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92 SMT007 MAGAZINE I SEPTEMBER 2019 cycle to clean and activate the base copper. Electroless palladium was simply substituted for electroless nickel to create an EPIG finish. The singular preparation and plating sequence utilized are outlined in Table 1. Plating times were adjusted in the immersion gold step as the deposition is more vigorous on nickel versus palladium as predicted by the rel- ative corrosion potentials of -0.250V for nickel and +1.498V for gold, a delta of -1.748V com- pared to +0.987V for palladium, and +1.498V for gold a delta of -0.511V [3]. The larger delta negative potential between nickel and gold shows that, in the case of immersion plating, gold deposits more rapidly on to nickel than on to palladium. Post plating, solder coupons were dried and stored in a sealed and desiccated environment until steam stressing and solder wick testing was performed. Coupons were steam aged, as outlined in J-STD-002/003, for one to eight continuous hours. This was conducted in one- hour increments until solder failure or until eight hours of total steam-aging time had been achieved. Results and Discussion Test results show that ENIG is an outstand- ing final finish. ENIG, however, is susceptible to rapid solder non-wetting after steam expo- sure. On the contrary, EPIG shows little to no susceptibility in comparison. Speculation as to why such a difference between the two fin- ishes suggests a strong interaction of steam on exposed nickel phosphorus. The nickel phos- phorus is made available to steam penetration through the thin porous gold topcoat present in the ENIG deposit. Figures 7–12 show the results of solder wet- ting tests using the solder wetting balance. The X-axis measures contact time between the plated copper pads and the liquid solder. The time units are in seconds. Total contact time for each measurement is 10 or 20 seconds. The Y-axis of the wetting chart measures the wicking force of the solder as it wets the plated copper test pads. The force units are mN/mm for these tests. The force climbs as a greater area of the test pad is covered with solder. The initial dip in force represents the initial con- tact between the coupon and solder. As the test area wets with solder the force climbs to a maximum. That maximum is the wetting force for that particular test. The time needed for the force to cross or intersect the zero-force line is considered to be the total wetting time for that particular test. Results are interpreted as positive when the wetting force rapidly ascends to a maximum and holds at that level. Should the wetting force Table 1: Plating cycles used to prepare ENIG and EPIG test samples.