Solder paste is a combination of a powder made up of metal solder particles and sticky flux that has the consistency of putty. The flux not only does its usual job of cleaning the soldering surfaces of impurities and oxidation, but it also provides a temporary adhesive that holds the surface mount components in place. The solder powder used in the paste can vary in its chemical composition with different material types and percentages used depending on the needs of the board being soldered. For instance, solder paste is available in both lead and lead-free versions to satisfy the restriction of hazardous substances (RoHS) directive. Solder paste is also classified by the size of the metal particles that make up the solder powder.