Use IPC-4552 for your ENIG thickness.
Typically, ENIG has nickel deposit with medium [7-9 percent by weight] phosphorus content. That's what fabs do on rigid boards. On a flex with medium-P and IPC-4552 specified ENIG, you could see brittleness and cracking issues with the nickel. * If you reduce the thickness of nickel on flex to overcome brittleness issue, you open yourself to "black pad" failures. So, you're kind of blanked. * Work-around is to use high-P [10-13 percent phosphorus by weight] nickel to increase the ductility of the nickel.
This high-P nickel seems to limit gold thickness to 0.05�m, which is not the end of the world. But it's nice to know just how much gold is actually on the flex. Thickness measurements in this range are a bit dicey to perform, because of limited equipment accuracy [~+/- 0.01�m on a good day].
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