Rosin Resins for Soldering Solution

Modified Rosin Resins for Soldering Application

In general, liquid soldering fluxes for electronics have the ability to remove the oxidizing materials on the PCB surface. Gum rosin, modified rosin resins are the key raw materials, have 1~45 %Wt. in the formulation of Rosin Fluxes, and have 2~8 %Wt. in the Low-Solids/No-Clean Fluxes. The actives of rosin acid remove the oxides and extending tin material on the PCB surface. Also forming a coating on the solder joints to prevent reoxidation.

But there is some weakness by using gum rosin as the raw material in the soldering flux formulation. Comparing with the modified resin system, the sticky residual substances of gum rosin will dirty the PCB surface, requires more machine maintenance, the residuals also can pose difficulty for downstream processes. By using modified rosin resin system for Rosin Fluxes and  Low-Solids/No-Clean Fluxes can improve the issues.

Soldering Flux Composition
Gum Rosin System Modified Resin System
  • Gum rosin + Isopropyl alcohol
  • Dark colour
  • Sticky residual substances
  • High smoke density
  • Modified Rosin/Resin + Carboxylic acid + trace metal elements
  • Light colour
  • Low residue activity level
  • Low smoke density
  • Rosin actives
Pros & Cons Summary of Rosin-based Fluxes
Rosin Flux
  • Very-good to excellent soldering—wide process window
  • Very good to excellent for high reliability applications
  • Very good to excellent for high heat applications
  • Long process “Life”
  • Requires more preheat
  • Residues must be cleaned—process and floor space expensive
  • Requires more machine maintenance & can pose difficulty for downstream processes, ICT, AOI, etc.
  • Often requires optional fluxer upgrade
  • In some scenarios/applications residues can be reliability concern


Low Solids/No-Clean Flux
  • Good to very-good soldering
  • Less post-soldering residue-may allow for no cleaning†
  • Can allow for lower preheat processes (vs. wave & non water based)
  • Variety of flux choices and can work well on variety of applications
  • Easy on equipment
– Non-rosin, Alcohol based
  • Burn off very cleanly
– VOC-free
  • No volatile solvent; can be more active and have
  • Longer process “Life” vs. Non-Rosin, Alcohol flux
  • Less activity and smaller process window vs. Rosin or Water Soluble fluxes
  • Can pose difficulty and have reduced process window with high heat applications
  • Residues still may need to be cleaned by edict, or flux amount required leaves undesirable or unsafe amount of residues.
  • Water-based (VOC-free) can require more preheat
– Non-rosin, Alcohol based
  • Short process “life”
– VOC-free
  • Requires more preheat, can increase cycle time unnecessarily
  • Can not necessarily as clean as Non-Rosin, Alcohol flux


© Produced by: Eddie Groves – Selective Soldering Academy
Edited by: Jonathan Wol – Pillarhouse USA Inc.


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Halogen Free Formulation

When PCB is stored for longer or deeper oxidation, the activating agent is needed to enhance rosin activity. The alkalinity of ammonium salt can be neutralized with acid. However, halogen compounds such as chlorine and bromine will cause corrosion and lead to a decline in insulation resistance. Halogen compounds are the key controlled pollutants, halogen free will become the industry standard (RoHS). In the halogen-free formulation, small molecular organic acids can satisfy the soldering requirements. Fluxes can be partially sublimated or vaporized at the soldering temperature. And the insulation resistance of the residues is relatively high.

Soldering Paste

Soldering paste is mixed by solder powder, flux, solvent, surfactant, and the thixotropic agent. The thixotropic agent is used for adjusting the adhesion and printing performance of soldering paste, to prevent trailing and sticking. The solvent makes the soldering paste more uniform while stirring, it also affecting the soldering paste’s storage life. The surfactant can remove oxidizing substances from the surface of PCB and parts, and reduce the surface tension of tin and lead. Flux can prevent PCB reoxidation after soldering, and improve the adhesion of solder paste, and paste electronic components.

Rosin free is the new composition of soldering pastes. Comparing with the rosin system products, the rosin free system results new features, such as a) Low solid content level; b) No corrosion on the surface of PCB; c) Soldering substances are easily decomposing, sublimation, or volatilization at soldering temperature; d) No residue after soldering. The features make soldering more easier to meet the requirements for ion cleanliness testing in some special industries.

Rosin free soldering paste is mixed by surfactants (0.5% to 3.0%), forming agents (0.2% to 2.0%), cosolvents (5% to 20%) and solvents (75% to 90%). In some cases, the formulators select fatty acid, aromatic acid, or amino acid to use for the surfactant. Select hydrocarbons, alcohols, or esters to use for forming agents. Select ethers, esters, or terpenes to used for cosolvents. The cosolvent is used for improving the solubility of surfactants and forming agents. Reduce deposition, and increases the wettability of the pastes.

To improve the flux performance to meet the different needs of different uses, opt-inhibitor, foaming agent, brightener or matting agent.


Composition Ingredient
Surfactant (0.5~3.0%)
  • Succinic acid
  • Adipic acid
  • Sebacic acid
  • Fumaric acid
  • Benzoic acid
  • Salicylic acid
  • Glutamate
  • Lysine

Forming agent is the role of the flux solvent is volatilized after the active agent carrying a uniform film on the printed board, the ability to obtain better on the tin to prevent the tin solder splash and uneven.


Composition Ingredient
Forming agents (0.2~2.0%)

The cosolvent is used for improving the solubility of surfactants and forming agents. Reduce deposition, and increases the wettability of the pastes.


Composition Ingredient
 Cosolvents (5.0~20.0%)
Composition Ingredient
Solvent (75.0~90.0%)
  • Ethanol
  • Isopropyl alcohol

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