Method for manufacturing embedded cavity of LTCC (Low Temperature Co-Fired Ceramic) substrate

Abstract

The invention relates to a method for manufacturing an embedded cavity of an LTCC (Low Temperature Co-Fired Ceramic) substrate. The method comprises the following steps of: carrying out integral primary pre-pressing on a micro-cavity lower green ceramic blank forming a lower blank layer; performing laser cutting on the green ceramic chip forming the cavity layer to form a cavity, and performing secondary pre-pressing on the cavity layer with the cavity and the lower green ceramic blank after primary pre-pressing; printing sacrificial slurry in the cavity of the cavity layer by adopting a silk-screen printing technology, and performing drying and leveling treatment to complete cavity filling; the cavity upper layer raw ceramic chips forming the upper blank body layer are laminated above the cavity layer in a layer-by-layer alignment mode to form a raw ceramic laminated body, and the raw ceramic laminated body is subjected to overall isostatic pressing treatment; and carrying out glue discharging and sintering treatment on the raw ceramic laminated body subjected to isostatic pressing treatment to obtain the LTCC substrate with the embedded cavity. According to the method, the high-efficiency, high-precision and wide-adaptability batch manufacturing of the embedded cavity of the LTCC substrate is realized through a combined process of forming the cavity through laser cutting and filling the sacrificial slurry through silk-screen printing.

Description

Technical Field

[0001] This invention relates to the field of LTCC substrate technology, and specifically to a method for fabricating an embedded cavity in an LTCC substrate using screen printing technology. Background Technology

[0002] MEMS devices are widely used in aerospace and other fields due to their small size and light weight. LTCC substrates, with their multi-layer wiring and vertical interconnect structure, are particularly suitable as substrates for highly integrated MEMS device packaging. However, the thermal conductivity of LTCC substrates is much lower than that of alumina and aluminum nitride substrates. To solve the heat dissipation problem of LTCC substrates in highly integrated and miniaturized 3D packaging applications, the industry typically creates embedded channels (i.e., microcavities) inside the substrate and introduces coolant to carry away the heat generated by the chip, thereby achieving efficient heat dissipation.

[0003] Existing conventional methods for fabricating microcavities typically involve creating prefabricated solid sacrificial material blocks of corresponding shapes based on the cavity's internal structure and then filling them. These sacrificial materials act as supports during the isostatic pressing process on the substrate, preventing the cavity from collapsing. However, this method has the following significant drawbacks:

[0004] (1) The thickness of each green ceramic layer in the LTCC substrate is relatively thin, only 0.114 to 0.127 mm, while the pre-fabricated solid sacrificial block is prone to dimensional deviations in the X, Y, and Z directions. After isostatic pressing, negative tolerances in the X / Y directions of the sacrificial block will cause the cavity size to become smaller and deviate from the design value, while positive tolerances will cause substrate deformation; negative tolerances in the Z direction will cause the cavity to collapse, while positive tolerances will cause the substrate surface to arch. These problems will ultimately affect the final actual value of the microcavity, and thus affect the heat dissipation of the substrate.

[0005] (2) For cavity designs with complex shapes or small sizes (≤0.2mm), the processing difficulty of prefabricated solid sacrificial blocks will increase significantly, making it difficult to meet the requirements of precise processing.

[0006] (3) Precast solid sacrificial blocks need to be added and placed manually one by one, which not only makes it difficult to ensure the consistency between products, but also results in low production efficiency and cannot be adapted to mass production.

[0007] Therefore, there is an urgent need to develop a method for fabricating embedded cavities in LTCC substrates to address the problems of insufficient processing accuracy, limited adaptability, and low production efficiency in existing technologies, and to meet the practical application requirements of high-integration packaging for MEMS devices. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a method for fabricating an embedded cavity in an LCTT substrate.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A method for fabricating an embedded cavity in an LTCC substrate, wherein the LTCC substrate comprises an upper preform layer, a cavity layer, and a lower preform layer arranged sequentially from top to bottom; the cavity layer has a cavity formed therein; the fabrication method includes the following steps: S1. Primary pre-compression: The micro-cavity under-ceramic body constituting the under-body layer is subjected to overall primary pre-compression; S2. Cavity Forming and Secondary Pre-pressing: Cut all the green ceramic pieces that make up the cavity layer into cavity shapes, then perform alignment and stacking and isostatic pressing on the cut green ceramic pieces to form a cavity layer with cavities. Align and stack the cavity layer with cavities with the lower green ceramic body after the first pre-pressing and perform secondary pre-pressing. S3. Cavity filling: The sacrificial paste is printed into the cavity of the cavity layer using screen printing technology, and the cavity filling is completed after drying and leveling. Preferably, the sacrificial paste is a carbon-based organic material. S4. Isostatic pressing: The upper layer of green ceramic pieces constituting the upper body layer is stacked layer by layer on the cavity layer after step S3 to form a green ceramic composite. The green ceramic composite is then subjected to overall isostatic pressing. S5. Debinding and Sintering: The green ceramic laminate after isostatic pressing is subjected to debinding and sintering to obtain an LTCC substrate with an embedded cavity.

[0010] As a further improvement to the above technical solution, step S1 specifically includes: S11. Stacking and fixing: All the green ceramic pieces that make up the lower green ceramic body are stacked in a preset order and fixed on the stacking plate. S12. Sealing process: Cover the surface of the stacked green ceramic tiles with a layer of plastic film, put the whole thing into an aluminum foil bag, and vacuum seal it. S13. Isostatic pre-pressing: The sealed raw ceramic body is placed in an isostatic press, the isostatic parameters are set, and the raw ceramic body is subjected to isostatic pre-pressing, so that all the raw ceramic pieces generate preliminary adhesion under the action of their own adhesive, forming the raw body layer.

[0011] As a further improvement to the above technical solution, step S2 specifically includes: S21. Cavity cutting: A laser cutting machine is used to cut the cavity shape of each layer of green ceramic sheet that constitutes the cavity layer. After cutting, each layer of green ceramic sheet forms a cavity with a cavity outline. S22. Alignment and Lamination: After the cavity cutting is completed, first remove the plastic film on the surface of the pre-compressed lower ceramic body in step S1, and then align and laminate the first layer of cavity-bearing green ceramic sheet above the lower ceramic body with the lower ceramic body according to the preset lamination sequence; preferably, the plastic film is made of polyethylene.

[0012] S23. Isostatic pre-compression: Repeat the isostatic pre-compression operation in step S1 to pre-compress the lower green ceramic body and the green ceramic slab with cavity in this layer, so that the two are initially compacted to form a pre-compressed body with cavity layer.

[0013] As a further improvement to the above technical solution, step S3 specifically includes: S31. Printing preparation: Place the pre-pressed blank with cavity layer formed in step S2 on the worktable of the screen printing machine, and fill the cavity with sacrificial paste using a stainless steel screen that matches the shape of the cavity. S32, Sacrificial paste printing: Using a screen printing machine, the sacrificial paste is printed and filled into the cavity of the cavity layer according to the cavity contour; S33. Drying treatment: The preform after printing and filling with sacrificial paste is placed in an oven for drying treatment; S34. Leveling treatment: Place the dried preform on a leveling machine to level the sacrificial slurry protruding from the cavity surface. S35, Remove the film for later use: Remove the mylar film from the surface of the embryo.

[0014] As a further improvement to the above technical solution, step S4 specifically includes: S41. Cutting of the cavity inlet and outlet of the upper body layer: For all the green ceramic pieces that make up the upper body layer, use a laser cutting machine to cut out the cavity inlet and outlet that match the cavity of the cavity layer; S42. Alignment and stacking of the upper body layer: The upper body layer green ceramic pieces with the cut cavity inlet and outlet are aligned and stacked layer by layer in a preset order, covering the cavity layer after step S3, to form a complete green ceramic piece stack consisting of the lower green ceramic body, the cavity layer, and the upper body layer. S43. Vacuum sealing: Cover the surface of the green ceramic tile composite with a plastic film and perform vacuum sealing. S44. Isostatic pressing treatment: Place the plastic-sealed green ceramic tile assembly into an isostatic press, set the isostatic pressing parameters, and perform isostatic pressing treatment on the green ceramic tile assembly.

[0015] As a further improvement to the above technical solution, step S5 specifically includes: S51. Sintering preparation: The green ceramic sheet composite after isostatic pressing in step S4 is placed into a co-firing furnace for sintering. S52, Debinding Stage: Set the debinding parameters for the co-firing furnace. Under these parameters, the organic matter contained in the green ceramic body and the sacrificial slurry filling the cavity decompose and volatilize, forming a preset embedded cavity inside the substrate, i.e., forming a cavity. S53, Sintering Stage: After the binder is removed, the co-firing furnace continues to heat up to the sintering peak temperature and maintains the preset peak temperature for a set time. At this peak temperature, the green ceramic body shrinks, and each layer of green ceramic sheet is tightly bonded together, achieving complete densification of the ceramic body and forming an LTCC substrate with an embedded cavity.

[0016] As a further improvement to the above technical solution, the isostatic pressure parameters in step S13 are set as follows: temperature 45-55℃, pressure 1000-2000psi, and time 50-70s.

[0017] As a further improvement to the above technical solution, in step S31, the printing parameters of the screen printing machine are set as follows: pressure 70-90N, speed 10-20mm / s, and screen spacing 0mm. In step S33, the parameters of the drying oven are set as follows: drying temperature 70-90℃, drying time 40-50min.

[0018] As a further improvement to the above technical solution, the isostatic parameters in step S44 are set as follows: temperature 55-65℃, time 15-25min, and pressure 3000-4000psi.

[0019] As a further improvement to the above technical solution, in step S52, the parameters for debinding the co-fired furnace are: temperature 500~600℃, holding time 150~210min; In step S53, the peak temperature is 850-870℃, and the preset peak time is 25-35 minutes.

[0020] Compared with the prior art, the advantages of the present invention are: (1) The present invention uses a semi-automatic printing machine to fill the internal cavities of the substrate with sacrificial paste through screen printing. The cavity filling time for each layer of green ceramic sheet is only about 10 seconds per sheet. In contrast, the existing technology uses a manual placement of filling blocks, which requires manual operation at each cavity position in the green ceramic sheet. The cavity filling time for each layer of green ceramic sheet is about 30 seconds per sheet, and the filling time will further increase with the complexity and number of cavities. The method described in the present invention significantly shortens the processing time of a single layer, significantly improves production efficiency, and is more suitable for large-scale mass production.

[0021] (2) The manufacturing method described in this invention has wide applicability and can meet the processing requirements of complex shapes and extreme sizes of cavities. In this invention, the shape pattern of the substrate cavity is engraved on a screen using an exposure machine. The screen can achieve a minimum pattern size of 80µm. Then, the sacrificial paste is printed into the cavity of the green ceramic sheet according to the preset cavity pattern through the screen opening. This not only meets the filling requirements of cavities with any complex shape, but also achieves a minimum cavity size of 80µm, effectively solving the problem of high processing difficulty for complex shapes and small sizes (≤0.2mm) cavities in the prior art.

[0022] (3) In the prior art, solid sacrificial blocks are prone to processing errors in terms of horizontal dimensions and thickness during processing. The more layers are stacked, the more obvious the error accumulation becomes, resulting in bulges or depressions in the cavity of the substrate after sintering, which in turn affects the cavity flow rate. The sacrificial paste used in this invention has good fluidity. After being printed inside the cavity, it can be spread horizontally and completely fill all contour edges of the cavity. With the leveling treatment, the paste portion that protrudes vertically from the surface of the ceramic tile will be compacted inside the cavity. Finally, the cavity area of ​​the substrate after sintering is flat, and the cavity dimensions inside the cavity have high accuracy in both the horizontal and vertical directions. Attached Figure Description

[0023] Figure 1 This is a flowchart of the method for fabricating an embedded cavity in an LTCC substrate according to the present invention. Figure 2 This is a schematic diagram showing the outline of the cavity inside the substrate in step S2 of the present invention. Figure 3 This is a schematic diagram of the outline of the inlet and outlet of the cavity of the superstructure in step S4 of the present invention.

[0024] in: 1. Inlet and outlet of the cavity in the upper body layer; 2. Raw porcelain shards; 3. Cavity outline. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings: The LTCC substrate of this invention consists of 11 layers of green ceramic sheets: 4 layers of green ceramic preforms below the cavity, 3 layers of green ceramic sheets in the cavity layer, and 4 layers of green ceramic preforms on top of the cavity. The thinnest part of the cavity is 0.2 mm. This invention first pre-isostatically presses all the green ceramic sheets below the microcavity layer to form a single unit. Then, the green ceramic strips with a film are laser-pressed to form cavities, and subsequently placed layer by layer on the pre-pressed preforms for a second pre-pressing process. Next, sacrificial paste is filled into the cavity using screen printing. After drying, the sacrificial paste protruding from the cavity surface is leveled using a leveling machine. Finally, the green ceramic strips on top of the cavity are aligned and stacked layer by layer, and the entire substrate is isostatically pressed and sintered. During sintering, the sacrificial paste completely evaporates, ultimately forming the designed blind cavity inside the substrate, thus completing the microcavity formation.

[0026] like Figure 1 The method shown includes the following steps: An embedded cavity fabrication method for an LTCC substrate. S1. First pre-compression: The micro-cavity under-body ceramic body that constitutes the underbody layer is pre-compressed as a whole to avoid separation and scattering in subsequent processes.

[0027] S2. Cavity Forming and Secondary Pre-pressing: All green ceramic pieces constituting the cavity layer are cut to the cavity shape. The cut green ceramic pieces are then aligned, stacked, and isostatically pressed to form a cavity layer. This cavity layer is then aligned and stacked with the lower green ceramic body after the first pre-pressing, and a secondary pre-pressing is performed. This step serves two purposes: firstly, to provide a base for the lower body when printing the ink into the cavity layer; and secondly, to increase the mechanical strength of the cavity layer and prevent deformation during printing, which could affect subsequent alignment and stacking.

[0028] S3. Cavity Filling: Since the embedded cavity exists inside the substrate, if it is not filled, the superstructure will collapse at the cavity due to loss of support during the subsequent isostatic pressing process, ultimately resulting in the cavity failing to form and the substrate surface becoming concave. This invention uses screen printing technology to print sacrificial paste into the cavity of the cavity layer, and then dries it to form a solid. Preferably, the sacrificial paste is a carbon-based organic material. To ensure the filling paste inside the cavity is flat during isostatic pressing and to prevent protrusions from forming on the substrate surface, a leveling machine is used to level the surface of the dried paste.

[0029] S4. Isostatic Pressing: The upper layer of green ceramic sheets constituting the upper body layer is stacked layer by layer on top of the cavity layer treated in step S3, forming a green ceramic composite. The green ceramic composite is then subjected to isostatic pressing. This step is to press the upper body layer, cavity layer, and lower body layer together to form a whole for subsequent sintering.

[0030] S5. Debinding Sintering: The isostatically pressed green ceramic laminate is subjected to debinding sintering to decompose and volatilize the organic matter in the green ceramic body and the sacrificial paste in the cavity, resulting in an LTCC substrate with an embedded cavity. The purpose of this step is to remove the carbon-based organic sacrificial paste filling the cavity of the substrate.

[0031] The microcavity structure of the LTCC substrate, viewed in a vertical cross-section, consists of three parts: an upper preform layer, a cavity layer, and a lower preform layer. The cavity layer contains cavities, forming a cavity. The upper and lower preform layers are sintered together by sandwiching the cavity layer in between, creating the internal cavity of the substrate.

[0032] As a further improvement to the above technical solution, step S1 specifically includes: S11. Lamination and Fixing: All the green ceramic pieces that make up the lower green ceramic body are aligned and laminated in a preset order and fixed on the lamination plate.

[0033] S12. Vacuum sealing: Cover the surface of the stacked green ceramic tiles with a layer of plastic film, put the whole thing into an aluminum foil bag, and vacuum seal it.

[0034] S13. Isostatic Pressing: The sealed raw ceramic blank is placed in an isostatic press, and isostatic pressing parameters are set to perform isostatic pressing pre-compress on the raw ceramic blank. This allows all the raw ceramic pieces to form a preliminary adhesive force under the action of their own adhesives, preventing separation and scattering in subsequent processes. Preferably, the isostatic pressing parameters include: temperature of 45–55°C, pressure of 1000–2000 psi, and time of 50–70 s.

[0035] In this embodiment, firstly, the four layers of Ferro A6m green ceramic sheets comprising the lower ceramic body are peeled off their inherent mylar film and then sequentially aligned and stacked in the order of layers 1-4, and fixed on a stainless steel stacking plate, with the fourth layer located on top of the lower ceramic body. Next, a plastic film is placed over the surface of the fourth green ceramic sheet, and the entire assembly is placed in an aluminum foil bag and vacuum-sealed. Then, the sealed lower ceramic body is placed in an isostatic press for isostatic pre-pressing. The isostatic pressing parameters are set as follows: temperature 50°C, pressure 1500 psi, time 60 s. Finally, after pre-pressing, the four green ceramic sheets develop initial adhesion under the action of their inherent adhesives, thus forming a unified whole.

[0036] As a further improvement to the above technical solution, step S2 specifically includes: S21. Cavity Cutting: The cavity layers are typically designed to be 1-30 layers depending on functional requirements. Following the structural design, a laser cutting machine is used to cut each cavity layer of green ceramic sheet into its cavity shape. After cutting, each layer of green ceramic sheet forms a shape similar to... Figure 2 The cavity shown has a cavity profile.

[0037] S22. Alignment and Lamination: After the cavity cutting is completed, first remove the plastic film on the surface of the lower green ceramic body that has been pre-compressed in step S1, and then align and laminate the first layer of green ceramic pieces containing cavities above the lower green ceramic body according to the lamination sequence. When laminating, the side of the green ceramic piece covered with the Mylar film should face upward to ensure that the green ceramic piece body is in direct contact with the lower green ceramic body.

[0038] S23. Isostatic pre-compression: Repeat the isostatic pre-compression operation in step S1 to pre-compress the lower ceramic body and the hollow ceramic slabs in this layer, so that the two are initially compacted to form a whole.

[0039] In this embodiment, the three-layer hollow green ceramic sheet needs to be laser-cut with the mylar film intact. According to the structural design requirements, a laser cutter is used to cut the three layers of green ceramic sheet into cavity shapes layer by layer. After cutting, each layer of green ceramic sheet forms a cavity with a cavity outline 3, the thinnest part being only 0.2mm. Simultaneously, a cavity outline cavity is also formed at the position of the mylar film corresponding to the green ceramic sheet cavity. The purpose of using film-insulated cutting is to prevent excess sacrificial paste from overflowing outside the green ceramic sheet cavity due to printing misalignment during subsequent sacrificial paste printing, thereby preventing delamination caused by affecting the bonding between the green ceramic sheets. The laser cutting parameters are set according to the corresponding process standard. After cutting, the plastic film on the surface of the lower green ceramic body, which was pre-pressed in step S1, is first removed. Then, the first hollow green ceramic sheet (i.e., the 5th layer) above the lower green ceramic body is removed according to the stacking sequence and aligned for stacking. During the stacking process, the side of the green ceramic sheet covering the mylar film should face upwards, so that the green ceramic sheet itself is in direct contact with the lower green ceramic body. Then, the isostatic pre-compression operation of step S1 is repeated to pre-compress the lower ceramic body and the first layer of hollow ceramic pieces, so that the two are initially compacted to form a whole.

[0040] As a further improvement to the above technical solution, step S3 specifically includes: S31. Printing Preparation: Place the pre-pressed blank with the cavity layer from step S2 onto the worktable of the screen printing machine. Select a stainless steel screen corresponding to the shape of the cavity to fill the cavity with sacrificial paste. The printing parameters of the screen printing machine are set as follows: pressure 70-90N, speed 10-20mm / s, screen spacing 0mm.

[0041] S32. Sacrificial paste printing: Using a screen printing machine, the sacrificial paste is printed to fill the cavity according to the cavity contour, ensuring that the paste covers all contour edges of the cavity.

[0042] S33. Drying treatment: Place the printed and filled preform into an oven for drying. Set the drying temperature to 70-90℃ and the drying time to 40-50 minutes to allow the paste to initially solidify.

[0043] S34. Leveling treatment: After drying, the preform is placed on a leveling machine to level the sacrificial slurry protruding from the cavity surface, so that the slurry inside the cavity is filled more densely.

[0044] S35. Remove the film for later use: After leveling, remove the mylar film from the surface of the preform to prepare for subsequent cavity layer lamination.

[0045] The cavity structure is similar to a cavity embedded inside a substrate. If the cavity is not completely and densely filled, it will collapse under water pressure during the subsequent isostatic pressing process because the cavity lacks support inside the substrate. Therefore, this invention uses a carbon-based organic sacrificial slurry that can be decomposed during the sintering process as a filler.

[0046] In this embodiment, the pre-compressed preform with cavity layers (including layers 1-5, with layer 5 being the first cavity layer) that has been pre-compressed in step S2 is first removed from the stainless steel stacking plate and placed on the screen printing machine worktable. Then, the printing parameters are set: pressure 80N, speed 15mm / s, and screen spacing 0mm. A stainless steel screen corresponding to the shape of the cavity in layer 5 is used to fill the cavity with sacrificial paste. At this time, since the surface of the preform still retains the Mylar film, excess sacrificial paste will be separated from the preform by the film layer during printing, preventing overflow and affecting the bonding of the green ceramic sheet. After filling, the preform with sacrificial paste is placed in an oven for drying, with a drying temperature of 80℃ and a drying time of 45 minutes. After drying, the preform is placed on a leveling machine to level the sacrificial paste protruding from the cavity surface, making the paste filling inside the cavity more compact. The leveling pressure is 5T, and the leveling time is 30 seconds. After leveling, the Mylar film on the surface of the preform is peeled off. For the remaining two cavity layers (the 6th and 7th layers), repeat steps S2 and S3 layer by layer until all cavity layers and the underlying ceramic body are pre-compressed and bonded, and the sacrificial slurry inside each cavity is densely filled.

[0047] As a further improvement to the above technical solution, step S4 specifically includes: Before performing isostatic pressing, the cavity layer pretreatment is confirmed. The above steps S2 and S3 are repeated layer by layer for the remaining cavity layer green ceramic pieces to ensure that all cavity layers are pre-pressed and bonded to the underlying green ceramic body, and that the sacrificial slurry inside each cavity is densely filled.

[0048] S41. Cutting the inlet and outlet of the cavity in the upper body layer: For all the green ceramic pieces in the upper body layer, use a laser cutting machine to cut out the following... Figure 3 The cavity inlet / outlet 1 shown is a reserved channel for subsequent coolant flow.

[0049] S42. Upper body layer alignment and stacking: The upper body layer green ceramic pieces with the cut cavity inlet and outlet 1 are aligned and stacked layer by layer in a preset order, covering the cavity layer that has been processed, to form a complete green ceramic piece composite body composed of the lower green ceramic body, the cavity layer and the upper body layer.

[0050] S43. Vacuum Sealing: A plastic film is applied to the surface of the green ceramic tile assembly, followed by vacuum sealing. The plastic film ensures a good overall seal.

[0051] S44. Isostatic Pressing: The encapsulated green ceramic wafer stack is placed in an isostatic press. After setting the isostatic pressing parameters, the green ceramic wafer stack is subjected to isostatic pressing treatment. The isostatic pressing parameters are: temperature 55–65℃, time 15–25 min, and pressure 3000–4000 psi. Because the sacrificial paste filling the cavity provides stable support, the cavity position of the substrate will not collapse during the isostatic pressing process, thus maintaining the integrity of the cavity structure.

[0052] In this embodiment, the Mylar film on the surface of the top cavity layer (layer 7) is first removed. Then, a laser cutter is used to process all the green ceramic pieces in the upper body layer, cutting out the cavity inlet and outlet with a diameter of 1 mm. Next, the green ceramic pieces in the upper body layer (layers 8-11) are stacked layer by layer in a preset order, covering the 7th cavity layer, forming a complete 11-layer green ceramic piece composite consisting of the lower green ceramic body, the cavity layer, and the upper body layer. A plastic film is then placed on the surface of the 11-layer green ceramic piece composite, and the entire assembly is placed in a sealed container for vacuum sealing to ensure the composite is completely sealed. Finally, the sealed composite is placed in an isostatic press, and after setting the isostatic pressure parameters, all the green ceramic pieces undergo isostatic pressing treatment. The isostatic pressing parameters at this point are: temperature 60℃, time 1200s, and pressure 3500psi. These parameters, including time, temperature, and pressure, are all higher than the pre-pressing parameters, which significantly enhances the bonding force between the green ceramic sheets, resulting in a dense, unified substrate. Because the sacrificial paste completely fills the internal cavities of the substrate, these cavities do not collapse under water pressure during isostatic pressing, effectively ensuring the integrity of the cavity structure.

[0053] As a further improvement to the above technical solution, step S5 specifically includes: S51. Sintering preparation: The green ceramic sheet composite body after isostatic pressing in step S4 is placed into a co-firing furnace for sintering.

[0054] S52, Debinding Stage: Set the debinding parameters for the co-firing furnace, with a temperature of 500~600℃ and a holding time of 150~210min. The organic matter contained in the green ceramic body itself and the carbon-based organic sacrificial slurry filling the cavity decompose and volatilize, forming a cavity inside the substrate.

[0055] S53, Sintering Stage: After the binder is removed, the co-firing furnace continues to heat up to the sintering peak temperature and maintains the preset peak temperature for a set time. At this peak temperature, the green ceramic body shrinks, and each layer of green ceramic sheet is tightly bonded together, ultimately achieving complete densification of the ceramic body and forming a stable and cavitary LTCC substrate.

[0056] In this embodiment, the sintering curve can be divided into two main stages: binder removal and sintering. In the binder removal stage, the temperature is 500–600°C, and the time is 150–210 min. The organic matter contained in the green ceramic preform and the sacrificial slurry inside the cavities decompose and volatilize. After this stage, the cavities inside the substrate have been formed. In the sintering stage, after binder removal, the temperature continues to rise to the sintering peak temperature and is maintained for a preset time. The peak temperature is 850–870°C, and the peak time is 25–35 min. The green ceramic preform shrinks and achieves densification.

[0057] In this embodiment, after isostatic pressing, the green ceramic wafer stack containing cavities and sacrificial slurry is placed in a co-firing furnace for sintering. The debinding stage parameters are set as follows: temperature 550℃, holding time 180 min. After debinding, the sacrificial slurry filling the cavities completely decomposes and volatilizes, and the organic matter in the green ceramic preform also volatilizes simultaneously, forming cavities inside the substrate. After debinding, the peak sintering parameters are set as follows: peak temperature 860℃, peak time 30 min. Sintering continues under these conditions, causing the green ceramic preform to shrink and bond tightly, ultimately achieving complete densification. After sintering, the LTCC substrate with internal microcavities is completed.

[0058] Compared with the prior art, the innovation of this invention is as follows: (1) By performing overall secondary pre-compression on the cavity layer green ceramic sheet, the shape of the cavity is fixed in advance, which effectively solves the problem of deformation and misalignment of each layer of green ceramic sheet cavity during the subsequent cavity filling process, improves the shape accuracy of the final cavity forming, and lays a stable structural foundation for subsequent sacrificial slurry filling and overall forming.

[0059] (2) Compared with the traditional method of filling solid sacrificial blocks, this invention uses a printing method to fill liquid sacrificial slurry. The liquid sacrificial slurry, with its good fluidity, can fully cover all the positions and corners of the cavity, and can better meet the filling needs of cavities with more complex shapes (irregular shapes) or smaller sizes (≤0.2mm). The filling uniformity and filling accuracy are higher. Especially in the Z direction (thickness direction) of the substrate, the thickness of the traditional solid sacrificial block cannot be completely consistent with the thickness of the green ceramic sheet, which can easily lead to defects such as collapse or bulge inside the cavity after isostatic pressing. However, the printed liquid sacrificial slurry can accurately fit the thickness of the green ceramic sheet, perfectly solving this problem and ensuring the quality of cavity forming.

[0060] (3) The traditional method of manually placing solid sacrificial blocks has defects such as low cavity forming dimensional accuracy and limited cavity size and shape. The innovation of this invention is that it adopts a filling process of liquid sacrificial slurry printed by stainless steel mask screen printing, which can fill cavities of any shape and size. At the same time, based on the fluidity characteristics of liquid sacrificial slurry, it can fully fill all the space of the cavity, effectively solving the limitations of solid sacrificial block filling. After isostatic pressing, the dimensional accuracy of the cavity in the X, Y and Z directions can be improved, ensuring the forming quality and structural stability of the embedded cavity.

[0061] The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for fabricating an embedded cavity in an LTCC substrate, wherein the LTCC substrate comprises an upper preform layer, a cavity layer, and a lower preform layer arranged sequentially from top to bottom; the cavity layer has a cavity formed therein; characterized in that, The production method includes the following steps: S1. Primary pre-compression: The lower ceramic body that constitutes the lower body layer is subjected to overall primary pre-compression; S2. Cavity Forming and Secondary Pre-pressing: Cut all the green ceramic pieces that make up the cavity layer into cavity shapes, then perform alignment and stacking and isostatic pressing on the cut green ceramic pieces to form a cavity layer with cavities. Align and stack the cavity layer with cavities with the lower green ceramic body after the first pre-pressing and perform secondary pre-pressing. S3. Cavity filling: Sacrificial paste is printed into the cavity of the cavity layer using screen printing technology. After drying and leveling, the cavity filling is completed. S4. Isostatic pressing: The upper layer of green ceramic pieces constituting the upper body layer is stacked layer by layer on the cavity layer after step S3 to form a green ceramic composite. The green ceramic composite is then subjected to overall isostatic pressing. S5. Debinding and Sintering: The green ceramic laminate after isostatic pressing is subjected to debinding and sintering to obtain an LTCC substrate with an embedded cavity.

2. The method for fabricating an embedded cavity in an LTCC substrate according to claim 1, characterized in that, Step S1 specifically includes: S11. Stacking and fixing: All the green ceramic pieces that make up the lower green ceramic body are stacked in a preset order and fixed on the stacking plate. S12. Sealing process: Cover the surface of the stacked green ceramic tiles with a layer of plastic film, put the whole thing into an aluminum foil bag, and vacuum seal it. S13. Isostatic pre-pressing: The sealed raw ceramic body is placed in an isostatic press, the isostatic parameters are set, and the raw ceramic body is subjected to isostatic pre-pressing, so that all the raw ceramic pieces generate preliminary adhesion under the action of their own adhesive, forming the raw body layer.

3. The method for fabricating an embedded cavity in an LTCC substrate according to claim 2, characterized in that, Step S2 specifically includes: S21. Cavity cutting: A laser cutting machine is used to cut the cavity shape of each layer of green ceramic sheet that constitutes the cavity layer. After cutting, each layer of green ceramic sheet forms a cavity with a cavity outline. S22. Alignment and stacking: After the cavity cutting is completed, first remove the plastic film on the surface of the pre-compressed lower ceramic body in step S1, and then align and stack the first layer of cavity-bearing green ceramic pieces above the lower ceramic body with the lower ceramic body according to the preset stacking order. S23. Isostatic pre-compression: Repeat the isostatic pre-compression operation in step S1 to pre-compress the lower green ceramic body and the green ceramic slab with cavity in this layer, so that the two are initially compacted to form a pre-compressed body with cavity layer.

4. The method for fabricating an embedded cavity in an LTCC substrate according to claim 3, characterized in that, Step S3 specifically includes: S31. Printing preparation: Place the pre-pressed blank with cavity layer formed in step S2 on the worktable of the screen printing machine, and fill the cavity with sacrificial paste using a stainless steel screen that matches the shape of the cavity. S32, Sacrificial paste printing: Using a screen printing machine, the sacrificial paste is printed and filled into the cavity of the cavity layer according to the cavity contour; S33. Drying treatment: The preform after printing and filling with sacrificial paste is placed in an oven for drying treatment; S34. Leveling treatment: Place the dried preform on a leveling machine to level the sacrificial slurry protruding from the cavity surface. S35, Remove the film for later use: Remove the mylar film from the surface of the embryo.

5. The method for fabricating an embedded cavity in an LTCC substrate according to claim 4, characterized in that, Step S4 specifically includes: S41. Cutting of the cavity inlet and outlet of the upper body layer: For all the green ceramic pieces that make up the upper body layer, use a laser cutting machine to cut out the cavity inlet and outlet that match the cavity of the cavity layer; S42. Alignment and stacking of the upper body layer: The upper body layer green ceramic pieces with the cut cavity inlet and outlet are aligned and stacked layer by layer in a preset order, covering the cavity layer after step S3, to form a complete green ceramic piece stack consisting of the lower green ceramic body, the cavity layer, and the upper body layer. S43. Vacuum sealing: Cover the surface of the green ceramic tile composite with a plastic film and perform vacuum sealing. S44. Isostatic pressing treatment: Place the plastic-sealed green ceramic tile assembly into an isostatic press, set the isostatic pressing parameters, and perform isostatic pressing treatment on the green ceramic tile assembly.

6. The method for fabricating an embedded cavity in an LTCC substrate according to claim 5, characterized in that, Step S5 specifically includes: S51. Sintering preparation: The green ceramic sheet composite after isostatic pressing in step S4 is placed into a co-firing furnace for sintering. S52, Debinding Stage: Set the debinding parameters for the co-firing furnace. Under these parameters, the organic matter contained in the green ceramic body and the sacrificial slurry filling the cavity decompose and volatilize, forming a preset embedded cavity inside the substrate, i.e., forming a cavity. S53, Sintering Stage: After the binder is removed, the co-firing furnace continues to heat up to the sintering peak temperature and maintains the preset peak temperature for a set time. At this peak temperature, the green ceramic body shrinks, and each layer of green ceramic sheet is tightly bonded together, achieving complete densification of the ceramic body and forming an LTCC substrate with an embedded cavity.

7. The method for fabricating an embedded cavity in an LTCC substrate according to claim 2, characterized in that, The isostatic pressure parameters in step S13 are set as follows: temperature 45-55℃, pressure 1000-2000psi, and time 50-70s.

8. The method for fabricating an embedded cavity in an LTCC substrate according to claim 4, characterized in that, In step S31, the printing parameters of the screen printing machine are set as follows: pressure 70-90N, speed 10-20mm / s, and screen spacing 0mm. In step S33, the parameters of the drying oven are set as follows: drying temperature 70-90℃, drying time 40-50min.

9. The method for fabricating an embedded cavity in an LTCC substrate according to claim 5, characterized in that, The isostatic parameters in step S44 are set as follows: temperature 55-65℃, time 15-25min, and pressure 3000-4000psi.

10. The method for fabricating an embedded cavity in an LTCC substrate according to claim 6, characterized in that, In step S52, the parameters for debinding the co-fired furnace are: temperature 500~600℃, holding time 150~210min; In step S53, the peak temperature is 850-870℃, and the preset peak time is 25-35 minutes.

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