Super comprehensive PCB failure analysis technology

- Sep 01, 2018-

PCB, as the carrier of various components and the junction of circuit signal transmission, has become the most important and key part of electronic information products.With the miniaturization of electronic information products and the environmental protection requirements of lead-free and halogen-free, PCB is also developing towards high density and high Tg as well as environmental protection.However, due to the cost and technology, there are a lot of failures in the production and application process of PCB, resulting in a lot of quality disputes.In order to understand the cause of failure so as to find a solution to the problem and distinguish between responsibilities, failure analysis must be carried out on the failure cases that occurred. 

Super comprehensive PCB failure analysis technology

Basic program for failure analysis

In order 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, and the analysis cannot continue or may lead to wrong conclusions.The general basic process is that, first of all, the failure location and failure mode shall be determined through information collection, functional test, electrical performance test and simple appearance inspection, namely, failure location or fault location.For simple PCB or PCBA, the location of failure is easy to be determined. However, for relatively complex BGA or MCM encapsulated devices or substrate, the defect is not easy to be observed through a microscope, and it is not easy to be determined for a while. At this time, it needs to be determined by other means.Then, failure mechanism analysis will be carried out, that is, various physical and chemical methods will be used to analyze the mechanism causing PCB failure or defects, such as virtual welding, pollution, mechanical damage, moisture stress, medium corrosion, fatigue damage, CAF or ion migration, stress overload, etc.Then, the analysis of failure causes, that is, based on failure mechanism and process analysis, to find the causes leading to failure mechanism, to carry out test verification when necessary, and generally to carry out test verification as much as possible. Through test verification, the precise cause of induced failure can be found.This provides a definite basis for further improvement.Finally, based on the experimental data, facts and conclusions obtained in the analysis process, the failure analysis report is prepared, which requires clear facts, strict logical reasoning and strong organization. 

In the process of analysis, it should be noted that the analysis method should be used from simple to complex, from the outside to the inside, and from the sample to the basic principle of use destruction.Only in this way can we avoid losing key information and introducing new artificial failure mechanism.Just as in a traffic accident, if one of the parties destroys or escapes from the scene, it is very difficult for a wise policeman to make an accurate identification of the responsibility.The failure analysis of PCB or PCBA is also the same. If the failed soldering iron is used to repair the failed solder or the large scissors are used to make a strong cutting of the PCB, the re-analysis will be impossible. The failure site has been damaged.Especially in the case of few failure samples, once the failure site environment is damaged or damaged, the real failure cause cannot be obtained. 

 

Super comprehensive PCB failure analysis technology

Failure analysis technique

Optical microscope

Optical microscopy is mainly used to examine the appearance of PCB, look for the failure site and relevant physical evidence, and preliminarily determine the failure mode of PCB.Appearance inspection mainly examines PCB contamination, corrosion, the location of the bursting plate, circuit wiring and failure regularity, if batch or individual, whether always concentrated in a certain area, etc. 

X ray

X-ray fluoroscopy is used to check for defects in areas that cannot be visually inspected, as well as in the holes of the PCB and other internal defects.X-ray fluoroscopy system is to use different material thickness or different material density to X-ray absorbency or transmittance different principles to image.The technology is more used to check the localization of defects in PCBA solder joints, in through-holes, and in BGA or CSP devices in high density packages. 

Section analysis

Section analysis is the process of obtaining PCB cross section structure through a series of means and steps such as sampling, inlaying, slicing, polishing, corrosion and observation.Abundant information on microstructure of PCB(through-hole, coating, etc.) can be obtained by slicing analysis, providing a good basis for quality improvement in the next step.But the method is destructive, and once sliced, the sample is inevitably destroyed. 

Scanning acoustic microscope

At present, the ultrasonic scanning acoustic microscope in C mode is mainly used for electronic packaging or assembly analysis. It is used to image the amplitude and phase and polarity changes generated by the reflection of high-frequency ultrasonic wave on the discontinuous interface of materials. The scanning mode is to scan the information of x-y plane along the Z axis.Therefore, sem can be used to detect defects in components, materials and PCB and PCBA, including cracks, delamination, inclusions and cavities.If the frequency width of the scan acoustics is sufficient, the internal defect of the solder can be directly detected.Of a typical scanning acoustic image in color red alert said defects exist, because a large amount of plastic packaging components used in SMT process, by a lead into the process of lead-free technology, a large number of moisture reflow sensitive problem, namely the moisture absorption of powder coating devices will be at a higher temperature reflow lead-free process occurs within or substrate layer cracking phenomenon, under the high temperature of lead-free craft common PCB will often be a critical phenomenon.At this point, sem highlights its particular advantage in non-destructive detection of multilayered high-density PCB.In general, the obvious detonation plate can be detected only through visual appearance. 

Microinfrared analysis

Micro infrared analysis is to infrared spectroscopy combined with microscope analysis method, it use different material (mainly organic matter) on the principle of the infrared spectrum absorption, analyzing the compound composition of the materials, coupled with the microscope can make visible light and infrared light with the light path, as long as under the visual field, can look for analysis of trace organic pollutants.Without the combination of the microscope, the infrared spectrum can only analyze samples with large sample sizes.However, in electronic process, microcontamination can lead to poor solderability of PCB pad or lead foot. It can be imagined that it is difficult to solve the process problem without the infrared spectrum matching the microscope.The primary purpose of microinfrared analysis is to analyze organic contaminants on the solder surface or solder joint surface, and to analyze the causes of corrosion or poor weldability. 

Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) is one of the most useful large electron microscopic imaging systems for failure analysis. It is most commonly used for morphological observation. The current scanning electron microscopy is very powerful. 

In terms of failure analysis of PCB or solder joint, SEM is mainly used to analyze the failure mechanism. Specifically, it is used to observe the morphology and structure of the solder plate surface, the metallographic structure of solder joint, measure the metal interstitial, solder coating analysis and tin whisker analysis and measurement.Unlike optical microscopes, scanning electron microscopy (sem) produces an electron image, so it is only in black and white, and the sem samples require electrical conduction. Non-conductors and some semiconductors need to be treated with gold or carbon. Otherwise, charge accumulation on the surface of the sample will affect the observation of the sample.In addition, the depth of field of sem images is much larger than that of optical microscopy, which is an important analysis method for uneven samples such as metallographic structure, microfracture and tin whisker. 

Thermal analysis

Differential scanning calorimeter (DSC)

Differential Scanning calorim-etry is a method of measuring the temperature (or time) of power difference between an input substance and a reference substance under controlled temperature.It is an analytical method to study the relation between heat and temperature. According to this relation, physical, chemical and thermodynamic properties of materials can be studied and analyzed.DSC is widely used in PCB analysis, but it is mainly used to measure the curing degree and glass transition temperature of various polymer materials used in PCB. These two parameters determine PCB's reliability in the subsequent process. 

Thermal mechanical analyzer (TMA)

Thermal Mechanical Analysis is used to measure the deformation properties of solids, liquids and gels under Thermal or Mechanical forces at controlled temperature.It is a method to study the relation between thermal and mechanical properties. According to the relation between deformation and temperature (or time), physical, chemical and thermodynamic properties of materials can be studied and analyzed.TMA is widely used in PCB analysis. It is mainly used to measure the linear expansion coefficient and glass transition temperature of PCB.The failure of metallized holes is often caused by PCB of base material with excessive expansion coefficient after welding and assembly. 

TGA

Thermogravimetry Analysis (Thermogravimetry Analysis) is a method to measure the change of mass of a substance with temperature (or time) under controlled temperature.TGA can monitor the slight mass change of substance in the process of controlled temperature change by using a precise electronic balance.The physical, chemical and thermodynamic properties of materials can be studied and analyzed according to the relation of mass to temperature (or time).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 base material is too low, the PCB will have the flak or layer failure at the high temperature during the welding process.