Abstract:
The present embodiments and associated methods provide for an integrated EMI shield for effective shielding not only from emissions perpendicular to the integrated circuit (IC) chip carrier but also parallel (edgewise) to the carrier. In one embodiment, a method includes forming at least a portion of an internal ground layer along at least a portion of a chip carrier edge, applying an electrically conductive layer to at least a portion of the chip carrier edge, the conductive layer being applied over the exposed portion of the ground layer and in electrical contact with said ground layer, and forming at least one cavity within the top surface of the chip carrier, where the at least one cavity configured to hold one or more integrated circuit chips therein.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
   This application is a division of U.S. patent application Ser. No. 09/378,407, filed on Aug. 20, 1999, now U.S. Pat. No. 6,288,344, the specification of which is incorporated herein by reference. 

   TECHNICAL FIELD 
   The present description generally relates to hybrid circuit boards, and more particularly to edgewise electromagnetic interference (EMI) shielding utilizing an integrated peripheral EMI shield. 
   BACKGROUND 
   In many applications, sensitive electronic circuitry and components are susceptible to electromagnetic interference (EMI) emanating from other circuits and components. Electronics designers may incorporate EMI shields to isolate the sensitive circuits from the offending circuits. For example, in the design of implantable cardioverter defibrillators, highly sensitive circuitry is in very close proximity to the pulse generating case which is a major EMI source. Such shielding may be composed of electrically conductive metal, such as copper, and may be in electrical contact with electrical ground. 
   Integrated circuit (IC) chip carriers are specialized circuit panel structures that are frequently used to attach IC&#39;s to circuit boards. Chip carriers provide high density, complex interconnections between the IC and the circuit board. Separately attached peripheral or edge shielding is used to address cross-wise EMI emissions. Such shielding may be in the form of conductive tape or foil. The physical size of the tape is a limiting factor for miniaturizing systems and the extra labor involved in manufacturing does not provide for an efficient manufacturing process. It is not mechanically efficient or desirable to have a separate component that takes up valuable device space and volume. 
   SUMMARY 
   The present embodiments and associated methods provide for an integrated EMI shield for effective shielding not only from emissions perpendicular to the integrated circuit (IC) chip carrier but also parallel (edgewise) to the carrier. In one embodiment, an IC chip carrier comprises an insulating ceramic substrate and at least one internal electrically conductive layer being a circuit ground, portions of which extend to the edge of the substrate, the internal electrically conductive layer in electrical contact with an electrically conductive layer applied to a portion of the substrate edge. This provides a horizontal hybrid ground plane which can be used as an EMI shield that runs horizontally to the chip as well as a hybrid edge EMI shield perpendicular along a portion of the edge. This edge shield protects internal circuitry from exterior EMI emissions directed towards the chip carrier edge portion incorporating the shield. 
   In one embodiment, the method of applying the edge conductive layer comprises the steps of laying-up a plurality of ceramic green sheets, at least one green sheet having an applied electrically conductive first layer, applying heat and pressure to the lay-up to consolidate the green sheets and at least one conductive layer into a monolithic structure, exposing at least a portion of the conductive layer on the structure edge, application of metallized paste (such as that used to create the first conductive layer) to at least a portion of the structure edge and in electrical contact with the first conductive layer, applying heat and pressure to the structure a second time to metalize the paste and to create a consolidated chip carrier. 
   In another embodiment, metalized paste is applied to the entire chip carrier edge and in electrical contact with the first conductive layer. The chip carrier is treated to heat and pressure a second time to consolidate the unit. Thus, the chip carrier has a hybrid shield which provides EMI shielding of internal circuits from external EMI emissions directed towards any part of the edge of the chip carrier. 
   In another embodiment, the metalized paste is applied using a printing process, such as silk-screening. 
   In another embodiment, the metalized paste contains molybdenum. 
   In another embodiment, the metalized paste contains tungsten. 
   In another embodiment, the edge EMI shield is applied via a plating process not requiring a second heat and pressure treatment. 
   In other embodiments, where the chip carrier is made of a low-temperature calcined ceramic, the metalized paste may contain either Cu, Au, Ag, Al, Ni, Pb—Sn, among others, since the heat treatments may be conducted at a lower temperature. 
   In another embodiment, the edge shield is applied to a printed circuit board. 
   In another embodiment, the edge shield is applied to a printed wiring board. 
   This summary is a brief overview of some embodiments of an integrated EMI shield utilizing a hybrid edge and methods of use and is not intended to be exclusive or limiting and the scope of the invention is provided by the attached claims and their equivalents. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of one embodiment of a ceramic green sheet with through-holes. 
       FIG. 2  is a perspective view of one embodiment of a ceramic green sheet with a layer of electrically conductive paste. 
       FIG. 3  is a perspective view of one embodiment of a pre-fired stack of ceramic green sheets. 
       FIG. 4  is a top view of one embodiment of a post-fired pre-sawn stack of ceramic green sheets. 
       FIG. 5A  is a top view of one embodiment of a chip carrier. 
       FIG. 5B  is a side view of one embodiment of a chip carrier. 
       FIG. 5C  is a cross section view of one embodiment of a chip carrier. 
       FIG. 6A  is a top view of one embodiment of a chip carrier. 
       FIG. 6B  is a side view of one embodiment of a chip carrier with an integrated EMI edge shield. 
       FIG. 7  is a cross sectional view of one embodiment of a chip carrier with an integrated EMI edge shield. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings, which are not necessarily to scale, which form a part hereof, and in which is shown by way of illustrating specific embodiments in which the device may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the device, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural changes may be made without departing from the spirit and scope. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope is defined by the appended claims and their equivalents. In the drawings, like numerals describe substantially similar components throughout the several views. 
   The present embodiments and methods will be described in applications involving ceramic IC chip carriers. However, it is understood that the present apparatus and methods may be employed in other single or multi-layer wiring board applications. 
   The basic components of a chip carrier is a multi-layer integrated structure of electrically insulating ceramic sheets upon which a network of electrically conductive network or “paths” are placed. The pre-fired (green) ceramic sheet is produced by casting a thin layer of ceramic material onto a flexible sheet which is allowed to dry. The ceramic material is typically alumina powder in an organic binder, solvents, plastifiers, and resins. After drying, the ceramic film is stripped from its substrate and allowed to release the volatile components. The green ceramic film is then cut into desired dimensions. Sheets may be made large enough to produce a number of chip carriers after a final cutting or sawing procedure is performed on the post-fired unit. 
     FIG. 1  illustrates an embodiment of a ceramic green sheet  100 . Through-holes  110  are punched or cut in the green sheets  100  at predetermined locations which are subsequently filled with metalized paste to provide electric interconnections between the metallized patterns or networks which will be applied to the various green sheets  100 . Additional through-holes  112  may be punched or cut into the green sheet  100  to provide what would later become a cavity into which IC chips or other circuits are placed. 
     FIG. 2  illustrates a green sheet  200  with applied conductive metalized paste  220 . The conductive metallized paste  220  is deposited on the green sheet  200  by printing methods such as silk-screening through a mask. The conductive paste  220  materials consist of a mixture of metal and alumina particles and a solvent/binder. The metal particles are of a refractory and/or noble metal, such as molybdenum or tungsten. 
     FIG. 3  illustrates an embodiment of a multi-layer green sheet stack  314 . Several green sheets  300  with corresponding metalized conductive paths (not shown) are stacked together in such a way as to ensure that all conductive paths are established. The stacking of non-filled through-holes provides a cavity  318  for accepting IC or other circuits. The multi-layer stack  314  is laminated at sufficient pressure and temperature to cause the binder to evaporate and provide good intersheet bonding. A monolithic structure is thus obtained which is then sintered at the temperature required to fire the ceramic, thereby eliminating the organic components of the pastes and converting the conductive metalized paste to the metal state. Exposed though-holes filled with metalized paste provide electrical attachment pads  322  for attaching IC leads. 
     FIG. 4  illustrates a top view of an embodiment a post-fired pre-sawn sheet  414  of chip carriers  416 . Individual chip carriers  416  are sawed or cut from this monolithic structure  414 . 
     FIG. 5  illustrates various views of an embodiment of chip carrier  530 .  FIG. 5A  is a top view of one embodiment of a chip carrier. IC chip carriers  530  may incorporate a cavity  518  that holds integrated circuit chips or other circuits.  FIG. 5C  illustrates the chip carrier  530  cross-section showing the staircase-shaped steps  550  of cavity  518 . Other cavities may not have this structure as is illustrated by a second cavity  519 . As shown in  FIG. 5A , patterned electrical conductors terminating in bonding pads  522  lie on the surfaces of the ceramic layers and on the steps  550  of cavity  518 . The bonding pads  522  on the cavity steps  550  are connected to an integrated circuit (not shown) through either bonding pads or individual wire leads on the IC to carry signals to and from the chip. 
     FIG. 5B  shows an edge view of an embodiment of chip carrier  530  having portions of an electrically conductive layer, in this case a hybrid horizontal ground plane (HHGP)  524 , exposed on the edge of the chip carrier  530 . Portions of the HHGP  524  may be exposed as an engineered feature of the unit, or as a consequence of having sawed through portions of the HHGP  524  during the sawing process. 
   In one embodiment, metalized paste (such as that used to create the HHGP  524  itself) is applied to a portion of the edge  532  of the chip carrier  530  and in electrical contact with the HHGP  524 . The chip carrier  530  is then sintered a second time to metalize the paste forming the integrated hybrid edge EMI shield and to create a consolidated unit. 
     FIGS. 6A and 6B  shows, respectively, a top and edge view of an embodiment of chip carrier  630  incorporating an integrated hybrid edge EMI shield  626 . This embodiment provides an edge shield  626  that shields circuitry that is internally mounted in cavities  618  and  619  from external EMI emissions directed towards the portion of the chip carrier edge  632  incorporating the shield  626 . 
   A subsequent manufacturing step may be used where copper is introduced to the conductive paths through capillary action to fill any voids and provide a solid conduction path. The exposed interconnect pads  622  are then plated or coated with another metal layer, preferably a noble metal such as Pt, Pd, Au, or Ag, to provide an oxide-free surface to attach IC or other component leads. 
   Other subsequent manufacturing steps may be performed to create the final chip carrier. In some cases, a thick-film conductor, a thick-film resistor, glass and the like, are printed and baked repetitively on one or more surfaces of the multi-layered substrate. 
     FIG. 7  illustrates a cross section of an embodiment of a chip carrier  730  incorporating an integrated hybrid edge EMI shield  726 . The chip carrier  730  incorporates conduction paths  728  to allow electrical communication of the IC  740  to an external device, such as a circuit board (not shown). The IC  740  is in electrical communication with the conduction paths  728  through either wire bonding leads  742  or conduction pads on the IC  740  itself (not shown). The integrated hybrid edge EMI shield  726  is electrically connected with the integrated hybrid horizontal ground plane  724 , both of which may act as an EMI shield. This embodiment provides an EMI shield that shields circuitry that is internally mounted in cavities  718  and  719  from external EMI emissions directed towards the portion of the chip carrier edge incorporating the shield  726  and the chip carrier side incorporating the hybrid horizontal ground plane  724 . 
   Chip carriers made from alumina ceramic green sheets are baked at a temperature of a thousand and several hundred degrees with the application of pressure, thereby to obtain a multi-layered consolidated ceramic substrate. In order to withstand these high temperatures, the conductive paste material has to be composed of a refractory metal such as tungsten or molybdenum. In order to incorporate the integrated hybrid EMI edge shield, the edge shield must also be able to withstand these high temperatures while the chip carrier undergoes the second firing at similar temperatures. Therefore, the edge shield must also be composed of a refractory metal. 
   Alternatively, low calcined temperature ceramic substrate materials, such as glass or composite glass-ceramic, may be used instead of the alumina for the green sheets. This allows for the use of lower melting point metals such as Ag, Au, Ag—Pd, and Cu for the metalized layers. This would allow the EMI shield to be composed of these metals also. The preparation method is the same as that in the case of alumina. 
   It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.