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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 level determine the quality and reliability of the whole equipment. However, due to cost and technical reasons, a large number of failure problems have occurred in the production and application of PCB. For this kind of failure problem, we need to use some common failure analysis techniques to ensure the quality and reliability level of PCB during manufacturing. This article summarizes the top ten failure analysis techniques for reference. 01 Appearance inspection Appearance inspection is to visually inspect the appearance of PCB or use some simple instruments such as stereo microscope, metallographic microscope or even magnifying glass to find the failed parts and related physical evidence. The main function is to locate the failure and preliminarily judge the failure mode of PCB. The appearance inspection mainly checks the pollution, corrosion, location of the burst board, circuit wiring and the regularity of the failure of the PCB, such as batch or individual, whether it is always concentrated in a certain area, etc. In addition, many PCB failures are discovered after being assembled into PCBA. Whether the failure is caused by the assembly process and the influence of the materials used in the process also requires careful inspection of the characteristics of the failure area.
02 X-ray fluoroscopy inspection For some parts that cannot be inspected by appearance, as well as the internal defects of the through holes and other internal defects of the PCB, an X-ray fluoroscopy system has to be used for inspection. The X-ray fluoroscopy system uses the different principles of moisture absorption or transmittance of X-rays by different material thicknesses or different material densities to form images. This technology is more used to inspect the defects inside the PCBA solder joints, the internal defects of the through holes and the positioning of defective solder joints of high-density packaged BGA or CSP devices. The resolution of current industrial X-ray fluoroscopy equipment can reach less than one micron, and it is changing from two-dimensional to three-dimensional imaging equipment. There are even five-dimensional (5D) equipment for packaging inspection, but this 5D X-ray fluoroscopy system is very expensive and rarely used in the industry.
03 Slice analysis Slice analysis is the process of obtaining the cross-sectional structure of PCB through a series of means and steps such as sampling, inlaying, slicing, polishing, corrosion, and observation. Slice analysis can obtain rich information about the microstructure of the quality of PCB (through-holes, plating, etc.), providing a good basis for the next step of quality improvement. However, this method is destructive. Once slicing is performed, the sample will inevitably be destroyed; at the same time, this method has high sample preparation requirements and takes a long time to prepare, requiring well-trained technicians to complete. For detailed slicing process, please refer to IPC standard IPC-TM-650 2.1.1 and the process specified in IPC-MS-810.
04 Scanning Acoustic Microscope Currently, the main ultrasonic scanning acoustic microscope used for electronic packaging or assembly analysis is 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 form an image. Its scanning method is to scan the information of the XY plane along the Z axis. 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 shows the presence of defects in red warning color. Since a large number of plastic-encapsulated components are used in the SMT process, a large number of moisture reflow sensitivity problems arise during the process of converting from lead-free to lead-free processes. That is, hygroscopic plastic-encapsulated components will experience internal or substrate delamination and cracking when reflowing at higher lead-free process temperatures. Ordinary PCBs will often experience board explosions under the high temperatures of lead-free processes. At this time, the scanning acoustic microscope highlights its special advantages in non-destructive testing of multi-layer high-density PCBs. However, general obvious board explosions can be detected simply by visual inspection of the appearance. 05 Microscopic infrared analysis Microscopic infrared analysis is an analysis method that combines infrared spectroscopy with a microscope. It uses the principle that different materials (mainly organic matter) absorb infrared spectroscopy differently to analyze the compound composition of the material. Combined with a microscope, visible light and infrared light can be placed on the same optical path. As long as they are in the visible field of view, trace organic pollutants to be analyzed can be found. If there is no combination with a microscope, infrared spectroscopy can usually only analyze samples with a large amount of samples. In many cases in electronic processes, trace contamination can lead to poor solderability of PCB pads or lead pins. It can be imagined that it is difficult to solve process problems without infrared spectroscopy with a microscope. The main purpose of microscopic infrared analysis is to analyze organic pollutants on the surface of the soldered surface or solder joint, and to analyze the causes of corrosion or poor solderability. 06 Scanning electron microscope analysis Scanning electron microscope (SEM) is the most useful large-scale electron microscopic imaging system for failure analysis. Its working principle is to use the electron beam emitted by the cathode to accelerate through the anode, and then focus by the magnetic lens to form an electron beam with a diameter of tens to thousands of angstroms (A). Under the deflection of the scanning coil, the electron beam scans the sample surface point by point in a certain time and space sequence. This high-energy electron beam bombards the sample surface and stimulates a variety of information. After collection and amplification, various corresponding graphics can be obtained from the display screen. The excited secondary electrons are generated within 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 different characteristics of backscattered electrons are emitted with different atomic numbers of the substances. Therefore, the backscattered electron image has the ability to distinguish the morphological characteristics and atomic number. Therefore, the backscattered electron image can reflect the distribution of chemical element components. The current 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 joints, SEM is mainly used for failure mechanism analysis. Specifically, it is used to observe the morphological structure of the pad surface, the metallographic structure of the solder joint, measure intermetallic compounds, analyze the solderability coating, and perform tin whisker analysis and measurement. Different from optical microscope, scanning electron microscope forms an electron image, so it is only black and white. In addition, the sample of scanning electron microscope is required to be conductive. Non-conductor and some semiconductors need to be treated with gold or carbon. Otherwise, the charge will accumulate on the sample surface and affect the observation of the sample. In addition, the depth of field of scanning electron microscope image is much greater than that of optical microscope. It is an important analysis method for metallographic structure, microscopic fracture and uneven samples such as tin whiskers. 07 X-ray energy spectrum analysis The scanning electron microscope mentioned above is generally equipped with an X-ray energy spectrometer. When the high-energy electron beam hits the surface of the sample, the inner electrons in the atoms of the surface material are bombarded and escape, and the outer electrons will stimulate characteristic X-rays when they jump to the lower energy level. The characteristic X-rays emitted by different elements are different due to the 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 characteristic energy of the detected X-ray signal, the corresponding instruments are called wave dispersive spectrometer (abbreviated as spectrometer, WDS) and energy dispersive spectrometer (abbreviated as energy spectrometer, EDS). The resolution of the wave spectrometer is higher than that of the energy spectrometer, and the analysis speed of the energy spectrometer is faster than that of the wave spectrometer. Since the energy spectrometer is fast and low in cost, the general scanning electron microscope is equipped with an energy spectrometer. With different scanning methods of the electron beam, the energy spectrometer can perform point analysis, line analysis and surface analysis on the surface, and obtain information on different element distributions. Point analysis obtains all elements at a point; line analysis performs one element analysis on a specified line each time, and multiple scans are performed to obtain the line distribution of all elements; surface analysis analyzes all elements within a specified surface, and the measured element content is the average value of the measurement surface range. In the analysis of PCB, the energy spectrometer is mainly used for the composition analysis of the surface of the pad, and the element analysis of the surface contaminants of the pad and lead pin with poor solderability. The quantitative analysis accuracy of the energy spectrometer is limited, and the content below 0.1% is generally difficult to detect. The combination of energy spectrum and SEM can obtain information on surface morphology and composition at the same time, which is why they are widely used. 08 Photoelectron spectroscopy (XPS) analysis When the sample is irradiated by X-rays, the inner shell electrons of the surface atoms will break away from the bondage of the nucleus and escape from the solid surface to form electrons. By measuring their kinetic energy Ex, the binding energy Eb of the inner shell electrons of the atom can be obtained.Eb varies with different elements and different electron shells. It is the "fingerprint" identification parameter of atoms, and the spectrum formed is the photoelectron energy spectrum (XPS). XPS can be used to perform qualitative and quantitative analysis of shallow surface (several nanometers) elements on the sample surface. In addition, information about the chemical valence state of the element can be obtained based on the chemical shift of the binding energy. It can provide information such as the valence state of the surface layer atoms and the bonding with the surrounding elements; the incident beam is an X-ray photon beam, so it can be used to analyze insulating samples, and rapid multi-element analysis can be performed without damaging the analyzed sample; it can also perform longitudinal element distribution analysis on multiple layers under the condition of argon ion stripping (see the following case), and the sensitivity is much higher than that of energy spectrum (EDS). XPS is mainly used for the analysis of pad coating quality, contaminant analysis and oxidation degree in PCB analysis to determine the deep-seated reasons for poor solderability. 09 Thermal Analysis Differential Scanning Calorimetry A method for measuring the relationship between the power difference input to a substance and a reference substance and temperature (or time) under program temperature control. DSC is equipped with two sets of compensation heating wires under the sample and reference containers. When a temperature difference ΔT appears between the sample and the reference due to thermal effect during the heating process, the current flowing into the compensation heating wire can be changed through the differential thermal amplifier circuit and the differential thermal compensation amplifier. The heat on both sides is balanced, the temperature difference ΔT disappears, and the relationship between the difference in thermal power of the two electric thermal compensations under the sample and the reference and the temperature (or time) is recorded. Based on this relationship, the physical, chemical and thermodynamic properties of the material can be studied and analyzed. DSC is widely used, but in the analysis of PCB, it is mainly used to measure the degree of curing 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 Thermomechanical analysis technology (Thermal Mechanical Analysis) is used to measure the deformation properties of solids, liquids and gels under the action of heat or mechanical force under program temperature control. Common load methods include compression, needle penetration, tension, bending, etc. The test probe is supported by a cantilever beam and a coil spring fixed on it. The load is applied to the sample through a motor. When the sample deforms, the differential transformer detects this change, and after processing together with the temperature, stress and strain data, the relationship between the deformation and temperature (or time) of the material under negligible load can be obtained. According to the relationship between deformation and temperature (or time), the physical, chemical and thermodynamic properties of the material can be studied and analyzed. TMA is widely used. In the analysis of PCB, it is mainly used to measure the two most critical parameters of PCB: its linear expansion coefficient and glass transition temperature. PCB with a substrate with too large expansion coefficient often leads to fracture and failure of metallized holes after welding and assembly. Due to the development trend of high-density PCB and the environmental protection requirements of lead-free and halogen-free, more and more PCBs have various failure problems such as poor wetting, bursting, delamination, CAF, etc. Introduce the application of these analysis techniques in actual cases. The acquisition of PCB failure mechanism and cause will be conducive to the quality control of PCB in the future, so as to avoid the recurrence of similar problems. Some cases 01Scratches after board electrical connection and before graphic connection
Cut slices
The copper surface at the fracture is smooth and has no signs of corrosion The substrate at the OPEN has signs of damage (whitish) to a greater or lesser extent The shapes are mostly strips or blocks The nearby circuits may have seepage plating or poor circuits From the cut slices, the graphic layer will wrap the board electrical layer and the bottom copper 02 The dry film attached to the copper surface is broken
03 Anti-plating material with glue or glue-like material attached to the copper surface
04 Poor exposure
Slice diagram
The fracture is pointed, without sandy beach, and there are no fine lines on the board except the fine lines near the fracture The fracture is pointed or round, without sandy beach, and accompanied by poor lines nearby The fracture is pointed, without sandy beach, and accompanied by residual copper or short circuit caused by exposed garbage From the slice, the electrical layer will extend a hook, some long and some short 05 Erase the dry film
The area is large, often accompanied by short circuit The shape is irregular, but directional 06 Tin surface erode
Cut slices
There is no obvious sandy area on the fracture, which is caused by heavy scratches; when it is lighter, there is a sandy area or no corrosion. From the slice, the eroded area is relatively smooth and has a gentle slope.The beach area is larger 07 Poor tin dissolution or electrotinning
Cut picture
08 Unclear development
09 Scratches after electrotinning
The scratches after electroplating are generally rough on the substrate and copper surface. There will be copper particles on the substrate. There will be obvious scratch marks on the scratched circuit, and there will be protrusions along the scratch direction on the edge of the circuit From the slice, the circuit at the scratched position will be pressed toward the substrate, with obvious bending 10 Film throwing
Poor circuit caused by residual adhesive of dry film
Slice diagram
Poor circuit caused by residual adhesive of dry film, no residual copper on the substrate The bottom of the defective circuit is generally very flat, and the color of copper will be exposed, which is different from the color of the surrounding circuits From the slice, the board electrical layer and bottom copper are intact at the defective part of the circuit, but the second copper cannot be plated, and the surrounding electrical layer has a wrapping action 11 Diffusion plating
From the plane, the diffusion plating area will be shiny, and the diffusion plating area will have a smooth slope, without any signs of corrosion From the slice, there is a electrical layer at the diffusion plating area 12 The etched board is not clean (film clamped)
Slice diagram
The etched board is not clean (film sandwiched) will not be shiny, the bottom is very flat, there is no slope, it is stepped, and there will be some traces of etching From the slice, there is no electrical layer in the unclean etched board 13 Pinhole
Pinholes usually occur at the edge of the line or hole ring, and will not appear in the middle of the line The slice of the pinhole is a very smooth arc, with electrical layer 14 Green oil nail bed crushing
The green oil nail bed is a localized injury. The circuit is generally rounded and concave, with an extension at the bottom. The slice pattern is bow-shaped, and the electrical layer is squeezed toward the board electrical layer. 15 Circuit shrinkage
1. Features: Generally appears on densely arranged lines, in a bracket shape, and the edge of the line at the shrinkage is shiny. 2. Reasons: The appearance of circuit shrinkage is related to the structural type of the production board. This type of board has many densely arranged lines and few holes. When developing dry film, the developer is not easy to excrete, and oily substances are easy to adhere to the edge of the line, resulting in unclean development. Waist shrinkage occurs after electrical etching. 3. Solution: When developing, the board direction should be parallel to the direction of the densely arranged lines, and the board with more densely arranged lines should be placed downward. Source: RF BaihuatanThere are no signs of corrosion From the slice, there is a graphic layer at the infiltration plating 12 The etched board is not clean (film clamped)
Slice diagram
The etched board is not clean (film clamped) and will not shine. The bottom is very flat, without slope, and is stepped. There will be some signs of corrosion From the slice, there is no graphic layer at the etched board 13 Pinholes
Pinholes usually occur at the edge of the line or hole ring, and will not appear in the middle of the line The slice of the pinhole is a very smooth arc with a graphic layer 14 Green oil nail bed crushing
The green oil nail bed crushing is localized, the line crushing place generally presents a circular depression, there will be an extension protrusion at the bottom The slice pattern is bow-shaped, the graphic layer is squeezed toward the board layer 15 Line waisting
1. Features: Generally appears on densely arranged lines, in brackets, and the edge of the line is shiny at the waist. 2. Reason: The appearance of circuit shrinkage is related to the structural type of the production board. This type of board has densely arranged lines and few holes. When developing dry film, the developer is not easy to excrete, and oily substances are easy to adhere to the edge of the line, resulting in unclean development. The shrinkage appears after electro-etching. 3. Solution: When developing, the direction of the board should be parallel to the direction of the densely arranged lines, and the board with more densely arranged lines should be placed downward. Source: RF BaihuatanThere are no signs of corrosion From the slice, there is a graphic layer at the infiltration plating 12 The etched board is not clean (film clamped)
Slice diagram
The etched board is not clean (film clamped) and will not shine. The bottom is very flat, without slope, and is stepped. There will be some signs of corrosion From the slice, there is no graphic layer at the etched board 13 Pinholes
Pinholes usually occur at the edge of the line or hole ring, and will not appear in the middle of the line The slice of the pinhole is a very smooth arc with a graphic layer 14 Green oil nail bed crushing
The green oil nail bed crushing is localized, the line crushing place generally presents a circular depression, there will be an extension protrusion at the bottom The slice pattern is bow-shaped, the graphic layer is squeezed toward the board layer 15 Line waisting
1. Features: Generally appears on densely arranged lines, in brackets, and the edge of the line is shiny at the waist. 2. Reason: The appearance of circuit shrinkage is related to the structural type of the production board. This type of board has densely arranged lines and few holes. When developing dry film, the developer is not easy to excrete, and oily substances are easy to adhere to the edge of the line, resulting in unclean development. The shrinkage appears after electro-etching. 3. Solution: When developing, the direction of the board should be parallel to the direction of the densely arranged lines, and the board with more densely arranged lines should be placed downward. Source: RF Baihuatan
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