Types of Solder Flux

Flux improves the bond between soldered components

Solder does not always bond well to components, resulting in a bad solder joint, bridged pins, or no joint at all. Use a flux agent and the right temperature to overcome these problems.

Close up of hands soldering on computer component
Dmitriy Muravev / Getty Images

What Is Flux?

When solder melts and forms a joint between two metal surfaces, it forms a metallurgical bond by chemically reacting with the other metal surfaces. A good bond requires two things:

  • A solder that is metallurgically compatible with the metals being bonded.
  • Good metal surfaces free of the oxides, dust, and grime that prevent good bonding.

Remove grime and dust by cleaning the surfaces or preventing these with good storage techniques. Oxides, on the other hand, need another approach.

Oxides and Flux

Oxides form on almost all metals when oxygen in the air reacts with the metal. On iron, oxidation is commonly called rust. However, oxidation affects tin, aluminum, copper, silver, and nearly every metal used in electronics. Oxides make soldering more difficult or impossible, preventing a metallurgical bond with the solder. Oxidization happens all of the time. However, it happens faster at higher temperatures—as when soldering flux cleans metal surfaces and reacts with the oxide layer, leaving a surface primed for a good solder bond.

Flux remains on the surface of the metal while you're soldering, which prevents additional oxides from forming due to the high heat of the soldering process. As with solder, there are several types of flux, each with key uses and some limitations.

Types of Flux

For many applications, the flux included in the core of the solder wire is sufficient. However, additional flux is beneficial in some scenarios, such as surface-mount soldering and desoldering. In all cases, the best flux to use is the least acidic (least aggressive) flux that will work on the oxide on the components and result in a good solder bond.

Rosin Flux

Some of the oldest types of flux are based on pine sap—refined and purified—called rosin. Rosin flux is still used today, but modern rosin flux blends different fluxes to optimize its performance.

Ideally, flux flows easily when hot, removes oxides quickly, and helps remove foreign particles from the surface of the metal being soldered. Rosin flux is acid when liquid. When it cools, it becomes solid and inert. Because rosin flux is inert when solid, it can be left on a printed circuit board without harming the circuit unless the circuit warms to the point where the rosin may become liquid and eat away at the connection.

It's a good policy to remove rosin flux residue from a PCB. Also, if you intend to apply a conformal coating or if PCB cosmetics are important, flux residue should be removed with alcohol.

Organic Acid Flux

One of the more common fluxes is water-soluble organic acid flux. Common weak acids are used in organic acid flux, including citric, lactic, and stearic acids. The weak organic acids are combined with solvents like isopropyl alcohol and water.

Organic acid fluxes are stronger than rosin fluxes and clean the oxides off more quickly. Additionally, the water-soluble nature of the organic acid flux allows the PCB to be easily cleaned with regular water—just protect components that should not get wet. Because the OA residue is electrically conductive and affects the operation and performance of a circuit, remove the flux residue when you're done soldering.

Inorganic Acid Flux

Inorganic acid flux works better with stronger metals such as copper, brass, and stainless steel. It's a blend of stronger acids like hydrochloric acid, zinc chloride, and ammonium chloride. Inorganic acid flux requires complete cleaning after use to remove the corrosive residues from the surfaces, which 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 include several chemical compounds from the acids and their reaction with the oxide layers. Other compounds such as formaldehyde, toluene, alcohols, and acidic fumes are often present in the solder fumes. These fumes can lead to asthma and increased sensitivity to solder fumes. Ensure adequate ventilation and, as necessary, use a respirator.

Cancer and lead risks from solder fumes are low since the boiling point for solder is several times hotter than the boiling temperature of the flux and melting temperature of the solder. The greatest lead risk is the handling of the solder. 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 particulate from entering the body.