High-reliability and low-insertion-loss balun based on LTCC (Low Temperature Co-Fired Ceramic) process

Abstract

The invention relates to a high-reliability and low-insertion-loss balun based on an LTCC (Low Temperature Co-Fired Ceramic) process. The balun comprises a dielectric layer, a metal structure and a vertical interconnection structure, the dielectric layer is of an LTCC (Low Temperature Co-Fired Ceramic) multi-layer ceramic integrated structure formed by laminating and sintering multiple layers of green ceramic tapes; the metal structure comprises a surface metal layer and filling metal, and the surface metal layer is printed on the surface of the dielectric layer; the vertical interconnection structure is a plated-through hole structure penetrating through the dielectric layer and comprises a plurality of half holes and through holes, and the filling metal is located in the plated-through hole structure. According to the invention, the balun which has the advantages of high reliability, high performance and miniaturization is designed by adopting an LTCC (Low Temperature Co-Fired Ceramic)-based multilayer ceramic process and adopting unequal-width coupling lines, a hollow ground and a vertical semi-through hole structure.

Description

Technical Field

[0001] This invention relates to the field of microwave communication system technology, and specifically to a high-reliability, low-insertion-loss balun based on LTCC technology. Background Technology

[0002] With the rapid development of wireless communication systems, highly reliable, high-performance, and miniaturized passive devices have become a research hotspot in the industry. The balun (balanced-to-unbalanced converter), as a key component in microwave communication systems, plays a crucial role in converting balanced signals to unbalanced signals. The balun can decompose an input signal into two signals of equal amplitude and 180° phase difference through balanced-to-unbalanced conversion, thus playing a vital role in the system.

[0003] Traditional baluns typically employ a magnetically wound wire structure, but this results in a large footprint, making it difficult to meet current application demands for device miniaturization. To address these miniaturization requirements, the industry currently widely uses LTCC (Low Temperature Co-fired Ceramic) multilayer ceramic processes to fabricate baluns. However, conventional LTCC baluns still face several unresolved issues: (1) The coupling line adopts an equal width design. When the LTCC stacking process is misaligned, the coupling degree will decrease, which will affect the overall performance of the balun.

[0004] (2) The conventional coupling line adopts a fully grounded structure on the upper and lower layers. This design will increase the energy loss during signal transmission and make the insertion loss of the device higher.

[0005] (3) Traditional processes expose the output terminals of the inner RF traces by ball milling and use thick film paste printed on the sidewalls to connect the top and bottom surfaces with the inner RF traces. However, if the ball milling is not thorough or the inner metal is too thin, it is very easy to cause poor contact between the output terminals and the sidewall metal, which will not only lead to a decrease in balun performance, but in severe cases it will also cause it to fail completely, greatly reducing its reliability.

[0006] Therefore, in order to solve the problems of coupling performance stability, insertion loss control and connection reliability of baluns, it is urgent to realize a high-reliability, low-insertion-loss balun based on LTCC technology. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a highly reliable, low insertion loss balun based on LTCC technology.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A high-reliability, low-insertion-loss balun based on LTCC technology, the balun comprising: a dielectric layer, a metal structure distributed on or inside the dielectric layer, and a vertical interconnect structure penetrating the dielectric layer; The dielectric layer includes a multilayer dielectric substrate formed by stacking and sintering multiple layers of green ceramic tapes; The metal structure includes at least one pair of coupling wires and a grounding system; The coupling line group includes two coupling lines located on different layers and with different widths, wherein the projection range of the wider coupling line covers the projection range of the narrower coupling line. The grounding system includes an outer grounding layer located outside the coupling line group and an intermediate grounding layer located between a pair of coupling line groups; the outer grounding layer is a hollow ground structure, and its hollow area overlaps with the vertical projection of the corresponding coupling line; the intermediate grounding layer is a complete grounding structure used to isolate signals between upper and lower layers; The vertical interconnect structure includes a plurality of half-holes formed on the side of the multilayer dielectric substrate, and the half-holes are filled with conductive material.

[0009] Furthermore, the number of green ceramic tape layers in the dielectric layer is 10; the number of coupling line groups is 2.

[0010] The vertical interconnect structure includes half-holes V1 to V6, a first through hole, and a second through hole; wherein each half-hole and through hole is filled with a conductive material, which is any one or more metal pastes selected from gold, silver, and copper, and the metal pastes filled in the half-holes or through holes form a filler metal.

[0011] Furthermore, the metal structure also includes a radio frequency (RF) port layer; the RF port layer includes a top RF port layer disposed on the upper surface of the first green ceramic tape and a bottom RF port layer disposed on the lower surface of the tenth green ceramic tape. The top RF port layer includes six RF ports: input port P1, no-load port P2, RF ground P3, RF ground P4, output port P5, and output port P6. The bottom RF port layer includes six RF ports: output port P7, output port P8, RF ground P9, RF ground P10, no-load terminal P11, and input port P12. The six RF ports in the top RF port layer are connected one-to-one with the six RF ports in the bottom RF port layer through vertical half-holes.

[0012] Furthermore, the outer grounding layer includes a first grounding layer g1 printed on the surface of the second green ceramic tape and a third grounding layer g3 printed on the surface of the tenth green ceramic tape; the intermediate grounding layer includes a second grounding layer g2 printed on the surface of the sixth green ceramic tape.

[0013] Furthermore, both the first grounding layer g1 and the third grounding layer g3 are hollow ground structures. The hollow ground structure has a long strip-shaped hollow area. The length of the hollow area is consistent with the effective coupling length of the corresponding coupling line, and the width is 10-20um larger than the width of the adapted coupling line. The hollow area coincides with the projection range of the coupling line. The second grounding layer g2 is a complete grounding structure.

[0014] Furthermore, the two sets of coupling lines are an upper coupling pair and a lower coupling pair, respectively. The upper coupling pair includes a first coupling line and a second coupling line, and the lower coupling pair includes a third coupling line and a fourth coupling line. The four coupling lines form a first coupling layer, a second coupling layer, a third coupling layer, and a fourth coupling layer. The upper coupling pair is located between the first ground layer g1 and the second ground layer g2. The lower coupling pair is located between the second ground layer g2 and the third ground layer g3.

[0015] Furthermore, the linewidth of the second coupling line is greater than that of the first coupling line; the linewidth of the fourth coupling line is greater than that of the third coupling line.

[0016] Furthermore, the total thickness of the green ceramic strips between the first coupling line 2 and the second coupling line 3, and between the third coupling layer line and the fourth coupling line 5, after sintering is 45 μm. That is, the sum of the thicknesses of the coupling regions of the fourth and fifth green ceramic strips, and the sum of the thicknesses of the coupling regions of the seventh and eighth green ceramic strips, are all 45 μm. The thickness of each of the remaining eight green ceramic strips after sintering is 90 μm, that is, the non-coupling regions of the first to third, sixth, and ninth to tenth green ceramic strips, and the fifth and eighth green ceramic strips.

[0017] Furthermore, the signal transmission path of this high-reliability, low-insertion-loss balun based on LTCC technology includes a top-level signal transmission path and a bottom-level signal transmission path; The core signal transmission path at the top level is as follows: Input port P1 is connected to the third coupling line 4 through half-hole V1. The third coupling line 4 is connected to the second coupling line 3 through the first through-hole. The second coupling line 3 is connected to the unloaded terminal P2 through half-hole V2. At the same time, the first coupling line 2 and the second coupling line 3 are coupled. RF ground P4 is connected to the first coupling line 2 through half-hole V4. The first coupling line 2 is connected to the output port P6 through half-hole V6. The third coupling line 4 is coupled to the fourth coupling line 5. RF ground P3 is connected to the fourth coupling line 5 through half-hole V3. The fourth coupling line 5 is connected to the output port P6 through half-hole V6, thus completing the balanced-to-unbalanced signal conversion and stable output. The underlying extended signal transmission path is: (1) Output port P7 is vertically penetrated through 10 layers of green ceramic tape via half hole V5 and is connected to the output port P5 of the top layer; output port P5 is connected to the upper layer coupler, and output port P7 indirectly receives the first balanced signal output by the upper layer coupler and becomes the output terminal of the bottom layer balanced signal. (2) Output port P8 is vertically connected to the top layer output port P6 via half hole V6; output port P6 is connected to the signal path of the lower layer coupler, and output port P8 indirectly receives the second balanced signal output by the lower layer coupler, forming a bottom layer balanced output combination with output port P7 with equal amplitude and 180° phase difference. (3) Input port P12 is vertically connected to the top layer input port P1 through half hole V1; input port P1 is directly connected to the third coupling line 4 through half hole V1 to start the signal conversion of the double-layer coupling pair; input port P12 serves as the bottom layer input port, used to receive external unbalanced signals and transmit them to the inner layer coupling line, and is the bottom layer starting point of the entire signal transmission link. (4) The unloaded end P11 is vertically connected to the unloaded end P2 of the top layer through the half hole V2; the unloaded end P2 is connected to the second coupling line 3 through the through hole; (5) Radio frequency ground P9 is vertically connected to radio frequency ground P3 through half hole V3; radio frequency ground P3 is connected to the fourth coupling line 5 through half hole V3, and the projection range of radio frequency ground P9 coincides with the grounding area of ​​the third ground layer g3. (6) Radio frequency ground P10 is vertically connected to the top radio frequency ground P4 through half hole V4; radio frequency ground P4 is connected to the first coupling line 2 through half hole V4, and radio frequency ground P10 is connected to the non-cutout area of ​​the third ground layer g3 to provide the bottom ground support for the first coupling line 2, and together with the first ground layer g1 and the second ground layer g2, it forms a shielding structure of three-layer grounding and double-layer coupling to block inter-layer signal crosstalk.

[0018] Compared with the prior art, the advantages of the present invention are: (1) Based on LTCC multilayer ceramic technology, this invention innovatively integrates unequal width coupling lines, hollow ground structure and vertical half-hole vertical interconnect structure to design a balun device with the advantages of high reliability, high performance and miniaturization. The balun is composed of LTCC ceramic material and ceramic embedded metal structure. The ceramic material is a dielectric layer formed by stacking and sintering 10 layers of green ceramic tape, which serves as the structural carrier of the device. The metal structure includes metal layers printed on each surface of the dielectric layer and filler metal filling the half-holes and through holes. Together, they provide a solid foundation for the structural stability and performance of the device.

[0019] (2) The innovations of this invention are concentrated in four dimensions: coupling structure, grounding design, connection method, and thickness optimization. Specifically, they include: First, an unequal width coupling line design is adopted, setting the two coupling lines in the upper coupling pair to have different line widths. This design specifically solves the ±25μm misalignment problem that is prone to occur in the LTCC stacking process. Even if this deviation exists, the projection area of ​​the coupling line can still be kept consistent, so that the balun coupling performance remains stable and the coupling degree is avoided. Second, the first ground layer g1 and the third ground layer g3 adopt a hollow ground structure, with hollowing only in the projection area of ​​the coupling line, while the rest of the area remains fully grounded. The hollow ground structure replaces the traditional full-ground design, solving the problem of high signal energy loss and high insertion loss caused by conventional grounding methods. Experiments have verified that the insertion loss can be reduced by 0.2dB, significantly improving the device transmission efficiency. Next, a semi-through-hole structure is formed by seamless splicing of the device panel and a green cutting process, and metal paste is filled inside the semi-through-hole structure to achieve vertical direct connection between the top and bottom ports and the inner layer RF traces. The semi-through-hole vertical interconnect structure overcomes the technical pain point of poor contact between the sidewall metal and the inner layer RF traces in traditional ball milling processes. Structurally, it ensures the integrity and stability of the connection between the top and bottom surfaces and the inner layer conductors, significantly improving device reliability and increasing product yield from 75% in traditional processes to over 95%. Finally, the dielectric layer is designed with differentiated thicknesses according to functional zones. The vertical distance between the green ceramic tapes after sintering between coupling layers is 45μm, while the vertical distance between the remaining single-layer green ceramic tapes is 90μm. This design balances coupling efficiency and structural stability. The 45μm coupling interval shortens the signal coupling distance and improves coupling effect, while the 90μm structural layer ensures the load-bearing capacity of the green ceramic tapes and the overall structural strength, ultimately achieving device miniaturization, adapting to the 0603 package, with a size of only 1.6mm × 0.8mm. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the overall structure of a high-reliability, low-insertion-loss balun based on LTCC technology; Figure 2 This is a schematic diagram of the top layer L1 of the first layer of raw ceramic belt; Figure 3 This is a schematic diagram of the first grounding layer L2; Figure 4 This is a schematic diagram of layer L3; Figure 5 This is a schematic diagram of the first coupling layer, L4; Figure 6 This is a schematic diagram of the second coupling layer, L5; Figure 7 This is a schematic diagram of the second grounding layer L6; Figure 8 This is a schematic diagram of the third coupling layer, L7; Figure 9This is a schematic diagram of the fourth coupling layer, L8; Figure 10 This is a schematic diagram of the L9 output terminal of the 4th layer; Figure 11 This is a schematic diagram of the third grounding layer L10; Figure 12 This is a schematic diagram of the underlying L10_BOT; Figure 13 This is a graph showing the S-parameters of a high-reliability, low-insertion-loss balun based on LTCC technology. Figure 14 This is a graph showing the insertion loss parameters of a high-reliability, low-insertion-loss balun based on LTCC technology. Figure 15 shows the output phase difference parameters of a high-reliability, low-insertion-loss balun based on LTCC technology. Figure 16 is a comparison of insertion losses of different apertures for a high-reliability, low-insertion-loss balun based on LTCC technology. Detailed Implementation

[0021] The present invention will be further described below with reference to the accompanying drawings: This invention discloses a high-reliability, low-insertion-loss balun based on LTCC technology. The balun is fabricated using LTCC multilayer ceramic technology. Through the collaborative design of internal unequal-width coupled RF traces (50μm linewidth difference), a hollow ground structure (the hollow area coincides with the projection of the coupled line), and a vertical interconnect structure with half-holes on the top and bottom surfaces, it achieves the design goals of miniaturization, high yield, and low insertion loss. The overall structure of the balun is composed of ceramic material and metal. The ceramic material consists of a dielectric layer composed of 10 layers of green ceramic tape, and the metal includes a metal structure on the surface of the dielectric layer and metal paste within the vias / half-holes. The metal paste is any one or a combination of gold, silver, and copper. The RF traces lead the RF ports to the surface through vertical half-holes, meeting bonding and mounting requirements, thus satisfying the application requirements of high reliability and low insertion loss.

[0022] like Figure 1 As shown, the high-reliability, low-insertion-loss balun based on LTCC technology includes: a dielectric layer, a metal structure, and a vertical interconnect structure.

[0023] As a further improvement to the above technical solution, the dielectric layer is prepared using LTCC (Low Temperature Co-fired Ceramic) multilayer ceramic process. The dielectric layer is an integrated structure formed by stacking and sintering 10 layers of green ceramic strips, namely L1 layer, L2 layer, L3 layer, L4 layer, L5 layer, L6 layer, L7 layer, L8 layer, L9 layer and L10 layer.

[0024] like Figures 1-12 As shown, Figure 2This is a schematic diagram of the top layer L1 of the first green ceramic tape of the high-reliability, low-insertion-loss balun based on LTCC technology. It shows the top RF port layer set on the upper surface of the L1 layer, including input port P1, no-load terminal P2, RF ground P3, RF ground P4, output port P5 and output port P6. The positions of the half-holes V1-V6 corresponding to each port are also marked. Figure 3 This is a schematic diagram of the first ground layer L2 of the high-reliability, low-insertion-loss balun based on LTCC technology. It shows the first ground layer g1 printed on the surface of the L2 layer. Its long strip-shaped cutout area coincides with the projection range of the corresponding coupling line. The cutout length is consistent with the effective coupling length of the coupling line, and the width is 10-20μm wider than the coupling line. Figure 4 This is a schematic diagram of the L3 layer of a high-reliability, low-insertion-loss balun based on LTCC technology, showing the structural form of the L3 layer as a transition layer connecting the first ground layer g1 and the upper coupling pair. Figure 5 This is a schematic diagram of the first coupling layer L4 of the high-reliability, low-insertion-loss balun based on the LTCC process. It shows the first coupling line 2 printed on the upper surface of the L4 layer, with a linewidth of W (75 μm), which is a component of the upper coupling pair. Figure 6 This is a schematic diagram of the second coupling layer L5, which is based on the high reliability and low insertion loss balun of LTCC technology. It shows the second coupling line 3 printed on the lower surface of the L5 layer. Its line width is W+50μm (125μm). It forms an upper layer coupling pair with the first coupling line 2 with unequal width. The line width difference cancels the ±25μm deviation of LTCC stacking. Figure 7 This is a schematic diagram of the second ground layer L6 of the high-reliability, low-insertion-loss balun based on LTCC technology. It shows the second ground layer g2 printed on the surface of the L6 layer. This layer is a complete and continuous ground structure, used to separate the upper coupling pair from the lower coupling pair and block signal crosstalk. Figure 8 This is a schematic diagram of the third coupling layer L7 of a high-reliability, low-insertion-loss balun based on LTCC technology, showing the third coupling line 4 printed on the upper surface of the L7 layer, which is a component of the lower coupling pair; Figure 9 This is a schematic diagram of the fourth coupling layer L8 of a high-reliability, low-insertion-loss balun based on LTCC technology. It shows the fourth coupling line 5 printed on the lower surface of the L8 layer, which forms a lower layer coupling pair with the third coupling line 4. The coupling area of ​​the two corresponds to the 45μm thick green ceramic tape between L7 and L8. Figure 10 This is a schematic diagram of the L9 output terminal of a high-reliability, low-insertion-loss balun based on LTCC technology, showing the structural form of the L9 layer, which serves as a transition layer connecting the lower coupling pairs and the underlying structure. Figure 11 This is a schematic diagram of the third ground layer L10 of a high-reliability, low-insertion-loss balun based on LTCC technology. It shows the third ground layer g3 printed on the surface of the L10 layer. Its hollow structure coincides with the projection range of the lower coupling line, forming a symmetrical shielding design with the first ground layer g1.Figure 12 This is a schematic diagram of the bottom layer L10_BOT of a high-reliability, low-insertion-loss balun based on LTCC technology. It shows the bottom RF port layer set on the lower surface of the L10 layer, including output port P7, output port P8, RF ground P9, RF ground P10, no-load terminal P11 and input port P12. Each port is vertically connected to the top RF port one-to-one through a half-hole.

[0025] As a further improvement to the above technical solution, the metal structure includes a surface metal layer and a filler metal.

[0026] Specifically, the surface metal layer includes an RF port layer, three RF ground layers, and four sets of coupling lines. The coupling lines employ an unequal width design, with the first and third RF ground layers having a hollow structure to ensure coupling stability and low loss. The filler metal comprises a metal paste filling through-holes or half-holes, wherein the metal paste is any one or a combination of gold, silver, and copper.

[0027] Furthermore, the radio frequency port layer includes a top radio frequency port layer disposed on the upper surface of the first green ceramic tape and a bottom radio frequency port layer disposed on the lower surface of the tenth green ceramic tape; as shown Figure 2 As shown, the top RF port layer includes six RF ports: input port P1, no-load port P2, RF ground P3, RF ground P4, output port P5, and output port P6. The bottom RF port layer includes six RF ports: output port P7, output port P8, RF ground P9, RF ground P10, no-load port P11, and input port P12. The six RF ports in the top RF port layer are connected one-to-one with the six RF ports in the bottom RF port layer through vertical half-holes, such as between top port 1 and bottom port P12. The remaining ports are connected in the same manner. This balun has four input / output ports, specifically one input port, one isolation port, and two output ports. The ports are interconnected; when one port is determined, the status of the other three ports is also determined.

[0028] Furthermore, the three RF ground layers are a first ground layer g1 printed on the surface of the second green ceramic tape, a second ground layer g2 printed on the surface of the sixth green ceramic tape, and a third ground layer g3 printed on the surface of the tenth green ceramic tape. The first ground layer g1 and the third ground layer g3 are both hollow ground structures with elongated hollow areas. The length of the hollow area is consistent with the effective coupling length of the corresponding coupling line, and the width is 10-20µm wider than the width of the adapted coupling line. The hollow area coincides with the projection range of the coupling line to avoid overlap between the grounding metal and the coupling line. The second ground layer g2 is a complete ground structure, used to separate the upper coupling pair from the lower coupling pair, blocking signal crosstalk.

[0029] To address the issue of high insertion loss in devices caused by using a fully grounded structure on both the upper and lower layers of conventional coupling lines, this invention offers an innovative optimization: a cleverly designed... Figure 3 The first grounding layer g1 and Figure 11 The hollow ground structure of the third grounding layer g3 replaces the traditional full-surface grounding design. Experimental results are as follows: Figure 16 As shown, this structure can reduce the insertion loss of the balun by 0.2 dB, effectively improving the transmission performance of the device.

[0030] Furthermore, the four sets of coupling lines are designated as first coupling line 2, second coupling line 3, third coupling line 4, and fourth coupling line 5. These four sets of coupling lines form a four-layer coupling structure: a first coupling layer, a second coupling layer, a third coupling layer, and a fourth coupling layer. The first coupling line 2 and the second coupling line 3 form an upper coupling pair, located between the first ground layer g1 and the second ground layer g2; the third coupling line 4 and the fourth coupling line 5 form a lower coupling pair, located between the second ground layer g2 and the third ground layer g3. These two coupling pairs, together with the three RF ground layers, form a hierarchical layout of first ground layer g1 - upper coupling pair - second ground layer g2 - lower coupling pair - third ground layer g3, collectively constituting a double-layer coupling structure.

[0031] To address the performance degradation of balun coupling caused by the traditional equal-width coupling line design, this invention provides targeted optimizations. Figure 6 The linewidth ratio of the intermediate coupling line is set to be greater than that of the coupling line. Figure 5 The linewidth W of the intermediate coupling line is 50 μm. That is, the linewidth of the second coupling line 3 is 50 μm larger than that of the first coupling line 1, and the linewidth of the third coupling line 4 is 50 μm larger than that of the fourth coupling line 5. This design can effectively ensure that the ±25 μm deviation that may occur in the LTCC stacking process remains consistent. Even if this deviation exists, the projected area of ​​the coupling lines can still be kept consistent, thereby ensuring that the coupling performance of the balun remains unchanged.

[0032] Furthermore, the total thickness of the green ceramic tapes between the first coupling line 2 and the second coupling line 3, and between the third coupling layer line and the fourth coupling line 5, is 45 μm after sintering (i.e., the thickness between the coupling regions of the fourth and fifth green ceramic tapes, and the thickness between the coupling regions of the seventh and eighth green ceramic tapes are all 45 μm). The thickness of the remaining eight green ceramic tapes (the non-coupling regions of the first to third, sixth, ninth to tenth, and the fifth and eighth layers) is 90 μm after sintering, in order to precisely adapt to the signal transmission requirements of the double-layer coupling structure.

[0033] As a further improvement to the above technical solution, the vertical interconnect structure includes half-holes V1 to V6, a first through-hole, and a second through-hole. Each half-hole and through-hole is filled with one or more metal pastes selected from gold, silver, and copper, forming a filler metal. This vertical interconnect structure and filler metal achieve a one-to-one vertical connection between the top and bottom RF ports, and also ensure stable electrical connections between the ports and coupling lines and RF ground. Structurally, this completely solves the contact problems that easily occur in traditional connection methods, significantly improving the reliability of device connections and the stability of signal transmission.

[0034] To address the weak connection between the sidewall metal and the inner RF metal in traditional devices, this invention ingeniously designs semi-through-hole structures V1~V6, ensuring a complete and stable connection between the top and bottom surfaces and the inner conductor. During manufacturing, adjacent devices are seamlessly spliced ​​together, and a uniform cut is made along the center of the through-hole during the green cutting process, transforming the originally spliced ​​complete through-hole into a semi-through-hole structure. This design structurally solves the defects of traditional connection methods, effectively ensuring the RF performance stability of the balun and significantly improving the overall reliability of the device.

[0035] During device panelization, it is essential to ensure that the half-holes between adjacent devices can be combined to form a complete through-hole, laying the foundation for subsequent processing. The green ceramic tape undergoes processes including slicing, drying, drilling, via filling (150µm diameter, filled with gold, silver, copper, or other metal pastes), printing, stacking, and isostatic pressing. Strict precision control is required throughout the process, with both printing and stacking precision needing to be within 10%. The device is cut into individual pieces along the center of the through-hole, then co-fired at low temperature to form individual products, and finally completed through electroplating. This invention, through the vertical direct connection design of the half-holes, combined with the high-precision control of the LTCC co-firing process (printing and stacking precision ≤10%), ensures the integrity of the contact between the bottom RF ports P7-P12 and the inner metal structure (coupled lines, RF ground), significantly improving connection reliability. Simultaneously, the design of the hollow ground and unequal-width coupled lines enables the device to achieve excellent electrical performance with full-band insertion loss <1.0dB and phase difference <6.5°.

[0036] The high-reliability, low-insertion-loss balun based on LTCC technology described in this invention has a signal transmission path that includes a top-level signal transmission path and a bottom-level signal transmission path, which together form a complete and synchronous transmission system.

[0037] 1. Top-level core signal transmission path Input port P1 is connected to the third coupling line 4 via half-hole V1. The third coupling line 4 is connected to the second coupling line 3 via the first through-hole. The second coupling line 3 is connected to the unloaded terminal P2 via half-hole V2. Simultaneously, the first coupling line 2 and the second coupling line 3 are coupled. RF ground P4 is connected to the first coupling line 2 via half-hole V4. The first coupling line 2 is connected to the output port P6 via half-hole V6. The third coupling line 4 is coupled to the fourth coupling line 5. RF ground P3 is connected to the fourth coupling line 5 via half-hole V3. The fourth coupling line 5 is connected to the output port P6 via half-hole V6, ultimately completing the balanced-to-unbalanced signal conversion and stable output. This signal transmission path belongs to a high-reliability, low-insertion-loss balun based on LTCC technology. It is the key link that enables the balanced-to-unbalanced signal conversion function, including the complete process of signal input, inner layer coupling, ground adaptation, and output. It is perfectly matched with the device's three-layer RF ground, four sets of coupling lines, and vertical interconnect structure of half-holes / through-holes.

[0038] 2. Lower-level extended signal transmission path (1) Output port P7 passes vertically through 10 layers of green ceramic tape via half hole V5 and is connected to the output port P5 of the top layer; output port P5 is connected to the upper layer coupler, and output port P7 indirectly receives the first balanced signal output by the upper layer coupler, becoming the output end of the bottom layer balanced signal.

[0039] (2) Output port P8 is vertically connected to the top layer output port P6 via half hole V6; output port P6 is connected to the signal path of the lower layer coupler, and output port P8 indirectly receives the second balanced signal output by the lower layer coupler, forming a bottom layer balanced output combination with output port P7 with equal amplitude and 180° phase difference.

[0040] (3) Input port P12 is vertically connected to the top layer input port P1 through half hole V1; input port P1 is directly connected to the third coupling line 4 through half hole V1 to start the signal conversion of the double-layer coupling pair; input port P12 is the bottom layer input port, used to receive external unbalanced signals and transmit them to the inner layer coupling line, and is the bottom layer starting point of the entire signal transmission link.

[0041] (4) The unloaded end P11 is vertically connected to the unloaded end P2 of the top layer through the half hole V2; the unloaded end P2 is connected to the second coupling line 3 through the through hole. Therefore, the unloaded end P11 and the unloaded end P2 synchronously realize the impedance matching function, suppress signal reflection, provide a stable load environment for the second coupling line 3, and ensure stable coupling performance.

[0042] (5) Radio frequency ground P9 is vertically connected to radio frequency ground P3 through half hole V3; radio frequency ground P3 is connected to the fourth coupling line 5 through half hole V3, and the projection range of radio frequency ground P9 coincides with the grounding area of ​​the third ground layer g3. Therefore, radio frequency ground P9 provides a bottom grounding path for the fourth coupling line 5 on the one hand, and forms a double grounding collaboration with the third ground layer g3 on the other hand, enhancing the electromagnetic shielding of the lower coupling pair.

[0043] (6) Radio frequency ground P10 is vertically connected to the top radio frequency ground P4 through half hole V4; radio frequency ground P4 is connected to the first coupling line 2 through half hole V4, and radio frequency ground P10 is connected to the non-cutout area of ​​the third ground layer g3, providing the bottom ground support for the first coupling line 2, and forming a shield structure with the first ground layer g1 and the second ground layer g2 to block inter-layer signal crosstalk.

[0044] In summary, after receiving the signal at input port P12, it is transmitted to input port P1 via half-hole V1, and then connected to the third coupling line 4 to initiate double-layer coupling. The coupled balanced signal first passes through output ports P5 and P6, and then through half-holes V5 and V6, to output ports P7 and P8, forming a complete signal chain of bottom layer input → inner layer coupling → bottom layer output, which is completely synchronized with the top layer signal path. This design not only meets the diverse needs of bonding and mounting, but also further improves the reliability and stability of signal transmission.

[0045] The balun described in this invention comprises a dielectric layer composed of multiple layers of ceramic material and semi-holes and through-holes within the dielectric layer. The surface of the dielectric layer is printed with metals such as gold, silver, and copper, and the semi-holes and through-holes are filled with these metals. All components are tightly bonded together using a low-temperature co-firing process. The RF lines are arranged vertically to form a coupling structure, with grounding layers on both the top and bottom sides of the coupling layer. The two RF lines of the coupling layer are designed with unequal widths, effectively improving manufacturing reliability. Simultaneously, the first and third grounding layers employ a hollow structure, significantly reducing device insertion loss. The connection between the top and bottom surfaces and the internal RF traces is achieved through the filled semi-hole structure; this design not only improves device reliability but also reduces insertion loss. The dual-layer coupling structure balun invented using the above technical solution, packaged in a 0603 package, has a size of only 1.6mm × 0.08mm, meeting the core requirement of miniaturization in microwave communication systems.

[0046] In terms of electrical performance, this balun performs excellently; experimental data shows that it performs well across the entire operating frequency band. Figure 13 This is a graph showing the S-parameters of a high-reliability, low-insertion-loss balun based on LTCC technology. It visually presents the return loss (S-parameters) performance within the 3.5-8GHz operating frequency band. The return loss across the entire frequency band is <-15dB, demonstrating good signal input-output matching and no abnormal attenuation. Figure 13As shown, the return loss is less than -15dB, which means that the signal reflection is minimal, the input and output are well matched, and the S-parameter simulation shows that the signal transmission at each port is stable and there is no abnormal attenuation. Figure 14 This is an insertion loss metric chart for a high-reliability, low-insertion-loss balun based on LTCC technology, showing insertion loss data across the entire operating frequency band, such as... Figure 14 As shown, the insertion loss is less than 1.0dB, and the highest insertion loss in the 7GHz band is only 0.96dB, which is far superior to the insertion loss level of 1.2-1.5dB of traditional baluns. Figure 15 This is a graph showing the output phase difference of a high-reliability, low-insertion-loss balun based on LTCC technology, illustrating the output phase difference distribution; for example... Figure 15 As shown, the output phase difference is between 173.8° and 178.2°, with a phase difference fluctuation of less than 6.5°, which is close to the ideal 180° phase difference, ensuring the accuracy of the balanced-to-unbalanced conversion. Figure 16 This is a comparison chart of insertion losses for high-reliability, low-insertion-loss baluns with different apertures based on LTCC technology, such as... Figure 16 As shown, by comparing the insertion loss of different cutout widths (0.1mm, 0.5mm, 0.9mm), it is verified that the cutout ground structure (corresponding to the projection area of ​​the coupling line) can reduce the insertion loss by 0.2dB, directly verifying the effectiveness of the design.

[0047] The core innovation of this invention is not a single technical feature, but a multi-structure synergistic combination based on the characteristics of LTCC process, specifically embodied in the following three main features: (1) Design of coupling lines for asymmetric compensation The first coupling line 2 and the second coupling line 3 form an upper-layer coupling pair. They are designed with unequal widths, with the linewidth of the second coupling line 3 being 50 μm wider than that of the first coupling line 2. This linewidth difference compensates for dimensional deviations in the LTCC lamination process, ensuring the stability of the projected area of ​​the coupling lines.

[0048] In traditional constant-width designs, random misalignments of ±25μm often occur during the LTCC stacking process, leading to variations in the coupling area between upper and lower layers, resulting in decreased coupling and phase / amplitude imbalance. To address this LTCC stacking misalignment issue, this invention uses a preset linewidth difference of 50μm. Even with a ±25μm lateral misalignment, the projection of the narrower line always falls entirely within the range of the wider line. This ensures a constant effective coupling area, thereby maintaining high stability in coupling performance and phase difference (<6.5°) during mass production.

[0049] (2) "Sandwich" type hybrid grounding structure Conventional full-ground structures result in excessive parasitic capacitance between the coupling line and ground, leading to severe signal energy loss and high insertion loss during transmission. To address high insertion loss and achieve a balance between low insertion loss and high isolation, this invention employs a layered design of a hollow ground layer + a complete ground layer + a hollow ground layer. The first ground layer g1 and the third ground layer g3 both use a hollow structure, with the hollow area coinciding with the projection of the coupling line and its width slightly larger than the coupling line width by 10-20 μm. The middle second ground layer g2 is a complete ground structure without any hollow areas, thus forming a symmetrical shielding structure that separates the upper and lower coupling pairs to block signal crosstalk. The hollow structure design of the first ground layer g1 and the third ground layer g3 reduces parasitic capacitance, lowering the insertion loss from 1.2 dB to below 1.0 dB. The complete ground structure of the middle second ground layer g2 ensures signal isolation between the upper and lower coupling pairs, preventing signal crosstalk.

[0050] (3) Vertical half-hole vertical interconnect structure based on green cutting process Traditional ball milling and sidewall printing processes rely on the adhesion of metal layers to achieve interconnection. If the inner metal layer is thin or the ball milling is incomplete, poor contact and circuit breaks are highly likely to occur. This invention abandons this traditional process. It forms semi-holes V1-V6 through green cutting, fills the semi-holes with metal paste, and utilizes the material tightening effect during the LTCC co-firing process to form a structural bond between the filler metal and the semi-hole wall, inner layer traces, and ports, rather than an adhesive connection. This directly achieves vertical interconnection between the top and bottom layer ports and the inner layer traces. This structural design not only improves the device yield to over 95%, but also significantly enhances the thermal shock resistance of the connection points, fundamentally solving the reliability defects of traditional processes.

[0051] From the perspective of the specific application scenario, this invention addresses the three core requirements of microwave communication systems: miniaturization of baluns, high reliability, and low insertion loss. Existing technologies struggle to simultaneously meet all three indicators. The three inherent problems of LTCC technology—overlapping deviation, grounding loss, and connection reliability—are interconnected, forming a technical bottleneck that cannot be fundamentally solved by a single technological improvement. The four innovative points of this invention are not isolated but rather constitute a systematic solution to the aforementioned bottlenecks.

[0052] First, the unequal width coupling line is not simply a change in line width. Instead, it is based on the inherent deviation of ±25μm in the LTCC process. A precise 50μm line width difference is designed, and the deviation is offset by a structure in which the wide line is projected to cover the narrow line, thus avoiding the impact of overlay errors on coupling performance from the source.

[0053] Secondly, the hollowed-out structure is not a simple hollowing-out process, but rather a strict matching of the effective coupling length and width of the coupling line. The width of the hollowed-out area is set to be 10-20μm wider than the coupling line. This design not only reduces signal energy loss through local hollowing-out, but also utilizes the non-hollowed-out area and the complete grounding layer in the middle to form a shield, avoiding electromagnetic leakage. This size parameter was determined through multiple simulation optimizations and is not an arbitrary hollowing-out, achieving a balance between low loss and anti-crosstalk.

[0054] Furthermore, the semi-through hole structure is not a variation of the conventional through hole, but rather a combination of mass production panelization process and seamless splicing and raw cutting process to form the connection structure and production process. This solves the inherent defects of traditional processes that rely on ball milling precision. It is a collaborative innovation of structural design and manufacturing process, and not a simple variation of the conventional through hole structure.

[0055] Three major technical biases have long existed in the industry: first, the belief that coupling lines must be of equal width to ensure coupling balance; second, the belief that the grounding layer must be fully covered to achieve effective shielding; and third, the belief that interlayer connections can only be achieved through ball milling sidewall metal or through-holes. This invention breaks these preconceived notions, achieving a key technological breakthrough through targeted innovative design. It abandons the equal-width coupling line design and uses line width difference compensation to reduce process deviations, decreasing phase difference fluctuations from ±10° in existing technologies to ±6.5°, achieving more stable coupling balance. This invention innovatively adopts a hybrid grounding structure of partial hollowing and overall grounding. This not only separates the upper and lower coupling pairs and shields interlayer crosstalk through the complete second grounding layer g2, but also reduces insertion loss through the partial hollowing design of the outer grounding layer, balancing shielding effectiveness with low insertion loss requirements. This invention breaks the traditional path dependence of interlayer connections, adopting a semi-through-hole direct connection method, transforming connection reliability from relying on ball milling accuracy to relying on cutting accuracy, improving control accuracy from ±50μm to ±10μm, and providing the industry with a completely new approach to interlayer connections. In existing technologies, coupling performance, insertion loss, and connection reliability are mutually restrictive and contradictory. Conventional techniques can only optimize one single indicator and cannot achieve a balance. However, this invention, through the organic combination of four innovative points, forms a closed loop of deviation compensation, loss control, reliable connection, and structural optimization, achieving an overall synergistic effect.

[0056] In summary, the balun described in this invention has the characteristics of small size, high reliability, and superior performance, and is suitable for mass production, thus having broad application prospects.

[0057] The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A high-reliability, low-insertion-loss balun based on LTCC technology, characterized in that, The balun includes: a dielectric layer, a metal structure distributed on or inside the dielectric layer, and a vertical interconnect structure penetrating the dielectric layer; The dielectric layer includes a multilayer dielectric substrate formed by stacking and sintering multiple layers of green ceramic tapes; The metal structure includes at least one pair of coupling wires and a grounding system; The coupling line group includes two coupling lines located on different layers and with different widths, wherein the projection range of the wider coupling line covers the projection range of the narrower coupling line. The grounding system includes an outer grounding layer located outside the coupling line group and an intermediate grounding layer located between a pair of coupling line groups; the outer grounding layer is a hollow ground structure, and its hollow area overlaps with the vertical projection of the corresponding coupling line; the intermediate grounding layer is a complete grounding structure used to isolate signals between upper and lower layers; The vertical interconnect structure includes a plurality of half-holes formed on the side of the multilayer dielectric substrate, and the half-holes are filled with conductive material.

2. The high-reliability, low-insertion-loss balun based on LTCC technology according to claim 1, characterized in that, The dielectric layer contains 10 layers of green ceramic tape; the number of coupling line groups is 2.

3. The high-reliability, low-insertion-loss balun based on LTCC technology according to claim 2, characterized in that, The vertical interconnect structure includes half-holes V1 to V6, a first through hole, and a second through hole; wherein each half-hole and through hole is filled with a conductive material, which is any one or more metal pastes selected from gold, silver, and copper, and the metal pastes filled in the half-holes or through holes form a filler metal.

4. The high-reliability, low-insertion-loss balun based on LTCC technology according to claim 3, characterized in that, The metal structure further includes a radio frequency (RF) port layer; the RF port layer includes a top RF port layer disposed on the upper surface of the first green ceramic tape and a bottom RF port layer disposed on the lower surface of the tenth green ceramic tape. The top RF port layer includes six RF ports: input port P1, no-load port P2, RF ground P3, RF ground P4, output port P5, and output port P6. The bottom RF port layer includes six RF ports: output port P7, output port P8, RF ground P9, RF ground P10, no-load terminal P11, and input port P12. The six RF ports in the top RF port layer are connected one-to-one with the six RF ports in the bottom RF port layer through vertical half-holes.

5. The high-reliability, low-insertion-loss balun based on LTCC technology according to claim 4, characterized in that, The outer grounding layer includes a first grounding layer g1 printed on the surface of the second green ceramic tape and a third grounding layer g3 printed on the surface of the tenth green ceramic tape; the intermediate grounding layer includes a second grounding layer g2 printed on the surface of the sixth green ceramic tape.

6. The high-reliability, low-insertion-loss balun based on LTCC technology according to claim 5, characterized in that, Both the first grounding layer g1 and the third grounding layer g3 are hollow ground structures. The hollow ground structure has a long strip-shaped hollow area. The length of the hollow area is consistent with the effective coupling length of the corresponding coupling line, and the width is 10-20um larger than the width of the adapted coupling line. The hollow area coincides with the projection range of the coupling line. The second grounding layer g2 is a complete grounding structure.

7. The high-reliability, low-insertion-loss balun based on LTCC technology according to claim 6, characterized in that, The two sets of coupling lines are an upper coupling pair and a lower coupling pair. The upper coupling pair includes a first coupling line (2) and a second coupling line (3). The lower coupling pair includes a third coupling line (4) and a fourth coupling line (5). The four coupling lines form a first coupling layer, a second coupling layer, a third coupling layer and a fourth coupling layer. The upper coupling pair is located between the first ground layer g1 and the second ground layer g2. The lower coupling pair is located between the second ground layer g2 and the third ground layer g3.

8. The high-reliability, low-insertion-loss balun based on LTCC technology according to claim 7, characterized in that, The line width of the second coupling line (3) is greater than that of the first coupling line (1); the line width of the fourth coupling line is greater than that of the third coupling line (4).

9. The high-reliability, low-insertion-loss balun based on LTCC technology according to claim 7, characterized in that, The total thickness of the green ceramic strips between the first coupling line (2) and the second coupling line (3), and between the third coupling layer line (4) and the fourth coupling line (5) after sintering is 45μm. That is, the sum of the thicknesses of the coupling regions of the fourth and fifth green ceramic strips, and the sum of the thicknesses of the coupling regions of the seventh and eighth green ceramic strips are all 45μm. The thickness of each of the remaining eight green ceramic strips after sintering is 90μm, that is, the non-coupling regions of the first to third, sixth, and ninth to tenth green ceramic strips and the fifth and eighth green ceramic strips.

10. The high-reliability, low-insertion-loss balun based on LTCC technology according to claim 7, characterized in that, The signal transmission path of this high-reliability, low-insertion-loss balun based on LTCC technology includes a top-level signal transmission path and a bottom-level signal transmission path. The core signal transmission path at the top level is as follows: Input port P1 is connected to the third coupling line (4) through half hole V1. The third coupling line (4) is connected to the second coupling line (3) through the first through hole. The second coupling line (3) is connected to the unloaded end P2 through half hole V2. At the same time, the first coupling line (2) and the second coupling line (3) are coupled. RF ground P4 is connected to the first coupling line (2) through half hole V4. The first coupling line (2) is connected to the output port P6 through half hole V6. The third coupling line (4) is coupled to the fourth coupling line (5). RF ground P3 is connected to the fourth coupling line (5) through half hole V3. The fourth coupling line (5) is connected to the output port P6 through half hole V6. Finally, the conversion of balanced to unbalanced signal and stable output are completed. The underlying extended signal transmission path is: (1) Output port P7 is vertically penetrated through 10 layers of green ceramic tape via half hole V5 and is connected to the output port P5 of the top layer; output port P5 is connected to the upper layer coupler, and output port P7 indirectly receives the first balanced signal output by the upper layer coupler and becomes the output terminal of the bottom layer balanced signal. (2) Output port P8 is vertically connected to the top layer output port P6 via half hole V6; output port P6 is connected to the signal path of the lower layer coupler, and output port P8 indirectly receives the second balanced signal output by the lower layer coupler, forming a bottom layer balanced output combination with output port P7 with equal amplitude and 180° phase difference. (3) Input port P12 is vertically connected to the top layer input port P1 through half hole V1; input port P1 is directly connected to the third coupling line (4) through half hole V1 to start the signal conversion of the double-layer coupling pair; input port P12 serves as the bottom layer input port, used to receive external unbalanced signals and transmit them to the inner layer coupling line, and is the bottom layer starting point of the entire signal transmission link. (4) The unloaded end P11 is vertically connected to the unloaded end P2 of the top layer through the half hole V2; the unloaded end P2 is connected to the second coupling line (3) through the through hole; (5) Radio frequency ground P9 is vertically connected to radio frequency ground P3 through half hole V3; radio frequency ground P3 is connected to the fourth coupling line (5) through half hole V3, and the projection range of radio frequency ground P9 coincides with the grounding area of ​​the third ground layer g3. (6) Radio frequency ground P10 is vertically connected to the top radio frequency ground P4 through half hole V4; radio frequency ground P4 is connected to the first coupling line (2) through half hole V4, and radio frequency ground P10 is connected to the non-hole area of ​​the third ground layer g3 to provide the bottom ground support for the first coupling line (2)2, and forms a shield structure with three grounding and double coupling with the first ground layer g1 and the second ground layer g2.

Images