Why do the optoelectronic components on the PCB not work?

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. The quality and reliability of PCB determine the quality and reliability of the whole equipment.


With the miniaturization of electronic information products and the environmental protection requirements of lead-free and halogen-free, PCB have also developed in the direction of high density and high Tg and environmental protection. However, due to cost and technical reasons, a large number of failures have occurred in the production and application process of PCB, which has caused many quality disputes. In order to understand the cause of failure in order to find a solution to the problem and clarify responsibilities, failure analysis must be performed on the failure cases that occur.


Basic Procedure for Failure Analysis


To obtain the exact cause or mechanism of PCB failure or failure, basic principles and analysis procedures must be followed, otherwise valuable failure information may be missed, causing analysis to fail to continue or may lead to erroneous conclusions. The general basic process is that, first of all, based on the failure phenomenon, the failure location and failure mode must be determined through information collection, functional testing, electrical performance testing, and simple visual inspection, that is, failure location or fault location.


For a simple PCB or PCBA, the location of the failure is easy to determine. However, for more complicated BGA or MCM packaged devices or substrates, defects are not easy to observe through a microscope, and it is not easy to determine for a while. At this time, other means are needed to determine.


The next step is to analyze the failure mechanism, that is, use various physical and chemical methods to analyze the mechanism that causes PCB failure or defect generation, such as soldering, pollution, mechanical damage, wet stress, dielectric corrosion, fatigue damage, CAF or ion migration, Stress overload and so on.


Then there is the failure cause analysis, which is based on the failure mechanism and process analysis to find the cause of the failure mechanism. If necessary, perform test verification. Generally, test verification should be performed whenever possible. Through test verification, the exact induced failure cause can be found.


This provides a targeted basis for the next improvement. Finally, it is to compile a failure analysis report based on the test data, facts and conclusions obtained during the analysis process. It is required that the reported facts are clear, logical reasoning is strict, and the method is strong. Never imagine in vain.


In the process of analysis, pay attention to the basic principles of using analytical methods from simple to complex, from outside to inside, from not destroying samples to using damage. Only in this way can you avoid losing critical information and avoid introducing new artificial failure mechanisms.


It is like a traffic accident. If one side of the accident destroys or flees the scene, it is difficult for the police in smart to make an accurate responsibility determination. At this time, traffic regulations generally require the fugitive or the side of the scene to assume full responsibility.


The failure analysis of PCB or PCBA is also the same. If you use an electric soldering iron to repair the failed solder joints or use a large scissors to cut the PCB strongly, then the analysis can no longer be started, and the failure site has been destroyed. Especially in the case of a small number of failure samples, once the environment of the failure site is destroyed or damaged, the real cause of failure cannot be obtained.


Failure Analysis Technology


Optical microscope


The optical microscope is mainly used for the appearance inspection of the PCB, looking for the location of failure and related physical evidence, and preliminarily judging 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. If it is batch or individual, is it always concentrated in a certain area and so on.


X-ray (X-ray) has to use X-ray fluoroscopy system to inspect some parts that cannot be inspected by appearance and the inside of PCB through holes and other internal defects.


X-ray fluoroscopy systems use different material thickness 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 of high-density packaged BGA or CSP devices.


Slicing analysis Slicing 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. But this method is destructive, and once the section is performed, the sample must be destroyed.


Scanning acoustic microscope is currently used in electronic packaging or assembly analysis mainly in the C mode ultrasonic scanning acoustic microscope, which uses high-frequency ultrasonic reflection on the discontinuous interface of the material to generate amplitude and phase and polarity changes to image, its scanning method It is information scanning the XY plane along the Z axis.


Therefore, scanning acoustic microscopes can be used to detect various defects in components, materials, and PCB and PCBA, including cracks, delamination, inclusions, and voids. If the frequency width of scanning acoustics is sufficient, the 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 have internal or substrate delamination cracking phenomenon when reflowing at a higher lead-free process temperature, and ordinary PCB will often burst 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 PCB. In general, the obvious bursting board can be detected only by visual inspection.


Micro-infrared analysis 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), analyzes the composition of materials, and combines the microscope to make visible light. With the same optical path as infrared light, as long as it is in the visible field of view, you can find trace organic pollutants to be analyzed.


Without the combination of a microscope, usually the infrared spectrum can only analyze samples with a large number of samples. And in many cases in the electronic process, trace pollution can cause poor solderability of 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 microscopy (SEM) Scanning electron microscopy (SEM) is one of the most useful large-scale electron microscopy imaging systems for failure analysis. It is most commonly used for morphological observation. The current scanning electron microscopy is already very powerful, and any fine Structure or surface features can be enlarged to hundreds of thousands of times for observation and analysis.


In terms of failure analysis of PCB or solder joints, SEM is mainly used to analyze the failure mechanism. Specifically, it is used to observe the morphology and structure of the pad surface, the metallographic structure of the solder joints, the measurement of intermetallics, and the solderable coating. Analysis and measurement of tin whisker.


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. Non-conductors and some semiconductors need to be sprayed with gold or carbon. Otherwise, the accumulation of charge on the sample surface will affect Observation of the sample. In addition, the depth of field of SEM images is much larger than that of optical microscopes. It is an important analysis method for uneven samples such as metallographic structure, micro fractures and tin whiskers.


Thermal analysis


Differential Scanning Calorimeter (DSC)


Differential scanning calorimetry (Differential Scanning Calorimetry) is a method for measuring the relationship between the power difference and temperature (or time) input between a substance and a reference substance under a programmed temperature control. It is an analytical method to study the relationship between heat and temperature. Based on this relationship, physical and chemical and thermodynamic properties of materials can be studied and analyzed.


DSC is widely used, but in the analysis of PCB, it is mainly used to measure the curing 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.


Thermo Mechanical Analyzer (TMA) Thermal Mechanical Analysis technology is used to measure the deformation properties of solids, liquids and gels under the influence of heat or mechanical force under the temperature control of the program. It is a method to study the relationship between thermal and mechanical properties. According to the relationship between deformation and temperature (or time), the 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. PCB with substrates with excessive expansion coefficients often lead to failure of metallized holes after solder assembly.


Thermogravimetry (TGA) Thermogravimetry Analysis is a method for measuring the relationship between the mass of a substance and its temperature (or time) under a programmed temperature control. TGA monitors the subtle mass changes of a substance during a program-controlled temperature change process using a precision electronic balance.


According to the relationship between the mass of material and temperature (or time), the physical and chemical and thermodynamic properties of materials can be studied and analyzed. In terms of PCB analysis, it is mainly used to measure the thermal stability or thermal decomposition temperature of PCB materials. If the thermal decomposition temperature of the substrate is too low, board explosion or delamination failure will occur when the PCB is subjected to high temperatures during the soldering process.