Solder Flux for Printed Circuit Board Assemblies

By Akber Roy - Chief Technical Officer at RUSH PCB

Flux is Used for Soldering Printed Circuit Boards (PCB)

Needed for good soldering practices in an electronic assembly, preparing a circuit board with flux is critical to the soldering process. Solder flux prepares the copper soldering pads of PCBs and the leads of components so the applied molten solder will bond properly.
The Latin root word for flux is “fluxus,” meaning “flow.” In order to obtain good solder connections, and to prevent further oxidation to exposed copper, solder has to flow over, and attach to, all exposed copper on the PCB assembly.


When melted, solder attaches very well to bare copper, but attaches poorly or not at all to the various oxides of copper. The trouble is that exposed to air, copper tarnishes, or forms oxides on its surface. At room temperatures, this is a slow process. At soldering temperatures, however, the speed of tarnishing increases.
In the soldering process, flux has two functions: it dissolves the oxides on the metal surface, which allows the molten solder to flow over the metal, it blocks oxygen from combining with the metal while the metal is hot for soldering.
Flux is used with both tin-lead and lead-free solders. It is used on bare copper, solder or tin coated copper, or other metal.

Difference between Rosin Flux and Water Soluble Flux

Rosin Flux

Rosin comes from trees and is organic. It has an active ingredient that dissolves tarnish, which is less aggressive than the active ingredients used with water soluble fluxes.
Because rosin doesn’t dissolve in water, to clean off the printed circuit boards and components after soldering, an organic solvent is used. It carries away flux residues, including impurities left over from the soldering process, making a clean electronic assembly. Of course, the solvent needs to be used in a well-ventilated area.
After soldering, with some less aggressive or Mildly Activated Rosin Fluxes, some residue can be left on the printed circuit board and it will not corrode the conductors or components. When some areas of the circuit board are hard to clean off, like under Ball Grid Arrays, not being able to remove all the flux is a critical issue.
Whereas the residues from the Mildly Activated Rosin Fluxes do not have to be removed, because of their appearance, and the difficulty they poses when testing, most manufacturing processes remove the residue anyway.

Water Soluble Flux

With water soluble fluxes, the active ingredients are usually proprietary formulas suspended in non-water based solvents. After soldering the circuit board assemby, water soluble flux is removed from the PCB using water.
Because of the solvents used in the flux, applying and using these fluxes should be in well ventilated areas.
VThe active ingredients in the water soluble flux are more aggressive, as opposed to rosin base fluxes, and can dissolve tarnishes from copper on the pcb assembly more easily.
Because of the aggressiveness of water based flux, to prevent the active ingredients in the flux from attacking the metal surfaces of the printed circuit board or components, within a short time after soldering, all the flux has to be removed.
Of course, because water is used for removing the flux, all the components and the electronic assembly must be sufficiently sealed to withstand immersion in water.

Water Soluble Flux versus Rosin Flux

With higher levels of oxidation on component leads or printed circuit boards, water soluble flux is better than rosin because of its more aggressive active ingredients.

However, on components with open windings or with catch pockets, rosin is still a better alternative because, after cleaning, any left-over flux residue on the electronic assembly will be less harmful to the components or the printed circuit board (PCB).

Liquid or Paste Solder Flux

Solder Flux comes in two forms: Liquid and Paste

Liquid Flux

When the whole PCB assembly is going to be soldered at once, like when it is wave soldered, liquid flux is used. Before soldering, the printed circuit board can be dipped in the flux, foamed with the flux, or sprayed with the flux.
Liquid flux is usually water soluble.

Paste Flux

In the reflow process, where only one side of the PCB is soldered, paste flux is used.
Here, powdered solder is mixed with the paste flux. The mixture is applied to the PCB through a stencil, and only to the copper solder pads of the PCB. The solder/flux mixture becomes the reflow solder used to assemble the PCB.
When installing hand applied components, such as large scale connectors or Ball Grid Arrays after most of the PCB is assembled, or when reworking the electronic assembly, paste flux is applied judiciously and only where needed.

Flux Doesn’t Solve All Problems

Flux isn’t the answer to all soldering problems; it only affects the surface of the metal.
The trouble is that oxidation is a time-cumulative process. As time progresses, the tarnish caused by oxidation accumulates and affects the metal of the component leads or the PCB Assembly deeper into the surface. The tarnish starts on the first day after manufacture, but it sometimes takes months or years to become visible.
If the oxidation has occurred over a long time and has become deep seated in the metal, the Flux can’t dissolve the tarnish and the solder won’t bond with the metal.
Corrosion, like tarnish, is a problem and is also time cumulative.
When exposed to chemicals in the air, all metals corrode. Depending on what chemicals are in the air, problems with corrosion can take many years, or just a few weeks. Again, the Flux can’t dissolve the corrosion and the solder won’t bond with corroded metal.

Solder Flux

Solder flux, when properly used in an electronic assembly, helps obtain clean and shiny looking solder connections to be used in a full turnkey assembly.

References: "
Sparkfun – Liquid Flux Pen – Water Soluble "
AIM- Global Manufacturer of Solder Assembly Materials for the Electronics Industry"
circuitnet - We search for industry news, so you don't need to.|pcrid|41566056141|plid|&CMP=KNC-GPLA"

Contact RUSH PCB

For further information on reducing voids in solder connections, contact the engineers at RUSH PCB .