The most common solderable coating on leaded components is tin/lead coatings between 5-10um. If the lead is made from some materials there will also be a copper layer between the base material and the tin/lead. In this example the tin/lead plating is so thin that the solder could not wet the leads during testing.

During assembly with palladium plated components the visual appearance of the soldering may look poor due to the non fusible coating but the joints may be perfectly sound.