Soldering Flux Type and Classification for Electronics PCBs

In this article, I’ll tell you about soldering flux classifications and No-Clean fluxes. I will explain why it’s important to understand what type of flux you’re using and touch a bit on soldering flux cleaning.

Soldering Flux Activity

There are several classes of soldering flux used in the electronics industry: low-activity, medium-activity, and high-activity fluxes. To understand which flux you’re using, check its datasheet or label.

Cheap China Soldering Fluxes

Many soldering fluxes sold on online marketplaces don’t have any classification at all. A lot of these soldering fluxes come from China, typically as cheap, generic noname products with strange names. Their manufacturers and characteristics are unknown.
Most cheap Chinese fluxes must be thoroughly cleaned. These fluxes can remain active after soldering. It means that repaired electronics may start oxidizing or even faster when exposed to operating current.

No-Clean Term

Many brands use the term No-Clean, but this is misleading and doesn’t provide any real information. No-Clean doesn’t really mean anything. Just possible no-clean properties. No-Clean alone doesn’t tell us the flux class. However, if the classification is labeled ROL0, REL0, ORL0, then it’s truly No-Clean.

Soldering Flux Oxidation

This leads to another issue - How well can you clean these residues from under the components? There’s no guarantee. You need to be certain that flux residues will not cause oxidation. To ensure this, you should use high-quality fluxes and solder pastes that have clearly defined classifications.

Active Soldering Flux Terms

If the classification ends in 1, the flux must be cleaned. Fluxes with a 1 at the end contain active compounds and halides that promote oxidation and current leakage.
If the third letter of the classification is M or H, it means the flux is active and must be cleaned. It is critical that M and H fluxes are fully washed out from under components, or oxidation can occur.
Use quality soldering flux with a clear classification. Don't use strange soldering fluxes and solder pastes – you might ruin your work. Future problems will cost much more than buying a high-quality flux with a clear classification.

Here’s a classification table for different flux types: The first two letters indicate the base on which the soldering fluxes are made.
• RO – Rosin-based
• RE – Synthetic resin-based
• OR – Organic-based (water- or alcohol-based)

The third letter in the classification defines the flux’s activity level:
• L – Low
• M – Medium
• H – High

Only ROL0, REL0, and ORL0 are considered No-Clean.

Organic fluxes ORL0 and ORL1 are more common in industrial use because they are easier to clean with water-based washing systems.

The last digit (0 or 1) in the abbreviation refers to the amount of halides in the flux:
• 0 – Halide-free
• 1 – Contains halides

Last digit directly impacts how well the flux removes oxide films from PCB pads and components during soldering. 1 indicates stronger oxide removal properties. When using active fluxes classified as 1 or containing M or H, you must ensure proper cleaning. If active flux residues remain on the PCB under microchips or components, oxidation will begin in the coming months. And your device will be covered with gray oxides spots.
For new PCBs and components, it’s best to use L0 low-activity soldering fluxes - they are sufficient for most soldering and repair work.
If PCB pads or components covered slight oxidation, M or H fluxes can be used. These provide better oxide removal and improve solder joint quality.
High-activity fluxes primarily used in mass production where very small components soldering is required. They ensure complete soldering of all pads and components, especially on modern PCBs with very small contact areas.
Medium-activity, high-activity, and digit 1-class fluxes must be washed off after soldering.

Soldering Flux Resistance and Cleaning

A very important point! - Unwashed flux residues change their resistance when heated. If the microcircuits heat up during operation, the flux residues can start conducting electricity. This will cause failures or short circuits in the operation of the microcircuit. Remember this and try to wash off any fluxes.

RMA Term

Many manufacturers still use the term RMA—this is old abbreviation. It is mostly used as a marketing trick because people recognize it. The correct modern classification for RMA is ROM0. Rely on the modern flux classification.

Recomendation

For manual and automated soldering, I recommend safe soldering fluxes and solder pastes classified as ROL0. The flux form—liquid or gel—doesn’t matter - choose what works best for you.

Final tip: Always clean your PCB after soldering!
Dust and other particles will stick to the flux residues, which can lead to current leakage in the working device.

Thank you for reading!