Hello Hongwei,

I haven't worked on copper plating for quite some time but let me try to
answer some of your questions.  I do not think there would be any problem
in terms of copper plating onto aluminum.  Copper has a standard electrode
potential of 0.34, whereas that value for aluminum is about -1.66.  A more
negative value simply implies that copper is more noble so if you were to
electroplate copper onto an aluminum coated surface, there is no fear of
aluminum getting plated.  A simple example to consider:

case 1) Insert a Zn rod into a CuSO4 bath.  You will see that Zn will plate
out and copper will plate onto the rod simply because Cu has a higher
electrode potential value.

Case 2) Now just reverse the setup.  Insert a Cu rod into a bath of ZnSO4
solution.  You will see NOTHING will happen because Zn solution would like
to remain in diluted form simply because it has a lower standard potential.

You can try to coat Cu onto Al, and I am quite positive you will not
encounter into too many problems pertinent to just plating.  What you might
need to worry about is the adhesion of copper onto al-coated
surfaces.  Adhesion is a major phenomenon in material science and if Al
offers poor adhesion to Cu, your coating will easily peel off or not even
stick at all.  That is of serious consequences so I suggest you try to read
some literatures concerning the adhesion compatibility between Al and
Cu.  Another problem to consider would be unwanted diffusion of Cu ions
into the Al layer.  Copper is not necessarily the most noble metal when it
comes to contamination.  So you might seriously consider a
DIFFUSION-BARRIER layer onto the Al surface before you coat copper.  This
will dramatically solve the contamination problem.  I hope this
helps.  Should you have further question, feel free to drop me an
email.  Good luck,