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PCB failure analysis

As the carrier of various components and the hub of circuit signal transmission, PCB has become the most important and critical part of electronic information products. Its quality and reliability determine the quality and reliability of the whole 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 during the production and application of PCBs, which has caused many quality disputes. In order to find out 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 occurred.

Basic procedure of failure analysis

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 difficult to determine for a while. At this time, it is necessary through by other means 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 defects, such as pseudo soldering, pollution, mechanical damage, moisture stress, medium corrosion, fatigue damage, CAF or ion migration, stress overload and so on.

Then there is failure reason 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 exact cause of induced failure can be found through test verification.

This provides a targeted basis for the next improvement. Finally, according to the test data, facts and conclusions obtained in the analysis process, the failure analysis report is compiled, and the reported facts are required to be clear, logical reasoning, and coherent. Do not imagine out of thin air.

In the process of analysis, pay attention to the basic principles of using analytical methods from simple to complex, from outside to inside, from no damage sample to damage. 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, no matter how smart the police are, it is difficult 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 also 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 in the case of 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 the PCB, looking for the failure location 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 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

For some parts that cannot be visually inspected, as well as the internal and other internal defects of PCB through holes, X-ray fluoroscopy system has to be used for inspection.

X-ray fluoroscopy system is to use different material thickness or different material density to X-ray moisture absorption or different principles to image. This technology is more used to check the internal defects of PCBA solder joints, the internal defects of through-holes, and the location of defective solder joints of BGA or CSP devices in high-density packaging.

Micronsection coupon analysis

Microsection coupon analysis is the process of obtaining PCB cross-sectional structure through a series of methods and steps such as sampling, inlaying, slicing, polishing, corrosion, and observation. Through microsection coupon analysis, we can get rich information of the microstructure reflecting 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 is bound to be destroyed.

Scanning acoustic microscope

Currently used for electronic packaging or assembly analysis is mainly the C-mode ultrasonic scanning acoustic microscope, which uses the amplitude, phase and polarity changes generated by the reflection of high-frequency ultrasonic waves on the discontinuous interface of the material to image. The scanning method is along the the Z axis scans the information on the XY plane.

Therefore, the scanning acoustic microscope can be used to detect various defects in components, materials, and PCBs and PCBAs, 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 problems are generated during the conversion from lead to lead-free process. That is to say, moisture-absorbing plastic packaged devices will have internal or substrate delamination cracking during reflow at a higher lead-free process temperature, and ordinary PCBs will often experience board bursts at the high temperature of the lead-free process.

At this time, the scanning acoustic microscope highlights its special advantages in the non-destructive multi-layer high-density PCB. Generally, obvious broken can be detected by visual inspection.

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 the same The light path, as long as it is in the visible field of view, you can find the trace organic pollutants to be analyzed.

Without the combination of microscope, infrared spectroscopy can only analyze samples with a large amount of samples. However, in many cases in electronic technology, slight 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 (SEM) is one of the most useful large-scale electron microscopy imaging systems for failure analysis. It is most commonly used for morphology observation. The current scanning electron microscope is already very powerful. Any fine structure or surface feature can be magnified to hundreds of thousands of times for observe and analysis.

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, the solderability coating analyze and do tin whisker analysis and measurement.

Unlike the optical microscope, the scanning electron microscope produces an electronic image, so there are only two colors in black and white, 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 charge accumulation 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 much 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 Calorim- etry

Differential scanning calorimetry (DSC) is a method of measuring the relationship between the power difference between the input material and the reference material and the temperature (or time) under the temperature control of the program. 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 is widely used, 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.

Thermal Mechanical Analysis

Thermal Mechanical Analysis (TMA) 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 PCB analysis for the two most critical parameters of PCB: measuring its linear expansion coefficient and glass transition temperature. PCBs with substrates with excessive expansion coefficients often lead to fracture failure of the metallized holes after soldering and assembly.

Thermogravimetry Analysis

Thermogravimetry analysis (TGA) is a method of measuring the relationship between the material quality 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 process 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 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 broken or delamination etc. failure phenomenon during the high temperature of the soldering process.