What you can do is perforate the board holding rim by drilling holes of 0.6mm (similar to via's). This will reduce the mass and heatsinking capacity, allow excess flux drainage and facilitate convection heat contacting the board through the holes. This will also allow flux vapors (from direct contact with wave) otherwise condensing onto the board, escaping trough the holes. Depending on the flux characteristics this might not totally eliminate the problem, but will definitely reduce the amount of residues. And don't worry solder can not penetrate holes that size.
Good luck Pat
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