Abstract:
A method and packaging device which provide a magnetically permeable shield for integrated circuits containing magnetic field sensitive circuit elements are disclosed. One or more magnetic shields are provided on, or embedded within, a printed circuit board and additional shielding elements are provided over the printed circuit board.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a shielded printed circuit board for mounting integrated circuits which are sensitive to external magnetic fields. 
     BACKGROUND OF THE INVENTION 
     Recently, very high-density magnetic memories, such as magnetic random access memories (MRAMs), have been proposed as storage devices for digital systems. Magnetic random access memories employ one or more ferromagnetic films as storage elements. A typical multilayer-film MRAM includes a plurality of bit or digit lines intersected by a plurality of word lines. At each intersection, a ferromagnetic memory element is interposed between the corresponding bit line and word line to form a storage cell. 
     When in use, an MRAM cell stores information as digital bits, the logic value of which depends on the states of magnetization of the thin magnetic multilayer films forming each memory cell. As such, the MRAM cell has two stable magnetic configurations, high resistance representing, for example, a logic state 0 and low resistance representing, for example, a logic state 1. The magnetization configurations of the MRAMs depend in turn on the magnetization vectors which are oriented as a result of electromagnetic fields applied to the memory cells. The electromagnetic fields used to read and write data are generated by associated CMOS circuitry. However, stray magnetic fields, which are generated external to the MRAM, may cause errors in memory cell operation when they have sufficient magnitude. 
     Very high-density MRAMs are particularly sensitive to stray magnetic fields mainly because the minuscule MRAM cells require relatively low magnetic fields which generate magnetic vectors used in read/write operations. These magnetic vectors are, in turn, easily affected and have a magnetic orientation which can be changed by such external stray magnetic fields. 
     To diminish the negative effects of the stray magnetic fields and to avoid lo sensitivity of MRAM devices to stray magnetic fields, the semiconductor industry could produce memory cells requiring higher switching electromagnetic fields than a stray field which the memory cells would typically encounter. However, the current requirements for operating such memory cells is greatly increased because higher internal fields necessitate more current. Thus, both the reliability and scalability of such high current devices decrease accordingly, and the use of MRAMs which may be affected by stray magnetic fields becomes undesirable. 
     Accordingly, there is a need for an improved magnetic memory shielding which reduces the effects of external magnetic fields on internal memory cell structures and operations, as well as a method for fabricating such magnetic shielding. There is further a need for minimizing the cost of shielding a magnetic random access memory IC chip from external magnetic fields. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus which provide a shielding structure for a printed circuit board supporting magnetic field sensitive integrated circuits, such as MRAMs, which shield such circuits from external magnetic fields. The invention employs one or more magnetic shields, preferably formed of non-conductive magnetic oxides, which are on, or embedded within, a printed circuit board and, optionally, an additional magnetic shielding structure which surrounds and covers integrated circuit chips mounted on the printed circuit board. 
    
    
     These and other features and advantages of the invention will be more clearly apparent from the following detailed description which is provided in connection with accompanying drawings and which illustrates exemplary embodiments of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a printed circuit board assembly with surface-mounted integrated circuits. 
     FIG. 2 is a cross-sectional view of the printed circuit board assembly of FIG. 1 at a subsequent stage of processing to that shown in FIG.  1  and in accordance with a first embodiment of the present invention. 
     FIG. 3 is a cross-sectional view of the printed circuit board assembly of FIG. 1 at a subsequent stage of processing to that shown in FIG.  1  and in accordance with a second embodiment of the present invention. 
     FIG. 4 is a cross-sectional view of the printed circuit board assembly of FIG. 1 at a subsequent stage of processing to that shown in FIG.  1  and in accordance with a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following detailed description, reference is made to various specific embodiments in which the invention may be practiced. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that structural and electrical changes may be made without departing from the spirit or scope of the present invention. 
     The present invention provides a printed circuit board assembly which shields magnetic memory structures, such as MRAMs, from external stray magnetic fields. The invention employs one or more magnetically permeable shielding layers, preferably formed of electrically non-conductive magnetic oxides, which are provided on, or embedded within, a printed circuit board. Magnetic shielding material may also surround integrated circuit chips which include such magnetic memory structures and which are supported by the printed circuit board. 
     Referring now to the drawings, where like elements are designated by like reference numerals, FIGS. 1-4 illustrate exemplary embodiments of the present invention. FIG. 1 depicts a printed circuit board assembly  10  which includes an array of surface-mounted magnetically sensitive integrated circuits  12  which, in turn, include internal electromagnetic structures, such as MRAM cells and access circuitry. The integrated circuits  12  also include conductive leads  14  that are attached to a plurality of electrically conductive patterns  15 . The electrically conductive patterns  15  are laid out on a top surface  18  (or on internal layers) of a printed circuit board  20 . 
     Referring now to FIG. 2, which for simplicity illustrates only a cross-sectional view of the printed circuit board assembly  10  of FIG. 1, a magnetic shield is provided for shielding the integrated circuits  12  from external magnetic field disturbances. According to an exemplary embodiment of the present invention, a magnetic shielding layer  22  is formed on the bottom surface of the printed circuit board  20 , that is the surface that is opposite to the top surface  18  that directly supports the integrated circuits  12 . 
     The magnetic shielding layer  22  is formed of a magnetically permeable shielding material which comprises, for example, an electrically non-conductive material with permeability higher than that of air or silicon. As such, the preferred choice for the magnetic shielding material is a non-conductive magnetic oxide, for example, a ferrite such as MFe 2 O 4 , wherein M=Mn, Fe, Co, Ni, Cu, or Mg, among others. Manganites, chromites and cobaltites may be used also, depending on the device characteristics and specific processing requirements. Further, the magnetic shielding layer  22  may also comprise conductive magnetic particles, for example nickel, iron or cobalt particles, which are incorporated into a non-conductive base material, such as a glass sealing alloy or polyimide. Also, the magnetic shielding layer  22  may also comprise conductive magnetic materials, such as nickel, iron, cobalt, Permalloy, or Mumetals, among others. 
     The printed circuit board  20  with the integrated circuits  12  may also be surrounded with a magnetic shielding packaging material  33  (FIG.  2 ), which may be similar to, or different than, the magnetic shielding material for the magnetic shielding layer  22 . In any event, the magnetic shielding packaging material  33  may be formed of a non-conductive magnetic oxide, for example a ferrite such as MFe 2 O 4 , wherein M=Mn, Fe, Co, Ni, Cu, or Mg, among others. However, manganites, chromites and cobaltites may be used also, depending on the device characteristics and processing requirements. Further, conductive Mumetal alloys comprising approximately 77% nickel (Ni), 4.8% copper (Cu), 1.5% chromium (Cr) and 14.9% iron (Fe) may be used also, as well as conductive magnetic particles, such as nickel, iron or cobalt particles, incorporated into a base material, for example a glass sealing alloy, a polyimide, or a polymer. 
     In addition, the magnetic shielding packaging material  33  may also comprise a mold compound, such as a plastic compound, with conductive magnetic particles therein. For example, conductive magnetic particles of, for example, nickel, iron, and/or cobalt, may be suspended in a matrix material, such as a plastic compound, at a concentration that does not allow the particles to touch each others. Alternatively, the packaging material  33  may comprise a mold compound, such as a plastic compound, including non-conductive particles of, for example, non-conductive magnetic oxides and/or Mumetal alloys. 
     The magnetic shielding packaging material  33  surrounds the integrated circuits  12  of the printed circuit board  20 , so that the integrated circuitry including magnetic memory structures, such as MRAMs, are shielded by the magnetic shielding layer  22  and surrounded in the magnetic shielding packaging material  33  for maximum protection from external stray magnetic fields. 
     Although FIG. 2 shows the magnetic shielding material in the form of the layer  22  on the back side of the printed circuit board  20 , it is also possible to apply a continuous or patterned layer of shielding material on the top surface  18  of the printed circuit board  20 , provided it is arranged so that it does not interfere with the electrical connections on the printed circuit board  20 . Thus, as shown in FIG. 3, a top magnetic shielding layer  44  can be formed and patterned on the top surface  18  of the printed circuit board  20  so that portions of magnetic shielding layer  44  are formed beneath the integrated circuits  12 . 
     FIG. 4 illustrates yet another exemplary embodiment of the present invention, in which a flat magnetic shielding layer  55  is embedded within the printed circuit board  20 , which can be otherwise formed of a resin compound, for example, or other known printed circuit board material. The material for the flat magnetic shielding layer  55  may be similar to, or different than, that of the magnetic shielding layer  22  and the shielding packaging material  33 . In any event, the preferred material for the flat magnetic shielding layer  55  is a non-conductive magnetic oxide, for example a ferrite such as MFe 2 O 4 , wherein M=Mn, Fe, Co, Ni, Cu, or Mg, among others. 
     Although the exemplary embodiments described above refer to specific magnetic shielding materials, it must be understood that the invention is not limited to the materials described above, and other magnetic shielding materials, such as ferromagnetics like nickel-iron (Permalloy), cobalt-nickel-iron, nickel or iron may be used also. 
     Further, although the exemplary embodiments described above refer to specific locations where the shielding material is applied to a printed circuit board, it is also possible to selectively apply the shielding material at desired locations, for example, corresponding to below or above the locations of the integrated circuits  12 . In addition, the shielding material shown in the various embodiments may be used alone or in combination. For example, two or more layers of material could be employed for shielding the magnetic memories structures, one on each surface of the printed circuit board  20 , or multiple layers of the same or different shielding material which overlap each other may be used on one or both surfaces of the printed circuit board  20 , or on one surface and in between the surfaces of the printed circuit board  20 . Moreover, the specific shape of the shielding material is not limited to that shown in FIGS. 2-4 and other shapes, configurations, or geometries may be employed, particularly for the magnetic shielding packaging material  33 . 
     The present invention is thus not limited to the details of the illustrated embodiments and the above description and drawings are only to be considered illustrative of exemplary embodiments which achieve the features and advantages of the present invention. Modifications and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.