The subject matter disclosed herein relates to integrating transformers into integrated circuits or other packaged electronic devices. Wire wound transformers provide electrical isolation with good performance, but these devices are large and expensive. In power supplies and other applications, it is desirable to reduce the cost and size of magnetic components and to integrate isolation transformers into a packaged electronic device or module with a small footprint, while providing high isolation voltage ratings. Magnetic films can be added on the top and bottom of a lamination with conductive windings separated by an isolation barrier to improve performance of an integrated air core transformer with a small footprint. However, this approach suffers from AC winding loss due to fringing effects caused by the airgap between two magnetic plates, which limits power delivery capability through the transformer. A lamination can also be assembled within magnetic core structures to provide an isolated closed magnetic loop transformer. However, assembling the components with interconnecting material introduces and traps air bubbles or other voids which inhibit achievement of the desired isolation performance rating of the transformer.
One aspect of the present disclosure 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.
In another aspect, 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
The method 100 begins with positioning a laminate strip on a Teflon plate or other carrier structure at 102, and a stencil is positioned on the laminate strip at 104. The method 100 further includes printing a magnetic paste (e.g., ferrite paste) on a top side of, and into an opening of, the laminate strip at 106 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 108, the method 100 further includes curing, such as by a thermal heating process, to harden the magnetic paste top transformer core piece. The method 100 further includes separating the laminate strip at 110 into individual laminate pieces having respective top transformer core pieces.
At 112, the method 100 includes dispensing magnetic paste on a lead frame pad. At 114, the method 100 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 116, the method 100 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 100 further includes dispensing magnetic paste at 118 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 120. At 122, the method 100 further includes curing, such as by a thermal heating process, to finish the isolated transformer assembly. A pre-bond plasma treatment is performed at 124 in one example. At 126, the method 100 further includes wire bonding. The method 100 further includes molding at 128, and package separation at 130.
In one example, the first transformer core piece 214 includes a first portion that extends along a side of the laminate structure 212, and a second portion that fills an opening of the laminate structure 212, for example, to form a core leg of a T or U or E-shaped core piece. The second transformer core piece 216 extends along a side of the second portion of the first transformer core piece 214. The laminate structure 212 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 214 and the second transformer core piece 216.
The example laminate structure 212 includes multiple layers or levels, each including a dielectric material layer. This example also includes conductive features and conductive inter-level 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 212. In one example, one or both respective windings PW and SW extend on multiple levels of the multi-level laminate structure 212. In the example of
In one implementation, the printing process 900 is a screen printing or silk screening process that uses a dispensing apparatus (not shown) to dispense or otherwise deposit the magnetic paste 214 onto the first side 303 and into the openings 306, 307, and 308 of the laminate structure 212, preferably to a level above the top of the stencil 702, and a blade or squeegee (not shown) is moved with applied downward pressure across the top side of the stencil 702 (e.g., screen) to fill the open stencil apertures with the printed magnetic paste 214 and create a smooth (e.g., substantially planar) top side of the printed magnetic paste 214.
The processing continues in
The transformer 210 includes the laminate structure 212 with the first side 303 and the opposite second side 305, as well as the first transformer core piece 214 formed of cured magnetic paste. The first transformer core piece 214 has the first portion that extends along the first side 303 of the laminate structure 212, and the second portion that fills the opening 308 of the laminate structure 212. The closed magnetic circuit also includes the second transformer core piece 216 that extends along the lower side of the second portions of the E-shaped first transformer core piece 214. The magnetic circuit also includes the cured magnetic paste 1602 between the first side of the second transformer core piece 216 and the bottom sides of the second portions of the first transformer core piece 214. The transformer 210 in
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 100 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 100 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 106 in
Modifications are possible in the described examples, and other implementations are possible, within the scope of the claims.
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Number | Date | Country | |
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20210398736 A1 | Dec 2021 | US |