Finckelstein reaction:
$\ce{RX + X' <=> RX' + X-}$
For Fluoride preparation:
treatment of alkyl halides with:
1) Anhydrous HF (for allylic and benzylic only)
2) $\ce{AgF}$
3) $\ce{KF}$
4) $\ce{HgF2}$
5) $\ce{Et_3N.2HF}$
-> Equilbrium shifted to right because of $\ce{F}$'s poor leaving tendency