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84 PCB007 MAGAZINE I SEPTEMBER 2020 will be difficult to detect in a simple light mi- croscope picture and require a high-resolution SEM for reliable detection. Also, the applica- tion of a protective layer applied on top of the gold layer can help to provide better contrast and reduce the risk of misinterpretation of arti- facts created during the cross-section prepara- tion. This type of surface corrosion, as shown in Figure 3, finally bears a high risk for solder- ing as well as for bonding applications. Due to the shape of the corrosion, not deep but distributed over large areas, this may in- hibit the formation of the IMC, and by that, lead to solder dewetting after the reflow step. For the bonding application, the "sponge"-like nickel surface acts as a kind of predetermined breaking point where the gold lacks the adhe- sion to the nickel and easily peels off. Com- pared to a pad with single and deep corrosion events, it appears that the single corrosion lo- cations are far less critical (Figure 4). Even though the corrosion events almost penetrate the full nickel layer, the IMC forms densely and thoroughly, and the pad is fully wetted with solder. This serves as an indica- tion that the appearance of single corrosion events is judged to be less critical for the solder joint reliability or the bonding adhesion than the appearance of large area surface corro- sion. Therefore, to create a realistic judgment of the potential risk of the corrosion for the solder or bonding joint, it is highly recommend- ed to select a representative and well-distrib- uted investigation spot and confirm the light microscope evaluation by complementary high- resolution SEM study. Summary and Conclusion ENIG corrosion is one of the most discussed topics for ENIG finishes in the PCB industry and is continuously being negotiated to define clear criteria for the acceptance of the final fin- ish. One of the key targets in electrolyte de- velopment is to reduce the corrosive attack as much as possible. The method for the evalu- ation of corrosion, which is presented in this article, allows a statistical judgment and com- parison of different final finishes. In particular, Figure 4: IMC formation at a single corrosion location on a soldered pad.

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