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20 The PCB Magazine • February 2015 tion is quite evident. Further evaluation of the etched area shows a trench at the copper gold junction. Research suggests that the type of mi- cro-etch plays a significant role in the galvanic cell formation. As an example, the oxidation redox potential of sodium persulphate in an aqueous medium is: (1) S 2 O 8 2- + 2e- 2SO 4 2- E=2.1 V The oxidation potential of hydrogen per- oxide in an aqueous acidic medium is approxi- mately 1.7 V. Thus the persulphate anion is a much stronger oxidizer and provides a much stronger potential with which to effect the gal- vanic cell potential. In addition to the type of micro-etching chemistry, the galvanic potential is affected by the ratio of the area of the anode to cathode. A rule of thumb (assuming design rules permit) in minimizing the galvanic coupling of metals is keeping the surface area ratio of the anode to cathode as large as possible. The large anode surface area (i.e., the corroding surface) results in a much smaller driving potential. However as shown in Figure 8, the gold circuit feature surface area is significantly larger than copper, resulting in a greater potential. Copper is the corroding surface. In each case the hydrogen peroxide sulfuric acid micro-etch gave more favorable results than the persulphate based etchants. This is explained by the fact that hy- drogen peroxide sulfuric acid etchant have a lower potential than persulphate. Lessons Learned with Mixed-Metal Finish Designs First and foremost, select the EN process that will provide the best corrosion resistance under the various processing conditions. How- ever, one should not default to a high phos- phorous content system without first qualify- ing the process for solderability and reliability. Figure 7: Example of galvanic etch with mixed-metal finish PCB (left) and close-up view of actual corrosion area known as the "trench" (right). Figure 8: Comparison of three different micro- etches with three different anode to cathode areas. OSP AND SELECTIVE ELECTROLESS NICKEL continues feature

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