Surface treatment method of DBC substrate for power module

Abstract

The invention provides a surface treatment method of a DBC substrate for a power module. The surface treatment method comprises the following steps that S1, the DBC substrate with an oxide layer on the surface is cleaned; s2, uniformly coating the surface of the oxide layer with a soldering flux; s3, placing in a vacuum environment, activating at a first temperature, heating to a second temperature to carry out a first-stage reduction reaction, heating to a third temperature to carry out a second-stage reduction reaction, and vacuumizing to discharge reaction byproducts; and S4, after cooling, residues on the surface of the DBC substrate are removed. According to the method, the scaling powder is used for carrying out thermal reduction reaction in a vacuum environment, an oxide layer is reduced into pure copper, and the original microstructure and roughness of the oxide layer are reserved; after treatment, the copper content is increased to 100% from 96.3%, the oxygen content is greatly reduced, the chip welding void ratio reaches 0.3%, the shearing strength of a welding interface reaches up to 38.5 Mpa, and the reliability of power module packaging is fundamentally improved.

Description

Technical Field

[0001] This invention belongs to the field of power semiconductor device packaging technology, and in particular relates to a surface treatment method for a DBC substrate for power modules. Background Technology

[0002] Power modules (such as IGBT modules and SiC power modules) are core power conversion units in fields such as new energy vehicles, rail transit, and photovoltaic energy storage inverters. Their reliability, power density, and efficiency directly affect the performance of the end system. As a core component in power modules, responsible for electrical interconnection, insulation isolation, and critical heat dissipation channels, direct copper-clad ceramic (DBC) substrates are widely used. DBC substrates typically consist of a ceramic insulating layer (such as Al2O3, AlN, or Si3N4) and copper foil layers directly bonded to both sides using high-temperature processes. The surface quality of the copper foil serves as the direct interface for chip mounting (such as soldering and silver sintering) and wire bonding.

[0003] However, during the storage, transportation, and packaging processes after the DBC substrate is manufactured, the surface of its copper foil is highly susceptible to reaction with oxygen and water vapor in the air, forming a surface oxide layer mainly composed of cuprous oxide (Cu2O) and copper oxide (CuO). The presence of this oxide layer will severely degrade the quality of subsequent packaging processes and the final performance of the module. When the chip is soldered to the DBC substrate, the interface void ratio increases significantly, affecting the reliability of heat dissipation. The oxide layer has extremely poor conductivity, which will increase the contact resistance between the chip and the substrate, leading to increased conduction losses and reduced efficiency of the module. At the same time, it will also seriously affect the bonding strength and reliability of the lead bonding points, easily causing desoldering or cracking.

[0004] To address the oxidation problem on the copper foil surface of DBC substrates, the existing technologies mainly employ the following methods, but all of them have significant limitations. For example, chemical pickling uses strong acid solutions such as hydrochloric acid and sulfuric acid to corrode and remove the oxide layer. While this method is effective to some extent, it poses risks of wastewater pollution and chemical residues, and the acid may alter the micro-roughness and morphology of the copper surface. Mechanical physical removal uses physical methods such as sandpaper grinding and sandblasting to peel off the oxide layer. Although this method can remove the oxide layer, it inevitably damages the micro-morphology of the copper surface, destroys surface anchors, and reduces solder wettability. Plasma cleaning uses active particles in plasma to physically bombard and chemically react with the surface to remove contaminants and thin oxide layers. This method is relatively clean, but its effectiveness is limited for thicker (e.g., exceeding 100 nanometers) or dense oxide layers. Furthermore, the equipment is expensive and the processing efficiency is low, making it unsuitable for large-scale, low-cost production needs. Direct scrapping and replacement is the most direct solution for DBC substrates that have undergone significant oxidation. However, for large-area DBC substrates or those with complex customized patterns, this method will result in extremely high material cost waste.

[0005] Existing technologies lack a surface treatment method that can efficiently and thoroughly remove the oxide layer from the copper foil surface of a DBC substrate without damaging the copper foil surface, causing no environmental pollution, and suitable for large-scale industrial production. Therefore, there is a need to provide an improved technical solution that addresses the shortcomings of the existing technologies. Summary of the Invention

[0006] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a surface treatment method for a DBC substrate for power modules, which solves the problems of poor welding quality and low reliability caused by the presence of an oxide layer on the copper foil surface of the DBC substrate in the prior art, as well as the problems of pollution, damage to the copper surface, or high cost during the oxide layer treatment process.

[0007] To achieve the above and other related objectives, the present invention provides a surface treatment method for a DBC substrate for a power module, comprising the following steps:

[0008] S1. A DBC substrate is provided, the DBC substrate including a ceramic sheet and a copper foil sheet on the surface of the ceramic sheet, an oxide layer is formed on the side of the copper foil sheet away from the ceramic sheet, and the DBC substrate with the oxide layer on the surface is cleaned.

[0009] S2. Apply flux evenly to the surface of the oxide layer;

[0010] S3. Place the DBC substrate coated with the flux in a vacuum environment, first activate it at a first temperature, then raise the temperature to a second temperature for a first-stage reduction reaction, and then raise the temperature to a third temperature to continue the second-stage reduction reaction, while removing the reaction byproducts by vacuuming.

[0011] S4. After cooling down, remove the residue on the surface of the DBC substrate.

[0012] Preferably, the acid value of the flux in step S2 is 80 mg KOH / g to 120 mg KOH / g.

[0013] Preferably, the flux in step S2 includes rosin resin, organic acid activator and thixotropic agent, wherein the organic acid activator includes at least one of C4~C10 dicarboxylic acid and hydroxy aromatic acid.

[0014] Preferably, in step S2, the amount of flux applied is 15 mg / cm² to 25 mg / cm² on a wet weight basis.

[0015] Preferably, the flux in step S2 has a viscosity of 50 Pa·s to 200 Pa·s at 25°C.

[0016] Preferably, in step S3, the temperature is increased to a first temperature at a heating rate of 0.2℃ / s to 3℃ / s, where the first temperature is 50℃ to 150℃, and the activation treatment time is 60s to 120s.

[0017] Preferably, in step S3, the second temperature is 150~200℃, the time of the first stage reduction reaction is 60s~120s, and the vacuum degree of the vacuum environment of the first stage reduction reaction is not higher than 50mbar.

[0018] Preferably, the third temperature in step S3 is 200~250℃, and the vacuum degree of the vacuum environment of the second stage reduction reaction is not higher than 30mba.

[0019] Preferably, during the first and second stage reduction reactions in step S3, a reducing gas is also introduced into the vacuum environment, wherein the reducing gas is formic acid vapor or a hydrogen-containing reducing protective gas.

[0020] Preferably, the cooling in step S4 is performed under a protective atmosphere at a rate of 0.2℃ / s to 3℃ / s.

[0021] As described above, the surface treatment method for the DBC substrate for power modules of the present invention has the following beneficial effects:

[0022] This invention provides a simple, environmentally friendly, and industrially applicable surface treatment method for DBC substrates. This method utilizes a programmed thermal reduction reaction of flux in a vacuum environment to chemically reduce the oxide layer on the copper foil surface to pure copper without damaging the copper substrate. It completely preserves the original microstructure and surface roughness of the copper foil, restoring its solderability. This fundamentally solves the problems of poor soldering quality and low reliability caused by the oxide layer in existing technologies, as well as pollution, damage, or high costs during the processing. EDS analysis verifies that the atomic percentage of copper before treatment significantly increased from approximately 96.3% to approximately 100%, while the oxygen content was greatly reduced, indicating that the oxide layer was efficiently and thoroughly removed. Furthermore, this process is compatible with existing vacuum reflow ovens, requiring no additional investment in specialized equipment. The solid flux is evenly coated, the dosage is controllable, and there is no risk of dripping. Multiple DBC substrates can be processed in a single batch, making it particularly suitable for industrial-scale batch repair. This fundamentally solves the problems of low processing efficiency, high cost, or environmental damage associated with existing technologies.

[0023] The surface treatment method in this invention can obtain a clean and highly active copper foil surface, providing a foundation for subsequent chip soldering and wire bonding. When soldering is performed using the treated DBC substrate, the chip soldering porosity reaches 0.3%, and the average shear strength of the soldering interface is as high as 38.5 MPa. This invention not only successfully removes the oxide layer, but also fundamentally improves the reliability of power module packaging, providing a key guarantee for power modules to achieve higher power density, longer service life, and more stable operating performance. Attached Figure Description

[0024] Figure 1 The diagram shows a schematic of the surface treatment process of the DBC substrate for the power module of this invention.

[0025] Figure 2 The diagram shown is a schematic representation of the structure of the DBC substrate of this invention.

[0026] Figure 3 The graph shown is a temperature curve of the DBC substrate for the power module in Embodiment 1 of the present invention undergoing surface treatment in a vacuum reflow oven.

[0027] Figure 4 The image shown is a photograph of the DBC substrate before surface treatment in Embodiment 1 of the present invention.

[0028] Figure 5 The image shown is a photograph of the DBC substrate after surface treatment in Embodiment 1 of the present invention.

[0029] Figure 6a The image shown is an SEM image of the DBC substrate after surface treatment in Embodiment 1 of the present invention.

[0030] Figure 6b Displayed as Figure 6a EDS energy spectrum of two feature points.

[0031] Figure 7a The image shown is a SEM image of the DBC substrate before surface treatment in Embodiment 1 of the present invention.

[0032] Figure 7b Displayed as Figure 7a EDS energy spectrum of two feature points.

[0033] Explanation of reference numerals in the attached figures: 10, copper-clad ceramic substrate; 100, ceramic sheet; 101, first copper foil sheet; 102, second copper foil sheet; 201, first oxide layer; 202, second oxide layer. Detailed Implementation

[0034] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0035] Before further describing specific embodiments of the present invention, it should be understood that the scope of protection of the present invention is not limited to the specific embodiments described below; it should also be understood that the terminology used in the embodiments of the present invention is for describing specific embodiments and not for limiting the scope of protection of the present invention. Test methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the respective manufacturers.

[0036] When numerical ranges are given in the embodiments, it should be understood that, unless otherwise stated in the present invention, both endpoints of each numerical range and any value between the two endpoints may be selected. Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. In addition to the specific methods, apparatus, and materials used in the embodiments, based on the knowledge of the prior art possessed by one of ordinary skill in the art and the description of this invention, any prior art methods, apparatus, and materials similar to or equivalent to those described, apparatus, and materials in the embodiments of this invention may be used to implement the present invention.

[0037] It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the drawings only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components. In actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0038] See Figure 1 This invention provides a surface treatment method for a DBC substrate for power modules, comprising the following steps:

[0039] S1. Provide a DBC substrate, the DBC substrate including a ceramic sheet 100 and a copper foil sheet located on the surface of the ceramic sheet 100, an oxide layer is formed on the side of the copper foil sheet away from the ceramic sheet 100, and the DBC substrate with the oxide layer on the surface is cleaned.

[0040] S2. Apply flux evenly to the surface of the oxide layer;

[0041] S3. The DBC substrate coated with the flux is placed in a vacuum environment, first activated at a first temperature, then heated to a second temperature for a first-stage reduction reaction, and then heated to a third temperature for a second-stage reduction reaction, while removing reaction byproducts by vacuuming.

[0042] S4. After cooling down, remove the residue on the surface of the DBC substrate.

[0043] The surface treatment method of the DBC substrate for the power module in this invention will be described in detail below with reference to the specific accompanying drawings.

[0044] First, please refer to Figure 1 and Figure 2 Step S1 is performed, providing a DBC substrate, the DBC substrate including a ceramic sheet 100 and a copper foil sheet located on the surface of the ceramic sheet 100, the side of the copper foil sheet away from the ceramic sheet 100 having an oxide layer formed, and cleaning the DBC substrate with the oxide layer on the surface.

[0045] For details, please refer to Figure 2 The DBC substrate is a double-sided copper-clad ceramic substrate 10, wherein the copper foil includes a first copper foil 101 and a second copper foil 102, which are separately disposed on opposite sides of the ceramic sheet 100. Correspondingly, the oxide layer includes a first oxide layer 201 and a second oxide layer 202. The first oxide layer 201 is formed on the side of the first copper foil 101 away from the ceramic sheet 100, and the second oxide layer 202 is formed on the side of the second copper foil 102 away from the ceramic sheet 100. The oxides of the oxide layer are mainly Cu2O and CuO.

[0046] Specifically, the cleaning process involves wiping the copper foil of the DBC substrate away from the ceramic sheet 100 with a lint-free cloth soaked in anhydrous ethanol or isopropanol, and then drying it for later use.

[0047] Next, step S2 is performed to uniformly coat the surface of the oxide layer with flux.

[0048] Specifically, an oil painting brush is used to evenly coat the surface of the oxide layer with flux. The oil painting brush is made of pig bristles or nylon bristles. Utilizing its elasticity, it can form a uniform and dense flux film on the microscopically rough surface of the copper foil. Compared with scraping or screen printing, it can better fill the micro-grooves without damaging the copper foil under the oxide layer.

[0049] As an example, the acid value of the flux mentioned in step S2 is 80 mg KOH / g to 120 mg KOH / g.

[0050] Specifically, the acid value of the flux can be any value within a range such as 80mgKOH / g, 90mgKOH / g, 100mgKOH / g, 110mgKOH / g, or 120mgKOH / g, and can be adjusted according to actual needs.

[0051] As an example, the flux in step S2 includes rosin resin, organic acid activator and thixotropic agent, wherein the organic acid activator includes at least one of C4~C10 dicarboxylic acid and hydroxy aromatic acid.

[0052] Specifically, the C4-C10 dicarboxylic acids are succinic acid, glutaric acid, adipic acid, pimelic acid, octanoic acid, azelaic acid, and sebacic acid, respectively; the hydroxy aromatic acids include salicylic acid, etc.; preferably, the organic acid activator is selected from at least one of succinic acid, adipic acid, sebacic acid, and salicylic acid. Furthermore, no excessive restrictions are placed on the rosin resin and thixotropic agent; those commonly used by those skilled in the art can be used.

[0053] As an example, in step S2, the amount of flux applied is 15 mg / cm² to 25 mg / cm² on a wet weight basis.

[0054] Specifically, based on wet weight, the amount of flux applied can be any value within the range of 15 mg / cm², 18 mg / cm², 20 mg / cm², 22 mg / cm², 25 mg / cm², etc., and can be adjusted according to the actual situation.

[0055] In practical applications, after applying flux, the flux film should be visually inspected to ensure it is a semi-transparent thin film without accumulation or missed areas.

[0056] As an example, the flux described in step S2 has a viscosity of 50 Pa·s to 200 Pa·s at 25°C.

[0057] Specifically, the viscosity of the flux at 25°C can include any value within the range of 50 Pa·s, 100 Pa·s, 150 Pa·s, 200 Pa·s, etc., and can be adjusted according to actual conditions. Those skilled in the art will understand that as long as the viscosity of the solid flux is within an appropriate range (50 Pa·s~200 Pa·s), a uniform coating can be formed by the selected coating method, and the wet weight can be controlled to fall within the range of 15~25 mg / cm², the objective of this invention can be achieved.

[0058] The flux used in this specific embodiment of the invention is a solid flux with a coating viscosity of 30-100 cps under heating, and a solid content of 85wt%-95wt%. Preferably, the solid flux used in this specific embodiment of the invention is FLUXSF64, with a density of 1.0-1.2 g / cm³ and a softening point of 70-85°C. Of course, those skilled in the art will understand that any flux with a similar acid value range, composition, and physical state (solid) can be used in this invention, and is not limited to the specific grades mentioned above.

[0059] Of course, paste flux can also be used. Compared with liquid flux, solid or paste flux has no risk of dripping and will not contaminate the edge of the ceramic sheet. Moreover, the coating thickness is uniform. On large-scale production lines, in order to ensure process stability and result consistency, a standard amount of flux can be specified for a specific model and size of DBC substrate, so that the amount of flux used is precise and controllable.

[0060] Next, step S3 is performed, in which the DBC substrate coated with the flux is placed in a vacuum environment, firstly activated at a first temperature, then heated to a second temperature for a first-stage reduction reaction, and then heated to a third temperature to continue the second-stage reduction reaction, while simultaneously removing reaction byproducts by vacuuming.

[0061] Specifically, in this embodiment of the invention, the vacuum environment used is the existing vacuum reflow oven for power modules, requiring no additional investment in dedicated equipment; the model used is VacL1030. However, in other examples, the model of the vacuum reflow oven is not specifically limited, as long as it meets the actual usage requirements. The equipment used in this vacuum environment includes a vacuum pumping device and a programmed temperature control device. Those skilled in the art will understand that any equipment capable of programmed heating and cooling, and capable of establishing and maintaining a vacuum environment during heating, can be used in this invention, such as a vacuum sintering furnace, a vacuum brazing furnace, or other customized vacuum heat treatment equipment.

[0062] As an example, in step S3, the temperature is increased to a first temperature at a heating rate of 0.2℃ / s to 3℃ / s, where the first temperature is 50℃ to 150℃, and the activation treatment time is 60s to 120s.

[0063] Specifically, before the reduction reaction, the flux should be activated and spread evenly. This involves preheating from room temperature to a first temperature. The heating rate can be any value within the range of 0.2℃ / s, 0.5℃ / s, 1℃ / s, 2℃ / s, 2.5℃ / s, 3℃ / s, etc. The first temperature can be any value within the range of 50℃, 60℃, 80℃, 100℃, 120℃, 140℃, 150℃, etc. The activation time can be any value within the range of 60s, 70s, 75s, 80s, 90s, 100s, 110s, 120s, etc., and can be adjusted according to the actual situation.

[0064] In addition, during the activation stage, there are no strict restrictions on the vacuum level of the vacuum environment, but the vacuum level during the activation stage should not be too low to avoid flux boiling or evaporating too quickly.

[0065] As an example, in step S3, the second temperature is 150~200℃, the time of the first stage reduction reaction is 60s~120s, and the vacuum degree of the vacuum environment of the first stage reduction reaction is not higher than 50mbar.

[0066] Specifically, the second temperature can be any value within the range of 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, etc., and the reduction reaction time can be any setting within the range of 50s, 60s, 70s, 80s, 90s, 100s, 110s, 120s, etc., which can be adjusted according to actual needs. During the first stage of the reduction reaction, the vacuum level of the vacuum environment shall not exceed 50mbar, such as 50mbar, 40mbar, 30mbar, 20mbar, 1mbar, 0.1mbar, etc., but not 0.

[0067] As an example, the third temperature in step S3 is 200~250℃, and the vacuum degree of the vacuum environment of the second stage reduction reaction is not higher than 30mba.

[0068] Specifically, the third temperature includes any value within the range of 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, etc., and the vacuum degree of the vacuum environment during the second-stage reduction reaction is not higher than 30mbar, such as 30mbar, 20mbar, 10mbar, 5mbar, 1mbar, etc., but not 0.

[0069] Preferably, the vacuum level of the vacuum environment during the second-stage reduction reaction is 10 mbar to 30 mbar (e.g., 10 mbar, 15 mbar, 20 mbar, 25 mbar, 30 mbar, etc.).

[0070] Specifically, in the second stage reduction reaction process of step S3, it is also necessary to remove the reaction byproducts by evacuation. In a specific embodiment of the present invention, the vacuum is continuously evacuated for 5 to 10 seconds at the peak temperature of the second stage reduction reaction so that the vacuum degree in the vacuum environment does not exceed 30 mbar.

[0071] Specifically, in a vacuum environment, the rosin resin in the flux softens and flows, and the organic acid activator undergoes a reduction reaction with CuO. The chemical reaction formula is as follows:

[0072] CuO + 2H⁺ → Cu²⁺ + H₂O↑

[0073] In practical applications, when the temperature is raised to the third temperature, the reduction reaction rate also increases accordingly. At this time, by drawing a vacuum to remove the water vapor generated by the reaction, the reaction is promoted to proceed to the right, so that the reduction reaction can be completed. Secondly, drawing a vacuum can also remove reaction byproducts, such as low-valence copper oxide volatiles, thereby reducing the oxygen partial pressure and preventing the re-oxidation of copper at high temperatures. It can also promote the removal of residual solvents and high-boiling-point volatiles in the flux. At the same time, the vacuum environment can improve the wetting and spreading of liquid flux on the copper foil surface.

[0074] Moreover, the reduction reaction is used to remove the oxide layer without involving mechanical friction or strong corrosion, and the surface roughness (Ra value) of the copper foil is completely preserved, which is beneficial to solder climb and bonding strength in the subsequent welding stage; finally, the surface oxide layer of the DBC substrate treated by the method in the specific embodiment of the present invention is reduced to Cu.

[0075] As an example, in step S3, during the first stage reduction reaction and the second stage reduction reaction, a reducing gas is also introduced into the vacuum environment, wherein the reducing gas is formic acid vapor or a hydrogen-containing reducing protective gas.

[0076] Specifically, during the reduction reaction stage, the purpose of introducing formic acid vapor or a hydrogen-containing reducing protective gas into the vacuum environment is to enhance the reducing atmosphere, allowing the organic acid activator in the flux to better react with the oxide layer. The hydrogen-containing reducing protective gas is specifically a mixture of hydrogen and nitrogen, or a mixture of hydrogen and argon; preferably, the hydrogen-containing reducing protective gas is a mixture of hydrogen and nitrogen, with a hydrogen to nitrogen volume ratio of 19:1.

[0077] Finally, in step S4, after cooling down, the residue on the surface of the DBC substrate is removed.

[0078] As an example, the cooling in step S4 is performed under a protective atmosphere at a rate of 0.2℃ / s to 3℃ / s.

[0079] Specifically, the protective atmosphere includes at least one of nitrogen, argon, and helium to prevent oxidation during cooling; the cooling rate can be any value within the range of 0.2℃ / s, 0.5℃ / s, 1℃ / s, 2℃ / s, 2.5℃ / s, 3℃ / s, etc., and can be adjusted according to actual conditions.

[0080] Specifically, the method for removing residues from the surface of the DBC substrate in step S4 is as follows: immersion cleaning with flux cleaning solution to remove most of the rosin resin and flux residue, followed by spraying with undiluted flux cleaning solution to rinse away the remaining small amount of rosin resin and flux residue, resulting in a clean DBC substrate with high soldering activity; wherein the flux cleaning solution is dichloromethane or a dichloromethane-containing cleaning solution; preferably, immersion cleaning with flux cleaning solution for 720s is performed first, followed by spraying for 240s.

[0081] To better understand the surface treatment method for the DBC substrate of the power module in this invention, the surface treatment method for the DBC substrate of the power module in this invention will be described below with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the invention in any way.

[0082] The flux used in the following examples is FLUXSF64 flux, with a viscosity of 150 Pa·s at 25°C, a solid content of ≥90 wt%, and an acid value of 100 mg KOH / g; the DBC substrate is 50 mm × 60 mm in size; the standardized amount of flux is 30 mg / piece, which is equivalent to 20 mg / cm² wet weight.

[0083] Example 1

[0084] This embodiment provides a surface treatment method for a DBC substrate used in power modules, including the following steps:

[0085] S1. Provide a DBC substrate (provide 10 samples of the same specifications). The DBC substrate includes a copper foil sheet on the surface of a ceramic sheet 100. An oxide layer is formed on the side of the copper foil sheet away from the ceramic sheet 100. Wipe the DBC substrate with the oxide layer on the surface with a lint-free cloth soaked in isopropanol and let it air dry in clean air for 5 minutes. The ceramic sheet 100 is an Al2O3 ceramic sheet 100, and the DBC substrate is an Al2O3 ceramic DBC substrate with a size of 50mm × 60mm. Place the DBC substrate in an environment with a temperature of 20~30°C and a relative humidity of 50~70%RH for 30 days. The copper foil sheet of the DBC substrate shows obvious oxidation and discoloration, turning dark red, that is, there is an oxide layer on the surface of the copper foil sheet.

[0086] S2. Use an oil painting brush to evenly coat the flux onto the surface of the oxide layer to form a flux film, and control the wet weight (area density after flux coating) to be 20 mg / cm².

[0087] S3. Place the DBC substrate coated with flux on the tray of the vacuum reflow oven. The programmed temperature parameters are shown in Table 1 below. Under a vacuum environment of 30 mbar, heat to the first temperature of 50~150℃ (preheating zone) at a heating rate of 1.2℃ / s to activate and spread the flux (the actual measured time is 75s). Continue to heat to the second temperature of 150℃~200℃ (first reduction zone) to start the first stage of reduction reaction and hold for 120s. Continue to heat to 200~240℃ (second reduction zone) to carry out the second stage of reduction reaction and hold for 120s until the reaction is complete. At the peak temperature of the second reduction zone, vacuum is applied to remove the reaction byproducts and maintain the vacuum at 20 mbar.

[0088] S4. Under nitrogen protection, the DBC substrate is cooled at a rate of 1.2℃ / s for 180s (cooling zone). After cooling, the DBC substrate is immersed in dichloromethane cleaning solution for 720s and then sprayed with dichloromethane cleaning solution for 240s to remove the residue on the surface of the DBC substrate. After cleaning, it is air-dried or oven-dried to obtain 10 surface-treated DBC substrates.

[0089] Table 1 shows the programmed temperature rise parameters in this embodiment.

[0090]

[0091] See Figure 3 This is a temperature profile of a DBC substrate undergoing surface treatment in a vacuum reflow oven. The numbers 1-10 represent the 10 temperature zones of the vacuum reflow oven, the black horizontal line represents the set temperature for each zone, and the curves show the measured temperature-time relationship obtained from thermocouples at different locations. Figure 3The temperature curves in the figure and the parameters in Table 1 verify the uniformity of the temperature distribution in the vacuum reflow oven and the feasibility of the process parameters.

[0092] See Figure 4 and Figure 5 The images show actual photos of the DBC substrate before and after surface treatment. The left image shows the upper surface of the DBC substrate, and the right image shows the lower surface of the DBC substrate. After surface treatment, the copper foil surface of the DBC substrate regains its metallic luster and has a bright copper color, which intuitively demonstrates the effect of oxide layer removal.

[0093] Referring to GB / T 3505-2009, the surface roughness of multiple DBC substrates before and after surface treatment in this embodiment was measured using a 3D profilometer. The average surface roughness Ra of the copper foil of the DBC substrate before surface treatment was 0.4 μm. In this embodiment, the surface roughness Ra of the copper foil of the DBC substrate after surface treatment was not significantly different from that before treatment. The surface roughness of the copper foil is preserved, which is beneficial to the subsequent solder creep and bonding strength.

[0094] To quantitatively evaluate the surface treatment effect, an energy-dispersive X-ray spectroscopy (EDS) coupled with SEM was used to analyze the surface composition of the copper foil sheets on the DBC substrate before and after treatment. Under the conditions of an accelerating voltage of 15 kV and a data acquisition time of 100 seconds, point analysis was performed on characteristic points in the SEM images, with an analysis depth of approximately 1–3 μm. The ZAF correction method was then used for quantitative calculation. (See reference...) Figure 6a The processed SEM image. Figure 6b for Figure 6a The EDS spectra of the two selected feature points, Spc_003 and Spc_004, show that when a high-energy electron beam bombards copper atoms, inner-shell (K-shell) electrons are ejected, and outer-shell (L-shell) electrons transition to fill the vacancies, releasing X-rays. Figure 6b The CuLa, CuLl, CuKa, and CuKb marked in the figure are characteristic X-ray spectral lines of Cu. They represent X-rays with specific energies released when copper atoms undergo electronic transitions. CuLa is the L-series α line of copper, with an energy of approximately 0.9 keV, and is the strongest peak in the figure, indicating a high copper content in the sample. CuLl is the L-series β line of copper, with an energy of approximately 0.95 keV, and its intensity is weaker than CuLa; it is a companion peak of the L-series. CuKa is the K-series α line of copper, with an energy of approximately 8.04 keV, and is the most prominent characteristic peak of the K-series. CuKb is the K-series β line of copper, with an energy of approximately 8.90 keV, and its intensity is weaker than CuKa; it is a companion peak of the K-series. Quantitative calculations show that the atomic percentage of Cu at Spc_003 and Spc_004 is 100%, and the atomic percentage of oxygen is 0%. (See also...) Figure 7a The image is the SEM image before processing. Figure 7b for Figure 7a The EDS spectra of the two selected feature points Spc_006 and Spc_008 are compared with... Figure 6b , Figure 7b Additional oxygen and carbon peaks appeared. Oka is the K-series α line for oxygen, with an energy of approximately 0.525 keV, indicating the presence of oxygen on the surface of the copper foil of the DBC substrate before treatment. Cka is the K-series α line for carbon, with an energy of approximately 0.277 keV, typically originating from contamination or organic residues on the surface of the copper foil of the DBC substrate before treatment. Quantitative calculations revealed that the atomic percentage of Cu at Spc_006 was 95.59%, oxygen was 0.59%, and C was 3.81%; the atomic percentage of Cu at Spc_008 was 97.01%. The atomic percentages of oxygen and carbon are 0.45% and 2.54%, respectively. Calculating the average of two characteristic sites, the atomic percentages of Cu are 96.3%, oxygen is 0.52%, and carbon is 3.18%. That is, after surface treatment using the method described in this embodiment, the atomic percentage of copper on the surface of the DBC substrate copper foil significantly increases from approximately 96.3% to approximately 100%, while the oxygen content decreases substantially (from 0.52% to 0%). The EDS results of the DBC substrate after surface treatment are shown in [Figure number missing]. Figure 6b The EDS results of the DBC substrate before surface treatment are shown in [reference needed]. Figure 7b This indicates that the copper oxide on the surface has been effectively reduced and removed, resulting in a high-purity metallic copper surface, which lays a solid foundation for its subsequent high-reliability welding.

[0095] Ten surface-treated DBC substrates and untreated DBC substrates obtained in this embodiment were used for chip soldering (the solder was SnAgCu eutectic solder, the soldering temperature was 240℃, and the holding time was 60s). The chip soldering void rate was measured after soldering. The test showed that the soldering void rate of the untreated DBC substrate was greater than 15%. However, the average chip soldering void rate of the surface-treated DBC substrate in this embodiment was 0.3%, which meets the industry standard that the chip soldering void rate should be controlled within 1%. That is, the surface treatment significantly reduced the chip soldering void rate.

[0096] Ten chips, each 3mm × 3mm in size, were welded together and subjected to shear strength testing using a push-pull tester (Dage 4000) according to MIL-STD-883H Method 2019 to test the welding strength of the chips. The test results showed that the average shear strength of the ten chips welded from the surface-treated DBC substrates obtained in this embodiment was 38.5MPa, with a minimum of 34.8MPa, both exceeding the minimum requirement of standard MIL-STD-883H (≥25MPa), indicating that the copper surface after reduction treatment forms an excellent metallurgical bond with the solder.

[0097] In summary, this invention provides a simple, environmentally friendly, and industrially applicable surface treatment method for DBC substrates. This method utilizes a programmed thermal reduction reaction of flux in a vacuum environment to chemically reduce the oxide layer on the copper foil surface to pure copper without damaging the copper substrate. It completely preserves the original microstructure and surface roughness of the copper foil, restoring its solderability. This fundamentally solves the problems of poor soldering quality and low reliability caused by the oxide layer in existing technologies, as well as pollution, damage, or high costs during the processing. EDS analysis verifies that the atomic percentage of copper before treatment significantly increased from approximately 96.3% to approximately 100%, while the oxygen content was significantly reduced, indicating that the oxide layer was efficiently and thoroughly removed. Furthermore, this process is compatible with existing vacuum reflow ovens, requiring no additional investment in specialized equipment. The solid flux is evenly coated, the dosage is controllable, and there is no risk of dripping. Multiple DBC substrates can be processed in a single batch, making it particularly suitable for industrial-scale batch repair. This fundamentally solves the problems of low processing efficiency, high cost, or environmental damage associated with existing technologies. The surface treatment method in this invention can obtain a clean and highly active copper foil surface, providing a foundation for subsequent chip soldering and wire bonding. Using the treated DBC substrate for soldering results in a chip soldering porosity of 0.3% and an average shear strength at the solder interface as high as 38.5 MPa. This invention not only successfully removes the oxide layer but also fundamentally improves the reliability of power module packaging, providing a key guarantee for achieving higher power density, longer lifespan, and more stable operating performance of power modules. Therefore, this invention effectively overcomes the various shortcomings of existing technologies and has high industrial application value.

[0098] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A surface treatment method for a DBC substrate for a power module, characterized by, Includes the following steps: S1. A DBC substrate is provided, the DBC substrate including a ceramic sheet and a copper foil sheet on the surface of the ceramic sheet, an oxide layer is formed on the side of the copper foil sheet away from the ceramic sheet, and the DBC substrate with the oxide layer on the surface is cleaned. S2. Apply flux evenly to the surface of the oxide layer; S3. Place the DBC substrate coated with the flux in a vacuum environment, first activate it at a first temperature, then raise the temperature to a second temperature for a first-stage reduction reaction, and then raise the temperature to a third temperature to continue the second-stage reduction reaction, while removing the reaction byproducts by vacuuming. S4. After cooling down, remove the residue on the surface of the DBC substrate.

2. The surface treatment method of a DBC substrate for a power module according to claim 1, characterized by: The flux described in step S2 has an acid value of 80 mg KOH / g to 120 mg KOH / g.

3. The surface treatment method of a DBC substrate for a power module according to claim 1 or 2, characterized in that: The flux mentioned in step S2 includes rosin resin, organic acid activator and thixotropic agent, wherein the organic acid activator includes at least one of C4~C10 dicarboxylic acid and hydroxy aromatic acid.

4. The surface treatment method for a DBC substrate for a power module according to claim 1, characterized in that: In step S2, the amount of flux applied is 15 mg / cm² to 25 mg / cm² by wet weight.

5. The surface treatment method for a DBC substrate for a power module according to claim 4, characterized in that: The flux described in step S2 has a viscosity of 50 Pa·s to 200 Pa·s at 25°C.

6. The surface treatment method for a DBC substrate for a power module according to claim 1, characterized in that: In step S3, the temperature is increased to a first temperature at a heating rate of 0.2℃ / s to 3℃ / s, where the first temperature is 50℃ to 150℃, and the activation treatment time is 60s to 120s.

7. The surface treatment method for a DBC substrate for a power module according to claim 1, characterized in that: In step S3, the second temperature is 150~200℃, the time of the first stage reduction reaction is 60s~120s, and the vacuum degree of the vacuum environment of the first stage reduction reaction is not higher than 50mbar.

8. The surface treatment method for a DBC substrate for a power module according to claim 1, characterized in that: The third temperature mentioned in step S3 is 200~250℃, and the vacuum degree of the vacuum environment of the second stage reduction reaction is not higher than 30mba.

9. The surface treatment method for a DBC substrate for a power module according to claim 1, characterized in that: In step S3, during the first and second stage reduction reactions, a reducing gas is introduced into the vacuum environment, wherein the reducing gas is formic acid vapor or a hydrogen-containing reducing protective gas.

10. The surface treatment method for a DBC substrate for a power module according to claim 1, characterized in that: The cooling process described in step S4 is carried out under a protective atmosphere at a rate of 0.2℃ / s to 3℃ / s.

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