Soldering is a core method for permanently joining metals, widely used both in industry and by hobbyists. Achieving a strong, durable and clean joint is only possible when the surface to be soldered is properly prepared and accompanied by the right materials, tools and process-enhancing agents — such as soldering fluid. So, what does precise soldering with soldering fluid involve?
Metal oxides – the primary obstacle in soldering
Precision soldering requires preparatory steps, primarily cleaning the surface of unwanted chemical compounds. The main “enemies” of the soldering process are metal oxides, which must be thoroughly removed. These compounds form naturally when metal is exposed to oxygen. The most common oxides include:
- aluminum oxide (Al₂O₃),
- sodium oxide (Na₂O),
- iron(III) oxide (Fe₂O₃) – the main component of rust and a major cause of corrosion,
- copper(II) oxide (CuO).
As explained earlier, there is a difference between soft and hard soldering. In soft soldering, the metal surface is wetted by molten solder, which spreads evenly to form a solid soldered joint. Metal oxides interfere with this process — therefore, they must be removed promptly.
What is soldering fluid?
Soldering fluid is a rosin-free flux designed for soft soldering. This technique uses alloys that melt at relatively low temperatures (such as tin or tin-based alloys) and relies on water-based fluxes — like soldering fluid — for optimal results.
Its action is straightforward: it removes thick layers of metal oxides from the surface being soldered. Additionally, it enhances solder adhesion and improves the overall quality of the joint.
Composition and form
AG TermoPasty soldering fluid is a clear, water-based solution. Its formula is built around active ingredients — a blend of inorganic salts (e.g., chlorides) and organic acids with reducing properties, responsible for:
- removing oxides from metal surfaces (regular application is recommended),
- improving wettability by reducing the surface tension of molten solder — enabling smoother, more uniform spreading,
- protecting cleaned surfaces from reoxidation during the soldering process,
- enhancing solder adhesion to the base metal.
Unlike traditional rosin-based fluxes, this soldering fluid contains no rosin or resins, making it easier to use and clean off.
Is 100 ml of AG TermoPasty soldering fluid enough?
Yes — it’s a highly efficient solution. At AG TermoPasty, the soldering fluid is offered in two practical sizes: a 50 ml or 100 ml plastic bottle with an integrated applicator brush.
Certifications
AG TermoPasty soldering fluid is a premium-quality product. This is confirmed not only by hundreds of positive reviews from distributors and customers, but also by its certified quality control standards. Every bottle complies with strict European industry regulations, including:
- ISO 9454, Type 3.1.1.4 – confirms the composition (contains chemically active yet corrosive zinc or ammonium chlorides).
- J-STD-004, Type INH1 – defines the physicochemical properties (halogen-free, water-based and non-corrosive in certain conditions — once properly removed from the soldered surface).
When and why to use AG TermoPasty soldering fluid?
Our rosin-free flux is designed for soldering galvanized surfaces. It significantly improves surface wettability, enhances the quality of the joint and helps prevent oxide formation at the soldering pointv — vall essential functions for any flux. But there’s more.
What makes AG TermoPasty soldering fluid stand out:
- minimal smoke emission during soldering,
- no need for additional rosin support,
- easy cleaning of metal surfaces treated with this acid-based flux.
Soldering fluid proves especially effective in soft soldering of metals with poor wettability.
Application – not for SMD, but ideal for galvanized steel soldering
This product is intended exclusively for soft soldering in installation, workshop and industrial settings — commonly used in plumbing and refrigeration systems — for tasks such as:
- soldering galvanized steel and components (e.g., galvanized sheet metal),
- soft soldering of zinc,
- joining pipes in plumbing installations,
- industrial and workshop repair work,
Important: AG TermoPasty soldering fluid is an active acid-based flux and not suitable for soldering electronic components (such as SMDs, integrated circuits, or printed circuit boards). It may damage delicate parts and lead to track corrosion. Instead, choose a rosin-based flux — such as Flux RF800 — or a solder paste, both of which are ideal for SMD applications.
Soldering paste or AG TermoPasty soldering fluid (100 ml)?
Our soldering paste contains rosin, hence the term rosin-based flux. While effective for electronics, it has limitations in more demanding industrial applications (e.g., soldering galvanized materials). In such cases, a better solution is an active water-based flux — like our rosin-free soldering fluid, which is safer, easier to clean and better suited for challenging tasks.
We also recommend similar alternatives, such as Waart 100 ml soldering fluid.
Other notable AG TermoPasty products:
- Solder Paste Easy Print /Sn62Pb36Ag2/ (lead-based),
- Solder Paste Easy Print /Sn96.5Ag3Cu0.5/ (lead-free, suitable for SMD soldering).
A key distinction between Easy Print flux pastes and soldering fluid lies in cleaning requirements:
- Easy Print pastes feature “No-Clean” technology (no residue removal needed).
- Soldering fluid is “water-soluble”, meaning post-soldering residue must be rinsed away.
Step-by-step: soft soldering with soldering fluid for zinc
Soft soldering involves evenly heating the metal and the filler material (such as solder wire) to a specific working temperature — typically around 230–250°C. Performing the process correctly with soldering liquid requires precision and proper technical conditions. So how does precise soft soldering with soldering fluid look in practice?
Step 1: Preparing tools and materials
For soft soldering with soldering fluid at home, you’ll need a soldering iron — either a heating element type or a transformer model with 60–100 W of power. This kind of iron can also be used for larger SMD components and basic assembly tasks.
Essential tools also include accessories for mechanically cleaning the metal surface, such as sandpaper or a wire brush.
You’ll also need solder wire (e.g. a tin-based alloy), a heat source (like a propane-butane torch or microflame burner) and the flux — in this case, a bottle of soldering fluid (e.g. 100 ml). Our product comes with an integrated brush applicator, but if you’re using a flux that doesn’t include one, be sure to have a separate brush or dropper on hand for applying the soldering fluid.
Don’t forget a water-based cleaner (e.g. water PCB cleaner) to clean residues afterward.
And of course, personal protective equipment is a must — especially gloves and safety goggles!
Step 2: Surface cleaning and degreasing
To ensure a strong and reliable solder joint, the work surface must first be cleaned — specifically, cleared of oxides — and degreased. Experts confirm that one of the most common causes of defective soldering is insufficient surface preparation, which can lead to poor adhesion, porosity or cold joints. The process should include:
- Mechanical cleaning – scrub the metal surface with a wire brush or sandpaper (preferably with 180–240 grit) until you achieve a smooth, matte finish. This step removes oxides, grease, dust, corrosion and general debris.
- Degreasing – use isopropyl alcohol or technical-grade acetone to remove oils, lubricants and organic contaminants.
Step 3: Precise application of soldering fluid
Soldering fluid serves as a flux — its primary role is to remove micro-oxides, enhance surface wettability and protect the metal during heating. Apply the fluid to the cleaned soldering area using a brush or dropper. Keep in mind: a thin, even layer is all you need — no excess!
For optimal performance, timing is key: apply the soldering fluid right before assembling the joint. The active ingredients in the flux will begin working instantly upon contact with the solder, aiding its melting and flow across the metal.
These reactive compounds ensure that the soldering fluid activates immediately, improving the molten solder’s spread and adhesion.
Step 4: Heating and soldering galvanized components
Start by preheating your soldering iron to the appropriate working temperature.
Heat the soldering area until the liquid flux begins to activate. At this stage, the solder wire should begin to melt and spread evenly across the surface — thanks to the presence of soldering fluid. Avoid excessive heating time, as the flux is only effective until it evaporates.
During the heating process, keep the following key principles in mind:
- Do not apply heat directly to the solder wire — the joint itself should be hot, not just the solder. Use a torch to evenly heat the entire connection area.
- Avoid localized overheating, as it may degrade the flux or damage the surface.
- Apply solder only once the joint reaches the alloy’s melting temperature (this varies based on the flux used). At that moment, the solder will flow naturally through capillary action and penetrate the joint.
Lack of proper solder flow usually indicates either a low temperature or insufficient surface preparation.
Step 5: Cooling
After soldering, set the soldering iron aside and allow the joint to cool down naturally. It’s important not to move the components during this time.
Wondering why you shouldn’t speed up the cooling process with cold water or compressed air? Doing so can cause thermal stress and microcracks in the delicate structure of the solder joint.
Step 6: Cleaning off flux residue
Once the solder joint has fully cooled, it’s time for a thorough second cleaning of the soldered surface — this step is especially important in plumbing or heating system installations. Even a thin layer of leftover soldering fluid may contain acidic compounds with undesirable properties (such as accelerating corrosion on galvanized steel, particularly in humid environments). Additionally, leaving active liquid flux behind may eventually lead to metal joint failure due to seal degradation.
Following the water-soluble cleaning standard, we recommend two effective methods for flux residue removal:
- Rinse the soldered area with warm deionized water (approx. 50 °C) using a soft antistatic brush (e.g. nylon) to gently scrub away any residue. Then, rinse the surface again with cold water. This two-step rinsing process is crucial and is considered the most effective “at-home” method for removing post-soldering contaminants.
- If you prefer a more advanced approach, consider using our professional-grade water PCB cleaner — it delivers even better results. Compared to plain deionized water, it provides superior protection against corrosion and contamination on soldered components.
Common mistakes in soft soldering with soldering fluid
While soft soldering might seem simple at first glance, it’s deceptively complex — it requires precision and a good grasp of both chemistry and physics. The most common mistakes when soldering galvanized components include:
- inadequate surface cleaning,
- excessive or insufficient application of soldering fluid,
- incorrect heating temperature,
- moving components during soldering,
- failing to remove flux residue after soldering galvanized materials,
- using the wrong type of solder, an unsuitable flux (e.g. rosin-based) or low-quality materials.
Summary
To sum up, soldering fluid is an essential part of the soft soldering process. As an effective and safe active flux, it ensures clean, strong metal joints. The liquid flux is easy to rinse off — no need for harsh chemicals — and works even on challenging surfaces like copper, brass or steel.
Looking for the ideal flux? We invite you to explore our diverse range of soldering products — we’re confident you’ll find the right formula to match your needs. If you’re interested in soldering and welding or have questions about other products, like solder pastes or flux blends, contact us. We’ll be happy to help you choose the best solution or clarify any technical concerns.