PCB circuit board has become the most important part of electronic products as its carrier of various components and signal transmission,PCB quality and reliability determines the quality and reliability of the whole machine.

With the miniaturization of electronic products and environmental protection requirements of lead-free and halogen-free,PCB also develops towards high-density, high Tg and environmental protection.However, due to cost and technical reasons, a large number of failures have been occurred in the production and application of PCB, and many quality disputes have been triggered.In order to find out the cause of the failure and to find a solution to the problem and to distinguish responsiblility,failure analysis of failure case must be carried out.

Basic procedures for failure analysis

To obtain PCB circuit board failure or poor accuracy or mechanism, must abide by the basic principle and the analysis process, otherwise the failure may miss valuable information, analysis can not continue or might get the wrong conclusion caused. The basic process is generally must first be based on the failure phenomenon, through the collection of information, function test, electrical performance test and simple visual inspection, determine the failure position and failure mode, the failure location or fault location.

For simple PCB or PCBA, the failure of the site is easy to determine, but for the more complex BGA or MCM package devices or substrate defects through the microscope, the moment is not easy to determine, this time need to be determined by other means.

Then it is necessary to analyze the failure mechanism, namely the use of various physical and chemical means to analyze the mechanism leading to PCB failure or defects, such as solder, pollution, mechanical damage, moisture stress, corrosion and fatigue damage, CAF or ionic migration, stress overload etc..

Then analyze the reasons for the failure, the failure mechanism and process based on process analysis, find the cause of the failure mechanism of the test should be carried out as necessary, general test may, through the test can find the reason of failure induced by the accurate.

This provides a definite basis for the next step of improvement. Finally, according to the analysis process to obtain test data, facts and conclusions, the preparation of failure analysis report, the requirements of the report of the facts clear, logical reasoning, strict, coherent, and should not be imagined.

In the process of analysis, attention should be paid to the basic principles of analytical methods, from simple to complex, from outside to inside, from never destroying samples to using them. Only in this way can we avoid losing the key information and avoid introducing new artificial failure mechanism.

The failure analysis of PCB or PCBA is the same. If an electric iron is used to repair the missing solder joints or to shear the PCB strongly, then the analysis cannot be done. The failure site has been destroyed. Especially in the case of less failure samples, once the damage or damage to the site of the environment, the real failure reason can not be obtained.

Failure analysis technique

1.Optical microscope

The optical microscope is mainly used for the appearance inspection of PCB, looking for the failure parts and relevant material evidence, and preliminarily judging the failure mode of PCB. The appearance inspection mainly examines the PCB pollution, corrosion, bursting plate location, circuit wiring and failure regularity, such as batch or individual, whether it is always concentrated in a certain area and so on.

2.X ray

For certain parts that cannot be visually examined, as well as through and through the hole and other internal defects of the PCB, the X ray fluoroscopy system is used. X-ray system is the use of different materials, thickness or density of different materials to X light moisture absorption or transmittance of different principles to imaging. This technique is used to check the internal defects of PCBA solder joints, the defects in through holes and the defects of BGA or CSP devices in high-density packaging.

3.Slice analysis

Slice analysis is the process of obtaining PCB cross section structure by means of sampling, mosaic, slicing, grinding, etching and observation. Through slicing analysis, we can get abundant information about microstructure of PCB (through hole, plating, etc.), and provide a good basis for quality improvement in the next step. But the method is destructive, and once it is sliced, the sample is bound to suffer damage.

4.Scanning acoustic microscope

At present, for the analysis of electronic packaging or assembly is mainly ultrasound scanning acoustic microscope C mode, which is reflected in the material discontinuous interface using high frequency ultrasonic amplitude and phase and polarity changes to imaging, its scanning mode is along the Z axis scanning X Y plane information.

Therefore, scanning acoustic microscopy can be used to detect defects in components, materials, and PCB and PCBA, including cracks, delamination, inclusions, voids, and so on. If the frequency of the scanning acoustic is sufficient, the internal defects of the solder joint can also be detected directly.

The typical image scanning acoustic is the red warning color represents the existence of defects, due to the large number of plastic packaging components used in the SMT process, by a lead into a lead-free process in the process of a large number of wet reflux sensitive issues, namely the internal moisture of plastic devices or substrate delamination phenomenon in lead-free reflow higher process temperatures, common in high temperature lead-free process under the PCB will often appear in the phenomenon of explosion.

At this point, scanning acoustic microscopy has highlighted its special advantages in multi layer high-density PCB nondestructive testing. In general, the obvious burst plates can be detected only by visual inspection.

5.Micro infrared analysis

FTIR analysis is infrared spectroscopy and microscopy analysis together, using different materials (mainly organic) different absorption principle of the infrared spectrum analysis, compound material, combined with the microscope can make visible light and infrared light, as long as in the field, you can find the analysis the trace organic pollutants.

If there is no combination of microscopes, the infrared spectrum can usually be used to analyze samples with more samples. And in the electronic process, a lot of situations are micro pollution, which can lead to the poor weldability of the PCB welding pad or lead pin. It can be imagined that it is difficult to solve the process problem without the infrared spectrum provided by the microscope. The main purpose of micro infrared analysis is to analyze the organic contaminants in the welded surface or solder joint surface, and to analyze the cause of poor corrosion or weldability.

6.Scanning electron microscope analysis (SEM)

Scanning electron microscope (SEM) is one of the most useful failure analysis of large electron microscopic imaging system, the most commonly used morphology observation, scanning electron microscopy, the function is very powerful, any fine structure or surface characteristics can be enlarged to hundreds of thousands of times were observed and analyzed.

The failure analysis of PCB or SEM solder joints, mainly used for the analysis of the failure mechanism, specifically is used to observe the pad surface morphology, microstructure, solder joint measurement of intermetallic compound, solderable coating and analysis of tin whisker measurement and analysis etc..

Different with the optical microscope, scanning electron microscope and the electron is like, so only black and white, and the scanning electron microscope sample requirements for non conductive conductor and semiconductor part need to spray gold or carbon, or charge accumulation on the surface of the sample on the influence of sample observation. In addition, the depth of image of scanning electron microscope is much larger than that of optical microscope. It is an important analysis method for metallographic structure, micro fracture and tin paste.

7.Thermal analysis

Differential scanning calorimetry (DSC)

Differential Scanning Calorimetry is a method of measuring the relationship between the power difference between a substance and reference material and temperature (or time) under programmed temperature control. It is a method to study the relationship between heat and temperature. According to this change relation, the physical, chemical and thermodynamic properties of materials can be studied and analyzed.

DSC is widely used, but in the analysis of PCB is mainly used for curing degree, glass transition temperature conversion of various high polymer materials used to measure the PCB, the two parameter determines the reliability of the PCB in the follow-up process.

8.Thermal mechanical analyzer (TMA)

Thermal Mechanical (Analysis) is used to measure the deformation properties of solids, liquids, and gels under thermal or mechanical forces for program 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 physical, chemical and thermodynamic properties of materials can be studied and analyzed.

TMA is widely used in PCB analysis. It is mainly used for the two key parameters of PCB: measuring its linear expansion coefficient and glass transition temperature. The PCB of the base material with excessive expansion coefficient often results in fracture of the metallized hole after welding assembly.

9.Thermogravimetric analyzer (TGA)

Analysis (Thermogravimetry) is a method of measuring the relationship between the mass of a substance with temperature (or time) under programmed temperature control.