LTCC planar transformer and preparation method thereof

Abstract

The invention discloses an LTCC (Low Temperature Co-Fired Ceramic) planar transformer and a preparation method thereof, the transformer comprises an upper margin, a lower margin and a plurality of middle substrates, the upper margin, the lower margin and the plurality of middle substrates are formed by ferrite raw tapes, annular areas are arranged on the plurality of middle substrates, PET (Polyethylene Terephthalate) films at the bottoms of the raw tapes are reserved in the annular areas, and printing paste is used for filling in the annular areas. Silver paste is printed through a screen plate to form an internal winding, and a via hole is formed in the annular area of each middle substrate corresponding to the winding. After the transformer is prepared according to the above steps, each layer of the transformer is preliminarily fused through warm water isostatic pressing, glue discharging sintering is carried out through a sectional type sintering furnace, silver bonding is carried out on a leading-out terminal electrode through a special silver bonding jig after sintering, and the terminal electrode is formed through electroplating after silver sintering. According to the invention, the problems of cracking, voids and warping easily occurring in the sintering process are solved, and meanwhile, the coupling coefficient of the product is increased from 90% to 95% or above.

Description

Technical Field

[0001] The invention belongs to the technical field of microwave communication, and in particular relates to an LTCC planar transformer, and also relates to a preparation method of the LTCC planar transformer. Background Art

[0002] Planar transformer products designed based on LTCC technology have high integration, small size and high temperature resistance. However, the mainstream processing method in the industry is to use ferrite green tape to print silver electrodes and cover each layer of silver electrodes with low UI and high magnetic resistance materials to achieve high coupling of LTCC planar transformers. However, this processing method is prone to cracking, warping and voiding after sintering, with low yield and low coupling coefficient of only 90%. Summary of the invention

[0003] In order to solve the above technical problems, the present invention provides a LTCC planar transformer and a preparation method thereof.

[0004] The technical solution adopted by the present invention is a LTCC planar transformer, including an upper margin, a lower margin and multiple middle substrates formed by a ferrite green tape, annular areas are opened on the multiple middle substrates and the PET film at the bottom of the green tape is retained in the annular areas, printing paste is used as filling in the annular area, and internal windings are formed by screen printing silver paste on the filling, and vias are opened in the corresponding windings in the annular areas of each middle substrate.

[0005] Preferably, the ferrite green tape has a relative magnetic permeability ui value of ≥300, a thickness of 48 um, and is filled with a ferrite slurry with a magnetic permeability ui value of 2-10.

[0006] Preferably, the thickness of the winding is 10-15um.

[0007] A method for preparing a LTCC planar transformer comprises the following steps: S1. Use laser to remove a ring area from the green tape of high-UI material, but keep the PET film at the bottom of the green tape; S2, using printing to print the slurry of low ui material and fill it in the annular area to form a raw belt in which high ui material and low ui material are embedded with each other; S3, forming an internal winding on the surface of the embedded raw tape by screen printing silver paste, and the internal winding is printed in the low ui material area; S4, then according to the winding structure designed for each layer, holes are opened at corresponding positions of the low ui material area of ​​the raw tape; S5, stacking the embedded green tapes with the winding patterns printed on them layer by layer to form a plurality of middle substrates; S6. Use raw tapes of high ui materials to stack them to the designed thickness to form lower margins and lower margins; S7, stacking the upper margin, multiple middle substrates and the lower margin in sequence, performing isostatic pressing with warm water to initially fuse each layer, and using a cutting machine to cut the entire stacked green tape; S8. The cut products are passed through a segmented sintering furnace for debinding and sintering. After sintering, a special silver-bonding fixture is used to bond silver to the lead-out terminal electrodes. After silver sintering, the terminal electrodes are formed by electroplating.

[0008] In the above step S7, the water temperature is 50-80°C and the time is 0.5-2.5h.

[0009] In the above step S8, the debinding sintering temperature curve is to increase from 25°C to 465±10°C within 40-70h, maintain at 465±10°C for 1-6h, then increase to 890±20°C within 40-150min, maintain at 890±20°C for 40-200min, and then slowly decrease to 25°C.

[0010] Compared with the prior art, the present invention has the following beneficial effects: The LTCC planar transformer provided by the present invention adopts a middle substrate formed by a high ui ferrite green tape to open an annular area to fill low ui slurry, and the annular area retains a PET film at the bottom of the green tape, and the internal winding is printed on the low ui material area, so that the leakage magnetic flux between the electrode layers is reduced while ensuring the excellent electrical properties such as the inductance of the planar transformer, the magnetic permeability of the overlapping induction area is improved, and the magnetic flux between the electrodes is increased.

[0011] The present invention provides a method for preparing an LTCC planar transformer. In the transformer, only two materials are co-fired in the same area, and the green tape magnetic permeability ui value is ≥300, and the filling magnetic permeability ui value is 2-10. While reducing the number of interfaces, the two are tightly combined during the sintering process, solving the cracking, voids, and warping problems that are prone to occur in the sintering process of the prior art, and the coupling coefficient of the product is increased from 90% to more than 95%. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a schematic diagram of the structure of LTCC planar transformer; Figure 2 It is a schematic diagram of the structure of the substrate in the LTCC planar transformer; Figure 3 The schematic diagram of winding arrangement is shown below; Figure 4 Schematic diagram of multiple basic stacked structures. DETAILED DESCRIPTION

[0013] The present invention will be further explained below in conjunction with the accompanying drawings to facilitate better understanding by those skilled in the art.

[0014] Example 1 like Figure 1-4 As shown, an LTCC planar transformer includes an upper margin 1, a lower margin 2 and a plurality of middle substrates 3 formed by a ferrite green tape, an annular area 4 is opened on the plurality of middle substrates 3 and the annular area 4 retains a PET film at the bottom of the green tape, a slurry is printed in the annular area 4 as a filling 5, an internal winding 6 is formed on the filling 5 by screen printing silver paste, and a via hole is opened in the annular area 4 of each middle substrate 3 corresponding to the winding 6 for connecting the upper and lower windings 6.

[0015] The relative magnetic permeability ui value of the ferrite green tape is ≥300, the thickness is 48um, and the ferrite slurry with a magnetic permeability ui value of 2-10 is used for filling; the thickness of the winding is 10-15um.

[0016] The present invention adopts a middle substrate formed by a high ui ferrite green tape to open an annular area to fill with low ui slurry, and the annular area retains the PET film at the bottom of the green tape, and the internal winding is printed in the low ui material area, so as to reduce the leakage magnetic field between the electrode layers while ensuring the excellent electrical properties such as the inductance of the planar transformer, improve the magnetic permeability of the overlapping induction area, and increase the magnetic flux between the electrodes.

[0017] Example 2 A method for preparing a LTCC planar transformer comprises the following steps: S1. Use laser to remove a ring area from the green tape of high-UI material, but keep the PET film at the bottom of the green tape; S2, using printing to print the slurry of low ui material and fill it in the annular area to form a raw belt in which high ui material and low ui material are embedded with each other; S3. Form an internal winding on the surface of the embedded raw tape by screen printing silver paste. The internal winding is printed in the low ui material area, and the internal winding printing thickness is 10-15um; S4, then according to the winding structure designed for each layer, holes are opened at corresponding positions of the low ui material area of ​​the raw tape for connecting the upper and lower windings; S5, stacking the embedded green tapes printed with winding patterns layer by layer to form a plurality of middle substrates; S6. Use raw tapes of high ui materials to stack them to the designed thickness to form lower margins and lower margins; S7, stack the upper margin, multiple middle substrates and the lower margin in sequence, and perform isostatic pressing with warm water at a temperature of 50-80°C for 0.5-2.5 hours to initially fuse each layer; then use a cutting machine to cut the entire stacked raw tape (which contains many products) to separate each product; S8. The cut products are passed through a segmented sintering furnace for debinding and sintering. The debinding and sintering temperature curve is as follows: the temperature is increased from 25°C to 465±10°C within 40-70h, maintained at 465±10°C for 1-6h, then increased to 890±20°C within 40-150min, maintained at 890±20°C for 40-200min, and then slowly decreased to 25°C. After sintering, a special silver bonding jig is used to bond silver to the lead-out terminal electrodes, and the terminal electrodes are formed by electroplating after silver burning.

[0018] The present invention provides a method for preparing an LTCC planar transformer. In the transformer, only two materials are co-fired in the same area, and the green tape magnetic permeability ui value is ≥300, and the filling magnetic permeability ui value is 2-10. While reducing the number of interfaces, the two are tightly combined during the sintering process, solving the cracking, voids, and warping problems that are prone to occur in the sintering process of the prior art, and the coupling coefficient of the product is increased from 90% to more than 95%.

[0019] The above-described embodiments are only for describing the preferred implementation of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit and principles of the present invention, various modifications and improvements made by those skilled in the art to the technical solution of the present invention should all fall within the scope of protection determined by the claims of the present invention.

Claims

1. A LTCC planar transformer, characterized in that: The invention comprises an upper margin (1), a lower margin (2) and a plurality of middle substrates (3) formed by using a ferrite green tape, an annular region (4) is provided on the plurality of middle substrates (3) and the annular region (4) retains a PET film at the bottom of the green tape, a slurry is printed in the annular region (4) as a filler (5), an internal winding (6) is formed on the filling (5) by screen printing silver paste, and a via hole corresponding to the winding (6) is provided in the annular region (4) of each middle substrate (3).

2. The LTCC planar transformer according to claim 1, characterized in that: The ferrite green tape has a relative magnetic permeability ui value of ≥300, a thickness of 48um, and is filled with ferrite slurry with a magnetic permeability ui value of 2-10.

3. The LTCC planar transformer according to claim 1, characterized in that: The thickness of the winding (6) is 10-15 um.

4. A method for preparing a LTCC planar transformer according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. Use laser to remove a ring area from the green tape of high-UI material, but keep the PET film at the bottom of the green tape; S2, using printing to print the slurry of low ui material and fill it in the annular area to form a raw belt in which high ui material and low ui material are embedded with each other; S3, forming an internal winding on the surface of the embedded raw tape by screen printing silver paste, and the internal winding is printed in the low ui material area; S4, then according to the winding structure designed for each layer, holes are opened at corresponding positions of the low ui material area of ​​the raw tape; S5, stacking the embedded green tapes printed with winding patterns layer by layer to form a plurality of middle substrates; S6. Use raw tapes of high ui materials to stack them to the designed thickness to form lower margins and lower margins; S7, stacking the upper margin, multiple middle substrates and the lower margin in sequence, performing isostatic pressing with warm water to initially fuse each layer, and using a cutting machine to cut the entire stacked green tape; S8. The cut products are passed through a segmented sintering furnace for debinding and sintering. After sintering, a special silver-bonding fixture is used to bond silver to the lead-out terminal electrodes. After silver sintering, the terminal electrodes are formed by electroplating.

5. The method for preparing a LTCC planar transformer according to claim 4, characterized in that: In step S7, the water temperature is 50-80°C and the time is 0.5-2.5h.

6. The method for preparing a LTCC planar transformer according to claim 4, characterized in that: In step S8, the debinding sintering temperature curve is to increase from 25°C to 465±10°C within 40-70h, maintain at 465±10°C for 1-6h, then increase to 890±20°C within 40-150min, maintain at 890±20°C for 40-200min, and then slowly decrease to 25°C.

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