Reasons why optoelectronic components on PCB do not work

Nowadays, many PCB boards have optoelectronic components, so do you know some of the reasons for their failure? 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. Its quality The quality and reliability level determine the quality and reliability of the complete equipment.

With the miniaturization of electronic information products and the environmental requirements for lead-free and halogen-free, PCBs are also developing in the direction of high density, high Tg and environmental protection. However, due to cost and technical reasons, PCBs have experienced a large number of failure problems during the production and application process, which has led to many quality disputes. In order to clarify the cause of the failure so as to find solutions to the problem and assign responsibilities, a failure analysis must be performed on the failure cases that occurred.

1. Basic procedures for failure analysis

To obtain the accurate cause or mechanism of PCB failure or failure, basic principles and analysis procedures must be followed. Otherwise, valuable failure information may be missed, causing the analysis to not continue or reaching wrong conclusions. The general basic process is that first, 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 simple PCB or PCBA, the failure location is easy to determine. However, for more complex BGA or MCM packaged devices or substrates, defects are difficult to observe through a microscope and are not easy to determine at the moment. At this time, other means are needed to determine. Then we need to analyze the failure mechanism, that is, use various physical and chemical methods to analyze the mechanisms that cause PCB failure or defects, such as virtual soldering, contamination, mechanical damage, moisture stress, media corrosion, fatigue damage, CAF or ion migration, Stress overload, etc.

Then there is the failure cause analysis, that is, based on the failure mechanism and process process analysis, find the cause of the failure mechanism, conduct experimental verification when necessary, and generally conduct experimental verification as much as possible. Through experimental verification, the accurate cause of the induced failure can be found. This provides a targeted basis for the next improvement. Finally, a failure analysis report is prepared based on the test data, facts and conclusions obtained during the analysis process. The reported facts are required to be clear, the logical reasoning is strict, and the organization is strong, and avoid imagination.

During the analysis process, pay attention to the basic principles of using analysis methods from simple to complex, from outside to inside, and from never destroying the sample to using destruction. Only in this way can we avoid losing key information and avoiding the introduction of new artificial failure mechanisms. Just like a traffic accident, if one party to the accident destroys or flees the scene, it is difficult for a skilled police officer to make an accurate determination of responsibility. Traffic laws at this time generally require that the person who fled the scene or the party who destroyed the scene bear full responsibility.

The same goes for the failure analysis of PCB or PCBA. If you use a soldering iron to repair the failed solder joints or use scissors to forcefully cut the PCB, then there will be no way to start further analysis, and the failure site has been destroyed. Especially when there are few failed samples, once the environment at the failure site is destroyed or damaged, the real cause of the failure cannot be obtained.

2. Failure analysis technology

Optical microscope

Optical microscopes are mainly used for visual inspection of PCBs, looking for failure parts and related physical evidence, and preliminarily determining the failure mode of PCBs. The visual inspection mainly checks the PCB pollution, corrosion, the location of the burst board, circuit wiring and the regularity of failure, whether it is a batch or individual, whether it is always concentrated in a certain area, etc.

X-ray

For some parts that cannot be inspected through visual inspection, as well as the inside of PCB through holes and other internal defects, X-ray fluoroscopy systems have to be used to inspect. The X-ray fluoroscopy system uses different principles of moisture absorption or transmittance of X-rays from different material thicknesses or different material densities to create images. This technology is more commonly used to inspect defects within PCBA solder joints, defects within through holes, and positioning of defective solder joints in high-density packaged BGA or CSP devices.

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, etching, and observation. Through slice analysis, rich information on the microstructure that reflects the quality of PCB (through holes, plating, etc.) can be obtained, providing a good basis for the next step of quality improvement. However, this method is destructive, and once sectioned, the sample is inevitably destroyed.

scanning acoustic microscopy

Currently, C-mode ultrasonic scanning acoustic microscopy is mainly used for electronic packaging or assembly analysis. It uses the amplitude, phase and polarity changes caused by high-frequency ultrasonic waves reflected on discontinuous interfaces of materials to image. The scanning method is along the The Z axis scans the information of the XY plane. Therefore, scanning acoustic microscopy can be used to detect various defects within components, materials, and PCBs and PCBAs, including cracks, delaminations, inclusions, and voids. If the frequency width of the scanning acoustics is sufficient, internal defects of the solder joints can also be directly detected.

A typical scanning acoustic image uses 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 issues arise during the conversion from lead to lead-free processes. That is to say, moisture-absorbing plastic-sealed devices will suffer internal or substrate delamination cracking during reflow at higher lead-free process temperatures. Ordinary PCBs will often experience board explosions at high temperatures in lead-free processes.

At this time, the scanning acoustic microscope highlights its special advantages in non-destructive testing of multi-layer high-density PCBs. Generally, obvious bursts can be detected by visual inspection of the appearance.

Microscopic infrared analysis

Microinfrared 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 matter) to analyze the compound composition of the material. Combined with the microscope, visible light and infrared light can be simultaneously analyzed. As long as the optical path is within the visible field of view, trace amounts of organic pollutants to be analyzed can be found.

Without the combination of microscopy, infrared spectroscopy can usually only analyze samples with larger sample volumes. In many cases in electronic processes, trace amounts of contamination can lead to poor solderability of PCB pads or lead pins. As you can imagine, it is difficult to solve process problems without the infrared spectrum of a microscope. The main purpose of microscopic infrared analysis is to analyze organic contaminants on the surface to be welded or solder joints, and to analyze the causes of corrosion or poor solderability.

Scanning electron microscopy analysis (SEM)

Scanning electron microscope (SEM) is the most useful large-scale electron microscopy imaging system for failure analysis. It is most commonly used for morphology observation. The current scanning electron microscope is 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 PCB or solder joint failure analysis, SEM is mainly used to analyze the failure mechanism. Specifically, it is used to observe the morphological structure of the pad surface, the metallographic structure of the solder joint, and measure intermetallic compounds and solderability plating. Analysis and tin whisker analysis and measurement, etc.

Different from the optical microscope, the scanning electron microscope forms an electron image, so it only has black and white colors, and the sample of the scanning electron microscope is required to be conductive. Non-conductors and some semiconductors need to be sprayed with gold or carbon, otherwise the accumulation of charges on the surface of the sample will affect the Observation of samples. In addition, the depth of field of scanning electron microscope images is much larger than that of optical microscopes, making it an important analysis method for uneven samples such as metallographic structures, microscopic fractures, and tin whiskers.

3. Thermal analysis

Differential Scanning Calorimeter (DSC)

Differential Scanning Calorimetry is a method that measures the relationship between the power difference between the input material and the reference material and the temperature (or time) under programmed temperature control. It is an analytical method that studies the relationship between heat and temperature changes. Based on this changing relationship, the physical, chemical and thermodynamic properties of materials can be studied and analyzed.

DSC is widely used, but in PCB analysis it is mainly used to measure the degree of solidification and glass transition temperature of various polymer materials used on PCB. These two parameters determine the reliability of PCB in subsequent processes.

Thermal Mechanical Analyzer (TMA)

Thermal Mechanical Analysis technology is used to measure the deformation properties of solids, liquids and gels under the action of heat or mechanical force under programmed temperature control. It is a method to study the relationship between thermal and mechanical properties. Based on the relationship between deformation and temperature (or time), the physical, chemical and thermodynamic properties of materials can be studied and analyzed. TMA has a wide range of applications. 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. PCBs with base materials with excessive expansion coefficients often lead to fracture and failure of metallized holes after welding and assembly.

Thermogravimetric Analyzer (TGA)

Thermogravimetry Analysis is a method that measures the relationship between the mass of a substance and the change in temperature (or time) under programmed temperature control. TGA can monitor subtle mass changes in substances during programmable temperature changes through precision electronic balances.

According to the relationship between the change of material mass with temperature (or time), the physical, chemical and thermodynamic properties of the material can be studied and analyzed. In terms of PCB analysis, it is mainly used to measure the thermal stability or thermal decomposition temperature of the PCB material. If the thermal decomposition temperature of the base material is too low, the PCB will experience board explosion or delamination failure when it passes through the high temperature of the welding process. The above are the possible reasons for the failure of optoelectronic components on the PCB board. I hope it can be given for your reference.