A high-frequency analog transmission solution under ultra-low temperature

Abstract

The present application relates to printed circuit board manufacturing and signal transmission technical field, disclose a kind of high-frequency analog transmission solution under super-low temperature, including transmission soft board, adopt multilayer composite laminated structure, its with liquid crystal polymer core layer as core transmission medium;Resistance compensation composite layer is set by constantan foil layer and metal copper layer is formed, utilize the resistance stability of constantan under 5K low temperature provides distributed resistance and damping component, inhibit the signal nonlinear distortion caused by the decrease of conductor resistivity;Adopt the pure glue bonding layer of no substrate thermosetting to fill the gap of circuit and absorb interlaminar shear stress, prevent low-temperature delamination.In addition, it contains laser blind hole lead structure filled with solid metal copper, realize the electrical interconnection of inner layer circuit and outer layer and pressure contact resistance.This application solves the problems of signal distortion, structure delamination and interconnection failure under extremely low temperature environment through material property matching, etching compensation and controlled impedance design.

Description

Technical Field

[0001] This invention relates to the fields of printed circuit board manufacturing and signal transmission technology, specifically a solution for high-frequency analog transmission under ultra-low temperature conditions. Background Technology

[0002] With the development of quantum computing, radio frequency astronomical detection, and superconducting electronics, electronic systems need to transmit and process high-frequency analog signals in extremely low-temperature environments (such as the 5K temperature range) close to absolute zero. Transmission lines and interconnect structures that perform well under normal room temperature conditions face the dual challenges of electrical performance drift and mechanical failure when placed under such extreme temperature variations and low-temperature conditions.

[0003] In terms of electrical performance, conventional high-frequency transmission lines typically use copper as the conductor material. When the ambient temperature drops from room temperature to ultra-low temperatures around 5K, the lattice scattering effect of copper conductors weakens, and its residual resistivity decreases significantly. In analog signal transmission systems, this extremely low conductor loss leads to an abnormally high quality factor (Q value) in the transmission loop, resulting in poor flatness within the signal transmission passband and making it prone to parasitic oscillations or amplitude overshoot. This change in physical characteristics causes the system's signal transmission characteristics to deviate from the design operating point, inducing nonlinear problems such as carrier signal distortion and intermodulation distortion, making it difficult to meet the transmission requirements of high-fidelity analog signals.

[0004] In terms of mechanical reliability, transmission flexible circuit boards need to withstand drastic temperature changes from room temperature (approximately 300K) to ultra-low temperatures (5K). Due to the significant difference in the coefficients of thermal expansion between the metallic materials in the transmission line and the substrate insulating medium (such as liquid crystal polymers or polyimide), enormous interlaminar shear stresses are generated between different material layers during the cooling process. Existing multilayer board lamination systems typically use ordinary acrylic or modified epoxy adhesives, which are prone to embrittlement at extremely low temperatures and lack sufficient stress buffering capacity. This leads to delamination, microcracks, or blistering in the multilayer structure during cooling and shrinkage, compromising the insulation integrity of the circuit.

[0005] Furthermore, in terms of signal interconnection structure, multilayer transmission flexible circuit boards typically employ laser-guided blind via (LAB) technology to achieve high-density interconnection and signal lead-out. Traditional LAB processes often use conformal plating, resulting in hollow interiors or noticeable surface depressions in the blind vias. When such flexible circuit boards are used in scenarios requiring frequent testing or connection, the pogo pins of external connectors are directly pressed onto the blind via pads. Due to the lack of solid support beneath the blind vias, the pads are prone to collapse or deformation under high-frequency mechanical pressure, leading to unstable contact resistance or even open-circuit failure, thus compromising the long-term reliability of the signal transmission interface. Summary of the Invention

[0006] To achieve the above objectives, the present invention provides a solution for high-frequency analog transmission under cryogenic conditions, comprising the following steps:

[0007] Step S100, preparing the inner layer circuit pattern: Selecting the liquid crystal polymer core layer as the substrate material, machining positioning tool holes on the liquid crystal polymer core layer and performing pattern transfer, and using chemical etching to remove the uncovered copper foil to form the inner layer circuit; The fabrication process of the inner layer circuit introduces an etching compensation mechanism, pre-compensating the minimum line width of 0.09 mm to 0.11 mm to offset the etching side etching effect and meet the single-ended characteristic impedance matching requirements of high-frequency analog signal transmission;

[0008] Step S200: Perform a multilayer lamination process: On the surface of the liquid crystal polymer core layer with inner layer circuitry, an inner layer cover component, a pure adhesive layer, and a resistance compensation composite layer are sequentially laminated; the resistance compensation composite layer includes a constantan foil layer and a copper metal layer that are bonded together; the above layers are laminated and cured using a segmented lamination process.

[0009] Step S300, constructing an interlayer interconnect structure: using a laser drilling device to process laser blind holes, the laser blind holes sequentially penetrate the resistance compensation composite layer, the pure adhesive layer and the insulating medium of the inner layer covering component along the thickness direction until they connect to the inner layer circuit; then, through an electroplating filling process, metallic copper is deposited in the laser blind holes to achieve electrical conduction between the inner layer circuit and the outer resistance compensation composite layer;

[0010] Step S400: Fabricate the outer layer circuit and resistance compensation structure: Etch the outer surface of the resistance compensation composite layer to form the outer layer circuit pattern and blind via lead structure, and retain the constantan foil layer corresponding to the outer layer circuit pattern to construct the resistance compensation circuit structure.

[0011] Preferably, the resistance compensation composite layer adopts a bimetallic stacked structure;

[0012] The constantan foil layer is located between the pure adhesive layer and the metallic copper layer, and has a temperature coefficient of resistance of less than 40 × 10⁻⁻. 6 Made of / K copper-nickel alloy material;

[0013] In step S400, the constantan foil layer and the copper metal layer constitute a high-frequency composite conductor. The constantan foil layer utilizes its resistance stability at a low temperature of 5K to provide distributed resistance and damping components, thereby suppressing the nonlinear increase in signal amplitude caused by the decrease in resistivity of the copper metal layer.

[0014] Preferably, in step S200, the inner layer covering assembly includes a liquid crystal polymer film layer and an adhesive layer;

[0015] The adhesive layer of the inner layer cover assembly is disposed toward the inner layer circuitry;

[0016] The segmented pressing process includes a pre-pressing stage at 80 degrees Celsius and a main pressing stage at 190 degrees Celsius, which completely cures the pure adhesive layer and the adhesive layer of the inner covering component, achieving physical bonding and insulating encapsulation.

[0017] Preferably, the single-ended characteristic impedance of the inner layer circuit is matched to 50 ohms or 75 ohms by controlling the following parameters:

[0018] The parameters include the equivalent relative permittivity of the composite dielectric material between the inner layer circuit and the constantan foil layer, the sum of the thicknesses of the inner layer cover assembly and the pure adhesive layer, the actual conductive width of the inner layer circuit, and the thickness of the metal conductor of the inner layer circuit.

[0019] The sum of the thicknesses of the inner layer covering assembly and the pure adhesive layer constitutes the total thickness of the insulating medium between the inner layer circuit and the constantan foil layer, which serves as a reference plane.

[0020] Preferably, in step S300, the aperture of the laser blind hole is controlled to be 0.12 mm, and the laser ablation depth is controlled to expose the copper surface of the inner layer circuit without penetrating the substrate of the liquid crystal polymer core layer.

[0021] The electroplating filling process completely fills the laser blind hole with electroplated copper, forming a solid conductive copper pillar, and the top end face of the solid conductive copper pillar remains flush with the outer surface of the resistance compensation composite layer.

[0022] Preferably, in step S400, the etching process pre-compensates the minimum linewidth of the outer layer circuit pattern from 0.27 mm to 0.30 mm.

[0023] Preferably, the pure adhesive layer is a substrate-free thermosetting pure adhesive film. The pure adhesive layer fills the line gaps on the surface of the inner layer covering component and absorbs the interlayer shear stress caused by the mismatch of thermal expansion coefficients between the liquid crystal polymer core layer and the resistance compensation composite layer in a low-temperature environment.

[0024] Preferably, the single-ended characteristic impedance matching requirement of the inner layer circuit is determined according to the following logic: the single-ended characteristic impedance is inversely proportional to the square root of the sum of the equivalent relative permittivity and 1.41, and is directly proportional to the natural logarithm of a specific ratio; the specific ratio is the ratio of 5.98 times the total thickness of the insulating medium to 0.8 times the actual conductive width and the thickness of the metal conductor.

[0025] Preferably, the process also includes step S500, which involves surface treatment and molding testing: covering the surface of the board material after the circuit fabrication with a polyimide cover film and baking and curing it, and then performing chemical electroless nickel plating on the blind hole lead structure and the connector area, controlling the nickel layer thickness to be 80 to 120 microinches.

[0026] Preferably, a high-frequency analog transmission solution system for cryogenic environments, used to perform the above-described method, includes:

[0027] The transmission flexible board adopts a multi-layer composite stacked structure, which comprises, from the inside out:

[0028] The liquid crystal polymer core layer is made of liquid crystal polymer material and has an inner layer circuit formed on its surface; the inner layer cover assembly is bonded to the surface of the liquid crystal polymer core layer and is composed of a liquid crystal polymer film layer and an adhesive layer.

[0029] A pure adhesive layer is disposed on the outside of the inner layer covering component and is a substrate-free thermosetting pure adhesive film.

[0030] A resistance compensation composite layer is bonded to the outside of the pure adhesive layer, comprising a constantan foil layer and a copper metal layer bonded together, wherein the constantan foil layer is located between the pure adhesive layer and the copper metal layer;

[0031] The interlayer interconnect structure includes a laser blind via that passes sequentially through the resistance compensation composite layer, the pure adhesive layer, and the inner layer cover assembly along the thickness direction. The laser blind via is filled with metallic copper to achieve electrical conduction between the resistance compensation composite layer and the inner layer circuitry.

[0032] This invention provides a solution for high-frequency analog transmission at ultra-low temperatures. It offers the following advantages:

[0033] (1) This invention solves the signal distortion problem caused by the sharp drop in conductor resistivity under ultra-low temperature conditions by setting a resistance compensation composite layer composed of constantan foil layer and copper layer on the surface of the transmission line. Utilizing the low temperature coefficient of resistance of constantan material, it provides stable distributed resistance and damping components for the transmission line under low temperature of 5K and high frequency skin effect, effectively suppressing the abnormal rise of the transmission circuit quality factor (Q value); this mechanism stabilizes the linear operating range of the system and ensures that the carrier signal distortion (CSO) and intermodulation distortion (CTB) indicators of high frequency analog signals under ultra-low temperature conditions meet the stringent transmission specifications.

[0034] (2) The present invention adopts an inner core board structure wrapped with liquid crystal polymer (LCP) dielectric and combined with a substrate-free thermosetting pure adhesive layer, which takes into account both excellent high-frequency transmission performance and low-temperature mechanical reliability. The low dielectric loss characteristics of LCP material significantly reduce the transmission attenuation and dielectric loss of high-frequency signals, while the substrate-free pure adhesive layer completely fills the circuit gaps during the pressing process and acts as a flexible buffer medium to absorb the interlayer shear stress caused by the difference in thermal expansion coefficients between the core layer and the outer metal layer during cycling in a wide temperature range of 300K to 5K, effectively preventing the multilayer board from delamination or blistering failure in extremely low temperature environments.

[0035] (3) This invention ensures the electrical interconnection stability and mechanical strength between the high-density inner layer circuitry and the external interface by constructing a laser blind via lead structure filled with solid copper. This structure utilizes laser ablation to precisely penetrate multiple layers of dielectric material down to the inner copper surface, and eliminates the hole depression through an electroplating filling process, forming a rigid conductive copper pillar flush with the surface. This design not only provides a low-impedance vertical interconnection channel, but also effectively withstands the mechanical crimping force of the external connector pins, avoiding the risk of unstable contact resistance or open circuit caused by pad collapse. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the stacked structure of the present invention;

[0037] Figure 2 This is a partial circuit diagram of the present invention;

[0038] Figure 3 This is an enlarged schematic diagram of a portion of the circuitry of the present invention;

[0039] Figure 4 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] See attached document Figure 1 To be continued Figure 4 , Figure 1 This is a schematic diagram of a stacked structure according to an embodiment of the present invention. Figure 2 This is a partial circuit diagram according to an embodiment of the present invention. Figure 3 This is a partial enlarged circuit diagram according to an embodiment of the present invention. Figure 4This is a schematic diagram of the entire method flow of the present invention. The present invention provides a high-frequency analog transmission solution system under ultra-low temperature conditions, including a transmission flexible board. The transmission flexible board adopts a multi-layer composite stacked structure and is used to transmit analog signals in a 5K low temperature environment.

[0042] The multilayer composite stacked structure is laminated sequentially from the inside out. The multilayer composite stacked structure includes a liquid crystal polymer core layer located in the center. This liquid crystal polymer core layer is made of liquid crystal polymer (LCP) material, forming the sixth layer structure, and serves as the core medium carrier for high-frequency analog signal transmission.

[0043] The upper and lower surfaces of the liquid crystal polymer core layer are respectively bonded with inner layer cover components. The inner layer cover components are specifically selected from liquid crystal polymer (LCP) cover films, including stacked LCP film layers and adhesive layers, which constitute the 5th and 7th layer structures, respectively, to maintain the low dielectric loss characteristics of the inner layer dielectric environment.

[0044] The outer side of the inner layer covering assembly is bonded with a resistance compensation composite layer via a pure adhesive layer. The resistance compensation composite layer comprises a constantan foil layer and a copper layer bonded together. The constantan foil layer is located between the pure adhesive layer and the copper layer. The constantan foil layer is made of a copper-nickel alloy material with a low temperature coefficient of resistance, used to suppress resistivity drift at low temperatures.

[0045] The outer surface of the resistance compensation composite layer is covered with an outer insulating protection component. Specifically, the outer insulating protection component is a polyimide (PI) cover film, which includes a layered polyimide film layer and an outer adhesive layer, forming the outermost physical protection structure of the transmission flexible board.

[0046] The multilayer composite stacked structure contains an interlayer interconnect structure. The interlayer interconnect structure includes laser-guided blind vias and mechanical through-holes. The laser-guided blind vias sequentially penetrate the resistance compensation composite layer, the pure adhesive layer, and the inner layer cover assembly along the thickness direction until the inner layer circuitry on the liquid crystal polymer core layer is exposed, thereby electrically connecting the copper layer and constantan foil layer to the inner layer circuitry.

[0047] The signal output area of ​​the transmission flexible board is provided with a blind via lead structure. The blind via lead structure is formed in the circuit pattern composed of the resistance compensation composite layer and the metal copper layer, and the inner layer signal is led out through the blind via array, forming contact pads for bearing the pressure of the connector pins.

[0048] The circuit pattern formed by the resistance compensation composite layer and the copper metal layer has etched compensated line width and line spacing to meet the single-ended impedance matching requirements of 50 ohms or 75 ohms.

[0049] This invention also provides a solution for high-frequency analog transmission at ultra-low temperatures, comprising the following steps:

[0050] Step S100: Prepare the inner layer circuit pattern. A liquid crystal polymer core layer is selected as the substrate material, and positioning tool holes are machined on the liquid crystal polymer core layer using mechanical drilling. Photosensitive circuit patterns are transferred to the surface of the liquid crystal polymer core layer using direct laser imaging technology. Copper foil not covered by the resist layer on the surface of the liquid crystal polymer core layer is removed using a chemical etching solution to form the inner layer circuit. An etching compensation mechanism is introduced in the fabrication process of the inner layer circuit to compensate the designed minimum linewidth of 0.09 mm to 0.11 mm to meet the impedance matching requirements of high-frequency analog signal transmission.

[0051] Step S200: Perform a multilayer lamination process. On the upper and lower surfaces of the liquid crystal polymer core layer with inner layer circuitry, an inner layer cover assembly, a pure adhesive layer, and a resistance compensation composite layer are sequentially stacked according to a predetermined layer sequence. The resistance compensation composite layer comprises a pre-laminated constantan foil layer and a copper layer. The above-mentioned laminated assembly is pressed using a segmented pressing process in a vacuum hot-pressing device. The segmented pressing process includes a pre-pressing stage at 80 degrees Celsius and a main pressing stage at 190 degrees Celsius, ensuring complete curing of the adhesive in the pure adhesive layer and the inner layer cover assembly, achieving physical bonding and insulating encapsulation between the functional layers.

[0052] Step S300: Constructing the interlayer interconnect structure. Laser blind vias are fabricated at specific locations on the laminated board using a laser drilling machine. The diameter of the laser blind vias is controlled to 0.12 mm, and the depth is controlled to sequentially ablate the insulating medium of the resistance compensation composite layer, the pure adhesive layer, and the inner layer cover component, precisely exposing the copper surface of the inner layer circuitry without penetrating the substrate of the liquid crystal polymer core layer. Mechanical through-holes are then formed using a mechanical drilling process. The hole walls are subjected to plasma cleaning and adhesive residue removal treatment, followed by a patterned electroplating filling process. Conductive copper is deposited in the laser blind vias and mechanical through-holes, completely filling the laser blind vias to achieve electrical conductivity between the inner layer circuitry and the outer resistance compensation composite layer.

[0053] Step S400: Fabricate the outer layer circuitry and resistance compensation structure. An outer layer pattern transfer and etching process is performed on the outer surface of the resistance compensation composite layer. This step removes excess copper and constantan foil layers through etching, forming the outer layer circuitry and blind via lead structure. The etching process compensates the minimum outer layer linewidth from 0.27 mm to 0.30 mm, while retaining constantan foil layers in specific areas to construct a resistance compensation circuitry structure for low-temperature environments.

[0054] Step S500: Perform surface treatment and molding testing. Cover the surface of the board with an outer insulating protective component after the circuit fabrication is completed, and bake and cure at 160 degrees Celsius. Perform chemical nickel plating on the blind via lead structure and connector area, controlling the nickel layer thickness to 80 to 120 microinches to form an anti-oxidation coating. Use a flying probe tester to perform full-network electrical continuity testing on the transmission flexible board. Use an ultraviolet laser cutting device to cut and process according to the preset shape data, and clean the carbides on the cut surface to obtain the finished product high-frequency analog transmission flexible board under ultra-low temperature conditions.

[0055] Further, refer to the appendix. Figure 1 , Figure 1 This is a schematic diagram of the stacked structure parameters according to an embodiment of the present invention. The high-frequency analog transmission flexible circuit board for cryogenic environments provided by the present invention adopts a symmetrical material distribution in the vertical direction, wherein the core transmission region is composed of a liquid crystal polymer core layer and an inner layer covering component, and is used to construct a high-frequency signal transmission channel in a 5K cryogenic environment.

[0056] The liquid crystal polymer core layer is located in the middle layer of the stacked structure. The liquid crystal polymer core layer uses a double-sided copper-clad liquid crystal polymer (LCP) board as the substrate, and its insulating dielectric layer has a thickness of 50 micrometers. The liquid crystal polymer material has extremely low moisture absorption and stable dielectric constant (Dk) and dielectric loss tangent (Df) at low temperatures, thereby ensuring the structural dimensional stability and impedance consistency of the signal transmission layer under ultra-low temperature conditions.

[0057] An inner copper foil circuit is formed on the surface of the liquid crystal polymer core layer, and the base copper thickness of the inner copper foil circuit is 18 micrometers. This inner copper foil circuit undergoes a high-precision etching process to form conductive traces for transmitting high-frequency analog signals. The inner copper foil circuit is formed based on the original copper foil of the LCP copper-clad laminate, utilizing the low-loss characteristics of the LCP material to reduce skin effect loss and dielectric loss during high-frequency signal transmission.

[0058] The inner cover layer assembly is laminated and bonded to the outer surface of the liquid crystal polymer core layer and the inner copper foil circuitry. Specifically, the inner cover layer assembly is an LCP cover layer, consisting of a 12.5-micrometer-thick liquid crystal polymer film and a 25-micrometer-thick inner layer adhesive layer. The inner layer adhesive layer faces the inner copper foil circuitry, filling the gaps between the circuits and providing interlayer bonding after lamination. LCP material is chosen as the cover layer instead of traditional polyimide (PI) to ensure the consistency of the dielectric constant in the core transmission region and prevent signal reflection caused by heterogeneous material interface effects.

[0059] The liquid crystal polymer core layer and the inner layer cover assembly together form an inner core board structure fully encapsulated by LCP dielectric. This full LCP dielectric structure is connected to the outer resistance compensation composite layer via a pure adhesive layer. The thickness of the pure adhesive layer is controlled at 25 micrometers, serving as stress buffer and interlayer insulation, physically isolating the core high-frequency transmission functional area from the outer thermoelectric compensation functional area, ensuring the integrity of the inner layer's analog signal transmission.

[0060] The resistance compensation composite layer of this invention is respectively disposed in the top and bottom regions of the multilayer composite stacked structure and laminated to the outside of the pure adhesive layer. This resistance compensation composite layer adopts a bimetallic stacked structure, consisting of an inner constantan foil layer and an outer copper layer tightly bonded together.

[0061] The constantan foil layer is directly bonded to the surface of the pure adhesive layer, serving as the main material for resistance compensation. The constantan foil layer is selected with a temperature coefficient of resistance (TCR) of less than [value missing]. The copper-nickel alloy foil has a thickness of 18 micrometers and a resistivity of approximately At a low temperature of 5K, the phonon scattering effect of conventional copper conductors weakens, leading to a significant decrease in resistivity, which may cause excessively low transmission line loss and abnormally high signal levels. The constantan foil layer utilizes its low-temperature resistance stability to provide the basic ohmic loss and real impedance components for the transmission line.

[0062] The copper layer is applied to the outer surface of the constantan foil layer using a patterned electroplating process, with a thickness controlled to 10 micrometers. The copper layer and the constantan foil layer constitute a high-frequency composite conductor. At room temperature, the current is primarily distributed in the low-resistivity copper layer. During ultra-low temperature high-frequency transmission, due to the skin effect, the high-frequency signal current is redistributed between the copper layer and the constantan foil layer. At this time, the high resistivity of the constantan foil layer effectively suppresses the nonlinear increase in signal amplitude caused by the sudden drop in copper layer resistivity, maintaining the linearity of the system level.

[0063] The aforementioned resistance compensation composite layer constitutes a low-temperature resistance compensation network (i.e., the aforementioned overcurrent insulation structure) in terms of circuit function. This structure stabilizes the transmission attenuation characteristics of high-frequency analog signals by introducing thermoelectrically stable constantan material into the signal return path or impedance matching network. By arranging the resistance compensation composite layer on the top and bottom layers, the system can improve carrier signal distortion (CSO) and intermodulation distortion (CTB) indicators under low-temperature conditions, prevent parasitic oscillations or intermodulation interference caused by excessively high conductor Q values ​​at low temperatures, and ensure the electrical performance consistency of the transmission flexible circuit board within the temperature range of 300K to 5K.

[0064] The high-frequency analog transmission flexible board for ultra-low temperatures provided by this invention has a matching insulation and bonding system on the outermost layer of the structure and the interlayer bonding interface to ensure the mechanical integrity and electrical insulation of the multilayer material in a wide temperature range of 5K to 300K.

[0065] The outer insulating protection components symmetrically cover the outermost surface of the resistance compensation composite layer, forming the first and eleventh layers of the multilayer composite stacked structure. The outer insulating protection components are made of polyimide (PI) cover film, which is composed of a 25-micron-thick polyimide film layer and a 25-micron-thick outer adhesive layer. The outer adhesive layer is directly bonded to the circuit surface formed by the copper layer and constantan foil layer, filling the height difference caused by the circuit conductors after high-temperature pressing. Polyimide material has excellent wear resistance, mechanical strength, and solder resistance, serving as the outermost protective structure of the transmission flexible circuit board to isolate external physical damage and environmental moisture.

[0066] The pure adhesive layer is located between the inner cover assembly and the resistance compensation composite layer, forming the fourth and eighth layers of the multilayer composite stacked structure. The pure adhesive layer is a substrate-free thermosetting adhesive film with a thickness of 25 micrometers. During the lamination process, the pure adhesive layer acts as the sole bonding medium, firmly bonding the inner core structure of the all-LCP dielectric to the outer metal resistance compensation structure.

[0067] The pure adhesive layer exhibits compressive fluidity. During the lamination process of the multilayer board, the pure adhesive layer softens and flows upon heating to completely fill the uneven gaps on the surface of the inner layer covering components caused by the circuit distribution, achieving bubble-free interlayer lamination. Furthermore, the pure adhesive layer serves as a flexible buffer layer, absorbing the interlayer shear stress caused by the mismatch in the coefficient of thermal expansion (CTE) between the liquid crystal polymer core layer and the outer constantan / copper composite material during low-temperature shrinkage, preventing delamination failure of the flexible transmission board in extremely low-temperature environments.

[0068] See attached document Figure 2 and appendix Figure 3 , Figure 2 This is a partial circuit diagram according to an embodiment of the present invention. Figure 3This is a partial enlarged schematic diagram of the circuitry according to an embodiment of the present invention. The high-frequency analog transmission flexible circuit board for cryogenic applications provided by the present invention employs a controlled impedance design, wherein the inner layer circuitry located on the liquid crystal polymer core layer constitutes the main path for high-frequency signal transmission. The inner layer circuitry references the resistance compensation composite layer of the outer layer to form a stripline or embedded microstrip line structure. The stability of its characteristic impedance value is jointly determined by the circuit geometry, the total thickness of the composite dielectric layer, and the dielectric constant of the material.

[0069] In the vertical structure, the inner layer circuitry and the constantan foil layer serving as a reference plane are separated by an inner layer cover assembly and a pure adhesive layer. To ensure minimal transmission reflection loss of analog signals at a low temperature of 5K, the single-ended characteristic impedance of the inner layer circuitry is... Control is performed according to the following formula:

[0070]

[0071] in:

[0072] The characteristic impedance of a single end of a signal line ( The design target value is 50 ohms or 75 ohms.

[0073] The equivalent relative permittivity of the composite dielectric material between the signal line and the reference plane. This parameter is determined by the dielectric properties of the liquid crystal polymer material, the adhesive, and the pure adhesive layer in the inner cover assembly.

[0074] The total thickness (mm) of the insulating medium between the signal line layer and the reference plane (constantan foil layer). This thickness is equal to the sum of the thickness of the inner cover assembly and the thickness of the pure adhesive layer.

[0075] Actual conductive width of the signal line (mm).

[0076] : Thickness of the metal conductor of the signal line (mm).

[0077] In the manufacturing process, to counteract the side etching effect during chemical etching, the linewidth is adjusted. To mitigate the reduction effect, this invention implements linewidth pre-compensation. (See attached diagram.) Figure 3The densely packed circuit area shown is designed with a minimum linewidth of 0.09 mm. During the photoplotting film fabrication stage, the linewidth of the circuit pattern is pre-compensated to 0.11 mm. This 0.02 mm compensation margin ensures that the effective bottom width of the etched circuit meets the design requirements of the impedance model, thereby guaranteeing the signal integrity of the finished product in ultra-low temperature environments.

[0078] The high-frequency analog transmission flexible circuit board for ultra-low temperatures provided by this invention suppresses nonlinear signal distortion caused by the drift of conductor physical parameters under low-temperature conditions by matching the material properties of the resistance compensation composite layer.

[0079] At an ultra-low temperature of 5K, the resistance of a metallic conductor changes with temperature according to the temperature resistance characteristic formula. To quantitatively analyze the impedance behavior of a conductor in different temperature ranges, this invention constructs a model based on the following formula:

[0080]

[0081] The physical meanings of each symbol are as follows:

[0082] : Indicates the current operating temperature of the metallic conductor. Actual resistance value (unit: ohms) ).

[0083] : Represents the resistance value of a metallic conductor at a reference temperature (unit: ohms) ).

[0084] : Represents the real-time operating temperature of the environment (unit: Kelvin K). In this embodiment, The range of variation covers from room temperature (approximately 293 K) to ultra-low temperature (5 K).

[0085] : Indicates the reference temperature, which is usually set to room temperature 293K.

[0086] : Represents the temperature coefficient of resistance of a metallic material (unit: ℃). This parameter reflects the sensitivity of a material's resistivity to temperature changes.

[0087] For conventional copper layers in a multilayer structure, For a relatively large positive value (approximately When the operating temperature When the voltage is reduced to 5K, the residual resistivity of the copper layer decreases to an extremely low level. In low-loss, high-frequency transmission systems, the conductor resistance... Excessive reduction in Q value can lead to an abnormally high quality factor (Q value) in the transmission loop, which can easily cause resonance or amplitude overshoot in the signal during transmission, and thus cause the system level reference point to drift.

[0088] This level drift causes a nonlinear change in the transfer function of the analog signal transmission system. The relationship between the system's output signal and input signal can be expressed using a Taylor series expansion:

[0089]

[0090] The physical meanings of each symbol are as follows:

[0091] : Represents the analog output voltage signal of the transmission system.

[0092] : Represents the analog input voltage signal of the transmission system.

[0093] : Represents the linear gain coefficient of the system, which is the main component of ideal transmission.

[0094] : Represents the second-order nonlinear coefficient. The stability of this coefficient directly determines the carrier signal distortion (CSO, CompositeSecondOrder) index.

[0095] : Represents the third-order nonlinear coefficient. The stability of this coefficient directly determines the intermodulation distortion (CTB, Composite Triple Beat) index.

[0096] When the conductor resistance When an uncontrolled, large drop occurs at low temperatures, fluctuations in the real part of the system impedance will cause the coefficient to... and Unstable changes occur, resulting in significant harmonic distortion and intermodulation interference.

[0097] To address the aforementioned problems, this invention introduces a constantan foil layer. The constantan foil layer is made of a copper-nickel alloy and has an extremely low temperature coefficient of resistance (TCR). This means that in Under certain conditions, the resistivity of the constantan foil layer It can remain relatively stable and does not decay by orders of magnitude as the temperature decreases.

[0098] The constantan foil layer and the metallic copper layer are tightly bonded together, forming a high-frequency composite conductor structure electrically. During ultra-low temperature high-frequency transmission, due to the skin effect, the high-frequency signal current is distributed at the interface between the metallic copper layer and the constantan foil layer. At this time, the constantan foil layer provides distributed resistance and damping components by utilizing its resistive stability at a low temperature of 5K. This damping effect limits the excessive rise of the loop Q value and stabilizes the equivalent high-frequency surface resistance of the transmission line, thereby keeping the signal transmission system within a linear coefficient range. Within the dominant linear working region. By suppressing and The nonlinear fluctuations of the coefficients are compensated by this mechanism, which ensures that the transmission PCB maintains the integrity of the analog waveform in the extreme 5K environment, meeting the requirements of CSO and CTB.

[0099] Furthermore, the high-frequency analog transmission flexible board under ultra-low temperature conditions provided by the present invention has a blind via lead structure in the signal output terminal and interlayer connection area, which is used to realize the electrical transition and mechanical transfer between the inner high-density circuit and the external connection interface.

[0100] The blind via lead structure is constructed using laser-guided blind vias. These laser-guided blind vias penetrate vertically through the outer resistance compensation composite layer, the pure adhesive layer, and the insulating dielectric layer of the inner covering component, until they connect to the inner copper foil circuitry on the surface of the liquid crystal polymer core layer. The forming aperture parameter of the laser-guided blind via is set to 0.12 mm. During manufacturing, this aperture size, combined with laser energy control, ensures that the laser ablation depth precisely stops at the surface of the inner copper foil layer without penetrating the substrate of the liquid crystal polymer core layer.

[0101] The laser-guided blind via is filled with a conductive dielectric. Through a via-filling plating process, electroplated copper completely fills the internal cavity of the laser-guided blind via, forming a solid conductive copper pillar without voids. The top end face of the conductive copper pillar is flush with the outer surface of the resistance compensation composite layer. This capped structure eliminates the hole opening depressions common in traditional blind via processes, ensuring the flatness of the outer layer circuit pads.

[0102] The resistance compensation composite layer has contact pads formed at the positions corresponding to the laser blind vias. The contact pads are composed of stacked copper and constantan foil layers, forming an integrated metallurgical bond with the conductive copper pillars within the laser blind vias. The high-frequency analog signals transmitted by the inner layer circuitry are vertically transmitted to the outer layer contact pads through the laser blind vias with low impedance.

[0103] The blind via lead structure can withstand the mechanical crimping of external connector pogo pins. Due to its solid copper fill design, the contact pads are supported by a rigid metal structure. When external pogo pins apply vertical pressure for signal testing or connection, the blind via lead structure effectively resists localized mechanical deformation, preventing unstable contact resistance or open-circuit failure caused by pad collapse. Simultaneously, the flat pad surface increases the effective contact area between the pogo pin and the pad, reducing contact impedance.

[0104] like Figure 2 As shown, the inner layer circuitry exhibits a fan-out distribution in the signal output region. The blind via lead structure serves as the terminal node of the fan-out circuitry, converting the fine-linewidth signal path located in the inner layer into a connection point in the outer layer. The outer layer circuitry on the resistance-compensated composite layer is vertically interconnected with the inner layer circuitry through the blind via lead structure, thereby achieving the physical extraction of high-frequency signals while meeting 50-ohm or 75-ohm impedance matching requirements.

[0105] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A solution for high-frequency analog transmission under ultra-low temperature conditions, characterized in that, Includes the following steps: Step S100, preparing the inner layer circuit pattern: Selecting the liquid crystal polymer core layer as the substrate material, machining positioning tool holes on the liquid crystal polymer core layer and performing pattern transfer, and using chemical etching to remove the uncovered copper foil to form the inner layer circuit; The fabrication process of the inner layer circuit introduces an etching compensation mechanism, pre-compensating the minimum line width of 0.09 mm to 0.11 mm to offset the etching side etching effect and meet the single-ended characteristic impedance matching requirements of high-frequency analog signal transmission; Step S200: Perform a multilayer lamination process: On the surface of the liquid crystal polymer core layer with inner layer circuitry, an inner layer cover component, a pure adhesive layer, and a resistance compensation composite layer are sequentially laminated; the resistance compensation composite layer includes a constantan foil layer and a copper metal layer that are bonded together; the above layers are laminated and cured using a segmented lamination process. Step S300, constructing an interlayer interconnect structure: using a laser drilling device to process laser blind holes, the laser blind holes sequentially penetrate the resistance compensation composite layer, the pure adhesive layer and the insulating medium of the inner layer covering component along the thickness direction until they connect to the inner layer circuit; then, through an electroplating filling process, metallic copper is deposited in the laser blind holes to achieve electrical conduction between the inner layer circuit and the outer resistance compensation composite layer; Step S400: Fabricate the outer layer circuit and resistance compensation structure: Etch the outer surface of the resistance compensation composite layer to form the outer layer circuit pattern and blind via lead structure, and retain the constantan foil layer corresponding to the outer layer circuit pattern to construct the resistance compensation circuit structure.

2. The method for high-frequency analog transmission under ultra-low temperature conditions according to claim 1, characterized in that: The resistance compensation composite layer adopts a bimetallic laminate structure; The constantan foil layer is located between the pure adhesive layer and the metallic copper layer, and has a temperature coefficient of resistance of less than 40 × 10⁻⁻. 6 Made of / K copper-nickel alloy material; In step S400, the constantan foil layer and the copper metal layer constitute a high-frequency composite conductor. The constantan foil layer utilizes its resistance stability at a low temperature of 5K to provide distributed resistance and damping components, thereby suppressing the nonlinear increase in signal amplitude caused by the decrease in resistivity of the copper metal layer.

3. The solution for high-frequency analog transmission under ultra-low temperature conditions according to claim 1, characterized in that: In step S200, the inner layer covering assembly includes a liquid crystal polymer film layer and an adhesive layer; The adhesive layer of the inner layer cover assembly is disposed toward the inner layer circuitry; The segmented pressing process includes a pre-pressing stage at 80 degrees Celsius and a main pressing stage at 190 degrees Celsius, which completely cures the pure adhesive layer and the adhesive layer of the inner covering component, achieving physical bonding and insulating encapsulation.

4. The solution for high-frequency analog transmission under ultra-low temperature conditions according to claim 1, characterized in that: The single-ended characteristic impedance of the inner layer circuitry is matched to 50 ohms or 75 ohms by controlling the following parameters: The parameters include the equivalent relative permittivity of the composite dielectric material between the inner layer circuit and the constantan foil layer, the sum of the thicknesses of the inner layer cover assembly and the pure adhesive layer, the actual conductive width of the inner layer circuit, and the thickness of the metal conductor of the inner layer circuit. The sum of the thicknesses of the inner layer covering assembly and the pure adhesive layer constitutes the total thickness of the insulating medium between the inner layer circuit and the constantan foil layer, which serves as a reference plane.

5. The solution for high-frequency analog transmission under ultra-low temperature conditions according to claim 1, characterized in that: In step S300, the aperture of the laser blind hole is controlled to be 0.12 mm, and the laser ablation depth is controlled to expose the copper surface of the inner layer circuit without penetrating the substrate of the liquid crystal polymer core layer. The electroplating filling process completely fills the laser blind hole with electroplated copper, forming a solid conductive copper pillar, and the top end face of the solid conductive copper pillar remains flush with the outer surface of the resistance compensation composite layer.

6. The solution for high-frequency analog transmission under ultra-low temperature conditions according to claim 1, characterized in that: In step S400, the etching process pre-compensates the minimum linewidth of the outer layer circuit pattern from 0.27 mm to 0.30 mm.

7. The solution for high-frequency analog transmission under ultra-low temperature conditions according to claim 1, characterized in that: The pure adhesive bonding layer is a substrate-free thermosetting pure adhesive film. The pure adhesive bonding layer fills the line gaps on the surface of the inner layer covering component and absorbs the interlayer shear stress caused by the mismatch of thermal expansion coefficients between the liquid crystal polymer core layer and the resistance compensation composite layer in a low-temperature environment.

8. The solution for high-frequency analog transmission under ultra-low temperature conditions according to claim 1, characterized in that: The single-ended characteristic impedance matching requirement of the inner layer circuit is determined according to the following logic: the single-ended characteristic impedance is inversely proportional to the square root of the sum of the equivalent relative permittivity and 1.41, and is directly proportional to the natural logarithm of a specific ratio; the specific ratio is the ratio of 5.98 times the total thickness of the insulating medium to 0.8 times the actual conductive width and the thickness of the metal conductor.

9. The method for high-frequency analog transmission under ultra-low temperature conditions according to claim 5, characterized in that: It also includes step S500, which involves surface treatment and molding test: covering the surface of the board material after the circuit fabrication with a polyimide cover film and baking and curing it, and then performing chemical electroless nickel plating on the blind hole lead structure and the connection pad area, controlling the nickel layer thickness to be 80 to 120 microinches.

10. A high-frequency analog transmission solution system for cryogenic environments, used to perform the method according to any one of claims 1-9, characterized in that, include: The transmission flexible board adopts a multi-layer composite stacked structure, which comprises, from the inside out: The liquid crystal polymer core layer is made of liquid crystal polymer material and has an inner layer circuit formed on its surface; the inner layer cover assembly is bonded to the surface of the liquid crystal polymer core layer and is composed of a liquid crystal polymer film layer and an adhesive layer. A pure adhesive layer is disposed on the outside of the inner layer covering component and is a substrate-free thermosetting pure adhesive film. A resistance compensation composite layer is bonded to the outside of the pure adhesive layer, comprising a constantan foil layer and a copper metal layer bonded together, wherein the constantan foil layer is located between the pure adhesive layer and the copper metal layer; The interlayer interconnect structure includes a laser blind via that passes sequentially through the resistance compensation composite layer, the pure adhesive layer, and the inner layer cover assembly along the thickness direction. The laser blind via is filled with metallic copper to achieve electrical conduction between the resistance compensation composite layer and the inner layer circuitry.

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