Flux selection in PCB assembly is more than just a preference. It directly impacts the soldering process, influencing joint quality and long-term board stability. Unlike solder, flux is available in a vast array of chemical formulations. One of the most reliable options is ROL0, a low-residue, no-clean flux. These properties make it suitable for high-end electronics.
This article explains why proper flux selection matters, the factors to consider, and applicable standards. It also provides a detailed overview of ROL0 and why we use it for high-reliability soldering, including the upcoming optical equipment project.
Why is Flux Selection Critical in PCBA Manufacturing?
Flux plays a vital role in PCB assembly. It cleans, improves solder flow, and promotes wetting. Additionally, it prevents the formation of new oxides. After soldering, the type of flux dictates whether residues require cleaning.
That said, the product is available in a wide range of formulations. This diversity makes thoughtful selection crucial, especially when you must use a specific type of solder and soldering steps.
An improper choice can lead to defects, increase costs, or pose a corrosion risk. The right flux improves results, both during and after assembly. Joints form as desired: shiny, robust, and correctly shaped.
The right type will also reduce production costs by eliminating the need to clean residues. More importantly, it ensures the finished product meets specified acceptability standards. So, how do we select fluxes?
Flux Selection Factors
Fluxes fall into several categories, grouped by base materials, chemical activity, and halide content. These variations dictate the type of flux to use. With that in mind, the following are key factors that influence the final choice.
Product Reliability Needs: High-reliability electronics require fluxes that leave harmless residues. These are usually no-clean, low-activity types. High-activity fluxes can remain active long after assembly, posing electrical and corrosion risks.
Process Compatibility: The flux must be compatible with the assembly process. This includes the application method, thermal profiles, and board inspection methods. For instance, it must not evaporate prematurely, as that could encourage oxidation and hinder proper solder flow and wetting.
Residue Cleaning Costs: High-activity fluxes clean oxides better. However, they require removal, which increases production costs. No-clean residues may remain on the board; they are typically non-conductive and harmless.
ROL0 Flux for High-Reliability PCBA
ROL0 flux is a strong choice for high-reliability PCB assemblies. These are circuit boards designed for high-end mission-critical applications across industrial, automotive, medical, military, aerospace, and high-tech optical markets.
The first two letters in the name indicate that the flux contains rosin (an organic resin derived from trees), while the second letter (L) means it’s a low-activity type. The zero at the end implies it has no halides.
- RO — Rosin-Based
- L — Low Chemical Activity
- 0 — Halide-Free (<0.05% by weight)
The flux leaves a minimal, clear, and non-conductive residue that requires no cleaning. In cases where cleaning is recommended, such as when PCBs require protective coatings, solvents are available to perform the task.
ROL0 Flux vs. ROL1 Flux
ROL0 and ROL1 are common fluxes in PCBA manufacturing. Their chemical compositions are almost identical except for one slight difference: ROL0 contains no halides, while ROL1 does. Engineers choose between the two fluxes based on this difference.
The halide activators (Cl, Br) in ROL1 make it more aggressive, enhancing its cleaning abilities (oxide reduction). However, they leave ionic residues. The residues are corrosive and require cleaning.
Being halide-free makes ROL0 electrically safe. It has a low activity (uses weak acids as activators), and you can leave it on assembled boards. The right formulation will also have a wide process window, suiting most assembly needs.
- ROL0 is more preferred when a circuit board requires no cleaning or has higher reliability requirements; it leaves harmless residues. The residues are also clear and inspection-friendly.
- ROL1 is a good choice when surfaces are heavily oxidized or contaminated. It also suits applications that use lead-free solders, as the high assembly temperatures promote oxide formation, which the flux would help prevent.
ROL0 Flux Compatibility with Venture Assembly Processes
ROL0 flux is compliant with Venture’s PCB soldering processes, including the upcoming high-end optical equipment project. The processes include SMT and DIP assembly, vacuum nitrogen reflow, lead soldering, and board-cleaning technologies.
SMT and DIP Assembly
ROL0 is suitable for both SMT and selective DIP soldering. The flux is available in forms that support automated application methods (including selective fluxing) and fine-pitch printing. The qualities make it suitable for high-density SMT requiring high precision and accuracy.
The low activity reduces the risks of residues left under SMT components. The risks include corrosion and current leakage between adjacent component pins and other conductive surfaces.
Because it leaves a clear residue, the flux ensures assembled boards are easy to inspect. That’s in addition to being no-clean and suitable for components that may pose residue-removal challenges, such as BGAs.
Vacuum Nitrogen Reflow
This process combines vacuum pressure and nitrogen’s inert nature to improve joint formation and enhance first-pass yields. Applying a vacuum helps draw trapped gases and volatiles from the molten solder, reducing voiding to below 5%.
The introduction of nitrogen into the soldering furnace reduces oxygen to very low levels, typically below 1000 ppm. The lack of oxygen and the inert environment minimizes oxidation, improving solder flow and wetting.
Leaded Soldering
Our PCB assembly process uses leaded solder, primarily Sn62Pb36Ag2 and Sn63Pb37. The choice of solder aligns perfectly with the customer’s requirements for the optical equipment project.
Lead solder melts at lower temperatures (183°C-190°C) than lead-free solder (217°C-221°C), making it safer for delicate electronics. It also produces more robust joints, while the Pb content prevents the growth of tin whiskers.
The solder is exempt from RoHS restrictions when used to assemble PCBs for critical applications. The IPC-J-STD-001 (001xS) addendum, in particular, specifies its use in space and military circuit boards.
PCBA Cleaning
Because ROL0 is a low-activity flux, it doesn’t require cleaning, unless removal is necessary, such as when a PCB must meet strict residue standards. In such cases, the manufacturer ensures the cleaning method is effective, safe, and within a project’s budget.
Venture uses advanced cleaning equipment and liquid formulations to remove the flux residues. These prevent damage to the board and components. They also ensure the total removal of unwanted materials to meet specified cleanliness standards.
Flux Classification and Acceptability Standards
IPC standards provide guidelines that govern the entire PCBA production process. The IPC-J-STD-004 standard classifies fluxes, while the IPC-J-STD-001 outlines soldering requirements. The IPC-A-610 contains board inspection and acceptability criteria.
IPC-J-STD-004 Standard
The J-STD-004 standard categorizes fluxes using coded letters and numbers. These designators describe the product’s chemical composition, activity level, and the presence or absence of halides.
The first two letters indicate the base material, and can be rosin (RO), resin (RE), organic (OR), or inorganic (IN). The second letter is the chemical activity level, which ranges from low (L) to medium (M) to high (H).
The third designator, a number, denotes whether the flux contains halides (1) or doesn’t (0), indicating its activity level and cleaning requirements. The table below provides a glimpse of the flux types.
| Flux Composition | Activity Level (Low, Medium, High) | With Halides (>0.05%) | No Halides (<0.05%) |
| Rosin (RO) | ROL, ROL, ROH | ROL1, ROM1, ROH1 | ROL0, ROM0, ROH0 |
| Resin (RE) | REL, REM, REH | REL1, REM1, REH1 | REL0, REM0, REH0 |
| Organic (OR) | ORL, ORM, ORH | ORL1, ORM1, ORH1 | ORL0, ORM0, ORH0 |
| Inorganic (IN) | INL, INM, INH | INL1, INM1, INH1 | INL0, INM0, INH0 |
IPC-J-STD-001 Standard
The J-STD-001 standard defines how to solder printed circuit boards, focusing more on the process than the finished board. In other words, it tells you how to produce a high-quality electronic assembly.
The current document, J-STD-001J, is a comprehensive guide that covers everything from material selection to board inspection methods. It primarily contains instructions for the following:
- Soldering materials (solder alloys, fluxes, etc.).
- The assembly environment and process control parameters.
- Soldering rules for THT and SMT components, including BGAs and similar SMDs.
- Flux residue cleaning requirements for class 1, 2, and 3 circuit boards.
IPC-A-610 Standard
The IPC-A-610 describes how assembled circuit boards should look by providing visual acceptance guidelines. The guidelines include images and text. These serve as clear references, helping inspectors make better judgments.
The recently released version (A-610J) refines earlier visual cues and terminology. For instance, it replaces “target condition” images with “acceptable conditions” and has better images. The standard covers these main areas.
- Board and electronic components damage.
- Surface cleanliness and the coating condition.
- Solder joint quality for SMT and THT components.
- The condition of mechanical components.
Conclusion
Flux selection requires a deep understanding of the available options, their chemistries, and how each choice affects joint formation and the type of residue. The right type improves outcomes, reducing defects and keeping production costs low.
ROL0, ROL1, REL0, REL1, ORL0, and ORL1 are the most common flux formulations. Low-activity, no-clean types leave minimal residues. They are suitable for high-reliability electronics. ROL0 falls into this category, making it an excellent choice.
