Prototype pcb failure cause analysis (1)

As the hub of various components and circuit signal transmission, prototype PCB has become the most important and key part of electronic information products.Whichquality and reliability determine the quality and reliability of the whole equipment. With the miniaturization of electronic information products and the environmental requirements of lead-free and halogen-free, prototype PCB are also developing in the direction of high density and high Tg and environmentfriendly. However, due to cost and technical reasons, prototype PCB have experienced a large number of failures in the production and application process, and thus caused many quality disputes. In order to clarify the cause of the failureandfind a solution to the problem and to clarify the responsibility, a failure analysis must be performed.

Basic procedure for failure analysis

To obtain the exact cause or mechanism of prototype PCB failure, you must follow the basic principles and analysis procedures, otherwise valuable failure information may be missed, resulting in the analysis can not continue or may get wrong conclusions. The general basic process is to first based on the failure phenomena and failure modeto determine failure location and mode through information collection, functional testing, electrical performance testing, and simple visual inspection.

For a simple prototype PCB or PCBassembly, the location of the failure is easy to determine. However, for a more complex BGA or MCM packaged device or substrate, the defect is not easily observed by the microscope, and it is difficult to determine at a time. In this case, other means are needed to determine.

The failure mechanism is then analyzed by using various physical and chemical methods to analyze the mechanism that causes prototype PCB failure or defect generation, such as virtual welding, contamination, mechanical damage, moisture stress, dielectric corrosion, fatigue damage, CAF or ion migration, Stress overload and so on.

Then the failure cause analysis, that is, based on the failure mechanism and the process analysis, findingthe cause of the failure mechanism, and if necessary, carryingout test verification. Generally, the test verification should becarried outas muchas possible, and the cause of the induced failure can be found through the test verification.

This provides a targeted basis for the next step of improvement. Finally, the failure analysis report is compiledaccording to the test data, facts and conclusions obtained in the analysis process, It requiresthe facts of the report are clear, the logical reasoning is strict, the rules are strong, and it is forbidden to imagine.

In the process of analysis, pay attention to the basic principles of using analytical methods from simple to complex, from the outside to the inside, from not destruction of the sample to the destruction. Only in this way can we avoid losing key information and avoid introducing new artificial failure mechanisms.

Just like a traffic accident, if oneof the accidents destroys or flees the scene, it is difficult for the excellentpolice to make an accurate liability determination. At this time, the traffic regulations generally require that the party who flees or damage the scene tobear full responsibility.

The same for the failure analysis of prototype PCB or PCBA. If the soldering iron is used to repair the failed solder joints or the large scissors are used to strongly cut the PCB, then the analysis will not be possible, asthe failed scene has been destroyed. Especially in the case of a small number of failed samples, once the environment of the failed site is destroyed or damaged, the real cause of failure cannot be obtained.

Failure analysis technique

1. Optical microscope

The optical microscope is mainly used for the visual inspection of the prototype PCB, looking for the failed parts and related physical evidence, and initially determining the failure mode of the PCB. The visual inspection mainly checks the PCB contamination, corrosion, location of theboard, circuit wiring and the regularity of the failure, such as batch or individual, whether it’salways concentrated in a certain area and so on.

2,X-ray (X-ray)

For some parts that cannot be visually inspected, as well as the inside of the through hole of the PCB and other internal defects, it is necessary to use an X-ray system to check.

X-ray system principles:X-rayimagemoisture absorption or transmittanceare various due to different material thickness or different material density. This technique is used more to inspect defects inside PCBA solder joints, via internal defects, and the location of defective solder joints andhigh-density packaged BGA or CSP devices.

3, Slice analysis

Slice analysis is the process of obtaining the cross-sectional structure of a prototype PCB through a series of means and steps such as sampling, inlaying, slicing, polishing, etching, and observation. Through the slice analysis, a wealth of information reflecting the microstructure of the PCB (through holes, plating, etc.) can be obtained, which provides a good basis for the next step of quality improvement. However, this method is destructive, and once sliced, the sample is bound to be destroyed.

4, Scanning acoustic microscope

Currently it used for electronic packaging or assembly analysis, the main mode is the C-mode ultrasonic scanning acoustic microscope, which uses the amplitude and phase and polarity changes generated by the high-frequency ultrasonic reflection on the discontinuous interface of the material to image. The Z axis scans the information of the XY plane.

Therefore, scanning acoustic microscopy can be used to detect components, materials, and various defects inside the prototype PCBand PCBA, 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 detecteddirectly .

Typical scanning acoustic images represent the presence of defects in a red warning color. Due to the large number of plastic packaged components used in the SMT process, a large number of wet reflow sensitive problems occur during the conversion from lead to lead-free processes. That is, the hygroscopic plastic package device will cause internal or substrate delamination when reflowing at a higher lead-free process temperature, and the common PCB will often explode at the high temperature of the lead-free process.

At this time, scanning acoustic microscopy highlights its special advantages in multi-layer high-density PCB non-destructive testing. The general obvious blasting board can be detected by visual inspectiononly .