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60 PCB007 MAGAZINE I AUGUST 2018 Figure 2 is a graphic presentation of the data from Table 3. The data shows that the gold thickness at the lower palladium thickness was as high as 4.0 µin (0.1 µm) and contin- ued to diminish as the thickness of the layer increased and was limited to 2 µin (0.05 µm), when the thickness of the palladium was 8 µin (0.20 µm) or greater. The explanation is that the gold ions, at the lower thickness, had ac- cess to the underlying nickel and proceeded to deposit at an accelerated rate, producing nick- el corrosion. In absence of availability of the underlying nickel, the immersion gold reaction with phos palladium becomes self-limiting to 2 µin (0.05 µm). Figures, 3, 4 and 5 are SEM micrographs de- picting the level and type of corrosion at dif- ferent levels of phos palladium thickness. Fig- ure 3 shows shallow extensive corrosion and a thicker gold deposit; Figure 4 shows intermit- tent deep corrosion spikes and a gold thickness of 4.8 µin (0.12 µm); and Figure 5 has no cor- rosion spikes, however the gold thickness was limited to 2 µin (0.05 µm). Test #2 Non-phos palladium/immersion gold Test #2 followed the process sequence out- lined in Table 2. Six solder test coupons were plated in electroless nickel to fixed dwell time Table 2: Process Sequence. Table 3: Palladium thickness at different dwell times and the corresponding thickness of immersion gold for the different coupons for Test #1.