Low-stress integrated packaging process and structure for two-dimensional CMUT ultrathin film array

Abstract

The invention belongs to the technical field of ultrasonic transducer packaging and acoustic device manufacturing, and particularly relates to a low-stress integrated packaging technology and structure for a two-dimensional CMUT ultrathin film array, and the structure comprises a printed circuit board which is provided with the two-dimensional CMUT ultrathin film array. The surface of the printed circuit board is connected with a silicone rubber curvature acoustic lens serving as a packaging layer on one side of the emission surface of the two-dimensional CMUT ultrathin film array, the silicone rubber curvature acoustic lens covers the emission surface of the two-dimensional CMUT ultrathin film array, and the focus of the silicone rubber curvature acoustic lens is located on the two-dimensional CMUT ultrathin film array. The silicone rubber curvature acoustic lens further forms edge covering on the two-dimensional CMUT ultrathin film array on the printed circuit board along the surface periphery of the two-dimensional CMUT ultrathin film array, and sealing of the two-dimensional CMUT ultrathin film array is achieved. High-density filler, a light scattering film or a multi-layer gradient matching layer does not need to be adopted, the process is simplified, the stress is extremely low, and stable performance, safety and reliability of the CMUT array are ensured.

Description

Technical Field

[0001] This invention belongs to the field of ultrasonic transducer packaging and acoustic device manufacturing technology, specifically relating to a low-stress integrated packaging process and structure for two-dimensional CMUT ultrathin film arrays. "CMUT" refers to a capacitive micromachined ultrasonic transducer, and "two-dimensional CMUT ultrathin film array" refers to a two-dimensional planar array device composed of multiple micromachined ultrasonic diaphragm units. Background Technology

[0002] Ultrasonic transducers typically require an acoustic window or matching layer at their front end to protect the sensitive transducer unit and achieve effective ultrasonic coupling with external media (such as water, human tissue, etc.). To address the need for improved acoustic energy transmission efficiency and bandwidth, various encapsulation schemes have been proposed in existing technologies. For example, to reduce acoustic impedance mismatch and broaden bandwidth, multilayer or gradient acoustic matching structures are often used. This involves placing multiple layers of material with progressively decreasing acoustic impedance between the transducer and the medium, allowing for a gradual transition of sound waves. However, the fabrication process of such multilayer gradient structures is complex, and problems such as poor interlayer bonding, porosity defects, or uneven filler distribution often lead to additional acoustic scattering and reflection, thus negating the expected performance improvement. Another example is the use of high-density fillers (such as metal oxide powder) in silicone rubber to increase the acoustic impedance of lenses, thus approximating the impedance of biological tissue. These filled acoustic lenses can reduce interface reflection to some extent, but require the addition of a large amount (tens of parts by mass) of micron / nano particles. High filler content not only increases the viscosity and processing difficulty of silicone rubber mixtures, potentially leading to uneven filler dispersion and increased acoustic attenuation inside the lens after curing, but also introduces residual stress due to the difference in thermal expansion properties between particles and the matrix, affecting the reliability of the transducer. For example, in composite probes for photoacoustic imaging, some literature proposes depositing a thin film of light-scattering particles on the front surface of the acoustic lens to improve the uniformity of photoacoustic excitation. However, for pure ultrasonic applications, adding such a light-scattering film is unnecessary and may introduce additional interfaces, complicating the lens material properties.

[0003] On the other hand, the transducer units of the two-dimensional CMUT ultrasonic array are ultra-thin films with a thickness of only a few micrometers, making them extremely sensitive to mechanical and thermal stresses during the packaging process. Using traditional packaging techniques, such as high-pressure clamping of components with fixtures or curing adhesives at high temperatures, can cause deformation or even damage to the array film, reducing device performance. Therefore, the industry urgently needs a simple and reliable packaging method that provides necessary acoustic coupling and protection while minimizing packaging stress and avoiding complex solutions such as high-density fillers, multi-layer gradient structures, or light-scattering films. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a low-stress integrated packaging process and structure for two-dimensional CMUT ultra-thin film arrays. This invention abandons the practice of adding high-density fillers, setting light scattering films or constructing multi-layer gradient matching layers in the acoustically transparent medium. Instead, it achieves curvature-locked packaging in one step through a special process, forming an ultra-thin and uniform front acoustic window that is firmly bonded to the array and provides electrical insulation and acoustic coupling functions.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A low-stress integrated packaging structure for a two-dimensional CMUT ultra-thin film array includes a printed circuit board (PCB). The PCB is mounted with the CMUT ultra-thin film array. A silicone rubber curvature acoustic lens, serving as an encapsulation layer, is connected to one side of the emitting surface of the CMUT ultra-thin film array on the surface of the PCB. The silicone rubber curvature acoustic lens covers the emitting surface of the CMUT ultra-thin film array, and its focal point is located on the CMUT ultra-thin film array. The silicone rubber curvature acoustic lens also forms an edge around the CMUT ultra-thin film array along the periphery of the surface of the CMUT ultra-thin film array on the PCB, thereby sealing the CMUT ultra-thin film array.

[0006] Preferably, the top surface of the silicone rubber curvature acoustic lens is part of a sphere.

[0007] Preferably, the radius of curvature of the silicone rubber curvature acoustic lens is 10–15 mm, the aperture is 3.90–4.90 mm, and the effective thickness h at the center is one-quarter of the working wavelength of the two-dimensional CMUT ultrathin film array.

[0008] Preferably, the material of the silicone rubber curvature acoustic lens is room temperature vulcanized silicone rubber.

[0009] Preferably, the material includes RTV560 silicone rubber.

[0010] Preferably, the two-dimensional CMUT ultrathin film array is spatially coaxially aligned with the acoustic curvature window of the silicone rubber curvature acoustic lens.

[0011] This invention also provides a low-stress integrated packaging method for two-dimensional CMUT ultrathin film arrays, used to realize the processing of the low-stress integrated packaging structure for two-dimensional CMUT ultrathin film arrays as described above, including the following processes: Uncured silicone rubber, used to form a silicone rubber curvature acoustic lens, is filled into a mold with a preset curvature on the bottom surface, and then the silicone rubber is subjected to vacuum degassing treatment. Then, the emitting surface of the two-dimensional CMUT ultra-thin film array of the printed circuit board assembly with the two-dimensional CMUT ultra-thin film array is facing down, and the printed circuit board is placed on the mold, so that the lower surface of the printed circuit board is in contact with the mold, and the two-dimensional CMUT ultra-thin film array is immersed in the uncured silicone rubber in the mold, and the peripheral position of the two-dimensional CMUT ultra-thin film array on the printed circuit board is in contact with the uncured silicone rubber liquid surface in the mold. After the silicone rubber is cured, the mold is removed to obtain the low-stress integrated packaging structure for the two-dimensional CMUT ultrathin film array.

[0012] Preferably, the mold is provided with at least two positioning posts, and the printed circuit board is provided with at least two positioning holes that can be clearance-fitted with the positioning posts; The printed circuit board is placed on the mold, and the positioning pins are inserted into the positioning holes. When the lower surface of the printed circuit board is in contact with the mold, the focus of the bottom surface of the mold is located on the two-dimensional CMUT ultra-thin film array.

[0013] Preferably, the low-stress integrated packaging method for two-dimensional CMUT ultrathin film arrays further includes the following process: Before placing the printed circuit board on the mold, an ultra-thin flexible interface layer is first set on one side of the emitting surface of the two-dimensional CMUT ultra-thin film array. The ultra-thin flexible interface layer covers the emitting surface of the two-dimensional CMUT ultra-thin film array and the surface of the printed circuit board. The coverage area of ​​the ultra-thin flexible interface layer includes the coverage area of ​​the silicone rubber curvature acoustic lens. The thickness of the ultra-thin flexible interface layer does not exceed one-tenth of the working wavelength of the two-dimensional CMUT ultra-thin film array. The ultrathin flexible interface layer is formed by pre-coating, in-mold preforming, or pre-film application. The material of the ultrathin flexible interface layer is any one or a combination of several of the following: silicone gel, hydrogel, and PU film; when it is a combination, it is set layer by layer.

[0014] Preferably, the low-stress integrated packaging method for two-dimensional CMUT ultrathin film arrays further includes the following process: Before injecting uncured silicone rubber into the mold to form the silicone rubber curvature acoustic lens, a release agent layer is applied to the surface of the mold cavity inside the mold, and then the uncured silicone rubber for forming the silicone rubber curvature acoustic lens is injected into the mold.

[0015] Compared with the prior art, the present invention has the following beneficial effects: In this invention, a low-stress integrated packaging structure for two-dimensional CMUT ultra-thin film arrays is presented. A silicone rubber curvature acoustic lens is directly applied to the emitting surface of the two-dimensional CMUT ultra-thin film array. Simultaneously, an edge is formed along the periphery of the two-dimensional CMUT ultra-thin film array on the printed circuit board, sealing the array. Therefore, in this invention, the silicone rubber curvature acoustic lens serves not only as an acoustic lens, providing focusing / near-field shaping, but also as a dielectric interface, directly contacting external media (such as water, coupling adhesive, or human tissue) to achieve efficient acoustic energy coupling. Furthermore, it acts as an electrical insulating layer, providing high dielectric strength isolation for the array and ensuring safe operation under high-voltage bias conditions. The dielectric breakdown field strength of silicone rubber material can reach approximately 20 kV / mm, thus enabling it to withstand high-voltage DC bias without breakdown.

[0016] This invention presents a low-stress integrated packaging method for two-dimensional CMUT ultrathin film arrays. It employs in-mold molding and one-time curing processes, eliminating the cumbersome layered assembly and particle doping steps of traditional multi-layer matching or fill-based impedance tuning schemes, significantly simplifying the process. Secondly, by locking the curvature of the silicone rubber curvature acoustic lens using a mold and vacuum degassing, internal bubbles and interface defects are eliminated during the molding of the silicone rubber curvature acoustic lens, resulting in a homogeneous and dense silicone acoustic medium and improving sound transmission efficiency. Thirdly, since no additional pressure curing or high-temperature processing is required, the mechanical and thermal stress acting on the CMUT ultrathin film during packaging is significantly reduced, protecting the structural integrity and performance stability of the array. The "self-weight bonding + room temperature curing" packaging method ensures that the ultrathin sensitive components are protected from stress concentration, achieving truly low-stress packaging. The precise thickness of the lens's front window makes the acoustic impedance transition smoother, reducing sound reflection and attenuation, thus potentially maintaining a wide operating bandwidth and high transmission and reception efficiency. Finally, the silicone rubber curvature acoustic lens serves as both an acoustic window and electrical insulation, eliminating the need for an additional isolation layer. This ensures ultrasonic coupling while enhancing the safety and reliability of the device in practical applications. Attached Figure Description

[0017] Figure 1 The figure shown is a schematic diagram of the overall structure of the two-dimensional CMUT ultrathin film array after low-stress packaging according to an embodiment of the present invention. Figure 2 The first stage of the packaging process of the present invention is shown: RTV560 curvature locking injection and in-mold degassing process.

[0018] Figure 3 The second stage of the packaging process of the present invention is shown: the array component self-weight bonding, positioning alignment and room temperature curing molding steps.

[0019] In the figure: 1-2D CMUT ultrathin film array; 2-printed circuit board; 3-silicone rubber curvature acoustic lens; 4-mold; 5-mold cavity curvature surface; 6-positioning post; 7-positioning hole; 8-ultrathin flexible interface layer; 10-uncured silicone rubber fluid material. Detailed Implementation

[0020] The present invention will be further described clearly and in detail below with reference to specific embodiments and the accompanying drawings. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0021] Figure 1 The diagram shows the overall structure of a two-dimensional CMUT ultrathin film array after low-stress packaging according to an embodiment of the present invention.

[0022] like Figure 1 and Figure 3 As shown, the two-dimensional CMUT ultra-thin film array 1 is mounted and electrically connected to the printed circuit board 2. The working surface of the two-dimensional CMUT ultra-thin film array 1 is entirely covered by an integrally molded encapsulation layer (i.e., a silicone rubber curvature acoustic lens 3). The silicone rubber curvature acoustic lens 3 is formed by curing room temperature vulcanized silicone rubber (preferably RTV560) and is a red encapsulation area. This silicone rubber curvature acoustic lens 3 covers both the emitting surface of the two-dimensional CMUT ultra-thin film array 1 and forms a rim along the periphery of the surface of the two-dimensional CMUT ultra-thin film array 1, sealing the two-dimensional CMUT ultra-thin film array 1, thereby achieving: (1) The rubber curvature acoustic lens 3 forms an acoustic front-end window with a predetermined curvature, which plays a focusing / near-field shaping role; (2) The rubber curvature acoustic lens 3 serves as a medium interface, directly contacting the external medium (such as water, coupling adhesive or human tissue) to achieve efficient coupling of acoustic energy; (3) The rubber curvature acoustic lens 3 serves as an electrical insulation protective layer, providing high dielectric strength isolation for the array and ensuring safe operation under high voltage bias conditions. The dielectric breakdown field strength of silicone rubber material can reach approximately 20 kV / mm, thus it can withstand high voltage DC bias without breakdown.

[0023] In the above scheme, the radius of curvature R of the rubber curvature acoustic lens 3 is recommended to be 13.15 mm, which can be adjusted within the range of 10–15 mm according to the array focal length requirements. The larger the R value, the gentler the lens curvature and the farther the focal point. The aperture D of the rubber curvature acoustic lens 3 is recommended to be 4.40 mm, which should be slightly larger than the size of the effective emitting surface of the array. D can be optimized within ±0.5 mm of the array size to ensure that the entire array area is covered by silicone. The front window thickness h of the rubber curvature acoustic lens 3 is recommended to be 0.18 mm, which is one-quarter of the typical operating wavelength (i.e., the operating wavelength of the two-dimensional CMUT ultra-thin film array 1). h can be appropriately adjusted within the range of 0.1–0.3 mm: reducing the thickness can reduce acoustic impedance mismatch but weaken mechanical strength, while increasing the thickness can improve durability but may slightly increase acoustic attenuation.

[0024] In this embodiment, the curvature radius R of the curvature acoustic lens 3 is 13.15 mm, the aperture D is 4.40 mm, and the effective thickness h at the center is 0.18 mm. R represents the spherical curvature radius of the curvature acoustic lens 3, D is its aperture size, and h is the minimum thickness (acoustic window thickness) directly in front of the array radiation area. By controlling h to be on the order of 0.18 mm, the acoustic impedance transition between the array and the external medium is smoother, reducing interface reflection and suppressing waveform distortion, which is beneficial for maintaining broadband and high-sensitivity output.

[0025] In addition, other structures adapted to the two-dimensional CMUT ultra-thin film array 1 are also arranged on the upper end of the printed circuit board 2, such as a high-voltage DC bias interface (DC port) and an AC transmit / receive interface (AC port), implemented for example through an SMA coaxial connector. These two interfaces are used to apply a DC bias voltage to the two-dimensional CMUT ultra-thin film array and to inject / acquire ultrasonic pulse or echo signals, respectively. To achieve high-voltage isolation, AC coupling, and decoupling protection, a protection / matching network composed of chip resistors and capacitors is provided between the DC port and the AC port. This network may include high-value resistors, DC blocking capacitors, voltage divider monitoring elements, etc., for: High voltage bias is stably superimposed onto the CMUT array (i.e., two-dimensional CMUT ultra-thin film array). Implement an AC drive / receive path within the operating bandwidth; Suppress the direct coupling of high voltage to the downstream low-voltage electronic system; During testing, the DC bias link and the RF / ultrasonic signal link are separated.

[0026] Mounting holes are provided at the four corners of the printed circuit board 2. Figure 1 The central fixing point (with screws / copper pillars) facilitates the mounting and positioning of the probe module with subsequent fixtures, immersion test brackets, or system housings, while also defining the array's orientation relative to the external platform during subsequent imaging / measurement. Through this structure, Figure 1This corresponds to "a packaged and directly testable two-dimensional CMUT array module", which is the final low-stress packaged finished product state obtained by this invention.

[0027] Figure 2 The first stage of the packaging process of the present invention is shown: RTV560 curvature locking injection and in-mold degassing process.

[0028] like Figure 2 As shown, the room temperature vulcanizing silicone rubber (e.g., RTV560) used to form the encapsulation layer 3 is first prepared. RTV560 is a two-component system containing a main agent and a curing agent. The main agent and curing agent are weighed separately using a precision electronic balance (for example, the main agent is weighed to approximately 24,000 mg, and then the curing agent is added at a mass ratio of 100:0.3, for example, the curing agent corresponds to 75 mg) to ensure accurate proportioning.

[0029] Subsequently, multi-stage mixing is performed sequentially: 1. Magnetic stirring: After adding the curing agent dropwise to the main agent, use a magnetic stirrer ( Figure 2 Use the "Magnetic Stirrer" step above to initially homogenize the mixture.

[0030] 2. Ultrasonic stirring / ultrasonic dispersion: Place the mixture in an ultrasonic stirring device ( Figure 2 The "Ultrasonic Stirrer" in the upper right corner uses high-frequency cavitation and shearing to further break up agglomerates and reduce the size of microbubbles.

[0031] 3. Second magnetic stirring: The ultrasonically dispersed mixture is returned to the magnetic stirrer for second stirring to achieve both macroscopic and microscopic uniformity in "two-way / multi-way mixing". Figure 2 As indicated by the arrow at the bottom right that says "Magnetic Stir Again / Multi-directional Mix".

[0032] The uncured silicone rubber fluid material 10, after multi-directional mixing, is slowly poured into a mold 4 with a cavity having the target curvature. The bottom surface of the inner cavity of the mold 4 (i.e., Figure 3 The bottom surface of the inner cavity of mold 4 in the indicated orientation is a spherical concave cavity (i.e., a portion of a sphere). The bottom surface of the inner cavity of mold 4 is pre-designed with a required radius of curvature R = 13.15 mm, diameter D = 4.40 mm, and central concave depth of 0.18 mm, corresponding to the geometric shape of the finished product's encapsulation layer (i.e., Figure 1 The curvature and thickness distribution of the outer surface of the red encapsulation layer. In this embodiment, the mold 4 is also machined with positioning posts 6 for subsequent positioning (see...). Figure 3 This column serves as a reference point during subsequent alignment.

[0033] A key point of this invention is that it does not first vacuum the material before pouring it in (i.e., the uncured silicone rubber fluid material 10), but instead directly pours the mixed uncured silicone rubber fluid material 10 into the mold cavity with the final curvature constraint, and then puts the whole thing into a vacuum environment for degassing.

[0034] Specifically, such as Figure 2 As shown in the lower left section, a mold 4 containing liquid silicone rubber (i.e., uncured silicone rubber fluid material 10) is placed in a vacuum chamber, and a vacuum is directly drawn inside the mold 4. Preferably, the vacuum level is drawn to -0.09 MPa (28-inch mercury vacuum) and maintained for about 30 minutes, allowing the bubbles introduced during the mixing process to grow, rise, burst, and escape under the constrained curvature geometry. Because the uncured silicone rubber fluid material 10 is already within the space defined by the target radius of curvature R, the material will not undergo secondary disturbances such as "re-pouring → re-encapsulating bubbles → surface collapse / dentation" during the subsequent high viscosity stage during the degassing process, thus avoiding problems such as local depressions, bubble sandwiching, or incomplete free liquid surfaces.

[0035] go through Figure 2 The aforementioned steps result in "uncured silicone rubber fluid material 10 that has been fully degassed and is at the target curvature". This uncured silicone rubber fluid material 10 will directly serve as the body of the final encapsulation layer, determining... Figure 1 The curvature, diameter range, and minimum thickness h of the red-enclosed region.

[0036] Figure 3 The second stage of the packaging process of the present invention is shown: the array component self-weight bonding, positioning alignment and room temperature curing molding steps.

[0037] like Figure 3 As shown in the lower left, the array assembly, which has already undergone front-end bonding / interconnection, is prepared on a PCB substrate (i.e., printed circuit board 2) with a two-dimensional CMUT ultrathin film array 1. The array has been electrically connected to the traces, voltage divider / bias networks, etc., of the printed circuit board 2 (corresponding to...). Figure 1The process of routing multiple lines between the lower red area and the upper SMA interface is a conventional technique in this field and will not be described in detail here. Before encapsulation and bonding, an ultra-thin flexible interface layer can be formed on the emitting surface side of the two-dimensional CMUT ultra-thin film array 1. The ultra-thin flexible interface layer covers the emitting surface of the two-dimensional CMUT ultra-thin film array 1 and the surface of the printed circuit board 1. The area covered by the ultra-thin flexible interface layer includes the coverage area of ​​the silicone rubber curvature acoustic lens 1. In this embodiment, an ultra-thin flexible interface layer 8 is introduced between the surface of the two-dimensional CMUT ultra-thin film array 1 and the main package (lens body), and between the printed circuit board and the main package (lens body). The ultrathin flexible interface layer 8 can be achieved through pre-coating, pre-fabricated film application, or in-mold preforming. The material of the ultrathin flexible interface layer 8 can be any one or a combination of several biocompatible and acoustically transmissive media such as extremely soft silicone gel, hydrogel, and PU film. When in a combined form, it is layered sequentially. The ultrathin flexible interface layer 8 employs a controllable weakly bonded / debonded interface on the side facing the array surface, and an activated adhesive layer is provided on the side facing the silicone rubber curvature acoustic lens to cross-link and cure with the uncured silicone rubber to form a strong bond. The thickness of the ultrathin flexible interface layer 8 is controlled to be less than one-tenth of the typical operating wavelength of the two-dimensional CMUT ultrathin film array 1, typically set to 0.10–0.20 mm, to reduce acoustic impedance mismatch and provide good coupling. To facilitate later maintenance, the ultrathin flexible interface layer 8 adopts a reworkable design: preferably, the ultrathin flexible interface layer 8 forms a strong bond with the lens body (i.e., the silicone rubber curvature acoustic lens 3), while forming a controllable weak bond or a debonding interface with the surface of the two-dimensional CMUT ultrathin film 1 array and the surface of the printed circuit board 2 (e.g., achieved through low surface energy undercoating, UV debonding, thermal softening / swelling peelable formulation, etc.). When the silicone rubber curvature acoustic lens 3 ages due to increased use, the thin flexible interface layer 8 can be locally heated (generally ≤50°C) or wet-swelled to significantly reduce the adhesion of the thin flexible interface layer 8, thereby allowing the entire "thin flexible interface layer 8 + silicone rubber curvature acoustic lens 3" assembly to be peeled off; the array surface only needs gentle cleaning to remove residues, maintaining the integrity of the chip and metal interconnection and quickly entering the same secondary packaging process as the above packaging process. The interface layer (i.e., the thin flexible interface layer 8) serves as a stress buffer layer, which can buffer curing shrinkage and thermal-humid stress, and protect the ultra-thin film array from curing tension. On the other hand, it serves as a transition acoustic layer, which stabilizes and enhances the coupling between the lens and the CMUT, stabilizes the acoustic impedance transition between the array and the lens, and at the same time serves as a one-time "sacrificial layer" to achieve reversible encapsulation.

[0038] like Figure 3 As shown above, the previous step ( Figure 2The resulting mold 4 has now undergone in-mold vacuum degassing, and the mold cavity is filled with still-flowing (uncured) silicone rubber (i.e., uncured silicone rubber fluid material 10). Two or more positioning posts 6 are provided on the mold 4.

[0039] Subsequently, as Figure 3 As shown by the arrow on the right, the PCB substrate is flipped so that the 2D CMUT ultrathin film array 1 faces downwards. Mechanical alignment and guidance are achieved using the positioning holes 7 on the PCB and the positioning posts 6 on the mold 4. This alignment method ensures that: The two-dimensional CMUT ultrathin film array 1 accurately falls into the center of the target area of ​​the defoamed silicone rubber; The thickness of the encapsulation layer is uniform throughout the entire working aperture of the two-dimensional CMUT ultra-thin film array 1. The multi-channel array unit is spatially coaxially aligned with the expected acoustic curvature window.

[0040] After completing the hole-to-pillar alignment, the PCB board is lowered further, allowing the front side of the array to gently contact and immerse itself in the silicone rubber. This process does not require external clamping pressure; instead, it relies primarily on the weight of the PCB assembly itself to allow the array to slowly sink into the silicone rubber and be "attached" and fixed by the surface tension of the liquid. Because there is no external load, the CMUT's ultra-thin diaphragm does not experience additional static mechanical stress, avoiding the deformation, warping, or even damage to the diaphragm that is easily caused by traditional press-fit packaging, thus achieving "low-stress bonding."

[0041] After completing the self-weight bonding and positioning as described above, keep the PCB board and mold 4 aligned and allow it to cure at room temperature. Since the material used is room temperature vulcanizing silicone rubber, curing can be completed at around 25°C (i.e., 22-25°C), and generally, a curing time of no less than 24 hours is sufficient to achieve full cross-linking. If necessary, curing can be slightly accelerated under gentle heating conditions not exceeding 50°C, but high temperatures should be avoided to reduce thermal stress accumulation.

[0042] After curing, the PCB board, along with the formed and firmly bonded encapsulation layer, is removed from mold 4. Because the inner wall of mold 4 can be pre-treated with a release agent layer (e.g., silicone-based release agent), the cured lens / encapsulation layer easily detaches from the mold without being torn or scratched during demolding. The resulting structure is... Figure 1 The finished product shown: The working surface of the array is completely covered by an integrated curvature acoustic lens / insulating encapsulation layer. The outer periphery can be fixed to the test fixture / bracket through the mounting hole 7. The top is connected to the external high voltage bias and ultrasonic transmission and reception system through the DC port and AC port. The module can be directly immersed in a water tank or coupling medium for acoustic testing and imaging verification.

[0043] In the above packaging process, the parameters and process window recommended in this application are as follows: Silicone rubber dosage and ratio: 25 g of RTV560 silicone rubber base is required for each encapsulation, with 0.075 g of curing agent added at 0.3%. The ratio of base agent to curing agent can be finely adjusted within the range of 100:0.2 to 100:0.5 to affect the curing speed and hardness. During mixing, it is recommended to use magnetic stirring for at least 5 minutes, followed by ultrasonic degassing for 1–2 minutes to obtain a uniform colloid.

[0044] Vacuum degassing conditions: It is recommended to degas at -0.08 to -0.09 MPa (equivalent to 24–28 inHg vacuum) for 20–30 minutes. Insufficient vacuum will result in residual bubbles affecting acoustic performance; maintaining vacuum for more than 30 minutes will not significantly improve the effect of further bubble removal. The bubble removal process can be monitored through a transparent mold or observation window to determine the end time of degassing.

[0045] Curing time at room temperature: It is recommended to cure at room temperature (22–25℃) for at least 24 hours to ensure complete curing. If a shorter production cycle is required, curing can be carried out at 30–50℃ for 4–8 hours under gentle heating, but avoid exceeding 50℃ to prevent thermal expansion of the silicone rubber and the generation of additional stress. The lens surface should be dry and non-sticky after curing before demolding.

[0046] Primer application: It is recommended to use a silicone rubber primer such as SS4004P, and apply it evenly to the array surface to form a film 5–10 μm thick. After coating, allow it to stand at room temperature for 5–15 minutes to allow the solvent to evaporate. The best results will be achieved when the surface is slightly tacky before proceeding to the next encapsulation step. An excessively thick primer layer may affect sound transmission performance, while an excessively thin layer will result in poor adhesion. The coating should be controlled to be uniform and continuous.

[0047] Mold Release Treatment: It is recommended to apply a thin layer of silicone oil or fluorine-based release agent (such as 717 release agent) to the mold cavity surface. Before use, ensure the mold cavity is clean and dust-free. After evenly applying the release agent, allow it to dry naturally for at least 10 minutes. Reapply the release agent before each use to ensure effective release. Do not use release agents that may chemically react with silicone.

[0048] Positioning pin and positioning hole design: It is recommended that the gap between the diameter of the positioning pin and the diameter of the PCB positioning hole be controlled within 0.05–0.1mm to ensure both smooth insertion and positioning accuracy. The height of the positioning pin should be slightly lower than the thickness of the silicone filler to prevent the PCB board from being suspended during bonding. The positioning pin material can be metal or rigid plastic, and must have good dimensional stability and reusability after demolding.

[0049] External Coupling and Use: Before use, the encapsulated array can be coated with an ultrasonic coupling agent (such as water or ultrasonic gel) on the lens surface to eliminate air gaps. This will further improve the efficiency of ultrasonic energy coupling from lens 3 into the external medium 9. During use, avoid scratching the lens surface with hard objects to prevent affecting acoustic performance.

[0050] Summary of the technical effects of this invention (corresponding) Figure 1 – Figure 3 ): 1. Curvature locking + in-mold vacuuming ( Figure 2 ) First, silicone rubber is injected into the mold cavity with the final curvature R, and then a vacuum is drawn to ensure that the silicone rubber 10 is degassed under the target geometry, forming an acoustic window with no bubbles, low internal defects, and a thickness of h=0.18 mm. This avoids the secondary air bubble roll, free liquid surface collapse, and uneven thickness caused by the traditional "vacuuming first and then pouring material" method.

[0051] 2. Positioning pin / positioning hole guide + self-weight fit ( Figure 3 ) The PCB board is precisely aligned with the positioning post 6 of the mold 4 through the positioning hole 7. The array naturally sinks into the still liquid silicone rubber and achieves bonding and fixation by its own weight without applying external pressure. This significantly reduces the static mechanical stress on the ultra-thin film CMUT diaphragm, ensuring the stable performance and safety of the CMUT array.

[0052] 3. Integrated curing molding + dual functions of electrical insulation and acoustics ( Figure 1 ) The encapsulation layer formed after room temperature curing simultaneously serves multiple functions, including an acoustic lens, a liquid / human body coupling interface, a high-voltage electrical insulation layer, and a mechanical protection layer. This eliminates the need for an additional casing or front-end window layer, resulting in a compact structure, controlled thickness, and clear interfaces (DC bias 4 / AC signal 5).

[0053] 4. High-density fillers, light-scattering films, or multilayer coupling agent gradient structures were not used.

[0054] 5. The silicone rubber is bonded to the array by its own weight, and guided and positioned using the positioning posts on the mold and the positioning holes on the printed circuit board, without the need for external pressure pressing. The encapsulation is completed in one step at room temperature, so that the silicone rubber forms an acoustic lens / coupling surface with curvature that matches the front end of the array after curing, and achieves permanent bonding with the array. At the same time, it forms an electrical insulation barrier with a dielectric strength of about 20 kV / mm and an acoustic coupling interface with the external coupling medium. The thickness of the front end window is precisely controlled by the mold cavity thickness, avoiding human processing errors. After curing, the encapsulated array assembly is removed from the mold.

[0055] 6. The lens material is room temperature curing silicone rubber (such as RTV560), which does not contain any high-density fillers or delamination structures. The outer surface of the lens is the front window of the transducer, which is in direct contact with external media such as water, ultrasonic coupling adhesive or biological tissue for the transfer of ultrasonic energy; at the same time, the silicone rubber curvature acoustic lens (i.e., curvature silicone rubber curvature acoustic lens 3) itself has high dielectric strength, which can reliably electrically isolate the electrodes of the array from the outside world, meeting the requirements for safe operation of CMUT devices under high voltage bias.

[0056] In summary, this invention achieves low-stress, high-performance encapsulation of two-dimensional CMUT ultrathin film arrays through ingenious process design. The key lies in utilizing a curved mold cavity to constrain the shape of the silicone rubber, combined with vacuum degassing, to allow the material to solidify and form in its final form; employing weight-bearing bonding and primer adhesion methods to avoid stress concentration caused by mechanical extrusion; and using room-temperature curing flexible silicone rubber to perform multiple functions of lens, coupling layer, and insulating encapsulation in a single step. Compared with existing technologies, this invention significantly reduces component interfaces, eliminates internal bubbles, and features a simple, integrated encapsulation structure, ensuring the stable performance and lifespan of the CMUT array. It can be widely applied in the packaging and manufacturing of high-density ultrasonic array devices.

[0057] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A low-stress integrated packaging structure for two-dimensional CMUT ultrathin film arrays, characterized in that, The device includes a printed circuit board (2), on which a two-dimensional CMUT ultra-thin film array (1) is mounted. A silicone rubber curvature acoustic lens (3) serving as an encapsulation layer is connected to one side of the emitting surface of the two-dimensional CMUT ultra-thin film array (1) on the surface of the printed circuit board (2). The silicone rubber curvature acoustic lens (3) covers the emitting surface of the two-dimensional CMUT ultra-thin film array (1). The focal point of the silicone rubber curvature acoustic lens (3) is located on the two-dimensional CMUT ultra-thin film array (1). The silicone rubber curvature acoustic lens (3) also forms an edge around the two-dimensional CMUT ultra-thin film array (1) along the periphery of the surface of the two-dimensional CMUT ultra-thin film array (1) on the printed circuit board (2), thereby sealing the two-dimensional CMUT ultra-thin film array (1).

2. The low-stress integrated packaging structure for two-dimensional CMUT ultrathin film arrays according to claim 1, characterized in that, The top surface of the silicone rubber curvature acoustic lens (3) is part of a sphere.

3. The low-stress integrated packaging structure for two-dimensional CMUT ultrathin film arrays according to claim 2, characterized in that, The radius of curvature of the silicone rubber curvature acoustic lens (3) is 10–15 mm, the aperture is 3.90–4.90 mm, and the effective thickness h at the center is one-quarter of the working wavelength of the two-dimensional CMUT ultra-thin film array (1).

4. The low-stress integrated packaging structure for two-dimensional CMUT ultrathin film arrays according to claim 1, characterized in that, The material of the silicone rubber curvature acoustic lens (3) is room temperature vulcanized silicone rubber.

5. The low-stress integrated packaging structure for two-dimensional CMUT ultrathin film arrays according to claim 4, characterized in that, The product includes RTV560 silicone rubber.

6. The low-stress integrated packaging structure for two-dimensional CMUT ultrathin film arrays according to claim 1, characterized in that, The two-dimensional CMUT ultrathin film array (1) is spatially aligned with the acoustic curvature window of the silicone rubber curvature acoustic lens (3).

7. A low-stress integrated packaging method for two-dimensional CMUT ultrathin film arrays, characterized in that, The fabrication process for realizing the low-stress integrated packaging structure for any one of claims 1-6, oriented towards a two-dimensional CMUT ultra-thin film array, includes the following steps: Uncured silicone rubber for forming a silicone rubber curvature acoustic lens (3) is filled into a mold (4) with a preset curvature on the bottom surface, and then the silicone rubber is subjected to vacuum degassing treatment. Then, the emitting surface of the two-dimensional CMUT ultra-thin film array (1) of the printed circuit board (2) assembly with the two-dimensional CMUT ultra-thin film array (1) is facing down, and the printed circuit board (2) is placed on the mold (4) so ​​that the lower surface of the printed circuit board (2) is in contact with the mold (4), and the two-dimensional CMUT ultra-thin film array (1) is immersed in the uncured silicone rubber in the mold (4), and the peripheral position of the two-dimensional CMUT ultra-thin film array (1) on the printed circuit board (2) is in contact with the uncured silicone rubber liquid surface in the mold (4); After the silicone rubber is cured, demolding is performed and the mold (4) is removed to obtain the low-stress integrated packaging structure for the two-dimensional CMUT ultra-thin film array.

8. A low-stress integrated packaging method for two-dimensional CMUT ultrathin film arrays according to claim 7, characterized in that, The mold (4) is provided with at least two positioning posts (6), and the printed circuit board (2) is provided with at least two positioning holes (7) that can be clearance-fitted with the positioning posts (6). The printed circuit board (2) is placed on the mold (4) and the positioning post (6) is inserted into the positioning hole (7). When the lower surface of the printed circuit board (2) is in contact with the mold (4), the focus of the bottom surface of the mold (4) is located on the two-dimensional CMUT ultra-thin film array (1).

9. A low-stress integrated packaging method for two-dimensional CMUT ultrathin film arrays according to claim 7, characterized in that, It also includes the following processes: Before placing the printed circuit board (2) on the mold (4), an ultra-thin flexible interface layer is first set on one side of the emitting surface of the two-dimensional CMUT ultra-thin film array (1). The ultra-thin flexible interface layer covers the emitting surface of the two-dimensional CMUT ultra-thin film array (1) and the surface of the printed circuit board (2). The coverage area of ​​the ultra-thin flexible interface layer includes the coverage area of ​​the silicone rubber curvature acoustic lens (3). The thickness of the ultra-thin flexible interface layer does not exceed one-tenth of the working wavelength of the two-dimensional CMUT ultra-thin film array (1). The ultrathin flexible interface layer is formed by pre-coating, in-mold preforming, or pre-film application. The material of the ultrathin flexible interface layer is any one or a combination of several of the following: silicone gel, hydrogel, and PU film; when it is a combination, it is set layer by layer.

10. A low-stress integrated packaging method for two-dimensional CMUT ultrathin film arrays according to claim 7, characterized in that, It also includes the following processes: Before injecting uncured silicone rubber into the mold (4) to form the silicone rubber curvature acoustic lens (3), a release agent layer is applied to the surface of the mold cavity in the mold (4), and then the uncured silicone rubber is injected into the mold (4) to form the silicone rubber curvature acoustic lens (3).

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