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78 PCB007 MAGAZINE I SEPTEMBER 2020 which leads to a dark appearance if the gold layer is stripped off. As it has been observed, in some cases the IMC formation can be disturbed in such areas of hyper corrosion, which may lead to poor solder joints, and the observation of nickel cor- rosion has always been a parameter to control the quality of the final ENIG finish. Basical- ly, depending on the nickel and gold electro- lytes being used, the shape and intensity of the corrosion events may vary strongly. In gener- al, it can be distinguished between single small corrosion events in bite or spike shape, which do not necessarily impact the IMC formation due to their marginal area portion, as shown in Figure 1 on the top right. When comparing the different types of corrosion, the obvious target for the chemical process development appears to create a process with a minimum attack of the nickel layer during the gold plating step. Corrosion Evaluation in Product Development One of the main challenges during the de- velopment of the electrolyte, besides the iden- tification of the best electrolyte formulation is to implement objective and statistical evalua- tion criteria to characterize the performance of the deposit and identify positive and negative trends in the development process. In the Atotech process development, a broad catalog of criteria has been implemented, which Excessive nickel corrosion through entire nickel layer. Nickel corrosion through entire nickel deposit. Single corrosion events in SEM with a protection layer. Surface corrosion in SEM with a protection layer. Figure 1: Exemplary pictures of different types of ENIG corrosion.