Laser drilling method for thin copper HDI circuit board

Abstract

The invention relates to a laser drilling method for a thin copper HDI circuit board. The laser drilling method comprises the following steps that a circuit board to be machined is positioned and installed on a machining station of laser machining equipment; a first section of laser drilling is carried out, a first laser parameter is adopted, uncovering drilling is carried out on a preset machining area of the outer copper foil layer, and an initial hole structure is formed; performing second-section laser drilling, namely performing compensation drilling on the dielectric layer exposed at the bottom of the initial hole structure by adopting a second laser parameter of which the energy is lower than that of the first laser parameter to form a transition hole structure; and in the third section of laser drilling, a third laser parameter with energy lower than that of the second laser parameter is adopted, the bottom of the transition hole structure is trimmed, and a final hole channel is formed. According to the method, drilling is conducted in three sections, namely the first section of uncovering drilling, the second section of compensation drilling and the third section of finishing treatment. Therefore, the bottom cushion copper cannot absorb a large amount of heat in a short time, the base material layer below the bottom cushion copper can be protected from being burnt, and the product quality is improved.

Description

Technical Field

[0001] The present invention relates to the field of printed circuit board processing, and more particularly to a laser drilling method for a thin copper HDI printed circuit board. Background Art

[0002] With the popularization of HDI (High Density Interconnection) printed circuit boards, users are increasingly inclined to the thin copper design of the signal transmission layer. The original copper thickness of the bottom pad copper is usually above 1OZ, while it is usually designed as HOZ now. As the copper thickness becomes thinner, the original production process of the printed circuit board cannot adapt to the new printed circuit board design. For example, in the laser drilling process flow, the existing process usually uses high-energy laser for one-time drilling. Since the copper thickness of the bottom pad copper has become at least half thinner, it is easy to cause the bottom pad copper to be heated in a short time, and the copper heat is transmitted to the substrate layer under the bottom pad copper, resulting in the substrate layer being burned and blackened. When the substrate under the bottom pad copper can be burned, when the finished product is subjected to reliability tests such as thermal shock test and HCT test, cracks and other defects are likely to appear in the burned and blackened parts, causing reliability problems. Summary of the Invention

[0003] In view of this, the present invention provides a laser drilling method for a thin copper HDI printed circuit board that can effectively avoid burning the substrate layer under the bottom pad copper during laser drilling.

[0004] The object of the present invention is achieved by the following technical solutions: A laser drilling method for a thin copper HDI printed circuit board, comprising the following steps: S1: Position and install the printed circuit board to be processed on the processing station of the laser processing equipment. The printed circuit board at least includes a substrate layer, a bottom pad copper layer, a dielectric layer, and an outer copper foil layer that are sequentially stacked; S2: Perform the first-stage laser drilling. Using the first laser parameters, perform open-top drilling on the predetermined processing area of the outer copper foil layer to form an initial hole structure; S3: Perform the second-stage laser drilling. Using the second laser parameters with an energy lower than that of the first laser parameters, perform compensation drilling on the dielectric layer exposed at the bottom of the initial hole structure to form a transition hole structure; S4: Perform the third-stage laser drilling. Using the third laser parameters with an energy lower than that of the second laser parameters, perform trimming on the bottom of the transition hole structure to form a final hole; Wherein, the cumulative depth of the initial hole structure, the transition hole structure, and the final hole is equal to the preset drilling depth.

[0005] In the above technical solution, when opening the cover and drilling holes with the first laser parameters of higher energy in the first stage, the outer copper foil layer can be quickly removed. Since there are subsequent compensation drilling and trimming processes, there is no need to worry about the problems of over - deep drilling or irregular shape in this step. In the second stage, the second laser parameters of lower energy are used to perform compensation drilling on the dielectric layer. This decreasing energy setting can avoid penetrating the dielectric layer due to excessive energy during dielectric layer drilling and contacting the bottom pad copper, and prevent a large amount of heat from being generated in the bottom pad copper in a short time. In the third - stage trimming process, the third laser parameters of even lower energy are used, which can finely adjust the final hole path, making the size and shape of the final hole path more in line with the preset requirements and improving the overall drilling accuracy.

[0006] Therefore, in the present invention, the drilling operation is carried out in three stages: the first - stage cover - opening drilling, the second - stage compensation drilling, and the third - stage trimming process. Lasers with different energy parameters are used in each stage, which can more precisely control the depth and shape of the drilling. Among them, the energy of the third - stage laser drilling is the lowest. Therefore, the bottom pad copper will not absorb a large amount of heat in a short time, thereby protecting the substrate layer under the bottom pad copper from being burned and improving the quality and test yield of the product.

[0007] Optionally, in a possible implementation manner, the first laser parameters are set as: the pulse width is 12 - 16 μs, and the number of pulses is 1; the depth of the initial hole structure is 50 - 60% of the preset drilling depth.

[0008] In the above technical solution, the setting of the first laser parameters makes the laser action time short and the energy concentrated, enabling efficient operation in the cover - opening drilling stage, and avoiding excessive damage to the circuit board that may be caused by multiple laser actions while meeting the cover - opening requirements. The depth ratio of the initial hole structure ensures that in the first - stage laser drilling, most of the outer copper foil layer and part of the dielectric layer can be removed, while leaving enough space for the subsequent compensation drilling and trimming processes.

[0009] Optionally, in a possible implementation manner, the second laser parameters are set as: the pulse width is 4 - 6 μs, and the number of pulses is 2; the depth of the transition hole structure is 35 - 45% of the preset drilling depth.

[0010] In the above technical solution, compared with the first - stage laser drilling, the second - stage laser drilling uses a lower pulse width, which is more suitable for drilling materials such as the dielectric layer that are relatively thin and have different textures from the copper foil layer. It will neither instantaneously penetrate the dielectric layer due to excessive energy, causing the bottom pad copper to quickly absorb heat, nor fail to reach the expected drilling depth due to insufficient energy.

[0011] Optionally, in a possible implementation manner, the third laser parameters are set as: the pulse width is 3 - 5 μs, and the number of pulses is 1.

[0012] In the above technical solution, the setting of the third laser parameter enables the laser to precisely act on the bottom of the transition hole structure, removing a small amount of material to meet the precise shape and size requirements of the final hole. At the same time, due to the low energy, the bottom pad copper will not absorb a large amount of heat in a short time, thus protecting the substrate layer under the bottom pad copper from being burned.

[0013] Optionally, in a possible implementation manner, after the third-stage laser drilling is completed in step S4, the residues in the holes are removed by a plasma treatment process.

[0014] In the above technical solution, the plasma treatment process is efficient in removing residues in the holes. The active particles in the plasma can undergo physical and chemical reactions with the residues, decomposing the residues into smaller particles or directly vaporizing them. This treatment method can reach every corner of the holes. Whether it is copper chips, dielectric layer fragments or other impurities generated during the laser drilling process, they can be effectively removed.

[0015] Optionally, in a possible implementation manner, the plasma treatment process includes two treatment stages: the first stage uses high-energy argon plasma to remove the carbonized layer, and the second stage uses oxygen plasma to activate the surface of the hole wall.

[0016] In the above technical solution, the plasma treatment process in the two stages complements each other, comprehensively improving the quality of the hole wall. From removing the carbonized layer to activating the surface of the hole wall, it covers the key links to improve the physical and chemical properties of the hole wall, making the hole wall not only clean and free of carbonized layer, but also having good activity, which is convenient for subsequent copper deposition and electroplating treatment.

[0017] Optionally, in a possible implementation manner, the thickness of the bottom pad copper layer is designed as HOZ, and the thickness of the outer copper foil layer is designed as 1 / 3OZ.

[0018] In the above technical solution, the bottom pad copper layer with a thickness of HOZ can provide appropriate conductivity and meet the design requirements of the inner layer impedance value. The outer copper foil layer mainly undertakes functions such as signal transmission and shielding. The thickness of 1 / 3OZ is sufficient to meet these functional requirements and is more conducive to manufacturing fine circuits.

[0019] Optionally, in a possible implementation manner, the preset drill hole is a blind hole, and the blind hole extends to the surface of the bottom pad copper layer.

[0020] In the above technical solution, the structure of the blind hole helps to reduce electromagnetic interference to a certain extent. Since the blind hole does not penetrate the entire circuit board, the electromagnetic radiation generated when it conducts current is relatively small. Moreover, when the blind hole is connected to the bottom pad copper layer, the bottom pad copper layer can play a certain shielding role, limiting the electromagnetic interference to a local area, thereby improving the electromagnetic compatibility of the entire circuit board.

[0021] Optionally, in a possible implementation, a preprocessing process is further included before the step S2: using a laser to process alignment marking points at predetermined positions and performing coordinate calibration through a machine vision system.

[0022] In the above technical solution, alignment marking points are processed at predetermined positions by a laser, providing an accurate positioning basis for subsequent processes. Laser processing can achieve high-precision marking point production, and the size and position of its marking points can be precisely controlled, facilitating subsequent drilling operations.

[0023] Optionally, in a possible implementation, a quality inspection process is further included after the step S4: identifying drilling defects through an automatic optical inspection system, marking abnormal hole positions, and feeding back to the laser processing equipment.

[0024] In the above technical solution, the automatic optical inspection system has high sensitivity. It can utilize the principle of optical imaging to accurately identify various defects generated during the drilling process, such as abnormal hole wall roughness, aperture deviation, excessive residues in the hole, etc., ensuring comprehensive monitoring of the quality of all drilled holes. Description of the Drawings

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative efforts.

[0026] Figure 1 Flowchart of the laser drilling method for an embodiment Detailed Implementation Modes

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. Usually, the components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations.

[0028] Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the present application to be protected, but merely represents the selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0029] Please refer to Figure 1, this embodiment provides a laser drilling method for a thin copper HDI circuit board, including the following steps: S1: Position and install the circuit board to be processed on the processing station of the laser processing equipment. The circuit board at least includes a substrate layer, a bottom pad copper layer, a dielectric layer, and an outer copper foil layer that are sequentially stacked; S2: Perform the first-stage laser drilling. Using the first laser parameters, perform an opening drilling on the predetermined processing area of the outer copper foil layer to form an initial hole structure; S3: Perform the second-stage laser drilling. Using the second laser parameters with an energy lower than the first laser parameters, perform a compensation drilling on the dielectric layer exposed at the bottom of the initial hole structure to form a transition hole structure; S4: Perform the third-stage laser drilling. Using the third laser parameters with an energy lower than the second laser parameters, perform a trimming process on the bottom of the transition hole structure to form the final hole; Among them, the cumulative depth of the initial hole structure, the transition hole structure, and the final hole is equal to the preset drilling depth.

[0030] In this embodiment, when performing the opening drilling with the first laser parameters of higher energy in the first stage, the outer copper foil layer can be quickly removed. Since there are subsequent compensation drilling and trimming processes, there is no need to worry about the problems of over-drilling or irregular shape in this step. In the second stage, the second laser parameters with lower energy are used to perform compensation drilling on the dielectric layer. This decreasing energy setting can avoid piercing the dielectric layer and contacting the bottom pad copper due to excessive energy when drilling the dielectric layer, and avoid generating a large amount of heat in the bottom pad copper in a short time. In the third-stage trimming process, the third laser parameters with even lower energy are used, which can fine-tune the final hole, making the size and shape of the final hole more in line with the preset requirements and improving the overall drilling accuracy.

[0031] By adopting the laser drilling method with gradually decreasing energy, compared with the single high-energy drilling method, the thermal impact on each layer of the circuit board is smaller. During the drilling process, the lower-energy laser will not cause excessive thermal stress on the substrate layer, bottom pad copper layer, etc. of the circuit board during the subsequent compensation drilling and trimming processes, thereby reducing problems such as deformation, warping, and burning of the circuit board, and ensuring the flatness and overall quality of the circuit board.

[0032] Therefore, in this embodiment, the drilling operation is carried out in three stages: the first-stage opening drilling, the second-stage compensation drilling, and the third-stage trimming process. Each stage uses a laser with different energy parameters, which can more accurately control the depth and shape of the drilling. Among them, the energy of the third-stage laser drilling is the lowest, so the bottom pad copper will not absorb a large amount of heat in a short time, thereby protecting the substrate layer under the bottom pad copper from being burned and improving the quality and test yield of the product.

[0033] In this embodiment, the first laser parameters are set as follows: the pulse width is 12 - 16 μs, and the number of pulses is 1; the depth of the initial hole structure is 50 - 60% of the preset drilling depth. Specifically, preferably, the pulse width is 15 μs, and the high - energy open - cover method is adopted to open the cover of the preset drill hole. At this time, through slice confirmation, the first - stage laser drilling drills the depth of the hole to 55%.

[0034] The setting of the first laser parameters enables the laser to have a short action time and concentrated energy, which can achieve efficient operation during the open - cover drilling stage, and while meeting the open - cover requirements, it avoids the excessive damage to the circuit board that may be caused by multiple laser actions. The depth ratio of the initial hole structure ensures that in the first - stage laser drilling, most of the outer copper foil layer and part of the dielectric layer can be removed, and at the same time, enough space is left for subsequent compensation drilling and trimming processing.

[0035] In this embodiment, the second laser parameters are set as follows: the pulse width is 4 - 6 μs, and the number of pulses is 2; the depth of the transition hole structure is 35 - 45% of the preset drilling depth. Specifically, preferably, the pulse width is 5 μs and the depth is 40%. At this time, the preset hole is drilled through to near the bottom - pad copper, and 5% of the dielectric layer remains at the bottom of the hole.

[0036] Compared with the first - stage laser drilling, the second - stage laser drilling uses a lower pulse width, which is more suitable for drilling materials such as the dielectric layer that are relatively thin and have different textures from the copper foil layer. It will neither instantaneously penetrate the dielectric layer due to excessive energy, causing the bottom - pad copper to rapidly absorb heat, nor fail to reach the expected drilling depth due to insufficient energy. In addition, this depth ratio provides a suitable starting state for the third - stage laser drilling. The third - stage laser drilling needs to trim the bottom of the transition hole structure, and the reasonable setting of the depth of the transition hole structure enables the third - stage laser drilling to be finely adjusted on this basis to meet the precise requirements of the final hole.

[0037] In this embodiment, the third laser parameters are set as follows: the pulse width is 3 - 5 μs, and the number of pulses is 1. The setting of the third laser parameters enables the laser to precisely act on the bottom of the transition hole structure, removing a small amount of material to meet the precise shape and size requirements of the final hole. At the same time, due to the low energy, the bottom - pad copper will not absorb a large amount of heat in a short time, thus protecting the substrate layer under the bottom - pad copper from being burned.

[0038] The third laser parameters cooperate with the laser parameters of the first two stages of laser drilling. After the first - stage high - energy open - cover drilling and the second - stage medium - energy compensation drilling, the third stage uses this low - energy and single - pulse - number laser parameter for trimming. This decreasing parameter setting from high energy to low energy makes the entire drilling process coherent and efficient, and each stage can fully play its role, finally forming a high - quality final hole.

[0039] It should be noted that the clear laser parameters and the corresponding processing depth ratios make the entire laser drilling process have higher repeatability. In mass production, whether it is the processing of circuit boards in different batches or the process conversion between different devices, this clear parameter setting can ensure the consistency of drilling quality.

[0040] In this embodiment, after the third-stage laser drilling is completed in step S4, the residues in the holes are removed by a plasma treatment process. Among them, the plasma treatment process includes two treatment stages: the first stage uses high-energy argon plasma to remove the carbonized layer, and the second stage uses oxygen plasma to activate the surface of the hole wall.

[0041] The plasma treatment process is efficient in removing residues in the holes. The active particles in the plasma can undergo physical and chemical reactions with the residues, decomposing the residues into smaller particles or directly vaporizing them. This treatment method can reach every corner of the hole, and whether it is copper chips, dielectric layer fragments or other impurities generated during the laser drilling process, they can be effectively removed.

[0042] In addition, the plasma treatment process in two stages complements each other, comprehensively improving the quality of the hole wall. From removing the carbonized layer to activating the surface of the hole wall, it covers the key links for improving the physical and chemical properties of the hole wall, making the hole wall not only clean and free of carbonized layer, but also have good activity, which is convenient for subsequent electroless copper plating treatment.

[0043] It should be noted that the thickness of the bottom pad copper layer in this embodiment is designed to be HOZ (half ounce), and the thickness of the outer copper foil layer is designed to be 1 / 3OZ (one-third ounce).

[0044] The bottom pad copper layer with HOZ thickness can provide appropriate conductivity and meet the design requirements of the inner layer impedance value. The outer copper foil layer mainly undertakes functions such as signal transmission and shielding. The thickness of 1 / 3OZ is sufficient to meet these functional requirements and is more conducive to manufacturing fine circuits.

[0045] In this embodiment, the preset drilled holes are blind holes, and the blind holes extend to the surface of the bottom pad copper layer. The blind holes do not penetrate the entire circuit board and are opened to the bottom pad surface according to the design requirements.

[0046] The structure of the blind holes helps to reduce electromagnetic interference to a certain extent. Since the blind holes do not penetrate the entire circuit board, the electromagnetic radiation generated when they conduct current is relatively small. Moreover, when the blind holes are connected to the bottom pad copper layer, the bottom pad copper layer can play a certain shielding role, limiting the electromagnetic interference to a local area, thereby improving the electromagnetic compatibility of the entire circuit board.

[0047] Before step S2 of this embodiment, there is also a pretreatment process: using a laser to process alignment marking points at predetermined positions and performing coordinate calibration through a machine vision system. Processing alignment marking points at predetermined positions by a laser provides an accurate positioning basis for subsequent processes. Laser processing can achieve high-precision marking point production, and the size and position of its marking points can be precisely controlled, facilitating subsequent drilling operations.

[0048] In addition, after step S4, there is also a quality inspection process: identifying drilling defects through an automatic optical inspection system, marking abnormal hole positions and feeding back to the laser processing equipment.

[0049] The automatic optical inspection system has high sensitivity. It can utilize the principle of optical imaging to accurately identify various defects generated during the drilling process, such as abnormal hole wall roughness, aperture deviation, excessive residues in the hole, etc., ensuring comprehensive monitoring of the quality of all drilled holes.

[0050] In the description of the present invention, it should be understood that terms such as "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present invention.

[0051] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality of" means two or more, unless otherwise specifically defined.

[0052] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A laser drilling method for a thin copper HDI circuit board, characterized in that, Including the following steps: S1: Position and install the circuit board to be processed on the processing station of the laser processing equipment. The circuit board at least includes a substrate layer, a bottom pad copper layer, a dielectric layer, and an outer copper foil layer that are sequentially stacked; S2: Perform the first-stage laser drilling. Using the first laser parameters, perform open-hole drilling on the predetermined processing area of the outer copper foil layer to form an initial hole structure; S3: Perform the second-stage laser drilling. Using the second laser parameters with energy lower than the first laser parameters, perform compensation drilling on the dielectric layer exposed at the bottom of the initial hole structure to form a transition hole structure; S4: Perform the third-stage laser drilling. Using the third laser parameters with energy lower than the second laser parameters, perform trimming processing on the bottom of the transition hole structure to form a final hole; Wherein, the cumulative depth of the initial hole structure, the transition hole structure, and the final hole is equal to the preset drilling depth.

2. The laser drilling method of the thin copper HDI circuit board according to claim 1, characterized in that The first laser parameters are set as: pulse width 12 - 16 us, number of pulses 1; the depth of the initial hole structure is 50 - 60% of the preset drilling depth.

3. The laser drilling method for the thin copper HDI circuit board according to claim 2, wherein, The second laser parameters are set as: pulse width 4 - 6 us, number of pulses 2; the depth of the transition hole structure is 35 - 45% of the preset drilling depth.

4. The laser drilling method of the thin copper HDI circuit board according to claim 3, characterized in that, The third laser parameters are set as: pulse width 3 - 5 us, number of pulses 1.

5. The laser drilling method for the thin copper HDI circuit board according to claim 1, characterized in that After the third-stage laser drilling in step S4 is completed, the residues in the holes are removed through a plasma treatment process.

6. The laser drilling method of the thin copper HDI circuit board according to claim 5, characterized in that, The plasma treatment process includes two treatment stages: the first stage uses high-energy argon plasma to remove the carbonized layer, and the second stage uses oxygen plasma to activate the surface of the hole wall.

7. The laser drilling method for the thin copper HDI circuit board according to claim 1, characterized in that, The thickness of the bottom pad copper layer is designed as HOZ, and the thickness of the outer copper foil layer is designed as 1 / 3OZ.

8. The laser drilling method for the thin copper HDI circuit board according to claim 1, characterized in that, The preset drilling is a blind hole, and the blind hole extends to the surface of the bottom pad copper layer.

9. The laser drilling method of the thin copper HDI circuit board according to claim 1, characterized in that, Before step S2, there is also a pretreatment process: using a laser to process alignment mark points at predetermined positions and performing coordinate calibration through a machine vision system.

10. The laser drilling method for the thin copper HDI circuit board according to claim 1, characterized in that, After step S4, there is also a quality inspection process: identifying drilling defects through an automatic optical inspection system, marking abnormal hole positions, and feeding back to the laser processing equipment.

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