A blind hole residual glue detection device and a detection method

Abstract

The application discloses a kind of blind hole residual glue detection devices and detection methods, belong to printed circuit board detection technical field, comprising: selecting broadband light source, the light emitted by broadband light source is split into sample light and reference light by optical fiber coupler;Reference light is transmitted according to the set path, and is reflected back to optical fiber coupler by reflector;Sample light is transmitted to the blind hole of PCB according to the set path, and returns to optical fiber coupler after backscattering;Returning sample light and reference light interfere in optical fiber coupler and form coherent light, read coherent light parameters using spectrometer, whether the irradiation point position exists residual glue is judged according to the light intensity of coherent light.The wavelength, output power and bandwidth of the present application are controlled by controlling broadband light source, so that the detection device can realize the detection of 0.007mm~10mm thickness residual glue, the blind hole residual glue detection device in the present application has good penetrability, and the detection of minimum thickness in 7um residual glue can be realized, with higher detection precision.

Description

Technical Field

[0001] This invention belongs to the field of printed circuit board testing technology, specifically relating to a blind hole residual adhesive detection device and detection method. Background Technology

[0002] Printed circuit boards (PCBs) provide electrical connections in electronic components. With increasingly faster digital signal transmission rates and higher frequencies, ensuring the integrity of signal transmission is a key technology in PCB manufacturing. During PCB manufacturing, the inspection of blind vias can promptly identify issues such as whether drilling parameters are appropriate and whether electroplating processes are functioning correctly. By checking and providing feedback on blind vias, production processes can be adjusted and optimized, thereby improving PCB production efficiency and quality.

[0003] During the inspection of blind vias on PCBs, the detection of residual adhesive inside the blind vias is also a very important step. When drilling, if the drill bit speed, feed rate, or other parameters are not appropriate, it may cause local overheating of the board, causing the resin to melt and adhere to the via wall, forming residual adhesive. At the same time, if the pressure or temperature is not suitable during film application, air bubbles or poor adhesion may exist between the dry film and the board. During exposure, some areas may receive uneven light, and after development, dry film may remain in the blind vias, forming residual adhesive.

[0004] For residual adhesive inside blind vias on PCBs, current conventional detection methods include optical microscopy, scanning electron microscopy, and AOI (Automated Optical Inspection). While optical microscopy and scanning electron microscopy offer high accuracy, they suffer from low imaging depth and slow detection speeds. AOI, on the other hand, cannot detect deep micro-vias. Therefore, existing PCB blind via detection technologies cannot accurately determine the specific location and morphology of residual adhesive. Summary of the Invention

[0005] In view of one or more of the above-mentioned defects or improvement needs of the prior art, the present invention provides a method for detecting residual adhesive in blind holes, which is used to detect the location and morphology of residual adhesive in blind holes of PCB boards.

[0006] To achieve the above objectives, the present invention provides a method for detecting residual adhesive in blind holes of PCB boards, comprising the following steps:

[0007] S1. Select a broadband light source and split the light emitted by the broadband light source into sample light and reference light through an optical fiber coupler.

[0008] S2. The reference light is transmitted along the set path and reflected back to the fiber coupler by the mirror; the sample light is transmitted along the set path to the blind hole on the PCB board and returns to the fiber coupler after backscattering.

[0009] S3. The returned sample light and reference light interfere with each other in the fiber optic coupler and form coherent light. The parameters of the coherent light are read by a spectrometer, and the presence of residual adhesive at the irradiation point is determined based on the intensity of the coherent light.

[0010] As a further improvement of the present invention, the selection of the broadband light source in step S1 is determined according to the thickness of the residual adhesive in the blind hole of the PCB board to be inspected.

[0011] As a further improvement of the present invention, the thickness of the residual adhesive in the blind hole of the PCB board to be inspected is 0.007mm~10mm, the wavelength of the broadband light source is determined according to the maximum value of the thickness of the residual adhesive to be inspected, so that the light emitted by the broadband light source can penetrate the residual adhesive and be received by the fiber optic coupler; the bandwidth of the broadband light source is determined according to the minimum value of the thickness of the residual adhesive to be inspected, so that the residual adhesive can be detected and identified.

[0012] As a further improvement of the present invention, the wavelength of the broadband light source is calculated according to the Beer-Lambert law, and the wavelength of the broadband light source is derived from the absorption coefficient of the residual glue at a set wavelength.

[0013] As a further improvement of the present invention, the absorption coefficient of the residual adhesive at a set wavelength is calculated as follows:

[0014] (Formula 1)

[0015] Where α is the absorption coefficient of the residual adhesive at a set wavelength, d is the penetration depth, I is the light intensity at the laser penetration depth, and I0 is the initial light intensity when the laser is incident on the resin surface.

[0016] The residual adhesive is epoxy resin. When the laser intensity is attenuated to 36.8% of its initial intensity, the resin can be penetrated. The wavelength of the broadband light source is selected based on the absorption coefficient of the epoxy resin at different wavelengths.

[0017] As a further improvement of the present invention, the wavelength of the broadband light source is 840nm.

[0018] As a further improvement of the present invention, the bandwidth of the broadband light source is calculated based on the Ruili length.

[0019] As a further improvement of the present invention, the bandwidth of the broadband light source is calculated as follows:

[0020] (Formula 2)

[0021] Where Δλ is the bandwidth of the broadband light source; λ0 is the center wavelength of the broadband light source; and Δz is the axial resolution.

[0022] As a further improvement of the present invention, the broadband light source is composed of two light sources with a bandwidth of 50nm.

[0023] This application also includes a blind hole residual adhesive detection device, which includes:

[0024] A broadband light source, the output of which is connected to an optical fiber coupler, which can split the light emitted by the broadband light source into reference light and sample light;

[0025] The end of the fiber optic coupler that is away from the broadband light source is connected to the first fiber optic collimator and the second fiber optic collimator, respectively.

[0026] The first fiber collimator is provided with a lens and a reflector in the optical transmission path. The first fiber collimator is aligned with the lens and the reflector in the optical path, so that the light reflected back by the reflector can be transmitted back to the first fiber collimator.

[0027] The second fiber collimator has a galvanometer in the optical transmission path, and a scanning lens is provided in the light output direction of the galvanometer, with the scanning lens facing the blind via of the PCB board under test.

[0028] The fiber optic coupler is also connected to a spectrometer, which is used to analyze the light returning to the fiber optic coupler to determine the intensity of the coherent light.

[0029] The broadband light source has a wavelength of 840nm, an output optical power greater than 20mW, and a bandwidth greater than 100nm; the insertion loss of the fiber coupler, the first fiber collimator, and the second fiber collimator is less than 3dB, and the acquisition speed of the spectrometer is greater than 250kHz.

[0030] The aforementioned improved technical features can be combined with each other as long as they do not conflict with each other.

[0031] In summary, the beneficial effects of the above-described technical solutions conceived by this invention compared with the prior art include:

[0032] (1) The blind hole adhesive residue detection device of the present invention is based on the principle of Michelson interferometer. It uses light emitted from a low-coherence broadband light source and splits the light into reference light and sample light through an optical fiber coupler. The reference light is reflected by a mirror, and the sample light generates backscattered or reflected light after irradiating the copper plate or adhesive residue in the blind hole. When the optical path difference between the reference light and the sample light is within the coherence length of the light source, the two beams will interfere, and the spectrometer will then receive the interference signal. When the sample light hits the copper plate in the blind hole of the PCB board, due to the extremely high reflectivity of the bottom copper layer, the beam will be backscattered when it hits the adhesive residue in the blind hole. Most of the beam will continue to penetrate to the surface of the copper layer and backscatter, which makes the intensity of the reflected light hitting the copper plate and hitting the adhesive residue significantly different. The inspector can determine whether there is adhesive residue in the blind hole of the PCB board based on the strength of the interference signal received by the spectrometer. Meanwhile, by controlling the wavelength, output power, and bandwidth of the broadband light source, this application enables the detection device to detect residual adhesive with a thickness of 0.007mm to 10mm, that is, the penetration thickness can reach 10mm and the minimum detection thickness can reach 7um. The blind hole residual adhesive detection device in this application has good penetration and can detect residual adhesive with a minimum thickness of 7um, and has high detection accuracy.

[0033] (2) The blind hole residual adhesive detection method of the present invention utilizes the principle of Michelson interferometer. By interfering the reference light and the sample light in the fiber optic coupler, and using a spectrometer to obtain the light intensity of the coherent light, the reflection or backscattering of the sample light in the blind hole of the PCB board is obtained, thereby analyzing whether there is residual adhesive in the PCB board. At the same time, according to the residual adhesive detection requirements, the wavelength and bandwidth of the broadband light source are obtained by using the Beer-Lamber law and the Rayleigh length calculation formula, thereby realizing the penetration detection of 10mm thick residual adhesive and the resolution of residual adhesive with a minimum thickness of 7um, ensuring the accuracy of residual adhesive detection in blind holes. Furthermore, this application uses two 50nm bandwidth light sources combined to obtain light with a bandwidth of more than 100nm, which improves the axial resolution of the system by 100%, greatly enhancing the ability of the blind hole residual adhesive detection method to identify residual adhesive, so that residual adhesive can be identified even when it is very thin. Attached Figure Description

[0034] Fig. 1 This is a schematic diagram of light transmission at the blind hole in the blind hole residual adhesive detection device in this embodiment of the invention;

[0035] Fig. 2 This is a schematic diagram of the overall structure of the blind hole residual adhesive detection device in an embodiment of the present invention;

[0036] Fig. 3 This is a schematic flowchart of the blind hole residual adhesive detection method in an embodiment of the present invention.

[0037] In all the accompanying drawings, the same reference numerals denote the same technical features, specifically:

[0038] 1. Broadband light source; 2. Fiber optic coupler; 3. First fiber optic collimator; 4. Lens; 5. Reflector; 6. Second fiber optic collimator; 7. Galvanometer; 8. Scanning lens; 9. PCB board under test; 10. Spectrometer; 11. Control components. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0040] In the description of this invention, it should be understood that, unless otherwise stated, the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0041] Furthermore, unless otherwise stated, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0042] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0043] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0044] Example:

[0045] Please see Figs. 1-3 The blind hole adhesive detection device in a preferred embodiment of the present invention includes a broadband light source 1, the output end of which is connected to an optical fiber coupler 2, and the optical fiber coupler 2 can split the light emitted by the broadband light source 1 into reference light and sample light; the end of the optical fiber coupler 2 facing away from the broadband light source 1 is connected to a first optical fiber collimator 3 and a second optical fiber collimator 6 respectively; wherein, the first optical fiber collimator 3 is provided with a lens 4 and a reflector 5 in the light transmission path, and the first optical fiber collimator 3, the lens 4 and the reflector 5 are aligned in the optical path, so that the light reflected back by the reflector 5 can be transmitted back to the first optical fiber collimator 3; at the same time, the second optical fiber collimator 6 is provided with a galvanometer 7 in the light transmission path, and a scanning lens 8 is provided in the light output direction of the galvanometer 7, which is directed toward the blind hole of the PCB board 9 to be tested; at the same time, the optical fiber coupler 2 is also connected to a spectrometer 10, which is used to analyze the light returning to the optical fiber coupler 2 to determine the intensity of the coherent light.

[0046] Meanwhile, the broadband light source 1 of the blind hole residual adhesive detection device in this application has a wavelength of 840nm, an output optical power of more than 20mW, a bandwidth of more than 100nm, and an insertion loss of less than 3dB for the fiber coupler 2, the first fiber collimator 3, and the second fiber collimator 6, and an acquisition speed of more than 250kHz for the spectrometer.

[0047] The blind via adhesive residue detection device in this application is based on the Michelson interferometer principle. It utilizes light emitted from a low-coherence broadband light source 1, which is split into reference light and sample light via an optical fiber coupler 2. The reference light is reflected by a mirror 5, while the sample light, upon illuminating the copper plate or adhesive residue within the blind via, generates backscattered or reflected light. When the optical path difference between the reference light and the sample light is within the coherence length of the light source, the two beams interfere, and the spectrometer 10 receives the interference signal. When the sample light strikes the copper plate within the blind via of the PCB board, due to the extremely high reflectivity of the bottom copper layer, and the partial backscattering of the beam upon hitting the adhesive residue, most of the beam continues to penetrate to the copper layer surface, resulting in a significant difference in the intensity of the reflected light striking the copper plate and the adhesive residue. The inspector can determine the presence of adhesive residue within the blind via of the PCB board based on the strength of the interference signal received by the spectrometer 10. Meanwhile, by controlling the wavelength, output power, and bandwidth of the broadband light source 1, this application enables the detection device to detect residual adhesive with a thickness of 0.007 mm to 10 mm, meaning the penetration thickness can reach 10 mm, and the minimum detection thickness can reach 7 μm. The blind hole residual adhesive detection device in this application has good penetration and can detect residual adhesive with a minimum thickness of 7 μm, thus exhibiting high detection accuracy.

[0048] Furthermore, as an optional embodiment of the present invention, the broadband light source 1 in this application is composed of two light sources with a bandwidth of 50nm. The combination of two light sources with a bandwidth of 50nm results in a bandwidth of more than 100nm, which doubles the axial resolution of the system and greatly enhances the ability to identify residual adhesive, enabling the residual adhesive to be identified even when it is very thin.

[0049] Furthermore, as an optional embodiment of the present invention, this application also includes a control component 11, which is communicatively connected to the spectrometer 10 and the galvanometer 7. The control component 11 can acquire the detection parameters of the spectrometer 10 and calculate the intensity of the coherent light accordingly. After calculating the intensity of the coherent light, the intensity of the coherent light is identified and compared with the intensity of the coherent light hitting the copper plate under standard conditions to confirm whether there is residual adhesive in the blind holes of the PCB board.

[0050] Optionally, the broadband light source 1 in this application refers to a laser capable of emitting laser light. The fiber optic coupler 2 in this application is a 2*2 fiber optic coupler 2 with a splitting ratio of 50 / 50. The first fiber optic collimator 3 and the second fiber optic collimator 6 in this application are both used to collimate the laser light emitted from the fiber optic cable and to receive the laser light returning to the fiber optic coupler 2. Furthermore, the broadband light source 1 and the fiber optic coupler 2, the fiber optic coupler 2 and the first fiber optic collimator 3, the fiber optic coupler 2 and the second fiber optic collimator 6, and the fiber optic coupler 2 and the spectrometer 10 are all connected and transmitted via optical fibers; the first fiber optic collimator 3, the lens 4, and the reflector 5 are in the same optical transmission direction.

[0051] Furthermore, regarding the blind via adhesive residue detection device in this application, this application also includes a blind via adhesive residue detection method, which is used to detect adhesive residue inside blind vias of a PCB board, and specifically includes the following steps:

[0052] S1. Select broadband light source 1, and split the light emitted by broadband light source 1 into sample light and reference light through fiber optic coupler 2.

[0053] S2. The reference light is transmitted along the set path and reflected back to the fiber coupler 2 by the reflector 5; the sample light is transmitted along the set path to the blind hole of the PCB board and returns to the fiber coupler 2 after backscattering.

[0054] S3. The returned sample light and reference light interfere with each other in the fiber coupler 2 and form coherent light. The coherent light parameters are read by the spectrometer 10, and the presence of residual adhesive at the irradiation point is determined based on the intensity of the coherent light.

[0055] Furthermore, as an optional embodiment of the present invention, the selection of the broadband light source 1 in step S1 is determined based on the thickness of the residual adhesive to be inspected in the blind vias of the PCB board. Specifically, the wavelength and bandwidth of the broadband light source 1 determine the laser's penetration rate and resolution for the residual adhesive. By adjusting the parameters of the broadband light source 1 according to the requirements, high laser penetration rate and high resolution for the residual adhesive can be achieved.

[0056] Furthermore, as an optional embodiment of the present invention, the thickness of the residual adhesive inside the blind via of the PCB board to be inspected is 0.007mm to 10mm. The wavelength of the broadband light source 1 is determined according to the maximum value of the residual adhesive thickness to be inspected, so that the light emitted by the broadband light source 1 can penetrate the residual adhesive and be received by the fiber optic coupler 2; the bandwidth of the broadband light source 1 is determined according to the minimum value of the residual adhesive thickness to be inspected, so that the residual adhesive can be detected and identified.

[0057] Furthermore, as an optional embodiment of the present invention, the wavelength of the broadband light source 1 in this application is calculated according to the Beer-Lambert law, and the wavelength of the broadband light source 1 is derived from the absorption coefficient of the residual adhesive at a set wavelength.

[0058] Furthermore, as an optional embodiment of the present invention, the absorption coefficient of the residual adhesive at a set wavelength in this application is calculated as follows:

[0059] (Formula 1)

[0060] Where α is the absorption coefficient of the residual adhesive at a set wavelength, d is the penetration depth, I is the light intensity at the laser penetration depth, and I0 is the initial light intensity when the laser is incident on the resin surface.

[0061] Specifically, when a beam of parallel monochromatic light passes perpendicularly through a uniform, non-scattering absorbing material, its absorbance is directly proportional to the concentration of the absorbing material and the thickness of the absorbing layer. Once the type of residue is identified, the wavelength of the light can be derived by working backward from the maximum thickness of the residue to be tested and the absorbance of light within the residue.

[0062] Specifically, the Beer-Lambert law formula is as follows:

[0063] (Formula 3)

[0064] Formula 3 is transformed into solving for the absorption coefficient of the residual adhesive at a set wavelength, resulting in Formula 1.

[0065] Meanwhile, the residual adhesive in this application is epoxy resin, and the absorption level at which the laser attenuates to 36.8% of its initial intensity is set as the level at which the resin penetrates. This allows the absorption coefficient of the epoxy resin residue at a set wavelength to be obtained. Then, by reverse-engineering a reference table of epoxy resin absorption coefficients at different wavelengths, the wavelength of light emitted by the broadband light source is selected.

[0066] Optionally, in this application, the wavelength of the light emitted by the broadband light source 1 is 840 nm, and the penetration depth is calculated accordingly:

[0067] (Formula 4)

[0068] When the penetration depth is greater than 20mm, that is, when the thickness of the epoxy resin residue is within 10mm, the light emitted by the broadband light source 1 can be reflected after entering the residue and can be received by the fiber optic coupler 2.

[0069] Furthermore, as an optional embodiment of the present invention, the bandwidth of the broadband light source 1 in this application is calculated based on the Rayleigh length. The axial resolution (resolution in the direction of residual adhesive thickness) of the light emitted by the broadband light source 1 is mainly determined by the bandwidth of the light emitted by the broadband light source 1. When the minimum resolvable thickness of the residual adhesive is defined, the bandwidth of the broadband light source 1 can be calculated in reverse according to the Rayleigh length formula.

[0070] Specifically, the bandwidth of broadband light source 1 is calculated as follows:

[0071] (Formula 2)

[0072] Where Δλ is the bandwidth of broadband light source 1; λ0 is the center wavelength of broadband light source 1; and Δz is the axial resolution.

[0073] In this application, Δz is defined as 7µm, and the bandwidth of the broadband light source 1 is calculated to be 100nm.

[0074] When the blind via residual adhesive detection method in this application detects the thickness of the residual adhesive, the wavelength and bandwidth of the broadband light source 1 are determined, and epoxy resin is used as the residual adhesive. In this case, the intensity of the correlated light received by the spectrometer 10 is positively correlated with the thickness of the residual adhesive. Therefore, in addition to detecting the presence of residual adhesive in blind vias of a PCB board, this application can also determine the thickness of the residual adhesive in blind vias based on a correlation table between the thickness of the residual adhesive and the intensity of the coherent light.

[0075] Specifically, the blind hole residual adhesive detection method in this application further includes the following steps:

[0076] A table comparing the thickness of the residual adhesive in the blind hole with the intensity of the coherent light was established. The spectrometer 10 acquired the coherent light in the fiber coupler 2 and obtained the intensity of the coherent light. The thickness of the residual adhesive in the blind hole was obtained according to the table comparing the thickness of the residual adhesive in the blind hole with the intensity of the coherent light.

[0077] Specifically, the comparison table of blind hole residual adhesive thickness and coherent light intensity can be obtained by first inserting a set thickness of residual adhesive into the blind hole, and then detecting it through a blind hole residual adhesive detection device.

[0078] Furthermore, as an optional embodiment of the present invention, it is assumed that the residual adhesive in the blind vias of the PCB board in this application is all in droplet form, based on the volume calculation formula for droplets on a plane:

[0079] (Formula 5)

[0080] Given the residual adhesive thickness *h*, the radius *r* of the spherical cap formed by the droplet on the plane can be calculated, and the volume of the residual adhesive can then be determined. This application can also determine the impact of the residual adhesive on subsequent PCB board processing based on the detected spherical cap volume. Generally, when the theoretical thickness of the residual adhesive is less than 7 μm and the overall shape is spherical, even if residual adhesive exists inside the blind via, stable conductivity can still be formed when the external structure contacts the bottom of the blind via. Therefore, this application can further determine whether residual adhesive exists inside the blind via and whether the amount of residual adhesive substantially affects the conductivity of the PCB board based on the thickness and morphology of the residual adhesive.

[0081] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for detecting residual adhesive in blind vias, used for detecting residual adhesive in blind vias of PCB boards, characterized in that, Includes the following steps: S1. Select a broadband light source and split the light emitted by the broadband light source into sample light and reference light through an optical fiber coupler. S2. The reference light is transmitted along the set path and reflected back to the fiber coupler by the mirror; the sample light is transmitted along the set path to the blind hole of the PCB board and returns to the fiber coupler after backscattering. S3. The returned sample light and reference light interfere in the fiber coupler and form coherent light. The parameters of the coherent light are read by a spectrometer, and the presence of residual adhesive at the irradiation point is determined based on the intensity of the coherent light. The thickness of the residual adhesive inside the blind via of the PCB board to be inspected is 0.007mm to 10mm. The wavelength of the broadband light source is determined according to the maximum value of the residual adhesive thickness to be inspected, so that the light emitted by the broadband light source can penetrate the residual adhesive and be received by the fiber optic coupler. The bandwidth of the broadband light source is determined according to the minimum value of the residual adhesive thickness to be inspected, so that the residual adhesive can be detected and identified. The wavelength of the broadband light source is calculated according to Beer-Lambert's law, and is derived from the absorption coefficient of the residual adhesive at a set wavelength. The absorption coefficient of the residual adhesive at a set wavelength is calculated as follows: (Formula 1) Where α is the absorption coefficient of the residual adhesive at a set wavelength, d is the penetration depth, I is the light intensity at the laser penetration depth, and I0 is the initial light intensity when the laser is incident on the resin surface. The residual adhesive is epoxy resin. The resin can be penetrated when the laser attenuates to 36.8% of its initial intensity. The wavelength of the broadband light source is selected according to the absorption coefficient of epoxy resin at different wavelengths. The broadband light source is composed of two 50nm bandwidth light sources. The wavelength of the broadband light source is 840nm. The output optical power of the broadband light source is greater than 20mW and the bandwidth of the broadband light source is greater than 100nm. The acquisition speed of the spectrometer is greater than 250kHz.

2. The method for detecting residual adhesive in blind holes according to claim 1, characterized in that, The wavelength of the broadband light source is 840nm; the blind hole residual adhesive detection method can determine whether there is residual adhesive in the blind hole and whether the amount of residual adhesive will substantially affect the conductivity of the PCB board based on the thickness and morphology of the residual adhesive.

3. The method for detecting residual adhesive in blind holes according to claim 2, characterized in that, The bandwidth of the broadband light source is calculated based on the Ruili length.

4. The method for detecting residual adhesive in blind holes according to claim 3, characterized in that, The bandwidth of the broadband light source is calculated as follows: (Formula 2) Where Δλ is the bandwidth of the broadband light source; λ0 is the center wavelength of the broadband light source; and Δz is the axial resolution.

5. A blind hole residual adhesive detection device, used in the blind hole residual adhesive detection method according to any one of claims 1 to 4, characterized in that, include: A broadband light source, the output of which is connected to an optical fiber coupler, which can split the light emitted by the broadband light source into reference light and sample light; The end of the fiber optic coupler that is away from the broadband light source is connected to the first fiber optic collimator and the second fiber optic collimator, respectively. The first fiber collimator is provided with a lens and a reflector in the optical transmission path. The first fiber collimator is aligned with the lens and the reflector in the optical path, so that the light reflected back by the reflector can be transmitted back to the first fiber collimator. The second fiber collimator has a galvanometer in the optical transmission path, and a scanning lens is provided in the light output direction of the galvanometer, with the scanning lens facing the blind via of the PCB board under test. The fiber optic coupler is also connected to a spectrometer, which is used to analyze the light returning to the fiber optic coupler to determine the intensity of the coherent light. The broadband light source has a wavelength of 840nm, an output optical power greater than 20mW, and a bandwidth greater than 100nm; the insertion loss of the fiber coupler, the first fiber collimator, and the second fiber collimator is less than 3dB, and the acquisition speed of the spectrometer is greater than 250kHz.

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