Solder Flux (Rosin Flux, Water Soluble Flux, No-Clean Flux)
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Explanation and review of solder flux for soldering electronic components onto the PCB (Printed Circuit Board). I try to present complete information on solder flux including – types of solder flux, liquid solder flux, uses and application of solder flux in electronics and pcb assembly and rework.
What is solder flux?
Solder flux or simply flux is a kind of chemical used by electronic companies in electronics industry to clean surfaces of PCB before soldering electronic components onto the board. The main function of using flux in any PCB 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 deoxidize 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
The electronics industry standards, J-STD-004, requirements for soldering fluxes, classify solder fluxes into 3 major categories based on their flux activity:
1. Rosin Flux 2. Water Soluble Flux 3. No-clean Flux Let us now discuss these three types of flux in detail.
Rosin Flux
Rosin flux is primarily composed of natural resin extracted from the oleoresin of pine trees and refined. The the rosin category there are three subcategories: Rosin (R) Flux, Rosin Mildly Activated (RMA) Flux, and Rosin Activated (RA) Flux.
RA Flux or RA Flux Paste are rarely used because of their high activity level. RMA Flux and RMA Flux Paste 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 (R) Flux has the least activity. These fluxes with ronin need to be cleaned from the printed circuit assembly after soldering. Since solvents containing chlorinated fluorocarbon compounds (CFCs) have been determined to cause depletion of the ozone layer, alternative solvents are now available to clean rosin fluxes. (Check: Solvent Cleaning in SMT)
Water Soluble Flux
Water soluble fluxes are also known as organic acid (OA) fluxes. Water-soluble flux is primarily composed of organic materials other than rosin or resin. Most water solubles are formulated with glycol bases. Glycols tend to be the polar solvents, and the polyethylenes and polypropylenes, with heavy molecular weights, can become insoluble. The result is that they tend to bond to the board and become difficult to remove from low clearance areas under components. However, water-soluble fluxes can be cleaned and are widely used.
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.
Read More
Surface Mount Technology (SMT): Tutorial
Circuit Board: Design, Diagram and Assembly
SMT Equipments
Soldering Guide – How to Solder
BGA (Ball Grid Array): Repairing and Soldering BGA
SMD Soldering - An Illustrated Guide
Solder: Tin-Lead (Sn/Pb) and Lead-Free (Pb-Free) Solder
ROHS Lead-Free (Pb-Free) – Taking the Lead Out
No-Clean Flux
No-clean flux is primarily composed of natural resins other than rosin types, and/or synthetic resins. Typically RMAs have a resin content of 55-65% and no-cleans have a resin content of 35%-45%. So the no cleans have less body than RMAs and depend more on binders and other materials for their rheological characteristics.
No-clean flux is formulated with various levels of solids. Some of the no-clean fluxes have high levels of residue. Even if they pass electrical tests, they may leave excessive residue on the board and may be unacceptable cosmetically. Such fluxes provide good solderability and do not require the use of inert environments for reflow. There are also no-clean fluxes that will not leave any visible residue. No-clean flux and paste save not only cleaning costs but also capital expenditure and floor space. However, no-clean flux requires a very clean assembly environment and good board and component solderability. In addition, they may also require use of inert environment during reflow.Conclusion
No matter which category of flux is used, it needs to provide good solderability, meet board cleanliness requirements, and be cost effextive.
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