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76 PCB007 MAGAZINE I SEPTEMBER 2020 Article by Britta Schafsteller, Gustavo Ramos, Mario Rosin, Sebastian Weissbrod, and Timo Schlosser ATOTECH DEUTSCHLAND GMBH Abstract The electroless nickel/immersion gold (ENIG) finish is one of the most mature final finishes accepted in the market for decades. As the nature of the gold deposition is an immer- sion reaction, the dissolution of nickel and the risk of extensive nickel corrosion is process im- manent. Under standard conditions, this does not cause issues in the solderability or reliabil- ity of the coating. The key target for the development of im- mersion gold electrolytes is always to create a solution with the lowest possible corrosive at- tack avoiding (e.g., the creation of surface cor- rosion, which might cause defects in soldering and bonding applications). Several teams have studied the gold immer- sion reaction and the influencing factors on ENIG corrosion. The target of this article is to show how the ENIG layer can be rated during development, which types of corrosion can be really critical for the final application, and how misinterpretation of light microscope or SEM data can lead to false judgment. This study shows where the focus in gold electrolyte development, as well as PCB judg- ment, has to be put to allow a reliable evalu- ation of a forecast for the production perfor- mance. Introduction: Why 'No Obvious' Nickel Corrosion Does Not Necessarily Mean 'No Problem' The ENIG finish is still covering a major por- tion of the sales volume in the surface fin- ish market, offering a reliable finish to allow soldering as well as Al wire bonding. Due to the long experience in the market, it has been proven to be a robust process leading to reli- able solder connections. Nevertheless, due to the nature of the process where an immersion plating electrolyte is used in the second step, it implements the dissolution of nickel dur- ing the gold deposition, which may impact the performance of the finish in case the dissolu- tion reaction is uncontrolled. The nickel dis- solution reaction is usually named as "nick- el corrosion" or a greater extent "nickel hyper corrosion," which is sometimes referred to as "black pad." This becomes particularly true when the corrosive attack of the gold electro- lyte dissolves the nickel layer homogeneous- ly on the upper surface of the nickel deposit, ENIG: Corrosion and Learning From Failures

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