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22 The PCB Magazine • January 2017 tion may dissolve enough palladium which may allow a pathway for the gold to attack the nickel deposit. Lateral attack of the nickel un- derneath the palladium has been seen in some cases where the immersion gold layer has be- come very thick. The resultant lateral nickel at- tack in extreme cases may cause delamination between nickel and palladium, especially after wire bonding. The delamination results in wire lifting and connection failure. Figure 9 shows the lateral corrosion from an ENEPIG deposit which contained 5.1 µin of gold and 3.4 µin of palladium. The gold used in this instance was the standard displacement process. SEM cross section examination was per- formed for the three samples with thick gold to detect any deposit damage from the immersion gold reaction. Figures 10 and 11 show cross-section images from the different deposit at the BGA pad edge and pad center. The BGA pad was connected to the ground plane. In these three examples, the thicker gold deposit was examined for each of the three different types of immersion gold solutions. The BGA pad edges showed no nickel corrosion. The BGA pad center showed no corrosion for the reduction-assisted immersion gold while small corrosion spikes were seen with both the standard immersion gold and high efficiency immersion gold. The small corrosion spikes seen are not believed to be of consequence based on the wetting balance and wire bond data collected in this paper but can be considered a process indicator. No corrosion spikes were seen from any of the samples with thinner gold deposits. While wetting balance testing showed ex- cellent wetting time and wetting force, cross sections were performed on the solder samples from the three thick immersion gold deposits to examine the intermetallic formed. In all cases a uniform, continuous intermetallic layer was observed as shown in Figure 12. Figure 9: Example of ENEPIG corrosion under the gold and palladium deposit at 5000x. Figure 8: SEM images top-down of the ENEPIG deposit at 100x, 7500x. Figure 10: Cross-section on BGA pad edge at 5000x. A - Reduction assisted thick immersion gold B - Standard immersion thick immersion gold C - High-efficiency thick immersion gold Figure 11: Cross-section on BGA pad centers at 5000x. A - Reduction assisted thick immersion gold B - Standard displacement thick immersion gold C - High-efficiency thick immersion gold STUDY OF IMMERSION GOLD PROCESSES USED FOR BOTH ENIG AND ENEPIG