Abstract:
Hermeticity of microcircuit packages is achieved in one embodiment by recognition that water can penetrate the ceramic structure and thus the ceramic structure is sealed and the edges of the ceramic package are metallically bonded to the electronic package. In one embodiment, a clear ceramic dielectric compound is sprayed on the ceramic and then the package is glazed.

Description:
TECHNICAL FIELD 
     This invention relates to ceramic microcircuit packages and more particularly to systems and methods for making and using low cost hermetic ceramic microcircuit packages. 
     BACKGROUND OF THE INVENTION 
     High frequency ceramic microcircuit packages were introduced in the 1980&#39;s, examples of which are shown in U.S. Pat. Nos. 4,942,076 and 5,089,881. Such packages allow fine pitch microcircuit packaging with multi-layer and metal filled vias. The multi-layer ceramic substrate with metal filled vias provided the “water tight” hermeticity of the base of the microcircuit. Hermeticity of the device mounted inside this package was achieved by soldering onto the package a metal wall around the device and then soldering a metal lid to the metal wall. Alternatively, a combination metal lid and sidewall could be soldered into place. Hermeticity in this context is defined as achieving a leak rate less than 1×10E-8 CC/Second of Helium. The soldered (or brazed) metal sidewall and lid solution is expensive to manufacture due to the fine metal detail needed to manufacture the covering package lid. Precision soldering the covering package lid to the circuit also adds to the cost. 
     One solution to the cost issue that has been used where mechanical protection is required for the device but hermeticity is not required is to epoxy a low cost ceramic package lid over the device using a dry nitrogen atmosphere. Ceramic packages have been found to allow Gallium Arsenide (GaAs) ICs circuits to fail because water vapor penetrates the package lid (housing) around the circuit. This water vapor has been thought to pass through the epoxy seal but not through the walls of the housing itself. 
     BRIEF SUMMARY OF THE INVENTION 
     Hermeticity of microcircuit packages is achieved in one embodiment by recognition that water can penetrate the ceramic structure as well as the epoxy seal. To prevent water vapor from entering a sealed package, the ceramic structure is sealed and the edges of the ceramic package are metallically bonded to the substrate around the device to be protected. In one embodiment, a clear dielectric compound is sprayed on the interior of the ceramic package and then the package is fired to glaze the dielectric compound into the material of the ceramic lid. Solder is added to the edges of the package to form the metallic seal. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  shows one embodiment of a low cost hermetically sealable package lid; 
         FIG. 1A  shows a cross-section of the package lid of  FIG. 1 ; 
         FIG. 1B  shows another embodiment of a low cost hermetically sealable package lid. 
         FIG. 2  shows one embodiment of an electronic circuit substrate to Be coverd by the package lid of  FIG. 1 ; and 
         FIG. 3  shows one embodiment of a printed circuit board having thereon. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows one embodiment  100  of a low cost hermetically sealable package lid. In this embodiment, package lid  100  is a ceramic housing, or cover, designed to be placed over a microcircuit, such as RF microcircuit  202 , shown in  FIG. 2 . 
       FIG. 1A  shows a cross-section view of package lid  100  taken through section  1 A of  FIG. 1  showing inside surfaces  102  and  104  which will be sealed. The height of sides  102  can be made to fit over circuit  202  and can be made any height as required. A metallic bonding material, such as, for example, solder bead  103 , is shown on edge  101  and this bead could be on the outside edge as shown, or on the inside edge, or could be flat along the full edge. 
     The inside surface, including surfaces  104  and  102  of package lid  100 , in one embodiment, is coated with a ceramic dielectric material which is then fired at a temperature of 1250° C. for one minute. Currently, the dielectric material being used is applied by Max Levy Autograph, Inc., 220 West Roberts Avenue, Philadelphia, Pa.; and is a material proprietary to Max Levy Autograph, Inc. Any glaze of any color could be used and if increased reflectivity is desired the glass could be clear or otherwise made reflective. 
     A gold binding material  105 , shown in  FIG. 1B , such as a ceramic gold mixture, is put on (by screening, sputtering, or otherwise) the bottom edges  101  and the package is again fired, this time to a temperature of 850° C. for one minute. The gold/tin solder  103  is put over the gold binding and the package lid is again fired, this time at 320° C. Note that gold/tin solder can be any material provided it will bond with the gold plated (or other) material of electronic circuit substrate  201  ( FIG. 2 ) in a manner to avoid water vapor from penetrating the seal. Since in our example electronic circuit  202  is assumed to be mounted on a gold plated platform, such as platform  201  ( FIG. 2 ), then solder bead  103  (shown best in  FIG. 1A ) could be gold/tin having a ratio of 80% gold to 20% tin. Alternatively, instead of applying the solder to the package lid, a separate solder preform could be used with a package lid to make the hermetic seal to the package substrate. 
     Edges  101  of package  100  are then brought into contact with substrate  201  between the broken line and the edge of substrate  201  as shown in  FIG. 2 . The metal (in this example, gold/tin) is bonded with the gold plating of the substrate at a temperature of 300° C. 
     One method of accomplishing such bonding is to bring the two pieces ( 100  and  200 ) into contact with one another in a carbon holding fixture and then applying electricity to the package. A weight can be used to force ceramic package lid  100  into better contact with substrate  201 . The electricity heats the package and solder  103  from the ceramic package bonds with the gold plating of substrate  201 . Note that the composition of the solder material on the lid controls the melting point of the seal and can be adjusted as necessary. Soldering, or other bonding, such as laser welding, brazing, welding, etc., of package  100  to substrate  201  should be performed in a dry nitrogen (or other drying) atmosphere. The assembled package should be tested for both gross and fine leakage. 
     Using this system, the clear seal inside the ceramic package allows internal LED light to be reflected onto the GaAs device, if desired, thereby increasing operating system performance. 
     The reason for glazing the inside surface of package lid  100  is to prevent moisture from wicking through the ceramic. Until recently it has been thought that the ceramic package lid itself was water tight. In actuality, a typical ceramic package lid is sufficiently grainy so as to allow water molecules to pass through. This realization has led to the glazing of inner surfaces of package lid  100 . If desired, the outer surface could be glazed instead, or both inner and outer surface could be glazed. 
       FIG. 2  shows one embodiment  200  of an electronic device, such as electronic device  202 , to be covered by package lid  100 . Solder preform  103 , as discussed above, could be on the outside edge of package lid  100 , as shown, or on the inside edge (not shown), or across the entire edge (not shown). The inner edge of package lid  100  mates with substrate  201  along dotted line  205  thereby covering (protecting) circuit  202  and its various elements  203  from mechanical damage as well as from moisture. Substrate  201  could have terminals  204  for connections to other system components. 
     Electronic device  200 , with package lid  100  soldered in place, then can be added to a printed circuit board or to another substrate, if desired, (such as substrate  301 ,  FIG. 3 ) using a solder compound requiring less heat, for example 230° C., to control the soldering process. The solder used for such a bonding could be, for example tin/lead. 
       FIG. 3  shows one embodiment  300  of a printed circuit board or a substrate, such as substrate  301 , having several electronic circuits, such as circuits  200 ,  302 , and components, such as components  303 , mounted thereon. Components  200  are shown uncovered, while component  302  is essentially component  200  with package lid  100  sealed thereto. Substrate  301  could have terminals  304 , or flat planar terminals (not shown) for connection to other system components. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, as new epoxy glues become available it is possible that they have increased moisture resistance. Some of these epoxies can then be used to attach the glazed package lid to the package substrate, achieving much better moisture resistance than previously possible. Such a package lid may not be able to achieve hermetically, as defined above, but may be entirely appropriate for electronic circuit protection in some instances. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.