As the hub of various component carriers and circuit signal transmission, PCB has become the most important and key 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 requirements of lead-free and halogen-free, PCBs are also developing in the direction of high density and high Tg and environmental protection. However, due to cost and technical reasons, PCBs have experienced a large number of failures in the production and application process, and thus caused many quality disputes. In order to clarify the cause of the failure in order to find a solution to the problem and to clarify the responsibility, a failure analysis must be performed on the failure case that occurred.
Basic procedure for failure analysis
To obtain the exact cause or mechanism of PCB failure or failure, you must follow the basic principles and analysis procedures, otherwise valuable failure information may be missed, resulting in the analysis can not continue or may get wrong conclusions. The general basic process is to first determine the failure location and failure mode, ie, failure location or fault location, based on failure phenomena, through information collection, functional testing, electrical performance testing, and simple visual inspection.
For simple PCB or PCBA, the failed part is easy to determine. However, for a more complex BGA or MCM packaged device or substrate, the defect is not easy to observe through the microscope, and it is difficult to determine at a time. This time, other means are needed to determine.
Next, an analysis of the failure mechanism is performed, using various physical and chemical means to analyze the mechanism that causes PCB failure or defect generation, such as virtual welding, contamination, mechanical damage, moisture stress, dielectric corrosion, fatigue damage, CAF or ion migration, Stress overload and so on.
Then the failure cause analysis, that is, based on the failure mechanism and the process analysis, find the cause of the failure mechanism, and if necessary, carry out test verification. Generally, the test verification should be possible as possible, and the cause of the induced failure can be found through the test verification.
This provides a targeted basis for the next step of improvement. Finally, according to the test data, facts and conclusions obtained in the analysis process, the failure analysis report is compiled, and the facts of the report are required to be clear, the logical reasoning is strict, the rules are strong, and it is forbidden to imagine.
In the process of analysis, pay attention to the basic principles of using analytical methods from simple to complex, from the outside to the inside, from the destruction of the sample to the use of damage. Only in this way can we avoid losing key information and avoid introducing new artificial failure mechanisms.
Just like a traffic accident, if one of the accidents destroys or flees the scene, it is difficult for the Gaoming police to make an accurate responsibility. At this time, the traffic regulations generally require the party who fled the scene or the site to be damaged to assume full responsibility.
The same is true for the failure analysis of PCB or PCBA. If the soldering iron is used to repair the failed solder joints or the large scissors are used to strongly cut the PCB, then the analysis will not be possible, and the failed scene has been destroyed. Especially in the case of a small number of failed samples, once the environment of the failed site is destroyed or damaged, the real cause of failure cannot be obtained.
Failure analysis technique
Optical microscope
The optical microscope is mainly used for the visual inspection of the PCB, looking for the failed parts and related physical evidence, and initially determining the failure mode of the PCB. The visual 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, is always concentrated in a certain area and so on.
X-ray (X-ray)
For some parts that cannot be visually 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 used more to inspect defects inside PCBA solder joints, via internal defects, and the location of defective solder joints in high-density packaged BGA or CSP devices.
Slice analysis
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 sliced, 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 high-frequency ultrasonic reflection on the discontinuous interface of the material to image. The Z axis scans the information of the X-Y plane.
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 sufficient, the internal defects of the solder joints can also be directly detected.
Typical scanning acoustic images represent the presence of defects in a red warning color. Due to the large number of plastic packaged components used in the SMT process, a large amount of moisture reflow sensitive problems occur during the conversion from lead to lead-free processes. That is, the hygroscopic plastic sealing device will cause internal or substrate delamination when reflowing at a higher lead-free process temperature, and the ordinary PCB will often explode at the high temperature of the lead-free process.
At this point, scanning acoustic microscopy highlights its unique advantages in non-destructive testing of multilayer high-density PCBs. The general obvious explosion plate can be detected only by visual inspection.
Microscopic infrared analysis
Micro-infrared analysis is an analytical method that combines infrared spectroscopy with a microscope. It uses different materials (mainly organic matter) to absorb different infrared spectra, analyzes the compound composition of the material, and combines the microscope to make visible light and infrared light The light path, as long as it is visible in the field of view, can be found to analyze trace amounts of organic pollutants.
If there is no microscope combination, usually the infrared spectrum can only analyze samples with a larger 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 the process problem without the infrared spectrum of the microscope. The main purpose of microscopic infrared analysis is to analyze the organic contaminants on the surface of the soldered surface or solder joints and analyze the causes of poor corrosion or 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. Observe and analyze 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 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 optical microscopes, SEMs are electronic images, so only black and white, and SEM samples require electrical conduction. Non-conductors and some semiconductors need to be sprayed with gold or carbon, otherwise the charge will accumulate on the surface of the sample. Observation of the sample. In addition, the depth of field of the SEM image is much larger than that of the optical microscope. It is an important analytical method for irregular samples such as metallographic structure, micro-fracture and tin whiskers.
Thermal analysis
Differential Scanning Calorimeter (DSC)
Differential Scanning Calorimography is a method of measuring the relationship between the power difference between a substance and a reference substance and temperature (or time) at program temperature control. It is an analytical method for studying the relationship between heat and temperature. According to this relationship, the physical and chemical properties of materials 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 process.
Thermomechanical Analyzer (TMA)
Thermal Mechanical 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 and 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 solder assembly.
Thermogravimetric Analyzer (TGA)
Thermogravimetry Analysis is a method of measuring the mass of a substance as a function of temperature (or time) under program temperature control. TGA monitors the subtle mass changes that occur during the programmed temperature change process through a sophisticated electronic balance.
Physicochemical and thermodynamic properties of materials can be studied based on the relationship between mass and temperature (or time). In the analysis of PCB, 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 delainate when it passes through the high temperature of the soldering process.
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