SHEETMETAL FABRICATION TECHNICAL TIPS FOR SOLDERING Flux 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 metallurgy and metal joining. In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder (e.g.) attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. The role of a flux in joining processes is typically dual: 1. As cleaning agents, fluxes facilitate soldering by removing oxidation from the metals to be joined by dissolving the oxides on the metal surface, which facilitates wetting by molten solder. The flux also acts as an oxygen barrier by coating the hot surface, preventing further oxidation during the soldering process. 2. Additionally, flux allows solder to flow easily on the working piece rather than forming beads as it would otherwise. In some applications molten flux also serves as a heat transfer medium, facilitating heating of the joint by the soldering tool or molten solder. FLUX TYPES There are two types of fluxes used for soft soldering. These are organic (rosin based) fluxes or inorganic fluxes, usually based on halogenides and/or acids. 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 or braze-welding ferrous metals. Rosin Flux Some of the oldest types of flux used is based off pine sap (refined and purified) called rosin. Rosin flux is still used today, mostly in electronics soldering and now typically is a blend of fluxes to optimize the flux, its performance, and characteristics. Ideally flux will flow easily, especially when hot, removes oxides quickly, and helps to remove foreign particles from the surface of the metal SHEETMETAL FABRICATION TECHNICAL TIPS FOR SOLDERING Rosin Flux (continued) being soldered. Rosin flux is acidic when liquid, but when it cools it become solid and inert. Since rosin flux is inert when solid, it can be left on a Printed Circuit Board without harming the circuit unless the circuit will warm to the point where the rosin may become liquid and start eating away at the connection. For this reason it is always a good policy to remove rosin flux reside from a PCB. Also, if a conformal coating will be applied or PCB cosmetics are important, flux residue should be removed. Rosin flux residue can be removed with alcohol. Express Flux #855 for Lead Free Applications Organic Acid Flux One of the more common fluxes used is water soluble organic acid (OA) flux. Common weak acids are used in organic acid flux, such as citric, lactic, and stearic acid among others. The weak organic acids are combined with solvents like isopropyl alcohol and water. Organic acid fluxes are stronger that rosin fluxes and clean the oxides off much quicker. Organic acid fluxes are most frequently used in electronics assemblies. Inorganic Acid Flux A stronger option that organic flux is inorganic flux, which is typically a blend of stronger acids like hydrochloric acid, zinc Express’ #855 Flux does not generate toxic smoke and does chloride, and ammonium chloride. Inorganic acid flux is targeted not contain zinc chloride. It can be use in all types of lead more towards stronger metals such as copper, brass, and free soldering on zinc, copper, tin, tinned steel and stainless steel. Inorganic acid flux requires complete cleaning galvanized steel after use to remove all of the corrosive residue from the surfaces which will weaken or destroy the solder joint if left in place. Inorganic acid flux should not be used for electronic assembly work or electrical work. Solder Fumes The smoke and fumes released while soldering is not great to inhale. It includes several chemical compounds from the acids and their reaction with the oxide layers. Often compounds such as formaldehyde, toluene, alcohols, and acidic fumes are present in the solder fumes. These fumes can lead to asthma and increase sensitivity to solder fumes. Cancer and lead risks from solder fumes are very low since the boiling point for solder is several times hotter than the boiling temperature of the flux and the melting temperature of the solder. The greatest lead risk is the handling of the solder itself. Care should be taken when using solder, with a focus on washing hands and avoiding eating, drinking, and smoking in areas with solder to prevent solder from entering the body. SHEETMETAL FABRICATION TECHNICAL TIPS FOR SOLDERING Express Fluxes Product Item Code Appearance Type Uses Alternate uses Soldering Flux New Copper, Zinc Coated for Clean 840E Inorganic Acid New Zinc, Lead Copper, Lead Coated materials Copper Soldering Flux for Oxidized 845E Inorganic Acid Oxidized Zinc Oxidized Copper materials Soldering Flux Copper, Galvanized Steel, for Natural 847E Inorganic Acid New Zinc Lead, Lead Coated Copper Zinc Soldering Flux for Pre- 850E Inorganic Acid Pre Weathered Zinc Galvanized Steel Weathered Zinc Premium Zinc, Oxidized Copper, Soldering Flux 851E Inorganic Acid New Copper Oxidized Zinc, Lead, Lead for Copper Coated Copper Premium Soldering Flux 853E Inorganic Acid Stainless Steel Terne Coated Stainless Steel for Stainless Steel Premium Galvanized Aluminum Soldering Flux 854E Inorganic Acid Aluminum (Galvalume) for Aluminum Premium Soldering Flux Organic Galvanized Steel, Terne 855E Copper, Zinc, Brass, lead for Lead Free Acid Free Coated Steel & Copper Solders Pre-tinning Galvanized Steel, Terne 852E Copper, Zinc and Tin Solder Paste Coated Stainless Steel For more information go to www.guilbert-express.com .