PCB Technical - What are the causes of PCB board failure

As the carrier of various components and the hub of circuit signal transmission, the PCB board has become an important and key 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, PCB boards 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 PCB boards, 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 that has occurred.

Basic procedure of failure analysis
To obtain the exact cause or mechanism of the failure or failure of the PCB board, 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 board or PCB board A, the location of the failure 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. Sure. Then we need to analyze the failure mechanism, that is, use various physical and chemical methods to analyze the mechanism
that causes the failure or defect of the PCB board, 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 failure cause analysis,
that is, based on 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., Is to compile failure analysis based on the experimental data, facts and conclusions obtained in the analysis process, the required facts are clear, the
logical reasoning is strict, and the rationality is strong. 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. Just 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 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 board or PCB board A is also the same. If you use an electric soldering iron to repair the failed solder joints or use large scissors to cut the PCB board strongly, 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.

Failure Analysis Technology
Optical microscope: The optical microscope is mainly used for the appearance inspection of PCB boards, looking for failure parts and related physical evidence, and preliminarily judging the failure mode of PCB boards. The appearance inspection
mainly checks the PCB board pollution, corrosion, the location of the broken board, 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 (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 board, an X-ray fluoroscopy system has to be used for inspection. The X-ray fluoroscopy system uses different material thicknesses or different material densities based on the different principles of moisture absorption or transmittance of X-rays for imaging. This technology is more used to check the internal defects of PCB board A solder joints, the internal defects of through-holes, and the location of defective solder joints of high-density packaging BGA or CSP devices.

Slicing analysis: Slicing analysis is the process of obtaining the cross-sectional structure of the PCB board 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 the PCB board (through holes, plating, etc.), which provides a good basis for the next quality improvement. However, this method is destructive, and once the sectioning is carried out, the sample will inevitably be destroyed.

Scanning acoustic microscope: At present, the C-mode ultrasonic scanning acoustic microscope is mainly used for electronic packaging or assembly analysis. It uses the amplitude, phase and polarity changes caused by high-frequency ultrasonic reflection on the discontinuous interface of the material to image, and its scanning The method is to scan the information in the XY plane along the Z axis. Therefore, the scanning acoustic microscope can be used to detect components, materials,
and various defects inside the PCB and PCB A, 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. A typical
scanning acoustic image uses a red warning color to indicate the existence of defects. Because a large number of plastic packaged components are used in the SMT process, a large number of moisture reflow sensitivity issues are generated during
the conversion from lead to lead-free process. That is to say, moisture-absorbing plastic packaged devices will appear internal or substrate delamination cracking during reflow at a higher lead-free process temperature, and ordinary PCB boards
will often burst at the high temperature of the lead-free process. At this time, the scanning acoustic microscope highlights its special advantages in non-destructive flaw detection of multi-layer high-density PCB boards. Generally, obvious bursts
can be detected only by visual inspection of the appearance.

Micro-infrared analysis: Micro-infrared analysis is an analysis method that combines infrared spectroscopy and microscope. It uses the principle of different absorption of infrared spectra by different materials (mainly organic matter) to analyze the
compound composition of the material, and combined with the microscope can make Visible light and infrared light have the same optical path, as long as they are in the visible field of view, it is possible to find trace organic pollutants to be
analyzed. Without the combination of a microscope, infrared spectroscopy can usually only analyze samples with a large amount of samples. However, in many cases in electronic technology, micro-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 with a microscope. The main purpose of micro-infrared analysis is to analyze the organic contaminants on the welded surface or the
surface of the solder joint, and analyze the cause of corrosion or poor solderability.

Scanning electron microscope analysis: Scanning electron microscope is a useful large-scale electron microscopy imaging system for failure analysis. It is often used for topography observation. The current scanning electron microscope is already
very powerful, and any fine structure or surface feature can be magnified. Observe and analyze hundreds of thousands of times. In the failure analysis of PCB boards 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, and measure the intermetallic compound and solderability. Coating analysis and tin whisker analysis and measurement. Unlike an
optical microscope, the scanning electron microscope produces an electronic image, so only black and white colors are produced. The sample of the scanning electron microscope requires conductivity. Non-conductors 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.

Thermal analysis: Differential Scanning Calorimeter (DSC)
Differential Scanning Calorimetry (Differential Scanning Calorimetry) 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 that studies 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 the analysis of PCB boards, it is mainly used to measure the curing degree and glass transition temperature of various polymer materials used on the PCB board. These two parameters determine the reliability of
the PCB board in the subsequent process.

Thermal Mechanical Analyzer (TMA): Thermal Mechanical Analysis is used to measure the deformation properties of solids, liquids and gels under thermal or mechanical force under 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 has a wide
range of applications. It is mainly used in the analysis of PCB boards for two key parameters of PCB boards: measuring its linear expansion coefficient and glass transition temperature. The PCB board of the base material with too large expansion
coefficient will often lead to the fracture and failure of the metallized hole after welding and assembly.

Thermogravimetry Analysis (TGA): Thermogravimetry Analysis 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 in the process of program-controlled temperature change through a sophisticated electronic balance. According to the relationship between material quality and temperature (or time), the physical, chemical and
thermodynamic properties of materials can be studied and analyzed. In terms of PCB board analysis, it is mainly used to measure the thermal stability or thermal decomposition temperature of the PCB board material. If the thermal decomposition
temperature of the substrate is too low, the printed circuit boards will burst or delamination during the high temperature of the soldering process.