The addition of which substance during soldering can remove oxide films but leave a residue?

The correct choice is flux. During the soldering process, flux is a crucial additive that helps clean the metal surfaces by removing oxidation and other impurities, promoting better adhesion of solder. Flux works by lowering the surface tension of the liquid solder and making it flow more easily across the surfaces being joined. While it effectively removes oxide films that can prevent proper soldering, it does tend to leave a residue, which can be either acidic or non-acidic based on the type of flux used.

In contrast, resin is often a component of a specific kind of flux, particularly in rosin-core solder, but its primary purpose is to aid in the soldering process and not to remove oxides specifically. Solder itself is the metal used to create a joint between two materials and does not have cleaning properties. Tinning refers to the process of applying a layer of solder to a component or surface before making a joint, which lays the groundwork for proper soldering but does not directly remove oxide films in the way flux does.

Thus, flux is the substance that specifically addresses the need to clean surfaces during soldering, highlighting its important role in achieving effective and reliable solder joints while leaving behind some residue.