The situation when a refrigerant leak is detected in the refrigerator often leaves the owner with a choice: call a technician or try to fix the problem yourself. In most cases, repairs come down to restoring the tightness of the circuit where copper or steel tubes are connected to each other or to heat exchangers. The key point here is not only the quality of stripping, but also the correct choice connecting material.
Incorrect selection of components for soldering can lead to repeated leakage after a few months or, worse, to clogging of the capillary system with flux combustion products. Modern refrigeration units work with various types of refrigerants, such as R600a or R134a, which place increased demands on the cleanliness of the internal cavity of the system. Therefore, the question of what exactly to solder holes or joints with is fundamental for the durability of the repair.
In this article we will analyze in detail the physical and chemical properties of various alloys, consider the nuances of working with copper and steel, and also point out the critical mistakes that beginners make. You will learn why ordinary rosin is not suitable for steel tubes and what melting temperature the ideal solder for your refrigerator model should have.
Classification of solders for the refrigeration circuit
All materials for creating permanent connections in refrigeration technology are divided into two main groups: soft and hard solders. The boundary between them is the melting point of 450 degrees Celsius. Soft solders they melt at lower temperatures and are more often used for low-temperature circuits or repair of evaporators, where thin walls cannot be overheated.
Hard solders, on the contrary, require a powerful flame and provide a connection that is often stronger than the copper tube itself. They are used for soldering capacitors and discharge lines, where pressure and temperature can be high. The choice between these groups depends on which tubes you are going to connect and in which node the leak occurs.
It is important to consider that the composition of the alloy directly affects its fluidity and adhesion. For copper, alloys with a high content of this metal are optimal, while for steel, additions of silver or phosphorus are required. The wrong choice will lead to the fact that the solder will simply roll into balls without penetrating into the gap.
- 🔹 Tin-lead (PIC): classical soft solders that require the mandatory use of flux.
- 🔹 Silver (PSr): hard solders with excellent fluidity and high strength seam.
- 🔹 Phosphorus-copper (PMF): self-fluxing alloys for copper that do not require additional chemicals.
- 🔹 Copper-zinc: used less frequently, mainly for brass, but also applicable for some steels.
Soft solders: composition and scope
Soft solders, the melting point of which varies in the range of 200–300 degrees, are traditionally made on the basis of tin and lead. The alloy POS-61containing 61% tin remains the most popular in household repairs. However, for refrigeration equipment, the cleanliness of the connection is critical, so the use of acid-containing fluxes is unacceptable here due to the risk of corrosion.
The main advantage of soft solders is the ability to work without high-intensity open fire, which reduces the risk of deformation of thin-walled aluminum evaporators. However, the strength of such a seam is relatively low, and it cannot withstand high mechanical loads or vibrations. Therefore, their use is limited to low pressure areas.
There are also lead-free options for soft solders, which are more environmentally friendly and food safe in the event of hypothetical contact, although this is secondary in a closed loop. The main requirement when using them is perfect cleaning of surfaces, since an oxide film on copper forms instantly when heated.
⚠️ Attention: Never use active acid fluxes (for example, soldering acid) inside the refrigeration circuit. Remaining acid will cause rapid corrosion of the tubes from the inside and coke the compressor, which will lead to expensive repairs.
When working with soft solders, it is important to control the heating temperature. If you overheat the soldering area, the tin can burn out and the weld will become porous and brittle. A visually high-quality connection should have a characteristic metallic sheen, and not be matte and grainy.
Hard solders: silver and phosphorus
For professional repairs of lines made of copper tubes, hard solders are most often used. The leaders in this category are alloys containing silver, for example PSr-15 or PSr-45. The number indicates the percentage of precious metal, which significantly increases the fluidity of the melt and the strength of the connection.
Phosphorus-copper solders deserve special attention, such as PMF-7 or PMF-9. Their uniqueness lies in the fact that phosphorus acts as a reducing agent, removing the oxide film from copper without the use of additional flux. This makes the soldering process cleaner and faster, eliminating the entry of foreign substances into the system.
However, phosphorus solders have a serious limitation: they absolutely cannot be used for soldering ferrous metals (steel) and copper alloys with a nickel content of more than 9%. In reaction with steel, phosphorus forms brittle phosphides, and the seam will collapse under the influence of vibration of the compressor almost immediately after turning on.
The melting point of hard solders reaches 700–800 degrees, which requires the use of a gas burner. When heated this way, copper becomes very plastic, so it is important not to overheat the tube so that it does not lose its shape. A properly made seam on hard solder can withstand pressure of up to 30 atmospheres or more.
Features of soldering steel and dissimilar tubes
In modern refrigerators, especially in models with the system No Frost, steel tubes or copper-steel connections are often found (for example, at the outlet of the compressor). Soldering such connections requires a special approach, since standard copper solders do not work here.
To connect copper to steel or two steel elements, it is necessary to use special hard solders with a high silver content (at least 30-45%) or copper-zinc alloys. A prerequisite is the use of flux, since steel oxidizes much more intensely than copper. Without flux, solder will not spread over the surface.
The technological process is complicated by the difference in the thermal conductivity of the materials. Copper removes heat faster, so you need to heat mainly the steel part to equalize the temperature in the contact zone. If you heat only copper, the steel will remain cold and the solder will not stick.
| Type of connection | Recommended solder | Necessity of flux | Melting point |
|---|---|---|---|
| Copper - Copper | PMF-7, PMF-9 | Not required | ~650-750°C |
| Copper - Steel | PSR-30, PSR-45 | Required | ~700-800°C |
| Steel - Steel | PSR-45, L-CuP | Required | ~800-900°C |
| Copper (low pressure) | POS-61 (soft) | Required (neutral) | ~220°C |
When soldering dissimilar metals, it is important to create conditions for the capillary effect. The gap between the tubes should be minimal (0.1–0.2 mm) so that the molten metal itself is drawn into the joint under the action of surface tension forces.
The role of fluxes and surface preparation
Even the most expensive solder will not stick to a dirty or oxidized surface. Preparation is 80% of soldering success. The tubes must be cleaned to a shine using fine sandpaper or a special cord brush. There should be no traces of oil, paint or oxides left on the surface.
Flux acts as a solvent for oxides and improves the wettability of the metal. For brazing copper, borax (sodium tetraborate) is often used in powder or paste form. When heated, the borax turns into a glassy mass, which protects the seam from oxygen.
For copper-steel joints, more active fluxes are required, often containing fluoride compounds. You need to work with them extremely carefully, as they are aggressive. After soldering, the remnants of such fluxes must be carefully removed with hot water or special solvents to prevent corrosion.
☑️ Preparation for soldering
There is a rule: there should not be a lot of flux. Excess during combustion can produce soot that gets inside the system. Apply a thin layer to the joint area only. If you use self-fluxing solder (phosphorus), additional flux will only do harm.
Safety precautions and common mistakes
Working with open fire and gas requires strict adherence to safety precautions. There should be no flammable materials around the work area, such as refrigerator insulation, plastic panels, or oil-soaked rags. The torch flame must be directed strictly at the soldering area.
One of the most common mistakes is overheating the joint. If copper is heated to a cherry red color, it becomes brittle and may burst when it cools. In addition, with excessive heating, the flux burns out ahead of time, and the solder does not adhere well.
⚠️ Attention: Before starting soldering, make sure that there is no refrigerant vapor left in the system. Some gases can form toxic phosgene when in contact with an open flame. Purge the system with nitrogen or simply make sure there is no pressure.
They also often make mistakes with the amount of solder. Its excess can flow inside the tube and block the passage of refrigerant, especially in narrow places in the capillary system. The solder should only fill the gap between the walls, and not be packed inside.
What to do if the solder does not flow?
If the solder does not flow, most likely the metal temperature is not high enough or the surface is poorly cleaned. Do not try to heat the solder itself with a torch - it should melt from the hot tubes. Try adding a little flux and increasing the heating of the joint area, but without fanaticism, so as not to burn through the thin wall.
Tools and equipment for high-quality soldering
To perform the work you will need not only the solder itself, but also the appropriate equipment. The main tool is a gas torch. For hard soldering, mixtures of propane-butane with oxygen or ready-made gas mixtures in cans (MAPP gas), which give a higher temperature than ordinary household gas, are best suited.
An important element is nitrogen purge. Professionals always solder copper tubes by purging them with nitrogen inside. Nitrogen displaces oxygen, preventing the formation of an oxide film (scale) on the inner surface of the weld. The scale, falling off, can clog the filter drier or capillary.
In addition, you will need:
1. Pipe cutter (scissor only, not a hacksaw, so that there are no chips).
2. Rimmer (for chamfering and calibration).
3. Clamps (crimps) for clamping tubes.
4. Fireproof mat.
The quality of soldering directly depends on experience and preparation. If you are taking on this for the first time, practice on scraps of copper pipes of different diameters. Learn to feel the moment when the metal has reached the desired temperature, and the solder will go into the gap itself.
Completing the work and checking the tightness
After the seam has cooled naturally (do not cool it with water or air, this will cause cracks!), it is necessary to remove the remaining flux. Copper connections can simply be wiped with a damp cloth, steel connections can be degreased with a solvent.
The next mandatory step is nitrogen pressure testing. The system is pumped up with pressure (usually 10–15 bar for domestic refrigerators) and left under the control of a pressure gauge for several hours or days. A drop in pressure will indicate the presence of a fistula or poor-quality soldering.
⚠️ Attention: Never check the tightness with an open flame or spark if air or oxygen may remain in the system. Use only a soap solution or electronic leak detectors.
Only after making sure that the system is completely sealed can you evacuate the system and charge it with refrigerant. Compliance with technology at every stage ensures that your refrigerator will work reliably for many years.
Is it possible to solder a refrigerator with a regular soldering iron?
It is impossible to solder refrigerator tubes with a conventional electric soldering iron for radio components (power 40–100 W). The thermal conductivity of copper is so high that a soldering iron will not be able to heat the tube to the melting temperature of the hard solder. Soft solder (SOS) is theoretically possible, but only to eliminate micro-leaks in accessible places and only if the stripping is perfect. For a complete repair, you need a gas burner.
How to replace nitrogen when soldering at home?
Ideally, you should always blow with nitrogen. If there is no nitrogen cylinder, some craftsmen try to blow into the tube with their mouth (through a long hose to prevent moisture from getting in) or use compressed air. However, this is worse because there is oxygen in the air, which causes oxidation. The best compromise is to use carbon dioxide (CO2) from a siphon can, after releasing the gas to displace the air, although nitrogen is still preferable.
Why does solder roll up into balls and not stick?
This happens for two reasons: either the surface is not heated enough, or it is poorly cleaned/flux is not used. The copper should be red hot (for hard soldering). If the flux is black and does not work, it needs to be washed off and fresh applied. It is also possible that you are trying to solder steel with phosphorus solder, which simply does not fit on it.
Which solder is best for a beginner?
For a beginner who solders copper to copper, phosphorus-copper solder (PCS) in the form of a rod is best suited. It forgives some mistakes with temperature, does not require the purchase of a separate flux, and produces a durable seam. The main thing is not to overheat the connection. Silver solders are more expensive, and tin-lead solders require ideal preparation and produce a less durable seam.