Sintering method

Abstract

The invention belongs to the technical field of power module packaging, and particularly provides a sintering method. The invention aims to solve the problems of poor connection strength and low reliability after the existing power module and the heat dissipation plate are sintered. Therefore, the sintering method provided by the invention comprises the following steps: S1, printing silver paste on the back surface of the power module, and forming a first printing layer; s2, silver paste is printed on the surface of the heat dissipation plate, and a second printing layer is formed; s3, the power module is placed on the heat dissipation plate, the power module and the heat dissipation plate are fixed through a clamp, and the first printing layer makes contact with the second printing layer; and S4, sintering the fixed power module and the heat dissipation plate. The power module and the heat dissipation plate sintered through the sintering method are high in connection strength and high in reliability.

Description

Technical Field

[0001] The invention belongs to the technical field of power module packaging, and specifically provides a sintering method. Background Art

[0002] In the packaging process of power modules, the use of large-area silver sintering is a new process that all mainstream manufacturers are trying. The sintering process is still under exploration, and is limited to sintering on metal surfaces such as gold / silver / copper. However, since the diffusion and bonding ability between the same metals is stronger, most silver sintering is done on the silver surface.

[0003] The existing sintering method generally involves applying silver paste on one of the two workpieces to be sintered, and then attaching and fixing the two workpieces and sintering them. However, the product obtained by this sintering method has low connection strength and poor reliability.

[0004] Accordingly, this field requires a new technical solution to solve the above technical problems. Summary of the invention

[0005] The present invention aims to solve the above technical problem, that is, to solve the problem of low connection strength and poor reliability between the existing power module and the heat sink after sintering.

[0006] The present invention provides a sintering method, which is used for sintering and bonding a power module to a heat sink, and the sintering method comprises the following steps: S1: printing silver paste on the back of the power module to form a first printing layer; S2: printing silver paste on the surface of the heat sink to form a second printing layer; S3: placing the power module on the heat sink and fixing the two with a clamp, wherein the first printing layer is in contact with the second printing layer; S4: sintering the fixed power module and the heat sink.

[0007] In the preferred technical solution of the above sintering method, in step S1, the thickness of the first printed layer is 5 μm to 50 μm.

[0008] In a preferred technical solution of the above sintering method, the sum of the thickness of the second printing layer and the thickness of the first printing layer is 200 μm to 800 μm.

[0009] In a preferred technical solution of the above sintering method, in step S1, the thickness of the first printing layer is 10 μm to 20 μm; and / or the sum of the thickness of the second printing layer and the thickness of the first printing layer is 300 μm to 600 μm.

[0010] In a preferred technical solution of the above sintering method, the ratio of the thickness of the second printing layer to the thickness of the first printing layer is (15-39):1.

[0011] In the preferred technical solution of the above sintering method, in step S3, before assembling the power module and the heat sink, the second printed layer is dried first, and the first printed layer is not dried.

[0012] In the preferred technical solution of the above sintering method, in step S4, the sintering temperature during the sintering process is 200° C. to 250° C., the sintering time is 5 min to 30 min, and the pressure is 8 MPa to 20 MPa.

[0013] In the preferred technical solution of the above sintering method, before printing the silver paste on the power module, the back side of the power module is cleaned.

[0014] In the preferred technical solution of the above sintering method, the cleaning treatment includes using any one of alcohol cleaning, reflow furnace treatment or ion bombardment, or a combination of any two of the methods.

[0015] In the preferred technical solution of the above sintering method, the back side of the power module is silver-plated, and the thickness of the silver plating layer is 0.4μm to 0.6μm; and / or the surface of the heat sink is silver-plated, and the thickness of the silver plating layer is 3μm to 5μm.

[0016] In the case of adopting the above technical scheme, the sintering method of the present invention prints silver paste on the back of the power module to form a first printed layer, and prints silver paste on the surface of the heat sink to form a second printed layer. During sintering, the first printed layer is brought into contact with the second printed layer for sintering. During the sintering process, the silver pastes are sintered and connected to each other, which can strengthen the connection strength between the silver particles, thereby improving the connection strength of the product and achieving high sintering reliability.

[0017] Furthermore, the thickness of the first printed layer is controlled to be between 5 μm and 50 μm. After the first printed layer of this thickness is printed on the back of the power module, firstly, the wetting agent and activator in the silver paste can process the back state of the power module, thereby improving the wettability of the surface of the power module, so that the silver particles in the silver paste and the silver plating of the power module can be better contacted and sintered, thereby improving the sintering performance and the connection strength of the product; secondly, the thickness of the first printed layer is relatively thin, and during the heating process, the excess solvent in the silver paste can be quickly evaporated, and there is no need to dry the power module, thereby reducing the drying process and reducing the cost.

[0018] Furthermore, the sum of the thickness of the second printing layer and the thickness of the first printing layer is controlled to be between 200 μm and 800 μm, which can ensure that the sintered product has sufficient connection strength while maintaining a relatively low cost.

[0019] Furthermore, the thickness of the first printed layer is controlled to be 10 μm to 20 μm. After being printed on the back of the power module, the first printed layer of this thickness does not affect the flatness of the back of the power module, and can further improve the connection strength between the sintered power module and the heat sink.

[0020] Furthermore, the sum of the thickness of the second printing layer and the thickness of the first printing layer is controlled to be 300 μm to 600 μm, which has high connection strength, short sintering time and low cost.

[0021] Furthermore, the ratio of the thickness of the second printed layer to the thickness of the first printed layer is controlled to be (15-39):1, and the connection strength between the sintered power module and the heat sink is high and the reliability is high.

[0022] Furthermore, before assembling the power module and the heat sink, the second printed layer is first dried to remove the solvent in the second printed layer, which can avoid the formation of excessive pores and cracks due to solvent evaporation during the sintering process, thereby ensuring that the power module and the heat sink maintain a high connection strength after sintering; in addition, since the first printed layer is thin, the solvent inside it is easy to evaporate. Not drying the first printed layer can ensure that the wetting agent and activator in the first printed layer first process the back state of the power module, and the excess solvent will evaporate by itself in a short time, saving a drying process, reducing costs, and improving the reliability of sintering.

[0023] Furthermore, before printing the silver paste on the power module, the back side of the power module is cleaned first, which can improve the surface cleanliness of the back side of the power module, thereby improving the connection strength after sintering.

[0024] Furthermore, the back of the power module is chemically silver-plated. During the sintering process, the silver-plated silver and the silver particles in the silver paste can be better diffused and sintered, thereby improving the connection strength between the sintered power module and the heat sink.

[0025] Furthermore, the surface of the heat sink is electroplated with silver. During the sintering process, the silver plating and the silver particles in the silver paste can be sintered better, thereby improving the connection strength between the power module and the heat sink after sintering. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings, in which:

[0027] Figure 1 is a flow chart of the sintering method of the present invention;

[0028] Figure 2 This is a c-sam scan of the sintering area of ​​the product of Example 1 of the present invention before the TST high and low temperature impact test;

[0029] Figure 3 This is a c-sam scan of the sintering area of ​​the product of Example 2 of the present invention before the TST high and low temperature impact test;

[0030] Figure 4 This is a c-sam scan of the sintering area of ​​the product of Example 3 of the present invention before the TST high and low temperature impact test;

[0031] Figure 5 This is a c-sam scan of the sintering area of ​​the product of Comparative Example 1 of the present invention before the TST high and low temperature impact test;

[0032] Figure 6 This is a c-sam scan of the sintering area of ​​the product of Comparative Example 2 of the present invention before the TST high and low temperature impact test;

[0033] Figure 7 It is a c-sam scan of the sintering area of ​​the product of comparative example 3 of the present invention before the TST high and low temperature impact test. DETAILED DESCRIPTION

[0034] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood by those skilled in the art that these embodiments are only used to explain the technical principles of the present invention and are not intended to limit the protection scope of the present invention.

[0035] The invention provides a sintering method for sintering and bonding a power module and a heat sink. The power module and the heat sink are sintered by the sintering method, and the obtained product has high connection strength and high sintering reliability.

[0036] Specifically, see Figure 1 The sintering method of the present invention comprises the following steps:

[0037] S1: Printing silver paste on the back side of the power module to form a first printed layer.

[0038] S2: Printing the silver paste on the surface of the heat sink to form a second printed layer.

[0039] S3: placing the power module on the heat sink and fixing the two using a clamp, wherein the first printed layer is in contact with the second printed layer.

[0040] S4: Sintering the fixed power module and the heat sink.

[0041] The sintering method of the present invention prints silver paste on the back of the power module to form a first printed layer, prints silver paste on the surface of the heat sink to form a second printed layer, and makes the first printed layer contact with the second printed layer during sintering, so that during the sintering process, the silver pastes are sintered and connected, and the connection strength of the sintered product is high. The sintering method of the present invention can improve the sintering quality, improve the reliability of sintering, and improve the connection strength of the sintered product.

[0042] It should be noted that the present invention does not impose any restrictions on the type of power module. In practical applications, those skilled in the art can set the type of power module according to actual needs. For example, the power module is a half-bridge module, or the power module is a full-bridge module, etc. Such adjustments and changes to the type of power module do not deviate from the basic principles of the present invention and should be limited within the scope of protection of the present invention.

[0043] Preferably, the back of the power module is chemically silver plated, and the thickness of the silver plated layer is 0.4 μm to 0.6 μm. The back of the power module is chemically silver plated, and the silver particles in the silver paste are better diffused during sintering, thereby improving the connection strength between the sintered power module and the heat sink.

[0044] Preferably, the surface of the heat sink is silver-plated, and the thickness of the silver-plated layer is 3 μm to 5 μm. The surface of the heat sink is silver-plated, and the silver-plated silver and the silver particles in the silver paste are better sintered during sintering, thereby improving the connection strength between the power module and the heat sink after sintering.

[0045] In a preferred embodiment, in step S1, the thickness of the first printing layer is 5 μm to 50 μm.

[0046] Silver paste is printed on the back of the power module to form a first printed layer. The wetting agent and activator in the silver paste can process the back state of the power module, thereby improving the wettability between the silver paste and the power module, so that the silver particles of the silver paste and the silver plating of the power module can better contact and sinter, thereby improving the sintering performance and the connection strength of the product; in addition, the thickness of the first printed layer is controlled at 5μm to 50μm, and there is no need to dry the power module, and the solvent inside it can also evaporate quickly, reducing the drying process and effectively reducing costs.

[0047] In a further preferred embodiment, in step S1, the thickness of the first printed layer is 10 μm to 20 μm. The thickness of the first printed layer is controlled to be 10 μm to 20 μm, and after the silver paste is printed on the back of the power module, the flatness of the back of the power module is not affected, thereby further improving the connection strength between the sintered power module and the heat sink.

[0048] In another embodiment, in step S1 , the thickness of the first printing layer is 60 μm to 100 μm.

[0049] It should be noted that the present invention does not impose any restrictions on the thickness of the first printed layer. As long as the silver paste is printed on the surface of the power module to form the first printed layer, in practical applications, those skilled in the art can set the thickness of the first printed layer according to actual needs. The specific implementation form of the thickness of the first printed layer in the above-mentioned embodiment shall not limit the protection scope of the present invention.

[0050] Of course, it is preferred to control the thickness of the first printed layer to 10 μm to 20 μm, which maintains the flatness of the back of the power module without the need for drying, and can improve the connection strength between the sintered power module and the heat sink, with high sintering reliability.

[0051] In a preferred embodiment, the sum of the thickness of the second printing layer and the thickness of the first printing layer is 200 μm to 800 μm.

[0052] The sum of the thickness of the second printing layer and the thickness of the first printing layer is controlled to be between 200 μm and 800 μm, which can ensure sufficient connection strength and maintain relatively low cost.

[0053] In a further preferred embodiment, the sum of the thickness of the second printed layer and the thickness of the first printed layer is 300 μm to 600 μm. The sum of the thickness of the second printed layer and the thickness of the first printed layer is controlled to be 300 μm to 600 μm, which has high connection strength, meets product requirements, and has a short sintering time and low cost.

[0054] In a preferred embodiment, the ratio of the thickness of the second printed layer to the thickness of the first printed layer is (15-39): 1. By controlling the ratio of the thickness of the second printed layer to the thickness of the first printed layer to be (15-39): 1, the connection strength between the sintered power module and the heat sink is high and the reliability is high.

[0055] In a preferred embodiment, in step S3, before assembling the power module and the heat sink, the second printed layer is dried first, and the first printed layer is not dried.

[0056] The second printed layer is dried to remove the solvent in the second printed layer, so as to avoid the formation of excessive pores and cracks due to the evaporation of the solvent during the sintering process, thereby ensuring the connection strength between the power module and the heat sink after sintering; since the first printed layer is thin, the solvent inside it is easy to volatilize. Not drying the first printed layer can ensure that the wetting agent and activator in the first printed layer can first process the back state of the power module, and the excess solvent will also evaporate by itself in a short time, saving a drying process, reducing costs, and ensuring the reliability of sintering.

[0057] In a preferred embodiment, in step S4, the sintering temperature during the sintering process is 200°C to 250°C, the sintering time is 5min to 30min, and the pressure is 8mpa to 20mpa. The sintering method of the present invention has a relatively low sintering temperature, does not require high-temperature sintering, and increases the pressure to 8mp to 20mpa to assist in sintering, which can reduce the sintering time, control the sintering time to 5min to 30min, shorten the sintering time, and improve the sintering efficiency.

[0058] Preferably, before printing the silver paste on the power module, the back side of the power module is cleaned.

[0059] Since the power module undergoes multiple packaging processes before sintering, the state of its back is easily affected, and the surface tension and surface cleanliness will change, which can easily affect the sintering performance. Therefore, before printing the silver paste, the back of the power module is cleaned first to improve the surface cleanliness of the back of the power module to ensure the subsequent sintering effect.

[0060] Further preferably, the cleaning process includes using any one of alcohol cleaning, reflow oven treatment or ion bombardment, or a combination of any two of the methods.

[0061] Among them, alcohol cleaning specifically refers to using alcohol solution to clean the back of the power module. Reflow oven treatment specifically refers to placing the power module in a reflow oven and introducing reducing gas to perform reducing treatment on the power module. Ion bombardment specifically refers to using Ar, N 2 or H 2 The back side of the power module is bombarded.

[0001] It should be noted that those skilled in the art can flexibly adjust the execution order of the above steps in practical applications. For example, step S1 and step S2 can be executed simultaneously, or step S2 can be executed first and then step S1, etc. Such flexible adjustments and changes do not deviate from the basic principles of the present invention and should be limited within the scope of protection of the present invention.

[0062] Example 1

[0063] In this embodiment, the power module and the heat sink are sintered by the following steps:

[0064] S1: Printing silver paste on the back of the power module to form a first printed layer; wherein the thickness of the first printed layer is 10 μm, and before printing the silver paste, the back of the power module is treated by alcohol cleaning and reflow oven treatment.

[0065] S2: printing the silver paste on the surface of the heat sink to form a second printed layer, and drying the second printed layer; wherein the thickness of the second printed layer is 290 μm.

[0066] S3: placing the power module on the heat sink so that the first printed layer contacts the second printed layer, and fixing the power module to the heat sink using a clamp.

[0067] S4: Sintering the fixed power module and the heat sink, wherein during the sintering process, the sintering temperature is 200° C. to 250° C., the pressure is 8 MPa, and the sintering time is 5 minutes.

[0068] In this embodiment, the area where the power module contacts the heat sink for sintering is 6000mm 2 The sintered product is subjected to ultrasonic scanning (c-sam scanning) to obtain the following Figure 2 The actual sintering area of ​​the scanned image is 6000mm 2 , and the calculated sintering area percentage is 100%.

[0069] Example 2

[0070] In this embodiment, the power module and the heat sink are sintered by the following steps:

[0071] S1: Printing silver paste on the back of the power module to form a first printed layer; wherein the thickness of the first printed layer is 15 μm, and before printing the silver paste, the back of the power module is treated by alcohol cleaning and reflow oven treatment.

[0072] S2: printing the silver paste on the surface of the heat sink to form a second printed layer, and drying the second printed layer; wherein the thickness of the second printed layer is 585 μm.

[0073] S3: placing the power module on the heat sink so that the first printed layer contacts the second printed layer, and fixing the power module to the heat sink using a clamp.

[0074] S4: Sintering the fixed power module and the heat sink, wherein during the sintering process, the sintering temperature is 200° C. to 250° C., the pressure is 20 MPa, and the sintering time is 10 min.

[0075] In this embodiment, the area where the power module contacts the heat sink for sintering is 6000mm 2 The sintered product is subjected to ultrasonic scanning (c-sam scanning) to obtain the following Figure 3 The actual sintering area of ​​the scanned image is 6000mm 2 , and the calculated sintering area percentage is 100%.

[0076] Example 3

[0077] In this embodiment, the power module and the heat sink are sintered by the following steps:

[0078] S1: printing silver paste on the back of the power module to form a first printed layer; wherein the thickness of the first printed layer is 20 μm, and before printing the silver paste, the back of the power module is treated by alcohol cleaning and reflow furnace treatment.

[0079] S2: printing the silver paste on the surface of the heat sink to form a second printed layer, and drying the second printed layer; wherein the thickness of the second printed layer is 300 μm.

[0080] S3: placing the power module on the heat sink so that the first printed layer contacts the second printed layer, and fixing the power module to the heat sink using a clamp.

[0081] S4: Sintering the fixed power module and the heat sink, wherein during the sintering process, the sintering temperature is 200° C. to 250° C., the pressure is 12 MPa, and the sintering time is 30 minutes.

[0082] In this embodiment, the area where the power module contacts the heat sink for sintering is 6000mm 2 The sintered product is subjected to ultrasonic scanning (c-sam scanning) to obtain the following Figure 4 The actual sintering area of ​​the scanned image is 6000mm 2 , and the calculated sintering area percentage is 100%.

[0083] Comparative Example 1

[0084] In this comparative example, the power module and the heat sink are sintered by the following steps:

[0085] S1: Printing silver paste on the back of the power module to form a first printed layer, and drying the first printed layer; wherein the thickness of the first printed layer is 300 μm, and before printing the silver paste, the back of the power module is treated by alcohol cleaning and reflow oven treatment.

[0086] S2: Place the power module on the heat sink so that the first printed layer contacts the surface of the heat sink, and fix the power module to the heat sink using a clamp.

[0087] S3: Sintering the fixed power module and the heat sink, wherein during the sintering process, the sintering temperature is 200° C. to 240° C., the pressure is 8 MPa, and the sintering time is 5 minutes.

[0088] In this comparative example, the area where the power module contacts the heat sink for sintering is 6000mm 2 The sintered product is subjected to ultrasonic scanning (c-sam scanning) to obtain the following Figure 5 The actual sintering area of ​​the scanned image is 5400mm 2 , and the calculated sintering area percentage is 90%.

[0089] Comparative Example 2

[0090] Compared with Example 1, in this comparative example, in step S1, the first printed layer is not dried.

[0091] In this comparative example, the area where the power module contacts the heat sink for sintering is 6000mm 2 The sintered product is subjected to ultrasonic scanning (c-sam scanning) to obtain the following Figure 6 The actual sintering area of ​​the scanned image is 4800mm 2 , and the calculated sintering area percentage is 80%.

[0092] Comparative Example 3

[0093] In this comparative example, the power module and the heat sink are sintered by the following steps:

[0094] S1: The back side of the power module is processed by alcohol cleaning and reflow oven treatment.

[0095] S2: printing the silver paste on the surface of the heat sink to form a second printed layer, and drying the second printed layer; wherein the thickness of the second printed layer is 300 μm.

[0096] S3: Place the power module on the heat sink so that the second printed layer contacts the back of the power module, and fix the power module to the heat sink using a clamp.

[0097] S4: Sintering the fixed power module and the heat sink, wherein during the sintering process, the sintering temperature is 200° C. to 240° C., the pressure is 8 MPa, and the sintering time is 5 minutes.

[0098] In this comparative example, the area where the power module contacts the heat sink for sintering is 6000mm 2 The sintered product is subjected to ultrasonic scanning (c-sam scanning) to obtain the following Figure 7 The actual sintering area of ​​the scanned image is 5400mm 2 , and the calculated sintering area percentage is 90%.

[0099] Test example

[0100] The products sintered in the above-mentioned embodiment and comparative example were subjected to performance tests respectively, and the test results are shown in Table 1.

[0101] The performance test process is specifically as follows: subjecting the product to a TST high and low temperature impact test at 125°C and -40°C, wherein the number of impact cycles is 1000 times; after the TST high and low temperature impact test, subjecting the product to a C-SAM scan again, observing the sintering area of ​​the heat sink and the power module again, and calculating the percentage of the sintering area after the TST high and low temperature impact test, so as to judge the connection strength between the power module and the heat sink based on the percentage of the sintering area after the TST high and low temperature impact test.

[0102] Table 1 Performance test data table

[0103] From the test data in Table 1, we can see that:

[0104] (1) Before the TST high and low temperature impact test, the sintering area percentages of the products obtained in Example 1, Example 2 and Example 3 were all 100%, while the sintering area percentages of the products obtained in Comparative Example 1, Comparative Example 2 and Comparative Example 3 were 90%, 80% and 90% respectively. It can be seen that the sintering methods of Example 1, Example 2 and Example 3 can achieve comprehensive sintering, and the actual sintering areas of the products obtained therefrom are much larger than the actual sintering areas of the products obtained in Comparative Example 1, Comparative Example 2 and Comparative Example 3. Therefore, the reliability of sintering using the sintering method of the present invention is high.

[0105] (2) After the TST high and low temperature impact test, the sintering area percentages of the products obtained in Example 1, Example 2 and Example 3 were reduced to 91%, 95% and 94%, respectively. The sintering area percentages of the products obtained in Example 1, Example 2 and Example 3 were also higher than 90%, while the sintering area percentages of the products obtained in Comparative Example 1, Example 2 and Example 3 were reduced to 65%, 30% and 60%, respectively. The reduction in the sintering area percentages of the products obtained in Example 1, Example 2 and Example 3 was much smaller than the reduction in the sintering area percentages of the products obtained in Comparative Example 1, Example 2 and Example 3. That is, the products prepared by the sintering method of the present invention still have high connection strength after passing the TST high and low temperature impact test. It can be seen that the products sintered by the sintering method of the present invention have high connection strength and high reliability.

[0106] So far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims

1. A sintering method for sintering and bonding a power module to a heat sink. It is characterized in that The sintering method comprises the following steps: S1: printing silver paste on the back side of the power module to form a first printed layer; S2: printing the silver paste on the surface of the heat sink to form a second printed layer; S3: placing the power module on the heat sink and fixing the two using a clamp, wherein the first printed layer is in contact with the second printed layer; S4: Sintering the fixed power module and the heat sink.

2. The sintering method according to claim 1, It is characterized in that In step S1, the thickness of the first printing layer is 5 μm to 50 μm.

3. The sintering method according to claim 2, It is characterized in that The sum of the thickness of the second printing layer and the thickness of the first printing layer is 200 μm to 800 μm.

4. The sintering method according to claim 3, It is characterized in that In step S1, the thickness of the first printing layer is 10 μm to 20 μm; And / or, the sum of the thickness of the second printing layer and the thickness of the first printing layer is 300 μm to 600 μm.

5. The sintering method according to any one of claims 2 to 4, It is characterized in that The ratio of the thickness of the second printing layer to the thickness of the first printing layer is (15-39):

1.

6. The sintering method according to any one of claims 2 to 4, It is characterized in that In step S3, before assembling the power module and the heat sink, the second printed layer is dried first, but the first printed layer is not dried.

7. The sintering method according to claim 1, It is characterized in that In step S4, the sintering temperature during the sintering process is 200° C. to 250° C., the sintering time is 5 min to 30 min, and the pressure is 8 MPa to 20 MPa.

8. The sintering method according to claim 1, It is characterized in that Before printing the silver paste on the power module, the back side of the power module is cleaned.

9. The sintering method according to claim 8, It is characterized in that The cleaning process includes any one of alcohol cleaning, reflow oven treatment or ion bombardment, or a combination of any two of the methods.

10. The sintering method according to claim 1, It is characterized in that The back side of the power module is silver-plated, and the thickness of the silver-plated layer is 0.4 μm to 0.6 μm; And / or, the surface of the heat sink is electroplated with silver, and the thickness of the silver plating layer is 3 μm to 5 μm.

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