FS Tech will learn the knowledge of PCB flux with you

Overview: Flux is an indispensable substance in the soldering process, but do you know how to choose the right flux and what to pay attention to when using it? Next, FS Tech will work with you to understand comprehensive flux knowledge.

What is PCB Flux

Flux is the chemical cleaning agent that is used in PCBs before and during the soldering process on the circuit board. In the PCB assembly process, flux is used in manual and automatic soldering processes, that is, when the assembly company completes the DIP assembly and is ready to go into wave soldering to fix the solder paste. It is used to clean and remove any impurities and oxide present in the PCB board and make it prepare the metal surface for soldering. Furthermore, flux helps the soldering process by altering the surface tension of the molten solder during the soldering process to prevent reoxidation of the metal surfaces. 

In flux, the base material and the activator are combined to remove oxides from the metal, thereby improving solder wetting. Soldering as well as corrosion prevention are also provided by the mixture’s solvents and additives. Solder joints may not be good because of these oxide deposits on the board. The circuit will have poor electrical conduction or flow if the solder joint is inadequate.

PCB flux is available in various forms including solid, semi-solid, and liquid, and is of three types; Rosin Flux, No-clean Flux, and Water-Soluble Flux.

Selection of Flux for soldering

The types of flux to be used during the soldering are determined using various factors like:

  • PCB types could be single-sided, double sides, or multilayer.
  • Density and types of components
  • Soldering process
  • Solderability of the metals 

Selection of Flux based on Applications

Rosin based Solder

Rosin-based solder is considered the best choice, for removing the oxidation from the wires. The use of corrosive flux for this purpose can be the worst decision as the connecting wires are more fragile and thinner resulting in damaging and shorting of the circuit.

Acid flux when soldering pipes

Soldering with acid flux will remove oxidation from larger areas and generate a much stronger joint. Acid or tinning flux is more corrosive which solves the work to clean the pipes as less corrosive rosin-based flux is not enough to get the job done. 

Leaded solder for electronics

In the case of fragile electronic components, leaded solder is an excellent choice. Soldering is easier due to the low melting point. A solid core without rosin oxidizes more quickly, while others with rosin can help in adding flux coating.

How to use PCB Flux?

PCB flux is available as paste and liquid in the form of small jars/tin cans and jars/bottles respectively. For the thin-electrical applications, organic acid-based flux is used whereas for soft soldering, organic flux is used. There are some proper steps for using the solder flux which is listed as:

  • Soldering flux should be applied to metal contacts after they have been cleaned with a good solvent to remove dust, grime, and excessive oxidation.
  • The metal contact should be cleaned with a good solvent before using the soldering flux to remove the dust, grime as well as excessive oxidation that may be present.
  • Then, the flux should be applied evenly to the surfaces where the actual soldering is slated to happen. It should be heated at this stage.
  • When the tip of the soldering gun is hot and ready, it should be placed over the metal contacts that are covered with flux. As the flux melts and spreads, it will remove the oxide layer from the metal surface, removing the oxide layer.
  • As soon as the flux has been vaporized, the soldering wire should be introduced and melted, ensuring that the bonds are formed before oxidizing again to ensure effective soldering.

How to clean flux?

Flux is acidic and it can corrode the components damaging the PCB over time. Some fluxes require very less or no cleaning after the applications. In order to prevent inhibiting printed wiring, or obscuring other components, flux paste or solder flux residue is usually removed. Some fluxes are water soluble while others require a solvent. Some of the best possible ways to remove the flux from PCB include:

Use of isopropyl alcohol

Using isopropyl alcohol is the best way to clean PCBs. The acid brush can be used by dipping into alcohol and applying gentle pressure for avoiding breaking the solder joints. 

Use of Flux cleaner

Flux cleaner can be used by spraying. After then the brush can be used to clean the flux residue. Surclean is one of the available flux cleaners.

Use of Poly Clens

Poly Clens can also be used for cleaning flux off PCBs. The assembled board should be submerged into the Poly Cleans solution and stir them gently for some time. Then the circuit board should be rinsed with distilled water after removing the flux from the board that should be finally dried with a heat gun. 

Use of Hot Water

Hot water can be used for the water-soluble flux. The process can be helped by a saponifier which itself is alkaline. It makes it easier to wash the flux and get away from hot water.

PCB Flux Handling Precautions

The development of PCBA is an iterative process as quality is the concern. The flux used in PCB is somehow toxic if inhale and swallow. It can be irritating to the respiratory system and can cause headaches and drowsiness. Thus, the flux should be handled and used properly taking suitable precautions. 

Some of the precautious measures while handling PCB flux is given below:

  • Before working with the flux, work areas should be well ventilated
  • When prolonged or repeated contact is required, neoprene gloves are recommended
  • Eye protection against chemicals should be worn

More relevant PCBA knowledge:

How to desolder components from PCB?