Patent Publication Number: US-11640873-B1

Title: Method of manufacturing a self-aligned planar magnetic structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 14/822,561, filed Aug. 10, 2015, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure describes apparatus and methods for constructing efficient planar transformers and inductors of the kind that include a magnetic core. 
     BACKGROUND 
     A planar inductive element may include a printed circuit board (PCB) with conductive windings positioned for magnetic coupling with other windings, a magnetic core, or both. 
     SUMMARY 
     One exemplary method of making a planar magnetic component includes providing a multilayer printed circuit board (PCB) including conductive features arranged on conductive layers of the PCB to form one or more windings around one or more predetermined axes. The method further includes forming a hole in the PCB at each of the one or more predetermined axes to accommodate one or more core legs. For each hole, an inner edge of one of the windings overlaps an edge of the hole in a lateral direction after the hole is formed. The method further includes assembling a magnetically permeable core including the one or more core legs, each core leg extending into one of the holes at the one or more predetermined axes. 
     Another exemplary method includes providing a multilayer printed circuit board (PCB) including conductive features arranged on conductive layers of the PCB to form one or more windings around one or more predetermined axes. The method further includes forming a hole in the PCB at each of the one or more predetermined axes to accommodate one or more core legs. For each hole, an inner edge of one of the windings overlaps an edge of the hole in a lateral direction after the hole is formed. The method further includes, for each hole, etching the PCB after forming the hole to create a setback between the inner circumferential edge of a respective one of the conductive features and the hole, the setback being created by etching from within the hole radially outward along the inner circumferential edge to remove a setback portion of the conductive feature. 
     According to another exemplary embodiment, a circuit is formed according to operations including providing a multilayer printed circuit board (PCB) including conductive features arranged on conductive layers of the PCB to form one or more windings around one or more predetermined axes. The operations further include forming a hole in the PCB at each of the one or more predetermined axes to accommodate one or more core legs. For each hole, an inner edge of one of the windings overlaps an edge of the hole in a lateral direction after the hole is formed. The operations further include assembling a magnetically permeable core comprising the one or more core legs, each core leg extending into one of the holes at the one or more predetermined axes. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows an exploded perspective view of an electronics assembly comprising a planar transformer according to an exemplary embodiment; 
         FIG.  2    shows a perspective view of the electronics assembly of  FIG.  1    after assembly according to an exemplary embodiment; 
         FIG.  3    shows an exploded perspective view of a portion of a multilayer PCB comprising etched windings according to an exemplary embodiment; 
         FIG.  4 A  shows an exploded perspective view of a portion of a multilayer PCB prior to the forming of apertures according to an exemplary embodiment; 
         FIG.  4 B  shows an exploded perspective view of the multilayer PCB of  FIG.  4 A  after the forming of apertures according to an exemplary embodiment; 
         FIG.  5    shows a multilayer PCB with apertures according to an exemplary embodiment; 
         FIG.  6    shows an exploded perspective view of an electronics assembly comprising a planar transformer comprising a magnetic core element with integral legs according to an exemplary embodiment; 
         FIG.  7    shows an exploded perspective view of an electronics assembly comprising a planar transformer comprising a plurality of separate magnetic core legs according to an exemplary embodiment; and 
         FIG.  8    shows a flow diagram of a process for forming a circuit (e.g., planar magnetic component) according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A power architecture using modular power converter components is described in Vinciarelli,  Factorized Power Architecture with Point of Load Sine Amplitude Converters , U.S. Pat. No. 6,984,965, issued Jan. 10, 2006, assigned to VLT, Inc. and incorporated herein by reference (the “FPA” patent). Efficient, high-frequency, high-density, and modular power converters are described in: Vinciarelli,  Factorized Power Architecture with Point of Load Sine Amplitude Converters , U.S. Pat. No. 6,930,893, issued Aug. 16, 2005 (hereinafter the “SAC I” patent); and Vinciarelli,  Point of Load Sine Amplitude Converters and Methods , U.S. Pat. No. 7,145,786, issued Dec. 5, 2006, (hereinafter the “SAC II” patent); each of which is assigned to VLT, Inc. and incorporated herein by reference (collectively the “SAC patents”). As described in the SAC patents, fixed ratio converters based upon the SAC technology fabricated using planar magnetics, e.g. for the power transformer(s), may achieve very high power densities. 
     Planar inductive elements including inductors and transformers (referred to herein, without loss of generality, as a “planar transformer”) may include a printed circuit board (“PCB”) including a plurality of conductive layers with intervening insulating layers arranged in a stack, i.e. a multilayer PCB. The PCB in a planar transformer may include one or more conductive windings (formed in one or more of the conductive layers) located to enable magnetic coupling with other windings (formed in other conductive layers in the PCB stack). One or more apertures may be provided in the PCB to accept a magnetic core that may enhance magnetic flux coupling between the various windings. The benefits of planar transformers relative to alternative means of construction, such as turns of wire arranged around a magnetic core structure, may include improved thermal performance, reduced leakage inductance, lower cost, and improved repeatability of operating characteristics. 
     An improved planar transformer configuration suitable for use in high-frequency power converters, such as those based on the SAC technology, is described in Vinciarelli,  Printed Circuit Transformer , U.S. Pat. No. 7,187,263, issued Mar. 6, 2007, assigned to VLT, Inc., (the “Planar Transformer” patent) incorporated herein by reference. It is sometimes beneficial to construct the individual transformer windings, e.g. a primary winding or a secondary winding, using a plurality of individual conductive turns that may be connected together, e.g. in parallel for lower voltage and higher current or series for higher voltage and lower current. For example, the Planar Transformer patent shows several examples of planar transformers having primary and secondary windings comprising multiple conductive turns (formed in several conductive layers) connected together in parallel and series to form desired number of turns in each the winding. For example,  FIG.  6    of the Planar Transformer patent shows 9 individual secondary windings,  54   a - 54   i , (some having a single turn and some having two turns) connected together to form a single center-tapped secondary winding shown schematically in  FIG.  7   .  FIGS.  9 - 21    of the Planar Transformer patent show another example in which a fourteen (14) conductive layer PCB stack includes four (4) individual turns formed in four (4) respective layers connected in series to form the four (4) turn primary winding schematically shown in  FIG.  23   ; and thirty two (32) individual turns formed in eight (8) respective layers connected in series and parallel to form the four (4) two turn center-tapped secondary windings shown schematically in  FIG.  22   . 
     An electronic assembly  10  including a planar transformer is illustrated in the exploded perspective view of  FIG.  1    and the assembled view of  FIG.  2    according to an exemplary embodiment. As shown, the assembly  10  typically includes a multilayer PCB  12  having apertures, e.g. holes  18 - 1  through  18 - 8 , for receiving a respective core leg, a magnetically permeable core plate  14 , and a magnetically permeable element  15  having a plurality of core legs, e.g. core legs  17 - 1  through  17 - 8 . The multilayer PCB  12 , which may include a plurality of conductive layers arranged in a stack with insulating material there between, may include windings formed in selected ones of the conductive layers. The core pieces  14  and  15 , may be assembled onto the PCB  12  as shown in  FIGS.  1  and  2    with the core legs  17 - 1  through  17 - 8  inserted through the respective apertures,  18 - 1  through  18 - 8 . The legs may be in direct contact with, or to set a gap spaced apart by a predetermined distance from, the matching core piece thereby completing the magnetic circuits. 
     In some implementations, a multiplicity of secondary windings may be coupled to a multiplicity of core legs connected in a series-parallel arrangement. It can be shown that in such converters, e.g. those based upon the SAC technology, the output resistance decreases in inverse proportion to the number of core legs and further that using a multiplicity of core legs whose cross-section varies in inverse proportion to the output voltage of the SAC and its resonant frequency may be beneficial. Magnetic flux density in a core leg is inversely proportional to the leg cross-sectional area and core losses in ferrite cores operating at frequencies in the megahertz range increases at a rate approaching the cube of increases in flux density. 
     Referring to  FIG.  3   , an exploded perspective view of a portion of one multilayer PCB  12  configured for use in a planar transformer is shown according to an exemplary embodiment. As shown, the PCB  12  is constructed by assembling a plurality of individual PCB laminates,  20 - 1 ,  20 - 2 ,  20 - 3 , each of which may typically include an insulation layer having one, or sandwiched between two, conductive metal layer(s) (not unlike simple single or double-sided PCBs). Two exemplary single-turn windings  22 - 1 ,  22 - 2  are formed on the top layer of laminate  20 - 2  as shown in  FIG.  1   . (As discussed above, each individual conductive layer may include one or more windings.) Each winding,  22 - 1 ,  22 - 2 , surrounds a respective leg aperture,  18 - 2 ,  18 - 4 , into which a magnetically permeable core leg (not shown) may be inserted. As shown in  FIG.  3   , the inner circumferential edge of each winding  22  is spaced apart, i.e. set back from, the edge of the respective aperture  18 . The resulting setback region is an area free of conductive material shown in the figure as bands around the apertures  18  having a width dimension labeled “S” in  FIG.  3   . As discussed below, the setback region serves several purposes including providing a minimum distance for insulation between the core legs  16  and the windings  22  and to protect against creep corrosion. 
     Conventional PCB fabrication techniques used to manufacture multilayer PCBs such as PCB  12  shown in  FIG.  3   , depending on the process and equipment, may introduce variations in (a) the features formed in each conductive layer, e.g. conductive traces or turns; (b) the relative position of each conductive layer with respect to the other conductive layers in the multilayer PCB; the (c) size and (d) relative position of mechanical milling and drilling operations, e.g. to form the edges of the finished PCB and apertures in PCB, e.g. for the core legs. The PCB manufacturing tolerances drive PCB layout and design rules requiring, for example, minimum spacing between conductive features in the various layers and the edges or apertures of the finished PCB. Additionally, manufacturing tolerances associated with production of the magnetic core elements introduce variations in (e) the diameter and (f) relative position of the legs. By way of example, the location of each core leg relative to other core legs may exhibit a 6 mil tolerance (“mil” herein refers to a milli-inch; 1 mil=0.001 inch); the location of each leg aperture relative to other core leg apertures in the PCB may exhibit a 3 mils tolerance; and the location of each PCB winding pattern relative to the corresponding core leg aperture may exhibit a 3 mil tolerance, which together add up to a worst case cumulative tolerance of 12 mils. 
     The cumulative tolerances may therefore require larger apertures in the PCB to accommodate variations in size and spacing of the legs and greater setback of the windings from the apertures to accommodate PCB manufacturing, both of which reduce the area (diameter) available for the core legs. For example, a one-volt output SAC operating at 1.5 MHz may ideally require a core leg having a diameter of 60 mils. If an aperture slightly larger than 60 mils were used in each individual PCB, accommodating the cited tolerances would require that the core leg diameter be reduced by approximately 40% (e.g. to less than 40 mils). Alternatively, the aperture diameter may be increased to accommodate the desired core leg (60 mils diameter), requiring larger inner-diameter windings, which would come at the expense of narrower width conductive turns, i.e. increased winding resistance, or increased leg-to-leg spacing, i.e. greater core size. In either case the overall efficiency of the planar transformer will be reduced relative to the efficiency that might be achieved if the mechanical tolerances were not a factor. 
     The present disclosure provides exemplary techniques for fabricating circuits (e.g., multilayer PCBs) that address the limitations of conventional PCB fabrication processes noted above. 
     I. Self-Aligned Windings 
     A portion of a multilayer PCB  28  that is adapted for use in an improved planar transformer is shown in exploded perspective view at different stages of processing in  FIGS.  4 A and  4 B , which respectively show the PCB before and after the leg apertures are formed. The multilayer PCB  28  as shown includes several individual laminates  30 - 1 ,  30 - 2 ,  30 - 3  and two exemplary winding features  26 - 1 ,  26 - 2  formed in the conductive layer of laminate  30 - 2 . (It should be understood that each conductive layer may include one or more of such winding features.) The winding features  26 - 1  and  26 - 2  have been formed having the desired outer circumferential shape and dimensions for the finished winding and include the desired gap between the two ends of the winding, the gap being shown as a slot extending from the center to the ends of the respective winding features in  FIG.  4 A . Notably, the winding features  26 - 1 ,  26 - 2  may be formed as shown without any setback from the leg apertures (the regions encircled by broken lines  32 - 1 ,  32 - 2  in  FIG.  4 A ), and in fact may extend into or completely cover those regions. After the PCB laminates, e.g.  30 - 1 ,  30 - 2 ,  30 - 3 , are assembled together, the leg apertures may be formed, e.g. drilled or milled, in the locations  32 - 1 ,  32 - 2  shown forming the inner circumference of the windings (in all of the layers). 
       FIG.  4 B  shows the PCB layers after the leg apertures have been formed. Although the layers are shown exploded in  FIGS.  4 A and  4 B , non-plated holes such as those used for core legs are generally formed as one of the last fabrication steps in the PCB manufacturing process, well after the plurality of laminate layers are bound together into the final multilayer stack. Eliminating the winding setback and using the leg aperture formation step to cut the inner circumference of the conductive windings produces windings having inner edges  42 - 2 ,  42 - 4  that are coaxial and coterminous with their respective apertures as shown in  FIG.  4 B . Windings formed in this manner may be called “self-aligned.” Forming the apertures and the windings inner-circumference in the same manufacturing step, e.g. by drilling or milling, eliminates some of the variations arising from limitations in the PCB manufacturing process, e.g. variations in the conductive features formed in each layer and in the relative position (e.g. lateral position) of each layer with respect to other layers. Additionally, some or all of the volume previously occupied by the eliminated winding setback may be used to increase the leg diameter and thus reduce flux density and as a result, core losses. A finished PCB having self-aligned windings and holes (leg apertures  40 - 1  through  40 - 8 ) is shown in  FIG.  5   . 
     Referring to  FIG.  6   , an example of an apparatus  50  including a planar transformer consisting of a self-aligned PCB  28 , i.e. having self-aligned windings ( FIGS.  4 A,  4 B,  5   ), and a magnetic circuit comprising a magnetic core plate  14  and core element  15  of the kinds shown in  FIGS.  1  and  2   . As described earlier, core element  15  includes integral core legs  17  that pass through apertures  18 . Assembly of the core element  15  and core plate  14  with the PCB  28  may be as described earlier. However, use of the self-aligned windings in the PCB  28  helps reduce the tolerances as noted above, allowing relatively larger core legs  17  or wider winding traces or both to be used to reduce transformer losses. 
     It should be noted that not all of the windings need be formed using the self-aligned process. In other words, some of the windings, e.g. higher voltage primary windings, may be formed traditionally with normal setback features and other windings in the transformer, e.g. low voltage secondary windings, may be formed using the self-aligned methods. 
     While the leg apertures and inner edges of the windings are shown in the Figures as being substantially cylindrical, it should be understood that, in various implementations, the leg apertures and/or inner edges of the windings may have any type of shape (e.g. ovular, rectangular, etc.). In some implementations, the shapes of the leg apertures and the inner edges of the windings may be complementary, such that the shapes are substantially similar and/or the winding setback is substantially the same around the edge/circumference of the leg aperture. In some implementations, the shapes may not be complementary, such that the shapes may be at least partially different and/or the winding setback is different adjacent to different areas of the leg aperture. All such implementations are contemplated within the scope of the present disclosure. 
     II. Self-Aligned Legs 
     While the apparatus  50  in  FIG.  6    benefits from the elimination of some manufacturing tolerances, other tolerances associated with variations in the relative locations of the core legs  17 - 1  through  17 - 8  with respect to each other must still be addressed in the PCB design. Referring to  FIG.  7   , an apparatus  55  includes an improved planar transformer which also uses a self-aligned PCB  28 , but has a magnetic circuit formed by eight individual core legs,  67 - 1  through  67 - 8  between two magnetically permeable core plates  64 - 1 ,  64 - 2 . 
     The apparatus  55  ( FIG.  7   ) may be assembled by (a) fastening one of the core plates (e.g. core plate  64 - 2 ) to a first surface of multilayer PCB  28  with the core plate overlaying the apertures  40 - 1  through  40 - 8 ; (b) inserting core legs into apertures  40 - 1  through  40 - 8 ; (c) fastening a second core plate (e.g. core plate  64 - 1 ) to the other surface of multilayer PCB  28  with the core plate overlaying the ends of the core legs  67 - 1  through  67 - 8 . One or the other of the core plates may be placed in contact with the legs or may be spaced away from the legs to form a gap in the magnetic circuit. Fastening may be by means of an epoxy adhesive or other adhesive or fastening technique. 
     Because each core leg  67 - 1  through  67 - 8  is independent of other core components, its placement into a respective leg aperture  40 - 1  through  40 - 8  in the PCB before being assembled to the core plates, eliminates the need to account for variations in relative leg position, allows for smaller leg apertures to be used, and results in better spacing and alignment between the windings and the core legs. The core structure may thus be said to have self-aligned legs. The apparatus  55  with self-aligned legs therefore improves upon the apparatus  50  ( FIG.  6   ) eliminating the tolerances required by variations in the relative positions of integral core legs, e.g. legs  17 - 1  through  17 - 8  ( FIG.  6   ), and allowing further reductions in planar transformer losses. Relative to a prior art SAC converter, the structure  55  of  FIG.  7    may allow reduction in core losses (e.g. of 20% or 30%) and a reduction in converter output resistance (e.g. of 5% or 10%). 
     It should be noted that while the combination of the self-aligned cores with self-aligned windings may be preferable in some embodiments, the self-aligned core legs may be used independently of the self-aligned windings in other embodiments. 
     III. Self-Aligned Insulation 
     After forming the apertures (e.g.  40 - 1  through  40 - 8 ) in a self-aligned PCB, the inner edges of the windings may be coterminous with the inner edge of the aperture (e.g. such that the inner edges of the windings substantially overlap the inner edge of the aperture and/or are positioned in substantially a same position in a lateral direction). The self-aligned windings being thus exposed in the leg apertures may be acceptable for some low voltage applications using core material having a sufficiently high resistivity. Alternatively, a modified PCB manufacturing process may be used to create a small setback or clearance between the winding and the aperture without adversely affecting the self-aligned position of the windings and apertures. The leg apertures may be formed after PCB laminates are assembled together in the PCB stack but before the outer conductive layers are etched. Doing so may allow the etchant bath access to the exposed inner circumference of the windings through the unmasked leg apertures. In the time it takes the etchant to remove unmasked material from the outer conductive layers having a 1 to 2 mils thickness, a 1 to 2 mils setback may be formed between the inner winding circumference and the leg apertures thus providing self-aligned setback between windings and the legs, in some embodiments. 
     The final stages of the manufacturing process for a multilayer PCB may include electrodeposition of copper onto unmasked regions of a copper foil or seed layer on outer surfaces of the laminated PCB assembly, followed by etching away of the undesired, previously masked, portion of the copper layer. If the leg apertures are formed after the electrodeposition step, the subsequent etching step may create the self-aligned setback (e.g. remove copper from the exposed inner circumferential edges of the self-aligned windings, e.g. edges  42 ,  FIG.  4 B ). Among the benefits of this approach is the uniformity of the setback dimension around the inner edge of the leg aperture. 
     Etching as a means of providing the self-aligned setback is a predictable and repeatable process that may produce uniform spacing between a self-aligned winding and the inner edge of the leg aperture. According to some embodiments, a PCB process may form the outer conductive layers using the following steps: (1) laminate copper foil (e.g. ½ oz. (0.65 mils thick)) onto the PCB stack; (2) drill blind via holes; (3) electrodeposition of copper (e.g. 0.5 to 0.6 mils) to fill the blind via holes and build up the outer copper layers to a particular thickness (e.g. approximately 1.2 mils); (4) apply and pattern photoresist film to cover the portions of the copper that will be subsequently removed and leave exposed portions that will remain; (5) electroplate copper, e.g. to a total thickness of 2 mils, and then tin as an etch resistant; and (6) etch away the exposed copper. In some embodiments, the outer conductive layers before the final etch step (6) in such a process may have approximately 2.0 mils thick copper (covered with tin) for the pattern that remains, and 1.2 mils thick copper (without tin covering) of the pattern may be removed by the final etch step. In such a process, the leg apertures may preferably be formed, e.g. by drilling, before the final etch step 6 (which removes the exposed unprotected copper layer) and either after deposition of the patterned copper and etch-resistant tin in step 5 for PCBs including self-aligned windings on the outer conductive layers or after electrodeposition in step 3 for PCBs not having self-aligned windings in the outer conductive layers. The final etch step may leave the tin-plated copper pattern remaining on the outer layers and create a nominal 2.0 mil (1.0 mil minimum) setback from the leg apertures to the edges of the self-aligned windings on the inner and if present outer conductive layers. The 1.0 mil setback may be sufficient insulation for low voltage (e.g. 1 V) applications. 
     Windings situated on the outer conductive layers of the PCB may be formed using the above described self-aligned winding process (using the leg aperture formation step to cut the inner circumference of the conductive windings) and the above-described self-aligned setback process (using the final etch of the outer layers to form set back from the leg apertures) or alternatively using standard lithographic techniques which will impose some setback dimensions specific to the process used. In the latter alternative, the leg apertures may be formed before and covered by the film applied in step 4 (preventing plating in the leg apertures during the electroplating in step 5). The film will be stripped before the final etch in step 6 allowing the formation of the self-aligned setback for winding features in the inner conductive layers. 
     In addition, or as an alternative, to the self-aligned set back process, insulation between the windings and the core legs may be provided for example by applying a uniform coating of electrically insulating material, e.g. parylene or epoxy, to the core legs or the inner surface of the leg apertures, or both. 
     IV. Assembly 
     Referring now to  FIG.  8   , a method  800  that may be used to form a circuit (e.g. planar magnetic component) is shown according to an exemplary embodiment. A multilayer PCB may be provided that includes conductive features ( 805 ). The conductive features may be arranged on conductive layers of the PCB to form windings around one or more axes (e.g. axes perpendicular to the layers of the PCB). One or more holes may be formed in the PCB at the axes ( 810 ). After the holes are formed, there may be no clearance provided between an inner edge of the conductive features (e.g. windings) and an edge of the holes. For example, the edges may be coterminous, such that, for example, an inner edge of the windings overlaps an edge of the hole in a lateral direction. 
     In some implementations, method  800  may include etching one or more of the conductive features/windings to form a clearance or setback between the edges of the hole(s) and inner edges of the conductive feature(s) ( 815 ). For example, in some embodiments, the inner edge of the conductive feature(s) may be etched radially outward from within the hole(s) to form the setback by removing a setback portion of the conductive feature(s). 
     In some implementations, method  800  may include assembling a core including one or more core legs ( 820 ). The core legs may be structures to extend through the hole(s) of the PCB (e.g. perpendicular to layers of the PCB). In some implementations, the core legs may be structured to mate or couple with a second core portion, such as a core plate, on an opposite side of the PCB. 
     In some particular implementations, one or more of the following steps may be used, alone or in combination, for making an improved planar transformer using magnetic cores with integral legs and plates:
         a) Build the multilayer PCB having winding features extending into the intend leg aperture regions, i.e. without setback;   b) Form the leg apertures, e.g. by drilling or milling;   c) Optionally etch the inner circumference of the exposed windings after the apertures are formed;   d) Assembling a first core piece from one side of the PCB with any legs of the first core piece extending into the leg apertures, optionally affixing the core piece to the PCB, e.g. with an adhesive, such as epoxy, optionally applied to the PCB or the core piece, or both, preferably applied before mating the two pieces, and optionally curing the adhesive after mating;   e) Optionally dispense adhesive to some or all of the following: the other side of the PCB, into apertures, or the second core piece, optionally setting desired gaps between the core pieces, e.g. optionally using gap glue in the apertures;   f) Assemble the second core piece from the other side of the PCB, with any legs of the second core piece extending into the respective leg apertures, and optionally cure the adhesive and/or gap glue.       

     In some particular implementations, one or more of the following steps may be used, alone or in combination, for making a planar transformer using plates and a plurality of separate core legs:
         a) Build the multilayer PCB having winding features extending into the intend leg aperture regions, i.e. without setback;   b) Form the leg apertures, e.g. by drilling or milling;   c) Optionally etch the inner circumference of the exposed windings after the apertures are formed;   d) Assemble a first core piece from one side of the PCB with any legs of the first core piece extending into the leg apertures, optionally affixing the core piece to the PCB, e.g. with an adhesive, such as epoxy, optionally applied to the PCB or the core piece, or both, preferably applied before mating the two pieces, and optionally curing the adhesive after mating;   e) Optionally dispense gap glue in the apertures;   f) Place core legs into apertures, optionally dispensing gapping material in the apertures before placing the legs, after placing the legs, or both;   g) Deposit core attach adhesive (e.g. epoxy) to the other outer surface of the multilayer PCB for use in attaching a second magnetically permeable plate;   h) Assemble the second core piece from the other side of the PCB over the core legs, optionally affixing the core piece to the PCB, e.g. with an adhesive, such as epoxy, optionally applied to the PCB or the core piece, or both, preferably applied before mating the two pieces, and optionally curing the adhesive after mating.       

     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the following features each may be used independently or in any combination: self-aligned windings, self-aligned core structures, self-aligned spacing, and self-aligned insulation. A wide variety of magnetic core configurations may be used, including, but not limited to: one or more U-cores combined with plates; pairs of symmetrical cores with legs inserted from both sides of a multilayer PCB; and a plurality of core pieces with multiple legs. The aperture may be of any shape required to accommodate a chosen set of magnetic elements. 
     The disclosure is described above with reference to drawings. These drawings illustrate certain details of specific embodiments that implement the systems, apparatus, and/or methods of the present disclosure. However, describing the disclosure with drawings should not be construed as imposing on the disclosure any limitations that may be present in the drawings. No claim element herein is to be construed as a “means plus function” element unless the element is expressly recited using the phrase “means for.” Furthermore, no element, component or method step in the present disclosure is intended to be dedicated to the public, regardless of whether the element, component or method step is explicitly recited in the claims. 
     It should be noted that although the disclosure provided herein may describe a specific order of method steps, it is understood that the order of these steps may differ from what is described. Also, two or more steps may be performed concurrently or with partial concurrence. It is understood that all such variations are within the scope of the disclosure. 
     The foregoing description of embodiments of the disclosure have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosure. The embodiments were chosen and described in order to explain the principles of the disclosure and its practical application to enable one skilled in the art to utilize the disclosure in various embodiments and with various modifications as are suited to the particular use contemplated.