PCB failure analysis technology

As the hub of various components and circuit signal transmission, PCB has become the most important and critical part of electronic information products. Its quality and reliability determine the quality and reliability of the whole equipment. However, due to cost and technical reasons, PCBs have experienced a large number of failures during production and application.

For this failure problem, we need to use some common failure analysis techniques to ensure the quality and reliability of the PCB during manufacturing. This paper summarizes the top ten failure analysis techniques for reference.

1. Visual inspection

Visual inspection is to visually test or use some simple instruments, such as stereo microscope, metallographic microscope or even magnifying glass to check the appearance of the PCB, find the faulty part and related physical evidence. The main function is to locate the fault and determine the failure mode of the PCB. The visual inspection mainly checks the PCB contamination, corrosion, location of the blasting board, circuit wiring and the regularity of the failure, such as batch or individual, is always concentrated in a certain area and so on. In addition, many PCB failures are discovered after assembly into PCBA. Failures caused by the assembly process and the materials used in the process also require careful examination of the characteristics of the failure zone.

2. X-ray fluoroscopy

For some parts that cannot be visually inspected, as well as the inside of the through hole of the PCB and other internal defects, it is necessary to use an X-ray system to check. X-ray system is the use of different material thickness or different material density to image the different principles of X-ray moisture absorption or transmittance. This technique is used more to inspect defects inside PCBA solder joints, via internal defects, and the location of defective solder joints in high-density packaged BGA or CSP devices. The current industrial X-ray equipment has a resolution of less than one micron and is being transformed from a two-dimensional to three-dimensional imaging device. Even five-dimensional (5D) devices have been used for package inspection, but this 5D X The fluoroscopy system is very expensive and rarely has practical applications in industry.

3. Slice analysis

Slice analysis is the process of obtaining the cross-sectional structure of a PCB through a series of means and steps such as sampling, inlaying, slicing, polishing, etching, and observation. Through the slice analysis, a wealth of information reflecting the microstructure of the PCB (through holes, plating, etc.) can be obtained, which provides a good basis for the next step of quality improvement. However, this method is destructive. Once sliced, the sample is inevitably destroyed. At the same time, the method requires high sample preparation, and the sample preparation takes a long time, which requires a well-trained technician to complete. A detailed slicing process is required, which can be referred to the IPC standard IPC-TM-650 2.1.1 and IPC-MS-810.

4. Scanning acoustic microscope

Currently used for electronic packaging or assembly analysis, the main mode is the C-mode ultrasonic scanning acoustic microscope, which uses the amplitude and phase and polarity changes generated by high-frequency ultrasonic reflection on the discontinuous interface of the material to image. The Z axis scans the information of the X-Y plane. Therefore, scanning acoustic microscopy can be used to detect components, materials, and various defects inside the PCB and PCBA, including cracks, delamination, inclusions, and voids. If the frequency width of the scanning acoustics is sufficient, the internal defects of the solder joints can also be directly detected. Typical scanning acoustic images represent the presence of defects in a red warning color. Due to the large number of plastic packaged components used in the SMT process, a large amount of moisture reflow sensitive problems occur during the conversion from lead to lead-free processes. That is, the hygroscopic plastic sealing device will cause internal or substrate delamination when reflowing at a higher lead-free process temperature, and the ordinary PCB will often explode at the high temperature of the lead-free process. At this point, scanning acoustic microscopy highlights its unique advantages in non-destructive testing of multilayer high-density PCBs. The general obvious explosion plate can be detected only by visual inspection.

5. Microscopic infrared analysis

Micro-infrared analysis is an analytical method that combines infrared spectroscopy with a microscope. It uses different materials (mainly organic matter) to absorb different infrared spectra, analyzes the compound composition of the material, and combines the microscope to make visible light and infrared light The light path, as long as it is visible in the field of view, can be found to analyze trace amounts of organic pollutants. If there is no microscope combination, usually the infrared spectrum can only analyze samples with a larger sample volume. In many cases in electronic processes, trace contamination can lead to poor solderability of PCB pads or lead pins. It is conceivable that it is difficult to solve the process problem without the infrared spectrum of the microscope. The main purpose of microscopic infrared analysis is to analyze the organic contaminants on the surface of the soldered surface or solder joints and analyze the causes of poor corrosion or solderability.

6. Scanning electron microscopy

Scanning Electron Microscopy (SEM) is one of the most useful large-scale electron microscopy imaging systems for failure analysis. Its working principle is to use the electron beam emitted by the cathode to accelerate through the anode. After focusing by the magnetic lens, a bundle of several tens of diameters is formed. The electron beam current of several thousand angstroms (A), under the deflection of the scanning coil, the electron beam is subjected to a point-by-point scanning motion on the surface of the sample in a time and space sequence. This high-energy electron beam bombards the surface of the sample and excites A variety of information, through the collection and amplification, can get a variety of corresponding graphics from the display. The excited secondary electrons are generated in the range of 5-10 nm on the surface of the sample. Therefore, the secondary electrons can better reflect the surface morphology of the sample, so it is most often used as a morphology observation; and the excited backscattered electrons are generated on the surface of the sample. In the range of 100-1000 nm, different characteristics of backscattered electrons are emitted with the atomic number of the substance, so the backscattered electron image has the ability of discriminating the morphology and atomic number, and therefore, the backscattered electron image can reflect the chemical element. Distribution of ingredients. The functions of current scanning electron microscopes are already very powerful, and any fine structure or surface features can be amplified to hundreds of thousands of times for observation and analysis.

In the failure analysis of PCB or solder joints, SEM is mainly used for the analysis of failure mechanism, specifically to observe the surface structure of the pad surface, the metallographic structure of the solder joint, the measurement of intermetallic compounds, and the solderability coating. Analysis and do tin whisker analysis and measurement. Unlike optical microscopes, SEMs are electronic images, so only black and white, and SEM samples require electrical conduction. Non-conductors and some semiconductors need to be sprayed with gold or carbon, otherwise the charge will accumulate on the surface of the sample. Observation of the sample. In addition, the depth of field of the SEM image is much larger than that of the optical microscope. It is an important analytical method for irregular samples such as metallographic structure, micro-fracture and tin whiskers.

7. X-ray energy spectrum analysis

The scanning electron microscope described above is generally equipped with an X-ray energy spectrometer. When a high-energy electron beam strikes the surface of the sample, the inner electrons in the atoms of the surface material are bombarded, and the outer electrons are excited to a low-energy level to excite characteristic X-rays. The characteristics of the atomic energy levels of different elements are different. X-rays are different, so characteristic X-rays emitted from the sample can be analyzed as chemical components. At the same time, according to the X-ray detection signal as the characteristic wavelength or characteristic energy, the corresponding instruments are called the spectrum dispersive spectrometer (referred to as the spectrometer, WDS) and the energy dispersive spectrometer (referred to as the spectrometer, EDS), the resolution of the spectrometer The spectrometer is faster than the spectrometer. Since the spectrometer is fast and low in cost, the general scanning electron microscope is configured with an energy spectrometer.

With the different scanning modes of the electron beam, the energy spectrometer can perform point analysis, line analysis and surface analysis on the surface, and information about different distribution of elements can be obtained. Point analysis yields all the elements of a point; line analysis performs an elemental analysis on a specified line each time, multiple scans to obtain the line distribution of all elements; face analysis analyzes all elements in a specified face, and the measured element content is Measure the average of the surface range.

In the analysis of the PCB, the spectrometer is mainly used for component analysis of the pad surface, elemental analysis of the solderability of the pad and the surface of the lead pin. The accuracy of the quantitative analysis of the spectrometer is limited, and the content below 0.1% is generally not easily detected. The combination of energy spectrum and SEM can simultaneously obtain information on surface topography and composition, which is why they are widely used.

8. Photoelectron spectroscopy (XPS) analysis

When the sample is irradiated by X-rays, the inner shell electrons of the surface atoms will break away from the nucleus and escape from the solid surface to form electrons. The kinetic energy Ex can be measured to obtain the binding energy Eb of the inner shell electrons of the atoms, Eb due to different elements and Different from the electronic shell, it is the "fingerprint" identification parameter of the atom, and the formed line is photoelectron spectroscopy (XPS). XPS can be used for qualitative and quantitative analysis of shallow surface (several nanoscale) elements on the surface of the sample. In addition, information about the chemical valence of the element can be obtained from the chemical shift of the binding energy. It can give information such as the bond between the valence state of the surface layer and the surrounding elements; the incident beam is an X-ray photon beam, so the analysis of the insulating sample can be performed without damaging the rapid multi-element analysis of the analyzed sample; in the case of argon ion stripping Perform longitudinal elemental distribution analysis on multiple layers (see the case below) and the sensitivity is much higher than the energy spectrum (EDS). XPS is mainly used for the analysis of PCB coating quality, contaminant analysis and oxidation degree analysis in the analysis of PCB to determine the deep cause of poor solderability.

9. Thermal Analysis Differential Scanning Calorimetry (Differential Scanning Calorim-etry)

A method of measuring the relationship between the power difference between a substance and a reference substance and temperature (or time) at program temperature control. The DSC is equipped with two sets of compensation heating wires under the sample and reference container. When the sample has a temperature difference ΔT between the thermal effect and the reference during the heating process, the differential thermal amplifier circuit and the differential heat compensation amplifier can be passed. The current flowing into the compensation heating wire is changed.

The heat balance on both sides, the temperature difference ΔT disappears, and the difference between the thermal power of the two electrothermal compensations under the sample and the reference is recorded as a function of temperature (or time). According to this change relationship, the physical properties of the material can be studied. Chemical and thermodynamic properties. DSC is widely used, but in the analysis of PCB, it is mainly used to measure the curing degree of various polymer materials used on PCB, and the glass transition temperature. These two parameters determine the reliability of the PCB in the subsequent process.

10. Thermomechanical Analyzer (TMA)

Thermal Mechanical Analysis is used to measure the deformation properties of solids, liquids and gels under thermal or mechanical forces under program temperature control. Commonly used load methods include compression, needle insertion, stretching and bending. The test probe is supported by a cantilever beam and a coil spring fixed thereto, and a load is applied to the sample by the motor. When the sample is deformed, the differential transformer detects the change and processes the data together with temperature, stress and strain. The relationship between the deformation of the material and the temperature (or time) under negligible load is obtained. According to the relationship between deformation and temperature (or time), the physicochemical and thermodynamic properties of the materials can be studied. TMA is widely used in PCB analysis and is mainly used for the two most critical parameters of PCB: measuring its linear expansion coefficient and glass transition temperature. PCBs of substrates with excessive expansion coefficients often cause fracture failure of metallized holes after solder assembly.

Due to the high density development trend of PCBs and the environmental requirements of lead-free and halogen-free, more and more PCBs have various failure problems such as poor wetting, bursting, delamination, CAF and so on. Introduce the application of these analytical techniques in practical cases. The acquisition of PCB failure mechanisms and causes will facilitate future quality control of the PCB, thus avoiding the recurrence of similar problems.