助焊剂，也称焊剂，有膏状和液态两种，在冶金术上是一种利用化学方法清洁被焊金属表面以便于锡焊、铜焊、或者定位焊接进行的物质。焊接不同的金属所使用的焊剂也有所不同，例如：焊接锡用氯化铵或者松香；焊接镀锌铁以及其他以锌为表面的材料用氢氯酸或者氯化锌；铜焊或者以铜定位焊接黑色金属则用硼砂。

焊接材料在高温下熔融后容易黏附于干净的相应焊接金属表面，但这些金属表面在高温下很容易形成氧化层，使焊接材料难以黏附在表面。焊剂在室温中稳定，在高温下具有很强的还原性，能够清除金属表面的氧化层。此外，焊剂在锡焊与铜焊过程中还能起到润湿剂的作用。

In metallurgy, a flux (derived from Latin fluxus meaning “flow”) is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.

Some of the earliest known fluxes were carbonate of soda, potash, charcoal, coke, borax, lime, lead sulfide and certain minerals containing phosphorus. Iron ore was also used as a flux in the smelting of copper. These agents served various functions, the simplest being a reducing agent which prevented oxides from forming on the surface of the molten metal, while others absorbed impurities into the slag which could be scraped off the molten metal. As cleaning agents, fluxes facilitate soldering, brazing, and welding by removing oxidation from the metals to be joined. Common fluxes are: ammonium chloride or rosin for soldering tin; hydrochloric acid and zinc chloride for soldering galvanized iron (and other zinc surfaces); and borax for brazing, braze-welding ferrous metals, and forge welding.