PCB Tech - Perform failure analysis on failure cases of PCB

As the carrier of various components and the hub of circuit signal transmission, PCB board 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, PCBs are also developing in the direction of high density, high Tg and environmental protection. However, due to cost and technical reasons, a large number of failure problems have occurred in the production and application of PCBs, which has caused many quality disputes. In order to clarify the cause of the failure in order to find a solution to the problem and distinguish the responsibilities, it is necessary to conduct a failure analysis on the failure cases that have occurred.

Basic procedure of failure analysis

To obtain the accurate cause or mechanism of PCB failure or failure, the basic principles and analysis process must be followed, otherwise valuable failure information may be missed, causing the analysis to be unable to continue or may get 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 failure location.

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

Then we must 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 virtual welding, pollution, mechanical damage, moisture stress, medium corrosion, fatigue damage, CAF or ion migration, Stress overload and so on.

Then there is the failure cause analysis, that is, based on the failure mechanism and process analysis, to find the cause of the failure mechanism, and test verification if necessary. Generally, test verification should be performed as much as possible, and the accurate cause of induced failure can be found through test verification.

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 in the analysis process, requiring clear facts, strict logical reasoning, and strong organization. Do not imagine out of thin air.

In the process of analysis, pay attention to the basic principles that the analytical method should be from simple to complex, from outside to inside, never destroying the sample and then using it. Only in this way can we avoid the loss of key information and the introduction of new man-made failure mechanisms.

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

X-ray (X-ray)

For some parts that cannot be visually inspected, as well as the internal and other internal defects of the through holes of the PCB, X-ray fluoroscopy system has to be used for inspection.

X-ray fluoroscopy systems use different material thicknesses or different material densities based on different principles of moisture absorption or transmittance of X-rays for imaging. This technology is more used to check the internal defects of PCBA solder joints, the internal defects of through-holes, and the positioning of defective solder joints of BGA or CSP devices in high-density packaging.

Slice analysis

Slicing analysis is the process of obtaining the cross-sectional structure of the PCB through a series of methods and steps such as sampling, inlaying, slicing, polishing, corrosion, and observation. Through slice analysis, we can get rich information of the microstructure that reflects the quality of PCB (through holes, plating, etc.), which provides a good basis for the next quality improvement. However, this method is destructive, once the sectioning is carried out, the sample will inevitably be destroyed.

Scanning acoustic microscope

At present, the C-mode ultrasonic scanning acoustic microscope is mainly used for electronic packaging or assembly analysis. It uses the amplitude, phase and polarity changes generated by the reflection of high-frequency ultrasonic waves on the discontinuous interface of the material to image. The scanning method is along the The Z axis scans the information on the XY plane.

Therefore, the scanning acoustic microscope can be used to detect various defects in components, materials, and PCBs and PCBAs, including cracks, delamination, inclusions, and voids. If the frequency width of the scanning acoustics is sufficient, the internal defects of the solder joints can also be directly detected.

A typical scanning acoustic image uses a red warning color to indicate the existence of defects. Because a large number of plastic packaged components are used in the SMT process, a large number of moisture reflow sensitivity issues are generated during the conversion from lead to lead-free process. That is to say, moisture-absorbing plastic packaged devices will experience internal or substrate delamination cracking during reflow at a higher lead-free process temperature, and common PCBs will often explode under 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 PCBs. Generally, obvious bursts can be detected only by visual inspection of the appearance.

Micro-infrared analysis

Micro-infrared analysis is an analysis method that combines infrared spectroscopy and microscope. It uses the principle of different absorption of infrared spectra by different materials (mainly organic matter) to analyze the compound composition of the material, and combined with the microscope can make visible light and infrared light the same. The light path, as long as it is in the visible field of view, you can find the trace organic pollutants to be analyzed.

Without the combination of a microscope, infrared spectroscopy can usually only analyze samples with a large amount of samples. However, in many cases in electronic technology, micro-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 with a microscope. The main purpose of micro-infrared analysis is to analyze the organic contaminants on the welded surface or the surface of the solder joint, and analyze the cause of corrosion or poor solderability.

Scanning electron microscopy (SEM)

Scanning electron microscope (SEM) is one of the most useful large-scale electron microscopy imaging systems for failure analysis. It is most commonly used for topography observation. The current scanning electron microscopes are already very powerful. Any fine structure or surface feature can be magnified. Observe and analyze hundreds of thousands of times.

In the failure analysis of PCB or solder joints, SEM is mainly used to analyze the failure mechanism. Specifically, it is used to observe the topographic structure of the pad surface, the metallographic structure of the solder joint, measure the intermetallic compound, and the solderability coating Analyze and do tin whisker analysis and measurement.

Unlike the optical microscope, the scanning electron microscope produces an electronic image, so it only has 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 accumulation of charges on the surface of the sample will affect Observation of the sample. In addition, the depth of field of the scanning electron microscope image is far greater than that of the optical microscope, and it is an important analysis method for uneven samples such as metallographic structure, microscopic fracture and tin whisker.

Thermal analysis

Differential Scanning Calorimeter (DSC)

Differential Scanning Calorimetry (Differential Scanning Calorimetry) is a method of measuring the relationship between the power difference between the input material and the reference material and the temperature (or time) under program temperature control. It is an analytical method for studying the relationship between heat and temperature. According to this relationship, the physical, chemical and thermodynamic properties of materials can be studied and analyzed.

DSC has a wide range of applications, but in PCB analysis, 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.