Filling method of polyimide vertical through hole and preparation method of flexible probe

Abstract

The invention relates to the technical field of flexible device manufacturing, in particular to a filling method of a polyimide vertical through hole and a preparation method of a flexible probe. The filling method provided by the invention comprises the following steps: placing a polyimide substrate with a vertical through hole in a sodium hydroxide solution for ring-opening reaction to obtain a first intermediate; placing the first intermediate in a seed layer solution for ion exchange to obtain a second intermediate; the second intermediate is placed in a solution containing thiourea and sodium borohydride for a reduction reaction, a seed layer is formed on the hole wall of the vertical through hole, and a third intermediate is obtained; placing the third intermediate in a chemical plating solution for metal filling; ultrasonic waves are applied in the metal filling process. The filling method is simple, the cost is low, and the coating is uniform.

Description

Technical Field

[0001] This invention relates to the field of flexible device manufacturing technology, and in particular to a method for filling vertical through-holes in polyimide and a method for preparing flexible probes. Background Technology

[0002] In the fields of flexible electronics and high-end packaging, polyimide (PI) has become an important substrate due to its excellent high-temperature resistance and mechanical flexibility. To achieve high integration of flexible circuits, vertical interconnection of PI is often required.

[0003] Typically, vertical interconnection of polyimide (PI) requires the fabrication of vertical vias, followed by electroless or electroplating to deposit a metal layer on the via walls to achieve electrical connection. Electroless plating requires a noble metal catalyst (such as Pd), which is costly; while electrochemical plating is complex and its coating uniformity is difficult to control due to uneven current density. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a method for filling vertical through-holes in polyimide and a method for preparing flexible probes. The filling method of this invention is simple, low-cost, and produces a uniform coating.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for filling vertical through-holes in polyimide, comprising the following steps: A polyimide substrate with vertical through-holes was placed in a sodium hydroxide solution to carry out a ring-opening reaction to obtain the first intermediate. The first intermediate was placed in a seed layer solution for ion exchange to obtain the second intermediate; The second intermediate was placed in a solution containing thiourea and sodium borohydride for a reduction reaction, forming a seed layer on the pore wall of the vertical through-hole to obtain the third intermediate; The third intermediate is placed in a chemical plating solution for metal filling; ultrasound is applied during the metal filling process. The composition of the seed layer is the same as that of the filling metal.

[0006] Preferably, the concentration of the sodium hydroxide solution is 2~8 mol / L.

[0007] Preferably, the ring-opening reaction is carried out at a temperature of 30~80℃ for a time of 30min~2h.

[0008] Preferably, the seed layer solution is a nickel sulfate solution; the concentration of the nickel sulfate solution is 0.1~0.4 mol / L.

[0009] Preferably, the concentration of thiourea in the solution containing thiourea and sodium borohydride is 0.8~1.2 ppm, and the concentration of sodium borohydride is 40~80 mmol / L.

[0010] Preferably, the temperature for metal filling is 70~130℃ and the time is 1~5min.

[0011] Preferably, the ultrasonic power is 100~500W, the frequency is 100~800Hz, and the duration is 1~2min.

[0012] Preferably, when the filling metal is Ni, the electroless plating solution is a mixed solution of nickel sulfate and sodium phosphite; the concentration of nickel sulfate in the mixed solution is 0.3~0.5 mol / L, and the concentration of sodium phosphite is 0.1~0.3 mol / L.

[0013] Preferably, the diameter of the vertical through hole is 100~600μm.

[0014] This invention provides a method for preparing a flexible probe, comprising the following steps: preparing circuit patterns on both sides of a polyimide substrate to obtain a patterned flexible substrate; drilling holes in the patterned flexible substrate to obtain a patterned substrate with vertical through holes. The patterned substrate with vertical through holes is filled using the filling method described above to obtain a flexible probe.

[0015] This invention first places PI in a sodium hydroxide solution, causing ring opening on the pore wall surface and forming (-COONa) functional groups. Then, it is placed in a seed solution for ion exchange (taking nickel sulfate as an example, sodium ions exchange with nickel ions, binding the nickel ions to the pore wall surface). Following this, a reduction reaction occurs, reducing the nickel ions to metallic nickel, which serves as the seed layer. Subsequently, it is placed in a chemical plating solution for metal filling. This invention adds thiourea as an additive in the surface catalysis stage. The sulfur atoms in the thiourea molecule can adsorb onto the metal surface, promoting the reduction and deposition of metal ions, eliminating the need for precious metal catalysts and reducing preparation costs. Furthermore, this invention applies ultrasound during the metal filling process, using ultrasound to reduce bubbles within the pores and improve the uniformity of the coating. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the process for filling vertical through-holes in polyimide in Example 1; Figure 2 This is a photograph of the actual openings in a PI double-sided copper-clad laminate. Figure 3 A physical image of a single metal column prepared in Example 1; Figure 4 A physical image of the metal pillars of the entire PI double-sided copper-clad laminate prepared in Example 1; Figure 5 The graph shows the results of the resistance bending test. Figure 6 The resistance values ​​of the metal pillars prepared in Examples 1-2 and Comparative Example 1; Figure 7 This is a diagram illustrating the photolithography and metallization pattern of Example 3; Figure 8 A photograph of the flexible probe prepared in Example 3; Figure 9 This is a physical image of a flexible probe test array sensor. Detailed Implementation

[0017] This invention provides a method for filling vertical through-holes in polyimide, comprising the following steps: A polyimide substrate with vertical through-holes was placed in a sodium hydroxide solution to carry out a ring-opening reaction to obtain the first intermediate. The first intermediate was placed in a seed layer solution for ion exchange to obtain the second intermediate; The second intermediate was placed in a solution containing thiourea and sodium borohydride for a reduction reaction, forming a seed layer on the pore wall of the vertical through-hole to obtain the third intermediate; The third intermediate is placed in a chemical plating solution for metal filling; ultrasound is applied during the metal filling process. The composition of the seed layer is the same as that of the filling metal.

[0018] Unless otherwise specified, all raw materials used in this invention are commercially available products well known in the art.

[0019] In this invention, a polyimide substrate with vertical through-holes is placed in a sodium hydroxide solution to perform a ring-opening reaction to obtain a first intermediate.

[0020] In this invention, the polyimide substrate is preferably a polyimide copper-clad laminate. In this invention, the diameter of the vertical through-hole is preferably 100-600 μm, and in specific embodiments it can be 100, 200, 300, 400, 500, or 600 μm. By adding a thiourea solution, this invention can achieve complete filling of larger pore sizes (up to 1.5-2 times that of conventional processes, where the conventional pore size is 300 μm).

[0021] In this invention, the concentration of the sodium hydroxide solution is preferably 2-8 mol / L, and in specific embodiments it can be 2, 4, 6, or 8 mol / L; the temperature of the ring-opening reaction is preferably 30-80℃, and in specific embodiments it can be 30, 40, 50, 60, 70, or 80℃; the time of the ring-opening reaction is preferably 30 min-2 h, and in specific embodiments it can be 30, 60, 90, or 120 min. During the ring-opening reaction, polyimide undergoes ring-opening to form polyamic acid (PAA) containing an active functional group (-COOH), and -COOH reacts with sodium ions to form a -COONa functional group.

[0022] After obtaining the first intermediate, the present invention places the first intermediate in a seed layer solution for ion exchange to obtain the second intermediate.

[0023] In this invention, the composition of the seed layer solution is determined according to the type of metal to be filled. When nickel is filled, the seed layer solution is preferably a nickel sulfate solution. The concentration of the nickel sulfate solution is preferably 0.1~0.4 mol / L, and in specific embodiments it can be 0.1, 0.2, 0.3 or 0.4 mol / L.

[0024] In this invention, the preferred temperature for the ion exchange reaction is 30-80°C, and in specific embodiments, it can be 30, 40, 50, 60, 70, or 80°C; the preferred time for the ion exchange reaction is 10-30 minutes, and in specific embodiments, it can be 10, 20, or 30 minutes. Taking nickel sulfate solution as an example, this invention uses ion exchange to replace sodium ions in the PAA layer with nickel ions.

[0025] After obtaining the second intermediate, the present invention places the second intermediate in a solution containing thiourea and sodium borohydride for a reduction reaction, forming a seed layer on the pore wall of the vertical through hole to obtain the third intermediate.

[0026] In this invention, the concentration of thiourea in the solution containing thiourea and sodium borohydride is preferably 0.8~1.2 ppm, and in specific embodiments it can be 0.8, 0.9, 1, 1.1 or 1.2 ppm; the concentration of sodium borohydride is preferably 40~80 mmol / L, and in specific embodiments it can be 40, 50, 60, 70 or 80 mmol / L. In this invention, the temperature of the reduction reaction is preferably 30~80℃, and in specific embodiments it can be 30, 40, 50, 60, 70 or 80℃; the time of the reduction reaction is preferably 10~30 min, and in specific embodiments it can be 10, 20 or 30 min.

[0027] After obtaining the third intermediate, the present invention places the third intermediate in a chemical plating solution for metal filling; and applies ultrasound during the metal filling process.

[0028] In this invention, when the filling metal is Ni, the electroless plating solution is preferably a mixed solution of nickel sulfate and sodium phosphite; the concentration of nickel sulfate in the mixed solution is 0.3~0.5 mol / L, and the concentration of sodium phosphite is 0.1~0.3 mol / L. In specific embodiments, the concentration of nickel sulfate in the mixed solution can be 0.3, 0.4, or 0.5 mol / L, and the concentration of sodium phosphite can be 0.1, 0.2, or 0.3 mol / L. In this invention, the metal filling temperature is preferably 70~130℃, and in specific embodiments it can be 70, 80, 90, 100, 110, 120, or 130℃; the metal filling time is preferably 1~5 min, and in specific embodiments it can be 1, 2, 3, 4, or 5 min.

[0029] In this invention, thiourea, acting as a surfactant, preferentially adsorbs sulfur atoms within its molecules onto the concave regions of the pores, promoting selective deposition of metal ions at the pore bottom and walls through a "superfilling mechanism." Thiourea adsorbs onto the inner surface of the pores, providing more nucleation sites. As a deposition accelerator, thiourea significantly reduces the activation energy of metal deposition (e.g., the activation energy for nickel deposition decreases from -59.16 kJ / mol to -79.56 kJ / mol), thereby increasing the deposition rate within the pores. This invention uses sodium borohydride as a reducing agent, enabling rapid metal filling within the pores.

[0030] Experiments have shown that the metal column can be filled within 5 to 10 seconds, but the resulting structure is loose and porous. Microscopic analysis shows that there are a large number of dendritic fractal structures inside. This invention solves this problem by using ultrasound.

[0031] In this invention, the ultrasonic power is preferably 100-500W, and in specific embodiments it can be 100, 200, 300, 400 or 500W; the frequency is preferably 100-800Hz, and in specific embodiments it can be 100, 200, 300, 400, 500, 600, 700 or 800Hz; the duration is preferably 1-2 minutes. Preferably, the ultrasonic wave is activated at the start of the filling process.

[0032] During metal filling, the generation of hydrogen gas, a reaction byproduct, is inevitable. If these bubbles remain in the microporous structure, they will not only hinder the diffusion and deposition of metal ions but also form persistent porosity defects in the final structure. This invention solves the bubble problem in microporous filling through ultrasonic assistance and promotes surface diffusion of metal atoms through the local high temperature and high pressure environment generated by cavitation, resulting in a more uniform grain size in the deposited layer. This invention introduces high-frequency ultrasonic-assisted technology, achieving efficient bubble removal through the cavitation and acoustic flow effects generated by ultrasound in the liquid medium. When the ultrasonic transducer is working, alternating compression and expansion pressure waves are formed in the solution, generating a large number of micron-sized cavitation bubbles. These cavitation bubbles expand rapidly during the negative pressure cycle and collapse violently during the positive pressure cycle, instantly creating an extreme environment of local high temperature and high pressure. This intense cavitation produces two key effects: firstly, the high-speed microjet (up to 100 m / s) generated by the collapse of cavitation bubbles directly impacts and breaks up the hydrogen bubbles produced in the reaction; secondly, the strong acoustic flow effect induced by ultrasound (flow velocity of approximately 0.5 m / s) forms a directional vortex inside the micropores, forcibly expelling the broken microbubbles from the pores. Experimental data shows that under the action of ultrasound, the bubble residence time in the micropores is shortened from 120 seconds in conventional treatment to less than 5 seconds, and the pore filling integrity is increased from 75% to over 98%. This invention not only solves the bubble problem in micropore filling but also promotes the surface diffusion of metal atoms through the local high temperature and high pressure environment generated by the cavitation effect, resulting in a more uniform grain size in the deposited layer.

[0033] In summary, the filling method provided by this invention is simple to operate, low in cost, and produces a uniform coating.

[0034] This invention provides a method for preparing a flexible probe, comprising the following steps: preparing circuit patterns on both sides of a polyimide substrate to obtain a patterned flexible substrate; drilling holes in the patterned flexible substrate to obtain a patterned substrate with vertical through holes. The patterned substrate with vertical through holes is filled using the filling method described above to obtain a flexible probe.

[0035] This invention prepares circuit patterns on both sides of a polyimide substrate to obtain a patterned flexible substrate.

[0036] Before preparing the circuit pattern, the polyimide substrate is preferably pretreated. In this invention, the pretreatment is preferably done by sanding. Sanding removes the oxide layer from the surface of the polyimide substrate and increases its surface roughness.

[0037] In this invention, the preferred steps for fabricating circuit patterns on both sides of a polyimide substrate include: first, coating one side of the polyimide substrate with photoresist, and then performing a first exposure and a first development in sequence; coating the other side of the polyimide substrate with photoresist, aligning the two sides, and then performing a second exposure and a second development in sequence; after completing the transfer of the double-sided pattern, performing wet etching to form the circuit pattern shown above.

[0038] In this invention, the photoresist can specifically be a blue film; this invention does not have special requirements for exposure and development, and exposure and development processes well known in the art can be used.

[0039] In the manufacturing process of flexible printed circuit boards (FPCBs), photolithography and etching processes are the core steps in patterning metal circuits, and their precision directly determines the performance and reliability of the devices. Especially when realizing double-sided interconnect structures, the alignment accuracy of the upper and lower conductive patterns needs to be controlled within ±2μm, posing a severe challenge to traditional single-sided exposure technology. This invention achieves high-precision double-sided alignment using high-end photolithography equipment (Sichuan Nanguang Vacuum Technology Co., Ltd. H94-30 4'' double-sided photolithography machine). The high-end photolithography equipment employs an innovative double-sided exposure system, whose key technological breakthrough lies in the integrated dual-microscope visual alignment module. The precise operation process of this system can be broken down into the following key steps: First, a high-resolution microscope with a long working distance at the top of the device acquires a microscopic image of the upper alignment mark, and the mark's position is recorded by a computer. After exposure, the polyimide undergoes a precise 180° rotation on a vacuum adsorption platform. To achieve sub-micron alignment, the device is equipped with a Chessman six-degree-of-freedom precision stage, which uses piezoelectric ceramic drive technology. Position calibration is first performed on a macroscopic scale: rapid movement along the X / Y axes, through coarse positioning, reduces the mark center deviation to within ±20μm.

[0040] For correction of rotational deviation, the system employs a unique Q-rotation module. This module, based on a high-precision torque motor (angular resolution 0.001°), drives the vacuum chuck, performing a continuous 360° rotation while maintaining a constant center position. This ensures precise alignment of the marker.

[0041] After obtaining the patterned flexible substrate, the present invention drills holes in the patterned flexible substrate to obtain a patterned substrate with vertical through holes.

[0042] The present invention does not have any special requirements for the drilling method; any drilling method well known in the art can be used.

[0043] After obtaining a patterned substrate with vertical through holes, the present invention fills the patterned substrate with vertical through holes according to the above filling method to obtain a flexible probe.

[0044] The flexible probe prepared by this invention can be used to detect the electrical connectivity of the prepared array electrodes. This invention involves directly attaching the obtained flexible probe to the corresponding array sensor electrode location using a solder ball bonding technique to form tin-based bumps at the sensor electrode position, achieving reliable contact with the sensor electrode, thus enabling rapid and non-destructive testing of the sensor electrode.

[0045] This invention enables rapid and non-destructive testing of sensor electrodes, effectively solving the problems of contact damage and efficiency bottlenecks of traditional probes.

[0046] The following detailed description, in conjunction with embodiments, of the method for filling vertical through-holes in polyimide and the method for preparing flexible probes provided by the present invention, should not be construed as limiting the scope of protection of the present invention.

[0047] Example 1 according to Figure 1 The process shown is for filling with metallic nickel: Nine 300μm diameter through-holes (e.g., drilled on a PI double-sided copper-clad laminate) were drilled. Figure 2 The image shows one of the through-holes. Finally, surface metallization and pore filling are performed. Specifically, the metal column is sequentially immersed in a 4 mol / L NaOH solution at 50°C for 30 min, then treated in a 0.1 mol / L nickel sulfate solution at 50°C for 10 min for ion exchange. Next, it is treated in a mixed solution of 40 mmol / L sodium borohydride and 1 ppm thiourea at 50°C for 10 min. Thiourea can adhere to the nickel metal surface, lower the activation energy, and promote the reduction and deposition of metallic nickel. Then, it is heated in a chemical plating solution (a mixed solution of 0.3 mol / L nickel sulfate and 0.1 mol / L sodium phosphite) at 120°C for 3 min, while simultaneously ultrasonicating the solution at 200W, 200Hz for 1 min to reduce air bubbles remaining in the pores, improve the quality of the metal column, and achieve a denser structure, thus completing the preparation of the metal column. A physical image of a single metal column is shown below. Figure 3 As shown in the image, the physical diagram of the metal pillars of the entire PI double-sided copper-clad laminate is as follows. Figure 4 As shown.

[0048] The fabricated metal pillars were subjected to bending tests. The test method was as follows: using double-sided photolithography, a wire and electrode were led out from the upper left and lower right sides of the metal pillar. After energizing, current flowed through the metal pillars. The resulting flexible circuit board was then continuously bent from 0 to 360 degrees for 35 seconds, and the change in resistance during the bending process was observed. The results are as follows: Figure 5 As shown. By Figure 5 It can be seen that the resistance change rate is low, indicating that the prepared metal column has excellent mechanical stability and electrical reliability.

[0049] Example 2 The only difference from Example 1 is that the ultrasound duration is 30 seconds.

[0050] Comparative Example 1 The only difference from Example 1 is that ultrasound is not performed.

[0051] The resistivity of the metal pillars prepared in Examples 1-2 and Comparative Example 1 is as follows: Figure 6 As shown. By Figure 6 It is evident that the metal column prepared under ultrasonic conditions exhibits significantly lower resistance than that prepared without ultrasonication, and the resistance reduction is more pronounced for metal columns prepared by ultrasonication for 1 minute compared to those prepared by ultrasonication for 30 seconds. This is because hydrogen bubbles, a byproduct, are generated during the electroless plating process. By employing ultrasonication, this invention eliminates porosity defects and, through the localized high-temperature and high-pressure environment created by cavitation, promotes surface diffusion of metal atoms, resulting in a more uniform grain size in the deposited layer. Therefore, the metal column prepared under ultrasonic conditions has a lower resistance.

[0052] Example 3 Fabrication of flexible probes: First, the 3×3cm PI double-sided copper-clad board is pretreated by using 800-grit sandpaper to evenly polish the surface to remove the oxide layer and increase the surface roughness.

[0053] During the first pattern transfer process, a blue film of the same size is cut and one side of the protective film is removed before being bonded to the surface of the PI copper-clad laminate. A hot press is used at 100℃ and 0.5MPa for 60 seconds to ensure tight adhesion between the blue film and the substrate. The substrate is then exposed for 15 seconds under a film mask using a photolithography machine to form the first-side circuit pattern. It is then developed in a 1wt% sodium carbonate solution for 30 seconds. During the second pattern transfer, the blue film bonding and hot pressing steps are repeated. A high-precision horizontal microscope (100X magnification) is used for precise alignment of the two-side patterns. First, a Chessman platform is used for coarse positioning, then the Q-switching function is used for fine-tuning the angle (accuracy ±0.1°) to ensure the alignment error between the two sides is less than 5μm. Another 15-second exposure is performed followed by development. After completing the double-side pattern transfer, the substrate is placed in a 20wt% dilute nitric acid solution (25±1℃) for 90 seconds for wet etching to remove the copper layer in unprotected areas, forming the circuit pattern (e.g., ...). Figure 7 As shown, where a is the front and b is the back.

[0054] Subsequently, 300 μm diameter through-holes are drilled at locations requiring vertical interconnection. Finally, surface metallization and hole filling are performed. Specifically, the probe is sequentially immersed in a 4 mol / L NaOH solution at 50°C for 30 min, treated in a 0.1 mol / L nickel sulfate solution at 50°C for 10 min for ion exchange, followed by treatment in a mixed solution of 40 mmol / L sodium borohydride and 1 ppm thiourea at 50°C for 10 min. Thiourea can adhere to the nickel metal surface, lower the activation energy, promote the reduction and deposition of metallic nickel, and increase the hole filling diameter. Then, it is heated in a 120°C electroless plating solution (a mixed solution of 0.3 mol / L nickel sulfate and 0.1 mol / L sodium phosphite) for 3 min, while simultaneously ultrasonicating the solution at 200 W and 200 Hz for 1 min to reduce air bubbles remaining in the holes, improve the quality of the metal column, and achieve a denser structure, thus completing the fabrication of the flexible probe (e.g., Figure 8 (As shown).

[0055] The obtained flexible probe is directly applied to the corresponding array sensor electrode, and tin-based bumps are formed at the corresponding sensor electrode locations using solder ball bonding technology to achieve reliable contact with the sensor electrode. This allows for rapid and non-destructive testing of the sensor electrode. Because some circuits in devices fabricated using blue film lithography may break during the dilute nitric acid etching process, the fabricated flexible probe can detect the electrical connectivity of the fabricated array electrodes.

[0056] Figure 9 This is a physical image of a flexible probe test array sensor.

[0057] Inspection Process: This invention develops a high-efficiency multi-channel sensor data acquisition and defect detection system based on the LabVIEW platform. The system constructs a stable and reliable data acquisition link through the collaborative work of a high-speed data acquisition module and a digital I / O module. LabVIEW is used to process and analyze the array sensor signals acquired by the flexible probe in real time, ensuring the accuracy of the detection results. Operation is simple; just align the sensor electrodes with the probe protrusions and start the automatic test mode, and the system can quickly complete various inspection procedures. Test results are displayed through an intuitive 3D graphical interface, enabling operators to clearly identify various defects. Practical application has proven that this system significantly improves inspection efficiency and accuracy, providing reliable technical support for sensor quality control.

[0058] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for filling vertical through-holes in polyimide, characterized in that, Includes the following steps: A polyimide substrate with vertical through-holes was placed in a sodium hydroxide solution to carry out a ring-opening reaction to obtain the first intermediate. The first intermediate was placed in a seed layer solution for ion exchange to obtain the second intermediate; The second intermediate was placed in a solution containing thiourea and sodium borohydride for a reduction reaction, forming a seed layer on the pore wall of the vertical through-hole to obtain the third intermediate; The third intermediate is placed in a chemical plating solution for metal filling; ultrasound is applied during the metal filling process. The composition of the seed layer is the same as that of the filling metal.

2. The filling method according to claim 1, characterized in that, The concentration of the sodium hydroxide solution is 2~8 mol / L.

3. The filling method according to claim 1 or 2, characterized in that, The ring-opening reaction is carried out at a temperature of 30~80℃ for a time of 30min~2h.

4. The filling method according to claim 1, characterized in that, The seed layer solution is a nickel sulfate solution; the concentration of the nickel sulfate solution is 0.1~0.4 mol / L.

5. The filling method according to claim 1, characterized in that, The concentration of thiourea in the solution containing thiourea and sodium borohydride is 0.8~1.2 ppm, and the concentration of sodium borohydride is 40~80 mmol / L.

6. The filling method according to claim 1, characterized in that, The metal filling temperature is 70~130℃, and the time is 1~5min.

7. The filling method according to claim 1, characterized in that, The ultrasound power is 100~500W, the frequency is 100~800Hz, and the duration is 1~2min.

8. The filling method according to claim 1 or 5, characterized in that, When the filling metal is Ni, the electroless plating solution is a mixed solution of nickel sulfate and sodium phosphite; the concentration of nickel sulfate in the mixed solution is 0.3~0.5 mol / L, and the concentration of sodium phosphite is 0.1~0.3 mol / L.

9. The filling method according to claim 1, characterized in that, The diameter of the vertical through hole is 100~600μm.

10. A method for preparing a flexible probe, characterized in that, The process includes the following steps: fabricating circuit patterns on both sides of a polyimide substrate to obtain a patterned flexible substrate; drilling holes in the patterned flexible substrate to obtain a patterned substrate with vertical through holes. The patterned substrate with vertical through holes is filled according to any one of claims 1 to 9 to obtain a flexible probe.

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