Application of the eddy current method for flaw detection of conductive tracks of printed circuit boards

The article discusses the creation and testing of a hardware-software complex aimed at examining the conductive paths on printed circuit boards. The complex includes a tiny eddy current transducer and a measuring system specifically designed to work with the transducer. The proposed system, which combines software and hardware components, allows for the examination of small sections of metal objects. It enables the monitoring of electrical conductivity and the determination of conductivity distribution across the object’s surface and depth. The article provides detailed explanations of the key features of the measuring system and presents the experimental findings obtained by testing different printed circuit boards. These boards were examined under various conditions, including defect-free ones and those intentionally modified to simulate defects. Dependences between the eddy current transducer signal and the different conductive tracks were established, elucidating the relationship between the transducer’s response and the properties of the tracks.


Introduction
Advancing and enhancing techniques for investigating and evaluating the physicochemical and functional properties of materials utilized in contemporary industry and research is a significant and practical objective within the realm of physical materials science.The manufacturing and utilization of numerous cutting-edge products, indispensable in industries such as aerospace, transportation, nuclear energy, and demanding mining environments, rely heavily on effective quality control measures.
Control and diagnostics serve as fundamental elements in addressing security concerns.Control involves the verification of object parameters to ensure their adherence to predefined technical specifications.In this context, non-destructive quality control methods assume great importance as they enable the analysis of objects without compromising their internal structure or altering their operational characteristics.This capability allows for comprehensive quality assessments to be conducted during the actual usage of the objects, if required.By employing non-destructive methods, it becomes possible to perform thorough quality checks without interfering with the object's functionality or performance.
Printed circuit boards remain one of the topical objects of research in non-destructive testing.This is due to the expansion of the range of electronic products in the modern consumer and industrial sphere and the constant spread and cheapening of devices based on microprocessors, which are almost always used together with technological equipment placed on a printed circuit board.
As it is widely acknowledged, the dependability of printed circuit boards (PCBs) exerts a direct influence on the overall profitability of electronic equipment manufacturing.Despite the meticulous execution of etching procedures and scrupulous material selection, the chemical processes employed in etching occasionally give rise to flaws in PCBs, such as discontinuities, short-circuited conductors, cracks, and similar deviations.Today, the task of identifying dangerous technological defects at the stages of production to limit the admission into operation of potentially unreliable equipment is very relevant [1][2][3][4][5][6][7][8][9].Defects in the conductive tracks of printed circuit boards are divided into functional and cosmetic [2][3][4].Functional defects can seriously affect the quality of the board, which may cause it to malfunction.Such defects are the most serious.Cosmetic defects mainly affect only the appearance of the printed circuit board, but with long-term use of the board, they also worsen its performance, due to violations of heat dissipation and current distribution [1].
In recent years, there has been significant research conducted on a range of image processing algorithms aimed at detecting defects in printed circuit boards (PCBs).These algorithms include binary morphological image processing [5], similarity measurement methods [6], and segmentationbased approaches.However, the application of deep learning algorithms in detecting PCB defects has certain limitations due to the inherent characteristics of local convolutional neural network (CNN) features.This limitation arises from the fact that defective areas typically occupy only a small fraction of the image depicting a flawed PCB [7].
Contemporary methodologies for defect detection facilitate the identification and localization of a majority of conspicuous defects, as well as certain latent anomalies in rare earth elements (REE).
Ensuring the reliability of electronic equipment necessitates the prompt identification of concealed defects and malfunctions that may arise during operation.These issues encompass various concerns, including the emergence of microcracks, microroughness, and microgaps in communication lines and connectors, as well as the aging or malfunctioning of printed circuit board components.The timely detection of such issues is vital to maintain the dependability of electronic equipment [8].An important direction in modern quality control is the automation of control.Automation of control is relevant, since manual control is too slow and has a high percentage of errors [9].
A large amount of memory, high performance and low cost of modern computer technology make it possible to provide significant opportunities for automating the process of scanning [10,11].Consequently, the objective of this research was to create and examine an innovative category of eddy current measuring systems.These systems enable automated qualitative analyses of the conductive paths found on printed circuit boards, facilitate the detection of minor defects within these paths, and offer the capability to examine closely positioned tracks within multilayer printed circuit boards situated at varying depths.

Materials and methods
The proposed system, which combines software and hardware components, allows for the examination of small sections of metal objects.It enables the monitoring of electrical conductivity and the determination of conductivity distribution across the object's surface and depth.The system consists of an eddy current transducer (ECT) connected to an Arduino circuit and computer software written in Python, compatible with Windows operating systems.To establish a connection between the Arduino circuit and the computer, a virtual COM port is utilized.The software performs various functions, including regulating the voltage in the converter's exciting circuit, controlling the positioning system of the ECT, recording output voltage in standard units, calculating electrical conductivity based on calibrated data, and generating images of the inspected parts using computer vision technology.The developed hardware is designed to be as portable as possible while remaining within its functional limitations.The sensors in the positioning system are ECT [12][13][14][15].The electromagnetic field of the ECT is produced by means of a system generating signals for the exciting winding.A generator based on a highly integrated AD9850 chip is used for program.Measuring system: ECC, ECT; ADC, analogdigital converter with fast Fourier transformation.Generator Amplifier ECC ADC Filter Amplifier Control PC Analysis and visualization Positioning system of ECC mable control.This integrated circuit utilizes an amalgamation of enhanced direct digital synthesis (DDS), a high-fidelity analog-todigital converter, and a comparator unit, which guarantees frequency synthesis and clock signal generation.The AD9850 chip incorporates a highly accurate reference clock signal source that yields a steady sinusoidal analog output signal, allowing for precise specification of its frequency and phase.When the frequency of the clock signal source is 125 MHz, a highly stable sinusoid (0-40 MHz) with low noise may be obtained at the module's output; the supply voltage is 3.3-5 V.The AD9850 module requires a control element.The Arduino computing platform is used to connect the computer and generator.This platform consists of an input-output circuit and is developed in the Processing/Wiring language.The device employs based on an ATmega 328 (32 kV) for storage of the program code and 2 kB of RAM.Due to the relatively low voltages generated by the eddy currents in the measuring winding, it is necessary to amplify the output signal of the ECT.Similarly, the generator module also requires signal amplification because the generator winding of the ECT has high resistance, and the generator itself does not regulate the signal amplitude.To address this, an inverting amplifier utilizing the AD8051 operational amplifier is employed to amplify the signal.In order to digitize the output signal of the ECT, an RTL2832U chip is utilized, which incorporates an analog-to-digital converter, a digital processor, a USB interface, and a module for fast Fourier transformation.In the software component of the system, the frequency of the exciting signal transmitted to the generator is established, and a sinusoidal signal with the specified frequency is generated.The generation of a set of signal values relies on the utilization of the NumPy library, which facilitates the organization of sets and corresponding mathematical operations.The specific frequency of the signal is determined through the implementation of the defsetfreq(self, freq) function.The generated sine wave array is subsequently routed through a virtual communication port.The establishment of interaction with the virtual communication port is achieved through the utilization of communication protocols employed in communicating with the positioning system via a communication port.The communication class integrates specific directives enabling the transmission of commands to the positioning system, thus facilitating the efficient synchronization of signal transmission.
To ensure the desired signal amplitude, the generated signal is routed to an amplifier.The amplification factor of the amplifier can be adjusted within a predetermined range.The amplified signal is subsequently conveyed to the exciting winding of the converter, resulting in the generation of eddy currents within the object under investigation.The resultant magnetic field induces an electromotive force (emf) in the measuring winding, representing the output signal that carries information about the object.In order to enhance the output signal, which corresponds to the recorded emf, it undergoes amplification and is then subjected to a Delian filter combined with a selective signal amplifier.Following the amplification process, the signal is directed to a module responsible for data collection, digitization, and initial processing.The voltage of the signal is measured, and the resulting data undergoes analog-to-digital conversion, enabling further analysis and interpretation.The voltage values obtained are sent through a serial port to a personal computer with an RtlSdr analog-digital converter.The results are written in the fft array.The amplitude of the signal components (Re2 + Im2) 0.5 is calculated using the abs function.The data from the sensors are displayed on the screen as the distribution of the part's surface conductivity.

Experimental results
Experimental investigations were conducted to examine defect-free conductive tracks on printed circuit boards, specifically focusing on a motherboard produced by Dell.The study involved the utilization of four distinct conducting paths (referred to as figure 1) with varying widths ranging from 3.3 to 0.7 mm.These paths were positioned at different distances from each other, measuring 0.8 and 0.6 mm respectively.The scanning process was conducted in the direction indicated by the arrow, allowing for comprehensive analysis and evaluation of the selected tracks.

Investigation of defective conductive tracks of printed circuit boards
Experimental investigations were conducted to examine defect-free conductive tracks on printed circuit boards, focusing on a specific motherboard manufactured by Dell.The study involved the selection of a conducting track with a model defect (referred to as figure 3).The intentional placement of the defect in isolation from surrounding tracks aimed to guarantee the unimpeded acquisition of the eddy current testing (ECT) signal from the primary track under scrutiny, without any interference caused by adjacent tracks.During the scanning process, a frequency of 5000 Hz was employed, allowing for precise and accurate data acquisition and analysis.This frequency was chosen to ensure optimal performance and sensitivity in detecting potential defects or irregularities within the conductive track.

Scanning multilayer printed circuit boards
Multilayer circuit boards play a crucial role in the investigation of printed circuit boards.These PCBs are specifically designed for complex devices that necessitate a high component density.The number of layers incorporated in multilayer PCBs is determined by the intricacy of the task at hand, with the aim of accommodating the requirements of PCB designers.
In the case of multilayer PCBs, components are mounted on both sides of the board, while the internal layers serve the purpose of interconnecting the components.The conductor connections within these PCBs are established through the utilization of vias, facilitating efficient communication and electrical continuity.
Multilayer PCBs can comprise numerous layers, enabling a high specific density of printed conductors and contact pads.This characteristic contributes to several advantages, including the reduction of conductor lengths.Consequently, the speed of data processing in devices such as computers can be significantly enhanced.Furthermore, multilayer PCBs provide the capacity for effective AC circuit shielding, thereby ensuring proper signal integrity and minimizing interference.
Figure 5 presents an instance of a multilayer printed circuit board (PCB), wherein the scanning procedure was conducted along the direction indicated by the arrow.The upper layer of the PCB exhibited conductive tracks with widths ranging from 0.7 to 1.5 mm.In contrast, the lower layer possessed conductive tracks with a consistent width of 0.5 mm.Notably, the second layer was situated at a depth of 0.3 mm within the PCB structure.One notable characteristic of scanning such objects is the capability of the eddy current transducer to detect conductive tracks at various depths and differentiate their signals.However, achieving this requires altering the frequency of the current applied to the excitation coil of the transducer to modify the penetration depth of the magnetic field into the object being examined.Consequently, synchronous adjustments of the cutoff frequency within the filtering system and the selective amplification within the measurement system are essential, aligning with the frequency variations of the current applied to the excitation coil.The results of object scanning at a frequency of 5000 Hz are shown in figure 6.The signal from the first layer became much less intense, narrow paths are marked by small peaks.On the other hand, a significant peak marked the influence of the second layer on the signal of the measuring winding of the eddy current transducer.In the future, this will allow examining deep layers for defects and determining the integrity of the layers.

Conclusion
An eddy current measuring system was developed and investigated, employing eddy current transducers with diverse parameters.The influence of the core material and shape on the signal transmitted to the eddy current transducer {ECT} from various conductive tracks of printed circuit boards was found to be significant.These tracks exhibited distinct characteristics such as different sizes and varying distances from each other.Dependences between the ECT signal and the different conductive tracks were established, elucidating the relationship between the transducer's response and the properties of the tracks.Additionally, an original system encompassing filtering and selective amplification was devised, enabling the purification of the received signal from the eddy current transducer.The implementation of this system, coupled with the counter-connection of two eddy current transducers, facilitated substantial signal clarification by reducing interference.Consequently, this enhanced signal processing approach facilitated the identification of defects located within closely spaced conductive tracks on printed circuit boards.Furthermore, the feasibility of testing multilayer printed circuit boards at low frequencies was demonstrated, thereby broadening the potential applications of the developed eddy current measuring system.

Figure 1 .Figure 2 .
Figure 1.Defect-free tracks with a width of 0.7 mm -3.3 mm, located at distances of 0.8 mm and 0.6 mm

Figure 3 .Figure 4 .
Figure 3. Defective track, width -4.6 mm, width of defect No. 1 -0.3 mm, width of defect No. 2 -0.05 mm The experimental results concerning the defective track are depicted in Figure 4.The data showcases discernible dependencies, emphasizing the divergent signals acquired from two distinct types of eddy current testing (ECT).It is evident that the transducer utilizing annealed permalloy (Figure 4.b) offers a notably superior depiction of the defect in comparison to the transducer employing ferrite (figure 4.a).In figure 4.b, the defects are readily observable as a notable reduction in the signal voltage detected by the measuring winding of the eddy current transducer.The presence of these defects is distinctly discernible due to the pronounced decline in signal strength, indicating the presence of abnormalities within the track under investigation.In contrast, the signal obtained from the ferrite-based transducer in figure 4.a exhibits less clarity and fails to exhibit such pronounced indications of the defects.

Figure 5 .
Figure 5. Multilayer printed circuit board with conductive layer 0.2 mm below other conductive layers

Figure 6 .
Figure 6.Results of scanning a track of a multilayer printed circuit board without defects at a frequency of 5000 Hz In figure 6 all 4 tracks of the upper layer are clearly visible as the level of voltage introduced into the measuring winding increases.However, the influence of the track of the lower layer is not traced.

Figure 7 .
Figure 7. Results of scanning a track of a multilayer printed circuit board without defects at a frequency of 500 Hz Figure 7 clearly shows the influence of the second layer.The signal from the first layer became much less intense, narrow paths are marked by small peaks.On the other hand, a significant peak marked the influence of the second layer on the signal of the measuring winding of the eddy current transducer.In the future, this will allow examining deep layers for defects and determining the integrity of the layers.