As a carrier of various components and a hub for circuit signal transmission, PCB has become the most important and critical part of electronic information products. The quality and reliability of the PCB determine the quality and reliability of the entire equipment. However, due to cost and technical reasons, a large number of failures have occurred during the production and application of PCBs.
For this kind of failure problem, we need to use some commonly used failure analysis techniques to ensure the quality and reliability of the PCB during manufacturing. This article summarizes the top ten failure analysis techniques for reference.
Visual inspection is to visually inspect or use some simple instruments, such as stereo microscopes, metallographic microscopes, and even magnifying glasses, to check the appearance of PCBs, and to find the parts of failure and related physical evidence. The main functions are to locate failures and determine the failure mode of the PCB. The appearance inspection mainly checks the PCB's pollution, corrosion, the location of the explosion board, the circuit wiring and the regularity of the failure. In addition, many PCB failures are discovered only after being assembled into a 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.
For some parts that can not be inspected by appearance and the internal and other internal defects of the through holes of the PCB, the X-ray fluoroscopy system has to be used for inspection. X-ray fluoroscopy systems use different material thicknesses or different material densities to absorb X-rays or transmit light through different principles. This technique is more used to inspect the defects inside PCBA solder joints, the defects inside through holes and the positioning of defective solder joints for high-density packaged BGA or CSP devices. The resolution of current industrial X-ray fluoroscopy equipment can reach below one micron, and it is changing from two-dimensional to three-dimensional imaging equipment. Even five-dimensional (5D) equipment has been used for packaging inspection, but this 5D X Light fluoroscopy systems are very expensive and rarely have practical applications in industry.
Slice analysis is the process of obtaining the cross-section structure of the PCB through a series of methods and steps such as sampling, inlaying, slicing, polishing, corrosion, and observation. Through slice analysis, you can get rich information about the microstructure of the PCB (through-holes, plating, etc.), which provides a good basis for the next quality improvement. However, this method is destructive. Once the sectioning is performed, the sample will be destroyed. At the same time, the method requires high sample preparation, and the sample preparation takes a long time, which requires a trained technician to complete. For detailed slicing procedures, refer to the procedures specified in IPC's standard IPC-TM-650 2.1.1 and IPC-MS-810.
Currently, C mode ultrasonic scanning acoustic microscope is mainly used for electronic packaging or assembly analysis. It uses high frequency ultrasonic reflection on the discontinuous interface of materials to image the amplitude, phase and polarity changes. The scanning method is along the Z axis scans the information in the XY plane. 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 of the solder joints can also be detected directly. The typical scanning acoustic image is the red warning color to indicate the existence of defects. Because a large number of plastic-encapsulated components are used in the SMT process, a large number of moisture reflow sensitive problems occur during the process of converting lead to lead-free. That is, the hygroscopic plastic packaging device will crack internally or on the substrate when it is reflowed at a higher lead-free process temperature, and ordinary PCBs will often pop out at the high temperature of the lead-free process. At this time, the scanning acoustic microscope highlights its special advantages in non-destructive testing of multilayer high-density PCBs. In general, the obvious bursting board can be detected only by visual inspection.
Micro-infrared analysis is an analysis method that combines infrared spectrum with a microscope. It uses the principle of different absorption of infrared spectrum by different materials (mainly organic substances) to analyze the compound composition of the material. Combined with the microscope, visible light can be combined with infrared light, as long as it is in the visible field of view, the infinitesimal organic pollution which need to be analyzed can be find. Without the combination of a microscope, usually the infrared spectrum can only analyze samples with a large sample volume. In many cases in the electronic process, trace pollution can lead to poor solderability of the PCB pads or lead pins. It is conceivable that it is difficult to solve the process problem without the infrared spectrum of the microscope. The main purpose of micro-infrared analysis is to analyze the organic pollutants on the welded surface or the surface of the joint, and to analyze the cause of corrosion or poor solderability.
Scanning electron microscope (SEM) is one of the most useful large-scale electron microscopy imaging systems for failure analysis. Its working principle is to use the electron beam emitted by the cathode to be accelerated by the anode and focused by a magnetic lens to form a beam with a diameter of several tens to Under the deflection of the scanning coil, the electron beam current of several thousand angstroms (A) makes a point-by-point scanning movement on the sample surface in a certain time and space sequence. This high-energy electron beam will be excited when it is bombarded on the sample surface. A variety of information is produced, and various corresponding graphics can be obtained from the display screen after being collected and enlarged. The excited secondary electrons are generated in the range of 5-10 nm on the sample surface. Therefore, the secondary electrons can better reflect the morphology of the sample surface, so they are most commonly used for morphological observation; the excited backscattered electrons are generated on the sample surface. In the range of 100 ～ 1000nm, different characteristics of backscattered electrons are emitted with the different atomic number of the material. Therefore, the backscattered electron image has morphological features and the ability to discriminate the atomic number. Therefore, backscattered electron image can reflect the distribution of chemical element composition. The current scanning electron microscope is very powerful, and any fine structure or surface feature can be enlarged to hundreds of thousands of times for observation and analysis.
In terms of failure analysis of PCBs or solder joints, SEM is mainly used to analyze the failure mechanism. Specifically, it is used to observe the morphology of the surface of the pads, the metallographic structure of the solder joints, the measurement of intermetallic, the solderable coating and the analysis and measurement of tin whisker. Unlike optical microscopes, scanning electron microscopy is an electronic image, so there are only black and white colors, and the sample of the scanning electron microscope needs to be conductive, and non-conductors and some semiconductors need to be sprayed with gold or carbon, otherwise the accumulation of charge on the surface of the sample will affect the observation of the sample. In addition, the depth of field of the SEM image is much larger than that of the optical microscope, and it is an important analysis method for uneven samples such as metallographic structure, micro fracture and tin whisker.
The SEMs mentioned above are generally equipped with X-ray spectrometers. When a high-energy electron beam hits the sample surface, the inner electrons in the atoms of the surface material are bombarded and escaped. When the outer electrons transition to a low energy level, characteristic X-rays are excited, and the characteristics of the different atomic energy levels of different elements are emitted. X-rays are different, so characteristic X-rays emitted from a sample can be analyzed as a chemical composition. At the same time, according to the detection of X-ray signals as characteristic wavelengths or characteristic energies, the corresponding instruments are respectively called spectroscopic dispersive spectrometer (abbreviated as spectrometer, WDS) and energy dispersive spectrometer (abbreviated as energy spectrometer, EDS). The resolution of the spectrometer is higher than the spectrometer, the analysis speed of the spectrometer is faster than the spectrometer. Due to the fast speed and low cost of the energy spectrometer, the general SEM is configured with an energy spectrometer.
With the different scanning methods of the electron beam, the spectrometer can perform surface point analysis, line analysis, and surface analysis, and can obtain information on different element distributions. Point analysis obtains all elements of a point; line analysis performs an element analysis on a specified line at a time, and multiple scans obtain the line distribution of all elements; surface analysis analyzes all elements in a specified surface, and the measured element content is The average of the measurement area range.
In the analysis of PCBs, the energy spectrometer is mainly used for the component analysis of the surface of the pad, ```9aqsand the element analysis of the contamination on the surface of the pad and the lead pin with poor solderability. The accuracy of the quantitative analysis of the energy spectrometer is limited, and the content below 0.1% is generally not easy to detect. The combination of energy spectroscopy and SEM can simultaneously obtain surface morphology and composition information, which is why they are widely used.
When the sample is irradiated with X-rays, the inner shell electrons of the surface atoms will escape from the bondage of the atomic nucleus and escape from the solid surface to form electrons. The kinetic energy Ex of the electrons can be measured to obtain the binding energy Eb of the inner shell electrons of the atom due to different elements and Different electron shells are different, it is the "fingerprint" identification parameter of the atom, and the formed spectral line is the photoelectron spectroscopy (XPS). XPS can be used for the qualitative and quantitative analysis of shallow surface (several nanometers) elements on the surface of samples. In addition, information about the chemical valence of an element can be obtained based on the chemical shift of the binding energy. It can give information such as the atomic valence of the surface layer and surrounding elements; the incident beam is an X-ray photon beam, so the analysis of the insulating sample can be performed without damaging the analyzed sample; rapid multi-element analysis can also be performed; and in the case of argon ion peeling The longitudinal element distribution analysis is performed on multiple layers (see the following case), and the sensitivity is much higher than the energy spectrum (EDS). In the analysis of PCB, XPS is mainly used for the analysis of the quality of the pad coating, the analysis of the contamination, and the analysis of the degree of oxidation to determine the deep-seated cause of poor solderability.
A method of measuring the relationship between the power difference and temperature (or time) input between a substance and a reference substance under programmed temperature control. DSC is equipped with two sets of compensation heating wires under the sample and reference container. When the temperature difference ΔT between the sample and the reference occurs due to the thermal effect during the heating of the sample, the differential thermal amplification circuit and differential thermal compensation amplifier can be used. , So that the current flowing into the compensation heating wire changes.
And make the heat balance on both sides, the temperature difference ΔT disappears, and record the relationship between the difference in the thermal power of the two electrothermal compensations under the sample and the reference as a function of temperature (or time). Chemical and thermodynamic properties. DSC is widely used, but in the analysis of PCB, it is mainly used to measure the solidification 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 is used to measure the deformation properties of solids, liquids and gels under the action of heat or mechanical force under programmed temperature control. Common loading methods include compression, penetration, stretching, bending, etc. The test probe is supported by a cantilever beam and a coil spring fixed on it. A load is applied to the sample by the motor. When the sample deforms, the differential transformer detects this change and processes it with temperature, stress and strain data. The relationship between the deformation of a substance under a negligible load and temperature (or time) can be obtained. According to the relationship between deformation and temperature (or time), physical and chemical and thermodynamic properties of materials 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 the PCB: measuring its linear expansion coefficient and glass transition temperature. PCBs with substrates with excessive expansion coefficients often lead to failure of metallized holes after solder assembly.
Due to the development trend of high-density PCBs and environmental protection requirements of lead-free and halogen-free, more and more PCBs have encountered various failure problems such as poor wetting, bursting, delamination, and CAF. The application of these analysis techniques in practical cases is introduced. Obtaining the failure mechanism and cause of PCB will be beneficial to the quality control of PCB in the future, so as to avoid similar problems from happening again.
