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June 2014 • The PCB Magazine 61 THE DEgREES oF NICKEL HyPER-CoRRoSIoN AND MITIgATIoN STRATEgIES continues Figure 2: level 2 (l) and level 3 (r) corrosion spikes. section shows excellent wettability and solder joint formation. should control the phosphorous content of the nickel to 8.3 ±0.5%. Again, consult with the chemical supplier to ensure proper controls ex- ist to insure the consistent phosphorus content through five metal turnovers of the electroless nickel plating solution. Control the gold Content One does not have to be a metals trader or buyer to know that gold metal pricing has flirt- ed with the $2000 per troy ounce level in early 2012. Thus, gold cost is a significant driver in the total cost of the ENIG process. Fabricators are looking for ways to reduce the impact of gold. One way to accomplish this is to lower the concentration of the gold metal in the work- ing solution. While many immersion gold pro- cesses require a gold metal concentration range of 1.0–1.5 grams per liter, it is suggested that lower gold concentrations may be beneficial. Of course, this is only valid if the lower gold does not lead to hyper-corrosion of the nickel. One way to ensure that this is effective is to build a first article and inspect the boards after ENIG. In addition, some suppliers have made suffi- cient improvements in the gold plating solu- tion itself. These improvements have allowed for lower gold concentrations (0.4–0.75 grams/ liter) to be employed with significant savings Figure 3: conventional immersion gold at 1.0 gram/liter. note circles around the area of severe hyper-corrosion.