Hesperian Health Guides


Flux chemicals are used to clean electronic parts during the soldering, brazing, and welding of metal parts. When clean, metal parts stick together much better.

Different fluxes are used for different metals. Rosin and ammonium chloride are used with tin and tin/lead in electronics. Hydrochloric acid and zinc chloride are used with zinc-coated iron. Sodium borate is used with any metal containing iron. Flux chemicals are sometimes dissolved in other chemicals, such as isopropyl alcohol, to make a liquid flux solution.

Lead solder was very common before it was banned by the European Union. Water-soluble fluxes are used with lead-free solders.

Fluxes release dangerous fumes when heated during soldering. Extractors must be close to the soldering source to remove all the fumes.

Many chemicals are used in fluxes. For more on ammonium chloride, see Ammonia and Ammonium Compounds; for hydrochloric acid, see Acids.

The chart includes only some of the flux chemicals that exist. See Learn about chemicals used in your factory and how to find information about other fluxes. See the Index of chemical names to find alternative names for fluxes.

Prevent or reduce exposure:

  • Use extraction ventilation to remove flux fumes as close to the soldering process as possible. Use ventilation systems that extract fumes and replace or dilute dirty air with clean air (see Chapter 17: Ventilation).
  • Enclose operations whenever possible.
  • Avoid manual hand soldering if there is an alternative automated manufacturing process available.
  • Do not mix or pour fluxes by hand.
  • Use gloves when handling fluxes. Wear correct respirators that fit you. All protective clothing should be clean, available each day, put on before work, and never taken home with you (see Chapter 18: Personal protective equipment).
  • Have an emergency plan that includes first aid treatment and protective equipment for spills, splashes, and exposures. Keep necessary emergency supplies at the work site well stocked and accessible to workers.
  • Work areas where fluxes are used, stored, and mixed need to be controlled for heat and monitored for concentration of fumes and vapors. The areas should also have alarms, fire extinguishers, and a fire emergency plan (see Chapter 11: Fire).


Ammonium chloride (ammonium muriate, sal ammonia) CAS No. 12125-02-9

Might harm reproductive health

Hydrochloric acid (HCl, hydrogen chloride, muriatic acid) CAS No. 7647-01-0

Rosin (colophony, gum rosin) CAS No. 8050-09-7

fire or explosive

Sodium tetraborate decahydrate (Borax, sodium borate) CAS No. 1303-96-4

Might harm reproductive health

Zinc chloride (butter of zinc) CAS No. 7646-85-7

Might harm reproductive health

Ammonium chloride is a solid white powder that has no smell. Hydrochloric acid is a colorless liquid with a sharp smell. Rosin is a yellow-orange powder and can have a slight pine smell or no smell at all. Sodium tetraborate decahydrate and zinc chloride are white, sand-like powders with no smell.
Flux chemicals are used in the electronics industry during soldering, welding, and brazing of electronic parts.

They irritate your skin. You may develop a skin rash, redness, dryness, and blisters. Your skin might start peeling, itching, and cracking. When hydrochloric acid touches your skin, it will burn quickly, but the skin will feel cold and numb. Treat it quickly as a chemical burn. Zinc chloride causes burns and ulcers. See First Aid.


They irritate your eyes. Your eyes become watery and red. See First Aid.


They irritate your nose, throat, and lungs, causing coughing, wheezing, shortness of breath, and chest pain. Sodium tetraborate decahydrate causes sore throats and nosebleeds. Zinc chloride can create buildup of fluid in the lungs, called lung edema. See First Aid.


If ingested, they can cause nausea, stomach pain, vomiting, and diarrhea. Swallowing sodium tetraborate decahydrate can cause weakness and convulsions. Zinc chloride burns your digestive tract. See First Aid and seek medical attention.


Ammonium chloride and rosin can make your nose and lungs very sensitive and cause chronic asthma. Even after you stop working with these chemicals, they can give you an asthma attack.

Ammonium chloride may affect your kidneys and may damage a baby inside the womb.

Hydrochloric acid can damage and cause yellowing of the teeth.

Sodium tetraborate decahydrate can damage your liver, kidneys, and nervous system. It may damage a baby inside the womb and may reduce fertility in men and women.

Zinc chloride can scar your lungs and may damage a baby inside the womb.

Use gloves and a face shield when soldering and handling flux chemicals (see Chapter 18: Personal protective equipment).

Use a respirator with a filter. 
Sometimes soldering and the need for flux can be eliminated by using screws and wire to join metal parts. Use no-clean, rosin-free, and water-soluble flux alternatives if possible.