A low-warpage DBC ceramic copper-clad laminate and its preparation method

Abstract

This invention discloses a low-warpage DBC ceramic copper-clad laminate and its fabrication method, belonging to the field of power electronic packaging technology. The ceramic copper-clad laminate includes a ceramic substrate and copper layers bonded to its front and back sides. The improvement lies in that at least one stress-relieving trench is etched around the four corners of the back copper layer, penetrating the thickness of the copper layer, exposing the ceramic substrate, and not extending to the edge of the copper layer. This trench structure actively interrupts the transmission path of welding thermal stress within the continuous copper layer, thereby significantly reducing warpage deformation of the substrate after welding. This invention also provides a corresponding fabrication method; this trench structure can be simultaneously completed in existing patterned etching processes, exhibiting good process compatibility. This invention effectively solves the problem of DBC substrate welding warpage from the structural source, improving the reliability of power modules.

Description

Technical Field

[0001] This invention belongs to the field of power electronic packaging technology, specifically relating to a low-warpage DBC ceramic copper-clad laminate and its preparation method. Background Technology

[0002] Direct-bonded ceramic copper-clad laminates (DBCs) are a key substrate material for power modules, consisting of a ceramic substrate directly bonded to copper layers on both the top and bottom surfaces. During module packaging, the DBC substrate needs to be soldered to the base plate (usually copper) using solder paste. This soldering process is typically performed at high temperatures. Due to the significant differences in the coefficients of thermal expansion of the various DBC layers (copper, ceramic, copper), enormous thermal stress is generated during heating and cooling. This stress accumulates and is unevenly distributed within the substrate, easily leading to irreversible warping of the DBC substrate after soldering. Severe warping reduces soldering reliability, creates voids, and can even cause chip cracking or solder layer failure, severely impacting the lifespan and stability of the power module.

[0003] Traditional improvement methods include optimizing the soldering profile, using low-stress solder, or increasing the substrate thickness. However, these methods often have limited effectiveness or introduce additional costs and process complexity. Another approach is to release stress by designing the DBC copper layer pattern, but existing designs are mostly front-side circuit patterns, which have limited effect on suppressing overall substrate warpage, especially macroscopic deformation during the soldering process. Therefore, there is an urgent need for a new solution that starts from the DBC substrate's own structure, effectively interrupts the internal stress transmission path, improves the soldering stress distribution, and thus significantly reduces soldering warpage. Summary of the Invention

[0004] To address the problems existing in the prior art, the present invention aims to provide a low-warpage DBC ceramic copper-clad laminate and its preparation method, which can actively interrupt and redistribute welding thermal stress by optimizing the structural design of the substrate itself, thereby significantly reducing the macroscopic warpage deformation of the substrate after welding.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0006] On one hand, the present invention provides a low-warpage DBC ceramic copper-clad laminate, comprising a ceramic substrate and copper layers bonded to its front and back sides. At least one trench for releasing welding thermal stress is etched around the four corners of the copper layer on the back side. The trench penetrates the thickness of the copper layer on the back side, exposing the underlying ceramic substrate, and the end of the trench terminates inside the copper layer on the back side and does not extend to the edge of the copper layer on the back side.

[0007] Preferably, the grooves around each corner are two, which are either parallel straight grooves or concentric curved grooves.

[0008] Preferably, the intersection of a diagonal line drawn from a corner of the ceramic substrate and the rounded edge of the copper layer on the back is defined as a virtual corner point, and the outer edge of the trench is 2-5 mm away from its corresponding virtual corner point.

[0009] Preferably, the distance W between the two grooves is 3-10 mm.

[0010] Preferably, when the groove is a straight groove, the spacing W is 5mm; when the groove is a curved groove, the radius of curvature R1 of the inner groove wall is 7.0-8.0mm, the radius of curvature R2 of the outer groove wall is 12.0-13.0mm, and R2 - R1 = W, where W is 5mm.

[0011] Preferably, the width of a single groove is 0.5 mm.

[0012] Preferably, when the groove is a concentric curved groove, the curved groove is composed of two or more concentric circular arcs, and the center of the concentric circular arcs is determined as follows: Connect the midpoints of opposite sides of the square ceramic substrate to form two mutually perpendicular bisectors, which divide the substrate plane into four rectangular regions. For the rectangular area to which the corner belongs, draw two diagonals, and the center of the concentric arc is the intersection of the diagonals.

[0013] On the other hand, the present invention provides a method for preparing a low-warpage DBC ceramic copper-clad laminate as described in any of the preceding claims, comprising the following steps: S1. Pretreatment: Clean the surface of the copper-clad laminate that has completed copper-ceramic bonding; S2. Double-sided lamination: Photosensitive films are laminated onto both sides of the cleaned copper-clad laminate to obtain a coated ceramic substrate; S3. Graphic Design and Exposure: Using film with a back-side graphic design, the product is exposed to transfer the designed graphic onto the surface of the coated substrate; S4. Etching Pattern: The exposed substrate is developed, and then the exposed copper layer is etched away. After the stripping process, a back trench structure is formed.

[0014] Preferably, in step S3, the positioning reference for the back face graphic is the virtual corner point defined above.

[0015] Preferably, in step S3, the line width of the groove pattern on the film is designed by photolithography compensation so that the width of the finished groove is 0.1mm to 0.3mm wider than the film line.

[0016] More preferably, when the width of the target finished product groove is 0.5mm, the width of the film graphic line is designed to be 0.25mm.

[0017] Compared with the prior art, the beneficial technical effects of this invention are reflected in: Active stress management: Unlike passive stress bearing, this invention actively interrupts the continuous transmission path of thermal stress in the copper layer by pre-setting grooves in the high-stress area (around the four corners) of the back welding surface, dividing the large continuous stress field into several smaller, relatively independent areas, thereby avoiding the accumulation of thermal stress in local areas during the welding process.

[0018] Significantly Reduced Warpage: By altering the stress distribution through physical structure, the overall warpage deformation of the DBC substrate during high-temperature soldering and cooling processes can be significantly reduced. Tests have shown that samples with back-side etched trenches exhibit significantly lower warpage than unetched samples.

[0019] Optimized groove design: This invention reveals that, compared to straight grooves, curved grooves can more gently guide and disperse stress flow, avoiding the generation of new stress singularities at the groove tip, thus achieving superior anti-warping performance. Finite element analysis and actual experimental results both verify this conclusion.

[0020] Good process compatibility: The trenches can be completed in one step in the existing DBC etching process without adding any additional production steps, resulting in minimal cost increase and easy industrialization.

[0021] Improved reliability: Reduced warpage means lower stress in the weld layer, fewer voids, and stronger bonds, thereby significantly improving the long-term reliability of the power module under operating conditions such as thermal cycling. Attached Figure Description

[0022] Figure 1 This is a process flow diagram disclosed in an embodiment of the present invention; Figure 2 This is a schematic diagram of the back copper layer structure without etched trenches disclosed in an embodiment of the present invention; Figure 3 This is a schematic diagram of the back copper layer structure of the etched straight trench disclosed in an embodiment of the present invention; Figure 4 This is a schematic diagram of the back copper layer structure of the etched curve trench disclosed in an embodiment of the present invention; Figure 5 A comparison of the warpage of the three sample examples under welding conditions at 280°C; Figure 6 The deformation contour plot of Example 1 after welding cooling is shown in the finite element method analysis. Figure 7 The deformation cloud diagram of Example 2 after welding cooling is shown in the finite element method analysis. Figure 8 This is a deformation cloud diagram of Example 3 after welding cooling, analyzed using finite element software. Detailed Implementation

[0023] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0024] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0025] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0026] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0027] Example 1 (Comparative Example) Prepare a standard DBC ceramic copper-clad laminate with a completely flat copper layer on its back side (i.e., the soldering side) without any stress relief grooves, such as... Figure 2 As shown. Specific parameters are as follows: The ceramic substrate is made of Al2O3, is rectangular, and has a thickness of 0.32 mm. The back copper layer, bonded using a direct copper bonding (DBC) process, typically covers an area smaller than the ceramic substrate, and its four corners are rounded by the process. Both the top and bottom copper layers are 0.3 mm thick. This embodiment serves as a comparison benchmark for subsequent embodiments.

[0028] Example 2 (Straight Trench) A DBC ceramic copper-clad laminate with straight stress relief trenches was fabricated. The substrate parameters were the same as in Example 1. Stress relief trenches were etched around the four corners of the copper layer on the back side.

[0029] like Figure 3 As shown, the starting point for measuring the distance between the trench and the four corner edges is not the right-angle vertex of the ceramic substrate, but rather the intersection point of a diagonal line drawn from the corresponding corner point on the plane of the square ceramic substrate and the actual edge (rounded corner portion) of the copper layer on the back. This intersection point is considered the "virtual corner point" of the copper layer at that corner. The straight-line distance between the outer edge of the trench pattern and this "virtual corner point" is designed to be 3mm.

[0030] Based on this, a set of straight trenches with a width of 0.5 mm and a spacing of 5 mm between the inner sides of the two trenches was etched. These two trenches extend perpendicular to the diagonal and have lengths of 8 mm and 10 mm respectively. Figure 3As shown. The trench is etched through the copper layer on the back side until the ceramic substrate is exposed, and the end of the trench does not extend to the outer edge of the copper layer.

[0031] Example 3 (Curved Groove) A DBC ceramic copper-clad laminate with curved stress relief grooves was prepared. The main parameters of the substrate were the same as in Example 1.

[0032] A set of curved trenches consisting of two concentric arcs is etched at each corner of the copper layer on the back side. The specific structure and positioning method are as follows: First, the "virtual corner point" corresponding to the corner is determined (defined as in Example 2); further, two perpendicularly intersecting straight lines are drawn by connecting the midpoints of opposite sides of the square ceramic substrate, dividing the substrate plane into four rectangular regions of equal area. The intersection of the two diagonals of the region to which the corner belongs is the common center of the set of trenches. Figure 4 The red "X" mark in the center indicates the center of the circle. Using this center as a reference, two concentric arcs with radii of curvature of 7.5mm and 12.5mm are drawn respectively. Each arc-shaped groove is 0.5mm wide, and the inner distance between the two grooves is 5mm. The straight-line distance between the outer edge of the groove and the corresponding "virtual corner point" is set to 3mm. The arc lengths of the grooves extending along the arcs are 12mm and 15mm respectively, and their depth penetrates the back copper layer until the surface of the underlying ceramic substrate is completely exposed. The grooves terminate inside the copper layer at both ends without extending to the outer edge of the copper layer, thus achieving stress isolation while maintaining the structural integrity and support function of the welded surface. Its shape is as follows... Figure 4 As shown.

[0033] Test Example 1: Comparative Test of Weld Warpage Five samples from each of the three embodiments described above were soldered at 280°C for 10 minutes using the same type of SnAgCu solder paste. After soldering, the samples were cooled to room temperature, and the maximum warpage height of each sample was measured using a laser flatness measuring instrument (according to the 9-point sampling method, the maximum warpage height is the maximum vertical distance between the 9 sampling points and the reference plane).

[0034] Test results: such as Figure 5 As shown, the average warpage of Example 1 (without trenches) was 0.61 mm; the average warpage of Example 2 (straight trenches) was 0.30 mm; and the average warpage of Example 3 (curved trenches) was 0.19 mm. The results indicate that etching release trenches at the four corners of the back copper layer can effectively reduce solder warpage by more than 50%, and the improvement effect of curved trenches is better than that of straight trenches, further improving it by approximately 37%.

[0035] Experimental Example 2: Finite Element Simulation Analysis To investigate the mechanical mechanism, a three-dimensional thermo-structural coupling model of the three types of DBC substrates was established using finite element analysis software (Yundofutu). The model accurately reflects the geometric characteristics of the ceramic right angles and the copper layer rounded corners. The cooling process from 280℃ to room temperature of 25℃ was simulated to analyze the thermal stress distribution and overall deformation.

[0036] Simulation results: such as Figures 6 to 8 As shown, the model of Example 1 exhibits high stress concentration in the four rounded corner areas, leading to severe overall warping deformation, with a maximum warping of 0.59 mm. The model of Example 2 shows that the straight grooves effectively interrupt the direct stress transmission path at the corners, significantly improving warping, with a maximum warping of 0.40 mm. The model of Example 3 shows that the curved grooves not only interrupt the stress transmission path but also more smoothly guide and disperse the stress flow emanating from the rounded corners, avoiding the slight stress concentration that might occur at the ends of the straight grooves, resulting in the most uniform overall stress distribution and the smallest theoretically calculated warping deformation, with a maximum warping of 0.29 mm. The simulation results are consistent with the measured data trends of Example 1, verifying the effectiveness of the invention.

[0037] Example 4: Preparation Method Example The preparation method of the DBC ceramic copper-clad laminate includes the following steps: S1. Pretreatment: Clean the ceramic copper-clad laminate that has been sintered and bonded at high temperature (such as pickling and grinding) to obtain a clean copper surface; S2. Double-sided lamination: Photosensitive films are laminated on both sides of the cleaned ceramic copper-clad board to obtain a laminated ceramic substrate; S3. Graphic Design and Exposure: Based on the position of the back-side pattern, design the four corner stress-relief groove patterns on the film. The positioning reference for the back-side groove patterns is the "virtual corner point" defined above. The line width of the groove patterns needs to be compensated by photolithography; for example, when the designed finished groove width is 0.5mm, the film pattern line width can be 0.25mm. Place the coated ceramic substrate under the film for exposure to complete the pattern transfer. S4. Etching Pattern: The exposed ceramic substrate is developed, and then the exposed copper layer is etched using an etching solution (such as copper chloride solution) to form back trenches. Finally, the film is removed, cleaned and dried to obtain the finished product.

[0038] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

[0039] It should be particularly noted that the various embodiments listed in this specification and accompanying drawings are intended to illustrate the technical solutions and advantages of the present invention, and not to limit the scope of protection of the present invention. Without departing from the core ideas and technical effects of the present invention, those skilled in the art can make any form of improvement, substitution, combination, or modification to the structural arrangement, process parameters, material selection, control logic, etc., of the described embodiments; any obvious changes based on the same concept should be considered equivalent solutions of the present invention and should be included within the scope of protection defined by the claims of the present invention. The actual scope of protection of the present invention is determined by the appended claims and should be correctly understood in conjunction with the specification and accompanying drawings.

Claims

1. A low-warpage DBC ceramic copper-clad laminate, comprising a ceramic substrate and copper layers bonded to its front and back sides, characterized in that: At least one trench is etched around the four corners of the copper layer on the back side to release the heat stress of the soldering; the trench penetrates the thickness of the copper layer on the back side, exposes the underlying ceramic substrate, and the end of the trench terminates inside the copper layer on the back side, without extending to the edge of the copper layer on the back side.

2. The DBC ceramic copper-clad laminate according to claim 1, characterized in that, The grooves around each corner are two in number, and the grooves are either parallel straight grooves or concentric curved grooves.

3. The DBC ceramic copper-clad laminate according to claim 2, characterized in that, The intersection of a diagonal line drawn from a corner point of the ceramic substrate and the rounded edge of the copper layer on the back is defined as a virtual corner point, and the outer edge of the trench is 2-5 mm away from its corresponding virtual corner point.

4. The DBC ceramic copper-clad laminate according to claim 3, characterized in that, The distance W between the two grooves is 3-10mm.

5. The DBC ceramic copper-clad laminate according to claim 4, characterized in that, When the groove is a straight groove, the spacing W is 5mm; when the groove is a curved groove, the radius of curvature R1 of the inner groove wall is 7.0-8.0mm, the radius of curvature R2 of the outer groove wall is 12.0-13.0mm, and R2 - R1 = W, where W is 5mm.

6. The DBC ceramic copper-clad laminate according to claim 5, characterized in that, The width of a single groove is 0.5 mm.

7. The DBC ceramic copper-clad laminate according to claim 2, characterized in that, When the groove is a concentric curved groove, the curved groove is composed of two or more concentric circular arcs, and the center of the concentric circular arcs is determined as follows: Connect the midpoints of opposite sides of the square ceramic substrate to form two mutually perpendicular bisectors, which divide the substrate plane into four rectangular regions. For the rectangular area to which the corner belongs, draw two diagonals, and the center of the concentric arc is the intersection of the diagonals.

8. A method for preparing a DBC ceramic copper-clad laminate as described in any one of claims 1-7, characterized in that, Includes the following steps: S1. Pretreatment: Clean the surface of the copper-clad laminate that has completed copper-ceramic bonding; S2. Double-sided lamination: Photosensitive films are applied to both sides of the cleaned copper-clad laminate to obtain a coated ceramic substrate; S3. Graphic Design and Exposure: Using film with a back-side graphic design, the product is exposed to transfer the designed graphic onto the surface of the coated substrate; S4. Etching Pattern: The exposed substrate is developed, and then the exposed copper layer is etched away. After the stripping process, a back trench structure is formed.

9. The method according to claim 8, characterized in that, In step S3, the line width of the groove pattern on the film is compensated by photolithography so that the width of the finished groove is 0.1mm to 0.3mm wider than the film line.

10. The method according to claim 9, characterized in that, When the target finished product groove width is 0.5mm, the film graphic line width is designed to be 0.25mm.

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