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February 2015 • The PCB Magazine 21 Placing additional em- phasis on pH control of the nickel solution will help control the phos content. Exces- sively high phos con- tent in the EN deposit (10.2–11.5%) will be detrimental to solder- ability as the action of the flux is diminished. Secondly, higher phos content EN deposits will act to minimize the immersion gold thickness. With respect to OSP, there are a number of challenges. First, don't force the OSP process to deposit more than 2500–3500 angstroms in thickness (0.25–0.35 micron). Excessive OSP film thickness could lead to organic deposits on the gold. In addition, judicious selection of an OSP chemistry devoid of copper ions will mini- mize staining of the gold in mixed metal fin- ish designs. Some folks are of the opinion that a thicker OSP coating on the copper provides more oxidation protection (read, better solder- ability) than thinner coatings. The data does not support such an assertion. Thinner OSP coatings perform quite effectively under lead- free soldering conditions as long as the OSP itself is able to reduce oxygen penetration to the base metal copper (Figure 9). This has been substantiated in publications [3] . The amount of copper oxidation (Cu 2 O) was compared by the peak height at certain wavelengths (ap- prox. 640 cm -1 ) with FT-IR as shown in Figure 9. Longer heat treatment times will acceler- ate the oxidation of copper. Thicker coatings tend to minimize the oxidation as shown in Figure 9. However, OSP-A, newly designed for BGA substrate applications minimizes the cop- per oxidation significantly in comparison with OSP-B and C. This result leads to better solder ball spreadability as well as higher solder joint strength afterward [3] . While thickness of the overall organic film is important (to an extent), the OSP's ability to resist or minimize oxidation of the underlying is much more critical when subjected to heat cycles. Furthermore, the fabricator should use a micro-etch that has a lower redox potential as this will provide some protection of the galvanic cell. PCB References 1. M. Fontana and N. Greene, Corrosion En- gineering, ISBN 0-07-021461-1, McGraw Hill, 1978. 2. Department of Defense, MIL-HDBK-729, November 1983. 3. M. Carano and K. Saeki, IPC Components Conference, September, 2013. Figure 9: Oxygen penetration of OSP coated copper surface. Comparison of three commercially available OSP chemistries. Michael Carano is with OMG Electronic Chemicals, a devel- oper and provider of processes and materials for the electron- ics industry supply chain. to read past columns, or to con- tact the author, click here. OSP AND SELECTIVE ELECTROLESS NICKEL continues feature