PCB Blog - Overview of PCB board failure analysis technology

Printed circuit boards have become an important and critical part of electronic information products, and its quality and reliability level determine the quality and reliability of the entire equipment. However, due to cost and technical reasons, a large number of failure problems have occurred in the production and application of PCB boards. For this kind of failure problem, we need to use some common failure analysis techniques to ensure the quality and reliability of the PCB board during manufacturing.

1. Appearance inspection Appearance 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 the PCB board, find the failed parts and related physical evidence, the main role is to locate the failure and make a preliminary judgment on the PCB. failure mode of the board. The visual inspection mainly checks the PCB board for pollution, corrosion, the position of the explosion board, the circuit wiring and the regularity of failure, whether it is batch or individual, whether it is always concentrated in a certain area, and so on. In addition, there are many PCB board failures that are found after assembly into PCB board A. 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.

2. X-ray fluoroscopy inspection For some parts that cannot be inspected by visual inspection, as well as the inside of through holes of the PCB board and other internal defects, the X-ray fluoroscopy system has to be used for inspection. The X-ray fluoroscopy system uses the different principles of the moisture absorption or transmittance of X-rays by different material thicknesses or different material densities to image. This technology is more used to inspect the defects inside the solder joints of PCB board A, the internal defects of through holes and the positioning of defective solder joints of high-density packaged BGA or CSP devices. The resolution of the current industrial X-ray fluoroscopy equipment can reach one micron or less, and is changing from two-dimensional to three-dimensional imaging equipment, and even five-dimensional (5D) equipment is used for packaging inspection, but this 5D X-ray Optical see-through systems are very expensive and rarely have practical applications in industry.

3. Slice analysis Slice analysis is the process of obtaining the cross-sectional structure of the PCB board through a series of means and steps such as sampling, inlaying, slicing, polishing, corrosion, and observation. Through slice analysis, rich information on the microstructure reflecting the quality of the PCB board (through holes, plating, etc.) can be obtained, which provides a good basis for the next quality improvement. However, this method is destructive, and once the sectioning is performed, the sample will be destroyed; at the same time, this method requires high sample preparation, takes a long time to prepare the sample, and requires well-trained technicians to complete. For detailed slicing process, you can refer to the procedures specified in IPC's standard IPC-TM-650 2.1.1 and IPC-MS-810.

4. The scanning acoustic microscope currently used for electronic packaging or assembly analysis is mainly 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 materials to image. The scan method is to scan the information in the XY plane along the Z axis. Therefore, scanning acoustic microscopy can be used to detect various defects in components, materials, and PCB boards and PCB board A, including cracks, delaminations, inclusions, and voids. Internal defects in solder joints can also be detected directly if the frequency width of the scanning acoustics is sufficient. A typical scanned acoustic image is a red warning color to indicate the presence of defects. Since a large number of plastic-packaged components are used in the SMT process, a large number of moisture reflow sensitive problems are generated during the conversion from lead to lead-free process. That is to say, the hygroscopic plastic package will have internal or substrate delamination and cracking during reflow at a higher lead-free process temperature, and ordinary PCB boards will often burst at the high temperature of the lead-free process. At this point, the scanning acoustic microscope highlights its special advantages in non-destructive testing of multi-layer high-density PCB boards. The general obvious burst board can be detected only by visual appearance.

5. Microscopic infrared analysis Microscopic infrared analysis is an analysis method that combines infrared spectroscopy and microscopy. It uses the principle of different absorption of infrared spectroscopy by different materials (mainly organic substances) to analyze the compound composition of materials, and then combined with microscopy The visible light and the infrared light are in the same optical path, as long as they are in the visible field of view, the trace organic pollutants to be analyzed can be found. Without the combination of a microscope, infrared spectroscopy can usually only analyze samples with larger sample volumes. In many cases in electronic technology, trace pollution can lead to poor solderability of PCB pads or lead pins. It is conceivable that it is difficult to solve process problems without infrared spectroscopy supporting a microscope. The main purpose of micro-infrared analysis is to analyze the organic contamination on the surface of the welded surface or the solder joint, and to analyze the cause of corrosion or poor solderability.

6. Scanning Electron Microscope Analysis Scanning Electron Microscope (SEM) is a useful large-scale electron microscope imaging system for failure analysis. Its working principle is to use the electron beam emitted by the cathode to be accelerated by the anode and focus by a magnetic lens to form a beam The electron beam current with a diameter of tens to thousands of angstroms (A), under the deflection of the scanning coil, the electron beam scans the surface of the sample point by point in a certain time and space sequence. A variety of information will be excited on the surface of the sample, and various corresponding graphics can be obtained from the display screen after collection and amplification. The excited secondary electrons are generated in the range of 5-10 nm on the surface of the sample. Therefore, the secondary electrons can better reflect the surface morphology of the sample, so they are often used for morphology observation; while the excited backscattered electrons are generated at 100 nm on the sample surface. In the range of ~1000nm, backscattered electrons with different characteristics are emitted with different atomic numbers of substances, so the backscattered electron images have the ability to discriminate the morphology and atomic number. Therefore, the backscattered electron images can reflect the composition of chemical elements Distribution. The functions of the current scanning electron microscope are already very powerful, and any fine structure or surface feature can be magnified to hundreds of thousands of times for observation and analysis.

In terms of failure analysis of PCB boards or solder joints, SEM is mainly used to analyze the failure mechanism, specifically to observe the topography and structure of the pad surface, the metallographic structure of solder joints, measure intermetallic compounds, and solderability. Coating analysis and tin whisker analysis and measurement, etc. Different from the optical microscope, the scanning electron microscope forms an electronic image, so there are only black and white colors, and the sample of the scanning electron microscope needs to be conductive, and the non-conductor and some semiconductors need to be sprayed with gold or carbon, otherwise the charge will accumulate on the surface of the sample. observation of the sample. In addition, the depth of field of SEM images is much larger than that of optical microscopes, and it is an important analysis method for uneven samples such as metallographic structure, microscopic fractures and tin whiskers.

7. X-ray energy spectrum analysis The scanning electron microscopes mentioned above are generally equipped with X-ray energy spectrometers. 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 escaped, and the outer electrons transition to the lower energy level, which will excite characteristic X-rays, which are characteristic of the difference in atomic energy levels of different elements. X-rays are different, so the characteristic X-rays emitted by the sample can be analyzed as chemical constituents. At the same time, according to the characteristic wavelength or characteristic energy of the detected X-ray signal, the corresponding instruments are called spectral dispersive spectrometer (abbreviated as spectrometer, WDS) and energy dispersive spectrometer (abbreviated as energy spectrometer, EDS). Higher than the energy spectrometer, the analysis speed of the energy spectrometer is faster than that of the spectrometer. Due to the fast speed and low cost of the energy spectrometer, the general scanning electron microscope is equipped with an energy spectrometer. With the different scanning methods of the electron beam, the energy spectrometer can perform point analysis, line analysis and surface analysis of the surface, and can obtain information on the different distribution of elements. Point analysis obtains all elements of a point; line analysis performs one element analysis on a specified line each time, and scans multiple 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 measurement area range. In the analysis of PCB boards, the energy spectrometer is mainly used for the component analysis of the surface of the pads, and the elemental analysis of the surface contaminants of the pads and lead pins with poor solderability. The quantitative analysis accuracy of the energy spectrometer is limited, and the content of less than 0.1% is generally not easy to detect. The combination of energy spectroscopy and SEM can simultaneously obtain information on surface morphology and composition, which is why they are widely used.

8. When the photoelectron spectroscopy (XPS) 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. The kinetic energy Ex is measured, and the combination of the inner shell electrons of the atoms can be obtained. The energy Eb and Eb vary with different elements and different electron shells. It is the "fingerprint" identification parameter of the atom, and the formed spectral line is the photoelectron 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 states of elements can be obtained from the chemical shifts of the binding energies. It can give information such as the valence state of the surface layer and the bonding of surrounding elements; the incident beam is an X-ray photon beam, so the insulation sample can be analyzed without damaging the sample to be analyzed. Rapid multi-element analysis; it can also be used in the case of argon ion stripping Longitudinal elemental distribution analysis of multilayers (see later) is performed with much higher sensitivity than energy dispersive spectroscopy (EDS). 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 in the analysis of the PCB board to determine the deep-seated cause of poor solderability.

9. Thermal Analysis Differential Scanning Calorimetry (Differential Scanning Calorim- etry ): A method of measuring the relationship between the power difference and temperature (or time) between the input material and the reference material under programmed temperature control. DSC is equipped with two sets of compensating heating wires under the sample and reference container. When the temperature difference ΔT occurs between the sample and the reference due to the thermal effect during the heating process, the differential thermal amplifier circuit and the differential thermal compensation amplifier can be used., the current flowing into the compensating heating wire changes, and the heat on both sides is balanced, the temperature difference ΔT disappears, and the relationship between the difference of the thermal power of the two electric heating compensations under the sample and the reference material with temperature (or time) changes, according to This changing relationship can be used to study and analyze the physicochemical and thermodynamic properties of the material. DSC is widely used, but in the analysis of PCB boards, it is mainly used to measure the curing degree of various polymer materials used on the PCB board (such as Figure 2) and the glass transition temperature. These two parameters determine the subsequent use of the PCB board. Process reliability.

Thermomechanical Analyzer (TMA): 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-controlled temperature. Insert, stretch, bend, etc. The test probe is supported by a cantilever beam and a coil spring fixed on it, and a load is applied to the sample through a motor. When the sample is deformed, the differential transformer detects the change and processes it together with data such as temperature, stress and strain. The deformation of the material under negligible load as a function of temperature (or time) can be obtained. 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 and is mainly used in the analysis of PCB boards for two key parameters of PCB boards: measuring its linear expansion coefficient and glass transition temperature. PCB boards with substrates with excessive expansion coefficients often lead to fracture failure of metallized holes after soldering and assembly. Due to the development trend of high-density PCB boards and the environmental protection requirements of lead-free and halogen-free, more and more PCB boards have various failure problems such as poor wetting, explosion, delamination, and CAF. The acquisition of the failure mechanism and cause of the PCB board will be beneficial to the quality control of the printed circuit boards in the future, so as to avoid the recurrence of similar problems.