If your SAC alloy doesn't 'behave' as expected, there are several things to do:
- make sure your materials are clean in good shape, that is no excessive oxidization is present (dull copper)
- use more flux, maybe even a more aggressive flux
- increase the temperature. once you get 'instant charcoal' it is too high.
You'll have to do some testing. Personally I hate 'lead free' solder, and it hates me as well.