Manufacturing method of high-flatness enclosure frame welding area of copper-based LTCC substrate

Abstract

The invention provides a method for manufacturing a high-flatness enclosure frame welding area of a copper-based LTCC (Low Temperature Co-Fired Ceramic) substrate, which comprises the following steps of: step 1, embedding copper slurry: embedding the copper slurry at a preset enclosure frame welding area position in a green tape lamination layer to form an embedded copper structure with the same shape as the enclosure frame welding area; and 2, lamination and low-temperature co-firing: carrying out lamination and low-temperature co-firing on the green tape lamination layer embedded with the copper slurry in the step 1 to form the LTCC substrate. And (3) grinding and polishing: grinding and polishing the LTCC substrate obtained in the step (2), so that the surface of the embedded copper structure is exposed and high flatness is achieved. And fourthly, a nickel-gold layer is electroplated, specifically, the exposed surface of the embedded copper structure obtained in the third step is electroplated with the nickel-gold layer, and a surrounding frame welding area is formed. According to the method, the influence of substrate warping can be effectively avoided, the influence of substrate warping caused by direct printing on the warping surface is avoided by embedding the enclosure frame welding structure into the substrate and grinding and forming the enclosure frame welding structure after sintering, and the problem of unevenness of a welding area is fundamentally solved.

Description

Technical Field

[0001] This invention belongs to the field of electronic packaging technology, and relates to copper-based LTCC substrates, specifically to a method for fabricating a high-flatness frame soldering area on a copper-based LTCC substrate. Background Technology

[0002] With the development of microelectronics and optoelectronics technologies, multilayer ceramic packaging has been widely used in radio frequency devices, power modules, and optical communication devices due to its excellent electrical performance, thermal stability, and hermeticity. Currently, the mainstream high-temperature co-fired ceramic (HTCC) technology typically uses refractory metals such as tungsten and molybdenum as conductive materials, and then prints and welds the surrounding frame area onto the surface after the ceramic substrate is sintered. However, due to the inevitable sintering differences between ceramic and metal materials during the high-temperature sintering process, the substrate experiences a certain degree of warping, which affects the flatness and sealing of the subsequent frame welding.

[0003] The existing HTCC process flow is as follows: first, prepare a ceramic substrate base → screen print and weld a frame pattern on its surface → high-temperature co-firing → subsequent welding and encapsulation. Because the printed layer is located on the warped substrate surface, it is difficult to achieve high flatness in the frame welding area, which easily leads to defects such as uneven solder distribution, cold solder joints, and air leakage, seriously affecting product yield and long-term reliability.

[0004] In contrast, copper-based LTCC (Low-Temperature Co-fired Ceramic) technology uses copper as the inner layer wiring and functional structure material, exhibiting lower resistivity and better thermal conductivity, and requiring a lower sintering temperature (typically 850–900℃), making it suitable for co-firing with a wider range of functional materials. Compared to HTCC, copper-based LTCC can achieve the strength of HTCC sintering at 1600℃ using only a sintering temperature of 950℃, representing a promising emerging advanced packaging technology in China. However, there are currently no publicly disclosed patents regarding the shell structure and key processes of copper-based LTCC, particularly regarding the high-precision fabrication of the frame welding area, which remains a technological gap.

[0005] Therefore, there is an urgent need to propose a new manufacturing method that is suitable for copper-based LTCC processes, can overcome the effects of substrate warping, and can achieve high flatness in the frame welding area. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method for manufacturing a high-flatness frame welding area on a copper-based LTCC substrate, thereby solving the technical problem that the flatness of the frame welding area needs to be further improved in the prior art.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution.

[0008] A method for fabricating a high-flatness frame soldering area on a copper-based LTCC substrate, the method comprising the following steps.

[0009] Step 1, embedding copper paste: Copper paste is embedded in the predetermined frame welding area of ​​the green ceramic tape stack to form an embedded copper structure that matches the shape of the frame welding area.

[0010] Step 2, lamination and low-temperature co-firing: The green ceramic tapes with embedded copper paste from step one are laminated and co-fired at low temperature to form an LTCC substrate.

[0011] Step 3, Grinding and Polishing: The LTCC substrate obtained in step two is ground and polished to expose the surface of the embedded copper structure and achieve high flatness.

[0012] Step 4, Electroplating a nickel-gold layer: A nickel-gold layer is electroplated on the exposed surface of the embedded copper structure obtained in step three to form the frame welding area.

[0013] The present invention also has the following technical features.

[0014] Preferably, in step one, the embedding depth of the embedded copper structure is 100–300 μm.

[0015] More preferably, in step one, the embedding depth of the embedded copper structure is 200 μm.

[0016] Preferably, in step three, the high flatness refers to a flatness of ≤5μm and a surface roughness Ra of ≤0.1μm.

[0017] Preferably, in step four, the nickel layer has a thickness of 2.0–5.0 μm and the gold layer has a thickness of 0.5–1.5 μm.

[0018] More preferably, in step four, the nickel layer has a thickness of 3 μm and the gold layer has a thickness of 1.3 μm.

[0019] Preferably, in step four, the frame welding area is a ring-shaped or rectangular closed structure for hermetically sealed welding.

[0020] Compared with the prior art, the present invention has the following technical effects.

[0021] (I) The method of the present invention can effectively avoid the influence of substrate warping: by embedding the frame welding structure inside the substrate and grinding it after sintering, the influence of substrate warping caused by the mismatch between metal and ceramic sintering is avoided, and the problem of unevenness in the welding area is fundamentally solved.

[0022] (II) The method of the present invention can achieve high flatness and consistency: the grinding process can precisely control the surface morphology, ensuring that the flatness of the frame area is highly consistent, and meeting the requirements of high-precision packaging.

[0023] (III) The method of the present invention can improve welding reliability: the electroplated nickel-gold layer not only improves weldability, but also enhances corrosion resistance and migration resistance, which is beneficial to long-term airtight sealing.

[0024] (IV) The method of the present invention is compatible with existing LTCC production lines: the method does not require changes to the mainstream LTCC process flow, but only requires the addition of embedded printing and grinding processes, which makes it easy to achieve industrialization and promotion.

[0025] (V) The method of the present invention can fill the domestic technology gap: it is the first to propose a high flatness preparation scheme for the welding area of ​​the casing of copper-based LTCC tubes. Attached Figure Description

[0026] Figure 1 This is a partial cross-sectional structural diagram of the area containing the embedded copper paste.

[0027] Figure 2 This is a schematic cross-sectional view of the LTCC substrate after sintering and grinding.

[0028] Figure 3 This is a schematic diagram of the microstructure of the welding area of ​​the frame after electroplating.

[0029] Figure 4 This is a schematic diagram comparing the flatness of the LTCC casing frame prepared according to the present invention with that of a traditional HTCC casing frame.

[0030] The specific content of the present invention will be further explained in detail below with reference to the embodiments. Detailed Implementation

[0031] It should be noted that, unless otherwise specified, all components and methods in this invention are based on components and methods known in the prior art.

[0032] This invention aims to solve the technical problem of unevenness in the frame welding area caused by warping of the ceramic substrate after sintering in existing HTCC technology. By pre-embedding a copper paste structure before substrate sintering, and combining it with post-sintering grinding and electroplating processes, high flatness and consistency of the frame welding area are achieved, improving packaging reliability and product yield. The manufacturing method of this invention is applicable to various LTCC devices requiring high hermeticity packaging, such as RF modules, MEMS sensors, and optical transceiver components.

[0033] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments. All equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention.

[0034] Example: This embodiment provides a method for fabricating a high-flatness frame soldering area on a copper-based LTCC substrate, which includes the following steps.

[0035] Step 1, embedding copper paste: Based on the structural design of the target LTCC casing, multiple layers of green ceramic tape are cut and stacked to form a prototype substrate to be sintered. Copper paste is embedded in the predetermined frame welding area within the green ceramic tape stack to form an embedded copper structure with a shape consistent with the frame welding area.

[0036] In this embodiment, the local cross-sectional structure of the embedded copper paste area is as follows: Figure 1 As shown, Figure 1 The image shows the embedding position and depth of copper paste between the green ceramic strip layers, with the embedding depth of the embedded copper structure being 200 μm.

[0037] In this embodiment, the copper paste is a commonly used conductive copper paste suitable for low-temperature co-firing, known in the art. The conductive copper paste uses Cu-L401-YK33 grade copper paste, which is known in the art. The conductive copper paste has good rheological properties, adhesion, and co-firing compatibility.

[0038] In this embodiment, the specific process of embedding copper paste is as follows: at the predetermined frame welding area, copper paste is filled into the cavity between at least one layer of green ceramic tape to form an embedded copper structure that is consistent with the shape of the final frame welding area.

[0039] In this specific embodiment, 10 layers of copper-based LTCC YK33 green ceramic tape (each layer is 100μm thick) are selected and stacked according to the design pattern; on the surface of the first to third layers of green ceramic tape, a laser cavity is used to form a rectangular ring structure (i.e., the frame welding area), which is filled with copper paste, and the pattern width is 0.3 mm.

[0040] Step 2, lamination and low-temperature co-firing: The green ceramic tapes with embedded copper paste from step one are laminated and co-fired at low temperature to form an LTCC substrate.

[0041] In this embodiment, the specific process of lamination and low-temperature co-firing is as follows: the green ceramic tape stack containing the embedded copper structure is hot-pressed and co-fired at low temperature in an inert nitrogen atmosphere to densify the ceramic and sinter the copper conductor at the same time to form an integrated LTCC substrate.

[0042] In this embodiment, the laminate is placed in an isostatic press for hot pressing (temperature 70°C, pressure 20MPa); low-temperature co-firing is performed in a nitrogen atmosphere (peak temperature 950°C); after sintering, surface inspection of the substrate revealed a local warping of approximately 100μm.

[0043] Step 3, Grinding and Polishing: The LTCC substrate obtained in step two is ground and polished to expose the surface of the embedded copper structure and achieve high flatness.

[0044] In this embodiment, the specific process of grinding and polishing is as follows: mechanical grinding and polishing are performed on the sintered LTCC substrate to remove the uneven parts on the surface until the surface of the embedded copper paste area is exposed, and a highly flat frame welding area is obtained; the grinding and polishing process uses a multi-stage diamond grinding disc in conjunction with a precision planar grinding device to control the grinding rate and pressure, ensuring that the overall flatness of the substrate is ≤5μm and the surface roughness Ra≤0.1μm.

[0045] In this embodiment, a double-sided polishing machine is used to polish the upper surface of the substrate, with the removal amount controlled at 200μm, so that the surface of the embedded copper structure is fully exposed. The cross-section of the LTCC substrate after sintering and polishing is shown below. Figure 2 As shown, Figure 2 The image shows the flat, exposed welded area of ​​the copper frame; the flatness was measured to be 4.2 μm and the surface roughness Ra = 0.08 μm.

[0046] Step 4, Electroplating a nickel-gold layer: A nickel-gold layer is electroplated on the exposed surface of the embedded copper structure obtained in step three to form the frame welding area.

[0047] In this embodiment, the specific process of electroplating the nickel-gold layer is as follows: A nickel layer and a gold layer are sequentially electroplated on the exposed copper frame welding area to form an oxidation-resistant and highly solderable nickel-gold welding interface. An activation cleaning treatment is performed before electroplating to ensure the adhesion of the plating layers. The microstructure of the frame welding area after electroplating is as follows: Figure 3 As shown, Figure 3 The image shows a copper-nickel-gold three-layer structure.

[0048] In step four, the frame welding area is a closed structure of ring or rectangle, used for hermetic welding.

[0049] Specifically, in this embodiment, the exposed copper frame area is acid-washed and activated, followed by sequential electroplating of a nickel layer (3 μm thick) and a gold layer (1.3 μm thick); ultimately, a copper-based LTCC casing substrate with a highly flat nickel-gold solder joint is obtained. A schematic diagram comparing the flatness of the LTCC casing frame prepared in this embodiment with that of a traditional HTCC casing frame is shown below. Figure 4 As shown.

[0050] Actual welding tests showed that the high flatness frame welding area of ​​the copper-based LTCC substrate prepared in this embodiment was uniformly wetted with solder, with no poor soldering or misalignment, and the airtightness test pass rate reached 99.6%.

Claims

1. A method for making a high flatness surround pad of a copper-based LTCC substrate, comprising: The method comprises the following steps: ​ Step one, embedding copper paste: Embedding copper paste in the predetermined frame welding area position in the cavity opening and hole filling process of the green ceramic tape, forming an embedded copper structure consistent with the shape of the frame welding area; Step two, laminating and low-temperature co-firing: Laminating and low-temperature co-firing the green ceramic tape embedded with copper paste in step one, forming a copper-based LTCC substrate; Step three, grinding and polishing: Grinding and polishing the LTCC substrate obtained in step two to expose the surface of the embedded embedded copper structure and achieve high flatness; Step four, electroplating nickel and gold layers: Electroplating nickel and gold layers on the exposed surface of the embedded copper structure obtained in step three to form a frame welding area.

2. The method of claim 1, wherein the copper-based LTCC substrate has a thickness of 0.1 to 0.3 mm. In step one, the copper paste is filled in the cavity opening position of the frame in the hole filling stage to realize the copper substrate under the frame.

3. The method of claim 2, wherein the copper-based LTCC substrate is a green sheet. In step one, the embedding depth of the embedded copper structure is 200μm.

4. The method of claim 1, wherein the copper-based LTCC substrate has a thickness of 0.1 to 0.3 mm. In step three, the high flatness is a flatness of ≤5μm and a surface roughness Ra of ≤0.1μm.

5. The method of claim 1, wherein the copper-based LTCC substrate has a high flatness of the surrounding frame solder area. In step four, the nickel layer in the nickel-gold layer has a thickness of 2.0-5.0μm, and the gold layer has a thickness of 0.05-1.5μm.

6. The method of claim 1, wherein the copper-based LTCC substrate has a thickness of 0.1 to 0.3 mm. In step four, the frame welding area is a ring-shaped or rectangular closed structure for airtight packaging welding.

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