Solders and soldering fluxes

Solder - This is a low-melting metal alloy designed for connecting wires, leads, parts and components by soldering. Previously, solders were designated by three letters - POS (tin-lead solder), followed by a two-digit number showing the tin content in percent, for example POS-40, POS-60.

The best solder is pure tin. However, it is expensive and is used in exceptional cases. During radio installation, tin-lead solders are most often used. By the strength of soldering, they are not inferior to pure tin. Such solders are melted at a temperature of 180 - 200 ° C.

Solder Selection

The choice of solder is made depending on such factors: from the metals or alloys being joined, from the method of soldering, from temperature restrictions, from the size of parts, from the required mechanical strength, from corrosion resistance, etc.

For soldering thick wires use solder with a melting point higher than for soldering thin wires.

In some cases, it is necessary to take into account the conductivity of the solder (reminder: the specific resistance of tin is 0.115 Ohm x mm2 / m, and lead - 0.21 Ohm x mm2 / m).

Varieties of solders.

Solders are divided into three groups: refractory, low-melting and ultra-low-melting. Refractory solders (hams practically do not use them). Refractory alloys include melts with a melting point above 500 ° C, which create a very high mechanical strength of the joint (tensile strength up to 50 kg / mm2). Their disadvantage is precisely that they require a high heating temperature and, although the strength of such a soldering is very high, intense heating can lead to undesirable consequences: you can, for example, “let go” a steel part.

The disadvantage of brazing alloys is that they require a high heating temperature, and although the strength of such a soldering is very high, intense heating can lead to very undesirable consequences: you can overheat an expensive part and damage it (for example, a transistor or a microcircuit), ”, For example, a steel part (spring).

Fusible (amateur) solders. This category includes solders with a melting point up to 400 ° C, which have a relatively low mechanical strength (tensile strength up to 7 kg / mm2). In radio engineering installation works, mainly fusible solders are used. They include tin and lead in various proportions, for example, POS-61 solder, which contains 61% lead, 38% tin and 1% of various additives.

Ultra-fusible (amateur) solders. There are also alloys, which, in addition to tin and lead, include bismuth and cadmium. These alloys are the most fusible: some of them have a melting point of less than 100 ° C. The mechanical strength of the joint in such alloys is very small. Previously, they were used for soldering crystals in crystalline detectors. Currently, low-melting cadmium-bismuth alloys are used in the repair of printed circuit boards. They are also used for soldering transistors, since, according to technical conditions, it is recommended to solder them with solder with a melting point not exceeding 150 ° C.

For soldering transistors, the so-called Wood alloy with a melting point of 75 ° C can be used, which includes: tin - 13%, lead - 27%, bismuth - 50%, cadmium - 10%. Wood's alloy can be prepared according to the specified recipe yourself or bought at a pharmacy. Soldering is underway lightly heated soldering iron. Rosin is used as a flux.

Amateur Radio Solder Shape

In the last century, a tin rod with a cross section of 10 mm was recommended. Now they use solder wire with a cross section of 1 to 5 mm for soldering. The most common 1.5-2 mm multichannel solders.Multichannel means that several flux channels are located inside the tin wire, which ensures the formation of an even, shiny and reliable soldering.

Such solder is sold in skeins - on radio markets, in flasks - in which it is coiled, and in bobbins (the amount of solder in them is such that it will last for more than one year). It is recommended to purchase in the form of a wire, match-thick - it is more convenient to solder.

When soldering the mounting wires of radio equipment, it is convenient to use tin-lead solders cast in the form of thin rods with a diameter of 2 - 2.5 mm. Such rods can be made by yourself by pouring molten solder into a vessel, in the bottom of which a hole has been made in advance. The vessel should be kept above a sheet of tin or a metal plate. After cooling, the bars should be cut into pieces of the required length.

Modern solders used in the soldering of electronic circuits are produced in the form of thin tubes filled with a special resin (colophonium), which serves as a flux. Heated solder creates an internal connection with metals such as copper, brass, silver, etc., if the following conditions are met: the surfaces of the parts to be soldered must be cleaned, that is, they must remove oxide films formed over time, the part is in place solders must be heated to a temperature above the melting point of the solder. Certain difficulties arise in this case in the case of large surfaces with good thermal conductivity, since the power of the soldering iron may not be enough to heat it.

Self-soldering

For self-preparation of solder, the components of the composition (tin and lead) are weighed on a balance, the mixture is melted in a metal crucible above a gas burner and, having mixed the melt with a steel rod, a slag film is removed from the melt surface with a steel plate. Then the melt is carefully poured into molds - troughs made of tin, duralumin or gypsum.

Melting must be carried out in a well-ventilated area, wearing safety glasses, gloves and an apron of coarse cloth.

Soldering fluxes

Why do I need a flux when soldering? During soldering, the temperature of the parts to be joined rises significantly. In this case, the oxidation rate of metal surfaces increases. As a result, the solder wetts the connected parts worse. Therefore, it is necessary to use excipients, fluxes.

What is flux? Flux - this is an auxiliary material, which is designed during soldering to remove the oxide film from the parts subjected to soldering, and to ensure good wetting of the surface of the part with liquid solder. Without flux, solder may not adhere to the surface of the metal. Purpose of fluxes: reliably protect the surface of the metal and solder from oxidation, improve the wetting conditions of the metal surface with molten solder.

The action of the flux depends on its composition, the fluxes available: either dissolve oxide films on the surface of the metal (and sometimes the metal itself), or protect the metal from oxidation when heated. Thus, the flux forms a protective film over the soldering point.

The flux is already contained in modern solder in the form of a thin core. When melting the solder, it is distributed over the surface of the liquid metal. The flux is used to coat the surfaces of already tinned metals also before they are joined (in fact, by soldering). In this case, the flux is a surfactant, that is, a surface-active substance. After the parts come into contact, the excess flux between them creeps out and evaporates all the time because its evaporation temperature is lower than that of the solder.

Fluxes are different. For example, for the repair of metal utensils they use "soldering acid" - a solution of zinc in hydrochloric acid. It is impossible to solder radio structures with such a flux - over time, it destroys the solder. For radio installation it is necessary to use fluxes in which there is no acid, for example, rosin.

Amateur Flux Requirements

The choice of flux is an important issue.Previously, only rosin was used, there was no other flux. Why rosin is bad - rosin, alcohol rosin flux belong to the category of active fluxes. The first drawback is that at high temperatures not only the metal oxide is removed, but also the metal itself. The second drawback - cleaning the board after soldering with rosin is a big problem. You can wash off the residues only with alcohol or solvents (and even then, it is sometimes easier to pick something sharp).

The remainder of the flux on the board is not only ugly from an aesthetic point of view, but also harmful. On boards with small gaps between the conductors, dendrites (in other words, short circuits) may increase due to galvanic processes on a contaminated surface. What is the way out - in the modern materials market you can find a wide range of fluxes that are washed off with ordinary water, do not destroy the soldering iron tip and provide high quality soldering. Such fluxes are sold, as a rule, in syringes, which is very convenient for use.

Regardless of which flux is used, the finished solder must be wiped with a cloth moistened with rectified alcohol or acetone, and also cleaned with a stiff brush or brush moistened with solvent to remove flux residues and dirt. In some exceptional cases, instead of rosin, you can use its substitutes:

- rosin lacquer, commercially available in hardware stores. It can be used as a liquid flux instead of a solution of rosin in alcohol. The same varnish can also be used for anti-corrosion coating of metals.

- resin - resin of pine or spruce - accessible material, especially to lovers living in the countryside. You can prepare such a flux yourself. The resin collected in the forest from the trees must be melted in a tin can over low heat (over high heat, the resin can ignite). Pour the molten mass into matchboxes.

- An aspirin tablet available in any medicine cabinet. The disadvantage of this flux is the unpleasant smell of smoke released during the melting of aspirin.

Now a large number of various, so-called “non-laundering” fluxes are being produced, both liquid and in the form of a semi-liquid gel. Their peculiarity is that they do not contain components that cause oxidation and corrosion of the connected parts, do not conduct electric current and do not require washing the board after soldering. Although it is still better to remove all residual flux from soldered parts after soldering.

To apply liquid flux, you can use a brush, cotton swab or just a match, but it is more convenient to use the so-called "flux applicator". You can try to buy a branded flux applicator for about $ 20-30, but it’s much easier and cheaper to do it yourself. To do this, you need a piece of silicone or rubber hose with an inner diameter of 5 - 6 mm and a disposable medical syringe.

The syringe is cut into 2 parts. Both parts are inserted into the rubber tube. The needle is slightly shortened, it can be slightly bent for ease of use. Pressing the hose lightly, squeeze a drop of flux from the tip onto the soldered parts and solder. During storage, so that the needle does not dry out, a thin wire can be inserted inside it. It is also convenient to use a flux in the form of a gel or paste. For its application, you can also use a disposable syringe, only because of its density the syringe needle will have to be taken thicker.

Other articles from the soldering cycle:

What is soldering? Soldering Safety

How to choose a soldering iron and organize a soldering workstation

How to solder