Solder Alloys and Metallurgy

But the IMCs formed in soldering are strong enough, and their formation is at the root of why every solder alloy uses tin. Concretely, when moving from soldering with lead-based to lead-free solder, there are two big changes:

As this PDF by Kester indicates, the tin in solder joins not only to the solder joint, but also dissolves the copper and iron in the tip itself, which is why tips eventually wear out. A tip that is designed for 63/37 lead-based solder will consequently not last as long when used with lead-free solder such as Sn99.3Cu0.7 or Sn95.5Ag3.8Cu0.7 because the iron plating will dissolve into the tin significantly faster.

Source: hackaday.com