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SEPTEMBER 2020 I PCB007 MAGAZINE 81 croscope is used for screening evaluations and the SEM to confirm the performance of sin- gle settings with a higher resolution. Applying this method allows the creation of a far more comprehensive and conclusive data set than compared to the evaluation based on the IPC- 4552A. The major difference on one hand is the finer gradation of the rating and the higher resolution of the SEM image. In the history of the process development, however, it still re- quired adjustments, and it became clear that there are limitations in the detection of criti- cal samples. If the main target for an immersion gold elec- trolyte is to show no corrosion in the cross-sec- tion, it may happen that critical surface corro- sion is simply overseen; through the means of light microscopic evaluation, it is not possible to detect if the penetration depth to the nickel layer is low. This may even be true for SEM in- vestigations if the contrast is low, and now the protection layer applied to prevent the gold lay- er from delaminating. In such cases, it is pos- sible that a finish that is rated as "corrosion- free" may show critical soldering and bonding defects. Some exemplary images of what the failures may look like are collected in Figure 2. The bond lift-offs show that the gold layer is peeled off in the respective areas, even though from the top surface no clear indication for ex- cessive corrosion can be observed. Comparing the two soldered pads, the defect pads show a clear indication for solder dewetting with the nickel underneath appearing dark in some of the dewetted areas. For a better understanding of the failure, high-resolution SEM cross-sec- per may migrate to the surface or to the sol- der joint. Corrosion events penetrating the nickel lay- er more than 100% are so severe that, in such cases, the copper underneath is also attacked and dissolved, bearing an even higher risk to lead to solder joint failures. Such events are very rare and usually a clear indication that the ENIG process is not run in the specified range. Finally, a type of corrosion is rated, which we call "surface corrosion." It typically consists of a sponge-like nickel structure and does not pen- etrate the nickel in depth. Due to the sponge- like structure of the nickel, the increased light absorption leads to a darker appearance af- ter the gold stripping, which leads to the com- mon term of "black pad." Black pad corrosion is usually spread over larger areas up to several µm in diameter at the top surface bearing the highest risk to lead to soldering or bonding fail- ures. This is due to the fact that a large area is affected, and in this area, the formation of the IMC during the soldering is inhibited. Or, in re- gard to bonding applications, the adhesion be- tween nickel and gold is weakened, and a pre- determined breaking point is created, where the gold layer lifts off the nickel [1] . Limitations: When a Light Microscope is Not Enough In the internal product development process, the rating, as described previously, was imple- mented to allow a reliable, representative, and statistically relevant judgment of the ENIG cor- rosion. Usually, a combination of light micro- scope and SEM is applied, where the light mi- Figure 2: Exemplary pictures of failure modes of ENIG finishes in soldering and bonding. Pad showing dewetting after soldering. Full wetted defect-free pad Pads with gold peel off after wire bonding Detailed SEM image of the ENIG surface after bond lift-off