Ultra-comprehensive PCB failure analysis technology

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Ultra-comprehensive PCB failure analysis technology [Copy link]

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. With the miniaturization of electronic information products and the environmental protection requirements of lead-free and halogen-free, PCB is also developing in the direction of high density, high Tg and environmental protection. However, due to cost and technical reasons, a large number of failure problems have occurred in the production and application of PCB, which has caused many quality disputes. In order to find out the cause of failure, find a solution to the problem and clarify the responsibility, failure analysis must be carried out on the failure cases that have occurred.

Basic Procedure of Failure Analysis

To obtain the accurate cause or mechanism of PCB failure or defect, the basic principles and analysis process must be followed, otherwise valuable failure information may be missed, resulting in the inability to continue the analysis or possible wrong conclusions. The general basic process is that first, based on the failure phenomenon, the failure location and failure mode must be determined through information collection, functional testing, electrical performance testing and simple appearance inspection, that is, failure location or fault location. For simple PCB or PCBA, the failure location is easy to determine, but for more complex BGA or MCM packaged devices or substrates, the defects are not easy to observe through a microscope, and it is not easy to determine at the moment. At this time, other means are needed to determine. Then, the failure mechanism 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 soldering, pollution, mechanical damage, moisture stress, dielectric corrosion, fatigue damage, CAF or ion migration, stress overload, etc. The next is the failure cause analysis, that is, based on the failure mechanism and process analysis, find the cause of the failure mechanism, and conduct experimental verification when necessary. Generally, experimental verification should be carried out as much as possible, and the accurate cause of induced failure can be found through experimental verification. This provides a targeted basis for the next step of improvement. Finally, a failure analysis report should be prepared based on the test data, facts and conclusions obtained during the analysis process. The report should have clear facts, rigorous logical reasoning and strong organization, and should avoid arbitrary imagination.

During the analysis, pay attention to the basic principle that the analysis method should be used from simple to complex, from outside to inside, and from never destroying the sample to using destruction. Only in this way can we avoid losing key information and introducing new artificial failure mechanisms. Just like a traffic accident, if one party of the accident destroys or flees the scene, it is difficult for the best police to make an accurate determination of responsibility. At this time, traffic regulations generally require the person who flees the scene or destroys the scene to bear all responsibilities. The same is true for the failure analysis of PCB or PCBA. If a soldering iron is used to re-solder the failed solder joints or a large scissors is used to forcefully cut the PCB, then further analysis will be impossible to start, and 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 microscopes are mainly used for appearance inspection of PCBs, to find the failed parts and related physical evidence, and to preliminarily determine the failure mode of PCBs. Appearance inspection mainly checks the contamination, corrosion, location of the exploded board, circuit wiring, and regularity of failure, such as batch or individual, whether it is always concentrated in a certain area, etc.

X-ray

For some parts that cannot be inspected by appearance, as well as the inside of the through-holes and other internal defects of the PCB, we have to use an X-ray fluoroscopy system to inspect them. The X-ray fluoroscopy system uses the different principles of moisture absorption or transmittance of X-rays due to different material thicknesses or densities to create images. This technology is more used to inspect defects inside PCBA solder joints, internal defects of through-holes, and the location of defective solder joints of high-density packaged BGA or CSP devices.

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, inlaying, slicing, polishing, etching, and observation. Slice analysis can obtain rich information about the microstructure of the PCB (through-holes, plating, etc.) quality, providing a good basis for the next step of quality improvement. However, this method is destructive, and once slicing is performed, the sample will inevitably be destroyed.

Scanning Acoustic Microscope

At present, 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. Therefore, the scanning acoustic microscope can be used to detect various defects in components, materials, PCBs and PCBAs, 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. The 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 sensitivity problems arise during the process of converting from lead-free to lead-free processes, that is, hygroscopic plastic-encapsulated devices will have internal or substrate delamination and cracking when reflowing at a higher lead-free process temperature. Ordinary PCBs will often have board explosions at high temperatures in the lead-free process. At this time, the scanning acoustic microscope highlights its special advantages in non-destructive testing of multi-layer high-density PCBs. The general obvious board explosion can be detected by visual inspection of the appearance.

Microscopic infrared analysis

Microscopic infrared analysis is an analysis method that combines infrared spectroscopy with a microscope. 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 placed on 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 can be imagined that it is difficult to solve process problems without an 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 to analyze the causes of corrosion or poor solderability.

Scanning electron microscopy 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 morphology observation. The current scanning electron microscope is very powerful. Any fine structure or surface feature can be magnified to hundreds of thousands times for observation and analysis.

In the failure analysis of PCB or solder joints, SEM is mainly used for failure mechanism analysis, specifically, to observe the morphology of pad surface, metallographic structure of solder joints, measure intermetallic compounds, solderability coating analysis, and tin whisker analysis. Unlike optical microscopes, SEM produces electron images, so there are only black and white colors. In addition, SEM samples are required to be conductive, and non-conductors and some semiconductors need to be treated 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 images is much greater than that of optical microscopes, and it is an important analysis method for uneven samples such as metallographic structure, microscopic fracture, and tin whiskers.

Thermal Analysis

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) is a method for measuring the relationship between the power difference input to a substance and a reference substance and temperature (or time) under programmed 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 materials can be studied and analyzed. DSC is widely used, but in the analysis of PCB, it is mainly used to measure the degree of curing and glass transition temperature of various polymer materials used on PCB. These two parameters determine the reliability of PCB in subsequent process.

Thermomechanical Analyzer (TMA)

Thermomechanical analysis (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 thermal 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 and failure of metallized holes after welding and assembly.

Thermogravimetric Analyzer (TGA)

Thermogravimetry (Thermogravimetry Analysis) is a method of measuring the relationship between the mass of a substance and the temperature (or time) under programmed temperature control. TGA can monitor the subtle mass changes of a substance during programmed temperature change through a precise electronic balance. According to the relationship between the mass of a substance and the temperature (or time), the physical, chemical and thermodynamic properties of the material can be studied and analyzed. In the analysis of PCBs, 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 delaminate when it passes through the high temperature of the welding process.

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