As the carrier of various components and the hub of circuit signal transmission, PCB has become the most important and critical part of electronic information products. Its quality and reliability level determine the quality and reliability of the whole equipment. However, due to cost and technical reasons, there are a large number of failure problems in the production and application of PCB.

For this kind of failure problem, we need to use some commonly used failure analysis techniques to ensure the quality and reliability level of PCBs during manufacturing. This article summarizes the top ten failure analysis techniques for reference.

1. Visual inspection

Appearance inspection is to visually inspect or use some simple instruments, such as stereoscopic microscopes, metallographic microscopes, or even magnifying glasses, to inspect the appearance of PCBs, and to find failure locations and related physical evidence. The main function is to locate failures and preliminarily judge the failure mode of PCBs. The appearance inspection mainly checks the pollution, corrosion, position of the exploded board, circuit wiring and failure regularity of the PCB, whether it is batch or individual, whether it is always concentrated in a certain area, etc. In addition, many PCB failures are discovered after they are assembled into a PCBA. Whether the failure is caused by the assembly process and the materials used in the process also requires careful inspection of the characteristics of the failure area.

2. X-ray perspective inspection

For some parts that cannot be detected by visual inspection, as well as inside the through holes of PCB and other internal defects, X-ray perspective system has to be used for inspection. The X-ray fluoroscopy system uses the different principles of different material thicknesses or different material densities for X-ray moisture absorption or transmittance to image. This technology is more used to inspect defects inside PCBA solder joints, defects inside through holes and the positioning of defective solder joints of BGA or CSP devices in high-density packaging. The resolution of the current industrial X-ray fluoroscopy equipment can reach below one micron, and it is changing from two-dimensional to three-dimensional imaging equipment. There are even five-dimensional (5D) equipment used for packaging inspection, but this 5D X-ray Optical see-through systems are very expensive and have few practical applications in industry.

3. Slice analysis

Slicing analysis is the process of obtaining the PCB cross-sectional structure through a series of means and steps such as sampling, inlaying, slicing, polishing, corrosion, and observation. Through slice analysis, rich information of the microstructure reflecting the quality of 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, and once sliced, the sample will inevitably be destroyed; at the same time, this method requires high sample preparation and takes a long time for sample preparation, which requires well-trained technicians to complete. For the detailed slicing process, please refer to the procedures stipulated in IPC-TM-650 2.1.1 and IPC-MS-810.

4. Scanning Acoustic Microscope

At present, the C-mode ultrasonic scanning acoustic microscope is mainly used for the analysis of electronic packaging or assembly. It uses the amplitude, phase and polarity changes generated by the reflection of high-frequency ultrasonic waves on the discontinuous interface of materials to image. The scanning method is along the The Z axis scans the information of the X-Y plane. Therefore, scanning acoustic microscopy can be used to detect various defects in components, materials, and PCBs and PCBAs, including cracks, delaminations, inclusions, and voids. Internal defects of solder joints can also be detected directly if the frequency width of the scanning acoustics is sufficient. A typical scanning acoustic image is a red warning color to indicate the existence of defects. Since a large number of plastic-encapsulated components are used in the SMT process, a large number of moisture reflow sensitive problems arise during the process of converting from lead to lead-free process. That is, moisture-absorbing plastic-encapsulated devices will have internal or substrate delamination cracking when reflowing at a higher lead-free process temperature, and ordinary PCBs will often experience board explosion at high temperatures in the lead-free process. At this time, the scanning acoustic microscope highlights its special advantages in the non-destructive detection of multi-layer high-density PCB. The general obvious burst plate can be detected only by visual inspection.

5. Microscopic infrared analysis

Micro-infrared analysis is an analysis method that combines infrared spectroscopy with a microscope. It uses the principle of different absorption of infrared spectra by different materials (mainly organic matter) to analyze the compound components of materials. Combined with a microscope, visible light and infrared light can be simultaneously The light path, as long as it is under the visible field of view, can search for trace organic pollutants to be analyzed. Without the combination of a microscope, infrared spectroscopy can usually only analyze samples with large sample sizes. In many cases in electronic technology, trace pollution can lead to poor solderability of PCB pads or lead pins. It is conceivable that it is difficult to solve process problems without infrared spectroscopy equipped with a microscope. The main purpose of micro-infrared analysis is to analyze the organic pollutants on the surface to be welded or the surface of the solder joint, and to analyze the cause of corrosion or poor solderability.

6. Scanning electron microscope analysis

Scanning electron microscope (SEM) is the most useful large-scale electron microscopic imaging system for failure analysis. Its working principle is to use the electron beam emitted by the cathode to be accelerated by the anode, and then focused by a magnetic lens to form a beam with a diameter of tens to With the electron beam current of several thousand angstroms (A), under the deflection of the scanning coil, the electron beam scans point by point on the surface of the sample in a certain time and space sequence. A variety of information can be obtained, and various corresponding graphics can be obtained from the display screen after being collected and enlarged. The excited secondary electrons are generated within the range of 5-10nm on the surface of the sample. Therefore, the secondary electrons can better reflect the morphology of the sample surface, so they are most commonly used for morphology observation; while the excited backscattered electrons are generated on the sample surface In the range of 100-1000nm, backscattered electrons with different characteristics are emitted according to the atomic number of the substance, so the backscattered electron image has the ability to distinguish the shape characteristics and atomic number, and therefore, the backscattered electron image can reflect chemical elements The distribution of ingredients. The current scanning electron microscope is very powerful, and any fine structure or surface feature can be magnified to hundreds of thousands of times for observation and analysis.

In the failure analysis of PCB or solder joints, SEM is mainly used for failure mechanism analysis, specifically, it is used to observe the morphology and structure of the pad surface, the metallographic structure of solder joints, measure intermetallic compounds, and solderability coatings. Analysis and tin whisker analysis and measurement. Different from the optical microscope, the scanning electron microscope forms an electronic image, so there are only black and white colors, and the sample of the scanning electron microscope is required to be conductive, and non-conductors and some semiconductors need to be sprayed with gold or carbon, otherwise the charge will accumulate on the surface of the sample. Sample observation. In addition, the depth of field of the scanning electron microscope image is much larger than that of the optical microscope, and it is an important analysis method for uneven samples such as metallographic structure, microscopic fractures and tin whiskers.

7. X-ray energy spectrum analysis

The scanning electron microscope mentioned above is generally equipped with an X-ray energy spectrometer. When the high-energy electron beam hits the surface of the sample, the inner layer electrons in the atoms of the surface material are bombarded and escaped, and when the outer layer electrons transition to a lower energy level, characteristic X-rays will be excited, which are emitted by different atomic energy levels of different elements. X-rays are different, therefore, the characteristic X-rays emitted by the sample can be analyzed as chemical composition. At the same time, according to the characteristic wavelength or characteristic energy of the X-ray detection signal, the corresponding instruments are called Spectrum Dispersion Spectrometer (abbreviated as Spectrometer, WDS) and Energy Dispersive Spectrometer (abbreviated as Energy Spectrometer, EDS). Higher than the energy spectrometer, the analysis speed of the energy spectrometer is faster than that of the spectrometer. Due to the high speed and low cost of the energy spectrometer, the general scanning electron microscope is equipped with an energy spectrometer.

Depending on the scanning method of the electron beam, the energy spectrometer can perform point analysis, line analysis and surface analysis on the surface, and can obtain information on different distributions of elements. Point analysis obtains all elements at one point; line analysis conducts an element analysis on a specified line each time, and obtains the line distribution of all elements through multiple scans; surface analysis analyzes all elements in a specified surface, and the measured element content is Measures the average value of the face range.

In the analysis of PCB, the energy spectrometer is mainly used for component analysis on the surface of pads, and elemental analysis of pollutants on the surface of pads and lead pins with poor solderability. The accuracy of quantitative analysis by energy spectrometer is limited, and the content below 0.1% is generally not easy to detect. The combination of energy spectroscopy and SEM can obtain information on surface morphology and composition at the same time, which is why they are widely used.

8. Photoelectron spectroscopy (XPS) analysis

When the sample is irradiated by X-rays, the electrons in the inner shell of the surface atoms will break away from the shackles of the nucleus and escape from the solid surface to form electrons. By measuring its kinetic energy Ex, the binding energy Eb of the electrons in the inner shell of the atom can be obtained. Eb varies with different elements and Different electron shells are different, it is the "fingerprint" identification parameter of the atom, and the formed spectral line is the photoelectron spectrum (XPS). XPS can be used for qualitative and quantitative analysis of shallow surface (several nanometers) elements on the sample surface. In addition, information about the chemical valence state of an element can be obtained from the chemical shift of the binding energy. It can give information such as the atomic valence state of the surface layer and the bonding of surrounding elements; the incident beam is an X-ray photon beam, so it can be used for insulating sample analysis, without damaging the analyzed sample for rapid multi-element analysis; it can also be used in the case of argon ion stripping Longitudinal element distribution analysis of multiple layers (see the following case), and the sensitivity is much higher than that of energy spectroscopy (EDS). In the analysis of PCB, XPS is mainly used for the analysis of pad coating quality, pollutant analysis and oxidation degree analysis to determine the deep-seated reasons for poor solderability.

9. Thermal Analysis Differential Scanning Calorimetry

Under program temperature control, it is a method to measure the relationship between the power difference input to the substance and the reference substance and the temperature (or time). The DSC is equipped with two sets of compensation heating wires under the container of the sample and the reference object. When there is a temperature difference ΔT between the sample and the reference object due to the thermal effect during the heating process, it can pass through the differential heat amplifier circuit and the differential heat compensation amplifier. , so that the current flowing into the compensation heating wire changes.

The heat on both sides is balanced, the temperature difference ΔT disappears, and the relationship between the thermal power difference between the two electrothermal compensations under the sample and the reference object is recorded as a function of temperature (or time). According to this relationship, the physical properties of the material can be studied and analyzed. Chemical and thermodynamic properties. DSC is widely used, but in the analysis of PCB, it is mainly used to measure the curing degree and glass transition temperature of various polymer materials used on PCB. These two parameters determine the reliability of 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 force under programmed temperature control. Commonly used loading methods include compression, needle penetration, stretching, bending, etc. The test probe is supported by a cantilever beam and a coil spring fixed on it, and the load is applied to the sample by the motor. When the sample is deformed, the differential transformer detects this change and processes it together with data such as temperature, stress and strain. The relationship between the deformation of a substance and the temperature (or time) under a negligible load can be obtained. According to the relationship between deformation and temperature (or time), the physical, chemical and thermodynamic properties of materials can be studied and analyzed. TMA is widely used, and it is mainly used for the two most critical parameters of PCB in the analysis of PCB: measuring its linear expansion coefficient and glass transition temperature. PCBs with base materials with excessive expansion coefficients often lead to fracture failure of metallized holes after soldering and assembly.