Patent Description:
One aspect of this description provides a method. The method comprises performing a printing process that deposits a magnetic paste onto a first side of a laminate structure using a stencil. The printing process deposits the magnetic paste into and fills an opening that extends from the first side of the laminate structure to an opposite second side of the laminate structure. The method also comprises curing the magnetic paste to form a first transformer core piece having: a first portion that extends along the first side of the laminate structure, and a second portion that fills the opening of the laminate structure, and joining a second transformer core piece to a side of the second portion of the first transformer core piece to form a transformer.

In one example, performing the printing process deposits the magnetic paste into and fills multiple openings that respectively extend from the first side of the laminate structure to the second side of the laminate structure. In one implementation, the laminate structure includes windings that encircle the opening or one or more openings of the laminate structure.

In one example, the second transformer core piece is joined to the side of the second portion of the first transformer core piece by depositing a second magnetic paste onto a first side of the second transformer core piece, positioning the side of the second portion of the first transformer core piece on the second magnetic paste, and curing the second magnetic paste to join the second transformer core piece to the side of the second portion of the first transformer core piece.

One example further comprises joining the second transformer core piece to a lead frame before joining the second transformer core piece to the side of the second portion of the first transformer core piece. In one implementation, the second transformer core piece is joined to the lead frame by depositing a third magnetic paste onto a side of the lead frame, positioning a second side of the second transformer core piece on the third magnetic paste, and curing the third magnetic paste to join the second side of the second transformer core piece to the lead frame. In one example, the method further comprises joining the second transformer core piece to the second side of the laminate structure while joining the second transformer core piece to the side of the second portion of the first transformer core piece.

According to the invention, a transformer comprises a laminate structure and first and second core pieces. The laminate structure has a first side, an opposite second side, an opening that extends from the first side of the laminate structure to the second side of the laminate structure, and windings that encircle the opening. The first transformer core piece has a first portion that extends along the first side of the laminate structure, and a second portion that fills the opening of the laminate structure, and the first transformer core piece comprises a cured magnetic paste. The second transformer core piece extends along a side of the second portion of the first transformer core piece. In one example, the transformer further comprises a second cured magnetic paste between a first side of the second transformer core piece and the side of the second portion of the first transformer core piece. In another example, the first transformer core piece has an E-shape. In one example, the first transformer core piece has a T-shape. In another example, the first transformer core piece has a U-shape.

A further aspect provides an electronic device that comprises a transformer, and package structure, and conductive leads. The transformer comprises a laminate structure, a first transformer core piece, and a second transformer core piece. The first transformer core piece comprises a cured magnetic paste, a first portion that extends along a side of the laminate structure, and a second portion that fills an opening of the laminate structure. The second transformer core piece extends along a side of the second portion of the first transformer core piece, and the laminate structure comprises windings that encircle the opening. The package structure encloses the transformer, and the conductive leads are electrically coupled to the transformer. In one example, the transformer further comprises a second cured magnetic paste between a first side of the second transformer core piece and the side of the second portion of the first transformer core piece. In one example, the first transformer core piece has an E-shape. In another example, the first transformer core piece has a T-shape. In another example, the first transformer core piece has a U-shape.

In the drawings, like reference numerals refer to like elements throughout, and the various features are not necessarily drawn to scale. Also, the term "couple" or "couples" includes indirect or direct electrical or mechanical connection or combinations thereof. For example, if a first device couples to or is coupled with a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via one or more intervening devices and connections.

Referring to <FIG> and <NUM>, <FIG> shows a method <NUM> according to one aspect of this description. In one implementation, the method <NUM> of <FIG> is performed to manufacture an electronic device with an integrated transformer shown in FIG. <NUM>, and FIGS. <NUM>-<NUM> show the device of FIG. <NUM> at various stages of fabrication according to the method <NUM>.

The method <NUM> begins with positioning a laminate strip on a Teflon plate or other carrier structure at <NUM>, and a stencil is positioned on the laminate strip at <NUM>. The method <NUM> further includes printing a magnetic paste (e.g., ferrite paste) on a top side of, and into an opening of, the laminate strip at <NUM> to form an E, U, or T-shaped magnetic paste top transformer core piece (also referred to hereinafter as a first transformer core piece) that extends on and into laminate strip. At <NUM>, the method <NUM> further includes curing, such as by a thermal heating process, to harden the magnetic paste top transformer core piece. The method <NUM> further includes separating the laminate strip at <NUM> into individual laminate pieces having respective top transformer core pieces.

At <NUM>, the method <NUM> includes dispensing magnetic paste on a lead frame pad. At <NUM>, the method <NUM> further includes positioning a bottom transformer core piece (also referred to hereinafter as a second transformer core piece) on the lead frame pad, for example, over the dispensed magnetic paste. At <NUM>, the method <NUM> further includes curing, such as by a thermal heating process, to harden the magnetic paste to join the bottom transformer core piece to the lead frame pad.

The method <NUM> further includes dispensing magnetic paste at <NUM> on a top side of the bottom transformer core piece, as well as positioning a singulated laminate piece with a top transformer core piece on the top side of the bottom transformer core piece at <NUM>. At <NUM>, the method <NUM> further includes curing, such as by a thermal heating process, to finish the isolated transformer assembly. A pre-bond plasma treatment is performed at <NUM> in one example. At <NUM>, the method <NUM> further includes wire bonding. The method <NUM> further includes molding at <NUM>, and package separation at <NUM>.

<NUM> shows a packaged electronic device <NUM> having a transformer with an E-I core structure. The electronic device <NUM> includes a molded package structure <NUM> and conductive leads <NUM> with outer sides or surfaces that are exposed outside the package structure <NUM>. The electronic device <NUM> includes a transformer <NUM> at least partially enclosed by the package structure <NUM>. The transformer <NUM> includes a laminate structure <NUM>, a first transformer core piece <NUM>, and a second transformer core piece <NUM>. The electronic device <NUM> includes bond wires <NUM> that electrically couple respective ones of the conductive leads <NUM> to the transformer <NUM>. The first (e.g., top) transformer core piece <NUM> is or includes a cured magnetic paste <NUM> (e.g., printed at <NUM> and cured at <NUM> in <FIG> above, where the uncured magnetic paste printed at <NUM> is also referred to herein as <NUM>).

In one example, the first transformer core piece <NUM> includes a first portion that extends along a side of the laminate structure <NUM>, and a second portion that fills an opening of the laminate structure <NUM>, for example, to form a core leg of a T or U or E-shaped core piece. The second transformer core piece <NUM> extends along a side of the second portion of the first transformer core piece <NUM>. The laminate structure <NUM> in one example includes primary and secondary windings that encircle the opening, for example, to magnetically couple primary and secondary transformer windings with a magnetic circuit formed by the first transformer core piece <NUM> and the second transformer core piece <NUM>.

<NUM>-<NUM> show the electronic device <NUM> having a transformer with an E-I core structure, undergoing fabrication processing according to the method <NUM> of <FIG>. <NUM> and <NUM> respectively show partial sectional side and top plan views of the laminate structure <NUM> in the form of a laminate strip <NUM> with multiple sections, each including respective primary and secondary transformer windings PW and SW. The laminate structure <NUM> includes a first side <NUM> (e.g., the top side) of a laminate structure <NUM> with conductive (e.g., copper) bond pads <NUM>, as well as an opposite (e.g., bottom) second side <NUM>. As shown in FIG. <NUM>, the laminate strip <NUM> includes an array of prospective device segments or sections, each having respective primary and secondary transformer windings PW and SW. The section view of FIG. <NUM> shows one example section with laminate structure openings <NUM>, <NUM> and <NUM> that each extend through the laminate structure <NUM> from the first side <NUM> to the second side <NUM>.

The example laminate structure <NUM> includes multiple layers or levels, each including a dielectric material layer. This example also includes conductive features and conductive interlevel vias (not shown) to form conductive primary windings PW and conductive secondary windings SW. The windings PW and SW in one example are spiral winding structures or traces on individual levels of the multi-level laminate structure <NUM>. In one example, one or both respective windings PW and SW extend on multiple levels of the multi-level laminate structure <NUM>. In the example of FIGS. <NUM> and <NUM>, the individual sections of the laminate structure <NUM> includes top side bond pads <NUM> electrically coupled with respective ends of one or both the primary and secondary transformer windings PW and SW to allow bond wire connection to the transformer windings in the finished packaged electronic device <NUM> (FIG.

<NUM> and <NUM> show respective sectional side and top views of the laminate structure <NUM> positioned on a Teflon plate <NUM> (e.g., at <NUM> in <FIG>). The Teflon plate <NUM> allows printing (e.g., screen printing) of magnetic (e.g., ferrite) paste to fill the openings <NUM>, <NUM> and <NUM> of the laminate structure <NUM> and to form lower ends of the second portion or portions of the top transformer core piece that are generally flush with the second side <NUM> of the laminate structure <NUM> after printing and curing and after the Teflon plate <NUM> has been removed. The resulting second portion has a flat lower side that facilitates joinder to the second (e.g., bottom) transformer core piece (<NUM> in FIG. <NUM>) without air gaps or voids, thereby facilitating improved transformer performance and isolation voltage rating. As shown in the top view of FIG. <NUM>, a single Teflon plate <NUM> is used for all the device sections of the laminate structure <NUM> for printing and curing of all the sections concurrently.

<NUM> and <NUM> show respective sectional side and top views of the laminate structure <NUM> undergoing a process <NUM> that positions a stencil <NUM> on portions of the first side <NUM> of the laminate structure <NUM>. The example stencil <NUM> covers the bond pads (<NUM> in FIG. <NUM> above) and exposes portions of the first side <NUM> and the openings <NUM>, <NUM> and <NUM> of the laminate structure <NUM>. The openings of the stencil <NUM> can be tailored for any desired magnetic paste print coverage for a given design. In one example, the stencil <NUM> is or includes stainless steel. In another example, the stencil <NUM> is or includes nylon.

<NUM> and <NUM> show respective sectional side and top views of the laminate structure <NUM> and the stencil <NUM> during performance of a printing process <NUM> (e.g., at <NUM> in <FIG> above). The printing process <NUM> deposits <NUM> magnetic paste <NUM> onto exposed portions of the first side <NUM> of the laminate structure <NUM>. The printing process also deposits the magnetic paste <NUM> into the exposed openings <NUM>, <NUM>, and <NUM>. In the illustrated example, the printing process <NUM> fills the openings <NUM>, <NUM>, and <NUM> as shown in FIG. <NUM>, and forms substantially flat bottoms of the second portions of the printed magnetic paste <NUM> within the openings <NUM>, <NUM>, and <NUM> along the exposed top side of the Teflon plate <NUM>. In the illustrated example, the bottoms of the second portions of the printed magnetic paste <NUM> in the openings <NUM>, <NUM>, and <NUM> are substantially planar with the second side <NUM> of the laminate structure <NUM>.

In one implementation, the printing process <NUM> is a screen printing or silk screening process that uses a dispensing apparatus (not shown) to dispense or otherwise deposit the magnetic paste <NUM> onto the first side <NUM> and into the openings <NUM>, <NUM>, and <NUM> of the laminate structure <NUM>, preferably to a level above the top of the stencil <NUM>, and a blade or squeegee (not shown) is moved with applied downward pressure across the top side of the stencil <NUM> (e.g., screen) to fill the open stencil apertures with the printed magnetic paste <NUM> and create a smooth (e.g., substantially planar) top side of the printed magnetic paste <NUM>.

<NUM> shows the laminate structure <NUM> and the printed magnetic paste undergoing a thermal curing process <NUM> with the stencil removed (e.g., <NUM> in <FIG> above). The curing process <NUM> forms the cured first transformer core piece <NUM> (e.g., FIG. <NUM> above). The cured first transformer core piece <NUM> has a first portion that extends along the first side <NUM> of the laminate structure <NUM>, and second portions that fill the respective openings <NUM>, <NUM>, and <NUM> of the laminate structure <NUM>. <NUM> shows the laminate structure <NUM> and the cured first transformer core piece <NUM> undergoing a singulation process <NUM> (e.g., at <NUM> in <FIG>) that separates individual sections from the laminate strip structure of FIGS. <NUM>-<NUM> and creates a joined upper transformer assembly <NUM>. In one example, the singulation process <NUM> is a saw cutting process. In another example, the singulation process <NUM> is a laser cutting process.

<NUM> shows a side view of a portion of a lead frame <NUM> formed as a strip with multiple device sections. The lead frame <NUM> is positioned or supported in a carrier tray <NUM> while undergoing a magnetic paste dispense process <NUM> that deposits a second magnetic paste <NUM> on select portions of the lead frame <NUM> (e.g., at <NUM> in <FIG>). In one example, the second magnetic paste <NUM> is the same material used to print the first transformer core piece <NUM>. <NUM> shows the lead frame strip undergoing a process <NUM> that positions the second transformer core piece <NUM> on the deposited second magnetic paste <NUM> on a portion of the top side of the lead frame <NUM> (e.g., at <NUM> in <FIG>). <NUM> shows the lead frame strip undergoing a thermal curing process <NUM> (e.g., at <NUM> in <FIG>) that cures the deposited second magnetic paste <NUM> to join the bottom side of the second transformer core piece <NUM> to the lead frame <NUM>.

<NUM>-<NUM> show example processing to join the second transformer core piece <NUM> to the second side (e.g., bottom side) of the second portions of the first transformer core piece <NUM>. The joining processing in this example also joins the top of the second transformer core piece <NUM> to the bottom or second side <NUM> of the laminate structure <NUM>. <NUM> shows the lead frame <NUM> and the positioned second transformer core piece <NUM> undergoing a magnetic paste dispense process <NUM> that deposits magnetic paste <NUM> on select portions of the first side (e.g., top side) of the positioned bottom transformer core piece <NUM> (e.g., at <NUM> in <FIG> above). In one example, the magnetic paste <NUM> is the same material used to print the first transformer core piece <NUM>.

The processing continues in FIG. <NUM>, which shows the assembled lead frame <NUM> and positioned second transformer core piece <NUM> undergoing a process <NUM> (e.g., at <NUM> in <FIG>) that positions the assembled first transformer core piece <NUM> and the second (e.g., bottom) side <NUM> of the associated laminate piece <NUM> onto the first (e.g., top) side of the second transformer core piece <NUM> (e.g., on the dispensed magnetic paste <NUM>). In one example, the process <NUM> uses pick and place apparatus (not shown) to position the first transformer core piece <NUM> and the laminate structure piece <NUM> onto the second transformer core piece <NUM>. <NUM> shows the lead frame <NUM> and the first and second transformer core pieces <NUM> and <NUM> undergoing a thermal curing process <NUM> (e.g., at <NUM> in <FIG>). The process <NUM> cures the magnetic paste <NUM> to join the second transformer core piece <NUM> to the side of the second portion of the first transformer core piece <NUM>, and also joins the second side <NUM> of the laminate structure <NUM> to the second transformer core piece <NUM>, to form the transformer <NUM>.

The transformer <NUM> includes the laminate structure <NUM> with the first side <NUM> and the opposite second side <NUM>, as well as the first transformer core piece <NUM> formed of cured magnetic paste. The first transformer core piece <NUM> has the first portion that extends along the first side <NUM> of the laminate structure <NUM>, and the second portion that fills the opening <NUM> of the laminate structure <NUM>. The closed magnetic circuit also includes the second transformer core piece <NUM> that extends along the lower side of the second portions of the E-shaped first transformer core piece <NUM>. The magnetic circuit also includes the cured magnetic paste <NUM> between the first side of the second transformer core piece <NUM> and the bottom sides of the second portions of the first transformer core piece <NUM>. The transformer <NUM> in FIG. <NUM> is then processed for wire bonding, for example, to connect the bond wires <NUM> in FIG. <NUM> above, and the device is molded to form the molded package structure <NUM> of <FIG> to provide an integrated transformer electronic device <NUM>.

<NUM> shows another example transformer <NUM> with a U-T core structure having a T-shaped first transformer core piece <NUM>. In this example, the laminate structure <NUM> is as described above, and a U-shaped second transformer core piece <NUM> is positioned as shown and joined by cured magnetic paste <NUM>. <NUM> shows a further example transformer <NUM> with a U-I core structure. This example includes a first transformer core piece <NUM> with a U-shape, a laminate structure <NUM> as described above with windings PW and SW that encircle one vertical leg of the first transformer core piece <NUM>, and a generally flat (e.g., I-shaped) second transformer core piece <NUM>.

The described example transformers and packaged electronic devices provide efficient closed magnetic loop structures, such as E-I, U-I, T-U-shaped transformer core piece structures, while mitigating or avoiding air gaps or other voids to provide a small reluctance path for the transformer magnetic circuit. These examples and the fabrication method <NUM> reduce leakage flux and increase the transformer quality factor and inductance density, particularly compared to using non-magnetic interconnecting material to bond two pieces of E-I, U-I, or T-U shaped cores, through alternative integration methods which use screen printed magnetic paste to serve as E, or T or U-shaped core piece of the transformer and mitigate or eliminate voids in the closed loop transformer structures. The described method <NUM> facilitates consistent isolation performance and reliability by mitigating voids through the screen-printing assembly process. The described structures and methods also reduce manufacturing cost by reducing or eliminating fabrication yield loss previously associated with voids introduced when joining or assembling transformer core pieces. Moreover, the screen-printing process can easily vary the feed material (e.g., ferrite particles in the printed magnetic paste) allowing fine tuning of the properties of the magnetic pastes for new applications, for example, to provide high relative permeability, low loss transformer core pieces with high breakdown voltage, proper viscosity for screen printing as well as dispensing, and high resistivity using feed material mixtures to make magnetic composite into ink, or powder paste form or magnetic paste with ferrite particles to eliminate or mitigate voids in the magnetic circuit path and ensure the desired isolation performance and mechanical reliability of the isolated closed magnetic loop transformer. In certain examples, the printing at <NUM> in <FIG> uses Ajinomoto screen printed magnetic paste (AMP) to fill the openings <NUM>, <NUM>, and <NUM> of the laminate structure <NUM>. In certain example, the magnetic past of the first transformer core piece <NUM>, <NUM>, <NUM> includes Ferrite particles such as NiZn ferrite, MnZn ferrite and NiCuZn selected to formulate the magnetic paste for isolated closed loop transformer fabrication. The particle size in one example is in the range of a few hundred nanometers to tens of micrometers to facilitate screen printing using existing printing apparatus and systems. The main magnetic properties of one example of the paste after curing, such as permeability, is around <NUM>-<NUM>. The properties mainly depend on the composition and particle size of the magnetic paste in certain implementations. The peak curing temperature in one example is less than <NUM> degrees C, which varies with epoxy resin used for the magnetic paste. The described examples provide a low cost solution to isolation and reliability performance in miniature isolated closed magnetic loop transformers by reducing or avoiding voids trapped in interconnecting layers by screen printing the magnetic pastes to form a T, E or U-shaped core piece.

Claim 1:
A method of manufacturing a transformer, comprising:
using a stencil, performing a printing process that deposits a magnetic paste onto a first side (<NUM>) of a laminate structure (<NUM>), and that deposits the magnetic paste into and fills an opening (<NUM>) that extends from the first side (<NUM>) of the laminate structure (<NUM>) to an opposite second side (<NUM>) of the laminate structure (<NUM>), wherein the laminate structure (<NUM>) comprises a primary winding (PW) and a secondary winding (SW) that encircle the opening (<NUM>);
curing the magnetic paste to form a first transformer core piece (<NUM>) having: a first portion that extends along the first side (<NUM>) of the laminate structure (<NUM>), and a second portion that fills the opening (<NUM>) of the laminate structure (<NUM>);
joining a second transformer core piece (<NUM>) to a side of the second portion of the first transformer core piece (<NUM>) and the opposite side (<NUM>) of the laminate structure (<NUM>) to form the transformer.