As the carrier of various components and the hub of circuit signal transmission, PCB has become the most important and key part of electronic information products, its quality and reliability level determines the quality and reliability of the whole equipment. However, due to cost and technical reasons, there are a lot of failure problems in PCB production and application.

For this kind of failure problem, we need to use some commonly used failure analysis techniques to ensure the quality and reliability level of PCB in manufacturing. This paper summarizes ten failure analysis techniques for reference.

Mechanism and cause analysis of PCB failure

1. A visual inspection

Appearance inspection is to visually inspect or use some simple instruments, such as stereoscopic microscope, metallographic microscope or even magnifying glass, to check the appearance of PCB and find the failed parts and relevant physical evidence. The main function is to locate the failure and preliminarily judge the failure mode of PCB. Appearance inspection mainly checks PCB pollution, corrosion, the location of board explosion, circuit wiring and the regularity of failure, if it is batch or individual, whether it is always concentrated in a certain area, etc. In addition, the failure of many PCBS was discovered after the assembly of PCBA. Whether the failure was caused by the influence of the assembly process and materials used in the process also requires careful examination of the characteristics of the failure area.

2. X-ray fluoroscopy

For some parts that cannot be inspected by the appearance, as well as the inside of the PCB through hole and other internal defects, we have to use X-ray fluoroscopy system to check. X-ray fluoroscopy system is the use of different material thickness or different material density of X-ray hygroscopicity or transmittance of different principles to imaging. This technology is more used to check the location of defects in PCBA solder joints, through hole defects and defects in BGA or CSP devices with high density packaging. At present, the resolution of industrial X-ray fluoroscopy equipment can reach less than one micron, and is changing from two dimensional to three dimensional imaging equipment. There are even five dimensional (5D) equipment used for packaging inspection, but this 5D X-ray fluoroscopy system is very expensive, and rarely has practical application in the industry.

3. Section analysis

Slice analysis is the process of obtaining PCB cross section structure through sampling, Mosaic, slice, polishing, corrosion, observation and a series of methods and steps. Abundant information about the microstructure of PCB (through hole, coating, etc.) can be obtained by slice analysis, which provides a good basis for the next quality improvement. However, this method is destructive, once the slice is carried out, the sample will inevitably be destroyed; At the same time, the method of sample requirements are high, sample preparation time is also long, the need for trained technical personnel to complete. For detailed slicing procedures, please refer to IPC standards 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 makes use of the amplitude, phase and polarity changes generated by the reflection of high-frequency ultrasound on the discontinuous interface of materials to image, and its scanning mode is to scan the information in the X-Y plane along the Z-axis. Therefore, scanning acoustic microscopy can be used to detect various defects, including cracks, delamination, inclusions, and voids, in components, materials, and PCB and PCBA. Internal defects of solder joints can also be directly detected if the frequency width of scanning acoustics is sufficient. Of a typical scanning acoustic image in color red alert said defects exist, because a large amount of plastic packaging components used in SMT process, by a lead into the process of lead-free technology, a large number of moisture reflow sensitive problem, namely the moisture absorption of powder coating devices will be at a higher temperature reflow lead-free process occurs within or substrate layer cracking phenomenon, Under the high temperature of lead-free process, common PCB will often burst board phenomenon. At this point, the scanning acoustic microscope shows its special advantage in nondestructive detection of multi-layer high-density PCB. The general obvious bursting plate can be detected by visual inspection.

5. Microinfrared analysis

Micro infrared analysis is to infrared spectroscopy combined with microscope analysis method, it use different material (mainly organic matter) on the principle of the infrared spectrum absorption, analyzing the compound composition of the materials, coupled with the microscope can make visible light and infrared light with the light path, as long as under the visual field, can look for analysis of trace organic pollutants. In the absence of a microscope, infrared spectroscopy can usually analyze only large samples. In many cases, trace pollution in electronic process can lead to poor weldability of PCB pad or lead pin. It can be imagined that it is difficult to solve the process problem without the matching infrared spectrum of microscope. The main use of microscopic infrared analysis is to analyze the organic pollutants on the welding surface or solder spot surface, and analyze the causes of corrosion or poor solderability.

6. Scanning electron microscopy 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 form an electron beam with diameter of tens to thousands of angstroms (A) by focusing the electron beam emitted from the cathode accelerated by the anode. Under the action of deflection of the scanning coil, The electron beam scans the surface of the sample point by point in a certain time and space order. The high-energy electron beam bombards the surface of the sample and generates a variety of information, which can be collected and amplified to get various corresponding graphics on the display screen. The excited secondary electrons are generated within the range of 5 ~ 10nm on the surface of the sample. Therefore, the secondary electrons can better reflect the surface topography of the sample, so they are most commonly used for morphology observation. The excited backscattered electrons are generated in the range of 100 ~ 1000nm on the surface of the sample, and they emit different characteristics with the difference of atomic number of the substance. Therefore, the backscattered electron image has morphologic characteristics and atomic number discrimination ability, and therefore, the backscattered electron image can reflect the distribution of chemical elements. The present scanning electron microscope has been very powerful, any fine structure or surface features can be magnified to hundreds of thousands of times for observation and analysis.

In PCB or solder joint failure analysis, SEM is mainly used for failure mechanism analysis, specifically, is used to observe the surface morphology structure of the pad, solder joint metallographic structure, measurement of intermetallic compounds, solderable coating analysis and tin must be analyzed and measured. Different from the optical microscope, the scanning electron microscope produces electronic images, so it has only black and white colors. Moreover, the sample of the scanning electron microscope is required to conduct electricity, and the non-conductor and part of the semiconductor need to be sprayed with gold or carbon, otherwise the charge will gather on the surface of the sample and affect the sample observation. In addition, the depth of field of the scanning electron microscope image is much larger than that of the optical microscope, which is an important method for the analysis of the metallographic structure, microscopic fracture and tin whiskers.

7. X-ray energy spectrum analysis

The above mentioned scanning electron microscopy is usually equipped with an X-ray energy spectrometer. When the high-energy electron beam hit the surface, the surface material of the inner electrons in the atoms are bombarded escape, outer electrons to low energy level transition will inspire characteristic X ray, atomic energy level difference of different elements from different characteristic X ray is different, therefore, can send sample of the characteristics of X-ray as chemical composition analysis. At the same time, the corresponding instruments are respectively called spectrum dispersion spectrometer (WDS for short) and energy dispersion spectrometer (EDS for short) according to the characteristic wavelength or characteristic energy of the X-ray signal detection. The resolution of the spectrometer is higher than that of the energy spectrometer, and the analysis speed of the energy spectrometer is faster than that of the energy spectrometer. Because of the high speed and low cost of energy spectrometers, the general SCANNING electron microscopy is equipped with energy spectrometers.

With the different scanning mode of electron beam, the energy spectrometer can analyze the point, line and plane of the surface, and obtain the information of different distribution of elements.Point analysis yields all elements of a point; Line analysis One element analysis is performed on a specified line each time, and the line distribution of all elements is obtained by multiple scanning. Surface analysis The analysis of all elements in a given surface. The measured element content is the average of the range of surface measurements.

In the analysis of PCB, energy dispersive spectrometer is mainly used for the composition analysis of pad surface, and the elemental analysis of contaminants on the surface of pad and lead pin with poor solderability. The quantitative analysis accuracy of energy spectrometer is limited, and the content less than 0.1% is generally not easy to detect. The combination of energy spectrum and SEM can obtain the information of surface morphology and composition simultaneously, which is the reason why they are widely used.

8. Photoelectron spectroscopy (XPS) analysis

Samples by X ray irradiation, the surface of the inner shell electrons of the atom will escape from the bondage of the nucleus and solid surface forming, measuring its kinetic energy the Ex, the inner shell electrons of the atom can be obtained the binding energy of Eb, Eb varied from different elements and different electron shell, it is the “fingerprints” of the atom identification parameters, formation of spectral line is the photoelectron spectroscopy (XPS). XPS can be used for qualitative and quantitative analysis of elements on shallow surface (several nanometers) of sample surface. In addition, information about chemical valence states of elements can be obtained from chemical shifts of binding energy. It can give the information of the bond between the valence state of the surface layer and the surrounding elements. The incident beam is X-ray photon beam, so insulation sample analysis can be carried out, without damaging the analyzed sample rapid multi-element analysis; Multilayers can also be analyzed longitudinally by argon ion stripping (see the case below) with far greater sensitivity than energy spectrum (EDS). XPS is mainly used in the analysis of PCB coating quality analysis, pollution analysis and oxidation degree analysis, in order to determine the deep reason of poor weldability.

9. Differential Scanning Calorim-etry

A method of measuring the difference in power input between a substance and a reference substance as a function of temperature (or time) under programmed temperature control. DSC is equipped with two groups of compensation heating wire under the sample and reference container, when the sample in the heating process due to the thermal effect and reference temperature difference δ T, through the differential heat amplifier circuit and differential heat compensation amplifier, so that the current flowing into the compensation heating wire changes.

The temperature difference δ T disappears, and the relationship between the difference of the thermal power of the two electrically compensated samples and the reference material with temperature (or time) is recorded. According to this relationship, the physicochemical and thermodynamic properties of the material can be studied and analyzed. DSC is widely used in PCB analysis, but is mainly used to measure the curing degree of various polymer materials used in PCB and glass state transformation temperature, 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 gels under Thermal or Mechanical forces under programmed temperature control. Commonly used load methods include compression, pin insertion, stretching, bending, etc. Test probe consists of fixed on the cantilever beam and helical spring support, through the motor of the applied load, when the specimen deformation occurs, differential transformer to detect the change, and together with the data processing, such as temperature, stress and strain after the material can be obtained under the negligible load deformation relations with 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 and is mainly used in measuring the two most critical parameters of PCB: linear expansion coefficient and glass transition temperature. PCB with too large expansion coefficient will often lead to fracture failure of metallized holes after welding and assembly.