Flux types are classified in several groups. An obsolete standard, the QQ-S-571, was the last to use the rosin-based designations of type R (rosin un-activated), type RMA (rosin mildly activated), and type RA (rosin activated). These flux types are still called out in many customer specifications, so they are made available. The rosin-based flux types produce residues that may be left on the assembly without risk of corrosion due to the encapsulating function of the rosin in the formula. More recently, the term no-clean is used to classify these types of rosin-based fluxes but the typical modern no-clean flux will have a much lighter residue. Water-soluble is a type of flux that usually has a high activity level that can be used for more difficult-to-solder or excessively oxidized metals. The residues from a water-soluble flux must be cleaned; otherwise there is a risk of corrosion, and the cleaning can be done using only water. Inorganic acid fluxes are another type of water-soluble flux but they have a very high activity level to be used in non-electronic metal joining applications. Solid wire contains no flux; it must be used with an external flux.