Research Status and Progress on Non-Destructive Testing Methods for Defect Inspection of Micro-Electronic Packaging

Integrated circuit (IC) industry is the core of information technology industry and a strategic, basic and leading industry supporting economic and social development and ensuring national security. Micro-electronic packaging is critical for the manufacturing process of integrated circuits. The tendency of micro-electronic packaging toward three-dimensional (3D) system integration, miniaturization and high-density, makes stacked-die packages, and high-density packages based on flip chip interconnection become the mainstream processes of IC packages. Up to now, the wafer thickness is below 40 μm, and bump spacing is below 15 μm. Then, the minimum thickness of 3D packaging is less than 0.1 mm, the diameter of its internal interconnection is about 5–10 μm, and the I/O density of flip chip is more than 5000/cm². Advanced 3D packages and system in packages have become the trend of micro-electronic packaging technology to meet portable electronics demand of ultrathin, ultralight, high-density, high performance with low power consumption and low cost. Three-dimensional packaging will play a very important role in consumer electronics (especially mobile phones, tablet computers), robotics, biomedicine, and other fields in the future [1–3].

Advanced complicated packaging brings a great challenge to package reliability and quality control in the manufacturing process. The bonding quality directly affects the reliability of micro-electronic devices, and microdefects are crucial to the bonding quality. Most common microdefect types of micro-electronic packaging are delamination, cracks, voids, ball missing, and other defects, which will further extend under the combined action of temperature, humidity, and stress field during operation of the electronics, and lead to failure of electronic devices. For instance, a tiny debond defect and void in the stacking layer inside the chip package, or a microcrack in flip chip bonding, may lead to failure of electronics in use. Especially for electronic systems used in harsh environment, such as automotive electronics, aerospace electronics, etc., internal micromechanical defects and fatigue damage may cause a serious hidden danger [4,5]. Therefore, defect inspection is essential to improve the reliability of micro-electronic packaging. Non-destructive testing (NDT) methods have been widely used in reliability testing of micro-electronic packaging. However, advanced packaging technologies such as flip chip, package on package, wafer level package, through-silicon-via, system in packages, etc., bring a massive challenge to conventional NDT methods for internal defect detection of micro-electronic packaging. Thus, a breakthrough technology is required to measure and inspect these advanced micro-electronic packaging [6].

In this paper, the conventional NDT methods for defect inspection of micro-electronic packaging have been mentioned, and the principles, characteristics, and limitations of each method have been discussed. Considering the trend and demand of micro-electronic packaging technology in the future, a novel hybrid ultrasonic-laser digital holographic microscopy (DHM) imaging inspection approach is discussed in detail, so as to solve the problem of defect inspection and evaluation of advanced micro-electronic packaging.

For defect detection and evaluation in micro-electronic packaging, the conventional non-destructive detection methods mainly include optical visual inspection, X-ray inspection, scanning acoustic microscopy, atomic force microscopy, laser Doppler vibration measuring technique, active infrared thermography, scanning SQUID microscopy, electrical impedance spectroscopy, and scanning electron microscope.

As a common detection technology, optical imaging has been widely applied to defect detection and assessment of micro-electronic packaging, mainly including optical two-dimensional (2D) visual detection and 3D structured light visual detection. Optical 2D visual detection method uses a high-resolution camera to capture the clear images of the chips through optimizing the parameters of the light source. The detection system consists of the lighting system, the imaging system and the motion system. The tested chip is placed in the camera's field, and its clear image is obtained by setting the light source. Then, the structure and process characteristics of chips are calculated by using the relevant image processing algorithms to determine whether there will be defects after chip packages [5,7]. Optical triangulation is the core principle of 3D structured light visual detection. The beam emitted by the laser source is projected on the surface of the measured object, forming a light stripe whose shape is related to the surface morphology of the object. The reflected light stripe captured by the charge-coupled device (CCD) or complementary metal-oxide-semiconductor camera is processed by coordinate transformation and the 3D structure features of the measured object can be obtained by image processing. Figure 1 shows the configuration of 3D structured light visual detection system [8]. By using the full-field 3D measurement method and precise phase-shift control, the measurement results of sample of flip-chip solder balls in the larger inspection area of 21 × 21 mm² and with a fringe period of 28 pixels are shown in Fig. 2. Experimental results showed that the measurement accuracy can reach the micron level, and the computation time for a 640 × 480 image that contains almost 500 solder bumps is less than 1 s [9].

Optical imaging has the advantages of high-speed and high-resolution, especially for optical microscopic imaging [8–10]. Moreover, it can realize the on-line testing of micro-electronic packaging, and its optical path is simple. However, optical visual inspection methods can only be used for testing the defects of unpackaged chips and sample surface, and cannot detect the internal structure and defects of samples [5,11,12]. With the tendency of micro-electronic packaging technology toward high density, optical visual inspection method will be faced with the big challenges and difficulties, and it is difficult to access the solder joint defects.

Scanning acoustic microcopy (SAM) is an effective high-resolution non-destructive tool for defect detection and evaluation of micro-electronic packaging, which has been widely applied to detect and assess the inner defects of packages, such as debonding, crack, void, and so on. It is also a useful technique for imaging, location, and size distribution of defects in different micro-electronic components. In addition, it can measure and detect the thickness of layers, air gap defects in micro-electronic packaging [13], and its principle is shown in Fig. 3. SAM method utilizes a piezo-electric probe to generate the ultrasonic waves with a specified frequency, which are focused and transmitted to the sample through the acoustic lens and couplant (usually de-ionized water). When the ultrasonic waves propagate through the sample, they can reflect, refract, and scatter at the internal interfaces or defects. The reflected ultrasonic waves received by the probe carry the internal feature information of the sample, and are displayed as pixels with defined gray values [3,5,14–16]. Utilizing the probe to scan the whole detection area of the tested sample in the scanning plane, a tomography image of internal structures and defects of the sample are formed. In general, SAM provides three types of scan modes: A-scan, B-scan, and C-scan. A-scan shows a reflection signal from the internal structure of the sample at a single measurement point. According to the velocity of ultrasonic wave and the travel time in the tested sample, the location of internal defects or the thickness of the sample can be estimated from the A-scan graph. By the probe scanning the sample over the line, the B-scan image can be obtained, which can be used to assess the cross-sectional shape and the location of the defects. C-scan mode can obtain the images of the planes, which are parallel to the surface of the sample and from different depths in the sample [3,13,17].

In terms of SAM method for defect inspection and assessment on micro-electronic packaging, a large number of theoretical research and experimental work have been carried out by many researchers and scholars. Xu et al. developed a SAM detection system, which can be used to detect internal microdefects in micro-electronic packaging and evaluate the welding quality of the pins [18–20]. Su et al. [4,14,21], Fan et al. [16], Wang et al. [22], and Yu et al. [23] studied the defect detection and identification methods of flip chip based on SAM technology. Lee et al. [24,25], Yang et al. [26], and Olumide et al. [27] developed a super resolution 3D acoustic microscopic imaging technology, which has been widely recognized internationally in the fields of reliability testing of modern micro-electronic packaging and research of SAM. Brand et al. applied GHz-SAM to inspect and assess the defects like voids and delaminations in 3D-integration micro-electronic components [28,29]. Figure 4 gives the acoustic micrographs of Cu–Cu wire bonding contacts obtained by SAM. As can be shown from Fig. 4(b), the shape and the interface area of the contacts achieve a high resolution of approximately 1 μm. Figure 4(c) shows a high-resolution zoom-in of two wire bonds by the rectangle in the center graph. It is noted that the ball bond interface is not homogeneous, but shows bright spots of small dimensions. These features can represent voids or small delamination areas in the bond interface [29].

Scanning acoustic microscopy method is an efficient method for detecting the thickness of layers and delamination inside material, underfill delamination, and void defects in micro-electronic packaging. However, resolution and penetration are two technical problems for using SAM to detect advanced micro-electronic packaging. SAM is a far-field imaging technique, and its axial resolution is limited by the ultrasonic wavelength. The demand of axial resolution for detecting the defects at multiple bonding interfaces of the stacked layered structure of silicon wafers inside the packages has exceeded the SAM resolution limit [30–32]. In addition, its lateral resolution mainly depends on the frequency of ultrasonic probe. The lateral resolution of 175 MHz probe is 8 μm, and its penetration is only 0.3 mm for advanced complicated packages. At present, the lateral resolution of GHz (up to 2 GHz) probe can be 1 μm and below. But its penetration will reduce drastically, and as a result, the lateral resolution and penetration of SAM cannot simultaneously meet the detection requirements of advanced micro-electronic packaging [29,33]. Another limitation of SAM is that it requires a couplant to propagate acoustic energy from a piezo-electric probe to the tested sample, and the couplant is usually de-ionized water. Therefore, SAM is not suitable for in-line application.

X-ray imaging technology is another common NDT technique for defect inspection of micro-electronic packaging according to the characteristics of the X-ray beams being absorbed by different materials along the path. It is divided into 2D X-ray detection and 3D X-ray computed tomography (CT) techniques.

Two-dimensional X-ray imaging technique has been widely used in the micro-electronic packaging industry to inspect various parts of IC packages and detect hidden defects, such as voids, delaminations and cracks in micro-electronic packaging by providing top-down views of the samples [3]. Fein Focus X-ray imaging instrument can provide the lateral resolution up to 1 μm, while microfocus and nanofocus X-ray systems have achieved the resolution at the submicron level. The latest developed soft X-ray inspection system can reach the sub-15-nm spatial resolution [34]. Nevertheless, 2D X-ray imaging technique is the projection of a 3D tested sample, thus the above inspection systems have no 3D imaging capability (namely, no axial resolution), which cannot provide a thorough assessment of the internal defects of micro-electronic packaging.

Three-dimensional X-ray CT is a very powerful tool for detecting the defects of 3D packages in micrometer or submicrometer, which can provide the detailed 3D images about the internal structure of tested sample such as 3D micro-electronic packaging [3]. As shown in Fig. 5, 3D X-ray CT exploits the penetrating power of X-rays to obtain a series of 2D images of the sample at each angle through the full 180 deg rotations with a scanner providing angular displacement in equally spaced angles [35]. A computed reconstruction algorithm is then used to create a stack of cross-sectional slices from all the 2D images of the sample, and a 3D image of the internal structure of the sample is obtained, which is utilized to analyze the location and size of internal defects of the tested sample [2,36]. 3D X-ray CT technique can provide the high-resolution 3D information of micro-electronic packaging nondestructively and have been applied to inspection and characterization of defects inside micro-electronic packaging [35,37,38]. Figure 6 demonstrates that 3D X-ray CT distinctly captures voids and cracks in small substrate Cu vias [2]. The recently developed 3D X-ray CT system can offer an extremely high-resolution of 5 nm [39]. Nevertheless, 3D X-ray CT technology needs to cut the tested sample into small samples for imaging detection, so the samples are destroyed. Moreover, it has some drawbacks, such as low imaging efficiency, high-cost equipment and operation. X-ray imaging technology cannot detect the closed cracks and delamination defects which are perpendicular to X-ray beams inside the tested sample. In addition, it also requires safety protection measures and environmental standards in use, which limits its practical application [5,40].

Conventional atomic force microscopy (AFM) is a type of scanning-probe microscopy technology used for imaging detection for the surface topography of a sample with the lateral resolution of a few nanometers and the axial resolution below 0.1 nm [41], but its scanning speed is extremely low. High-speed atomic force microscopy can reach an axial resolution of ∼2 nm and a lateral resolution of ∼0.15 nm, which dramatically improves its scanning speed [42,43]. Atomic force acoustic microcopy can also carry out nanometer-scale mechanical imaging of the elastic properties and subsurface defects of the IC test structure. Atomic force acoustic microcopy uses the nonlinearity of the mechanical interaction between the probe tip and the sample to measure the changes of the resonance frequency of the cantilever caused by the changes of contact stiffness of the sample, imaging subsurface defects of the sample [44–46]. Even though AFM and the related technologies can offer the nanoscale resolution, it only scans a tiny area (typically, a few micrometers by a few micrometers), not detecting the internal defects of micro-electronic packaging because of low imaging efficiency and poor penetration.

Shekhawat et al. [47–50] developed a scanning near-field ultrasound holography (SNFUH) technique based on AFM, which provides depth information as well as spatial resolution at the 10- to 100-nanometer scale. In SNFUH, at first two near-field ultrasonic probes are used to generate a stationary acoustic field on the surface of the sample, and then the phases and amplitudes of the acoustic field are measured by scanning the sample with the AFM probe. Finally, the phases and amplitudes of the surface acoustic waves are applied to image the internal structures of the sample. Conventional AFM can only image the surface defects of the sample, and yet SNFUH technique not only obtains high spatial resolution but also achieves the subsurface defect image of the sample. SNFUH technique has been successfully applied to inspect the buried defects and voiding for copper interconnect structures in semiconductor chips [47], as shown in Fig. 7. Because SNFUH is an AFM based 2D imaging technique, it scans a tiny area of the tested sample, and its imaging speed is very slow. Thus, it cannot offer 3D defect image, only detecting the subsurface defects, not suitable for internal defect inspection of micro-electronic packaging.

In terms of evaluation and detection of defects in high-density micro-electronic packaging, a lot of theoretical and experimental studies have been carried out, and several novel NDT methods have been proposed.

Based on the laser Doppler vibration detection technique, Yang et al. [51] developed a laser ultrasound-interferometric inspection system, as shown in Fig. 8. It has been successfully applied to detect solder bump defects including missing, misaligned, open, and cracked solder bumps in flip chip packages, chip-scale packages, and land grid arrays. In this system, a pulsed Nd: YAG laser as an excitation source generates the laser pulses that are delivered to the tested sample surface through optic fibers. The laser pulses are used to induce ultrasound in the sample in the thermoelastic regime, and the transient out-of-plane displacement responses in nanometer scale with an ultrasonic arrival on the surface of the sample is measured by a laser Doppler vibrometer. Modal analysis for the measured laser ultrasound signals is performed to inspect and identify the defects inside the sample by means of finite element simulation and signal processing methods, such as time-domain error ratio analysis, frequency-domain spectral analysis, correlation coefficient analysis, wavelet analysis, signal feature extraction and pattern recognition, etc. [51–54]. The latest developed dual beam laser ultrasonic inspection system can generate high intensity ultrasound, which can produce a significant increase in the out of plane vibrations of the tested sample. So, this will improve the signal to noise ratio and measurement sensitivity, and it has been successfully applied to assess the reliability of solder balls in the multilayered flip chip packages, as shown in Fig. 9. For two samples, the measurement locations are selected on the chip surface on top of solder bumps. Solder bumps are marked as large circles and the stars denote the test positions, with the excitation laser point indicated by an ellipse, and Figs. 9(c) and 9(d) show the power spectra of time-domain signals [51]. By increasing the laser power and resolution of the inspection pattern, the sensitivity and accuracy of the system can be further improved. Therefore, the system has the potential to be used to evaluate defects in all packages, including 3D packages, by using two-fiber array beam probe [55].

Su et al. [56] developed an ultrasonic excitation inspection system for flip chip defects based on the laser Doppler vibration detection technique, as shown in Fig. 10. In this system, the signal generator is used to generate the electrical signals that are magnified by the power amplifier. The amplified signals are converted into ultrasonic signals by the air-coupled ultrasonic transducer, forced on the surface of the flip chip to excite the structural vibrations of the chip. The vibration signals are measured and sampled by the laser scanning vibrometer, and are stored by industrial control computer. Based on modal analysis and finite element simulation, the vibration signals are analyzed by means of signal processing techniques, such as genetic algorithm, principal component analysis, back propagation networks, support vector machine, radial basis function networks, etc., achieving detection and recognition of various defects in the flip chips [56–62]. Figure 11 shows the second-order modal shapes of chips with open solder joints. It can be observed that the defective chips with open solder joints is different from the good chips by the modal shape differences represented by the characteristic angles [58].

The laser ultrasonic-interferometric and air-coupled ultrasonic excitation inspection systems can offer high accuracy for flip chip defect detection in a fast, accurate, low-cost, and efficient way. However, the laser Doppler vibration detection technique mainly applies the extremely weak vibration signals to perform modal analysis, only detecting the defects of flip chips with large scale and large spacing. With the trend of flip chips toward ultrafine pitch and high density, the changes of the excited vibration signals caused by the defects in the tested sample are weaker and hard to assess. Moreover, this technique is also difficult to identify the location of defects.

Active infrared thermography inspection technique is a non-destructive, rapid, and full-field NDT technique, which is also widely applied to defect inspection of micro-electronic packaging. In active infrared thermography, an external heat source is used to stimulate the tested sample. During the thermal transmission process, the changes of the transient heat conduction characterization of the sample caused by the defects in the tested sample can result in different temperature distribution. Then, an infrared thermal imager is used to capture the temperature distribution of the sample surface, and the defects are identified utilizing the thermal images combined with the image processing algorithms [63].

Chai et al. applied active thermal infrared imaging for the detection and fault isolation of solder joint defect in high density flip chip package using a thermal imager, and put forward an active infrared detection method based on electrical current heating. This method utilizes the principle of active joule beating by electrical current passing through a potential defective interconnection in a daisy chained package. The electrical current passes through the daisy chained chip. When there exist the defects in the tested chip, the resistance of the defective solder joint is significantly higher than that of the normal solder joint, leading to different temperature distribution. Hence, the temperature distribution of the surface of the chip is captured by infrared thermal imager to form the thermal image, the existence and location of defects can be inspected and identified by using the image processing algorithms. This method is only suitable for voids and partial cracks detection, and moreover, the electrode pairs are required to contact the tested chip, which is not convenient in non-contact detection [64].

Xu et al. [63] developed a non-destructive detection system for identifying the defects of solder joints based on the pulsed phase thermography, and its inspection process is shown schematically in Fig. 12. In this system, the tested chip is heated by a non-contact heating source, and the internal thermoresistance differences of the chip caused by defects can lead to abnormal thermal distribution, which is captured by an infrared thermal imager. Figure 13 shows the temperature distribution image on the front side of the chip. It can be seen that it forms a hot spot over the missed bump due to the insufficient heat transfer path, and a dark over the cracked bump. For the void, a ring distribution is generated and the center temperature of the solder joint is higher than the surrounding area [65]. The obtained temperature distribution image is analyzed to identify the defects by using adaptive median filtering, image segmentation, principal component analysis, fast Fourier transform, K-Means algorithm, self-reference technology, improved fuzzy C-means clustering, modified support vector machine, probabilistic neural network, and improved tiny-YOLOV3 network [63,66–72].

Active infrared thermography inspection technique is effective for detection and identification of the defects in flip chips. However, with the development of micro-electronic packaging technology, the size of solder balls/bumps as well as the pitch are getting smaller and smaller, and the spatial resolution of infrared thermal imager is more difficult to meet the requirements of defect detection.

Scanning SQUID microscopy (SSM) is a powerful tool for imaging magnetic fields above sample surfaces with the advantages of high sensitivity and bandwidth. Magnetic field generated by the input current inside the failed unit is detected and processed using a Fourier transform inversion technique to obtain current density map of the sample [2]. It has been successfully applied to localize current leakages, short faults, and high resistance defects in micro-electronic packaging. However, the early SSM also has some limitations, such as relatively low spatial resolution, the requirement of a cooled sensor [73,74].

Dechert et al. developed a scanning SQUID microscope for measurements of samples at room temperature, using a low T_C SQUID sensor with the spatial resolution of 50 μm [75]. The later developed high-temperature superconductivity-SQUID microscope can image the samples at room temperature and atmospheric pressure, and it can obtain a spatial resolution of several tens of microns for detecting the packaging of a micro-electronic device [76]. Laser-SQUID microscopy is detection of a magnetic field produced by a laser-beam-induced current by using an high-temperature superconductivity-SQUID magnetometer. The intensity of magnetic flux detected by the SQUID is imaged, while scanning a laser beam and a sample relatively. This method can localize open defects, short defects, or more complicated defects on a large scale integrated chip, achieving micron-scale spatial resolution [77,78]. In addition, it can be used to detect a room temperature sample without any special treatment. In fiber-SQUID configuration, a magnetic fiber about the diameter of a human hair that can be etched to a 10 nm tip is coupled to a SQUID, and the fiber can be brought very close to the surface of the sample. Experimental results showed that fiber-SQUID can provide a resolution of about 10 μm [74]. SQUID photoscanning method is to detect photogenerated currents via their magnetic field by means of sensitive SQUID magnetometers, which enables the non-invasive evaluation of semiconductor wafers and photovoltaic devices, and allows for the localization of artifacts in photovoltaic devices [79]. Leitão et al. provided an overview on several techniques used for surface imaging, mainly including SQUIDs and magnetoresistive (MR) sensors, which has the potential to localize buried and non-visual field defects in integrated circuits. Compared to the widely used SQUID devices, MR sensors have the advantage of being relatively low cost and of easier implementation [80]. Although SSM/MR can obtain a micron-scale resolution, submicron spatial resolution is still a big challenge. Moreover, it has several limitations, including time-consuming, high-cost equipment, harsh experimental conditions, etc.

Electrical impedance spectroscopy is a useful and non-destructive tool for the diagnostics of the health and degradation of micro-electronic systems by measuring the linear electrical response of a low electromagnetic signal in a wide range of frequency with a sine wave [81], including time-domain reflectometry (TDR), S-parameter spectra in radio frequency (RF) circuits, and so on. The TDR method sends an electrical pulse and detect change in geometry or electromagnetic properties along a controlled-impedance transmission line that can be embedded in or bonded to a material or a structure. The presence and propagation of crack cause great changes of these properties and hence it is easily detected [82]. Kwon et al. presented a TDR-based sensing method for interconnect failure mechanisms of electronics such as solder joint cracking and solder pad cratering. The test results demonstrated that the TDR reflection coefficient can be serve as a non-destructive means to determinate the interconnect failure mechanisms and to detect early stages of solder joint cracking [83]. The sequential comparative TDR can be used for isolating defects (shorts, opens, resistance defects) in complex IC packages by means of the comparison of the waveform acquired from the failing device to those acquired from a good device [84]. Matsumoto et al. applied TDR to the failure analysis of through-silicon vias, and the results showed that the failure points in through-silicon-vias can be identified using the TDR signals [85]. However, the application of TDR is limited by the bandwidth of probes (leading to a low spatial resolution), and baseline data are needed to evaluate the presence of defect.

The S-parameters can describe the electrical behavior of electrical networks when undergoing various steady-state stimuli using electrical signals, so the performance variation and defects of the RF circuits could be analyzed and detected by measuring the S-parameters [86]. Pascal et al. used S-parameters and TDR measurements to monitor the degradation of power electronics, which opens a new method for online condition monitoring of power modules [87]. Belaïd evaluated the hot-carrier reliability of the power RF devices by monitoring S-parameters before and after degradation [88]. Nevertheless, S-parameter method is mainly applied to monitor the degradation performance of the RF power electronics, and the interpretation of the measurement results is highly intricate due to the complexity of the power module structure.

Scanning electron microscope (SEM) is a high-resolution imaging analysis tool that provides a high magnification and a large depth of field, with the advantages of intuitive imaging and a strong sense of stereo. It has been widely applied in the field of the failure analysis of micro-electronic packages, such as microstructural characterization, intermetallic thickness measurement, electromigration damage evaluation, subtle defects detection (crack, void, delamination, etc.), buried electronic interfaces, revealing the details of the intermetallic compound, material analysis, and so on [2,89–94]. Although providing spatial resolution down to 1 nm, SEM can only be used to observe the surface morphology of the sample, and the structures below the surface cannot be detected. Moreover, it has some disadvantages of sample to be placed in vacuum environment, high operating, and maintenance costs. Therefore, SEM is commonly adopted to verify the detection results of other NDT methods.

With the trend of micro-electronic packaging technology toward system integration, miniaturization, high density, and ultrathin, the detection methods for embedded defects of micro-electronic packaging will suffer from a tremendous challenge. Especially, the quality evaluation of high-density bump flip chip bonding interface with submicron-scale defects is a difficult problem for the existing industrial NDT methods. According to international technology roadmap for semiconductors [6], a technology gap for NDT has been identified between a voxel size from tens of nanometer to a few micrometers, as shown in Fig. 14. Thus, NDT techniques for detecting the debond defects and voids between multiple layers and tiny cracks inside bonding bump of advanced 3D packages meet at least two requirements: high-resolution (up to submicron) and large penetration depth (more than a few millimeters). However, the current NDT methods for defect inspection of micro-electronic packaging cannot meet the above requirements at the same time [95,96]. Therefore, such a submicron resolution non-destructive inspection technology is urgently needed to solve the difficult problem of the safety and reliability, and quality control in the manufacturing process of micro-electronic packaging.

Since the evanescent wave in near-field ultrasound overcomes the diffraction limit of conventional far-field ultrasonic imaging, the lateral resolution of near-field ultrasonic imaging is no longer limited by ultrasonic wavelength. SNFUH technique has shown that a few-MHz probe can also achieve nanoscale lateral resolution for near-field ultrasonic imaging [31], so a low acoustic frequency can be used to increase the penetration without sacrificing the lateral resolution. As the latest optical holographic imaging technique, DHM can provide the lateral resolution in submicron level and the axial resolution in nanometer level [97,98]. The simulation results show that the amplitude of ultrasonic wave propagated to the chip package surface can easily achieve 1 μm, and reach more than 10 μm through increasing the excitation power of ultrasonic probe. Thus, compared with 1 μm ultrasonic amplitude, the nanoscale axial resolution fully meets the accuracy requirement of measuring the ultrasonic wavefield on the sample surface. The recently developed pulsed laser based lensless digital holographic microscopy is a promising technology for measuring the dynamic wavefield of high-frequency ultrasound on solid surface.

To overcome the problem that the existing NDT methods cannot simultaneously meet the requirements of high resolution and large penetration depth, Ma et al. [99,100] proposed a novel hybrid ultrasonic-laser DHM imaging inspection approach, combining the high lateral resolution and full field detection capability of DHM with the penetration capability and high resolution without diffraction limit of near-field acoustics. As shown in Fig. 15, its basic principle is as follows: an opaque solid tested sample is put on top of a piezo-electric acoustic transducer that generates a single frequency short-pulse ultrasonic wave. The near-field ultrasonic wave propagates through the sample and arrive at the sample surface. These ultrasonic wavefields carry information about the internal structures, internal defects, and materials of the sample. Then the dynamic ultrasonic wavefields on the surface of the sample are measured by the optical DHM acting as the receiving transducer array. By means of shifting the delay time between the ultrasonic excitation and CCD camera capture and repeating the optical measurement, 3D ultrasonic data are captured by recording multiple ultrasonic wavefields at a consecutive time sequence. The above acquired 3D ultrasonic wavefield data are used to reconstruct the 3D image of the internal structures and defects, so as to achieve the quantitative detection and evaluation of the internal defects of the sample.

Based on the idea of the proposed method shown in Fig. 15, the 3D ultrasonic wavefields measurement system was constructed as shown in Fig. 16, which be composed of the optical DHM subsystem, the near-field ultrasonic holography (NUH) subsystem, and the control and synchronization subsystem. The DHM subsystem consists of the CCD, the pulsed laser, and some optical components which form the off-axis digital holographic optical path, and it aims to precisely measure dynamic ultrasonic wavefields. The target of the NUH subsystem is to generate single frequency near-field ultrasonic waves, consisting of arbitrary waveform generator, power amplifier, and ultrasonic transducer. The control and synchronization subsystem consists of a host computer, a synchronous controller, an ultrasonic transducer, a CCD camera and a pulsed laser, and its main functions include: (1) to control the optical DHM subsystem and the NUH subsystem; (2) to accurately synchronize the ultrasonic transducer excitation and the optical NUH subsystem; (3) to precisely synchronize the pulsed laser and the CCD camera; and (4) to control the scanning stage [99–103].

On the basis of the above method and measurement system, “intelligent detection and control group” at Xian University of Science and Technology has carried out a lot of theoretical and experimental studies, such as propagation mechanism of near-field ultrasound in micronano structure [101,104,105], measurement and reconstruction of 3D ultrasonic wavefields based on digital holographic interferometry [100,102,103,106], phase correction and unwrapping technique for coherent optical measurement [103,107], digital holography parameter optimization [108,109], speckle noise reduction [110], etc. The measurement experiments of dynamic ultrasonic wavefields for a piezo-electric ceramic sheet and a tested sample with microdefects are carried out by using the above designed measurement system, as shown in Fig. 17. The tested sample is fabricated on a thin aluminum alloy plate of 0.3 mm thickness with a flat bottom hole with a diameter of 0.05 mm and a depth of 0.1 mm, representing a small internal defect and a groove with a diameter of 40 mm is used to embed an ultrasonic transducer into the tested sample. As can been seen, the surface topography of tested sample with defects is significantly different than that of tested sample without defects. Figure 17(d) shows the cross section data from Figs. 17(b) and 17(c). It can be observed that the maximum amplitude of ultrasonic field change caused by the internal defect is 0.44 μm [100,103]. Experimental results demonstrate that the proposed inspection method and the designed measurement system can not only accurately capture transient ultrasonic fields but also effectively identify the micron-scale internal defects in the tested sample. Moreover, the proposed inspection approach breaks through the resolution of the traditional ultrasonic NDT from the limitation of diffraction limit, and the lateral and longitudinal resolutions of the measurement system can reach tens of microns and hundreds of nanometers, respectively [100,101]. However, at present, this inspection approach is not a mature one, and the following key technical problems still remain to be solved:

1. Propagation mechanism of near-field ultrasound inside advanced micro-electronic packaging

   Due to the miniaturization, ultra-thin and 3D multilayer complex structure of advanced micro-electronic packaging, when propagating inside the package, ultrasonic waves can cause reflection, refraction, waveform conversion at the lamination boundaries and defects, and also occur attenuation, scattering and transmission, complicating the propagation mechanism of ultrasonic waves, especially evanescent waves. Based on finite element modeling for ultrasonic testing of micro-electronic packaging and the angular spectrum method, the propagation mechanism of ultrasonic waves in advanced micro-electronic packaging is investigated, which lays a theoretical foundation for parameter optimization of ultrasonic probe, selection of ultrasonic pulse period, and accurate measurement and reconstruction of ultrasonic transient sound field on the surface of the tested sample.

2. Reconstruct the phases and amplitudes of acoustic field precisely

   Analyzing the influence of reconstruction distance and reference light angle on the accuracy of holographic reconstruction, the parameter determination methods are developed using swarm intelligent optimization algorithms. Researching some novel approaches on digital holographic reconstruction and phase unwrapping using deep learning, the amplitudes and phases of ultrasonic sound field are accurately extracted from the holograms, and a 3D sound field sample carrying the internal structure and defect information of the specimen are obtained.

3. Three-dimensional image forming algorithm

   Analyzing the characteristics of 3D sound field sample and full matrix capture data, a total focus method (TFM) is investigated to obtain a set of full focus B-/C-scan images with defect location. According to the previously established finite element models of micro-electronic packaging, the impact of typical defects on TFM imaging accuracy is analyzed. A 3D image reconstruction approach based on TFM and isosurface algorithm is developed, achieving the high-precision 3D images of the internal defects of micro-electronic packaging.

4. Experimental researches for real specimens of advanced micro-electronic packaging

   Producing micro-electronic package specimens containing different sizes and types of real defects by accelerated thermal cycling test, and the inspection system is used to detect the above specimens, obtaining the 3D images of the defects, compared with the results of lossy anatomical analysis and 3D X-ray micro-CT imaging of the specimens.

For comparative analysis, Table 1 summarizes the advantages and disadvantages of the NDT methods for micro-electronic packaging.

Micro-electronic packaging is the bridge between IC and the electronic system. NDT methods have played a vital role in defect detection and evaluation of micro-electronic packaging. In this paper, a review on the past and current frequently used NDT methods for micro-electronic packaging inspection is provided with the basic principles, advantages, limitations, and application researches of each method. In addition, a lot of the most representative literatures and significant achievements have been addressed and discussed. The current trend of advanced micro-electronic packaging toward ultrathin, ultralight, and high density, leads to a massive challenge to the existing NDT methods applied to defect inspection for micro-electronic packaging. Thus, the novel NDT methods for defect inspection of advanced micro-electronic packaging must meet the requirements of high resolution and big penetration simultaneously. In this review, a hybrid optical-acoustic inspection approach for defect inspection of micro-electronic packaging has been introduced and discussed. The proposed method utilizes and combines the advantages of high spatial resolution optical measurement and unique penetration of acoustics, and overcomes the limitations of the existing NDT methods. Furthermore, it can obtain the 3D image of the internal structures and defects of micro-electronic packaging, so as to achieve the quantitative and location detection of the internal defects. Therefore, the hybrid inspection method can become a promising method for defect detection of advanced complicated 3D packages, but some technical problems still remain to be solved, and we will focus on tackling these issues in future research.

• National Natural Science Foundation of China (Grant No. 52175518; Funder ID: 10.13039/501100001809).

• Natural Science Basic Research Program of Shaanxi Province (Program Nos. 2019JM-024, 2019JQ-801, and 2021JLM-07; Funder ID: 10.13039/501100007128).

No data, models, or code were generated or used for this paper.