Tin Silver wettability problem after reflow in Copper Pillar Process

Q. Good morning everybody,
We've been running some Copper pillar process for a couple of years in my European company. The stack is made of Copper + Tin-Siver.
I face a big issue at this moment during the reflow process which consists of consolidating the Inter Metallic Compound (IMC).

At the end of the reflow, we notice that the solder has been totally removed from the copper surface. We can find some rest of Tin-Silver out of the Cu pillar on the substrate.
It looks like a very bad wettability of the solder once the liquidus reached.
This bad copper wettability finally leads to the total removal of the Tin Silver solder.

Did anybody already face such concern?
Any help would be welcome.

Regards,
Mike

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Ed. note: Readers who are not familiar with the technology & terminology of the copper pillar process may find http://en.wikipedia.org/wiki/Thermal_copper_pillar_bump interesting.