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Page 84 of 137

SEPTEMBER 2020 I PCB007 MAGAZINE 85 for product development, this is important to identify trends and distinguish between small effects. Comparing the corrosion rating with assem- bly data from solder and bonding performance, it appears that the surface corrosion is rated as the most critical type of corrosion in regards to the solder and bonding reliability—rather than the single corrosion events that were found to be non-critical, as the IMC can easily bridge and cover the defect and allows for a defect- free and complete solder wetting. For the re- liable judgment of the expected performance of an ENIG finish, it is recommended to not only screen the finish with a light microscope or low-resolution SEM but also focus on sur- face defects, which might be overlooked in the first view. In the final application, these can be the most critical ones. PCB007 Editor's Note: This article was originally titled "ENIG: Corrosion and Learning From Failures—Why 'No Obvious' Nickel Corrosion Does Not Necessarily Mean 'No Problem.'" Reference 1. R. Schmidt, M. Zwanzig, and M. Schneider-Ramelow, "Causes and Avoidance of the Black Pad Defect When Sol- dering SMDs," PLUS 2013 (5) 1080–1087. Britta Schafsteller, global product manager for selective finishing, Atotech in Berlin, Germany. Gustavo Ramos, global product director for selective finishing, Atotech in Berlin, Germany. Mario Rosin, application manager selective finishing high end, Atotech in Berlin, Germany. Sebastian Weissbrod, development engineer in the R&D department for selective finishing, Atotech in Berlin, Germany. Timo Schlosser, technical engineer for selective finishing high end at Atotech in Berlin, Germany. Researchers have identified one of the reasons why state-of-the-art nickel-rich battery materials become fa- tigued after prolonged use. Lithium-ion batteries used by EVs are likely to dominate the EV market for the foresee- able future, and nickel-rich lithium transition-metal ox- ides are the state-of-the-art choice for the positive elec- trode, or cathode, in these batteries. Currently, most EV batteries contain significant amounts of cobalt. However, cobalt can cause severe environmental damage, so researchers have been looking to replace it with nickel. Nickel-rich materials degrade much faster and require additional study to be com- mercially viable for applications, such as EVs. "To fully function, battery materials New Insights Into Lithium-Ion Battery Failure Mechanism need to expand and shrink as the lithium ions move in and out," said Dr. Chao, first author of the article. "However, after prolonged use, we found that the atoms at the sur- face of the material had rearranged to form new struc- tures that are no longer able to store energy." What's worse is that these areas of reconstructed sur- face apparently act as stakes that pin the rest of the ma- terial in place and prevent it from the contraction, which is required to reach the fully charged state. With this knowl- edge, the researchers are now seeking effective counter- measures, such as protective coat- ings and functional electrolyte ad- ditives, to mitigate this degradation process and extend the lifetime of such batteries. (Source: University of Cambridge)

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