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Solder Flux Basics

Solder flux is a kind of chemical used in PCB soldering and assembly. It is basically used to clean surface of PCB before soldering process. The main function of flux in any printed circuit board assembly or rework is to clean and remove any oxide from the board. These oxides deposited on the board may not permit good solder joints. If the solder joint is poor, there will be poor electrical conduction or poor flow of electricity through the circuit.

Solder flux helps to de-oxidize metals (copper tracks on the PCB and leads of electronic components) and helps better soldering and wetting. The activators in the flux promote wetting of the molten solder to the surface mount lands and component terminations or leads by removing oxides and other surface contaminants.

Types of Solder Flux

According to the electronics industry standards, J-STD-004, solder fluxes are classified into 3 major categories based on their flux activity:

  1. Rosin Flux: Rosin flux is primarily composed of natural resin extracted from the oleoresin of pine trees and refined. The rosin category is further divided three subcategories:Rosin (R) Flux: It has only rosin and is the least active. This type of flux is mostly used for surfaces that are clean. It leaves virtually no residue after soldering.Rosin Mildly Activated (RMA) Flux: It has sufficient activator to clean the solder-coated or plated lands and component terminations or leads, thereby enabling the molten solder to wet these areas.

    Rosin Activated (RA) Flux: Type RA is most active of the rosin fluxes, and leaves the most residues after soldering.

  2. Water Soluble Flux: Water soluble flux is also known as organic acid (OA) flux. It is is primarily composed of organic materials other than rosin or resin. Water-soluble flux provides good soldering results because they have good flux activity. They have good wetting action (ability to remove oxides and prepare a clean surface for soldering). However, they may be too aggressive, calling for extra precaution during cleaning of the assemblies to avoid flux contamination.
  3. No-clean Flux: These are flux that doesn’t need any cleaning after soldering process.


Choosing Flux for Soldering

Solder Flux

Solder Flux

Several factors determine the type of solder flux to be used during soldering:

  • Type of circuit board (single-sided, double-sided, multilayer)
  • Density and type of electronic components to be soldered.
  • Soldering process (hand soldering, wave soldering, SMT).
  • Solderability of metals to be joined.

Soldering Flux Cleaning

After the soldering process is over, it is important to clean the board to remove any unwanted residue that can affect the performance of the board and can even cause electrical short circuit. No-clean flux doesn’t need any cleaning. In case of fluxes that need cleaning after soldering, solvent cleaning or aqueous cleaners can be used. Most soldering flux manufacturers and vendors also supply flux cleaners.


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