readily dissolved by molten solder, the plated tip is more durable than a bare copper one, though it will eventually wear out and need replacing. This is especially important when working at the higher temperatures needed for modern lead-free solders. Solid iron and steel tips are seldom used because they store less heat, conduct it poorly, and rusting can break the heating element. Iron-plated tips may feature a layer of nickel between the copper core and the iron surface. A nickel-chrome outer plating may be used further back from the very tip, as solder does not stick well to this material: this avoids solder wetting parts of the tip where it would be unwanted. Some tips have a heater and a thermocouple-based temperature sensor embedded to facilitate a more precise temperature control (TS100 and T12, for instance). Cleaning When the iron tip oxidises and burnt flux accumulates on it, solder no longer wets the tip, impeding heat transfer and making soldering difficult or impossible; tips must be periodically cleaned in use. Such problems happen with all kinds of solder, but are much more severe with the lead-free solders which have become widespread in electronics work, which require higher temperatures than solders containing lead. Exposed iron plating oxidises; if the tip is kept tinned with molten solder oxidation is inhibited. A clean unoxidised tip is tinned by applying a little solder and flux. A wet small sponge, often supplied with soldering equipment, can be used to wipe the tip. For lead-free solder a slightly more aggressive cleaning, with brass shavings, can be used. Soldering flux will help to remove oxide; the more active the flux the better the cleaning, although acidic flux used on circuit boards that is not carefully cleaned off will cause corrosion. A tip which is cleaned but not retinned is susceptible to oxidation. Soldering iron tips are made of a copper core plated various metals including iron. The copper is used for heat transfer and the other platings are for durability. Copper is very easily corroded, eating away the tip, particularly in lead-free work; iron is not. Cleaning tips requires the removal of oxide without damaging the iron plating and exposing the copper to rapid corrosion. The use of solder already containing a small amount of copper can slow corrosion of copper tips. In cases of severe oxidation not removable by gentler methods, abrasion with something hard