Method for detecting internal defects of a plastic encapsulated component

Abstract

The application discloses a kind of detection methods for internal defects of plastic package component, it is related to defect detection technical field, comprising the following steps: the pretreatment of plastic package component;Photoacoustic signal acquisition;Time-domain waveform extraction and analysis.The application uses the above-mentioned detection method for internal defects of plastic package component, measures the photoacoustic image of different plastic package components using photoacoustic detection system based on photoacoustic effect, extracts the time-domain waveform at different positions of plastic package component along the propagation direction of laser and ultrasonic wave, realizes the identification, positioning and defect type analysis of internal defects of plastic package component by analyzing the reflection peak position, shape, intensity characteristics of time-domain waveform, distinguishes the defect area and defect type in plastic package component using photoacoustic effect, provides a new means for nondestructive testing of internal defects of plastic package component, and has important significance for improving the reliability of plastic package component.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of defect detection, in particular to a method for detecting internal defects of a plastic package component. BACKGROUND

[0002] Chips need to be packaged in the process to facilitate use and maintenance. At present, plastic packaging is the mainstream process in the integrated circuit market because of its low cost and simple process, which is suitable for mass production. The operation of plastic packaging is to put the chip and other components into a plastic packaging mold, pour plastic packaging materials such as epoxy resin into the mold, and then put the mold into a hot press with high temperature and high pressure to make the plastic packaging material deform and tightly seal the chip and the lead. Since this packaging method belongs to non-airtight packaging, the plastic package component after packaging is prone to internal defects, which reduces the reliability of the component, so the internal defect detection of the plastic package component before use is the key to ensure the long-term and efficient operation of the component.

[0003] The current internal defect detection technology of plastic package components mainly includes X-ray and ultrasonic detection technology. The X-ray detection technology has high detection rate for air holes and slag inclusions, and the defect display method can be directly observed through the negative, but this method cannot detect defects in the vertical irradiation direction, such as delamination. In addition, this method has high cost and is harmful to the human body. The ultrasonic detection technology is sensitive to defects caused by air and has good penetration, and it is harmless to the human body, but it needs to immerse the measured object in a coupling agent such as water, which requires subsequent drying treatment of the plastic package component to avoid secondary damage, and the imaging resolution is low.

[0004] Photoacoustic detection technology based on photoacoustic effect is a new type of non-destructive testing technology. Its basic principle is that when the periodic modulated light or pulsed light irradiates on the measured object, the measured object is periodically heated after absorbing light energy, thereby generating a pressure wave, i.e. photoacoustic signal. Since the lateral range of the generated ultrasonic signal depends on the size of the laser spot, compared with ultrasonic detection technology, photoacoustic detection technology can have higher lateral resolution, about 1-100 μm, under strong focusing of laser; and since the detection result is also affected by the optical absorption characteristics of the measured object, photoacoustic detection has higher imaging contrast than ultrasonic detection using only ultrasonic waves. Considering that the ultrasonic wave generated by photoacoustic effect transmits in the measured object, it is very sensitive to defects caused by air in the measured object, but photoacoustic detection only needs the surface of the measured object to have a layer of ultrasonic coupling agent, avoiding secondary damage or subsequent processing caused by sample immersion in water, and has the advantages of non-destructive, real-time, fast and high sensitivity. Therefore, photoacoustic detection technology based on photoacoustic effect has great application prospect and application value in the internal defect detection of plastic package components. SUMMARY

[0005] The application aims to provide a method for detecting internal defects of a plastic package component, solve the problems in the background art, and realize the determination of the internal defect position, size and defect type of the plastic package component by extracting the time-domain waveforms at different positions on the surface of the plastic package component and comparing them, and analyzing the position, shape and intensity characteristics of the time-domain waveforms.

[0006] To achieve the above-mentioned purpose, the application provides a method for detecting internal defects of a plastic package component, comprising the following steps:

[0007] Step 1: Pretreatment of the plastic package component: drop a coupling agent on the surface of the plastic package component;

[0008] Step 2: Photoacoustic signal acquisition: place the pretreated plastic package component on a test platform, and perform photoacoustic scanning detection on the plastic package component to obtain raw data;

[0009] Step 3: Time-domain waveform extraction and analysis: extract the time-domain waveforms at different positions on the surface of the plastic package component, and the size of the data value represents the strength of the ultrasonic signal at the test point; compare the time-domain waveform data, and determine whether the component has defects according to the position, shape and intensity characteristics of the abnormal time-domain waveforms; if there are defects, further obtain the position of the defects.

[0010] Preferably, the coupling agent covering the surface of the plastic package component in step 1 is ultrapure water or glycerol, which is placed in an ultrasonic vibration device to exclude air bubbles before use, so as to ensure that there is no air bubble between the ultrasonic probe and the plastic package component during the detection process.

[0011] Preferably, the photoacoustic signal acquisition in step 2 is performed by a photoacoustic detection system, which comprises a nanosecond laser source, the nanosecond laser beam of the nanosecond laser source is incident to a laser reflecting mirror through an optical path system, the ultrasonic probe and the reflected laser beam are close to each other, the laser reflecting mirror and the ultrasonic probe are placed on the side of a voice coil motor, the voice coil motor is used for scanning control to realize the scanning of the laser on the surface of the sample to be tested, the laser is focused and irradiated on the surface of the sample to be tested to excite ultrasonic signals, the ultrasonic signals are transmitted forward in the plastic package component, and when encountering an interface, a bubble or a hollow defect, the ultrasonic signals return to be measured by the ultrasonic probe.

[0012] The wavelength of the nanosecond laser source is set to 532 nm or 556 nm, and the single pulse energy and the peak power density will not cause damage to the surface of the sample to be tested, and the repetition frequency is 1 kHz.

[0013] The center frequency of the ultrasonic probe is 60 MHz.

[0014] Preferably, the specific steps of step 3 comprise:

[0015] Step S31, acquiring each column data along the laser and ultrasonic wave propagation direction;

[0016] Step S32, extracting the time-domain waveform at different positions on the surface of the plastic package component, and comparing and analyzing the reflection peak position, shape, intensity characteristics and the like of the time-domain waveform, wherein the abnormal position area of the waveform represents a defect area, and the normal area and the defect area can be distinguished, and the position of the defect is determined according to the position of the abnormal value in the time-domain waveform diagram, including the defect depth and the plane coordinate;

[0017] Step S33, mapping the position information to a two-dimensional plane diagram to form an x-y plane diagram and an x-z profile diagram, obtaining a two-dimensional photoacoustic image showing the defect position, size and shape, and determining the defect type.

[0018] Preferably, the plastic package component is one of an epoxy resin package, a silica gel package, a ceramic package, a glass package and a metal package.

[0019] Preferably, the time-domain waveform of the normal area and the defect area is obtained by a photoacoustic equation under the excitation of the delta (t) pulse energy in the step S32, and the expression is as follows:

[0020]

[0021] Wherein, v s is the transmission speed of the ultrasonic wave in the medium, p(r,t) is the photoacoustic pressure at r at t in the interior of the object to be measured, p0(r') is the photoacoustic pressure at a certain position of the sound source, delta (t) is an impulse function, and is used to represent a laser pulse, is a partial derivative symbol.

[0022] Preferably, the calculation formula of the defect depth in the step S32 is as follows:

[0023]

[0024] Wherein, d is the depth of the defect, v s is the transmission speed of the ultrasonic wave in the medium, N is the number of sampling points in the z-axis direction, and f s is the sampling frequency.

[0025] Therefore, the detection method for the internal defect of the plastic package component has the following beneficial effects:

[0026] (1) The photoacoustic effect is used to realize the internal defect detection of the plastic package component, and the method has the advantages of non-destructive, real-time and fast.

[0027] (2) The application only needs to drop ultra-pure water on the surface of the plastic package component to form a water film, has the advantages of simple operation, avoids secondary damage to the sample and complex subsequent operation, and saves time cost.

[0028] (3) The application determines the defect position by analyzing the time-domain waveform of the photoacoustic effect, and determines the defect size and type by combining the two-dimensional photoacoustic planar image and the sectional image, has the advantages of safety and high sensitivity, and provides a new detection technical means for internal defect detection of the plastic package component.

[0029] The technical solutions of the application will be further described in detail below with reference to the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 It is a structure schematic diagram of the photoacoustic detection system of the embodiment of the application, a method for detecting internal defects of a plastic package component.

[0031] Figure 2 It is a time-domain waveform diagram for photoacoustic detection of the plastic package component, wherein (a) is a time-domain waveform when a defect exists; and (b) is a time-domain waveform of a normal position.

[0032] Figure 3 It is a photoacoustic time-domain waveform diagram of a typical normal area and a defect area of the plastic package component, wherein (a) is an overall waveform diagram; and (b) is a detailed enlarged view.

[0033] Figure 4 It is a photoacoustic image of a typical plastic package component containing a defect, wherein (a) is an x-y planar image; and (b) is an x-z sectional image.

[0034] Figure 5 It is a photoacoustic image of a typical normal plastic package component, wherein (a) is an x-y planar image; and (b) is an x-z sectional image.

[0035] The drawing label: 1, control computer; 2, nanosecond laser source; 3, optical system; 4, laser reflecting mirror; 5, voice coil motor; 6, ultrasonic probe; 7, ultrasonic signal; 8, plastic package component; 9, ultra-pure water film. DETAILED DESCRIPTION

[0036] The technical solutions of the application will be further described in detail below with reference to the drawings and examples.

[0037] Unless otherwise defined, technical terms or scientific terms used in the present application shall have the meanings that can be commonly understood by a person having ordinary skill in the art to which the present application belongs. The terms "first", "second", and similar terms in the present application do not denote any order, quantity, or importance, but are used to distinguish different components. The terms "comprise", "include", and similar terms mean that the elements or objects before the term encompass the elements or objects listed after the term and their equivalents, and do not exclude other elements or objects. The terms "connect" or "connected" or similar terms are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. The terms "upper", "lower", "left", "right", and the like are used only to represent relative positional relationships, and when the absolute positions of the described objects are changed, the relative positional relationships can also be changed accordingly.

[0038] Embodiment

[0039] Please refer to Figures 1-5 The present application provides a method for detecting internal defects of a plastic encapsulated component, comprising the following steps:

[0040] Step one, pretreatment of the plastic encapsulated component 8: drop coupling agent on the surface of the plastic encapsulated component 8. The plastic encapsulated component 8 used for detection is one of the plastic encapsulated components 8 completed by epoxy resin packaging, silica gel packaging, ceramic packaging, glass packaging, and metal packaging. The coupling agent covering the surface of the plastic encapsulated component 8 is ultrapure water or glycerol or other coupling agents, which is used to ensure that there is no air bubble between the ultrasonic probe 6 and the plastic encapsulated component 8 during the detection process. The ultrapure water or glycerol coupling agent is placed in the ultrasonic vibration device to exclude air bubbles in the coupling agent before use. The coupling agent used in this embodiment is ultrapure water. First, a dropper is used to drop ultrapure water on the surface of the plastic encapsulated component 8 to form a layer of ultrapure water film 9, so that the ultrasonic probe 6 can completely cover the surface of the plastic encapsulated component 8 when it covers the surface, and air bubbles are avoided as much as possible during the contact process to ensure that there is no air bubble on the surface of the plastic encapsulated component 8 to be detected.

[0041] Step two, photoacoustic signal acquisition: place the pretreated plastic encapsulated component 8 on the test platform, and perform photoacoustic scanning detection on the plastic encapsulated component 8 to obtain raw data. The photoacoustic detection system is used for photoacoustic image signal acquisition, as shown in Figure 1 The wavelength of the nanosecond laser source 2 is set to 532 nm or 556 nm, and the single pulse energy and peak power density will not cause damage to the surface of the measured object, the single pulse energy is not higher than 50 μJ, and the repetition frequency is 1 kHz.

[0042] The rear of the nanosecond laser source 2 is provided with an optical path system 3, a laser reflecting mirror 4 and an ultrasonic probe 6, the center frequency of the ultrasonic probe 6 is 60MHz. The nanosecond laser beam of the nanosecond laser source 2 is incident to the laser reflecting mirror 4 through the optical path system 3, the ultrasonic probe 6 and the reflected laser beam are close to each other, the laser reflecting mirror 4 and the ultrasonic probe 6 are placed on the side of the voice coil motor 5, the voice coil motor 5 is used for scanning control to realize laser scanning on the surface of the sample to be measured, the laser irradiates the surface of the sample to be measured after focusing to excite an ultrasonic signal 7, the ultrasonic signal 7 is transmitted forward in the plastic package component 8, when encountering an interface, a bubble and a hollow defect, the ultrasonic signal 7 returns to be measured by the ultrasonic probe 6. The photoacoustic detection system and the control computer 1 are connected through a data transmission line.

[0043] The specific steps of photoacoustic signal collection include: (1) after completing the pretreatment, photoacoustic scanning is carried out, first, the position of the object table is adjusted to ensure that the surface of the plastic package component 8 is horizontal and the upper surface is at the laser focusing position; (2) according to the size of the detected plastic package component 8, the probe transverse sampling step is set to 15μm, and the scanning area is set to a square area with a side length of 9cm.

[0044] Step three, time domain waveform extraction and analysis: extract the time domain waveform at different positions on the surface of the plastic package component 8, the size of the data value represents the strength of the ultrasonic signal 7 at the measured point; by comparing the time domain waveform data, according to the position, shape and intensity characteristics of the abnormal time domain waveform, it is judged whether the device has defects, if there are defects, the position of the defects is further obtained. The specific steps of the above step three include:

[0045] Step S31, in the original data processing, the programming software on the control computer 1, such as Matlab, is used to directly extract one-dimensional data, and then reshape it into a three-dimensional data matrix. Get each column of time domain wavelength data in the three-dimensional data matrix along the laser and ultrasonic wave propagation direction.

[0046] Step S32, extract the time domain waveform along the z-axis direction of the plastic package component 8 at different positions on the surface along the laser and ultrasonic wave propagation direction. Since the data results of photoacoustic detection contain depth information, the size of each point value reflects the strength of the photoacoustic signal, therefore, the graph drawn along the z-axis direction is a time domain waveform graph. By comparing and analyzing the position, shape and intensity characteristics of the time domain waveform reflection peak, first, it is judged whether there is a defect in the plastic package component 8 according to the shape of the waveform, if there is a defect, the normal area and the defect area are distinguished, the abnormal position area of the waveform represents the defect area, the position of the defect is determined according to the position of the abnormal value in the time domain waveform graph, including the depth and plane coordinates of the defect.

[0047] Step S33, mapping the position information to a two-dimensional plane, drawing x-y plane and x-z profile to form two photoacoustic images, combining the depth information obtained by time-domain waveform to locate the defects, obtaining two-dimensional photoacoustic images showing the position, size and shape of the defects, and determining the type of the defects.

[0048] The time-domain waveform of the photoacoustic signal under the excitation of the delta (t) pulse energy is obtained by the photoacoustic equation to analyze the time-domain waveform of the normal region and the defect region, and the expression is as follows:

[0049]

[0050] Wherein, ν s is the transmission speed of the ultrasonic wave in the medium, p(r,t) is the photoacoustic pressure at r at t in the inside of the measured object, p0(r') is the photoacoustic pressure at a certain position of the sound source, delta (t) is an impulse function, and is used to represent the laser pulse, is the partial derivative symbol.

[0051] The signal amplitude in the above expression is mainly related to the sound speed, and when there is a defect in the inside of the measured object, it is generally considered that there is a bubble inside, the ultrasonic wave propagates through the epoxy resin, air, and then the epoxy resin, single crystal silicon or metal pin, at this time the sound speed of the medium first decreases and then rises, and then the corresponding signal amplitude first rises and then falls, as shown by the dashed box signal in (a). Figure 2 When there is no defect in the inside of the measured object, the first case is that the medium does not change, so the sound speed does not change, at this time the ultrasonic wave does not reflect, and the time-domain waveform also does not appear a reflected signal, the second case is that other materials in the inside of the medium are detected, here taking a plastic package chip as an example, mainly single crystal silicon or metal pin is detected, so the ultrasonic wave propagates through the epoxy resin, single crystal silicon or metal, and then the epoxy resin, at this time the sound speed of the medium first increases and then decreases, and then the corresponding signal amplitude first decreases and then rises, as shown by the dashed box signal in (b). Figure 2 Figure 2 The first reflected signal appearing in (a) and (b) is the reflected signal caused by the ultrasonic wave entering the surface of the epoxy resin material. Figure 2

[0052] The calculation formula of the defect depth is as follows:

[0053]

[0054] Wherein, d is the depth of the defect, ν s is the transmission speed of the ultrasonic wave in the medium, N is the number of sampling points in the z-axis direction, and f s ​​is the sampling frequency. Since the probe of the photoacoustic detection system receives reflected signals, the detected photoacoustic signals are transmitted according to the path of "generation → transmission in medium → detection of defect → reflection to probe", so the denominator is divided by 2 when calculating the defect depth. In the actual detection system, the number of sampling points in the z-axis direction is 1280, and the sampling frequency is 500 MHz.

[0055] This embodiment is the detection result of a plastic encapsulated component 8 with internal defect and a normal plastic encapsulated component 8 based on photoacoustic imaging. The two components are the same type and have the same size. The length, width and height of the epoxy resin encapsulation region are 10.43 mm, 7.41 mm and 2.19 mm respectively, and the actual figure is shown in Figure 3 (a). The test region is shown by the dashed square region in the figure.

[0056] Figure 3 (a) is the time-domain waveform diagram of the defect detection of the plastic encapsulated component 8 based on photoacoustic imaging. As can be seen from the figure, the reflection signals appearing in the depth range of 1.38-1.87 mm are derived from the reflection of the upper surface of the plastic encapsulated component 8; the second reflection signal appears in the depth range of 2.20-2.35 mm, and the waveforms are different. Figure 3 The second reflection signal in (a) is enlarged, as shown in Figure 3 (b). As can be seen from the figure, the time-domain waveform diagram represented by the light gray curve appears reflection signals in the depth range of 2.25-2.35 mm, and the reflection signal strength first decreases and then increases, which is caused by the incidence of ultrasonic waves from the epoxy resin to the encapsulated single crystal silicon; the time-domain waveform diagram represented by the black curve appears abnormal reflection signals in the depth range of 2.20-2.30 mm, and the reflection signal strength first increases and then decreases, which is caused by the incidence of ultrasonic waves from the epoxy resin to the internal air bubble. Since the acoustic impedance of air is smaller than that of epoxy resin, and the acoustic impedance of single crystal silicon is greater than that of epoxy resin, the waveforms of the reflection peaks caused by the defect region and the normal region in the plastic encapsulated component 8 present opposite shapes, wherein the waveform of the defect region presents a waveform that first increases and then decreases (also called positive peak), and the waveform of the normal region presents a waveform that first decreases and then increases (also called negative peak).

[0057] According to the photoacoustic time-domain waveform data, x-y plane and x-z cross-section graphs are drawn. Figure 4 (a) and (b) are the photoacoustic images of the defective plastic encapsulated component 8. Considering that Figure 3 the defect depth is about 2.25 mm as shown in Figure 4 (a) is the x-y photoacoustic plane graph at the depth of 2.2-2.3 mm, and as can be seen from the figure, there is an obvious abnormal region around the chip in the photoacoustic image, as shown by the part surrounded by the dashed line, so the size and range of the defect in the x-y plane can be determined. Figure 4(b) is the x-z photoacoustic profile at the y coordinate of 6.72 mm, and the defect position is in the dashed box, so the size and range of the defect in the x-z plane can be determined. In combination with the waveform analysis, the defect position on the waveform is close to the reflection signal position of the normal component, and the defect is located at the periphery of the chip from the x-y image, and the defect shape is a layered defect from the x-z image, and the defect type is a full-enclosed layered defect, which shows that the photoacoustic detection technology can complete the positioning and type judgment of the internal defects of the plastic package component 8.

[0058] Figure 5 (a) and (b) are photoacoustic images of the normal plastic package component 8. Considering that Figure 3 The chip depth shown in the above embodiment is about 2.3 mm, Figure 5 (a) is an x-y photoacoustic plane at a depth of 2.25-2.35 mm, and (b) is an x-z photoacoustic profile at a y coordinate of 6.54 mm. Figure 4 Compared with (a), no obvious abnormal area appears around the center chip. Figure 5 (b) is an x-z photoacoustic profile at a y coordinate of 6.54 mm, and (a) is an x-y photoacoustic plane at a depth of 6.54 mm. Figure 4 Compared with (a), Figure 5 The image in the dashed box in (b) has a significant difference, which shows that the photoacoustic detection technology realizes the internal defect recognition, positioning and defect type analysis of the plastic package component 8.

[0059] Therefore, the present application adopts the above-mentioned detection method for internal defects of a plastic package component, measures photoacoustic images of different plastic package components by using a photoacoustic detection system based on photoacoustic effect, extracts time-domain waveforms at different positions of the plastic package component along the propagation direction of laser and ultrasonic waves, realizes the recognition, positioning and defect type analysis of internal defects of the plastic package component by analyzing the reflection peak position, shape and intensity characteristics of the time-domain waveforms, and distinguishes the defect area and defect type inside the plastic package component by using photoacoustic effect, thereby providing a new means for nondestructive testing of internal defects of the plastic package component, and having important significance for improving the reliability of the plastic package component.

[0060] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application rather than limit them, and although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can still be modified or replaced by equivalents, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present application.

Claims

1. A method for detecting internal defects in plastic-encapsulated components, characterized in that, Includes the following steps: Step 1: Pretreatment of plastic-encapsulated components: Apply coupling agent to the surface of the plastic-encapsulated components; Step 2, Photoacoustic signal acquisition: Place the pre-processed plastic-encapsulated components on the test platform and perform photoacoustic scanning detection on the plastic-encapsulated components to obtain raw data; Step 3: Time-domain waveform extraction and analysis: Extract time-domain waveforms at different locations on the surface of the plastic-encapsulated component. The magnitude of the data value represents the strength of the ultrasonic signal at the test point. By comparing the time-domain waveform data, determine whether the component has defects based on the location, shape, and intensity characteristics of any anomalies in the time-domain waveform. If defects exist, further determine the location of the defects. Plastic-encapsulated components are one type of plastic-encapsulated component completed by epoxy resin encapsulation, silicone encapsulation, ceramic encapsulation, glass encapsulation, or metal encapsulation; In step two, photoacoustic signals are acquired using a photoacoustic detection system. The photoacoustic detection system includes a nanosecond laser source. The nanosecond laser beam from the nanosecond laser source is incident on a laser reflector through an optical path system. The ultrasonic probe is brought close to the reflected laser beam. The laser reflector and the ultrasonic probe are placed to the side of a voice coil motor. The voice coil motor is used for scanning control to realize the scanning of the laser on the surface of the sample to be tested. After being focused, the laser irradiates the surface of the sample to be tested and excites an ultrasonic signal. The ultrasonic signal is transmitted forward in the encapsulated component. When it encounters interface, bubble, or void defects, the ultrasonic signal returns and is measured by the ultrasonic probe. The wavelength of the nanosecond laser source is set to 532nm or 556nm, and the single pulse energy and peak power density will not damage the surface of the object to be tested, with a repetition frequency of 1kHz. The center frequency of the ultrasonic probe is 60MHz; The specific steps of step three include: Step S31: Obtain data for each column along the propagation directions of laser and ultrasound; Step S32: Extract time-domain waveforms at different locations on the surface of the plastic-encapsulated component, and compare and analyze the position, shape, intensity characteristics, etc. of the reflection peaks of the time-domain waveforms. Abnormal waveform locations represent defect areas, thus distinguishing between normal and defect areas. Determine the location of defects based on the location of abnormal values ​​in the time-domain waveform diagram, including defect depth and planar coordinates. Step S33: Map the location information onto a two-dimensional planar diagram to form an xy-plane diagram and an xz-section diagram, obtaining a two-dimensional photoacoustic image showing the location, size, and shape of the defect, and determining the defect type; δ(t) In step S32, the time-domain waveforms of the photoacoustic signal under δ(t) pulse energy excitation are obtained through the photoacoustic equation to analyze the normal and defective regions. The expressions are as follows: Among them, v s Let p(r,t) be the propagation speed of ultrasound in the medium, p(r,t) be the photoacoustic pressure at point r inside the object under test at time t, p0(r') be the photoacoustic pressure at a certain location of the sound source, and δ(t) be the impulse function used to represent the laser pulse. The sign for partial derivatives; In step one, the coupling agent covering the surface of the encapsulated component is ultrapure water or glycerin. The ultrapure water or glycerin is placed in an ultrasonic vibration device to remove air bubbles before use, in order to ensure that there are no air bubbles between the ultrasonic probe and the encapsulated component during the testing process. The formula for calculating the defect depth in step S32 is as follows: Where d is the depth of the defect, v s Let f be the speed of ultrasonic wave propagation in the medium, N be the number of sampling points along the z-axis, and f be the speed of ultrasonic wave propagation in the medium. s The sampling frequency.

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