PCB failure analysis technology and some cases

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PCB failure analysis technology and some cases [Copy link]

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.

1. Appearance inspection

Appearance inspection is to visually inspect the appearance of the PCB or use some simple instruments, such as stereo microscopes, metallographic microscopes, and even magnifying glasses to check the appearance of the PCB, find the failed parts and related physical evidence, and its main function is to locate the failure and preliminarily determine the failure mode of the PCB. Appearance inspection mainly checks the pollution, corrosion, location of the burst board, circuit wiring, and the regularity of the failure, such as batch or individual, whether it is always concentrated in a certain area, etc. In addition, many PCB failures are discovered after they are assembled into PCBAs. Whether the failure is caused by the assembly process and the materials used in the process, it is also necessary to carefully check the characteristics of the failure area.

2. X-ray fluoroscopy inspection

For some parts that cannot be inspected by appearance, as well as the inside of the through-holes and other internal defects of the PCB, an X-ray fluoroscopy system has to be used for inspection. X-ray fluoroscopy system uses the different principles of moisture absorption or transmittance of X-rays due to different material thickness or density to form images. This technology is more used to check defects inside PCBA solder joints, internal defects of through holes, and the location 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 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.

3. 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. Through slice analysis, rich information about the microstructure of the PCB (through-hole, plating, etc.) quality can be obtained, providing a good basis for the next step of quality improvement. However, this method is destructive. Once the slice is made, 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. If a detailed slicing process is required, refer to the procedures specified in IPC standards IPC-TM-650 2.1.1 and IPC-MS-810.

4. 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, scanning acoustic microscopes 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, internal defects in solder joints can also be directly detected. The typical scanning acoustic image uses a red warning color to indicate the presence of defects. 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 have internal or substrate delamination and cracking when reflowing at a higher lead-free process temperature. Ordinary PCBs will often explode at high temperatures in the lead-free process. At this time, the scanning acoustic microscope highlights its special advantages in non-destructive testing of multi-layer high-density PCBs. The general obvious explosion of the board can be detected by visual inspection of the appearance.

5. 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 spectra 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. Without the combination of a microscope, infrared spectroscopy can usually only analyze samples with a large sample volume. 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 an infrared spectroscopy equipped 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.

6. Scanning electron microscopy 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 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; while the excited backscattered electrons are generated within the range of 100~1000nm on the sample surface. Different characteristics of backscattered electrons are emitted with different atomic numbers of the material. Therefore, the backscattered electron image has the ability to distinguish morphological characteristics and atomic numbers. 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 morphology of the pad surface, the metallographic structure of the solder joint, measure intermetallic compounds, analyze solderability coatings, and perform tin whisker analysis. Unlike optical microscopes, scanning electron microscopes produce electron images, so there are only black and white colors. In addition, the specimens of scanning electron microscopes are required to be conductive. Non-conductors and some semiconductors need to be treated 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 scanning electron microscope images is much greater than that of optical microscopes. It is an important analysis method for metallographic structures, microscopic fractures, and uneven samples such as tin whiskers.

7. X-ray energy spectrum analysis

The scanning electron microscopes mentioned above are generally equipped with X-ray energy spectrometers. When a high-energy electron beam hits the surface of a 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 transition to lower energy levels. Different elements have different atomic energy levels, so the characteristic X-rays emitted are different. Therefore, the characteristic X-rays emitted by the sample can be used for 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 a wave spectrometer is higher than that of an energy spectrometer, and the analysis speed of an energy spectrometer is faster than that of an energy spectrometer. Since the energy spectrometer is fast and low in cost, general scanning electron microscopes are equipped with energy spectrometers.

Depending on the scanning mode of the electron beam, the 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 obtains the line distribution of all elements by multiple scans; 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 spectrometer is mainly used for the composition analysis of the pad surface, the element analysis of the contaminants on the surface of the pad with poor solderability and the lead pin. The accuracy of the quantitative analysis of the spectrometer is limited, and the content below 0.1% is generally not easy to detect. The combination of energy spectrum and SEM can obtain information on 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 break away from the bondage of the nucleus and escape from the solid surface to form electrons. By measuring their kinetic energy Ex, we can get the binding energy Eb of the inner shell electrons of the atoms. Eb varies with different elements and different electron shells. It is the "fingerprint" identification parameter of the atom. The spectrum formed is the photoelectron spectrum (XPS). XPS can be used to conduct qualitative and quantitative analysis of shallow surface (several nanometers) elements of the sample surface. In addition, information about the chemical valence state of elements can be obtained based on the chemical shift of binding energy. It can provide information such as the valence state of atoms in the surface layer and the bonding with surrounding elements. The incident beam is an X-ray photon beam, so it can be used to analyze insulating samples without damaging the analyzed samples for rapid multi-element analysis. It can also perform longitudinal element distribution analysis on multiple layers under the condition of argon ion stripping (see the following case), and its 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 causes of poor solderability.

[p=30, null,9. Thermal Analysis Differential Scanning Calorimetry

[color] [102, 102, 102] [font] [Microsoft YaHei] 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 material containers. When a temperature difference ΔT appears between the sample and the reference material 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 material 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. Thermo Mechanical Analyzer (TMA)

Thermo Mechanical Analysis (TMA) is used to measure the deformation properties of solids, liquids and gels under heat or mechanical force under programmed temperature control. Commonly used load methods include compression, needle penetration, tension, bending, etc. The test probe is supported by a cantilever beam and a spiral spring fixed on it. The load is applied to the sample through a motor. When the sample is deformed, the differential transformer detects this change and processes the temperature, stress and strain data to obtain the relationship between the deformation and temperature (or time) of the material under negligible load. 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 for the two most critical parameters of PCB: measuring its linear expansion coefficient and glass transition temperature. PCB with a substrate with too large expansion coefficient often leads to fracture failure of metallized holes after welding and assembly.

Due to the development trend of high-density PCBs 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. Introducing the application of these analysis techniques in actual cases. Obtaining the failure mechanism and causes of PCB will be beneficial to the quality control of PCB in the future, thus avoiding the recurrence of similar problems.

Some examples:

1. Board after electrical diagram before electrical rubbing

[color=rgb(51, 51,

Cut slice

1. The copper surface at the fracture is smooth and has no signs of corrosion.

2. The substrate at the OPEN has signs of damage (whitish) to varying degrees.

3. The shapes are mostly strips or blocks.

４. The nearby lines may have seepage plating or poor lines.

５. From the slice, the electrical layer will wrap the board electrical layer and the bottom copper.

2. Copper surface attached dry film fragments

1. The position of the beach at the fracture is consistent with the normal line or has a small difference

2. The copper surface at the fracture is flat and not shiny

3. Glue or glue-like anti-plating material attached to the copper surface

1. The copper surface at the fracture is uneven and shiny; sometimes it is jagged

2. Usually accompanied by short circuit or residual copper

4. Poor exposure

Slice diagram

1. The fracture is pointed, without sandy beach. Except for the fine lines near the fracture, there are no fine lines on other positions of the board

2. The fracture is pointed or round, without sandy beach, accompanied by bad lines nearby

3. The fracture is pointed, without sandy beach, accompanied by residual copper or short circuit caused by exposed garbage

４． From the slice, the electrical layer will extend a hook, some long and some short.

5. Dry film

1. Large area, often accompanied by short circuit

2. Irregular shape, but directional

6. Tin surface rubbing

Slice diagram

１. There is no obvious sandy beach on the fracture, which is caused by heavier scratches; when it is lighter, there is a sandy beach or no corrosion.

２. From the slice, the eroded part is relatively smooth, with a gentle slope,The beach is relatively large.

Seven, poor tin dissolution or electrotinning

Slice diagram

Eight, unclean development

1. Rarely occurs, generally covers a large area

2. The edge of the fracture and nearby lines are shiny,

9. After the figure is rubbed

Slice diagram

1. The scratches after electroplating. Generally, the base material and copper surface of the scratched area are relatively rough. There will be copper particles on the base material. There will be obvious traces of scratches on the scratched circuit, and there will be protrusions along the direction of the scratches on the edge of the circuit. 2. From the slice, the lines at the scratched part will be pressed toward the substrate, with obvious curvature.

10. Film throwing

Poor circuit caused by residual adhesive of dry film

[color=rgb(51, 51, 1. The poor circuit caused by residual adhesive of dry film will not have residual copper on the substrate.

2. The bottom of the bad circuit is usually very flat, and the copper color will be exposed, which is different from the color of the surrounding circuits.

3. From the slice, the board electrical layer and bottom copper of the bad circuit are intact, but the second copper cannot be plated, and the surrounding electrical layer has a wrapping action.

11. Plating

1. Viewed from a plane, the plated area will be shiny and have a smooth slope without any signs of corrosion.

2. From the slice, we can see that there is an electrical layer at the infiltration and plating area.

12. Etching is not clean (film clamping)

Slice diagram

1. The etched board is not clean (film clamped) and will not be shiny. The bottom is very flat, without slope, and is stepped. There will be some traces of being etched.

2. From the slice, there is no graphic layer in the unclean etched board.

13. Pinholes

1. Pinholes generally occur at the edge of the line or hole ring, and will not appear in the middle of the line.

2. The pinhole slice is a very smooth arc with an electrical layer.

14. Green Oil Nail Bed Crush

1. The green oil nail bed is localized. The line is generally rounded and concave at the line, with an extended protrusion at the bottom.

2. The slice pattern is bow-shaped, and the electrical layer is squeezed toward the board electrical layer.

15. Line waist reduction

1. Features:

It usually appears on densely arranged lines in a bracket shape, with shiny edges 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 the dry film, the developer is not easy to drain out, and the oily substance is easy to adhere to the edge of the circuit, resulting in unclean development. After the electro-etching, the shrinkage appears.

3. Solution:

During development, 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.

gmchen

Wow, so professional.