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AUGUST 2018 I PCB007 MAGAZINE 59 The expectation is that nickel corrosion would not occur in ENEPIG as the gold ions have no direct access to the nickel. This would be true if the palladium layer is impervious to the gold ions. If the Pd layer is thinner (< 4 µin/ 0.1 µm), it is not totally impervious, and the gold ions may have access to the underly- ing nickel offering an easier path to immersion gold deposition. Nickel corrosion would oc- cur. A thicker Pd layer (6–8 µin/0.15–0.2 µm), would go a long way towards preventing nick- el corrosion. The effect of the following attributes in cre- ating nickel corrosion were investigated: • Thickness of the electroless palladium layer • Type of electroless palladium (phos vs non-phos) • Type of immersion gold (standard immersion vs reduction assisted immersion gold) Experimental and Results The test vehicle (Figure 1) used in this study consisted of a double-sided, copper-clad lami- nated substrate which was copper plated to a thickness of 20 µm using an acid copper elec- troplating process. ENEPIG was deposited on the test vehicle using two different types of electroless palladium with two different types of gold. The nickel deposit (7-8 % phospho- rous) was a single source and was deposited at a fixed thickness of 225-275 µin (5.6–6.9 µm). The electroless palladiums were a phos Pd with ~4.0% P in the deposit and a non-phos Pd (0% P). Two different gold baths were cho- sen for this investigation; the first was a stan- dard immersion gold bath that ran at a mild- ly acidic pH of ~5.5 at a temperature of 180 o F, the second gold bath was a "reduction-assisted immersion gold" bath also known as a "mixed reaction" bath. This bath is both an immersion and an autocatalytic (electroless) bath. The bath composition includes a reducing agent; the deposition of gold does not depend on sub- strate oxidation. All the plating was done using plating chemicals commercially available from C. Uyemura & Co. The thickness of the palladium deposit was varied by changing the dwell time in the baths. The rate of deposition over time was recorded. The different thickness Ni-Pd layers were indi- vidually placed in the immersion gold bath for an exaggerated dwell time of 30 minutes. The exaggerated dwell in the gold bath was by de- sign, to ensure that some level of nickel cor- rosion would occur and there would be a way to evaluate the difference that the thickness of the Pd layer would play in Ni corrosion. Test #1: Varying thickness of phos palladium with standard immersion gold Test #2: Varying thickness of non-phos palla- dium with standard immersion gold Test #3: Varying thickness of phos palladium with reduction-assisted immersion gold After each test, cross-sections through the ENEPIG layer at different palladium thickness were evaluated for Ni corrosion using a Seiko SEA-5120 Element Monitor MX XRF. The cross- section images of the pads were observed us- ing a JEOL JSM-6010LA SEM. Test #1: Phos palladium/immersion gold Test #1 followed the process sequence out- lined in Table 2. Six solder test coupons were plated in electroless nickel to fixed dwell time and nickel thickness. This was followed by electroless phos-palladium. The dwell time in the palladium bath was 1, 2, 4, 6, 8, and 10 minutes, giving rise to thickness that varied from 2-8.8 µin. The holes were cross-sectioned and examined. Figure 1: Test vehicle (2.4 x 1.5 cm).