Types of Solder Flux

Open can of Kolophonium flux resin

 MdE/Wikimedia Common

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

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 and good metal surfaces, free of the oxides, dust, and grime that prevent good bonding. Remove grime and dust by cleaning the surfaces or preventing them with good storage techniques. Oxides, on the other hand, need another approach.

Oxides form on almost all metals when oxygen in the air reacts with the metal. On iron, oxidation is commonly called rust, but oxidation affects tin, aluminum, copper, silver and nearly every metal used in electronics. Oxides make soldering more difficult or even impossible, preventing a metallurgical bond with the solder. Oxidization happens all of the time but it happens much faster at higher temperatures—like 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 as well.

Types of Flux

For many applications, the flux included in the core of the solder wire is sufficient. However, there are several applications where additional flux is extremely beneficial, 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 off 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 (especially when hot), removes oxides quickly, and helps to remove foreign particles from the surface of the metal being soldered. Rosin flux is aciding when liquid, but when it cools it becomes solid and inert. Because rosin flux is inert when solid, it can be left on a PCB 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 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 much 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 will affect 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 all of the corrosive residues 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 you're 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 increase insensitivity to solder fumes. Ensure adequate ventilation and, as necessary, use a respirator.

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 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.