Abstract:
A method of manufacturing a substrate includes providing a substrate with a cavity and a post in the cavity, dispensing an elastic filling material in the cavity, inserting a magnetic core including a core hole such the post extends through the core hole, curing the elastic filling material, forming holes in the substrate outside of the cavity and in the post, and forming via-in-via structures in the holes.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to substrates with an embedded magnetic component. More specifically, the present invention relates to substrates with an embedded magnetic component surrounded by stress relieving material. 
         [0003]    2. Description of the Related Art 
         [0004]    It is known to provide a transformer by embedding a magnetic core  206 , also referred to as a ferrite core, in a printed circuit board  202  and by using conductors  208  and vias  210  to form windings around the magnetic core  206 , as shown, for example, in  FIG. 21  of this application which corresponds to a figure from U.S. Pat. No. 8,203,418. 
         [0005]    U.S. Pat. No. 8,203,418 provides an integrated planar transformer and electronic component that includes at least one wideband planar transformer disposed in a planar substrate. U.S. Pat. No. 8,203,418 uses cylindrical cavities  204  to hold the magnetic cores  206 . The magnetic cores  206  have an annular shape with a hole in the middle. The hole in the middle of the magnetic cores  206  is filled with an epoxy material with vias drilled through the epoxy material. The cavities  204  in U.S. Pat. No. 8,203,418 are cylindrical so that the center portion of the magnetic cores  206  is supported by the epoxy material and is exposed to lamination loads, which are the compressive loads applied to the planar substrate and the magnetic cores  206  when the different layers are bonded during a lamination process. Because the center portion of the magnetic cores  206  is epoxy, it is difficult to create vias in the center portion of the magnetic cores  206 . Mismatches in the coefficient of thermal expansion (CTE) of the materials used in the planar substrate and the center core will create thermal stresses that will result in the failure of the vias and/or the dielectric materials used therein. 
         [0006]    U.S. Pat. No. 7,271,697 provides miniature circuitry and inductor components in which multiple layers of printed circuitry are formed on each side of a planar substrate. U.S. Pat. No. 7,271,697 uses a pre-impregnated composite fiber material (prepreg material) to fill the space surrounding the magnetic core. The prepreg material is used in U.S. Pat. No. 7,271,697 because it facilitates the manufacturing process and because the coefficient of thermal expansion of the prepreg material is the same as the planar substrate because the prepreg and planar substrate are made from the same materials. However, the inventors of the present application have subsequently discovered that the prepreg material conforms to the space during lamination and imparts a certain amount of pressure on the magnetic core, which can negatively affect the magnetic permeability properties of the magnetic core due to magnetostriction. 
         [0007]    U.S. Patent Application Publication No. 2008/0816124 teaches a wireless inductive device and methods of manufacturing such an inductive device. The manufacturing methods in U.S. Patent Application Publication No. 2008/0816124 include forming conductors on two substrates (top and bottom) and joining the two substrates with a magnetic core between the two substrates to create an inductive device. U.S. Patent Publication No. 2008/0816124 does not use a cushioning material to prevent the compression of the magnetic core during the manufacturing process, which can lead to damage of the magnetic core and can also negatively affect the magnetic permeability properties of the magnetic core due to magnetostriction. 
         [0008]    U.S. Pat. No. 8,234,778 teaches substrate inductive devices and methods to make an inductive device comprised of three substrates: top, bottom, and middle. The top and bottom substrates contain conductors and the middle substrate contains a magnetic core(s) with electrical connectors. The three substrates are joined or assembled to create the device. This arrangement is difficult to manufacture in mass production and provides lower via hole density. 
       SUMMARY OF THE INVENTION 
       [0009]    To overcome the problems described above, preferred embodiments of the present invention provide a method of providing substrate with an embedded magnetic component that is surrounded by a stress relieving material that protects the magnetic component against mechanical stress and that eliminates magnetostriction effects on the magnetic component. 
         [0010]    A method of manufacturing a substrate according to a preferred embodiment of the present invention includes providing a substrate with a cavity and a post in the cavity, dispensing an elastic filling material in the cavity, inserting a magnetic core including a core hole such the post extends through the core hole, curing the elastic filling material, forming holes in the substrate outside of the cavity and in the post, and forming via-in-via structures in the holes. 
         [0011]    The method preferably further includes forming conductors connected to the via-in-via structures. The conductors and the via-in-via structures preferably provide primary and secondary windings of a transformer. 
         [0012]    A method of manufacturing a substrate according to a preferred embodiment of the present invention includes providing a substrate with a cavity and a post in the cavity, inserting a magnetic core including an elastic filing material coating and a core hole such that the post extends through the core hole, forming holes in the substrate outside of the cavity and in the post, and forming via-in-via structures in the holes. 
         [0013]    The method preferably further includes providing a prepreg ring or rings in the cavity before the step of inserting the magnetic core. The method preferably further includes forming conductors connected to the via-in-via structures. The conductors and the via-in-via structures preferably provide primary and secondary windings of a transformer. 
         [0014]    The step of forming via-in-via structures preferably includes forming a metal layer in the holes. The step of forming via-in-via structures preferably further includes forming an insulating coating over the metal layer in the holes. The step of forming via-in-via structures preferably further includes forming a metal layer over the insulating coating. 
         [0015]    The above and other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIGS. 1-12  show a method of manufacturing a substrate with an embedded magnetic component according to a first preferred embodiment of the present invention. 
           [0017]      FIG. 1  is perspective view of a substrate  10  with a cavity  11 . 
           [0018]      FIG. 2  is sectional side view showing a magnetic core  14  being inserted into a cavity  11 . 
           [0019]      FIG. 3  is perspective view of the substrate  10  with the magnetic core  14 . 
           [0020]      FIG. 4  is a sectional side view showing a copper foil  15  laminated to the substrate  10 . 
           [0021]      FIG. 5  is a sectional side view showing via holes  16  drilled in the substrate  10 . 
           [0022]      FIG. 6  is a sectional side view showing a copper plating  17  on the substrate  10  and inside via holes  16 . 
           [0023]      FIG. 7  is a perspective view showing conductors  18  on the substrate  10 . 
           [0024]      FIG. 8  is a side sectional view showing a parylene coating  19  on the substrate  10 . 
           [0025]      FIG. 9  is a side sectional view showing a predrilled adhesive  20   a  and copper layer  20   b  laminated on the substrate  10 . 
           [0026]      FIG. 10  is a side sectional view showing via hole openings  21  being formed in copper foil  20   b  on the substrate  10 . 
           [0027]      FIG. 11  is a side sectional view showing copper plating  22  formed on the substrate  10  and inside via holes  16 . 
           [0028]      FIG. 12  a perspective view showing conductors  23  formed on the substrate  10 . 
           [0029]      FIGS. 13-20  show a method of manufacturing a substrate with an embedded magnetic component according to a second preferred embodiment of the present invention. 
           [0030]      FIG. 13  is perspective view of a substrate  30  with a cavity  31 . 
           [0031]      FIG. 14  is sectional side view showing a magnetic core  34  being inserted into the cavity  31 . 
           [0032]      FIG. 15  is perspective view of the substrate  30  with the pre-coated magnetic core  34 . 
           [0033]      FIG. 16  is a sectional side view showing a prepreg layer  34   b  and a copper foil  35  on the substrate  30 . 
           [0034]      FIG. 17  is a sectional side view showing the prepreg layer  34   b  and the copper foil  35  laminated to the substrate  30 . 
           [0035]      FIG. 18  is a sectional side view showing via holes  36  drilled in the substrate  30 . 
           [0036]      FIG. 19  is a sectional side view showing a copper plating  37  on the substrate  30  and inside via holes  36 . 
           [0037]      FIG. 20  is a perspective view showing conductors  38  on the substrate  30 . 
           [0038]      FIG. 21  shows a known method of embedding a magnetic core in a printed circuit board. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0039]      FIGS. 1-12  show a method of manufacturing a substrate with an embedded magnetic component according to a first preferred embodiment of the present invention.  FIGS. 13-20  show a method of manufacturing a substrate with an embedded magnetic component according to a second preferred embodiment of the present invention. 
         [0040]    In contrast to U.S. Pat. No. 8,203,418, preferred embodiments of the present invention include a toroidal cavity including a support post of the planar substrate disposed in the middle of the toroidal cavity. The support post reduces the lamination loads on a magnetic core. In preferred embodiments of the present invention, the vias in the center of the magnetic core are formed in the support post of the planar substrate, and are not formed in an epoxy material as with U.S. Pat. No. 8,203,418. Because the vias are formed in the support post instead of an epoxy material in preferred embodiments of the present invention, the vias can be more reliably formed. Because the vias are disposed in the mostly homogeneous material of the planar substrate, thermal stresses are significantly reduced and prevented because there is no mismatch in coefficients of thermal expansion between the planar substrate and the epoxy material. The toroidal cavity significantly reduces and prevents any stresses from acting on the magnetic cores. The support post of the planar substrate will absorb the compressive loads imparted during laminating of the circuit layers so as to significantly reduce the compressive forces applied to the magnetic core. 
         [0041]    Preferred embodiments of the present invention solve the problems with the method of U.S. Pat. No. 7,271,697 by reducing the loads imposed on the magnetic core during the manufacturing steps of forming the transformer. Preferred embodiments of the present invention use a precise volume of elastic filling material dispensed in the space surrounding the magnetic core to prevent the compression of the magnetic material and to eliminate voids. Voids are known to result in delamination during the manufacturing process. The elastic filling material is preferably a silicon material with a low viscosity; however, other suitable materials, including other elastomeric materials that can withstand the conditions, such as the temperature, of the manufacturing process, can also be used. Typically, the volume of elastic filling material is determined before manufacturing begins. The volumes of the cavity and the magnetic cores can be determined using, for example, surface scans. Then, the volume of the elastic filling material can be determined by subtracting the volume of the magnetic core from the volume of the cavity, while considering the properties of the elastic filling material such as expansion or contraction during any curing process. The volume of the elastic filling material is preferably precisely determined because too much elastic filling material can cause cracking. The volume of the elastic filling material is equal to or substantially equal, within manufacturing tolerances, to the volume of the cavity minus the volume of the magnetic core. Manufacturing tolerances include the tolerances associated with the forming of the cavity and the magnetic core and with the specific properties of the type of elastic filling material used. 
         [0042]    By using a controlled volume of elastic filling material, preferred embodiments of the present invention ensure that the elastic filling material is located only in the space surrounding the magnetic core and does not migrate to the areas of the circuit where the vias will be formed. The volume of the elastic filling material to be dispensed is equal or substantially equal to the volume of the toroidal cavity minus the volume of the magnetic core. Using automatic dispensing equipment to apply the elastic filling material eliminates the possibility of over/under filling of the cavity containing the magnetic cores. This also prevents the elastic filling material from migrating to the via hole areas. 
         [0043]    The preferred embodiments of the present invention include methods of embedding a magnetic material in a planar substrate. More specifically, the preferred embodiments of the present invention include methods of embedding magnetic cores within a printed circuit board or a rigid flex circuit. The methods of the preferred embodiments of the present invention achieve a high-yield manufacturing process for creating miniature circuits with high functional reliability and with embedded magnetic cores for an inductor or a transformer. The embedding process and circuit configuration enable efficient and repeatable manufacturing of miniature circuits and miniature magnetic devices having high-voltage, high-current capabilities, as well as high tolerance to physical stress. The elastic filling material used to fill the toroidal cavity protects the magnetic core against mechanical stress and eliminates magnetostriction effects on the magnetic core. 
         [0044]    Some preferred embodiments of the present invention preferably include a via-in-via structure in which vias share the same via hole, which significantly reduces and prevents leakage inductance between the primary and secondary windings of a transformer and which can reduce the total number of via holes, which provides for further miniaturization. U.S. Pat. No. 8,203,418 B2 and/or U.S. Patent Publication No. 2008/0186124 A1 do not use a via-in-via structure, and thus cannot achieve the higher routing density made possible by the via-in-via structure. 
         [0045]    In the preferred embodiments of the present invention that use a via-in-via structure, multiple coaxial independent conductors are fabricated on the wall of the via hole drilled in the planar substrate near the magnetic core. The planar substrate is typically a printed circuit board or rigid flex circuit. The printed circuit board can be made of FR-4 epoxy laminate sheets or any other suitable material. Any suitable materials can be used including polyimide-based clad, copper-clad polyimide, epoxy, acrylic adhesives, copper-clad epoxy laminates, for example. 
         [0046]      FIGS. 1-12  show a method of manufacturing a substrate with an embedded magnetic component according to a first preferred embodiment of the present invention. 
         [0047]    To manufacture a substrate with an embedded magnetic component according to a first preferred embodiment of the present invention, a substrate  10  is provided. Substrate  10  preferably has a planar shape. Substrate  10  is typically a printed circuit board, e.g., an FR-4 epoxy-laminated sheet(s). As shown in  FIG. 1 , a cavity  11  is formed in the substrate  10  using a numerically controlled (NC) controlled-depth routing machine. The cavity  11  preferably has a toroidal shape with a post  12  in the center of the cavity  11 . It is possible to use any other suitable method of creating the cavity, including embossing and molding, for example. The possible methods that can be used depend on the type of substrate used. Instead of having a circular perimeter, the perimeter of the cavity  11  can have any suitable shape, including an oval or a square shape, for example. 
         [0048]    Next, as shown in  FIG. 2 , an elastic filling material  13  is dispensed into the cavity  11  using controlled-volume automatic-dispensing equipment. The elastic filling material  13  is preferably a low-viscosity silicon material. Any other elastomeric materials that can withstand the conditions, such as the temperature, of the manufacturing process, can also be used. 
         [0049]    A magnetic core  14  is inserted into the cavity  11 . A pick-and-place equipment is preferably used; however, the magnetic core  14  can be inserted into the cavity  11  using any suitable method, including manual insertion. The magnetic core  14  is typically a ferrite; however, other suitable magnetic permeable materials could also be used, such as powdered-iron core, for example. The choice of materials for the magnetic core  14  affects what type of materials can be used for the elastic filling material  13 . 
         [0050]    Additional elastic filling material  13  is dispensed on top of the magnetic core  14 . The additional elastic filling material  13  is dispensed using the same controlled-volume automatic-dispensing equipment used to dispense the original elastic filling material  13  in the empty cavity  11 . However, different equipment can be used to dispense the additional elastic filling material  13 . 
         [0051]    All of the elastic filling material  13  is then thermally cured. The conditions, including time and temperature, for thermally curing the elastic filling material depend on the material used for the elastic filling material. The curing results in the substrate  10  with the magnetic core  14  inserted into the cavity  11  as shown in  FIG. 3 . 
         [0052]    Copper foils  15  are laminated on the top and bottom surface of the substrate  10  as shown in  FIG. 4 . The copper foils  15  are preferably laminated using a vacuum lamination process; however other suitable processes could also be used. Although laminating copper is preferred, it is possible to use other suitable conductive materials and to use other suitable methods of providing the conductive materials. For example, instead of using copper, it is possible to use other conductive materials such as silver or aluminum, and instead of laminating copper, it is possible print conductive inks. 
         [0053]      FIG. 5  shows via holes  16  drilled into the substrate  10  around the magnetic core  14  and in the post  12 . The via holes  16  are preferably drilled using an NC drilling machine; however, the via holes  16  can be formed using any suitable method or machine. 
         [0054]      FIG. 6  shows plating the top and bottom surfaces of the substrate  10  and the via holes  16  with copper plating  17 . 
         [0055]    In  FIG. 7 , conductors  18  are formed on the top and bottom surfaces of the substrate  10 . The conductors  18  are preferably printed and etched using standard PCB processes. The conductors  18  can be used, for example, as the windings of a transformer. 
         [0056]    Next, as shown in  FIG. 8 , a parylene coating  19  is applied on the top and bottom surfaces of the substrate  10  and inside the via holes  16  to form an insulator so that a via-in-via structure can be formed. Epoxies, polymers, liquid polyamide, or any other insulating materials can be used instead of parylene. 
         [0057]    As shown in  FIG. 9 , a predrilled adhesive and copper layer  20  is laminated on top and bottom surfaces of the substrate  10 . The predrilled adhesive and copper layer  20  is preferably laminated using vacuum lamination; however, other suitable processes could be used. 
         [0058]    Via hole openings  21  are formed on the top and bottom surfaces of the substrate  10  as shown in  FIG. 10 . The via hole openings  21  are preferably printed and etched using standard PCB processes. 
         [0059]    The, as shown in  FIG. 11 , copper plating  22  is plated on the top and bottom surfaces of the substrate  10  and inside via holes  16  to form the via-in-via structure. 
         [0060]    Finally, conductors  23  are formed in  FIG. 12  on the top and bottom surfaces of the substrate  10 . The conductors  23  are preferably printed and etched using standard PCB processes. The conductors  23  can be the secondary windings of a transformer with a turns ratio of 5:1. 
         [0061]      FIGS. 13-20  show a method of manufacturing a substrate with an embedded magnetic component according to a second preferred embodiment of the present invention. One of the differences between the first and second preferred embodiments of the present invention is that magnetic core  34  is pre-coated with an elastic material and pre-impregnated (prepreg) rings are used to fill the cavity  31 . The second preferred embodiment eliminates the elastic filling material  13  dispensing step and the curing step and prevents getting silicone on the surfaces of the substrate  30 , which increases yields. 
         [0062]    To manufacture a substrate with an embedded magnetic component according to a second preferred embodiment of the present invention, a substrate  30  is provided. As with substrate  10 , substrate  30  preferably has a planar shape. Substrate  30  is typically a printed circuit board, e.g. an FR-4 epoxy-laminated sheet(s). As shown in  FIG. 13 , a cavity  31  is formed in the substrate  30  using a NC controlled-depth routing machine. The cavity  31  preferably has a toroidal shape with a post  32  in the center of the cavity  31 . It is possible to use any other suitable method of creating the cavity, including embossing and molding, for example. The possible methods that can be used depend on the type of substrate used. Instead of having a circular perimeter, the perimeter of the cavity  31  can have any suitable shape, including an oval or a square shape, for example. 
         [0063]    Next, as shown in  FIGS. 14 and 15 , a magnetic core  34  with an elastic material coating  33  is provided. The elastic material coating  33  is preferably a low-viscosity silicon material. Any other elastomeric materials that can withstand the conditions, such as the temperature, of the manufacturing process, can also be used. Prepreg rings  34   a  are also provided. The prepreg rings  34   a  are preferably composite-fiber weave that is impregnated with a resin. The preferred materials are medium or high Tg epoxy prepregs. 
         [0064]    The prepreg ring or rings  34   a  are inserted into the cavity  31 , and then the magnetic core  34  is inserted into the cavity  31 . Pick-and-place equipment is preferably used to insert the prepreg ring or rings  34   a  into the cavity  32 ; however, the prepreg ring or rings  34   a  can be inserted into the cavity  31  in any suitable manner, including manual insertion. Pick-and-place equipment is preferably used to insert the magnetic core  34   a  into the cavity  31 ; however, the magnetic core  34  can be inserted into the cavity  31  using any suitable manner, including manual insertion. The magnetic core  34  is typically a ferrite; however, other suitable magnetic permeable materials could also be used, such as powdered-iron core. The choice of materials for the magnetic core  34  affects what type of materials can be used for the elastic material coating  33 . 
         [0065]    The combination of a prepreg layer  34   b  layered on top of a copper foil  35  is preferably laminated to the top of the substrate  30  as shown in  FIG. 16 . The prepreg ring  34   a  and prepreg layer  34   b  are preferably made of the same material. As shown in  FIG. 17 , the prepreg layer  34   b  and the copper foil  35  are preferably laminated to the top surface of the substrate  30  under a prescribed pressure and at prescribed temperature. Other suitable processes could also be used. Although laminating copper is preferred, it is possible to use other suitable conductive materials and to use other suitable methods of providing the conductive materials. For example, instead of using copper it is possible to use other conductive materials such as silver or aluminum, and instead of laminating copper, it is possible to print conductive inks. Epoxy adhesives can also be used to laminate the copper foil  35  to the substrate  30 . During the lamination process, the melted resin from the prepreg ring  34   a  and the prepreg layer  34   b  fill the voids in the cavity between the magnetic core  34  and the substrate  30 . 
         [0066]      FIG. 18  shows via holes  36  drilled into the substrate  30  around the magnetic core  34  and in the post  32 . The vial holes  36  are preferably drilled using an NC drilling machine; however, the via holes  36  can be formed using any suitable method or machine. 
         [0067]      FIG. 19  shows plating the top and bottom surfaces of the substrate  30  and the via holes  36  with copper plating  37 . 
         [0068]    In  FIG. 20 , conductors  38  are formed on the top and bottom surfaces of the substrate  30 . The conductors  38  are preferably printed and etched using standard PCB processes. The conductors  38  can be used, for example, as the windings of a transformer. 
         [0069]    The via-in-via structure can be formed in substrate  30  using the steps discussed above for  FIGS. 8-12 . 
         [0070]    It should be understood that the foregoing description is only illustrative of the present invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the present invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications, and variances that fall within the scope of the appended claims.