CN114373580A - A flexible transmission line and its application - Google Patents

Abstract

The invention discloses a flexible transmission line and application thereof, wherein the flexible transmission line comprises a signal layer, a substrate layer and a ground layer which are sequentially stacked, the signal layer is in contact with the substrate layer, the signal layer is made of at least one of gold, silver or copper, the substrate layer comprises a liquid crystal polymer film layer, and the liquid crystal polymer film layer is a liquid crystal orientation film formed by curing raw materials comprising a liquid crystal polymer and a photoinitiator. The liquid crystal polymer film layers with different dielectric constants are obtained by adjusting the orientation of the liquid crystal polymer by utilizing the polymer alignment property of the liquid crystal polymer, so that the flexible transmission lines with different dielectric constants are further prepared.

Description

A flexible transmission line and its application

technical field

The invention relates to the field of materials, in particular to a flexible transmission line and its application.

Background technique

In recent years, with the rapid development of communication technology, in order to meet the needs of increasing communication rates, the working frequency bands of flexible electronic devices are getting higher and higher, from the frequency band of several GHz in the 4G era to the tens, several tens of millimeter waves and terahertz waves. In the 100 GHz frequency band, as a key way of signal transmission, the transmission line on the printed circuit board in flexible electronic equipment is the main part of realizing high-frequency signal transmission. At present, the global demand for high-speed communication applications is increasing, and various "5G industries" such as high-precision, low-latency autonomous driving, telemedicine, and smart cities need to study new transmission lines with high speed and low loss. High-speed, low-loss transmission lines require that the dielectric materials of the transmission line have the characteristics of low dielectric constant (Dk) and low dielectric loss (Df). However, the transmission line based on glass fiber epoxy resin board has very serious signal loss in high frequency transmission, which is completely unsuitable for the current high-speed high-frequency communication.

At present, materials with good application prospects in 5G communication technology mainly include: polytetrafluoroethylene (PTFE), liquid crystal polymer (LCP), modified polyphenylene (MPPE) and polyimide (PI) and other categories. Due to its low elastic modulus, PTFE film is difficult to process into a thin film with a small thickness, and its linear expansion coefficient is high, so it is not suitable for making ultra-thin circuit boards. In addition, the adhesion of PTFE to components such as metal conductors is weak. The MPPE surface substrate has excellent dielectric properties, but in practical applications, it is limited by its heat resistance and dimensional stability, and cannot meet the processing requirements of components in many cases. However, the dielectric constant of PI material is relatively large, the loss factor is relatively large, and the reliability is relatively poor, which leads to serious signal loss in the high-frequency transmission of PI material flexible circuit board (FPC). Unable to adapt. LCP has good thermal stability, dielectric properties, radiation resistance, corrosion resistance, and electrical insulation, and has good application prospects in the 5G field.

In order to solve the problem of serious signal loss during high-frequency transmission, a substrate material with low signal loss during high-frequency transmission is required.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned problems in the prior art, one of the objectives of the present invention is to provide a flexible transmission line.

The second purpose of the present invention is to provide an application of a flexible transmission line in 5G communication materials or circuit boards.

In order to achieve the above object, the technical scheme adopted by the present invention is:

A first aspect of the present invention provides a flexible transmission line, comprising a signal layer, a base material layer and a grounding layer that are stacked in sequence, the signal layer is in contact with the base material layer; the material of the signal layer is copper, gold or silver At least one of the above; the base material layer includes a liquid crystal polymer film layer, and the liquid crystal polymer film layer is a liquid crystal alignment film formed by curing a raw material including a liquid crystal polymer and a photoinitiator.

Preferably, the shape of the signal layer is at least one of a strip shape and a sheet shape; further preferably, the signal layer is a sheet shape.

Preferably, the signal layer is composed of more than one conductive metal sheet, all the conductive metal sheets are located in the same horizontal plane and all the conductive metal sheets are in contact with the base material layer, and the conductive metal sheet can be selected according to actual needs. quantity. The conductive metal sheet is at least one of copper foil, gold foil and silver foil.

Preferably, the signal layer, the base material layer and the ground layer are formed by pressing an adhesive.

Preferably, the flexible transmission line is a 5G flexible transmission line; further preferably, the flexible transmission line is a reconfigurable 5G flexible transmission line.

Preferably, the curing condition is ultraviolet light curing; further preferably, the curing condition is curing under the induction of ultraviolet light at 365 nm.

Preferably, the material of the signal layer is at least one of copper, silver and gold.

Preferably, the material of the ground layer is at least one of copper, silver and gold.

Preferably, the liquid crystal polymer includes at least one of E7 liquid crystal, 5CB liquid crystal, and RM257 liquid crystal. Further preferably, the liquid crystal polymer comprises RM257. RM257 (HCCH, Jiangsu Hecheng) is a white powdery solid at room temperature, and its molecular structure is as follows:

Preferably, the photoinitiator includes at least one of 2-hydroxy-2-methyl-1-phenyl-1-propanone and 1-hydroxycyclohexyl phenyl ketone. The 2-hydroxy-2-methyl-1-phenyl-1-propanone is also called 1173 photoinitiator, and its molecular formula is as follows:

1-Hydroxycyclohexyl phenyl ketone is also known as 184 photoinitiator.

Preferably, the mass ratio of the liquid crystal polymer to the photoinitiator is (40-50):1; further preferably, the mass ratio of the liquid crystal polymer to the photoinitiator is (42-48):1; further Preferably, the mass ratio of the liquid crystal polymer to the photoinitiator is (45-48):1.

Preferably, the liquid crystal polymer film layer is prepared by the following preparation method, including the following steps:

S1: mix the liquid crystal polymer and the photoinitiator in the dark;

S2: adding the product in step S1 into an alignment liquid crystal cell, and photoinitiating curing to obtain the liquid crystal polymer film layer.

Preferably, the mixing temperature of the light-proof mixing is 80-120°C; further preferably, the mixing temperature of the light-proof mixing is 90-110°C; still further preferably, the mixing temperature of the light-proof mixing is 100°C °C.

Preferably, the step S1 is specifically as follows: adding the liquid crystal polymer and the photoinitiator into a brown reagent bottle, placing the brown reagent bottle into a small beaker containing silicone oil, and heating the small beaker to a temperature of 80-120 °C by heating in a water bath. ℃, and stir for 10 to 30 minutes to make it evenly mixed.

Preferably, the step S2 is specifically as follows: adding the product in the step S1 into an oriented liquid crystal cell kept at a temperature of 80-120° C., and using 2-4 mW/cm ² and 360-370 nm of ultraviolet light to initiate curing for 2- 8 min to prepare the liquid crystal polymer film layer; further preferably, the step S2 is specifically as follows: adding the product in step S1 into an oriented liquid crystal cell kept at a temperature of 100°C, and at 95°C, using 3 mW/cm ² , 365 nm ultraviolet light initiates curing for 5 min to obtain the liquid crystal polymer film layer.

Preferably, the thickness of the liquid crystal polymer film layer is 200-400 μm; further preferably, the thickness of the liquid crystal polymer film layer is 200-350 μm; further preferably, the thickness of the liquid crystal polymer film layer is 200～300μm.

Preferably, the angle between the tilt angle of the liquid crystal molecules of the liquid crystal polymer and the horizontal direction is 0°˜90°. The invention obtains liquid crystal polymer film layers with different dielectric constants by changing the alignment and orientation of liquid crystal molecules of the liquid crystal polymer in the base material layer, thereby obtaining a transmission line with adjustable transmission speed.

Preferably, the thickness of the signal layer and the ground layer are both 60-150 μm; further preferably, the thickness of the signal layer and the ground layer are both 70-130 μm; still further preferably, the thickness of the signal layer and the ground layer is 70-130 μm. The thicknesses are all 90 to 110 μm.

Preferably, the width of the signal layer is 0.5-1.5 mm; further preferably, the width of the signal layer is 0.8-1.5 mm; further preferably, the width of the signal layer is 1-1.2 mm.

Preferably, a first adhesive layer is further provided between the signal layer and the base material layer; and a second adhesive layer is further provided between the base material layer and the ground layer.

Preferably, the materials of the first adhesive layer and the second adhesive layer are both hot-melt adhesives.

Preferably, the hot-melt adhesive includes at least one of polyolefin-based hot-melt adhesive, polyester-based hot-melt adhesive, polyamide-based hot-melt adhesive, and polyurethane-based hot-melt adhesive.

Preferably, the dielectric constant of the liquid crystal polymer film layer is 2.7˜3.3.

Preferably, the insertion loss of the flexible transmission line at 5-6 GHz is -0.2 to -0.3 dB,

The second aspect of the present invention provides an application of the flexible transmission line provided in the first aspect of the present invention in 5G communication materials or circuit boards.

The beneficial effects of the present invention are as follows: the present invention utilizes the liquid crystal polymer to have polymer alignment, and obtains liquid crystal polymer film layers with different dielectric constants by adjusting the orientation of the liquid crystal polymer, and then obtains flexible liquid crystal polymer films with different dielectric constants. Transmission line, the flexible transmission line has low dielectric loss, is reconfigurable, flexible and bendable, and different bending degrees do not change the performance of the transmission line, which can be suitable for high-frequency 5G signal transmission.

in particular:

In the present invention, a liquid crystal polymer film layer is formed in an oriented liquid crystal cell by photoinitiating and curing RM257 and 1173 photoinitiators, and the obtained liquid crystal polymer film layer has orientation, and the liquid crystal polymer can be adjusted by adjusting the molecular orientation of the liquid crystal polymer. The dielectric constant of the thin film layer can achieve the purpose of changing the transmission speed of the flexible transmission line by adjusting the dielectric constant of the flexible transmission line, and then adjust the performance of the flexible transmission line according to different usage requirements, and has a wider range of applications.

The dielectric loss of the liquid crystal polymer film layer prepared by the invention is smaller than that of the PI film and the glass fiber epoxy resin, and can be suitable for high-frequency 5G signal transmission lines with low dielectric loss.

The liquid crystal polymer in the base material layer of the flexible transmission line of the present invention is anisotropic, and the dielectric constant of the liquid crystal polymer can be changed by regulating the molecules of the liquid crystal polymer, and the liquid crystal polymer material in the base material layer can be obtained without changing the liquid crystal polymer material. Therefore, reconfigurable flexible transmission lines with different dielectric constants can be obtained, and the current transmission lines cannot change their dielectric constants under the same substrate layer material. The flexible transmission line in the present invention can change the base by adjusting the tilt angle of the liquid crystal molecules. The dielectric constant of the liquid crystal polymer film layer of the material layer changes the performance of the flexible transmission line.

Description of drawings

FIG. 1 is a schematic cross-sectional structure diagram of the flexible transmission line in Example 1. As shown in FIG.

FIG. 2 is a schematic cross-sectional structure diagram of the flexible transmission line in Example 3. FIG.

3 is a schematic structural diagram of different orientations of LCPs in the flexible transmission lines in Example 1 and Example 2.

FIG. 4 is a graph of the input return loss of the flexible transmission line in Example 1 and Example 2.

FIG. 5 is a graph of the insertion loss of the flexible transmission line in Example 1 and Example 2. FIG.

FIG. 6 is a schematic structural diagram of a flexible transmission line in which the substrate layers are LCPs with different orientations.

FIG. 7 is a graph showing the variation of characteristic impedance with frequency of the flexible transmission line whose base material layer is LCP with different orientations.

FIG. 8 is a schematic diagram of the structure of the flexible transmission line bent at an angle α in Example 1. FIG.

FIG. 9 is a graph of the input return loss after the flexible transmission line in Example 1 is bent at an angle α.

FIG. 10 is an insertion loss diagram of the flexible transmission line in Example 1 after bending an angle α.

Reference number:

The signal layer 101 ; the first adhesive layer 102 ; the base material layer 103 ; the second adhesive layer 104 ;

Detailed ways

The specific implementation of the present invention will be further described in detail below with reference to the accompanying drawings and examples, but the implementation and protection of the present invention are not limited thereto. It should be pointed out that, if the following processes are not specifically described in detail, those skilled in the art can realize or understand by referring to the prior art. If the reagents or instruments used do not indicate the manufacturer, they are regarded as conventional products that can be purchased in the market.

Example 1

The schematic cross-sectional structure of the flexible transmission line in this example is shown in FIG. 1 , including a signal layer 101 , a first adhesive layer 102 , a substrate layer 103 , a second adhesive layer 104 and a ground layer 105 that are stacked in sequence. The signal layer 101 is located in The top layer, the ground layer 105 is located at the bottom layer, the signal layer 101 is copper foil, the copper foil is sheet-like, the signal layer 101, the first adhesive layer 102, the substrate layer 103, the second adhesive layer 104 and the ground layer 105 are pressed together Form the flexible transmission line in this example. The base material layer 103 is a liquid crystal polymer film layer formed by curing liquid crystal polymer (LCP) poured into a liquid crystal cell arranged in parallel orientation, and its orientation direction is shown in FIG. 3( a ). FIG. 3 is a schematic structural diagram of different orientations of LCP in the flexible transmission line; wherein, FIG. 3(a) is a schematic structural diagram of the LCP film in the flexible transmission line when the orientations are arranged in parallel; the thickness of the signal layer 101 is 100 μm; the width of the signal layer 101 is 1 mm; the thickness of the ground layer 105 is 100 μm; the thickness of the first adhesive layer 102 and the second adhesive layer 104 are both 10 μm; the thickness of the liquid crystal polymer film layer is 300 μm;

In this example, the LCP base material layer 103 with parallel orientation is prepared according to the following method:

Using the rubbing alignment method, the glass substrate with the spin-coated alignment film was made into a surface-oriented glass substrate, and then the surface-oriented glass substrate was assembled into a liquid crystal cell with a cell thickness of 300 μm. Take 4mg1173 photoinitiator and 200mgRM257 liquid crystal into a 1ml brown reagent bottle, set the temperature of the magnetic stirrer to 100°C and adjust the speed, then put the brown reagent bottle in a small beaker with silicone oil and stir it on the magnetic stirrer for 15min , pour the mixed sample into the liquid crystal cell at 100 °C, and when the temperature is lowered to 95 °C, use 365nm ultraviolet light to induce curing, and separate the liquid crystal cell to obtain an LCP film with parallel orientation. The orientation-parallel configuration means that the orientation direction of the LCP molecules is parallel to the surface of the base material layer 103 .

In this example, the thermal deformation temperature of the LCP substrate layer is above 270°C, the continuous use temperature is above 200°C, and the LCP substrate layer will not be corroded in the presence of 90% acid and 50% alkali. , therefore, the LCP substrate layer in this example has good thermal stability and acid and alkali corrosion resistance.

Example 2:

The structure of the flexible transmission line in this example is the same as that in Example 1. The difference between this example and Example 1 is that the substrate layer 103 is made of liquid crystal polymer (LCP) poured into a liquid crystal cell with a vertical orientation. ) liquid crystal polymer film layer formed by curing; the orientation direction of LCP in this example is shown in Figure 3(b). 3(b) is a schematic structural diagram of the LCP film in the flexible transmission line when the orientation is vertically arranged; the LCP film with the vertical orientation in this example is prepared with reference to the preparation method in Example 1. The orientation vertical configuration means that the orientation direction of the LCP molecules is perpendicular to the surface of the base material layer 103 .

Example 3:

As shown in the schematic cross-sectional view of the flexible transmission line in FIG. 2 , the structure of the flexible transmission line in this example is different from that of Embodiment 1 in that there is no first adhesive layer 102 and second adhesive layer 104 in this example. The performance of the flexible transmission line in this example is basically the same as that of the flexible transmission line in Example 1 and Example 2.

Comparative Example 1:

The structure of the flexible transmission line in this example is the same as that in Example 1, and the difference between this example and Example 1 is that the base material layer 103 is a thin film layer formed of glass fiber epoxy resin.

Performance Testing:

The resonant cavity perturbation measurement method was used to test the dielectric constants of the liquid crystal polymer film layers in Examples 1 and 2 and the film layers formed by the glass fiber epoxy resin in Comparative Example 1. The specific test results are as follows: Example The dielectric constant of the liquid crystal polymer film layer with the parallel orientation in Example 1 is 2.7, the dielectric constant of the liquid crystal polymer film layer of the LCP with the vertical orientation in Example 2 is 3.3, while the glass fiber epoxy resin in Comparative Example 1 has a dielectric constant of 3.3. The dielectric constant of the thin film layer was 5.2. Compared with Comparative Example 1, the dielectric properties of the liquid crystal polymer thin film layers in Example 1 and Example 2 were more excellent.

The transmission performance input return loss and insertion loss of the flexible transmission lines in Example 1 and Example 2 were tested by using the two-port network analyzer E5071, wherein FIG. 4 is the input echo of the flexible transmission lines in Example 1 and Example 2. Loss ( S11 ) diagram; FIG. 5 is an insertion loss ( S21 ) diagram of the flexible transmission lines of Example 1 and Example 2. It can be seen from Figure 5 that the insertion loss S21 of the flexible transmission lines in Example 1 and Example 2 is -0.2dB to -0.3dB at 5-6GHz, while the glass fiber epoxy resin in Comparative Example 1 is the substrate layer The insertion loss of the flexible transmission line at 5-6GHz is -8dB to -9dB. It can be seen that the performance of the flexible transmission line in Example 1 and Example 2 is better than that in Comparative Example 1.

Test samples were prepared with reference to the preparation method of the oriented LCP substrate layer 103 and the structure of the flexible transmission line in Example 1, and the test samples were respectively: Sample 1: a flexible transmission line with an included angle β between the orientation direction of the LCP molecules and the x-axis of 90°; Sample 2: The flexible transmission line with the angle β between the orientation direction of the LCP molecules and the x-axis is 45°; Sample 3: The flexible transmission line with the angle β between the orientation direction of the LCP molecules and the x-axis is 0°; the orientation of the LCP molecules in samples 1 to 3 The directions are shown in Figure 6, wherein Figure 6(a) is a schematic diagram of the flexible transmission line in sample 1; Figure 6(b) is a schematic diagram of the flexible transmission line in sample 2; Figure 6(c) is a schematic diagram of the flexible transmission line in sample 3 schematic diagram. Then, the relationship between the characteristic impedance and frequency of the flexible transmission lines in samples 1 to 3 is tested respectively. The specific test method is as follows: using the RF module of the COMSOL software to simulate with the finite element method, the pre-tilt angles of the liquid crystal molecules can be simulated. The characteristic impedance as a function of frequency. The characteristic impedance is verified by the simulation method, which is the simulation data of three different liquid crystal molecular orientations of the liquid crystal polymer film layer, which reflects the reconfigurability of the LCP. The specific test results are shown in Figure 7, in which Figure 7(a) is the graph of the real part of the characteristic impedance of samples 1-3 with frequency; Figure 7(b) is the imaginary part of the characteristic impedance of samples 1-3 with frequency Change graph. It can be seen from Figure 7 that the flexible transmission line containing liquid crystal polymer film layers with different orientation directions has good reconfigurability.

The two ends of the flexible transmission line in Example 1 are welded to the input signal end by tin, and then connected to the network analyzer through the SMA interface. At this time, the bending angle of the flexible transmission line in Example 1 is α angle, and the α angle is 0 °, 15° and 30°, as shown in FIG. 8 , in which FIG. 8( a ) is a schematic cross-sectional view of the flexible transmission line in Example 1 after bending; FIG. 8( b ) is the flexible transmission line in Example 1. Schematic diagram of the structure bent at 0°; Figure 8(c) is a schematic diagram of the structure of the flexible transmission line bent by 15° in Example 1; Figure 8(d) is a schematic diagram of the structure of the flexible transmission line in Example 1 bent by 30°; and then tested separately The input return loss and insertion loss of the flexible transmission line after bending, the input return loss test chart is shown in Figure 9; the insertion loss test chart is shown in Figure 10, and it can be seen from Figure 9 and Figure 10 that the substrate layer 103 The bending degree of the flexible transmission line, which is LCP, does not affect the performance of the transmission line, indicating that the flexible transmission line in the present invention has excellent flexibility and the transmission performance of the transmission line does not change after bending.

To sum up, the present invention mainly changes the dielectric constant of the LCP base material layer 103 by changing the alignment and orientation of the LCP liquid crystal polymer to obtain the LCP base material layer 103 with adjustable dielectric constant and lower dielectric loss. The LCP substrate layer 103 is made into a reconfigurable 5G flexible transmission line with adjustable transmission speed. The dielectric constant of the LCP film layer in the base material layer of the flexible transmission line of the present invention is 2.7-3.3, which is far lower than that of glass fiber epoxy resin (the dielectric constant is 5.2), and the dielectric properties are more excellent. The insertion loss is -0.2～-0.3dB, while the insertion loss of the transmission line with glass fiber epoxy resin as the base material layer 103 at 5-6GHz is -8dB～-9dB, and the transmission line of the present invention has better performance.

The embodiments of the present invention have been described in detail above, but the present invention is not limited to the above-mentioned embodiments, and various changes can be made within the scope of knowledge possessed by those of ordinary skill in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and features in the embodiments may be combined with each other without conflict.

Claims (10)

1. A flexible transmission line, characterized in that: it comprises a signal layer, a base material layer and a ground layer that are stacked in sequence; the signal layer is in contact with the base material layer; the material of the signal layer is copper, gold or silver. At least one; the base material layer includes a liquid crystal polymer film layer; the liquid crystal polymer film layer is a liquid crystal alignment film formed by curing a raw material including a liquid crystal polymer and a photoinitiator. 2 . The flexible transmission line according to claim 1 , wherein the liquid crystal polymer comprises at least one of E7 liquid crystal, 5CB liquid crystal, and RM257 liquid crystal. 3 . 3. The flexible transmission line according to claim 1, wherein the photoinitiator comprises 2-hydroxy-2-methyl-1-phenyl-1-propanone, 1-hydroxycyclohexyl phenyl ketone at least one of. 4 . The flexible transmission line according to claim 1 , wherein the mass ratio of the liquid crystal polymer to the photoinitiator is (40-50):1. 5 . 5 . The flexible transmission line according to claim 1 , wherein the thickness of the liquid crystal polymer film layer is 200 μm-400 μm. 6 . 6 . The flexible transmission line according to claim 1 , wherein the angle between the tilt angle of the liquid crystal molecules of the liquid crystal polymer and the horizontal direction is 0°˜90°. 7 . 7 . The flexible transmission line according to claim 1 , wherein the thicknesses of the signal layer and the ground layer are both 60-150 μm. 8 . 8 . The flexible transmission line according to claim 1 , wherein the flexible transmission line is a 5G flexible transmission line. 9 . 9 . The flexible transmission line according to claim 8 , wherein a first adhesive layer is further provided between the signal layer and the base material layer; and a second adhesive layer is further provided between the base material layer and the ground layer. 10 . 10. Application of the flexible transmission line according to any one of claims 1 to 9 in 5G communication materials or circuit boards.

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