Application of Scanning Electron Microscopy (SEM) in Failure Analysis
The abbreviation of scanning electron microscope is scanning electron microscope, and the English abbreviation is SEM. It uses a finely focused electron beam to bombard the surface of the sample, and observes and analyzes the surface or fracture morphology of the sample through the secondary electrons and backscattered electrons generated by the interaction between the electrons and the sample.
In failure analysis, SEM has a wide range of application scenarios, and it plays a pivotal role in determining the failure analysis mode and finding the cause of failure.
working principle
The depth of focus of a scanning electron microscope is 10 times greater than that of a transmission electron microscope and hundreds of times greater than that of an optical microscope. Due to the large depth of field of the image, the scanned electronic image is full of three-dimensionality and has a three-dimensional shape. Provides more information than other microscopes.
electronic signal
Secondary electrons (SEI) refer to extranuclear electrons bombarded by incident electrons. It mainly comes from the shallow area less than 10nm away from the surface, which can effectively display the microscopic topography of the sample surface, and has little correlation with the atomic number, and is generally used to characterize the topography of the sample surface.
Backscattered electrons (BEI) refer to high-energy electrons that escape from the surface of the sample again after the incident electrons interact with the sample. Compared with secondary electrons, backscattered electrons are positively correlated with the atomic number of the sample, and the depth of collection is deeper, mainly used to reflect the elemental characteristics of the sample.
knowledge class
Q: What is failure analysis?
A: The so-called failure analysis is based on the failure phenomenon, through information collection, visual inspection, and electrical performance testing, etc., to determine the failure location and possible failure mode, that is, failure location;
Then, according to the failure mode, a series of analysis methods are adopted to carry out cause analysis and root cause verification;
Finally, according to the test data obtained in the analysis process, an analysis report is prepared and suggestions for improvement are put forward.
Practical Analysis Application Cases
1. Observation and measurement of intermetallic compound IMC
Welding needs to rely on the alloy layer formed on the joint surface, that is, the IMC layer, to achieve the connection strength requirements. The IMC formed by diffusion has a variety of growth forms, which have a unique impact on the physical and chemical properties of the junction, especially the mechanical properties and corrosion resistance. Moreover, if the IMC is too thick or too thin, it will affect the strength of the welding.
2. Observation and measurement of phosphorus-rich layer
For pads treated with chemical nickel gold (ENIG), after Ni participates in alloying, excess phosphorus will be enriched and concentrated at the edge of the alloy layer to form a phosphorus-rich layer. If the phosphorus-rich layer is thick enough, the reliability of the solder joints will be greatly compromised.
3. Metal fracture analysis
Through the shape of the fracture, some basic problems of fracture are analyzed: such as fracture origin, fracture property, fracture mode, fracture mechanism, fracture toughness, stress state in the fracture process, and crack growth rate. Fracture analysis has become an important method for failure analysis of metal components.
4. Observation of nickel corrosion (black plate) phenomenon
Corrosion cracks (mud cracks) and the surface of the nickel layer after gold stripping are observed from the fracture surface, and there are a large number of black spots and cracks, which is nickel corrosion. Observing the morphology of the section of the nickel layer, continuous nickel corrosion can be observed, further confirming that the poor weldability plate has nickel corrosion phenomenon, and the IMC growth at the nickel corrosion site is abnormal, resulting in poor weldability.
