PCB Technical - Super comprehensive pcb failure analysis technology

As the carrier of various components and the hub of circuit board 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 entire equipment. With the miniaturization of electronic information products and the environmental protection requirements of lead-free and halogen-free, PCBs are 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 PCBs, which has caused many quality disputes. In order to clarify the cause of the failure in order to find a solution to the problem and distinguish the responsibilities, it is necessary to conduct a failure analysis on the failure cases that have occurred.

To obtain the accurate cause or mechanism of PCB failure or failure, the basic principles and analysis process must be followed, otherwise valuable failure information may be missed, causing the analysis to be unable to continue or may get 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 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 are not easy to determine for a while. At this time, other means are needed to determine. Then we must analyze the failure mechanism, that is, use various physical and chemical methods to analyze the mechanism that causes PCB failure or defect generation, such as virtual welding, pollution, mechanical damage, moisture stress, medium corrosion, fatigue damage, CAF or ion migration, Stress overload and so on.

Then there is the failure cause analysis, that is, based on the failure mechanism and process analysis, to find the cause of the failure mechanism, and test verification if necessary. Generally, test verification should be performed as much as possible, and the accurate cause of induced failure can be found through test verification. This provides a targeted basis for the next improvement. Finally, it is to compile a failure analysis report based on the test data, facts and conclusions obtained in the analysis process, requiring clear facts, strict logical reasoning, and strong organization. Do not imagine out of thin air.

In the process of analysis, pay attention to the basic principles that the analytical method should be from simple to complex, from outside to inside, never destroying the sample and then using it. Only in this way can we avoid the loss of key information and the introduction of new man-made failure mechanisms. It is like a traffic accident. If the party involved in the accident destroys or escapes the scene, it is difficult for the wise police to make an accurate determination of responsibility. At this time, the 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 an electric soldering iron to repair the failed solder joints or use large scissors to forcefully cut the PCB, then there is no way to start the analysis, 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 failure cause cannot be obtained.

Basic procedure of failure analysis

Optical microscope

The optical microscope is mainly used for the appearance inspection of the PCB, looking for the failure parts and related physical evidence, and preliminarily determining the failure mode of the PCB. The visual inspection mainly checks the PCB pollution, corrosion, the location of the board burst, the circuit wiring and the regularity of the failure, if it is batch or individual, is it always concentrated in a certain area, etc.

X-ray

For some parts that cannot be visually inspected, as well as the internal and other internal defects of the through holes of the PCB, X-ray fluoroscopy system has to be used for inspection. X-ray fluoroscopy systems use different material thicknesses or different material densities based on different principles of moisture absorption or transmittance of X-rays for imaging. This technology is more used to check the internal defects of PCBA solder joints, the internal defects of through-holes, and the positioning of defective solder joints of BGA or CSP devices in high-density packaging.

Slice analysis

Slicing analysis is the process of obtaining the cross-sectional structure of the PCB through a series of methods and steps such as sampling, inlaying, slicing, polishing, corrosion, and observation. Through slice analysis, we can get rich information of the microstructure that reflects the quality of PCB (through holes, plating, etc.), which provides a good basis for the next quality improvement. However, this method is destructive, once the sectioning is carried out, the sample will inevitably be destroyed.

Scanning Electron Microscope Analysis (SEM)

Scanning electron microscope (SEM) is one of the most useful large-scale electron microscopy imaging systems for failure analysis. It is most commonly used for topography observation. The current scanning electron microscopes are already very powerful. Any fine structure or surface feature can be magnified. Observe and analyze hundreds of thousands of times.

In the failure analysis of PCB or solder joints, SEM is mainly used to analyze the failure mechanism. Specifically, it is used to observe the topographic structure of the pad surface, the metallographic structure of the solder joint, measure the intermetallic compound, and the solderability coating Analyze and do tin whisker analysis and measurement. Unlike the optical microscope, the scanning electron microscope produces an electronic image, so it only has black and white colors, and the sample of the scanning electron microscope needs to be conductive, and the non-conductor and some semiconductors need to be sprayed with gold or carbon. Otherwise, the accumulation of charges on the surface of the sample will affect Observation of the sample. In addition, the depth of field of the scanning electron microscope image is far greater than that of the optical microscope, and it is an important analysis method for uneven samples such as metallographic structure, microscopic fracture and tin whisker.

Differential Scanning Calorimeter (DSC)

Differential Scanning Calorimetry (Differential Scanning Calorim-etry) is a method to measure the relationship between the power difference between the input material and the reference material and the temperature (or time) under program temperature control. It is an analytical method for studying the relationship between heat and temperature. According to this relationship, the physical, chemical and thermodynamic properties of materials can be studied and analyzed. DSC has a wide range of applications, but in PCB analysis, it is mainly used to measure the curing degree and glass transition temperature of various polymer materials used on the PCB. These two parameters determine the reliability of the PCB in the subsequent process.

Thermogravimetric Analyzer (TGA)

Thermogravimetry (ThermogravimetryAnalysis) is a method of measuring the relationship between the mass of a substance and the temperature (or time) under program temperature control. TGA can monitor the subtle quality changes of the material during the program-controlled temperature change through a sophisticated electronic balance. According to the relationship of material quality with temperature (or time), the physical, chemical and thermodynamic properties of materials can be studied and analyzed. In terms of PCB analysis, it is mainly used to measure the thermal stability or thermal decomposition temperature of the PCB material. If the thermal decomposition temperature of the substrate is too low, the PCB will explode or fail delamination during the high temperature of the soldering process.