Guide to Using Soldering Flux as a Process Material

An earlier article introduced solder paste adhesive as a process material. This article discusses another important process material: soldering flux. It covers the functions, classification, testing, selection, and common defects associated with soldering flux.

The soft-soldering industry involves a broad range of disciplines, including metallurgy, physics, and chemistry. Common chemicals used in the process include soldering flux, cleaning agents, anti-oxidation agents, and tin dross reducers. Among these materials, soldering flux is one of the most important and critical.

1. Overview of Soldering Flux

In the early stages of the industry, liquid flux development was primarily driven by the need for post-soldering cleaning. Early rosin-based fluxes contained a high percentage of rosin, typically 20%–40%, and commonly used organic halide compounds as activators. These fluxes left large amounts of sticky residue and could cause corrosion. As a result, cleaning was required after soldering, and the cleaning agents used were environmentally harmful chlorofluorocarbons (CFCs).

Alternative cleaning agents were later developed, including hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). HCFCs still contain chlorine and may damage the ozone layer, while HFCs are greenhouse gases. Because of the environmental problems associated with solvent-based cleaning, water-soluble fluxes were developed. However, these fluxes created wastewater-treatment challenges, which eventually led to the concept of no-clean flux. No-clean flux has now become one of the most widely used flux types.

2. Standards Related to Soldering Flux

Many national and international standards apply to soldering flux. The following are commonly referenced standards.

Chinese Standards

• GB/T 9491-2021, Soldering Flux for Tin Soldering: Specifies the classification, technical requirements, test methods, inspection rules, labeling, packaging, transportation, and storage requirements for soldering flux used with electronic products.
• GB/T 31474-2015, Soldering Flux for High-Quality Internal Interconnections in Electronic Assemblies: Specifies the technical requirements, test methods, inspection rules, labeling, packaging, transportation, and storage requirements for flux used in high-quality internal interconnections.
• SJ/T 11273-2016, No-Clean Liquid Soldering Flux: Specifies the technical requirements and test methods for no-clean liquid flux.
• SJ/T 11389-2019, Soldering Flux for Lead-Free Soldering: Specifies the technical requirements, test methods, inspection rules, labeling, packaging, transportation, and storage requirements for flux used in lead-free soldering.
• SJ/T 2660-2021, Test Methods for Soldering Flux for Tin Soldering: Specifies test methods for soldering flux and provides a practical basis for flux quality inspection.

International Standards

• IPC/J-STD-004C, Requirements for Soldering Fluxes: Specifies requirements for flux composition, activity level, halide content, and other properties. Based on the primary composition of the flux, it divides fluxes into rosin, resin, organic, and inorganic categories. These categories are further classified according to copper-mirror test results and halide content.
• ISO 9455 series: Includes ISO 9455-1 for determining nonvolatile matter by gravimetric methods, ISO 9455-5 for the copper-mirror test, ISO 9455-9 for determining ammonia content, ISO 9455-11 for the solubility of flux residues, and ISO 9455-13 for determining flux spattering, among other test methods.
• IEC 61190-1-1, Attachment Materials for Electronic Assembly — Part 1-1: Specifies requirements for flux used in high-quality interconnections in electronic assemblies, including flux performance and test methods.
• ASTM D-465-15: An ASTM standard covering selected performance tests and requirements for soldering flux.

3. Functions of Soldering Flux

Soldering flux plays a critical role during soldering. Before soldering, metal surfaces may be covered by oxides and contaminants. These defects can prevent the surfaces from being properly soldered and from forming a strong bond. Flux acts as a cleaning agent, dissolving oxides and promoting wetting, which is the ability of molten solder to spread uniformly across a surface.

Removing Oxides

Metal surfaces readily form an oxide layer during soldering. This layer prevents the solder from contacting and bonding with the metal surface. The primary function of flux is to remove the oxide film through reactions between its active ingredients, such as organic acids, and the oxides.

The activity level must be selected carefully. Insufficient activity causes poor wetting, while excessive activity may leave corrosive residues that threaten long-term reliability. The flux formulation therefore needs to balance chemical activity and post-soldering safety.

Reducing Surface Tension

Flux reduces the surface tension of molten solder, allowing the solder to spread more easily across the surfaces being joined and helping form a sound solder joint. During soldering, surface tension at the interface between molten solder and a solid pad can reduce wetting and limit solder spreading.

A small amount of surfactant in the flux can effectively reduce interfacial tension. Surfactant molecules have an asymmetric structure, with one end attracted to water and the other attracted to oil. This substantially improves solder flow and wetting. If surfactant activity is insufficient, or if the surfactant decomposes and becomes ineffective during production, defects such as solder balls, cold or insufficient solder joints, and solder spikes may occur. Although the amount of surfactant is very small, it has a significant effect on soldering quality and reliability.

Preventing Reoxidation

At high temperatures, metals are more easily oxidized. In its molten state, flux covers the metal surface and isolates it from oxygen in the air, preventing further oxidation during soldering.

Supporting Heat Transfer

Flux helps transfer heat from the soldering equipment to the area being soldered. High soldering temperatures can create thermal shock in components. Although component materials are designed with thermal margins, process variation, such as an excessively fast heating rate, may still cause long-term material creep that is difficult to detect and potentially harmful to precision products.

Solvents and additives in the flux can absorb and carry away heat, slow abrupt temperature changes, and make heating more uniform. In this way, flux provides a buffering effect against thermal shock and assists heat transfer.

Promoting Molten-Solder Spreading

After preheating, viscous flux contacts the solder and becomes more active. Its viscosity decreases rapidly, allowing it to spread across the metal surface being soldered. This secondary spreading action adds a driving force to the molten solder and helps pull the liquid metal across the joint area.

Removing Light Oil and Surface Contamination

Flux can also help remove light contamination and oil from the surfaces being soldered, providing a cleaner surface for soldering.

The two most important functions of flux are removing oxides and reducing the surface tension of the materials being joined. In practical terms, flux uses its active ingredients to remove surface oxides while reducing the surface tension between the solder and the workpiece. This improves solder flow and wetting and enables the solder joint to form.

4. Classification of Soldering Flux

The internationally used flux classification system is specified in J-STD-004. First, fluxes are divided into four major categories according to the primary chemical composition of their nonvolatile matter, or solids content: rosin (RO), resin (RE), organic (OR), and inorganic (IN).

Fluxes are further classified according to the corrosivity or conductivity of the flux or its residues:

• Low activity (L): Low corrosivity; suitable for high-density, fine-pitch components such as circuit-board packages.
• Moderate activity (M): Balances activity and corrosivity and is suitable for soldering most electronic components.
• High activity (H): Provides strong chemical activity and is suitable for heavily oxidized metals such as stainless steel. Post-soldering cleaning is required to avoid corrosion.

According to GB/T 9491-2021, the quantitative halogen content is expressed as a mass fraction relative to the flux sample.

In flux type designations, 0 and 1 indicate whether the flux contains halides. A halide content below 0.05% is treated as halide-free. The L, M, H, 0, and 1 classifications must be determined using the corresponding test methods. A flux can be assigned to a category only when it satisfies all test requirements for that category.

Rosin-Based Flux

Rosin-based flux is a traditional type that contains a relatively high amount of rosin or resin. Its solids content is typically 15%–20% or higher. Conventional rosin flux generally contains a small amount of halide, providing strong solderability across a wide range of board materials. After soldering, a very thin protective film remains on the soldered surface, helping protect the joint from oxidation and moisture.

Low-Solids No-Clean Flux

This flux evolved directly from rosin-based flux. It contains rosin or resin, but in a lower concentration, generally with a solids content of 8%–10% or less. Most formulations contain a small amount of halide, although some are halide-free. The halide content is generally controlled below 0.2%.

Its soldering performance can approach that of conventional rosin flux, while the board surface remains relatively clean after soldering. The residue may be cleaned or left in place, depending on the process. Compared with low-residue no-clean flux, this type generally provides better solderability and reliability. Compared with conventional rosin flux, its performance is somewhat lower, but its post-soldering reliability can generally meet product requirements.

Low-Residue No-Clean Flux

Low-residue no-clean flux began to be adopted more widely in China in the mid-1990s. After 2000, more users began applying various no-clean fluxes and placing stricter requirements on them. Requirements progressed from a colorless and transparent appearance to more demanding requirements for solderability, surface residue, and ionic residue.

Current low-residue no-clean fluxes can be divided into rosin- or resin-containing and rosin- or resin-free types. Both types can have a solids content of approximately 2% or less.

Other Flux Types

Tinning Flux

Tinning flux is used to tin wires, transformers, coils, and component leads. It can be divided into no-clean and rosin-based types. Most manufacturers currently use no-clean fluxes. Typical requirements include no post-soldering residue, no lead corrosion, bright solder fillets, rapid tinning, good solder rise, and a smooth lead surface without localized pits or projections.

PCB Precoat Flux

After PCB manufacturing is completed, a coating may be applied to prevent oxidation of the exposed pads. The flux used for this purpose is called PCB precoat flux. Roller coating is commonly used. The flux must be distributed uniformly across the board, and the finished surface should be smooth and transparent without obvious rosin or resin coating marks.

Hot-Air-Leveling Flux

This type of flux is designed for the hot-air solder leveling process used to manufacture double-sided and multilayer PCBs. It improves molten-solder flow, promotes rapid and uniform tinning, and produces a thin coating that does not block through-holes. It also helps reduce pinholes and produce a dense solder layer.

Most hot-air-leveling fluxes can be removed with water after soldering. They may be applied by brushing, spraying, foaming, or roller coating.

Water-Washable Flux

In response to environmental requirements, water began to be considered as a cleaning medium. To support water cleaning, flux manufacturers modified their formulations so that post-soldering residues would be water-soluble and readily removable. Only residues with these characteristics can be effectively removed with water after soldering.

Water-Based Flux

In recent years, fluxes using water as the primary solvent have been developed. The main challenge is no longer flux formulation but compatibility with the user's process and equipment. Many existing systems, especially wave-soldering equipment, cannot provide the preheating conditions required by water-based flux.

If preheating is insufficient, water-based flux may remain on the soldering surface during soldering and cause large quantities of solder balls. If the actual soldering surface reaches more than 110 °C for 5 seconds, or approximately 130 °C for 3 seconds, the water in the flux can evaporate sufficiently. A subsequent drying zone of approximately 2–5 seconds should then allow soldering to proceed properly.

Flux for Lead-Free Soldering

In February 2003, the European Union issued two directives. The RoHS directive specified restrictions on hazardous substances in electronic information products, with implementation beginning July 1, 2006. The restricted substances included lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB), and polybrominated diphenyl ethers (PBDE). Lead is the substance most directly related to soldering.

Flux selection became a primary concern in the transition to lead-free soldering. Flux for lead-free soldering is designed specifically for lead-free processes. It generally has a medium solids content of approximately 10%–15%, good activator stability at high temperature, and sufficient wetting performance for lead-free soldering.

Flux for Solder Wire

Solder-wire flux is primarily rosin- or resin-based. It has a major effect on soldering speed and solderability. The following factors should be considered when developing flux for solder wire:

1. The activator must be selected appropriately.
2. The activator concentration must be properly controlled.
3. The surfactant must be selected appropriately.
4. The residue must be easy to clean.
5. When developing no-clean solder-wire flux, both low residue and post-soldering reliability must be ensured.

Flux for SMT Solder Paste

SMT solder paste consists primarily of alloy powder and flux, sometimes called soldering oil. Its major difference from wave-soldering flux is that solder paste flux contains a thixotropic agent, while ordinary liquid flux generally does not.

The main components of SMT solder-paste flux include the following:

• Activators: Remove oxide films from copper pads and component termination surfaces. Organic activators are commonly used and generally provide relatively mild activity to ensure electrical performance and reliability after soldering. Stronger activators are usually added only in very small quantities, while inorganic activators are generally avoided.
• Surfactants: High-efficiency surfactants are commonly used. Although the addition level is very low, they substantially reduce the surface tension between solder and the pad or component lead.
• Thixotropic agents: Adjust solder-paste viscosity and printing behavior and help prevent tailing and sticking during printing. This component is critical because many solder-paste problems are related to thixotropic behavior. Manufacturers continue to develop more effective thixotropic agents.
• Resin: Increases paste adhesion and protects the surface from reoxidation after soldering. It also plays an important role in holding components in position during placement.
• Solvents: Dissolve the other flux components and allow them to mix uniformly during production. During solder-paste mixing, solvents also help maintain uniform mixing between the metal powder and flux. Solvent selection affects the usable life of the paste during printing.

Special-Purpose Flux

Special-purpose flux is used primarily for soldering stainless steel, iron alloys, nickel alloys, and other difficult-to-solder materials. Its activators are generally strongly corrosive inorganic acids or inorganic acid salts, sometimes combined with small amounts of soluble tin-lead compounds.

The strong activity ensures tinning and solderability, but post-soldering corrosion cannot be avoided completely. To ensure component reliability, the assembly must be cleaned promptly after soldering. Most of these fluxes are compatible with water-washing processes, avoiding the higher cost associated with organic-solvent cleaning.

5. Flux Classification and Testing

Flux technical indicators can be divided into three groups:

1. Basic physical properties: Appearance, color, density, and solids content.
2. Soldering performance: Acid value and wetting capability.
3. Corrosion and electrical safety: Water-extract conductivity, halide content, copper-mirror corrosion, and surface insulation resistance.

The meanings, test methods, and requirements for these indicators are detailed in J-STD-004.

1. Copper-Mirror Test

Flux corrosivity should be determined according to IPC-TM-650 test method 2.3.32. A flux may be classified as L type only when no part of the copper film has been completely removed. If any copper film is removed and the background can be seen through the glass, the flux must not be classified as L type.

If only the copper film around the flux is completely removed, with penetration below 50%, the flux is classified as M type. If the copper film is completely removed with penetration above 50%, the flux is classified as H type.

2. Copper-Panel Corrosion Test

The corrosivity of flux residues should be determined according to IPC-TM-650 test method 2.6.15. For this test, corrosion is defined as a chemical reaction between copper, solder, and flux residue after soldering and exposure to the environmental conditions specified by the test method.

1. No corrosion: No visible sign of corrosion. Heating the test panel during soldering may intensify an initially developing color change; this condition may be disregarded.
2. Minor corrosion: Discrete white or colored spots in the flux residue, or a blue-green color change without surface depressions in the copper, is considered minor corrosion.
3. Severe corrosion: When blue-green stains or corrosion expand and depressions become visible in the copper panel, the condition is considered severe corrosion.

3. Quantitative Halide-Content Test

The concentration of chloride (Cl?), bromide (Br?), fluoride (F?), and iodide (I?) in liquid flux or extracted flux solution should be determined using a quantitative halide test. Total halide content is the sum of the measured Cl?, Br?, F?, and I? concentrations.

Flux solids, or nonvolatile matter, should be determined according to IPC-TM-650 test method 2.3.34 or an agreement between the supplier and user. For fluxes with a solids content below 10%, the deviation between the measured solids content and the supplier's nominal value should not exceed 10%. For all other fluxes, the deviation should not exceed 5%.

4. Surface Insulation Resistance Test

Except that the test duration should be 7 days, the surface insulation resistance (SIR) requirements should be determined according to IPC-TM-650 test method 2.6.3.7. The SIR test pattern should be prepared according to IPC-TM-650 test method 2.6.3.3 using the reflow or wave-soldering profile for the specific product.

When reporting SIR results, the supplier should clearly indicate whether cleaning is required before the SIR test and identify the cleaning process used.

The criteria for passing the SIR test are as follows:

1. All SIR measurements on the test pattern should be greater than 100 MΩ.
2. Electrochemical migration, including dendrite growth, should not reduce the conductor spacing by more than 20%.
3. There should be no conductor corrosion. Slight discoloration of one electrode of the comb pattern is acceptable.

5. Electrochemical Migration Test

Flux resistance to electrochemical migration should be evaluated according to IPC-TM-650 test method 2.6.14.1. The test temperature is 65 °C ± 2 °C, and the relative humidity is 88.5% ± 3.5% RH. The ECM test pattern should be prepared according to IPC-TM-650 test method 2.6.3.3 using the reflow or wave-soldering profile for the specific product.

When reporting ECM results, the supplier should clearly indicate whether cleaning is required before the ECM test and identify the cleaning process used.

The initial insulation resistance value, measured after the 96-hour stabilization period, and the final insulation resistance value should be reported. The criteria for passing the ECM test are as follows:

1. The final insulation resistance should be at least one-tenth of the initial insulation resistance: final IR ≥ initial IR/10.
2. Electrochemical migration, including dendrite growth, should not reduce the conductor spacing by more than 20%.
3. There should be no conductor corrosion. Slight discoloration of one electrode of the comb pattern is acceptable.

6. Selecting Soldering Flux

Flux selection should begin with a clear understanding of the actual process requirements. Users should evaluate and select flux based on actual production use.

For electrical safety, supplier-provided SIR data is only a reference. The user should conduct aging tests on soldered PCBs according to the company's own requirements, followed by electrical testing appropriate to the product. After the electrical-safety requirements have been satisfied, flux selection should focus primarily on solder-joint quality.

Flux evaluation generally considers the following factors:

1. Flux application method.
2. No-clean, solvent-cleaning, or water-cleaning process.
3. Number of reflow cycles before wave soldering.
4. Lead-free or leaded process.
5. Compatibility with the PCB solder mask and conformal coating.
6. Solder-joint quality and soldering yield.
7. Visual quality of flux residues.

Understanding the flux itself is equally important. Its three key attributes are activity, solids content, and material type. These properties determine the basic flux category and its process characteristics.

Low-Solids No-Clean Flux: 2%–8% Solids

• Alcohol-based, with or without rosin or resin.
• Water-based, generally without rosin or resin because rosin does not dissolve in water.
• Low to moderate activity.
• Short process life, meaning the active effect lasts for a relatively short time during processing.
• Cleaning may or may not be required, depending on the formulation and process.

Rosin Flux

• High-rosin formulations contain approximately 35%–45% rosin; medium-rosin formulations contain approximately 15%–20% rosin.
• Solvent-based.
• May have low activity, but is generally moderate to highly active.
• Short process life.
• Generally requires cleaning.

Water-Soluble Flux

• Usually has a high solids content and contains approximately 11%–13% rosin.
• Generally solvent-based, although some formulations are water-based.
• Usually highly active.
• Short process life.
• Must be cleaned after soldering.

7. Effects of Flux on Soldering and Common Defects

Flux affects many aspects of electronic assembly. The main defects associated with flux include the following.

1. Excessive Residue and a Dirty PCB Surface After Soldering

A high solids content and a high level of nonvolatile matter can cause excessive residue on the board. Other possible causes include:

• Conveyor speed is too high, resulting in insufficient preheating and incomplete evaporation of volatile flux components.
• Solder-pot temperature is too low, preventing relevant flux components from fully decomposing, evaporating, or sublimating during soldering.
• Anti-oxidation additives or oil in the solder pot transfer to the soldering surface and leave residue.
• Too much flux is applied, preventing complete evaporation.
• Large component holes allow flux to rise to the component side during preheating and soldering.
• A no-clean flux is used, but a noticeable residue remains because the soldering surface has a pre-applied rosin or resin protective layer. The initially uniform layer may not appear dirty before soldering, but it can be disrupted in the soldering zone and create a dirty surface.
• Too few vias are provided in the PCB design, preventing volatile flux components from escaping effectively during preheating and contact with molten solder.
• Diluent is not added for an extended period during use, causing the flux solids content to increase.

2. Poor Tinning, Insufficient Solder, Solder Bridges, or Open Solder Joints

• Flux activity is insufficient to remove oxides from the pads or component leads.
• Wetting performance is insufficient, preventing molten solder from fully wetting the soldering surfaces and component leads.
• In a dual-wave process, the effective flux components decompose completely during the first wave and cannot remove oxides or promote wetting during the second wave.
• Preheat temperature is too high, causing the activator to become active too early. Its activity is weak or exhausted by the time the board reaches the solder wave.
• Foaming or spraying is improperly adjusted, resulting in insufficient or uneven flux coverage.
• Some areas of the soldering surface do not receive flux and therefore cannot be properly tinned.
• An uneven wave or another process problem prevents parts of the soldering surface from contacting the solder.
• Some pads or leads are severely oxidized, and the flux activity is insufficient to remove the oxide film.
• The PCB conveyor direction is incorrect. Dense rows of solder joints are perpendicular to the wave direction, causing solder bridging.
• The tin content is insufficient, or copper and other impurity elements are excessive. This raises the liquidus temperature of the solder and reduces flow at the same temperature.
• Manual dip soldering is performed incorrectly, such as using an inappropriate immersion time or immersion direction.

3. Corrosion After Soldering, Green Components or Pads, and Black Solder Joints

• Flux activators are too strong and do not fully decompose after soldering, allowing corrosion to continue.
• Insufficient preheating, caused by low preheat temperature or excessive conveyor speed, leaves excessive flux and activator residue.
• Flux residue or ionic residue may not be inherently corrosive, but can absorb moisture and form substances that cause corrosion.
• A highly active flux that requires cleaning is used, but the assembly is not cleaned or is cleaned too late.a

4. Leakage Current and Poor Insulation After Soldering

• Excessive flux residue remains after soldering, and the flux itself has insufficient insulation resistance.
• Flux remains on the board as ionic residue. The residue absorbs moisture and becomes conductive.
• The PCB layout is inappropriate, with conductors placed too close together. Leakage may occur after high-temperature soldering or high-voltage stress.
• The solder mask quality is poor, and its insulation resistance decreases after exposure to soldering temperatures.

5. Solder Spatter and Solder Balls

• The flux contains excessive moisture, which does not evaporate sufficiently during preheating.
• The flux contains high-boiling-point or low-volatility substances that cannot evaporate adequately during preheating.
• Preheat temperature is too low, leaving part of the flux solvent unevaporated.
• Conveyor speed is too high to provide sufficient preheating.
• The conveyor angle is too small. Air trapped between the solder and the soldering surface forms bubbles, which burst and produce solder balls.
• Too much flux is applied, and the excess cannot flow away or evaporate completely.
• Manual dip soldering is performed incorrectly. For example, immersing the soldering surface vertically into the solder can cause solder splash and solder balls.
• In a humid environment, the PCB absorbs moisture immediately after passing through the preheat zone, resulting in spatter and solder balls when it contacts the solder.
• Component leads on the soldering side are too densely arranged, or vias and through-holes are poorly designed. Poor gas escape between the soldering surface and molten solder then causes spatter and solder balls.

6. Solder-Mask Blistering After Soldering

• The solder mask itself is of poor quality and has a low maximum temperature rating, causing blistering during high-temperature soldering.
• The flux contains additives that attack or damage the solder mask.
• Solder or preheat temperature is too high and exceeds the temperature range of the solder mask.
• The PCB is soldered repeatedly after an earlier defect. Excessive soldering cycles can also cause solder-mask blistering.
• During manual dip soldering, the soldering surface remains on the solder surface for too long. A typical immersion time is approximately 2–3 seconds; a longer time can cause blistering.