Soldering

SOLDERING PRACTICE THEORY

Soldering practice is a well known process for the fabrication of different electronic components in a Printed Circuit Board. The basic elements of the different essentials of soldering can be described briefly in the follow up next.

Solder A solder is a fusible metal alloy with a melting point or melting range of 90 to 450 °C (200 to 840 °F), used in a process called soldering where it is melted to join metallic surfaces. It is especially useful in electronics and plumbing. Alloys that melt between 180 and 190 °C are the most commonly used.

Lead solder Tin/lead solders, also called soft solders, are commercially available with tin concentrations between 5% and 70% by weight. The greater the tin concentration, the greater the solder’s tensile and shear strengths. At the retail level, the two most common alloys are 60/40 Sn/Pb which melts at 370 °F or 188 °C and 63/37 Sn/Pb used principally in electrical work. The 63/37 ratio is notable in that it is a eutectic mixture, which means: 1. It has the lowest melting point (183 °C or 361.4 °F) of all the tin/lead alloys; and 2. The melting point is truly a point — not a range At a eutectic composition, the liquid solder solidifies as a eutectic, which consists of fine grains of nearly pure lead and nearly pure tin phases, but in no way is it an intermetallic, since there are no tin/lead intermetallics

Soldering iron A soldering iron is a tool used for applying heat to two adjoining metal parts such that solder may melt and flow between those parts, binding them securely and conductively. A soldering iron is composed of a heated metal tip and an insulated handle. Heating is often achieved electrically, by passing a current, supplied through an electrical cord or a battery, through a heating element. Another heating method includes combustion of a suitable gas, which can either be delivered through a tank mounted on the iron (flameless), or through an external flame.

Some soldering irons heat up and cool down in a few seconds, but others take minutes. For electrical work, wires and various electronic components are soldered to printed circuit boards, other wires, and small terminals. A low-power iron (15-30 watts) is suitable for this work. In earlier days wires were frequently soldered to large chassis made of heavy metal, but this high-power requirement is now rare. Higher power is used for non-electrical metal-work.

Stands for irons A soldering iron stand keeps the iron away from flammable materials, and often also comes with a sponge and flux pot for cleaning the tip. Some soldering irons for continuous and professional use come as part of a soldering station, which allows the exact temperature of the tip to be adjusted, kept constant, and displayed.

Flux A flux is a chemical cleaning agent which facilitates soldering, brazing, and welding by removing oxidation from the metals to be joined. Common fluxes are: ammonium chloride or rosin for soldering tin; hydrochloric acid and zinc chloride for soldering galvanized iron (and other zinc surfaces); and borax for brazing or braze-welding ferrous metals. Different fluxes, mostly based on sodium chloride, potassium chloride, and a fluoride such as sodium fluoride, are used in foundries for removing impurities from molten nonferrous metals such as aluminum, or for adding desirable trace elements such as titanium. In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder (e.g.) attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Additionally, flux allows solder to flow easily on the working piece rather than forming beads as it would otherwise. In soldering of metals, flux serves a threefold purpose: it removes oxidation from the surfaces to be soldered; it seals out air thus preventing further oxidation, and by facilitating amalgamation improves wetting characteristics of the liquid solder. Flux is corrosive, so the parts have to be cleaned with a damp sponge or other absorbent material after soldering to prevent damage.

The function of flux is primarily to remove oxide, with the general formula being: Metal oxide + Acid → Salt + Water Salts are ionic in nature and can cause problems from metallic leaching or dendrite growth, with possible product failure. In some cases, particularly in high-reliability applications, flux residues must be removed.

Desoldering Desoldering is the removal of solder and components from a circuit for troubleshooting, repair purposes and to salvage components. Electronic components are often mounted on a circuit board and it is usually desirable to avoid damaging the circuit board, surrounding components, and the component being removed. Desoldering requires application of heat to the solder joint and removing the molten solder so that the joint may be separated. Desoldering may be required to replace a defective component, to alter an existing circuit, or to salvage high value components for possible re-use. Excess application of heat may damage electronic components, or destroy the bond between a printed circuit trace and the underlying substrate. Once the solder has been melted, the vacuum device is applied to withdraw solder from the joint by a rapid movement of air. Desoldering braid draws up the molten solder by capillary action; the braid usually contains the same flux as solder.