Introduction
Flux plays a critical role during the PCB soldering process, facilitating bonding between component leads, pads, and solder. Traditional clean fluxes require removal after soldering to prevent corrosion, while modern no-clean formulations can eliminate post-solder cleaning. This provides efficiency advantages, but no-clean flux has limitations too. Understanding the key differences allows selecting the optimal flux type for a particular application.
This guide covers:
- The composition, properties, and functions of clean and no-clean fluxes
- How the chemistry and residues differ
- Cleaning requirements and reliability considerations
- Usage recommendations for each flux category
- Proper handling, storage, and application methods
Gaining a deeper understanding of flux technology enables optimizing the soldering process for efficiency, quality, and reliability across the wide range of electronics assembly applications.
Flux Purpose and Function
Before comparing clean and no-clean fluxes, it helps to understand what flux does. The main purposes of flux during soldering are:
Removing Surface Oxides
Solder alloy and metal component surfaces form thin oxidized contamination layers. Flux chemically strips these oxides so solder can wet and adhere to the clean surfaces underneath.
Preventing Re-Oxidation
Once surface oxides are removed, flux provides a protective barrier preventing re-oxidation while the solder is molten during reflow or hand soldering.
Facilitating Wetting
Flux modifies surface tension properties to allow molten solder to spread out, cover, and wet the component and board surfaces thoroughly.
Residue Removal
Some flux residues must be cleaned after soldering to prevent harmful effects on the assembly. No-clean fluxes aim to not require cleaning.
Types of Flux Chemistry
Flux formulations utilize various chemistries, which dictate their properties and best usage. Common flux chemistry types include:
Rosin (RA) Flux
The most common traditional flux, composed of natural rosin extracted from pine sap. Lower activity and non-corrosive. Requires cleaning.
Organic Acid (OA) Flux
Synthetic carboxylic acids as the main activator. Moderate activity, suitable for soldering many metallic surfaces. Usually requires cleaning.
Inorganic Acid (IA) Flux
Contains strong mineral acids as activators. Highly active but much more corrosive. Typically requires cleaning.
Water-Soluble (WS) Flux
Blend of organic acids and amine hydrohalide activators. Moderate activity, but residues are water washable.
No-Clean (NC) Flux
Varied chemistries with mild organic activators. Low solids residue designed not to need cleaning after soldering.
There is some overlap between the properties of these categories, but this generalization highlights the key traits of each flux type.
Clean Flux Overview
Clean fluxes represent the traditional general-purpose option, encompassing rosin (RA), organic acid (OA) and inorganic acid (IA) formulations. Key characteristics include:
Medium to High Activity
Clean fluxes exhibit moderate to excellent soldering capability for surfaces with heavier oxidation or contamination. Rosin provides the gentlest clean flux, with increasing activity through OA and maximum activity from IA types.
Potentially Corrosive Residues
Flux activators are often acidic compounds that can damage board and component materials if residues remain. Even mildly activated rosin fluxes require removal.
Required Post-Solder Cleaning
The residues must be fully washed away after soldering using suitable solvents. Any remaining activators may continue attacking materials.
Readily Available
Clean fluxes have been used for decades in electronics assembly. Numerous suppliers offer a wide range of chemistries, gels, applicators, and supporting cleaning processes.
Clean fluxes provide strong soldering performance for difficult surfaces, but do necessitate process steps for post-solder residue removal.
No-Clean Flux Overview
No-clean fluxes were developed starting in the early 1990s to simplify assembly by eliminating the cleaning stage. Key attributes include:
Low Solids Content
No-clean formulations have just 1-5% flux residue content versus up to 35% for some paste fluxes. This reduces total residues.
Mild Activators
No-clean fluxes use relatively gentle organic activators that are minimally reactive after soldering when cooled. This reduces potential for damage.
Designed to Not Need Cleaning
If applied properly within limits, no-clean fluxes leave safe, benign residues that do not require removal after soldering.
Potential Reliability and Performance Tradeoffs
Reduced flux activity can increase defects. Residues may still cause issues in some environments over time.
No-clean flux provides production efficiencies but requires balancing tradeoffs in soldering quality, material compatibility, and long-term reliability.
Comparing Properties of Clean and No-Clean Fluxes
Clean and no-clean fluxes exhibit significant differences in their properties and performance which guide appropriate application:
| Property | Clean Flux | No-Clean Flux |
|---|---|---|
| Activity Level | Medium-high | Low-medium |
| Residue Amount | High | Very low |
| Corrosivity | Often high | Low |
| Residue Cleaning | Required | Designed to not need cleaning |
| Reliability Risk | High | Low-medium |
| Efficiency | Low | High |
| Availability | High | Medium |
Well-formulated no-clean fluxes can meet soldering, material compatibility, and reliability needs for many consumer, commercial, and industrial electronics assemblies while boosting efficiency.
Clean Flux Composition
Clean fluxes utilize fairly strong activators to promote effective soldering. Common compositions include:
Rosin Flux
- Basis: Pine tree rosin / colophony
- Activators: None or weak organic acids
- Flux solids: 15-35% by weight
- Activity: Mild
Organic Acid Flux
- Basis: Diethanolamine, ethylene glycol, glycerol
- Activators: Adipic, succinic, glutaric acids
- Flux solids: 2-20% by weight
- Activity: Moderate
Inorganic Acid Flux
- Basis: Isopropyl or ethyl alcohol
- Activators: Hydrochloric, hydrobromic, phosphoric acids
- Flux solids: 1-5% by weight
- Activity: Very high
Higher activity clean fluxes give better soldering performance but create more reactive residues requiring thorough removal.
No-Clean Flux Composition
In contrast, no-clean fluxes utilize much milder organic activators and low solids content:
Typical No-Clean Flux
- Basis: Propylene glycol, ethylene glycol, diethylene glycol
- Activators: Succinic, adipic, suberic acids
- Flux solids: 1-5% by weight
- Activity: Low-moderate
Water-Soluble No-Clean Flux
- Basis: Triethanolamine, ethylene glycol, glycerol
- Activators: Malic, citric, lactic acids
- Flux solids: 3-10% by weight
- Activity: Low-moderate
The relatively gentle organic acids allow no-clean fluxes to be left on assemblies after soldering without significant material risks.
Comparing Clean and No-Clean Flux Residues
Since flux residues remain on the PCB assembly after soldering, the properties of these residues determine whether cleaning is required:
| Metric | Clean Flux Residues | No-Clean Flux Residues |
|---|---|---|
| Acidity (pH) | 1-5, acidic | 5-9, neutral |
| Ionic Content | High | Very low |
| Conductivity | Conductive | Non-conductive |
| Corrosiveness | High | Low |
| Cleanability | Fairly easy | Difficult |
| Cosmetics | Very dull/discolored | Clear, shiny |
No-clean residues are designed to be benign when left on assemblies. But some reliability risks remain which are examined next.
Reliability Considerations: Clean vs. No-Clean
Since no-clean fluxes do not get removed, their residues must not interfere with circuit function or degrade the assembly over time. Here are key reliability factors to weigh for clean versus no-clean:
Surface Insulation Resistance (SIR)
No-clean residues must not substantially lower electrical resistance allowing potential leakage currents. Clean fluxes provide higher SIR.
Electrochemical Migration Resistance
Ionic residues absorb moisture and can create dendritic growths that electrically short traces. No-clean fluxes inhibit migration but risks remain.
Corrosion Potential
Reactive residues may continue attacking component leads and board metalization over time, eventually compromising connections. Less risk with no-clean but still possible.
Interfacial Compatibility
Flux activators could affect interfaces between component encapsulants or conformal coatings. No-clean residues are designed to be compatible.
Cosmetic Appearance
No-clean leaves clear, shiny residues versus discolored residues from cleaned fluxes. Cosmetics are unimportant for hidden solder joints.
While designed to mitigate reliability risks, no-clean fluxes demand tightly controlled processes and have limitations regarding sustainability and service environments.
Cleaning Requirements: Clean vs. No-Clean
The core difference between clean and no-clean fluxes is whether soldered assemblies require post-process cleaning:
| Metric | Clean Flux | No-Clean Flux |
|---|---|---|
| Cleaning agent | Solvents, surfactants | Not needed |
| Typical process | Batch cleaning | Not applicable |
| Cleanliness validation | SIR testing | Visual inspection |
| Cleaning challenges | Solvent handling, waste | None |
| Efficiency impact | High, slows production | No added steps |
Eliminating cleaning provides major time and efficiency benefits. But no-clean processes must be closely monitored to prevent issues.
Flux Application Methods
Applying the optimal amount of flux is critical for both clean and no-clean usage:
Application Methods
- Brush
- Dropper
- Spray
- Foam
- Automatic dispensing
Controlled Parameters
- Flux deposit weight and area coverage
- Uniformity of application
- Localized application options
Consistent Processes
- Specify flux application in assembly documentation
- Train operators on proper techniques
- Regularly validate and document flux application
Controlling flux application improves soldering yield while minimizing residues. This benefits both cleaned and no-clean assemblies.
Process Recommendations: Clean Flux
Here are guidelines to follow when using clean fluxes requiring post-solder cleaning:
- Select flux chemistry with appropriate activity level for the surfaces being soldered
- Apply minimum flux needed for effective soldering
- Use foam fluxes or controlled dispensing for tight process control
- Clean flux off boards as soon as possible after soldering
- Validate cleaning efficacy with SIR testing
- Work with environmental regulations regarding cleaning solvents
Closely following flux manufacturer recommendations enables utilizing cleaning fluxes effectively while mitigating risks.
Process Recommendations: No-Clean Flux
To reliably use no-clean fluxes, these are key process considerations:
- Match flux activity to needs of assembly materials and components
- Minimize flux application to reduce residues
- Maximize soldering heat and time above liquidus to fully volatize flux
- After soldering, allow flux residues to cool undisturbed until solidified
- Perform regular process monitoring, inspection, and SIR testing
- Adjust no-clean process limits based on field reliability
No-clean fluxes require disciplined process control and validation to avoid hidden reliability risks from residues.
Flux Handling and Storage
Best practices for flux storage and handling maximize shelf life and avoid process issues:
- Store flux in original containers away from heat, cold, and contaminants
- For refrigerated flux, allow it to reach room temperature before use
- Mix flux containers before use to re-disperse settled solids
- Keep flux containers covered during breaks in use to avoid evaporation
- Avoid contaminating fluxes with dust or metals
- Never mix used applicators back into flux containers
- Handle fluxes safely using proper PPE like gloves and eye protection
Proper flux handling prevents variability, maintains activation capability, and reduces defects.
Comparison Summary: Clean vs No-Clean Flux
| Metric | Clean Flux | No-Clean Flux |
|---|---|---|
| Activity Level | Medium-High | Low-Medium |
| Residue Amount | High | Very Low |
| Corrosivity | Often High | Low |
| Cleaning | Required | Not Needed |
| Reliability Risk | Higher | Lower |
| Process Control | Moderate | Very High |
| Efficiency | Lower | Higher |
Conclusion
Clean and no-clean fluxes each have characteristics making them preferable for certain applications:
Clean fluxes are best for:
- Very difficult to solder surfaces with heavy oxidation or coatings
- Mass soldering processes like wave soldering
- Applications where residues risks are unacceptable
- Lower volume production where cleaning costs are lower
No-clean fluxes are ideal for:
- High-volume consumer electronics assembly
- Field applications where cleaning is impractical
- Densely packed, complex board assemblies
- Safety-critical electronics where residues could not be tolerated
For specialized industrial, aerospace, automotive, and military applications, the risks of no-clean flux may outweigh the efficiency benefits. But no-clean has become the norm in high-volume consumer electronics where process controls can be implemented.
In summary, understanding clean versus no-clean flux properties allows selecting the optimal flux chemistry for each application’s performance, efficiency, and reliability requirements. Proper control of flux amount, soldering process, and cleaning or residue mitigation steps are also essential to unlock the benefits of each flux type.
Frequently Asked Questions
Here are some common questions about clean and no-clean fluxes:
Can no-clean flux residues be cleaned if needed?
It is difficult since no-clean fluxes are engineered to avoid dissolving in common cleaning solvents. Secondary cleaning processes may only partially remove no-clean residues. It is better to ensure residues are benign rather than try cleaning.
When would clean flux be used today?
For very high-reliability applications where no-clean chemistry limitations would risk product function – aerospace, military, medical, etc. Also in large solder bath processes where higher activity is needed.
Does no-clean flux leave visible residues?
Minimal visible residues if the soldering process is optimized. The residues are designed to become clear and practically invisible once solidified after soldering.
Can no-clean flux be used with water wash processes?
Yes, but the flux chemistry must be tailored to be water-soluble. General no-clean types are not readily washable with just water.
What is the shelf life of liquid fluxes?
Refrigerated shelf life is typically 1-2 years for most flux chemistries. For ambient storage, shelf life ranges from 3-9 months. Manufacturer guidelines should be followed.
Conclusion
Both clean and no-clean flux chemistries remain essential for electronics assembly. Understanding their composition, properties, reliability factors, and process recommendations allows engineers to select the optimal flux type for each application. With careful control of flux amount and the soldering process, high product quality and efficiency can be achieved.
