Basic Procedures for Failure Analysis
In order to obtain the exact cause or mechanism of PCB failure or failure, the basic principles and analysis process must be followed, otherwise, valuable failure information may be missed, resulting in the failure of 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 visual inspection, that is, failure location or failure location. For simple PCB or PCBA, the location of failure is easy to determine. However, for more complex BGA or MCM packaged devices or substrates, defects are not easy to observe through a microscope, and it is not easy to determine for a while. At this time, other means are needed to determine. Then it is necessary to analyze the failure mechanism, that is, to use various physical and chemical means to analyze the mechanism that leads to PCB failure or defects, such as virtual soldering, pollution, mechanical damage, moisture stress, medium corrosion, fatigue damage, CAF or ion migration, Stress overload and more. Then there is failure cause analysis, that is, based on the analysis of failure mechanism and manufacturing process, to find the cause of the failure mechanism, and to carry out test verification if necessary. Generally, test verification should be carried out as much as possible, and the exact cause of induced failure can be found through test verification. This provides a targeted basis for the next step of improvement. Finally, based on the test data, facts and conclusions obtained during the analysis process, a failure analysis report should be prepared, which requires clear facts, rigorous logical reasoning, and strong rationality, and should not be based on imagination.
During the analysis process, pay attention to the basic principles of using the analysis method from simple to complex, from the outside to the inside, never destroying the sample and then using it to destroy. Only in this way can the loss of key information and the introduction of new artificial failure mechanisms be avoided. Just like a traffic accident, if one party to the accident destroys or escapes from the scene, it will be difficult for a clever policeman to make an accurate determination of responsibility. At this time, traffic laws generally require the person who fled the scene or the party who destroyed the scene to bear full responsibility. The failure analysis of PCB or PCBA is the same. If you use a soldering iron to repair the failed solder joints or use large scissors to cut the PCB forcefully, then re-analysis will be impossible, and the site of failure has been destroyed. Especially in the case of few failure samples, once the environment of the failure site is destroyed or damaged, the real cause of failure cannot be obtained.
Failure Analysis Technique
Optical microscopes are mainly used for visual inspection of PCBs, looking for failure locations and related physical evidence, and preliminarily judging 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.
For some parts that cannot be detected by visual inspection, as well as inside the through hole 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.
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 sectioned, the sample must be destroyed.
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.
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.
Scanning Electron Microscopy (SEM)
Scanning electron microscope (SEM) is one of the most useful large-scale electron microscopic imaging systems for failure analysis. It is most commonly used for morphology observation. The current scanning electron microscope is already 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.
Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry (DifferentialScanningCalorim-etry) is a method of measuring the relationship between the power difference and the temperature (or time) between the input substance and the reference substance under programmed temperature control. It is an analysis method to study the relationship between heat and temperature. According to this change 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 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.
Thermomechanical Analyzer (TMA)
Thermal Mechanical Analysis (Thermal Mechanical Analysis) is used to measure the deformation properties of solids, liquids and gels under thermal or mechanical force under programmed 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, 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.
Thermogravimetric Analyzer (TGA)
Thermogravimetry (Thermogravimetry Analysis) is a method of measuring the relationship between the mass of a substance and the change of temperature (or time) under programmed temperature control. TGA can monitor the subtle mass changes of substances in the process of program-controlled temperature change through precise electronic balances. According to the relationship between the mass of the substance and the change of temperature (or time), the physical, chemical and thermodynamic properties of the material can be studied and analyzed. In terms of PCB analysis, 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 soldering process.