PCB Technical - Overview of PCB failure analysis technology

As the carrier of various components and the hub of circuit signal transmission, PCB has become an important and key 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 lot of failure problems in the production and application of PCB.

For this failure problem, we need to use some common failure analysis technologies to ensure the quality and reliability level of PCB during manufacturing. Therefore, as a senior enterprise in PCB proofing industry, Wang Gong of Shenzhen jieduobang Technology Co., Ltd. focused on summarizing nine technologies for PCB failure analysis when talking about failure analysis technology, Including: appearance inspection, X-ray fluoroscopy, metallographic section analysis, thermal analysis, photoelectron spectroscopy analysis, micro infrared analysis, scanning electron microscopy analysis and X-ray spectroscopy analysis.

Then some common failure analysis techniques will be used. Between the structural characteristics of PCB and the main failure mode, metallographic section analysis is a destructive analysis technology. Once these two technologies are used, the sample will be damaged and cannot be recovered; In addition, due to the requirements of sample preparation, scanning electron microscope analysis and X-ray energy spectrum analysis sometimes need to partially destroy the sample. In addition, in the process of analysis, due to the needs of failure location and verification of failure causes, it may be necessary to use test technologies such as thermal stress, electrical properties, weldability test and dimensional measurement, which will not be specially introduced here.

PCB

1. Appearance inspection appearance inspection is to visually inspect or use some simple instruments, such as stereomicroscope, metallographic microscope and even magnifying glass, to check the appearance of PCB and find the failed parts and relevant physical evidence. Its main function is to locate the failure and preliminarily judge the failure mode of PCB. The appearance inspection mainly checks the pollution, corrosion, position of PCB explosion, circuit wiring and regularity of failure, such as batch or individual, whether it is always concentrated in a certain area, etc. In addition, many PCB failures are found after PCBA is assembled. Whether the failure is caused by the influence of the assembly process and the materials used in the process also needs to carefully check the characteristics of the failure area.

2. X-ray fluoroscopy inspection for some parts that cannot be detected through visual inspection, as well as the through-hole internal and other internal defects of PCB, X-ray fluoroscopy system can only be used for inspection. X-ray fluoroscopy system uses the different principles of X-ray moisture absorption or transmittance of different material thickness or different material density to image. This technology is more used to check the internal defects of PCBA solder joints, internal defects of through holes and the positioning of defective solder joints of high-density packaged BGA or CSP devices. At present, the resolution of industrial X-ray fluoroscopy equipment can reach less than one micron, and it is changing from two-dimensional to three-dimensional imaging equipment. Even five-dimensional (5d) equipment has been used for packaging inspection, but this 5D X-ray fluoroscopy system is very valuable and has few practical applications in industry.

3. Slice analysis slice analysis is the process of obtaining the cross-sectional structure of PCB through a series of means and steps such as sampling, mosaic, slicing, polishing, corrosion and observation. Through slice analysis, we can get rich information about the microstructure reflecting the quality of PCB (through hole, coating, etc.), which provides a good basis for the next quality improvement. However, this method is destructive. Once sliced, the sample will be destroyed; At the same time, this method has high requirements for sample preparation and takes a long time, which needs well-trained technicians to complete. For detailed slicing process, refer to IPC-TM-650 2.1.1 and ipc-ms-810.

4. Scanning acoustic microscope at present, C-mode ultrasonic scanning acoustic microscope is mainly used for electronic packaging or assembly analysis. It uses the amplitude, phase and polarity changes generated by the reflection of high-frequency ultrasound on the discontinuous interface of materials to image. Its scanning method is to scan the information of X-Y plane along the Z axis. Therefore, scanning acoustic microscope can be used to detect various defects in components, materials and PCB and PCBA, including cracks, delamination, inclusions and cavities. If the frequency width of scanning acoustics is sufficient, the internal defects of solder joints can also be detected directly. The typical scanning acoustic image indicates the existence of defects with a red warning color. Because a large number of plastic encapsulated components are used in the SMT process, a large number of moisture reflux sensitive problems occur in the process of converting lead into lead-free process, that is, the moisture absorbing plastic encapsulated components will have internal or substrate delamination cracking when refluxing at a higher lead-free process temperature, Under the high temperature of lead-free process, ordinary PCB will often explode. At this time, scanning acoustic microscope highlights its special advantages in multi-layer high-density PCB nondestructive testing. The general obvious explosion plate can be detected only by visual appearance.

5. Micro infrared analysis micro infrared analysis is an analysis method combining infrared spectrum with microscope. It uses the principle of different absorption of infrared spectrum by different materials (mainly organic substances) to analyze the compound composition of materials. Combined with microscope, visible light and infrared light can be in the same light path, as long as they are in the visible field of view, We can find trace organic pollutants to be analyzed. Without the combination of microscope, infrared spectrum can only analyze samples with a large amount of samples. In many cases, micro pollution in electronic technology can lead to poor solderability of PCB pad or lead pin. It can be imagined that it is difficult to solve the process problem without infrared spectrum equipped with microscope. The main purpose of micro infrared analysis is to analyze the organic pollutants on the welded surface or solder joint surface, and analyze the causes of corrosion or poor weldability.

6. Scanning electron microscope analysis scanning electron microscope (SEM) is a useful large-scale electron microscopic imaging system for failure analysis. Its working principle is that the electron beam emitted by the cathode is accelerated by the anode and focused by the magnetic lens to form an electron beam with a diameter of tens to thousands of angstroms (a). Under the deflection of the scanning coil, The electron beam makes a point by point scanning movement on the sample surface in a certain time and space order. The bombardment of this high-energy electron beam on the sample surface will stimulate a variety of information. After collection and amplification, various corresponding graphics can be obtained from the display screen. The excited secondary electrons are generated in the range of 5 ~ 10 nm on the sample surface. Therefore, the secondary electrons can better reflect the morphology of the sample surface, so they are often used for morphology observation; The excited backscattered electrons are generated in the range of 100 ~ 1000 nm on the sample surface and emit backscattered electrons with different characteristics with different atomic numbers of substances. Therefore, the backscattered electron image has the ability to distinguish morphological characteristics and atomic numbers. Therefore, the backscattered electron image can reflect the distribution of chemical elements. At present, the function of scanning electron microscope is very powerful. 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 joint, SEM is mainly used to analyze the failure mechanism. Specifically, it is used to observe the morphology and structure of the pad surface, the microstructure of solder joint, measure intermetallic compounds, analyze the solderable coating, and measure the tin whisker. Different from the optical microscope, the scanning electron microscope is an electronic image, so it is only black and white. The sample of the scanning electron microscope is required to be conductive. Non conductors and some semiconductors need to be sprayed with gold or carbon, otherwise the charge will accumulate on the sample surface and affect the observation of the sample. In addition, the depth of field of SEM image is much larger than that of optical microscope. It is an important analysis method for uneven samples such as metallographic structure, micro fracture and tin whisker.

7. X-ray energy spectrum analysis the scanning electron microscope mentioned above is generally equipped with X-ray energy spectrometer. When the high-energy electron beam strikes the sample surface, the inner electrons in the atoms of the surface material are bombarded and escape, and the characteristic X-rays will be excited when the outer electrons transition to the low-energy level. The characteristic X-rays emitted by different atomic energy levels of different elements are different. Therefore, the characteristic X-rays emitted by the sample can be used for chemical composition analysis. At the same time, according to the characteristic wavelength or characteristic energy of the detected X-ray signal, the corresponding instruments are called spectral dispersion spectrometer (WDS) and energy dispersion spectrometer (EDS). The resolution of the spectrometer is higher than that of the spectrometer, and the analysis speed of the spectrometer is faster than that of the spectrometer. Because the energy spectrometer has high speed and low cost, the general scanning electron microscope is equipped with energy spectrometer.

With the different scanning modes of electron beam, the energy spectrometer can carry out point analysis, line analysis and surface analysis on the surface, and can obtain the information of different distribution of elements. All elements of a point are obtained by point analysis; Line analysis: perform an element analysis on a specified line each time, and scan multiple times to obtain the line distribution of all elements; Surface analysis analyzes all elements in a specified surface, and the measured element content is the average value of the measurement surface range.

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

8. Photoelectron spectroscopy (XPS) when the sample is irradiated by X-rays, the inner shell electrons of the surface atoms will escape from the bondage of the atomic nucleus and form electrons on the solid surface. By measuring their kinetic energy ex, the binding energy EB of the inner shell electrons of the atoms can be obtained. EB varies with different elements and different electronic shells. It is the "fingerprint" identification parameter of the atoms, and the formed spectral line is photoelectron spectroscopy (XPS) . XPS can be used for qualitative and quantitative analysis of shallow surface (several nanoscale) elements on the sample surface. In addition, information about the chemical valence of elements can be obtained according to the chemical shift of binding energy. It can give information about the bonding between the valence state of the surface layer atom and the surrounding elements; The incoming beam is X-ray photon beam, so it can be used for insulating sample analysis without damaging the analyzed sample for rapid multi-element analysis; In the case of argon ion stripping, the longitudinal element distribution analysis of multiple layers can also be carried out (see later), and the sensitivity is much higher than that of energy spectrum (EDS). In PCB analysis, XPS is mainly used for pad coating quality analysis, pollutant analysis and oxidation degree analysis to determine the deep-seated causes of poor solderability.

9. Thermal analysis differential scanning calorimetry (DSC) - a method for measuring the relationship between the power difference input to the substance and the reference substance and temperature (or time) under programmed temperature control. DSC is equipped with two groups of compensating heating wires under the sample and reference container. When there is a temperature difference between the sample and the reference due to thermal effect during heating Δ T, the current flowing into the compensation heating wire can be changed through the differential thermal amplification circuit and differential thermal compensation amplifier,

And balance the heat on both sides and reduce the temperature difference Δ T disappears, and the change relationship between the difference of thermal power compensated by two electrothermal compensations under the sample and the reference material with temperature (or time) is recorded. According to this change relationship, the physicochemical and thermodynamic properties of the materials can be studied and analyzed. DSC is widely used, but in the analysis of PCB, it is mainly used to measure the curing degree (such as Figure 2) and glass transition temperature of various polymer materials used on PCB. These two parameters determine the reliability of PCB in the subsequent process.

Thermomechanical analyzer (TMA): Thermal Mechanical Analysis (thermomechanical analysis technology) is used to measure the deformation properties of solid, liquid and gel under thermal or mechanical force under the control of temperature. The commonly used load modes are compression, needle insertion, stretching and bending. The test probe is supported by the cantilever beam and coil spring fixed on it, and the load is applied to the sample through the motor. When the sample is deformed, the differential transformer detects this change, and processes it together with the data of temperature, stress and strain, so as to obtain the relationship between the deformation of the material under negligible load and temperature (or time). According to the relationship between deformation and temperature (or time), the physicochemical and thermodynamic properties of materials can be studied and analyzed. TMA is widely used. In the analysis of PCB, it is mainly used for two key parameters of PCB: measuring its linear expansion coefficient and glass transition temperature. The PCB with too large expansion coefficient often leads to the fracture failure of metallized holes after welding and PCB assembly.

Due to the development trend of PCB high density and the environmental requirements of lead-free and halogen-free, more and more PCBs have various failure problems, such as poor wetting, plate explosion, delamination, CAF and so on. The application of these analytical techniques in practice is introduced. The acquisition of PCB failure mechanism and causes will be conducive to the quality control of PCB in the future, so as to avoid the recurrence of similar problems.