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SEPTEMBER 2019 I SMT007 MAGAZINE 93 not pass through zero for the duration of the test, then a non-wetting sol- der failure is assigned. Likewise, sol- der failure is also observed when the wetting force reaches a maximum and then decreases. That type of chart rep- resents solder de-wetting for the test. Comparing the solderability results of ENIG (Figure 7) and EPIG (Figure 8) show that both provide excellent wetting times and soldering forces in the as-plated condition. After just one hour of steam aging, the ENIG finished sample fails to wet solder (non-wet condition) (Figure 9). The EPIG sample continues to wet well to solder, unaffected by one-hour steam preconditioning (Figure 10). Through eight hours of steam aging, the EPIG samples solder well with a minor reduction in wetting force as seen in Figure 11, which shows a 20-second contact duration. These test results show that steam has a dramatic negative impact on the solderability of ENIG even after one-hour exposure. Meanwhile, EPIG remains quite solderable with almost no change in wetting times and force after eight hours of continuous steam exposure. The reason for the perfor- mance difference is thought to be a rapid formation of a nickel oxide on the electroless nickel phospho- rous surface during steam stressing, inhibiting wetting and the formation of a nickel-tin intermetallic, which reduced the ability to solder well. The formation of a tin-nickel inter- metallic is required for good solder wetting to occur. K. Yokomine, et al. [4] , describe results of testing the depth of oxy- gen found on the electroless nickel surface from an ENIG film as plated and after immersion in an aque- ous cleaner at 80°C. Using electron spectroscopy for chemical analysis (ESCA) equipment, a three-to-four- Figure 7: ENIG wetting time and force as plated. Figure 8: EPIG wetting time and force as plated. Figure 9: ENIG non-wet condition after one-hour steam exposure.