A vacuum reflow process for die bonding

Abstract

The application relates to the technical field of chip welding, and particularly discloses a vacuum reflow process for chip welding and mounting. The vacuum reflow process comprises the following steps: S1, a gas replacement stage; S2, a vacuum temperature rising stage 1: making the chip welding and mounting to be welded in a vacuum and rising to 100-120 DEG C which is higher than a formic acid activation temperature; S3, heat preservation and formic acid feeding: heat preservation for 6.5-9.5 min at the final temperature of S2, and feeding formic acid in the heat preservation process; S4, a vacuum temperature rising stage 2; S5, heat preservation and nitrogen feeding; and S6, cooling to room temperature. After the chip welding and mounting is welded by the process, the number of solder balls on the chip is remarkably reduced, and the welding quality is guaranteed.

Description

Technical Field

[0001] This application relates to the technical field of chip bonding, and more specifically, to a vacuum reflow process for chip bonding and mounting. Background Technology

[0002] In the process of soldering, solder is placed between the component to be soldered and the pad (solder is composed of two or more metals). The solder is heated and melted at the heated interface between the component and the pad. On the one hand, after the solder melts at high temperature, its alloying elements diffuse with the metal at the interface of the component to be soldered. On the other hand, the wetting force and surface tension of the solder after melting can also fuse the component to be soldered with the pad, thereby completing the connection process between the component and the pad. The stable connection between the component and the pad is achieved during the subsequent cooling process of the solder.

[0003] During this welding process, it is crucial to ensure that the surface of the active metal in the solder is not oxidized; otherwise, insufficient wettability of the solder may occur, affecting the stability of the weld. Currently, vacuum reflow soldering is commonly used. The vacuum reflow soldering process includes the following steps: gas replacement to remove as much air as possible from the reflow oven (or other vacuum reflow equipment); vacuuming and heating to reach the temperature required to melt the solder; maintaining a constant temperature to allow the solder to fully react with the components to be welded, alternating between nitrogen purging and vacuuming; and cooling. The advantages of vacuum reflow soldering are: the vacuum process and welding in an inert gas atmosphere (nitrogen) effectively reduce the possibility of solder oxidation; in addition, vacuum reflow soldering also results in a more uniform solder layer and eliminates the need for post-weld cleaning.

[0004] However, the current vacuum reflow soldering process still has the problem of producing a large number of solder balls on the chip surface after soldering, which affects the soldering stability; in addition, too many solder balls also make it difficult to clean them. Summary of the Invention

[0005] To reduce the number of solder balls on the chip surface after soldering, this application provides a vacuum reflow process for chip soldering and mounting.

[0006] The applicant discovered that during chip soldering using existing vacuum reflow processes, a significant number of solder balls often remain on the chip surface after soldering, resulting in a non-compliant soldering outcome. Further research revealed the following reasons for the presence of numerous solder balls on the chip surface: 1. Insufficient solder preheating prevents residual moisture and foreign organic components on the solder surface from reacting and releasing completely during melting. This gas release during the molten solder stage leads to solder splattering, forming solder balls on the chip surface; 2. Increasing the vacuum stage during solder melting causes the gas generated by insufficient preheating to rupture, resulting in solder splattering. Simultaneously, the sudden pressure drop under vacuum also contributes to solder splattering due to increased pressure on the chip.

[0007] Therefore, the vacuum reflow process for chip soldering and mounting provided in this application adopts the following technical solution: A vacuum reflow process for chip soldering and mounting includes the following steps:

[0008] S1, Gas Replacement Stage: The chip to be soldered is alternately placed in a vacuum and nitrogen atmosphere, wherein the chip to be soldered contains soft solder but does not contain flux.

[0009] S2, Vacuum Heating Stage 1: The chip to be soldered is placed in a vacuum and heated to 20-120°C above the formic acid activation temperature;

[0010] S3, Insulate and introduce formic acid: Insulate at the final temperature of S2 for 3-9.5 minutes, while introducing formic acid during the insulation process;

[0011] S4, Vacuum heating stage 2: After formic acid is introduced, the chip to be soldered is placed in a vacuum and heated to 20-50°C above the liquidus temperature line of the solder.

[0012] S5. Insulate and introduce nitrogen: Insulate at the final temperature of S4. After the insulation begins, introduce nitrogen first, and then continue to insulate without performing a vacuum operation.

[0013] S6. Cooling stage: Lower the temperature to room temperature.

[0014] At the start of step S5, the solder has gradually melted into a liquid state. In this state, vacuuming has a significant impact on the solder, as the vacuum environment exerts pressure on the chip, causing solder sputtering and resulting in numerous solder balls on the chip surface after soldering. Therefore, by adopting the above-mentioned technical solution, eliminating the need for vacuuming during this process will effectively reduce the number of solder balls on the chip.

[0015] Optionally, in step S2, the chip to be soldered is placed in a vacuum and heated to 100-120°C above the formic acid activation temperature.

[0016] This application first adds formic acid to the vacuum reflow process to reduce the possibility of solder oxidation by combining formic acid with a vacuum and nitrogen environment. With the addition of formic acid, the solution in this application is to increase the final temperature in step S2, adjusting the condition of "heating to 20-30°C above the formic acid activation temperature" in the traditional process to "heating to 100-120°C above the formic acid activation temperature." The applicant found that this improvement directly reduces the number of solder balls on the chip surface after soldering. This application selected this temperature not only considering the reaction of formic acid at high temperatures but also further considering the changes in the solder at high temperatures. Since the vacuum reflow process inevitably involves the melting of solder, this application considers the evaporation of water vapor and the generation of gases from external organic impurities in the solder at high temperatures during the formic acid activation process. Therefore, the temperature is increased from the first heating stage to ensure sufficient change and reaction between the formic acid and the solder. This, combined with the subsequent gas removal process, effectively removes impurity gases, further preventing these gases from remaining in the molten solder after melting, thus avoiding solder splattering caused by gas bursts and the formation of solder balls on the chip surface. It should also be noted that this endpoint temperature is much higher than the formic acid activation temperature, but lower than the solder liquidus temperature; therefore, the solder remains solid at this temperature.

[0017] Optionally, in step S2, the temperature is maintained at the final temperature of S2 for 6.5-9.5 minutes.

[0018] By adopting the above technical solution, in step S3, the applicant further extended the heat preservation time. This operation will also directly affect the number of solder balls on the chip after soldering. The applicant found that if the heat preservation time is short, the number of solder balls on the chip after soldering will increase significantly; while a longer heat preservation time will directly reduce the number of solder balls on the chip after soldering. The possible reason is that: water vapor evaporates from the soft solder at high temperature, and impurities from external organic impurities bring impurity gases. If the heat preservation time is insufficient, these impurity gases cannot be completely released. Therefore, in step S5, such gases exist in the molten soft solder. After heating, the gas breaks down, causing solder to splash, thereby causing the formation of solder balls.

[0019] Optionally, when formic acid is introduced in step S3, the formic acid is introduced in multiple batches, with vacuuming and nitrogen purging operations alternately performed between adjacent formic acid introduction steps.

[0020] By adopting the above technical solution, extending the heat preservation time and repeatedly performing gas replacement in step S3 will effectively remove this part of the impurity gas, thereby reducing the possibility of solder splashing caused by bubble rupture after the soft solder melts, resulting in a large number of solder balls forming on the chip surface.

[0021] Formic acid is introduced in batches, and after each introduction, the impurity gas introduced by the formic acid is promptly removed. This effectively reduces the amount of impurity gas present in the molten solder in step S5, thereby reducing the possibility of a large number of solder balls forming on the chip surface due to solder splashing caused by bubble rupture.

[0022] Optionally, in step S3, the vacuuming operation and nitrogen purging operation are alternately performed at least twice between two adjacent steps of introducing formic acid.

[0023] By adopting the above technical solution, impurity gases can be fully replaced.

[0024] Optionally, in step S3, the first continuous formic acid infusion time is 2-4 min, and the second continuous formic acid infusion time is 1-2 min; the formic acid infusion rate is 18-22 L / min.

[0025] Optionally, the formic acid activation temperature is 148-153℃, and the liquidus temperature line of the solder is 280-300℃.

[0026] Optionally, the solder is Pb-Sn. 10 -Ag2 or Pb-In5-Ag 2.5 .

[0027] Optionally, the nitrogen gas is introduced at a rate of 22-28 L / min during the vacuum reflux process.

[0028] Optionally, in step S6, constant-rate cooling or gradient cooling can be used to achieve cooling.

[0029] Optionally, for constant-rate cooling, the cooling rate is 1-2℃ / s; gradient cooling includes the steps of pre-cooling at a rate of 0.3-0.8℃ / s and then cooling to room temperature at a rate of 0.8-1.5℃ / s.

[0030] In summary, this application has the following beneficial effects:

[0031] 1. This application first increases the final temperature of the vacuum heating stage in step S2 and extends the holding time in step S3 to reduce the impurity gas introduced by formic acid, the water vapor brought by the soft solder, and the impurity gas generated after the reaction of organic impurities. Ultimately, it effectively reduces the amount of gas present in the molten soft solder, thereby reducing the possibility of solder splashing caused by the rupture of gas in the molten soft solder, which could lead to a large number of solder balls on the chip surface.

[0032] 2. In step S5 of this application, no vacuuming operation is performed to avoid the problem of solder splashing caused by the pressure of the vacuum environment on the chip, thereby effectively reducing the number of solder balls on the chip. Detailed Implementation

[0033] The present application will be further described in detail below with reference to the embodiments.

[0034] Example

[0035] A vacuum reflow process for chip soldering and mounting includes the following steps:

[0036] S1. Introducing nitrogen: Introducing nitrogen ensures that the chip to be soldered is in a nitrogen atmosphere. The chip to be soldered contains soft solder. The nitrogen introducing rate is 22-28 L / min.

[0037] The further steps can be as follows: first, evacuate for 1-1.8 minutes, then introduce nitrogen gas for 0.4-1 minutes; then evacuate for 1-2 minutes again, and then introduce nitrogen gas for 0.4-1 minutes again.

[0038] S2, Vacuum heating stage 1: Heat to 20-120℃ above the activation temperature of formic acid, with a heating rate of 18-22℃ / s. While heating, the chip to be soldered is placed in a vacuum state.

[0039] S3. Maintain the temperature and introduce formic acid and nitrogen: Maintain the temperature at the final temperature of S2. While maintaining the temperature, perform the following operations: After starting the temperature maintenance, first introduce formic acid at a rate of 18-22 L / min for 2-4 min, then evacuate for 0.5-1.5 min, then introduce nitrogen for 0.5-1.5 min, then evacuate for 0.5-1.5 min, and then introduce formic acid again at a rate of 18-22 L / min for 1-2 min.

[0040] S4. Heating stage 2: Under vacuum conditions, the temperature is raised to 20-50°C above the liquidus temperature of the solder, with a heating rate of 0.5-0.9°C / s.

[0041] S5. Maintain temperature and introduce nitrogen: Maintain temperature at the final temperature of S4, and perform the following operations while maintaining temperature: first introduce nitrogen at a rate of 22-28 L / min for 0.5-1.5 min, and then maintain temperature for 1.5-3 min.

[0042] S6. Cooling Stage: During gradient cooling, the temperature can be reduced to the holding temperature in step S3 at a rate of 0.3-0.8℃ / s, while nitrogen gas is continuously introduced at a rate of 22-28L / min for 2-3min; then the temperature can be reduced to room temperature at a rate of 0.8-1.5℃ / s. Alternatively, this cooling stage can be performed by directly cooling to room temperature at a rate of 1-2℃ / s.

[0043] Example 1

[0044] A vacuum reflow process for chip soldering and mounting, comprising the following steps:

[0045] S1, Gas Replacement Stage: The chips to be soldered are mounted (the mounting contains Pb-Sn solder). 10 The (Ag2 and flux-free) was placed in a vacuum reflow oven, and then the vacuuming and nitrogen purging operations were performed alternately. Specifically: first, vacuuming for 1 minute, followed by nitrogen purging for 0.4 minutes; then vacuuming for another 1 minute, followed by nitrogen purging for 0.4 minutes; nitrogen was purged at a rate of 22 L / min, and the vacuum was reduced to 5 Pa.

[0046] S2, Vacuum Heating Stage 1: Evacuate to 5 Pa, and simultaneously heat to 170°C at a rate of 18°C / s (the activation temperature of formic acid is approximately 150°C, and this temperature is 20°C higher than the activation temperature of formic acid).

[0047] S3. Heat preservation and formic acid introduction: Heat the temperature at 170℃ for 4.5 min. During this heat preservation process, formic acid is introduced at a rate of 18 L / min for 2 min. Then, the vacuum is evacuated to 5 Pa and held for 0.5 min. Nitrogen gas is introduced at a rate of 22 L / min for 0.5 min. The vacuum is then evacuated to 5 Pa and held for 0.5 min. Finally, formic acid is introduced again at a rate of 18 L / min for 1 min.

[0048] S4, Vacuum Heating Stage 2: Evacuate to 5Pa to place the chip to be soldered under vacuum. While maintaining this vacuum state, heat up to 320℃ (the liquidus temperature line of the solder is 280℃, and this temperature is 40℃ higher than the liquidus temperature line of the solder).

[0049] S5. Maintain temperature and introduce nitrogen: At the final temperature of S4, introduce nitrogen at a rate of 22 L / min for 0.5 min, and then maintain temperature for 1.5 min.

[0050] S6. Cooling stage: Cool directly to room temperature at a rate of 1.5℃ / s.

[0051] Example 2

[0052] A vacuum reflow process for chip soldering and mounting, comprising the following steps:

[0053] S1, Gas Replacement Stage: The chips to be soldered are mounted (the mounting contains Pb-Sn solder). 10 The (-Ag2 and without flux) was placed in a vacuum reflow oven, and then the vacuuming and nitrogen purging operations were performed alternately. Specifically: first, vacuuming was performed for 1.6 min, followed by nitrogen purging for 0.6 min; then vacuuming was performed for 1.6 min, followed by nitrogen purging for 0.6 min; nitrogen was purged at a rate of 25 L / min, and the vacuum was reduced to 5 Pa.

[0054] S2, Vacuum Heating Stage 1: Evacuate to 5 Pa, and simultaneously heat to 260°C at a rate of 20°C / s (the activation temperature of formic acid is approximately 150°C, which is 110°C higher than the activation temperature of formic acid).

[0055] S3. Incubate at 260℃ and introduce formic acid: Incubate at 260℃ for 7.5 min. During this incubation, introduce formic acid at a rate of 20 L / min for 3 min, then evacuate to 5 Pa and hold for 1 min, then introduce nitrogen at a rate of 25 L / min for 1 min, then evacuate to 5 Pa and hold for 1 min, and then introduce formic acid again at a rate of 20 L / min for 1.5 min.

[0056] S4, Vacuum Heating Stage 2: Evacuate to 5Pa to place the chip to be soldered under vacuum. While maintaining this vacuum state, heat up to 320℃ (the liquidus temperature line of the solder is 280℃, and this temperature is 40℃ higher than the liquidus temperature line of the solder).

[0057] S5. Maintain temperature and introduce nitrogen: At the final temperature of S4, introduce nitrogen at a rate of 25 L / min for 1 min, and then maintain temperature for another 2 min.

[0058] S6. Cooling stage: Use a gradient cooling method: first, reduce the temperature from 320℃ to 260℃ within 2 minutes, while continuously introducing nitrogen gas at a rate of 25L / min for 2 minutes during this cooling process; then, reduce the temperature from 260℃ to room temperature within 4 minutes.

[0059] Example 3

[0060] A vacuum reflow process for chip soldering and mounting, comprising the following steps:

[0061] S1, Gas Replacement Stage: The chips to be soldered are mounted (the mounting contains Pb-Sn solder). 10 The (Ag2 and flux-free) was placed in a vacuum reflow oven, and then the vacuuming and nitrogen purging operations were performed alternately. Specifically: first, vacuuming for 1.8 min, then nitrogen purging for 1 min; then vacuuming for 2 min, then nitrogen purging for 1 min; during which nitrogen was purged at a rate of 28 L / min, and the vacuum was reduced to 5 Pa.

[0062] S2, Vacuum Heating Stage 1: Evacuate to 5 Pa, and simultaneously heat to 270°C at a rate of 22°C / s (the activation temperature of formic acid is approximately 150°C, which is 120°C higher than the activation temperature of formic acid).

[0063] S3. Heat preservation and formic acid introduction: Heat the temperature at 270℃ for 10.5 min. During this heat preservation process, formic acid is introduced at a rate of 22 L / min for 4 min. Then, the vacuum is evacuated to 5 Pa and held for 1.5 min. Nitrogen gas is introduced at a rate of 28 L / min for 1.5 min. The vacuum is then evacuated to 5 Pa and held for 1.5 min. Finally, formic acid is introduced again at a rate of 22 L / min for 2 min.

[0064] S4, Vacuum Heating Stage 2: Evacuate to 5Pa to place the chip to be soldered under vacuum. While maintaining this vacuum state, heat up to 320℃ (the liquidus temperature line of the solder is about 280℃, and this temperature is 40℃ higher than the liquidus temperature line of the solder).

[0065] S5. Maintain temperature and introduce nitrogen: At the final temperature of S4, introduce nitrogen at a rate of 28 L / min for 1.5 min, and then maintain temperature for 3 min.

[0066] S6. Cooling stage: Use a gradient cooling method: first, reduce the temperature from 320℃ to 270℃ within 2.5 minutes, while continuously introducing nitrogen gas at a rate of 25L / min for 3 minutes during this cooling process; then, reduce the temperature from 270℃ to room temperature within 5.5 minutes.

[0067] Example 4

[0068] The difference between this embodiment and embodiment 2 is that a vacuum reflow process for chip soldering and mounting is provided. The step S2 is different, mainly because the temperature endpoint during heating is different. Other steps are the same as in embodiment 2.

[0069] Step S2 specifically involves: Vacuum heating stage 1: Evacuate to 5 Pa, and simultaneously heat to 170°C at a rate of 20°C / s (the activation temperature of formic acid is approximately 150°C, which is 20°C higher than the activation temperature of formic acid).

[0070] Example 5

[0071] The difference between this embodiment and embodiment 2 is that a vacuum reflow process for chip soldering and mounting is provided. The step S2 is different, mainly because the temperature endpoint during heating is different. Other steps are the same as in embodiment 2.

[0072] Step S2 specifically involves: Vacuum heating stage 1: Evacuate to 5 Pa, and simultaneously heat to 200 °C at a rate of 20 °C / s (the activation temperature of formic acid is approximately 150 °C, which is 50 °C higher than the activation temperature of formic acid).

[0073] Example 6

[0074] The difference between this embodiment and embodiment 2 is that a vacuum reflow process for chip soldering and mounting is provided, and step S3 is different, mainly in the holding time. Other steps are the same as in embodiment 2.

[0075] Step S3 specifically involves: maintaining the temperature and introducing formic acid: maintaining the temperature at 260℃ for 3 minutes, and introducing formic acid at a rate of 20L / min for 3 minutes during this maintenance process.

[0076] Example 7

[0077] The difference between this embodiment and embodiment 2 is that a vacuum reflow process for chip soldering and mounting is provided. Steps S2 and S3 are different, mainly because the temperature endpoint during heating in step S2 is different and the holding time in step S3 is different. Other steps are the same as in embodiment 4.

[0078] Step S2 specifically involves: Vacuum heating stage 1: Evacuate to 5 Pa, and simultaneously heat to 200 °C at a rate of 20 °C / s (the activation temperature of formic acid is approximately 150 °C, which is 50 °C higher than the activation temperature of formic acid).

[0079] Step S3 specifically involves: maintaining the temperature and introducing formic acid: maintaining the temperature at 200℃ for 3 minutes, and during this maintenance process, introducing formic acid at a rate of 20L / min for 3 minutes.

[0080] Example 8

[0081] The difference between this embodiment and embodiment 2 is that a vacuum reflow process for chip soldering and mounting is provided, wherein step S3 is different, and formic acid is introduced all at once, while the rest is the same as in embodiment 2.

[0082] Step S3 is as follows: heat preservation and formic acid introduction: heat preservation at 260℃ for 7.5 min. During this heat preservation process, formic acid is introduced at a rate of 20 L / min for 4.5 min, then vacuum is drawn to 5 Pa and held for 1 min, then nitrogen is introduced at a rate of 25 L / min for 1 min, then vacuum is drawn to 5 Pa and held for 1 min.

[0083] Comparative Example

[0084] The difference between this comparative example and Example 2 is that a vacuum reflow process for chip soldering and mounting is provided. Step S5 is different. When nitrogen is introduced during heat preservation, a vacuuming step is also included. The rest is the same as in Example 2.

[0085] Step S5 is as follows: heat preservation and nitrogen gas introduction: at the final temperature of S4, nitrogen gas is introduced at a rate of 25 L / min for 1 min, then the vacuum is evacuated to 5 Pa and held for 1 min, and finally the temperature is maintained for another 1 min.

[0086] Evaluation of different processes

[0087] The quality of the soldered chips was evaluated in accordance with the visual inspection requirements of GJB 548B Method 2010 and the chip shear strength of GJB 548B Method 2019.

[0088] Visual inspection requirements in Method 2010:

[0089] 1. Any chip mounting material that has accumulated and extended to or vertically extended to the top surface of the chip shall not be accepted.

[0090] 2. If the mounting material between the chip and the socket is not visible on at least two complete edges of the chip or on more than 75% of the area around the chip, the chip should be rejected.

[0091] 3. If the chip mounting material is spherical or clumped together, and the visible perimeter solder outline is less than 50% when viewed from above, or if the stacking height of the chip mounting material is greater than the longest dimension at the bottom, or if there is necking of the stacking material in any location, the chip should be rejected.

[0092] In response to the above visual inspection requirement 1, the number of solder balls on each chip (average value) was observed using a 200x microscope in this application. The chip size was 4.0×3.2mm. The specific results are shown in Table 1.

[0093] Table 1. Number of solder balls on the chip surface after welding chips using different processes.

[0094]

[0095] As can be seen from the data in Table 1, the method of this application can produce chips with a significantly reduced number of solder balls.

[0096] For Examples 2, 5, 6, 7 and the Comparative Example, additional parameters for evaluating chip soldering performance are provided, as detailed in Table 2:

[0097] Table 2 Shear strength and other visual inspection results of chips from different implementation schemes.

[0098]

[0099] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A vacuum reflow process for chip soldering and mounting, characterized in that, Includes the following steps: S1, Gas Replacement Stage: The chip to be soldered is alternately placed in a vacuum and nitrogen atmosphere, wherein the chip to be soldered contains soft solder but does not contain flux. S2, Vacuum Heating Stage 1: The chip to be soldered is placed in a vacuum and heated to 20-120°C above the formic acid activation temperature; S3, Insulate and introduce formic acid: Insulate at the final temperature of S2 for 3-9.5 minutes, while introducing formic acid during the insulation process; S4, Vacuum heating stage 2: After formic acid is introduced, the chip to be soldered is placed in a vacuum and heated to 20-50°C above the liquidus temperature line of the solder. S5. Insulate and introduce nitrogen: Insulate at the final temperature of S4. After the insulation begins, introduce nitrogen first, and then continue to insulate without performing a vacuum operation. S6. Cooling stage: Lower the temperature to room temperature.

2. The vacuum reflux process according to claim 1, characterized in that, In step S2, the chip to be soldered is placed in a vacuum and heated to 100-120°C, which is higher than the activation temperature of formic acid.

3. The vacuum reflux process according to claim 1, characterized in that, In step S2, the temperature is maintained at the final temperature of S2 for 6.5-9.5 minutes.

4. The vacuum reflux process according to claim 3, characterized in that, In step S3, the vacuuming operation and nitrogen purging operation are alternated at least twice between two adjacent steps of introducing formic acid.

5. The vacuum reflux process according to claim 3 or 4, characterized in that, In step S3, the first continuous formic acid infusion time is 2-4 min, and the second continuous formic acid infusion time is 1-2 min; the formic acid infusion rate is 18-22 L / min.

6. The vacuum reflux process according to claim 1, characterized in that, The activation temperature of the formic acid is 148-153℃, and the liquidus temperature line of the solder is 280-300℃.

7. The vacuum reflux process according to claim 1, characterized in that, The solder is Pb-Sn10-Ag2 or Pb-In5-Ag2.

5.

8. The vacuum reflux process according to claim 1, characterized in that, The nitrogen gas is introduced at a rate of 22-28 L / min in the vacuum reflux process.

9. The vacuum reflux process according to claim 1, characterized in that, In step S6, cooling can be achieved by constant-rate cooling or gradient cooling.

10. The vacuum reflux process according to claim 9, characterized in that, During constant-rate cooling, the cooling rate is 1-2℃ / s; gradient cooling includes pre-cooling at a rate of 0.3-0.8℃ / s and then cooling to room temperature at a rate of 0.8-1.5℃ / s.

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