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46 The PCB Magazine • February 2016 Corrosion Investigation Cross-sectional FIB image results are pre- sented in Figure 2. None of the samples showed corrosion damage in the electroless nickel layer. There were no visible black pad spikes that had penetrated into the electroless nickel region. Furthermore, surface corrosion by stripping off the gold layer also revealed no significant black pad attack (Figure 3). From these images, the underlying nickel layer is relatively intact. Surface damage is limited to very small pits on the surface of the grain and not along the grain boundaries. Conventional cyanide-containing gold plating would normally corrode along the grain boundaries and penetrate through the en- tire electroless nickel layer. Solder Performance The Pb-free solder performance of the ENIG finish are presented in the following section. The through-hole fill result is shown in Fig- ure 4. All of the samples had exceeded the IPC requirement of 95% fill. The thermal reflow had no detrimental effect on the solder hole-fill. In fact, samples that had been subjected to 2x thermal reflow have the same hole fill result as freshly plated samples. Figure 1: Plating-rate curve throughout the entire solution life. Figure 2: Cross section FIB images of the EnIG finish investigating for black pad corrosion (a) 0 mto and (b) 12 mto. Figure 3: SEM image of the surface after stripping the gold layer (a) 0 mto (b) 4 mto (c) 8 mto and (d) 12 mto. a b a b c d Cyanide-free iMMersion gold suitable for pCb surfaCe finishing