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For simple PCB or PCBA, the failure site is easy to determine. Then,pcba failure analysis is carried out, that is, various physical and chemical means are used to analyze the mechanism that causes PCB failure or defects, such as cold solder joints, pollution, mechanical damage, moisture stress, dielectric corrosion, fatigue damage, CAF or ion migration, stress overload, etc.
The same is true for failure analysis of PCB or PCBA. If a soldering iron is used to re-solder the failed solder joints or a large pair of scissors is used to forcefully cut the PCB, then further analysis will be impossible because the failure site has been destroyed. Especially when there are few failed samples, once the environment of the failure site is destroyed or damaged, the real cause of failure cannot be obtained.
Failure analysis technology
Optical microscope
Optical microscope is mainly used for appearance inspection of PCB, to find the failed parts and related physical evidence, and to preliminarily judge the failure mode of PCB. Appearance inspection mainly checks the pollution, corrosion, location of the burst board, circuit wiring, and regularity of failure of PCB, such as batch or individual, whether it is always concentrated in a certain area, etc.
pcba failure analysis
X-ray (X-ray)
For some parts that cannot be inspected by appearance, as well as the inside of the through-hole and other internal defects of the PCB, an X-ray fluoroscopy system has to be used for inspection.
The X-ray fluoroscopy system uses the different principles of moisture absorption or transmittance of X-rays by different material thicknesses or different material densities to form images. This technology is more used to check the defects inside the PCBA solder joints, the defects inside the through-holes, and the location of defective solder joints of high-density packaged BGA or CSP devices.
Slice analysis
Slice pcba failure analysis is the process of obtaining the cross-sectional structure of the PCB through a series of means and steps such as sampling, inlaying, slicing, polishing, corrosion, and observation. Through slicing analysis, rich information about the microstructure reflecting the quality of the PCB (through-holes, plating, etc.) can be obtained, providing a good basis for the next step of quality improvement. However, this method is destructive. Once the slice is made, the sample will inevitably be destroyed.
Scanning acoustic microscope
Currently, the main ultrasonic scanning acoustic microscope used for electronic packaging or assembly analysis is the C-mode ultrasonic scanning acoustic microscope, which uses the amplitude, phase and polarity changes generated by the reflection of high-frequency ultrasonic waves on the discontinuous interface of the material to form an image. Its scanning method is to scan the information of the X-Y plane along the Z axis.
Microscopic infrared analysis
Microscopic infrared analysis is an analysis method that combines infrared spectroscopy with microscopy. It uses the principle that different materials (mainly organic matter) absorb infrared spectra differently to analyze the compound composition of the material. Combined with a microscope, visible light and infrared light can be used in the same optical path. As long as it is in the visible field of view, trace organic pollutants to be analyzed can be found.
Without the combination of a microscope, infrared spectroscopy can usually only analyze samples with a large 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 process problems without infrared spectroscopy equipped with a microscope. The main purpose of microscopic infrared analysis is to analyze organic pollutants on the surface of the soldered surface or solder joint, and analyze the causes of corrosion or poor solderability.
Scanning electron microscope analysis (SEM)
Scanning electron microscope (SEM) is the most useful large-scale electron microscopic imaging system for failure analysis. It is most commonly used for morphological observation. The current 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 PCBs or solder joints, SEM is mainly used for failure mechanism analysis. Specifically, it is used to observe the morphological structure of the pad surface, the metallographic structure of the solder joint, measure intermetallic compounds, analyze solderability coatings, and perform tin whisker analysis measurements, etc.
Unlike an optical microscope, a scanning electron microscope produces an electron image, so it only has black and white colors. In addition, the specimens of a scanning electron microscope are required to be conductive. Non-conductors and some semiconductors need to be treated with gold or carbon, otherwise the charge will accumulate on the surface of the sample and affect the observation of the sample. In addition, the depth of field of a scanning electron microscope image is much greater than that of an optical microscope, and it is an important analysis method for metallographic structures, microscopic fractures, and uneven samples such as tin whiskers.
Thermal analysis
Differential scanning calorimeter (DSC)
Differential scanning calorimetry (DSC) is a method of measuring the relationship between the power difference input to a substance and a reference substance and temperature (or time) under program temperature control. It is an analytical method for studying the relationship between heat and temperature. Based on this relationship, the physical, chemical and thermodynamic properties of the material can be studied and analyzed.
DSC is widely used, but in the analysis of PCBs, it is mainly used to measure the degree of curing and glass transition temperature of various polymer materials used on PCBs. These two parameters determine the reliability of PCBs in subsequent processes.
pcba failure analysis
Thermomechanical Analyzer (TMA)
Thermomechanical analysis technology (TMA) is used to measure the deformation properties of solids, liquids and gels under heat or mechanical force under program temperature control. It is a method to study the relationship between heat and mechanical properties. 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. In the analysis of PCB, it is mainly used to measure the two most critical parameters of PCB: its linear expansion coefficient and glass transition temperature. PCB with a substrate with too large expansion coefficient often leads to fracture failure of metallized holes after welding and assembly.
As shown in the figure above, according to the ASTM E1641 standard method, TGA test is used to analyze the decomposition reaction kinetics of PCB, and evaluate the thermal stability of PCB under actual operating temperature or welding temperature conditions. First, four thermogravimetric curves are obtained by testing at four different heating rates, and the activation energy is calculated with a decomposition conversion rate of 10%, so that the service life of PCB at a specific temperature can be predicted. For example, assuming that 1% is the critical loss point for PCB failure, then the PCB cannot exceed 3 minutes under the actual solder bath temperature (260°C), otherwise the PCB will decompose and fail.
After these tests, your pcba failure analysis will maybe fine.