For nickel to be an effective diffusion barrier, the thickness of nickel is typically at 150 micro-inches.

The rule of thumb is no lower than 100 microinches and the nominal should be 150 microinches. It is ok for 200 microinches because electroless nickel barriers are sometimes hard to control and you see 175 to 200 microinches but as a general rule you will see 125 to 150 microinches.

The reason is you need to make sure the nickel is uniform across the board – pcb and that you do not see large swings in plating thickness across the board.

Same holds true for electrolytic nickel which is plated in an anode / cathode nickel bath. Again we want to see 150 microinches of nickel uniformily distributed across the board / pcb. This is a good safe plating margin.

The the normal spec for Nickel thickness is 100 micro-inches minimum (39.4 microinches/micron). Normally the Nickel minimum thickness is done to ensure no Gold migration into the copper. The thinner Nickel also is more nodular and this is not good.

I have not seen any studies that look at aging etc. as a function of minimum Nickel thickness but I would refer to the specs on this. You might want to check with a couple of companies, www.superior-processing.com and www.eftcircuits.com as they have some good information on their websites.