Bright tin is deposited from an acidic solution. The brightening system used is typically based on naphthalene compounds. These organic compounds codeposit with the tin. When the deposit is soldered, the temperature exceeds that required to oxidize the organic portion of the deposit and the codeposited brighteners burn. This is seen as a blackening or darkening on the surface of the deposit or solder joint. This charring prevents the formation of a proper solder joint.
To prevent this, the tin deposit must be devoid of codeposited organics. This is done in the plating systems which do not use any organic brightening systems. These processes are generically called “solderable” and plate out as a dull, matte finish, but they are highly solderable.