A normal PC board DOES NOT have a nickel plate under the tin/lead. I
suspect you are thinking about PC boards that have been gold plated, and
yes, those do have a nickel underplate.
From: "Dwight Elvey" <dwight.elvey at amd.com>
Normally the PC board will have a thin layer of
nickel over the copper before the solder. This also
produces a blue-ish green oxide.