What techniques are used for failure analysis? Summary of 10 methods

Date：2022-05-27 14:50:22Views：976

As an emerging technology, failure analysis has been popularized to mass enterprises in recent years. In fact, failure analysis is to reproduce the failure causes of products, and find out the specific failure causes and failure mechanisms through inspection and analysis. Therefore, the subject of failure analysis can improve the product quality while taking into account the product repair and technical improvement.

1. appearance inspection

Appearance inspection is to visually inspect or use some simple instruments, such as stereomicroscope, metallographic microscope and even magnifying glass, to check the appearance of PCB and find out the failed parts and relevant physical evidence. Its main function is to locate the failure and preliminarily judge the failure mode of PCB. The appearance inspection mainly inspects the pollution, corrosion, position of PCB explosion, circuit wiring and regularity of failure, such as whether the PCB is always concentrated in a certain area in batches or individually. In addition, many PCB failures are found only after PCBA is assembled. Whether the failure is caused by the influence of the assembly process and the materials used in the process also requires careful inspection of the characteristics of the failure area.

2. X-ray fluoroscopy

For some parts that cannot be detected by visual inspection, as well as internal defects of PCB through holes and other internal defects, the X-ray fluoroscopy system can only be used for inspection. X-ray fluoroscopy system is to use the different principles of moisture absorption or transmittance of X-ray with different material thickness or different material density to image. This technology is more used to check the internal defects of PCBA solder joints, internal defects of through holes and the positioning of defective solder joints of high-density packaged BGA or CSP devices. At present, the resolution of industrial X-ray fluoroscopy equipment can reach less than one micron, and it is changing from two-dimensional to three-dimensional imaging equipment, and even five-dimensional (5d) equipment has been used for packaging inspection. However, this 5D X-ray fluoroscopy system is very valuable and rarely has practical application in the industry.

3. slice analysis

Slice analysis is the process of obtaining PCB cross-section structure through a series of means and steps, such as sampling, inlaying, slicing, polishing, corrosion, observation, etc. Through slice analysis, we can get rich information about the microstructure reflecting the quality of PCB (through hole, coating, etc.), which provides a good basis for the next step of quality improvement. But this method is destructive. Once sliced, the sample will be destroyed; At the same time, this method has high requirements for sample preparation and takes a long time, which needs well-trained technicians to complete. For detailed slicing process, refer to IPC-TM-650 2.1.1 and ipc-ms-810.

4. scanning acoustic microscope

At present, C-mode ultrasonic scanning acoustic microscope is mainly used for electronic packaging or assembly analysis. It uses the amplitude, phase and polarity changes generated by high-frequency ultrasonic reflection on the discontinuous interface of materials to image. Its scanning method is to scan the information of X-Y plane along the Z axis. Therefore, scanning acoustic microscope can be used to detect various defects in components, materials, PCB and PCBA, including cracks, delaminations, inclusions and cavities. If the frequency width of the scanning acoustics is sufficient, the internal defects of the solder joint can also be directly detected. The typical scanning acoustic image indicates the existence of defects with red warning color. Since a large number of plastic encapsulated components are used in the SMT process, a large number of moisture reflow sensitive problems occur during the conversion from lead to lead-free process, that is, moisture absorbing plastic encapsulated components will have internal or substrate delamination cracking when reflow at higher lead-free process temperature, Under the high temperature of lead-free process, common PCB will often explode. At this time, the scanning acoustic microscope highlights its special advantages in the non-destructive testing of multi-layer high-density PCB. However, the general obvious bursting plate can be detected by visual inspection.

5. micro infrared analysis

Micro infrared analysis is an analysis method that combines infrared spectrum with microscope. It uses the principle of different absorption of infrared spectrum by different materials (mainly organic substances) to analyze the compound composition of materials. Combined with microscope, it can make visible light and infrared light have the same light path. As long as it is in the visible field of view, it can find trace organic pollutants to be analyzed. If there is no microscope, the infrared spectrum can only analyze the samples with a large amount of samples. In many cases, micro pollution in electronic process can lead to poor solderability of PCB pad or lead pin. It is conceivable that it is difficult to solve the process problem without infrared spectrum equipped with microscope. The main purpose of micro infrared analysis is to analyze the organic pollutants on the surface of the welded surface or solder joint, and to analyze the causes of corrosion or poor weldability.

6. scanning electron microscope analysis

Scanning electron microscope (SEM) is one of the most useful large-scale electron microscopic imaging systems for failure analysis. Its working principle is to use the electron beam emitted by the cathode to accelerate through the anode and form an electron beam with a diameter of tens to thousands of angstroms (a) after focusing by the magnetic lens. Under the deflection of the scanning coil, the electron beam makes a point-by-point scanning movement on the sample surface in a certain time and space sequence, The bombardment of this high-energy electron beam on the sample surface will stimulate a variety of information. After collection and amplification, various corresponding graphics can be obtained from the display screen. The excited secondary electrons are generated in the range of 5~10nm on the sample surface. Therefore, the secondary electrons can better reflect the morphology of the sample surface, so they are most commonly used for morphology observation; The excited backscattered electrons are generated in the range of 100~1000nm on the sample surface, and emit backscattered electrons with different characteristics with different atomic numbers of substances. Therefore, the backscattered electron image has the ability to distinguish the morphological characteristics and atomic numbers. Therefore, the backscattered electron image can reflect the distribution of chemical elements. At present, the function of scanning electron microscope is very powerful. Any fine structure or surface feature can be magnified to hundreds of thousands of times for observation and analysis.

In the failure analysis of PCB or solder joint, SEM is mainly used to analyze the failure mechanism. Specifically, it is used to observe the surface morphology and structure of the pad, the metallographic structure of the solder joint, measure the intermetallics, analyze the solderable coating, and measure the tin whisker. Different from the optical microscope, the scanning electron microscope produces an electronic image, so there are only black and white, and the sample of the scanning electron microscope is required to be conductive. Non conductors and some semiconductors need to be sprayed with gold or carbon, otherwise the charge will accumulate on the surface of the sample and affect the observation of the sample. In addition, the depth of field of SEM image is much larger than that of optical microscope, which is an important analysis method for uneven samples such as metallographic structure, micro fracture and tin whisker.

7.x-ray energy spectrum analysis

The scanning electron microscope mentioned above is generally equipped with X-ray energy spectrometer. When the high-energy electron beam strikes the surface of the sample, the inner electrons in the atoms of the surface material are bombarded and escaped, and the outer electrons will excite the characteristic X-rays when they transition to the low-energy level. The characteristic X-rays emitted by different elements are different due to different atomic energy levels. Therefore, the characteristic x-rays emitted by the sample can be used as chemical composition analysis. At the same time, according to the characteristic wavelength or energy of the X-ray detection signal, the corresponding instruments are called spectral dispersion spectrometer (WDS) and energy dispersion spectrometer (EDS). The resolution of the spectrometer is higher than that of the spectrometer, and the analysis speed of the spectrometer is faster than that of the spectrometer. Because of the high speed and low cost of the energy spectrometer, the general scanning electron microscope is equipped with the energy spectrometer.

With the different scanning modes of electron beam, the energy spectrometer can carry out point analysis, line analysis and surface analysis on the surface, and can obtain the information of different distribution of elements. All elements of a point obtained by point analysis; Line analysis perform an element analysis on a specified line each time, and scan for many times to obtain the line distribution of all elements; Surface analysis analyzes all elements in a specified surface, and the measured element content is the average value of the measured surface range.

In the PCB analysis, the energy spectrometer is mainly used for the composition analysis of the pad surface, and the element analysis of the pollutants on the pad and lead pin surface with poor solderability. The accuracy of quantitative analysis of energy spectrometer is limited, and the content below 0.1% is generally difficult to detect. The combination of EDS and SEM can obtain the information of surface morphology and composition at the same time, which is the reason why they are widely used.

8. photoelectron spectroscopy (XPS) analysis

When the sample is irradiated by X-rays, the inner shell electrons of the surface atoms will escape from the bondage of the atomic nucleus and form electrons on the solid surface. By measuring its kinetic energy ex, the binding energy EB of the inner shell electrons of the atoms can be obtained. EB varies with different elements and different electronic shells. It is the "fingerprint" identification parameter of the atoms, and the formed spectral line is photoelectron spectroscopy (XPS). XPS can be used for qualitative and quantitative analysis of shallow surface elements (several nanoscale) on the sample surface. In addition, information about the chemical valence states of elements can be obtained from the chemical shifts of binding energy. It can give information about the valence states of the surface layer atoms and the bonding of the surrounding elements; The incoming beam is X-ray photon beam, so the insulation sample can be analyzed without damaging the analyzed sample for rapid multi-element analysis; It is also possible to analyze the longitudinal element distribution of multiple layers in the case of argon ion stripping (see the following case), and the sensitivity is much higher than that of energy spectrum (EDS). XPS is mainly used to analyze the quality of pad coating, pollutants and oxidation degree in PCB analysis to determine the deep-seated causes of poor solderability.

9. differential scanning calorimetry for thermal analysis

Under programmed temperature control, a method for measuring the relationship between the power difference input to the substance and the reference substance and temperature (or time). DSC is equipped with two groups of compensating heating wires under the sample and reference containers. When the temperature difference between the sample and the reference occurs due to the thermal effect during the heating process Δ T, the current flowing into the compensation heating wire can be changed through the differential thermal amplification circuit and the differential thermal compensation amplifier.

And make the heat balance on both sides, and the temperature difference Δ T disappears, and record the relationship between the difference of the thermal power of the two electrothermal compensations under the sample and the reference with temperature (or time). According to this relationship, the physical, chemical and thermodynamic properties of the 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 PCB. These two parameters determine the reliability of PCB in the subsequent process.

10. thermomechanical analyzer (TMA)

Thermal mechanical analysis is used to measure the deformation properties of solids, liquids and gel under the action of thermal or mechanical forces under programmed temperature control. The commonly used loading methods include compression, penetration, tension, bending, etc. The test probe is supported by a cantilever beam and a coil spring fixed on it, and the load is applied to the sample through the motor. When the sample is deformed, the differential transformer detects the change, and processes the data together with temperature, stress and strain to obtain the relationship between the deformation of the material under negligible load and temperature (or time). According to the relationship between deformation and temperature (or time), the physicochemical and thermodynamic properties of materials can be studied and analyzed. TMA is widely used. In PCB analysis, it is mainly used to measure the two most critical parameters of PCB: its linear expansion coefficient and glass transition temperature. The PCB with too large expansion coefficient often leads to the fracture failure of metallized holes after welding and assembly.