Preparation method of selective nickel-palladium-gold spot plating and lead frame

Abstract

The invention discloses a preparation method for selective nickel-palladium-gold spot plating and a lead frame, and the method comprises the steps: S1, attaching a layer of first dry film on a substrate, and obtaining a first film-coated substrate; s2, carrying out selective exposure on the first film-coated substrate, so that the first film-coated substrate is subjected to a photochemical reaction to obtain a first exposed substrate with a pattern; s3, part of the first dry film on the first exposure substrate is removed through development, so that a to-be-electroplated area is exposed out of the first exposure substrate, and a first development substrate is obtained; and S4, the to-be-electroplated area on the first developing substrate is electroplated, so that a nickel plating layer, a palladium plating layer and a gold plating layer are sequentially formed on the spot plating area from inside to outside, and the electroplated substrate is obtained. The lead frame has the advantages that the anti-layering reliability of the lead frame is improved, the production cost of the lead frame is reduced, and the production efficiency of the lead frame is improved.

Description

Technical Field

[0001] The present application relates to the field of semiconductor device technology, and in particular, to a preparation method for selective nickel-palladium-gold dot plating and a lead frame. Background Art

[0002] A lead frame is the core metal structure in semiconductor packaging, usually made of a copper-based alloy and formed by stamping or etching processes. The structure of the lead frame generally includes a die pad and leads. The lead frame can fix the chip and protect the chip from physical damage, and can also transmit signals and power by bonding wires to connect the electrodes of the chip to external pins. The lead frame also uses a high thermal conductivity material to conduct the heat generated by the chip to the package housing or an external heat sink.

[0003] In the prior art, when electroplating a nickel-palladium-gold layer on a lead frame, a process of etching first and then electroplating is usually adopted. However, the disadvantage of this process is that it is impossible to completely block the contact between the electroplating solution and the semi-etched area during the electroplating process, resulting in problems such as silver back leakage and side leakage in QFN electroplated products, which to a certain extent restricts the improvement of product reliability. Moreover, the lead frame will undergo multiple baking processes during the packaging process, forming an oxide film structure of Cu-Cu2O-CuO on the product surface. The bonding forces between Cu and Cu2O and the molding compound are both stronger than the bonding force between the silver plating layer and the molding compound. At the same time, in the prior art, electroplating of the lead frame often involves electroplating all the lead frames first and then etching on the lead frame. Etching after full electroplating not only makes the production time of the lead frame longer, but also causes serious waste of precious metal materials and increases the production cost. Therefore, there is room for improvement. Summary of the Invention

[0004] In view of this, the present application provides a preparation method for selective nickel-palladium-gold dot plating and a lead frame, which can achieve excellent bonding force between the substrate and the plated metal by dot plating first and then etching, reduce the delamination phenomenon of the lead frame, and further make the dot plating on the lead frame highly reliable. Through precise deposition by dot plating, the production time of the lead frame is significantly reduced, and the usage of precious metals is reduced, thereby reducing the production cost.

[0005] In summary, in order to improve the reliability of the lead frame against delamination, reduce the production cost of the lead frame, and improve the production efficiency of the lead frame, the present application proposes a preparation method for selective nickel-palladium-gold dot plating and a lead frame.

[0006] The preparation method for selective nickel-palladium-gold dot plating and the lead frame provided by the present application adopt the following technical solution: A preparation method for selective nickel-palladium-gold dot plating, characterized in that:

[0007] Step S1: Attach a first dry film to the substrate to obtain a first film-covered substrate;

[0008] Step S2: Selectively expose the first film-covered substrate to cause a photochemical reaction on the first film-covered substrate, and obtain a first exposed substrate with a pattern.

[0009] Step S3: Remove a part of the first dry film on the first exposed substrate through development, so that the first exposed substrate exposes the area to be electroplated, and obtain a first developed substrate.

[0010] Step S4: Electroplate the area to be electroplated on the first developed substrate, so that a nickel plating layer, a palladium plating layer, and a gold plating layer are sequentially formed from the inside to the outside in the dot plating area, and obtain an electroplated substrate.

[0011] Step S5: After removing the remaining first dry film on the electroplated substrate, attach a second dry film to the electroplated substrate to obtain a second film-covered substrate.

[0012] Step S6: Selectively expose the second film-covered substrate to cause a photochemical reaction on the second film-covered substrate, and obtain a second exposed substrate with a pattern.

[0013] Step S7: Remove a part of the second dry film on the second exposed substrate through development, so that the second exposed substrate exposes the area to be etched, and obtain a second developed substrate.

[0014] Step S8: Etch the area to be etched on the second developed substrate to obtain an etched substrate.

[0015] Step S9: Remove the remaining second dry film on the etched substrate to obtain a lead frame with selective nickel-palladium-gold dot plating.

[0016] By adopting the above technical solution, a process of first selectively electroplating and then selectively etching the substrate through dry film, exposure, and development operations is carried out. Through selective dot plating, the electroplating area of the lead frame is greatly reduced, the effective electroplating area on the lead frame is increased, and through precise deposition by dot plating, not only the production time of the lead frame is saved in large quantities, the production efficiency is improved, but also the consumption of precious metals is greatly reduced, thereby reducing the production cost. Through selective etching, on the one hand, the edge boundary of electroplating is clarified, and on the other hand, the required shape is etched on the substrate. By electroplating first and then etching, the occurrence of silver side leakage and back leakage can be completely eliminated, the bonding force between the substrate and the plated metal is improved, the delamination phenomenon of the lead frame is reduced, and the reliability of the lead frame against delamination is improved.

[0017] Preferably, the step S2 includes: covering the patterned mask on the first coated substrate, irradiating the first coated substrate with ultraviolet light to cause a photochemical reaction on the first coated substrate, removing the mask, so that the shape of the area to be electroplated on the lead frame is formed on the surface of the first dry film, and a first exposed substrate with a pattern is obtained.

[0018] Preferably, the step S6 includes: covering the patterned mask on the second coated substrate, irradiating the second coated substrate with ultraviolet light to cause a photochemical reaction on the second coated substrate, removing the mask, so that the shape of the area to be etched on the lead frame is formed on the surface of the second dry film, and a second exposed substrate with a pattern is obtained.

[0019] Preferably, from step S1 to step S7, it is set for either a single side of the substrate, or both sides of the substrate, or selectively for a partial surface area of the substrate.

[0020] Preferably, the substrate is made of copper.

[0021] Preferably, during the exposure process, the light energy used is 60 - 150 mj / cm 2 , and the vacuum degree is greater than 650 Kpa; during the development process, the development concentration used is 3 - 12 g / L, the development temperature is 28 - 35 °C, and the development speed is 5 - 8 m / min.

[0022] Preferably, during the etching process, the etching temperature used is 45 - 55 °C, the etching pressure is 15 - 45 Psi, the etching speed is 0.5 - 3.5 M / min, the stripping concentration is 15 ± 5 g / L, and the stripping temperature is 30 - 42 °C.

[0023] A lead frame with selective nickel-palladium-gold dot plating is prepared by the preparation method as described above. The lead frame includes a plurality of lead frame units arranged in an array and non-functional areas connecting adjacent lead frame units. The nickel-palladium-gold plating layer is selectively provided on the lead frame units, and the side walls of the lead frame units and the non-functional areas do not include the nickel-palladium-gold plating layer.

[0024] By adopting the above technical solution, a process of first selectively electroplating and then selectively etching the substrate through dry film, exposure and development operations is carried out to obtain a lead frame. Through the selective plating process, the electroplating area of the lead frame is greatly reduced, the effective electroplating area on the lead frame is increased, and through precise deposition by plating, not only the production time of the lead frame is saved in large quantities, the production efficiency is improved, but also the consumption of precious metals is greatly reduced, thereby reducing the production cost. Through the selective etching process, on the one hand, the edge boundary of electroplating is clarified, and on the other hand, the required shape is etched on the substrate. By electroplating first and then etching, the occurrence of silver side leakage and back leakage can be completely prevented, the bonding force between the substrate and the plating metal is improved, the delamination phenomenon of the lead frame is reduced, and the reliability of the lead frame against delamination is improved.

[0025] Preferably, the nickel-palladium-gold plating layer includes a nickel plating layer, a palladium plating layer and a gold plating layer which are distributed in sequence from the inside to the outside. The thickness of the nickel plating layer is 0.5 - 2.0 μm, the thickness of the palladium plating layer is 0.02 - 0.15 μm, and the thickness of the gold plating layer is 0.003 - 0.015 μm.

[0026] By adopting the above technical solution, the nickel plating layer is used as the core plating layer. The nickel plating layer with a thickness of 0.5 - 2.0 μm not only enables the nickel-palladium-gold plating layer to have excellent adhesion to the copper substrate, but also can improve the wear resistance and mechanical shock resistance of the lead frame, reducing damage during transportation or packaging; the palladium plating layer with a thickness of 0.02 - 0.15 μm on the one hand effectively blocks the interdiffusion of nickel and gold, and on the other hand also facilitates the penetration of the palladium layer and the gold layer to form an alloy, enabling the nickel-palladium-gold plating layer to have excellent welding effects. At the same time, the palladium plating layer also has excellent corrosion resistance and can also be used as an alternative layer for gold, reducing the use of expensive gold while maintaining surface solderability and oxidation resistance; the gold plating layer with a thickness of 0.003 - 0.015 μm has extreme oxidation resistance, and also has high conductivity, low resistance, excellent welding and bonding properties, and long-term storage stability. Compared with the prior art, due to the adoption of the process of first selectively electroplating and then selectively etching, the plating time on the lead frame is shorter, the temperature change of the plating layer is larger during electroplating of the lead frame, and combined with the pressure during the dry film printing mask shape, electron migration occurs between the plating layers, thereby enabling the nickel-palladium-gold layers to penetrate each other in a certain area, strengthening the strength in the middle of the nickel-palladium-gold layer and strengthening the welding effect of the nickel-palladium-gold layer.

[0027] Preferably, the thickness range of the nickel-palladium-gold plating layer is 0.50 - 3.00 μm.

[0028] In summary, the present application includes at least one of the following beneficial technical effects:

[0029] 1. A process of first selectively electroplating and then selectively etching a substrate through dry film, exposure, and development operations. Through selective dot plating, the electroplating area of the lead frame is significantly reduced, increasing the effective electroplating area on the lead frame. Through precise dot plating deposition, not only can the production time of the lead frame be saved in large quantities, improving production efficiency, but also the usage of precious metals can be significantly reduced, thereby reducing production costs. Through selective etching, on the one hand, the edge boundary of electroplating is clarified, and on the other hand, the required shape is etched on the substrate. By electroplating first and then etching, silver side leakage and back leakage can be completely eliminated, the bonding force between the substrate and the plated metal can be improved, the delamination phenomenon of the lead frame can be reduced, and the reliability of the lead frame against delamination can be enhanced;

[0030] 2. The nickel plating layer serves as the core plating layer. The nickel plating layer with a thickness of 0.5 - 2.0μm not only enables the nickel palladium gold plating layer to have excellent adhesion to the copper substrate, but also can improve the wear resistance and mechanical shock resistance of the lead frame, reducing damage during transportation or packaging; The palladium plating layer with a thickness of 0.02 - 0.15μm, on the one hand, effectively blocks the interdiffusion of nickel and gold, and on the other hand, also facilitates the penetration of the palladium layer and the gold layer to form an alloy, making the nickel palladium gold plating layer have excellent welding effects. At the same time, the palladium plating layer also has excellent corrosion resistance properties and can also be used as a substitute layer for gold, reducing the usage of expensive gold while maintaining surface solderability and oxidation resistance; The gold plating layer with a thickness of 0.003 - 0.015μm has extreme oxidation resistance, high conductivity, low resistance, excellent welding and bonding properties, and long-term storage stability. Compared with the prior art, due to the adoption of the process of first selectively electroplating and then selectively etching, the dot plating time on the lead frame is shorter, resulting in a larger temperature change of the plating layer when the lead frame is electroplated. And combined with the pressure during the dry film printing mask shaping, electron migration occurs between the plating layers, thereby enabling a certain area of penetration between the nickel palladium gold layers, strengthening the strength in the middle of the nickel palladium gold layer and enhancing the welding effect of the nickel palladium gold layer;

[0031] 3. After the lead frame is placed at 400°C for 10 seconds, the lead frame is not oxidized, has stable properties, has excellent short-term high-temperature resistance ability, and also has excellent process compatibility. Detailed implementation mode

[0032] Example 1

[0034] The embodiment of the present application discloses a preparation method for selective nickel palladium gold dot plating.

[0035] Step S0: First, clean the substrate to remove large particle contaminants on the substrate surface. Then, thoroughly remove grease, fingerprints, and organic residues on the substrate surface through degreasing. Remove the residues of the degreasing step such as degreasing powder through water washing. Finally, remove the oxides on the substrate and activate the substrate surface through pickling to enhance the adhesion of the electroplated layer. Finally, continue water washing to thoroughly remove all chemical residues on the substrate. The material of the substrate is copper (or copper alloy). Copper (copper alloy) has advantages such as high electrical conductivity, high thermal conductivity, good mechanical properties, easy processability, corrosion resistance, low cost, and excellent welding performance. This makes copper (copper alloy) not only show good bonding ability with the plating metal during the plating process but also show excellent bonding ability with the packaging materials during the chip packaging process. Usually, during the degreasing process, the degreasing temperature is 55 - 65°C, the degreasing current is 20 - 45A, and the degreasing powder concentration is 60 - 90g / L; during the pickling process, the sulfuric acid concentration used is 30 - 50mL / L.

[0036] Step S1: Attach a layer of first dry film on the upper surface and the lower surface of the substrate respectively to obtain a first laminated substrate.

[0037] Step S2: Cover the patterned mask on the upper surface and the lower surface of the first laminated substrate. Irradiate the first laminated substrate with ultraviolet light to cause a photochemical reaction on the first laminated substrate. Remove the mask, so that the shape of the area to be electroplated on the lead frame is formed on the surface of the first dry film, and a first exposed substrate with a pattern is obtained. Among them, during the exposure process, the light energy used is 60 - 150mj / cm 2 , and the vacuum degree is greater than 650Kpa;

[0038] Step S3: Remove part of the first dry film on the upper surface and the lower surface of the first exposed substrate through development, so that the first exposed substrate exposes the area to be electroplated, and a first developed substrate is obtained. Among them, during the development process, the development concentration used is 3 - 12g / L, the development temperature is 28 - 35°C, and the development speed is 5 - 8m / min;

[0039] Step S4: Electroplate the areas to be electroplated on the upper surface and the lower surface of the first developed substrate, so that a nickel plating layer, a palladium plating layer, and a gold plating layer are formed in sequence from the inside to the outside on the electroplated area, and an electroplated substrate is obtained.

[0040] Step S5: After removing the remaining first dry film on the upper surface and the lower surface of the electroplated substrate, attach a layer of second dry film on the upper surface and the lower surface of the electroplated substrate respectively to obtain a second laminated substrate.

[0041] Step S6: Cover the patterned mask on the upper and lower surfaces of the second coated substrate, irradiate the second coated substrate with ultraviolet light to cause a photochemical reaction on the second coated substrate, and remove the mask, so that the shape of the etched area of the lead frame is formed on the surface of the second dry film, and a second exposed substrate with a pattern is obtained. During the exposure process, the light energy used is 60 - 150 mj / cm 2 , and the vacuum degree is greater than 650 Kpa;

[0042] Step S7: Remove part of the second dry film on the upper and lower surfaces of the second exposed substrate by developing, so that the second exposed substrate exposes the area to be etched, and a second developed substrate is obtained. During the developing process, the developing concentration used is 3 - 12 g / L, the developing temperature is 28 - 35 °C, and the developing speed is 5 - 8 m / min;

[0043] Step S8: Etch the areas to be etched on the upper and lower surfaces of the second developed substrate to obtain an etched substrate. During the etching process, the etching temperature used is 45 - 55 °C, the etching pressure is 15 - 45 Psi, the etching speed is 0.5 - 3.5 M / min, the stripping concentration is 15 ± 5 g / L, and the stripping temperature is 30 - 42 °C;

[0044] Step S9: Remove the remaining second dry film on the upper and lower surfaces of the etched substrate to obtain a selectively nickel-palladium-gold plated lead frame.

[0045] In this embodiment, through the processes of dry film, exposure and development operations on the substrate, first selective electroplating and then selective etching are carried out on the substrate. Through selective plating, the electroplating area of the lead frame is greatly reduced, so that the effective electroplating area on the lead frame is increased. Through precise deposition by plating, not only a large amount of production time of the lead frame is saved, the production efficiency is improved, but also the consumption of precious metals is greatly reduced, thereby reducing the production cost. Through selective etching, on the one hand, the edge boundary of electroplating is clarified, and on the other hand, the required shape is etched on the substrate. By electroplating first and then etching, the occurrence of silver side leakage and back leakage can be completely eliminated, the bonding force between the substrate and the plated metal is improved, the delamination phenomenon of the lead frame is reduced, and the reliability of the lead frame against delamination is improved.

[0046] Furthermore, in this embodiment, for the coating, exposure and development operations on the substrate, they can be set not only for the upper and lower surfaces of the substrate, but also for one side of the substrate, or for some areas on the substrate.

[0047] Embodiment 2

[0048] This application embodiment discloses a selectively nickel-palladium-gold plated lead frame.

[0049] Reference, prepared by the preparation method as in Example 1. The lead frame includes sub-units distributed in a matrix pattern and a rectangular frame for connecting adjacent sub-units. The sub-units include metal-based islands and pins. The metal-based islands and pins are located within the rectangular frame, and a nickel-palladium-gold layer is selectively plated on the sub-units.

[0050] The nickel-palladium-gold plating layer includes a nickel plating layer, a palladium plating layer, and a gold plating layer distributed in sequence from the inside out. The thickness of the nickel plating layer is 0.5 μm, the thickness of the palladium plating layer is 0.05 μm, and the thickness of the gold plating layer is 0.003 μm. After the lead frame is placed at a temperature of 400 °C for 10 seconds, the color of the lead frame does not change.

[0051] A process of first selectively electroplating and then selectively etching the substrate through dry film, exposure, and development operations is used to obtain the lead frame. Through the selective dot plating process, the electroplating area of the lead frame is significantly reduced, increasing the effective electroplating area on the lead frame. Through precise deposition by dot plating, not only is the production time of the lead frame greatly saved and the production efficiency improved, but also the usage of precious metals is significantly reduced, thereby reducing the production cost. Through the selective etching process, on the one hand, the edge boundary of electroplating is clarified, and on the other hand, the required shape is etched on the substrate. By electroplating first and then etching, the occurrence of silver side leakage and back leakage can be completely eliminated, the bonding force between the substrate and the plated metal is improved, the delamination phenomenon of the lead frame is reduced, and the reliability of the lead frame against delamination is improved. The nickel plating layer is the core plating layer. The 0.5-μm nickel plating layer not only gives the nickel-palladium-gold plating layer excellent adhesion to the copper substrate but also improves the wear resistance and anti-mechanical shock ability of the lead frame, reducing damage during transportation or packaging. The 0.05-μm palladium plating layer effectively blocks the interdiffusion of nickel and gold on the one hand and also facilitates the penetration of the palladium layer and the gold layer to form an alloy on the other hand, making the nickel-palladium-gold plating layer have excellent welding effects. At the same time, the palladium plating layer also has excellent corrosion resistance and can also be used as a substitute layer for gold, reducing the use of expensive gold while maintaining surface solderability and oxidation resistance. The 0.003-μm gold plating layer has extreme oxidation resistance, high conductivity, low resistance, excellent welding and bonding properties, and long-term storage stability. Compared with the prior art, due to the use of the process of first selectively electroplating and then selectively etching, the dot plating time on the lead frame is shorter, the temperature change of the plating layer is larger during electroplating of the lead frame, and with the pressure during dry film mask shaping, electron migration occurs between the plating layers, resulting in a certain area of penetration between the nickel-palladium-gold layers, strengthening the strength in the middle of the nickel-palladium-gold layer and enhancing the welding effect of the nickel-palladium-gold layer. At the same time, after the lead frame is placed at 400 °C for 10 seconds, the color of the lead frame does not change, indicating that the lead frame is not oxidized, has stable properties, has excellent short-term high-temperature resistance, and also has excellent process compatibility.

[0052] Further, in this embodiment, the nickel-palladium-gold plating layer includes a nickel plating layer, a palladium plating layer, and a gold plating layer that are sequentially distributed from the inside out. The thickness of the nickel plating layer is 0.5 - 2.0 μm, the thickness of the palladium plating layer is 0.02 - 0.15 μm, and the thickness of the gold plating layer is 0.003 - 0.015 μm.

[0053] Further, in this embodiment, the thickness range of the nickel-palladium-gold plating layer is 0.50 - 3.00 μm.

[0054] Further, in this embodiment, there are no specific restrictions on the specific form and plating positions of the lead frame in this application. The common lead frame forms and the positions of the lead frame to be electroplated in the prior art can be adopted.

[0055] It should be noted that in the case where the embodiments of this application do not conflict with each other and the technical solutions can coexist, they can be arbitrarily combined into new embodiments.

[0056] The above are all the preferred embodiments of this application. The protection scope of this application is not limited thereby. Therefore, all equivalent changes made according to the structure, shape, and principle of this application should be covered within the protection scope of this application.

Claims

1. A preparation method for selective nickel-palladium-gold dot plating, characterized in that: Step S1: Attach a first dry film to a substrate to obtain a first film-covered substrate; Step S2: Perform selective exposure on the first film-covered substrate to cause a photochemical reaction on the first film-covered substrate, obtaining a first exposed substrate with a pattern; Step S3: Remove a part of the first dry film on the first exposed substrate by development, so that the first exposed substrate exposes the area to be electroplated, obtaining a first developed substrate; Step S4: Electroplate the area to be electroplated on the first developed substrate, so that a nickel plating layer, a palladium plating layer and a gold plating layer are sequentially formed from the inside out in the dot plating area, obtaining an electroplated substrate; Step S5: After removing the remaining first dry film on the electroplated substrate, attach a second dry film to the electroplated substrate to obtain a second film-covered substrate; Step S6: Perform selective exposure on the second film-covered substrate to cause a photochemical reaction on the second film-covered substrate, obtaining a second exposed substrate with a pattern; Step S7: Remove a part of the second dry film on the second exposed substrate by development, so that the second exposed substrate exposes the area to be etched, obtaining a second developed substrate; Step S8: Etch the area to be etched on the second developed substrate to obtain an etched substrate; Step S9: Remove the remaining second dry film on the etched substrate to obtain a lead frame for selective nickel-palladium-gold dot plating.

2. The preparation method according to claim 1, wherein: The step S2 includes: covering a patterned mask on the first film-covered substrate, irradiating the first film-covered substrate with ultraviolet light to cause a photochemical reaction on the first film-covered substrate, removing the mask, so that the shape of the area to be electroplated on the lead frame is formed on the surface of the first dry film, obtaining a first exposed substrate with a pattern.

3. The preparation method according to claim 1, wherein: The step S6 includes: covering a patterned mask on the second film-covered substrate, irradiating the second film-covered substrate with ultraviolet light to cause a photochemical reaction on the second film-covered substrate, removing the mask, so that the shape of the area to be etched on the lead frame is formed on the surface of the second dry film, obtaining a second exposed substrate with a pattern.

4. The preparation method according to claim 1, characterized in that: From step S1 to step S7, it is set selectively for a single side of the substrate, or for both sides of the substrate, or for a partial surface area of the substrate.

5. The preparation method according to claim 1, characterized in that: The substrate is made of copper.

6. The preparation method according to claim 1, characterized in that: During the exposure process, the light energy used is 60 - 150 mj / cm 2 , and the vacuum degree is greater than 650 Kpa; during the development process, the development concentration used is 3 - 12 g / L, the development temperature is 28 - 35 °C, and the development speed is 5 - 8 m / min.

7. The preparation method according to claim 1, characterized in that: During the etching process, the etching temperature is 45-55 °C, the etching pressure is 15-45 Psi, the etching speed is 0.5-3.5 M / min, the stripping concentration is 15±5 g / L, and the stripping temperature is 30-42 °C.

8. A lead frame with selective nickel-palladium-gold dot plating, characterized in that: Prepared by the preparation method according to any one of claims 1-7, the lead frame includes sub-units and a rectangular frame for connecting adjacent sub-units, and a nickel-palladium-gold plating layer is selectively arranged on the sub-units.

9. The lead frame according to claim 8, wherein: The nickel-palladium-gold plating layer includes a nickel plating layer, a palladium plating layer and a gold plating layer sequentially distributed from the inside out, the thickness of the nickel plating layer is 0.5-2.0 μm, the thickness of the palladium plating layer is 0.02-0.15 μm, and the thickness of the gold plating layer is 0.003-0.015 μm.

10. The lead frame according to claim 8, characterized in that: The thickness range of the nickel-palladium-gold plating layer is 2.54 - 7.62 μm.