Pcb failure analysis technology
For a simple PCB or PCBA, the location of the failure is easy to determine. However, for a more complex BGA or MCM packaged device or substrate, the defect is not easily observed by the microscope, and it is difficult to determine at a short time, the following detailed description of several compensation pcb failure analysis technology.
The optical microscope is mainly used for the appearance inspection of the PCB, looking for the failed parts and related object evidence, and initially judging the failure mode of the PCB. The appearance 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, whether it’s always concentrated in a certain area and so on.
For some parts that cannot be appearance 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 often used to inspect defects inside PCBA solder joints, through hole internal defects, and the location of defective solder joints in high-density packaged BGA or CSP devices.
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, and once proceed slice, the sample is bound to be destroyed.
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 the high-frequency ultrasonic reflection on the discontinuous interface of the material to image. It scans the x-y plane information along the z-axis 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 big enough, the internal defects of the solder joints can also be directly detected. A typical scanning acoustic image is the presence of a defect in a red warning color. Due to the large number of plastic packaged components used in the SMT process, a large number of wet reflow sensitive problems occur during the conversion from lead to lead-free processes. That is, the hygroscopic plastic package device will cause internal or substrate delamination when reflowing at a higher lead-free craft temperature, and the ordinary PCB will often explode at the high temperature of the lead-free craft. At this point, scanning acoustic microscopy highlights its special advantages in non-destructive aspect of multilayer high-density PCBs. The general obvious blasting board can be detected only by appearance inspection.
Microscopic infrared analysis
Micro-infrared analysis is an analytical method that combines infrared spectroscopy with a microscope. It uses the principle of different absorption of different spectroscopy of different infrared materials to analyze the compound composition of the material. Combined with the microscope which can make the visible light and the infrared light be seen in the same way. Under the field of view, you can find trace amounts of organic pollutants. If there is no microscope combination, usually the infrared spectroscopy can only analyze samples with a large amount of sample. In many cases in electronic crafts, trace contamination can lead to poor solderability of PCB pads or lead pins. It is conceivable that it is difficult to solve the craft problem without the infrared spectroscopy of the microscope. The main purpose of microscopic infrared analysis is to analyze the organic contaminants on the surface of the welded surface or solder joints and analyze the causes of corrosion or poor solderability.
Scanning electron microscopy analysis (SEM)
Scanning Electron Microscopy (SEM) is one of the most useful large-scale electron microscopy imaging systems for failure analysis. It is most commonly used for morphological observation. The current scanning electron microscope is very powerful, and any fine structure or surface features can be amplified to observe and analyze for hundreds of thousands of times.
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 welding 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 an optical microscope, a scanning electron microscope formed is an electronic image, so it is only black and white, and the sample of the scanning electron microscope requires conduction. The non-conductor and part of the semiconductor need to be sprayed with gold or carbon treatment, otherwise the charge will accumulate on the surface of the sample which affect observation of the sample. In addition, the depth of the SEM image is much larger than that of the optical microscope, which is an important analytical method for irregular samples such as metallographic structure, micro-fracture and tin whiskers.
Differential Scanning Calorimeter (DSC)
Differential Scanning Calorim-etry is a method of measuring the relationship between the power difference and temperature (or time) input between substance and reference substance under program temperature control. It is an analytical method for studying the relationship between heat and temperature. According to this changing relationship, the physical and chemical properties of the material can be studied. 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 craft process.
Thermomechanical Analyzer (TMA)
Thermomechanical Analysis is used to measure the deformation properties of solids, liquids and gels under thermal or mechanical forces under programmed 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 physicochemical and thermodynamic properties of materials can be studied. TMA is widely used in PCB analysis. It 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 welding and assembly.
Thermogravimetric Analyzer (TGA)
Thermogravimetry Analysis is a method of measuring the quality of a substance along with temperature changing(or time) under program temperature control. TGA monitors the subtle quality changes that occur during the programmed temperature change process through a sophisticated electronic scales. Physicochemical and thermodynamic properties of materials can be studied based on the relationship that material quality along with temperature changing(or time). In the analysis of PCB aspect, it is mainly used to measure the thermal stability or thermal decomposition temperature of PCB materials. If the thermal decomposition temperature of the substrate is too low, the PCB will explode or layer when it passes through the high temperature of the welding process.
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