Dissimilar metals and thin layers

I have a question about dissimilar metals/galvanic corrosion.
I am designing a printed circuit board (PCB), which should be mounted on bare aluminium. For cost reasons, the aluminium part cannot be treated.
The PCB itself contains copper traces


Until now, I have used HAL plating (Hot Air Leveling) on the copper traces, which is a 1 to 5 um tin plating.
This tin plating should be sufficient to form a barrier between the copper and the aluminium.


The manufacturers are moving to ENIG plating (Electroless Nickel Immersion Gold), which consists of a 3 to 6 um nickel layer on the copper with a 0.1 to 0.2 um gold flash on top.


Gold and Aluminium do not behave too well together, but I heard that since the gold is a very thin layer, one can assume it is not there, and therefore in the galvanic series, only the nickel to aluminium voltage will exist.


It is hard for me to find literature/references to this subject, especially in combination with very thin layers. This may just be because it is not my field of expertise. I myself am an electronics design engineer


Could someone enlighten me about this subject?


PS: I was wondering whether to put this in Chemistry or in Physics. In neither I could find related questions.


