MEMS four-electrode conductivity sensor based on laser drilling back lead and preparation method of MEMS four-electrode conductivity sensor

Abstract

The invention discloses an MEMS (Micro Electro Mechanical System) four-electrode conductivity sensor based on a laser drilling back lead and a preparation method thereof. The MEMS four-electrode conductivity sensor comprises an LCP (Liquid Crystal Polymer) substrate and a four-electrode circular ring structure arranged on the front surface of the LCP substrate, the four-electrode circular ring structure sequentially comprises a first current electrode, a first voltage electrode, a second voltage electrode and a second current electrode from the center of the substrate to the outside; a micro through hole penetrating through the LCP substrate is formed in the non-electric field sensitive area of each electrode through laser drilling; the platinum wire penetrates into the micro-through hole from the back of the LCP substrate, the bottom end is sintered and fixed through a conductive material covering the micro-through hole area on the back of the LCP substrate, and the top end is in low-resistance electrical connection with the metal layer of each electrode. According to the sensor, a four-electrode structure with optimized electric field constraint is adopted, the technology of laser drilling-platinum wire threading-platinum paste sintering is innovatively applied, front face drilling and back face lead packaging is achieved, the sensor can be directly used as a standard surface-mounted device to be installed, and the measuring performance, the environmental adaptability and the integration convenience are improved.

Description

TECHNICAL FIELD

[0001] The application relates to the fields of ocean monitoring sensor technology and micro-electro-mechanical system packaging, and particularly relates to a MEMS four-electrode conductivity sensor based on laser punching back lead and a preparation method thereof. BACKGROUND

[0002] As a core characterization index of key parameters such as ocean salinity and temperature, the measurement accuracy of seawater conductivity directly determines the reliability of ocean environment monitoring data. The four-electrode measurement method can effectively suppress the electrode polarization effect and contact impedance interference by independently setting current injection and voltage measurement electrodes, significantly improve the stability and accuracy of long-term measurement, and has become the mainstream technical solution in the field of ocean observation.

[0003] With the development of ocean observation platforms towards miniaturization, networking and intelligentization, there is an urgent need for miniaturization, low power consumption and high reliability of sensors.

[0004] MEMS technology makes it possible to manufacture miniature conductivity sensors, but there are significant bottlenecks in realizing reliable interconnection and integrated packaging of sensors and backend circuits: Lead interference problem: traditional front lead bonding occupies the working surface of the sensor, interferes with the electric field distribution, and affects the full contact of the measured medium and the electrode; Connection reliability problem: in the ocean environment of high temperature, high humidity and high salt fog, the conventional metal lead soldering points or conductive glue connection places are prone to electrochemical corrosion and mechanical aging, resulting in signal drift or connection failure; Integration problem: it is difficult to realize direct and compact mounting of the sensor and the processing circuit board, and a complex adapter plate or an additional sealing shell is usually needed, which increases the system volume, cost and complexity.

[0005] Therefore, there is an urgent need for a new structure and packaging process that can maintain the integrity of the sensor working surface and realize stable electrical connection on the back. SUMMARY

[0006] The application aims to solve the problems of insufficient measurement accuracy, low packaging reliability and poor integration of seawater conductivity sensors in the prior art, and proposes a miniaturized four-electrode sensor structure for high-precision seawater conductivity measurement and a high-reliability and high-integration packaging preparation method. The sensor adopts an optimized four-electrode circular ring structure design with field constraint, and realizes stable packaging of "front punching and back lead" through the innovative introduction of a "laser punching-platinum wire threading-platinum paste sintering" composite process, so that the sensor can be directly installed on the circuit board as a standard surface-mounted device, significantly improving the measurement performance, environmental adaptability and system integration convenience.

[0007] In order to achieve the above object, the technical scheme adopted by the present application is as follows: A MEMS four-electrode conductivity sensor based on laser-drilled backwire, comprising an LCP substrate and a four-electrode ring structure arranged on the front surface of the LCP substrate, characterized in that the four-electrode ring structure comprises, from the center of the substrate outward, a first current electrode, a first voltage electrode, a second voltage electrode, and a second current electrode. The first current electrode is an inner ring electrode for injecting a measurement current. The first voltage electrode is spaced a certain distance from the first current electrode and is used to measure a voltage signal. The second voltage electrode is spaced a certain distance from the first voltage electrode and is used to measure the potential difference in the fixed gap between the first voltage electrode and the second voltage electrode. The second current electrode is an outer ring electrode for current return. The non-electric field sensitive area (inner or outer edge) of each electrode is formed into a microvia penetrating the LCP substrate by laser drilling. A platinum wire penetrates the microvia from the back surface of the LCP substrate, with the top end forming a low-resistance contact with the metal layer of each electrode, and the bottom end being fixed by sintering a high-conductivity platinum paste coated on the microvia area on the back surface of the LCP substrate. The high-conductivity platinum paste wraps the root of the platinum wire and penetrates the sidewall of the microvia.

[0008] Further, the radius of the first current electrode (inner ring) is 0.9-1.1 mm (preferably 1 mm), the ring width of the second current electrode (outer ring) is 0.4-0.6 mm (preferably 0.5 mm), the ring width of the first voltage electrode and the second voltage electrode is 0.2-0.4 mm (preferably 0.3 mm), the insulation gap between adjacent voltage-current electrodes is 0.2-0.4 mm (preferably 0.3 mm), and the insulation gap between adjacent voltage-voltage electrodes is 2.2-2.4 mm (preferably 2.3 mm). This size combination aims to constrain the sensing electric field to mainly distribute between adjacent current-voltage electrodes, the larger current electrode can reduce the error caused by electrode impedance, increase the current sensitivity and uniformity of the system, improve the sensitivity and spatial resolution of the sensor, and suppress the edge effect.

[0009] Further, through a high-temperature sintering process, the high-conductivity platinum paste is solidified and forms a strong and tough metallurgical-mechanical composite bond with the LCP substrate, the metal layer of each electrode, and the platinum wire, thereby forming a platinum paste pad in electrical communication with each electrode. As a standard surface-mounted device, the platinum paste pad can be directly connected to a circuit board through reflow soldering or thermal compression, realizing "chip-level" integration of the sensor.

[0010] Further, it further comprises a gold plating layer covering the surface of the LCP substrate and the entire sensing area to further improve the environmental corrosion resistance.

[0011] The application further provides a preparation method of the MEMS four-electrode conductivity sensor based on laser drilling back lead. S1, sensor structure manufacturing S1.1, select LCP film as a substrate, and use MEMS process to manufacture four concentric ring metal electrodes on the front surface of the substrate, the innermost ring is the first current electrode with a radius of 1 mm; after an insulating area with a spacing of 0.3 mm, the first voltage electrode is manufactured, with a ring width of 0.3 mm; after a spacing of 2.3 mm, the second voltage electrode is manufactured, with a ring width of 0.3 mm; finally, after a spacing of 0.3 mm, the outermost second current electrode is manufactured, with a ring width of 0.5 mm; S1.2, mark laser drilling points on the non-electric field sensitive area of each electrode; S2, laser drilling and interconnection packaging S2.1, drill at the laser drilling points to form micro-vias penetrating the LCP film, the micro-vias have a diameter of 80 μm, and the hole wall perpendicularity is ≤2°; S2.2, pass the platinum wire into the micro-via from the back surface, and make the top end of the platinum wire form low-resistance contact with the metal layer of each electrode; S2.3, coat high-conductivity platinum paste on the micro-via area of the LCP film to wrap the root of the platinum wire and penetrate into the micro-via; S2.4, sinter in air or inert atmosphere to make the platinum paste stably combined with the LCP film, the metal layer of each electrode and the platinum wire, and form a platinum paste pad in electrical conduction with each electrode; S3, system integration and application Solder the platinum paste pad and the circuit board pad by using tin paste or conductive glue, and complete the welding by reflow soldering or hot pressing.

[0012] Further, a gold plating layer is further formed on the surface of the LCP substrate and the sensing area to enhance the environmental corrosion resistance.

[0013] The application has the following characteristics and beneficial effects: 1) By optimizing the size design of the four-electrode ring, the electric field distribution is effectively constrained, the response sensitivity of the sensor to the change of seawater conductivity and the uniformity of the measurement area are improved, and the measurement precision is improved.

[0014] 2) The laser drilling realizes "front drilling and back lead", eliminates the interference and occupation of the front lead on the electrode working area, ensures the integrity and flatness of the front surface (working surface) of the sensor, enables the electrode to fully and unobstructed contact with seawater, and improves the transmission performance.

[0015] 3) Through the packaging process of platinum wire combined with platinum paste sintering, the excellent chemical stability of platinum material and the metallurgical bonding or strong mechanical bonding formed after the platinum paste sintering are utilized, so that the electrode lead connection point has extremely high mechanical strength, electrically conductive stability and seawater corrosion resistance, and the long-term reliability of the sensor in harsh environment is significantly improved.

[0016] 4) After the packaging is completed, a flat metal pad is formed on the back of the sensor, and the surface mounting technology can be directly used for welding on the special signal processing circuit board, so that the "chip level" integration of the sensor is realized, the system assembly is simplified, the parasitic parameters are reduced, and the miniaturization and modularization of the terminal equipment are facilitated. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 Figure 1 is a top view of a MEMS four-electrode conductivity sensor based on laser drilling back lead according to an embodiment of the present application; Figure 2 Figure 2 is a sectional view of a MEMS four-electrode conductivity sensor based on laser drilling back lead according to an embodiment of the present application; In the figure, 1 is an LCP substrate; 2 is a first current electrode; 3 is a first voltage electrode; 4 is a second voltage electrode; 5 is a second current electrode; 6 is a microvia; 7 is a platinum wire; 8 is a high-conductivity platinum paste; and 9 is a circuit board. DETAILED DESCRIPTION

[0018] The present application will be described in detail below with specific embodiments. The following embodiments will help those skilled in the art to further understand the present application, but do not limit the present application in any form. It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0019] As shown in the drawings, Figure 1 The first aspect of the present application is to provide a MEMS four-electrode conductivity sensor based on laser drilling back lead, which comprises an LCP substrate 1 and a four-electrode ring structure arranged on the front surface of the LCP substrate 1, the four-electrode ring structure comprises, from the center of the substrate outward, in order: The first current electrode 2 is an inner ring electrode with a ring radius of 0.9-1.1 mm (preferably 1 mm), which is used for injecting a measurement current; The first voltage electrode 3 is spaced apart from the first current electrode by 0.2-0.4 mm (preferably 0.3 mm) and has a ring width of 0.2-0.4 mm (preferably 0.3 mm), which is used for measuring a voltage signal; The second voltage electrode 4 is spaced apart from the first voltage electrode by 2.2-2.4 mm (preferably 2.3 mm) and has a ring width of 0.2-0.4 mm (preferably 0.3 mm), which is used for measuring the potential difference in the fixed gap between the first voltage electrode; Second current electrode 5: as an outer ring electrode, ring width 0.4-0.6mm (preferably 0.5mm), used for current return; The above electrode size combination can achieve the following technical effects by precisely controlling the current-voltage electrode spacing and ring width ratio: Electric field constraint: verified by finite element simulation (COMSOL), the sensing electric field is mainly distributed between adjacent current-voltage electrodes (electric field intensity ≥ 90% concentrated in the measurement area), and the spatial resolution is improved by 30%; Impedance optimization: the wide ring design of the inner / outer current electrode reduces the contact resistance to ≤5mΩ, the current injection efficiency is improved by 25%, and the measurement error caused by electrode impedance is reduced; Sensitivity improvement: the redundant voltage electrode design is verified by cross-validation, which enhances the detection ability of the system to weak voltage signals (sensitivity ≥ 0.1mV / μS), and is suitable for low conductivity medium (such as fresh water) measurement; Edge effect suppression: the wide ring design of the outer current electrode disperses the edge electric field, and the electric field distortion rate is reduced to ≤5%, ensuring the accuracy of the measurement data.

[0020] The non-electric field sensitive area (inner or outer edge) of each electrode is formed into a microvia 6 through the LCP substrate by laser drilling; the platinum wire 7 penetrates the microvia 6 from the back of the LCP substrate, the top end forms a low resistance contact (contact resistance ≤10mΩ) with the metal layer of each electrode through flip positioning technology, reduces signal transmission loss, and the bottom end is fixed by coating high-conductivity platinum paste 8 on the microvia area of the back of the LCP substrate and sintering, the high-conductivity platinum paste wraps the root of the platinum wire and penetrates into the sidewall of the microvia, the penetration depth is ≥50μm, through high-temperature sintering process, the high-conductivity platinum paste is solidified and forms a platinum paste pad with a diameter of 200-300μm which is in electrical conduction with the LCP substrate, the metal layer of each electrode and the platinum wire.

[0021] This design significantly optimizes the sensor structure through back lead technology: 1) Spatial integration advantage: the thickness of the sensor is controlled within 2mm, supporting high-density deployment, suitable for narrow space scenarios such as buoys, unmanned underwater vehicles, etc.; the diameter ratio of microvia and platinum paste pad (2.5:1) is matched, which improves the space utilization and reduces the sensor volume by 30%.

[0022] 2) Structural stability enhancement: high-temperature sintering process makes the platinum paste form a chemical bond with the LCP substrate, the bonding strength is ≥15N / mm², which ensures the structural integrity in the temperature cycle of-40℃~85℃; the platinum paste penetration depth is ≥50μm, forming a mechanical interlocking structure, which significantly suppresses the risk of pad falling off in a vibrating environment.

[0023] At the same time, in order to prolong the service life of the sensor in the marine environment, the surface of the LCP substrate and the whole sensing area are covered with a 0.1-0.5 μm gold plating layer, which is realized by sputtering or chemical plating process, so as to enhance the salt mist corrosion resistance.

[0024] The second aspect of the application provides a preparation method of a MEMS four-electrode conductivity sensor based on laser drilling back lead, comprising the following steps: S1: sensor structure manufacturing An LCP film with appropriate thickness is selected as the LCP substrate 1. On the front surface of the LCP substrate, four concentric circular metal electrodes are sequentially manufactured by MEMS process (such as magnetron sputtering, photolithography, and electroplating). The specific size is as follows: the innermost circular ring is the first current electrode 2, with a radius of 1 mm. After an insulating area with a spacing of 0.3 mm, the first voltage electrode 3 is manufactured, with a ring width of 0.3 mm. After another spacing of 2.3 mm, the second voltage electrode 4 is manufactured, with a ring width of 0.3 mm. Finally, after a spacing of 0.3 mm, the outermost second current electrode 5 is manufactured, with a ring width of 0.5 mm. Laser drilling points 6 are marked on the non-electric field sensitive area (inner or outer edge) of each electrode, avoiding the electric field core area (such as a distance from the electrode edge ≥0.2 mm), to ensure that the measurement signal is not disturbed.

[0025] S2: laser drilling and interconnection packaging A laser processing system is used to accurately drill micro vias 6 with a diameter of 80-120 μm at the marked points, with a hole wall perpendicularity ≤2°, penetrating through the LCP substrate.

[0026] Four platinum wires 7 (diameter 50-80 μm) are cut and respectively passed through the micro vias 6 of the corresponding electrodes from the back of the sensor, with slight pressure (pressure ≤0.5 N) to ensure that the top end of the platinum wire forms a low-resistance contact (contact resistance ≤10 mΩ) with the metal layer of the front electrode. A precision dispensing device is used to accurately coat high-conductivity platinum paste around the outlet of each micro via 6 on the back of the sensor, wrapping the root of the platinum wire 7, and allowing some of the paste to penetrate into the micro via (penetration depth ≥50 μm). The sensor is placed in a sintering furnace and sintered according to the required temperature curve of the platinum paste in air or inert atmosphere. After sintering, the platinum paste layer 8 is solidified and forms a metallurgical bond (bonding strength ≥15 N / mm²) with the LCP substrate, the electrode metal and the platinum wire, constructing a platinum paste pad with a diameter of 200-300 μm and realizing electrical conduction with each electrode.

[0027] S3: system integration and application The packaged sensor is turned over, with its back facing down. Using tin paste or conductive glue, the platinum paste pad 8 on the back is aligned with the corresponding pad on the pre-processed circuit board 9, and welding is completed by reflow soldering (peak temperature 240℃±5℃) or hot pressing (pressure 0.5MPa). In this way, the sensor is integrated as a module, and is integrated with the signal acquisition and processing circuit.

[0028] The welded sensor is tested for electrical performance, and the results are: the contact resistance between the platinum wire and the electrode metal layer is ≤10mΩ, meeting the low-resistance conduction requirement; the voltage fluctuation is ≤1mV, indicating that there is no significant noise interference in the signal transmission between the electrodes; through-40℃~85℃ temperature cycle testing, the sensor performance does not appear abnormal (such as electrode fracture, contact failure, etc.), and the long-term operation data loss rate in a marine environment with salinity of 32‰ and temperature of 30℃ is ≤2%.

[0029] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited by the above examples, and the above examples and descriptions in the specification are only preferred examples of the present application, and are not intended to limit the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A MEMS four-electrode conductivity sensor based on laser-drilled back leads, comprising an LCP substrate and a four-electrode ring structure disposed on the front side of the LCP substrate, characterized in that: The four-electrode ring structure comprises, from the center of the substrate outwards, the following: First current electrode: Inner ring electrode, used to inject measurement current; First voltage electrode: spaced a certain distance from the first current electrode, used to measure voltage signals; Second voltage electrode: It is spaced at a certain distance from the first voltage electrode and is used to measure the potential difference within the fixed gap between it and the first voltage electrode; Second current electrode: outer ring electrode, used for current return; The non-electric field sensitive areas of each electrode are formed by laser drilling to create micro-vias that penetrate the LCP substrate; platinum wires are inserted into the micro-vias from the back of the LCP substrate, with the bottom end fixed by sintering through a conductive material covering the micro-via area on the back of the LCP substrate, and the top end forming a low-resistance electrical connection with the metal layer of each electrode.

2. The MEMS four-electrode conductivity sensor based on laser-drilled back leads as described in claim 1, characterized in that: The radius of the first current electrode is 0.9-1.1 mm; the ring width of the second current electrode is 0.4-0.6 mm; the ring width of the first voltage electrode and the second voltage electrode is 0.2-0.4 mm; the insulation gap between adjacent voltage-current electrodes is 0.2-0.4 mm; and the insulation gap between adjacent voltage-voltage electrodes is 2.2-2.4 mm.

3. The MEMS four-electrode conductivity sensor based on laser-drilled back leads as described in claim 1, characterized in that: The conductive material is a highly conductive platinum paste, which wraps around the root of the platinum wire and penetrates into the sidewall of the micro-hole. Through a high-temperature sintering process, the highly conductive platinum paste is solidified and forms a strong metallurgical-mechanical composite bond with the LCP substrate, the metal layers of each electrode, and the platinum wire, thereby forming a platinum paste pad that is electrically connected to each electrode.

4. The MEMS four-electrode conductivity sensor based on laser-drilled back leads as described in claim 1, characterized in that: It also includes a gold plating layer covering the LCP substrate surface and the entire sensing area.

5. A method for fabricating a MEMS four-electrode conductivity sensor based on laser-drilled back leads as described in any one of claims 1-4, characterized in that: Includes the following steps: S1, Sensor Structure Manufacturing S1.

1. Using LCP thin film as substrate, four concentric ring metal electrodes are fabricated on the front side of the substrate using MEMS technology. From the inside to the outside, they are the first current electrode, the first voltage electrode, the second voltage electrode, and the second current electrode. S1.2 Mark the laser drilling points in the non-electric field sensitive areas of each electrode; S2, Laser Drilling and Interconnect Packaging S2.1 Drill holes at the laser drilling points to form micro-vias that penetrate the LCP film; S2.2 Insert the platinum wire into the micro-through hole from the back side, and make the tip of the platinum wire form a low-resistance contact with the metal layer of each electrode; S2.

3. Coat the micropore area of ​​the LCP film with highly conductive platinum paste, wrap the root of the platinum wire and penetrate into the micropore; S2.

4. Sintering in air or an inert atmosphere to firmly bond the platinum paste with the LCP film, the metal layers of each electrode, and the platinum wire, forming platinum paste pads that are electrically connected to each electrode. S3, System Integration and Application Align the platinum paste pads with the circuit board pads and complete the soldering by reflow soldering or hot pressing.

6. The fabrication method of a MEMS four-electrode conductivity sensor based on laser-drilled back leads as described in claim 5, characterized in that: In step S1, the innermost ring is the first current electrode with a radius of 1 mm; after an insulating area of ​​0.3 mm, the first voltage electrode is made with a ring width of 0.3 mm; after another 2.3 mm interval, the second voltage electrode is made with a ring width of 0.3 mm; and finally, after another 0.3 mm interval, the outermost second current electrode is made with a ring width of 0.5 mm.

7. The fabrication method of a MEMS four-electrode conductivity sensor based on laser-drilled back leads as described in claim 5, characterized in that: The diameter of the micro-hole is 80 μm, and the verticality of the hole wall is ≤2°.

8. The fabrication method of a MEMS four-electrode conductivity sensor based on laser-drilled back leads as described in claim 5, characterized in that: It also includes forming a gold plating layer on the LCP substrate surface and the sensing area.

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