Aluminum is quite reactive to most chemicals used for plated thru holes.  For
instance, the sodium hydroxide in conventional electroless copper violently
attacks aluminum.

The trick is to prepare a barrier over the exposed aluminum, to allow final
copper thru hole plating.  One way is treating with zinc chemicals -
zincating - as the first step. From there, a further barrier is plated -
perhaps nickel, similar to  aluminum hard drive disks, which are zincated and
then plated with electroless nickel.

Oversize holes in the aluminum heat sink, filled with epoxy, are sometimes
used. Then the plating chemicals see only a "normal" surface. Of course, some
boards are just bonded to a flat aluminum heat sink.  Can you use a copper
heat sink?

Dennis Fritz
MacDermid, Inc

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