gaps through which the microwave signals are transmitted. The MMIC is connected to the ground planes and signal traces on the surface of the substrate by methods such as wirebonding, solder bumps or flipped chips. Vias, e.g., holes or slots, in the ceramic substrate are filled with a conductive composite material such as copper-tungsten, which provides a transmission path for the signal from the metalization on the surface through the substrate itself. The composite material within the vias is in turn connected to input/output (I/O) ports of the MMIC package, which transmit the microwave signals therethrough. Existing packages are surface mounted to a mother PC board and are connected to the circuitry of the PC board through the I/O ports, which can include conductive balls (spheres), bumps (rounded protrusions), ribbons or leads. 
     The MMIC of the MMIC package is connected to the mother PC board in a way that is electrically transparent to microwave signals transmitted therebetween. Achieving electrical transparency requires matching the impedance of the microwave transmission path through all transition areas for the signal, i.e., areas between waveguides, from the mother board to the MMIC. Prior art transition areas for the signal include: 1) from the mother board through the I/O connection between the mother board and the package substrate, 2) through the substrate itself, and 3) from the surface where the MMIC is installed through the connection (e.g. wirebond, flip chip) to the MMIC. 
     Referring to FIG. 1, a prior art MMIC package  10  is illustrated where a mother PC board  12  has a waveguide  14  bonded to its top surface, and a substrate  16  has waveguides  18  and  20  bonded to its underside and topside respectively. The waveguide  14  includes signal conductor  30  and ground conductors  32  and  34 ; waveguide  18  includes signal conductor  36  and ground conductors  38  and  40 ; and waveguide  20  includes signal conductor  42  and ground conductors  44  and  46 . Transition area  1 , i.e., I/O ports of package  10 , comprises thin and wide conductive stubs  22 , transition area  2 , i.e., through substrate  16 , comprises narrow and wide slots (vias)  24  in the package substrate filled with an electrically conductive material, and transition area  3  comprises wire bonds  26  connected to the MMIC  28 . FIG. 1 illustrates where the dielectric constants of the substrate  16  and the mother board  12  are the same and the two transitions  1  and  2  of the waveguide structures are very close to the ideal. One of the factors in the representation of FIG. 1 that makes the transitions close to ideal are the conductively filled vias  24  and stubs  22  that match the width of the conductors with which they make contact ( 30 ,  32 ,  34  on the mother board;  36 ,  38 ,  40  on the underside of the package; and  42 ,  44  and  46  on the top surface of the substrate  16 ). The lack of variation in dielectric constant in this Figure enables the use of straight coplanar waveguide structures. 
     In order to provide the transmission path through the substrate  16  itself, the transition area  2 , the vias  24  are preferably drilled in the ceramic substrate with a carbon dioxide or YAG laser. The slots are then filled with the conductive composite material in a powder form, whereby the powder is melted in ovens and then cooled to provide conductively filled through structures, i.e., vias, for the microwave signals. However the ceramic substrate is brittle and can often be cracked by the thermal expansion and contraction of the composite material. This is especially problematic when there are a large number of vias for a complex circuit or the substrate is relatively thin. 
     Problematically, package substrate thickness is defined by a tradeoff between performance, mechanical strength and cost. The thicker the substrate the greater the mechanical strength and the less breakage that occurs during the manufacturing process. Yet it is generally required to minimize via length, by reducing the thickness of the substrate, in order to enhance high frequency performance and to facilitate impedance matching of the transition area. However, the brittle ceramic substrate can contain a high density of vias, which makes the substrate very fragile, difficult to handle and expensive. Therefore, the chances of losses due to thin substrate breakage are increased when the substrate is made thinner. Thus, the requirement for a thinner substrate may result in losses due to substrate breakage. 
     The deficiencies and limitations of the above package are eliminated or greatly alleviated by the present invention. 
     SUMMARY OF THE INVENTION 
     This invention offers advantages and alternatives over the prior art by providing a circuit package for a microwave signal having a reduced number of vias in the substrate. The substrate of the package is devoid of signal carrying vias. Advantageously, the invention eliminates a transition area for the signal through the substrate of the package. Moreover, because signal carrying vias are avoided the substrate can be made substantially thick so that manufacturing yield losses due to substrate breakage can be virtually eliminated. Additionally, high frequency performance is enhanced and impedance matching is facilitated. 
     These and other advantages are accomplished in an exemplary embodiment of the invention by providing a circuit package for a microwave signal. The circuit package comprises a substrate defining a MMIC surface of the substrate and an opposing non-MMIC surface of the substrate. The substrate is devoid of signal carrying vias. An arrangement of waveguides are disposed on the MMIC surface of the substrate. A MMIC is disposed on the MMIC surface of the substrate, and the MMIC is in electrical communication with the waveguide(s). An I/O port is in electrical communication with the waveguide wherein a transmission path for the signal is provided from the I/O port, through the waveguide and to the MMIC. 
     In a preferred embodiment of the invention, the MMIC surface of the substrate faces the PC board when the I/O port is electrically connected to the PC board. 
     In another exemplary embodiment, the circuit package includes at least one ground carrying via providing a conductive connection between a pair of ground planes for the signal. The pair of ground planes are disposed on the MMIC surface and the opposing non-MMIC surface of the substrate. 
     In yet another preferred embodiment of the invention, signal vias continue to be avoided but thermal vias are provided extending from the MMIC surface of the substrate to the non-MMIC surface of the substrate. The thermal vias are arranged to be in thermally conductive contact with the MMIC on one end and a heat sink attached to the non-MMIC surface of the substrate on the opposite end. Since the MMIC surface of the substrate faces the PC board the heat sink is located in a more unrestricted environment for air passage and easily dissipates heat to the environment. 
     The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
     FIG. 1 is a schematic perspective view of a prior art MMIC package; 
     FIG. 2 is an exploded schematic view of a MMIC package in accordance with the present invention; 
     FIG. 3 is an end view of a coplanar waveguide structure in accordance with the invention illustrating field lines; 
     FIG. 4 is a perspective view of the structure of FIG. 3; 
     FIG. 5 is a schematic perspective view of the concept of the invention; 
     FIG. 6 is perspective view of an exemplary embodiment of the invention showing conductive stubs in the I/O ports; 
     FIG. 7 is a cross section view of FIG. 6 taken along section line  7 — 7 ; 
     FIG. 8 is a perspective view of an exemplary embodiment of the invention showing conductive balls in the I/O ports; 
     FIG. 9 is a cross section view of FIG. 8 taken along section line  9 — 9 ; 
     FIG. 10 is a perspective view of an exemplary embodiment of the invention showing a recess in the substrate; 
     FIG. 11 is a cross section view of FIG. 10 taken along section line  11 — 11 ; 
     FIG. 12 is a perspective view of an exemplary embodiment of the invention showing a recess in the substrate and conductive balls in the I/O ports; 
     FIG. 13 is a cross section view of FIG. 12 taken along section line  13 — 13 ; 
     FIG. 14 is a perspective view of an exemplary embodiment of the invention showing a heat sink mounted to the substrate; 
     FIG. 15 is a cross section view of FIG. 14 taken along section line  15 — 15 ; 
     FIG. 16 is a perspective view of an exemplary embodiment of the invention showing a microstrip waveguide structure having vias conductively connecting the ground planes; 
     FIG. 17 is a perspective view of an exemplary embodiment of the invention showing a microstrip waveguide structure having a thin film metalization wraparound conductively connecting the ground planes; 
     FIG. 18 is a cross section view of FIG. 16 taken along section line  18 — 18 ; and 
     FIG. 19 is a cross section view of another embodiment of the invention illustrating thermal vias and a heat sink attached to the substrate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed toward connecting a MMIC to a mother PC board in a way that eliminates signal carrying vias through the substrate of the MMIC package. 
     MMIC packages of the invention are connected to PC boards in a way that is electrically transparent to microwave signals transmitted therebetween. Achieving electrical transparency requires matching the impedance of the transmission path through all transition areas from the mother board to the MMIC. The transition areas for a MMIC package in accordance with the invention are: 1) from the mother board through the input/output (I/O) connection between the mother board and the package substrate, and 2) from the surface where the MMIC is installed through the connection, e.g., wirebond, to the MMIC. 
     Referring to FIG. 2, the transition areas of an exemplary MMIC package  50  in accordance with the present invention are illustrated schematically. Therein  52  indicates the mother PC board,  54  indicates the first transition area,  56  indicates the package substrate,  58  indicates the second transition area and  60  indicates the MMIC. It is important to note that the transition area through the substrate  56  itself is eliminated. This is because the position of the substrate  56  and MMIC  60  relative to the mother board  52  have been reversed, eliminating the requirement for signal carrying vias to provide a transmission path for the signal through the substrate  56 . That is, the surface on which the MMIC is bonded (the MMIC surface)  62  faces the PC board  52 , and the opposing non-MMIC surface  64  faces away from the PC board  52 . 
     Referring to FIGS. 3 and 4, a flat coplanar waveguide is illustrated as it transmits microwave energy to the MMIC  60  (not shown). Waveguides are used to provide conductive transmission paths for microwaves from the mother board to the MMIC and back. 
     As best shown in FIG. 3, the electric field E and magnetic field H of the microwave energy are concentrated between the spaces  66  and  68 . Spaces  66  and  68  have a width (w) and a thickness (t). Changes in the width (w) and thickness (t) of spaces  66  and  68  affects the frequency of the microwave energy that can be transmitted and the value of the impedance to which the energy is subjected. 
     As best seen in FIG. 4, the center trace  70  is the signal conductor while the metalization  72 ,  74  on either side thereof is the ground plane. The width of center trace  70  is adjusted according to a dielectric constant of the substrate and at least one transition area that provides for communication between the various componentry of the MMIC package. The signal conductor  70  is spaced (width w) from the ground plane conductors  72  and  74  to control the inductance and capacitance of the circuit. Controlling those parameters by controlling the spacing (w) between the conductors is one of the factors to consider for controlling the impedance of the circuit in order to provide matched impedance pathways from the MMIC to the mother PC board and back. 
     RF currents in the respective conductors (signal and ground) flow in narrow regions adjacent the spacings  66 ,  68 . The regions are identified by the numerals  76 ,  78  for the signal conductor  70  and by  80  and  82  for the respective ground plane conductors  72  and  74 . Within spacings  66 ,  68  a spatial electric field configuration is shown by arrows  84  and  86 . Spacings  66 ,  68  are essentially gaps in the metalization which expose the dielectric material, e.g., the surface of a mother board or the surface of a substrate. This causes capacitance and inductance and modifies the electric field created between the conductors  70 ,  72  and  70 ,  74  as it passes therethrough. 
     Referring to FIG. 5, an exemplary embodiment of the present invention is illustrated wherein the concept of the flat coplanar waveguide is applied to a multilevel construction. Multilevel constructions are typical for MMIC packages because of the need for PC board  52 , substrate  56 , and the MMIC  60 . In industry, these components are built by different manufacturers or units. The MMIC, therefore, is not simply connected directly to the mother PC board. There are a number of transitions and different materials properties to contend with. These in addition to cross sectional dimensions become critical and complex in order to maintain impedance matching. 
     FIG. 5 broadly and schematically illustrates the exemplary embodiment of the present invention without any change in dielectric constant among the materials. Notice that this is a minimal two transition configuration. This is due to the elimination of vias through the substrate itself since the requirement to provide a transmission path through the substrate is eliminated. Transition path  1  is shown as an idealized, zero thickness layer of electrically conductive materials. This is particularly difficult to realize in practice, since transition  1  is between the mother PC board and the package. The conductor structure of transition is intended to bridge the gap (occupied by air) between the mother PC board and the package substrate and, therefore, requires considerable strength. 
     Referring to FIGS. 6 and 7, an exemplary embodiment of a MMIC package of the present invention is illustrated, where the dielectric constants of the of the substrate  56  and the mother board  52  are the same and transition  1 , i.e., the I/O ports, is very close to the ideal. It is important to note that one of the factors that makes transition  1  ideal are that the stubs  88  nearly match the width of the conductors with which they make contact ( 30 ,  32 ,  34  on the mother board and  90 ,  92 ,  94  on the substrate). Transition  2 , reference numeral  58 , is from the surface where the MMIC is installed (MMIC surface)  62  through the wire bonds  96  to the MMIC  60  itself. 
     Advantageously, by reversing the position of the substrate  56  and the MMIC  60  relative to the mother board  52  the requirement for vias to provide a transmission path through substrate  56  is eliminated. That is the MMIC surface  62  is oriented to face the PC board  52  and the non-MMIC surface  64  faces away. Therefore, a minimal number of two transition areas are required to complete the transmission path for the signal from the mother board  52  to the MMIC  60 . Moreover, it is an advantage of the package of the present invention over prior art packages which include signal carrying vias for a transmission path through the substrate itself, that the substrate thickness is not dictated by electrical performance design constraints. The substrate can be made substantially thick so that manufacturing yield losses due to substrate breakage can be virtually eliminated. Additionally, high frequency performance is enhanced and impedance matching is facilitated. Also, the potential introduction of contaminants and structural discontinuities within the substrate itself are virtually eliminated. 
     Referring to FIGS. 8 and 9, an alternate exemplary embodiment of the present invention shows stubs  88  substituted for by balls (or bumps)  98 . The balls  98  are a departure from the more ideal structure of FIG.  6 . They introduce a slight discontinuity in the electric (and magnetic) field spacial configuration, i.e., a slight impedance mismatch. The discontinuity is compensated for by extending the center conductor  90  as it emerges from the signal ball  98  by a small length of preferably less than ¼ of the wavelength of the highest frequency to be transmitted. This feature is labeled Δ1 in FIG.  8 . Note that this embodiment only contains two transition areas as signal transmission through the substrate  56  is not required. 
     Referring to FIGS. 10 and 11, another exemplary embodiment of the present invention is illustrated. In this case, the MMIC  60  is installed in a recess or cavity  100  as deep as the MMIC thickness plus the thickness of the die attached material, e.g., silver epoxy, used to bond the MMIC  60  to the substrate  56 . This substantially aligns the top substrate surface  102  with the MMIC surface  62 . Thus, the wire bonds  96  that connect the MMIC surface  62  with the substrate  56  are substantially shorter than those of the embodiments without the recess, thus reducing their inductance. 
     The substrate  56  is made of a fully sintered high aluminum material, i.e., greater than 96%. The recess in the substrate is typically of a depth of 0.004″ to 0.006″ (MMIC thickness plus the thickness of the die attached material). The coplanar waveguide structures are created by metallizing the substrate by means of vapor deposited or sputtered thick films of metals, such as Titanium and Nickel or suitable combinations of metals, such that there will be good adhesion to the substrate and good electrical conductivity. The circuitry patterns are created by a well known photoresist-etch process or by physically masking metal vapor deposition. The circuitry patterns are preferably coated with a thin layer of gold to provide high electrical conductivity for the microwave signals, which is conducive to low power losses. The structure can also be created by screen printing and firing thick film conductive pastes or inks. Alternatively, the substrate  14  can be a premolded piece of a low loss tangent (less than 0.0004) plastic material, such as a polytetrafluoroethylene (PTFE) composite or epoxy. 
     Referring to FIGS. 12 and 13, another exemplary embodiment of the present invention shows the transition of the coplanar waveguide from the substrate  56  to the mother board  52  as being made by conductive balls (or bumps)  98  attached to the mother board  52  by solder or a suitable electrically conductive material. The balls  98  themselves are attached to the waveguide structure of the substrate  56  by means of soldering or brazing using a material that has a melting point higher than that of the solder used to connect the balls  98  to the mother board  52 . The mother board  52  itself is made of alumina, Teflon composites, or other insulating material with a lost tangent in the vicinity of 0.0004 within the frequency range of the MMIC package. 
     Referring to FIGS. 14 and 15, another exemplary embodiment of the present invention is illustrated. The MMIC package body is built by bonding a piece of high thermal conductivity material  104  that functions as a heat sink, to a ceramic substrate  56 , which has an opening  106  generally at the center. The MMIC  60  is attached to the heat sink  104  within the ceramic substrate opening  106 . The heat sink-ceramic substrate assembly is metalized and patterned by thin film photo resist etch methods or by thick film screen printing of electrically conductive inks. Electrically conductive bumps  98  are attached to the metalized ceramic substrate for package attachment to a mother board (not shown). The entire circuitry of the package, including the bumps  98 , is coated with gold for high electrical conductivity, wire bondability and protection against the environment. 
     The opening in the ceramic substrate  106  can be produced by laser cutting, using a carbon dioxide or YAG laser, or by punching the ceramic in the green state, firing and lapping the substrate to achieve the required flatness. 
     The heat sink  106  is made of a material with high thermal conductivity and a coefficient of thermal expansion equal to or slightly larger than the MMIC (5.7 ppm/deg K for Gallium Arsenide) such as Tungsten-Copper, Copper-Molybdenum-Copper, or other adequate material. The heat sink is bonded to the ceramic substrate by direct copper bonding, anodic bonding, brazing, or other adequate methods. 
     A lid  108 , made of ceramic or plastic, is attached to the area indicated to protect the MMIC from the environment after attachment and wirebonding. 
     When properly designed according to the invention the package is capable of dissipating heat generated by typical power MMIC&#39;s from 1.0 to about 5.0 watts. The heat sink is connected to the package ground by means of wirebonds through cutouts in the ceramic substrate as shown. 
     Referring to FIGS. 16 and 17, an exemplary embodiment of the present invention utilizing a microstrip configuration is shown. The microstrip configuration contains a ground plane  110  on the surface of the substrate  56  where the MMIC is installed, i.e., the MMIC surface  62 , and a ground plane  112  on the opposing non-MMIC surface  64  of the substrate  56 . The two ground planes  110  and  112  are connected by means of ground carrying vias  114  filled with a metallic conductive material as shown in FIG. 16 or by means of a metalization wraparound  116  as shown in FIG.  17 . 
     Referring to FIG. 18, the microstrip configuration of FIG. 16 is shown connected to the mother board  52 . It is important to note that no other vias other than the ground carrying vias  114  are required to connect the two ground planes  110  and  112  through substrate  56 . Therefore all signal carrying vias are eliminated, and only two signal transition areas from the mother board  52  to the substrate  56 , i.e., the I/O ports, and from the substrate  56  to the MMIC  60  are required for this configuration. 
     In another preferred embodiment of the invention, the MMIC package is further enhanced by the provision of thermal vias  120  connecting to two ground planes  110  and  112  so that heat generated by MMIC  60  is drawn away therefrom by thermal vias  120 . Preferably a heat sink  122  is heat conductively attached (brazing, soldering, etc.) to ground plane  112  directly in line with thermal vias  120  such that heat conducted away from MMIC  60  along thermal vias  120  is conducted into and spread within heat sink  122 . Heat is dissipated from heat sink  122  into the surrounding environment. By removing heat from the MMIC, higher performance is achieved. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.