As the carrier of various components and the hub of circuit signal transmission, PCB has become the most important and vital part of electronic information products. The quality and reliability of the PCB determines the quality and reliability of the entire device. However, due to cost and technical reasons, a large number of failures occurred during the production and application of PCBs.
For this failure problem, we need to use some common failure analysis techniques to ensure the quality and reliability level of the PCB during the manufacturing process. Ten major failure analysis techniques are summarized for reference.
1. Visual inspection
Appearance inspection is to inspect the appearance of PCB by visual inspection or using some simple instruments (for example, stereo microscope, metallographic microscope or even magnifying glass) to find component failures and related physical evidence. The main function is to locate the fault and initially determine the failure mode of the PCB. The visual inspection mainly checks the PCB's pollution, corrosion, position of explosion board, circuit wiring and regularity of faults. If it is batch or single, it is always concentrated in a certain area and so on. In addition, there are many PCB failures that are only discovered after assembly into a PCBA. Whether the failure is caused by the assembly process and the effect of the materials used in the process also requires careful examination of the characteristics of the failure area.
2. X-ray perspective examination
For some parts that cannot pass the visual inspection, as well as through holes and other internal defects inside the PCB, we must use the X-ray perspective inspection system to inspect. 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 defects inside PCBA solder joints, defects inside 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 it has changed from two-dimensional imaging equipment to three-dimensional imaging equipment. Even five-dimensional (5D) devices have been used for package inspection, but such 5D X-fluoroscopy systems are very expensive and rarely find practical use in industry.
3. Slice analysis
Slicing analysis is through a series of methods and steps such as sampling, mosaicing, slicing, polishing, corrosion, and observation. Through slicing analysis, you can get rich information about PCB microstructure (via holes, plating, etc.), which is the next step Provides a good basis for quality improvement. However, this method is destructive. After sectioning, the sample is destroyed. At the same time, this method requires a lot of sample preparation, and the sample preparation takes a long time, which requires a well-trained technique Personnel to complete. For detailed slicing procedures, please refer to IPC-TM-650 2.1.1 and IPC-MS-810 procedures.
4. Scanning Acoustic Microscope
Currently, C-mode ultrasonic scanning acoustic microscopy is mainly used for electronic packaging or assembly analysis. It is the use of high-frequency ultrasonic reflection at the discontinuous interface between the material and the phase and pole. The imaging method is based on the change of the image, while the scanning 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 PCBs and PCBAs, including cracks, delaminations, inclusions, and voids. If the frequency width of the scanning sound is large enough, it can also directly detect the internal defects of the solder joints. A typical scan sound is a red warning color, indicating a defect. Since many plastic-encapsulated components are used in the SMT process, there are a number of issues that are sensitive to moisture reflow during the transition from lead to lead-free. That is, hygroscopic plastic packaging devices will have internal or substrate delamination cracking when reflowed at higher lead-free process temperatures, and regular PCBs often crack at the high temperatures of lead-free processes. At this time, the scanning acoustic microscope highlights its special advantages in the non-destructive testing of multi-layer high-density PCBs. Often, apparent cracked plates can only be found by visual inspection.
5. Micro-infrared analysis
Micro-infrared analysis is an analysis method that combines infrared spectroscopy and microscopy. It uses different materials (mainly organic substances) to absorb the infrared spectrum with different absorptivity. Principle: Analyze the chemical composition of the material, and combine the microscope to make visible light and infrared light have the same optical path. Trace organic contaminants can be found for analysis as long as they are within the visible field of view. Without the use of a microscope, infrared spectroscopy can usually only analyze large numbers of samples. In many cases in electronics processes, trace contamination can lead to poor solderability of PCB pads or pins. As you can imagine, it is difficult to solve process problems without infrared spectroscopy of a microscope. The main purpose of micro-infrared analysis is to analyze organic pollutants on the welding surface or joint surface, and to analyze the cause of corrosion or poor solderability.
6. Scanning electron microscope analysis
A scanning electron microscope (SEM) is one of the most useful large-scale electron microscope imaging systems for failure analysis. Its working principle is to use the electron beam emitted by the cathode to accelerate through the anode, and after the magnetic lens is focused, an electron beam with a diameter of tens to thousands of angstroms (A) is formed. Under the deflection of the scanning coil, the electron beam performs a point-by-point scanning motion on the sample surface in a certain time and space sequence. The bombardment of this high-energy electron beam on the sample surface will excite all kinds of information. After collection and magnification, various corresponding graphics can be obtained from the display screen. Excited secondary electrons are generated on the sample surface in the range of 5-10 nm. Therefore, secondary electrons can better reflect the topography of the sample surface, so they are most commonly used for topography observation. Scattered electrons after excitation are generated on the sample surface. In the range of 100~1000nm, backscattered electrons with different characteristics are emitted with different atomic numbers. Therefore, backscattered electron images have morphological characteristics and the ability to discern atomic number. Therefore, backscattered electron images can reflect chemical elements. Component distribution. Current scanning electron microscopes are so powerful that any fine structure or surface feature can be magnified hundreds of thousands of times for observation and analysis.
In terms of PCB or solder joint failure analysis, SEM is mainly used to analyze the failure mechanism. Specifically, it is used to observe the morphology of the pad surface, the metallographic structure of the solder joint, and measure intermetallic chemical analysis, solderability coating analysis, and tin whisker analysis. Unlike an optical microscope, a scanning electron microscope forms an electron image, so there are only black and white, and the sample for the scanning electron microscope needs to be conductive. Nonconductors and some semiconductors require sputtering of gold or carbon. Otherwise, the charge accumulation on the sample surface will affect the observation of the sample. In addition, the depth of field of a SEM image is much greater than that of an optical microscope. It is an important analytical method for inhomogeneous samples such as metallographic structure, microcracks and tin whiskers.
7. X-ray energy spectrum analysis
The aforementioned scanning electron microscope is usually equipped with an X-ray spectrometer. When a beam of high-energy electrons strikes the surface of a sample, internal electrons in atoms of the surface material are bombarded and escape. Characteristic X-rays are excited when external electrons transition to lower energy levels. The properties of different elements have different levels of atomic energy. X-rays are different, so the characteristic X-rays emitted by the sample can be analyzed for chemical composition. At the same time, according to the detected X-ray signal as the characteristic wavelength or characteristic energy, the corresponding instruments are called spectral dispersive spectrometer (abbreviated as WDS) and energy dispersive spectrometer (abbreviated as EDS). The resolution of the spectrometer is higher than that of the spectrometer, and the analysis speed of the spectrometer is faster than that of the spectrometer. Common SEMs are equipped with energy spectrometers due to their speed and low cost.
Using different scanning methods of the electron beam, the spectrometer can perform surface point analysis, line analysis and surface analysis, and can obtain information about the different distributions of elements. Point analysis can obtain all elements of a point; line analysis performs element analysis on a specified line each 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 Average value of the measurement range.
In the analysis of PCB, the energy spectrometer is mainly used for the component analysis of the surface of the pad, and the elemental analysis of the contamination of the pad surface and pins with poor solderability. The accuracy of quantitative analysis by energy spectrometers is limited, and levels below 0.1% are generally not easily detectable. The combination of energy spectroscopy and SEM can simultaneously obtain information about surface topography and composition, which is why they are widely used.
8. Photoelectron spectroscopy (XPS) analysis
When a sample is irradiated with X-rays, electrons on the inner shells of surface atoms will escape from the core and form electrons from the solid surface. The kinetic energy Ex and the binding energy Eb of the electrons in the inner shell of the atom can be obtained. Eb varies with different elements and different electron shells. It is the "fingerprint" identification parameter of the atom. The resulting spectral lines are photoelectron spectroscopy (XPS). XPS can be used for qualitative and quantitative analysis of shallow surface (several nanometers) elements on the sample surface. In addition, information about the chemical valence of an element can be obtained based on the chemical shift of the binding energy. It can provide information such as the atomic valence of the surface layer and surrounding elements; since the incident beam is an X-ray photon beam, the analysis of insulating samples can be carried out without damaging the analyzed sample. Fast multi-element analysis is also possible; in the case of argon ion stripping, longitudinal elemental distribution analysis has been performed on multiple layers (see case below), with much higher sensitivity than energy spectroscopy (EDS). In the analysis of PCB, XPS is mainly used to analyze the quality of pad coating, the analysis of contamination and the analysis of oxidation degree to determine the deep cause of poor solderability.
9. Thermal Analysis Differential Scanning Calorimetry
A method of measuring the power difference between a substance and a reference substance as a function of temperature (or time) input under programmed temperature control. The DSC is equipped with two sets of compensating heating wires below the sample and reference containers. When a temperature difference ΔT is generated between the sample and the reference due to thermal effects during heating of the sample, a differential thermal amplification circuit and a differential thermal compensation amplifier can be used. , so that the current flowing into the compensation heating wire changes.
In order to balance the heat on both sides, the temperature difference ΔT disappears, and the relationship between the thermal power difference between the two electrothermal compensation quantities under the sample and the reference with temperature (or time) is recorded. The physical, chemical and thermodynamic properties of materials can be studied and analyzed. DSC is widely used, but in the analysis of PCBs, it is mainly used to measure the degree of solidification and glass transition temperature of various polymer materials used on PCBs. These two parameters determine the reliability of the PCB in the subsequent process.
10. Thermodynamic Analyzer (TMA)
Thermal Mechanical Analysis is used to measure the effect of solids, liquids and gels on thermal or mechanical forces under temperature control. The program depends on the deformation properties, and the commonly used loading methods are compression, penetration, tension, bending, etc. The test probe is supported by a cantilever beam and a coil spring fixed to it, and a load is applied to the sample by an electric motor. When the sample deforms, the differential transformer detects this change and processes it with temperature, stress and strain data. It can be concluded that the deformation of a substance under negligible loads is temperature (or time) dependent. According to the relationship between deformation and temperature (or time), the physical, chemical and thermodynamic properties of materials can be studied and analyzed. TMAs are widely used. In PCB analysis, it is mainly used for the two most critical parameters of a PCB: measuring its linear expansion coefficient and glass transition temperature. PCBs with substrates that have an excessive expansion coefficient often result in failures of plated holes after assembly with solder.
Due to the high-density development trend of PCBs and the environmental requirements of lead-free and halogen-free, more and more PCBs encounter various failure problems, such as poor wetting, cracking, delamination, and CAF. The application of these analytical techniques to real cases is presented. Understanding the failure mechanism and reasons of PCB will be beneficial to the quality control of PCB in the future and avoid similar problems from happening again.
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