Types of Solder Flux

Rosin for soldering
MdE/Wikimedia Commons/CC BY-SA 3.0 DE

Soldering components is an essential element of electronics. Solder does not always bond well to components which can result in a bad solder joint, bridged pins, or even no joint at all. The solution to solder bonding issues is the use of a flux agent and the right temperature.

What is Flux?

When solder melts and forms a joint between two metal surfaces, it actually 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 oxides, dust, and grime that prevent good bonding. Grime and dust can easily be removed by cleaning or prevented with good storage techniques. Oxides, on the other hand need another approach.

Oxides form on almost all metals when oxygen reacts with the metal. On iron, oxidation is commonly called rust, but it happens to tin, aluminum, copper, silver, and nearly every metal used in electronics. Oxides make soldering much more difficult or even impossible, preventing a metallurgical bond with the solder. Oxidization happens all of the time, but 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 soldering which prevents additional oxides from forming due to the high heat of soldering. As with solder, there are several types of solder, 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 used is based off pine sap (refined and purified) called rosin. Rosin flux is still used today, but 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 being soldered. Rosin flux is aciding when liquid, but when it cools it become solid and inert. Since 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. Rosin flux can be removed with alcohol.

Organic Acid Flux

One of the more common fuxes 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. 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!). Cleaning organic acid flux is required since the residue is electrically conductive and will greatly impact the operation and performance of a circuit, if not lead to damage if the circuit is operate before the flux residue is cleaned off.

Inorganic Acid Flux

A stronger option that organic flux is inorganic flux, which is typically a blend of stronger acids like hydrocloric acid, zinc chloride, and ammonium chloride. Inorganic acid flux is targeted more towards stronger metals such as copper, brass, and stainless steel. Inorganic acid flux requires complete cleaning 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 insensitivity 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 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.