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Solderingmay be the right answerSolutions with Effective, Powerful Advice
What are these processes? When are they preferred to other joining methods? What is critical to their successful application? We would like to propose in the following some of the most important aspects of these processes which provide effective and economical solutions for a wide range of common joining problems.
The definition describes a joining process of metallic objects where coalescence or attachment is obtained by heating to a suitable low temperature, usually below 450 0C (840 0F), using nonferrous filler alloys (solders) which melt at temperatures lower than that of the base materials to be joined. Capillary attraction helps in spreading the solder between the properly fitted surfaces. What is in here for me? Soldering, solder, manual and automatic application, wetting, flux, resin-flux, low-temperature-joining, tin-lead-solders, lead-free-solders, wave-, flow-, reflow-, inert-, nitrogen-, drag-, dip-soldering, colophony, health-hazards, these are some of the issues developed in this page for the benefit of interested readers.
One of the oldest processes for keeping together different metals joined with a low temperature melting filler, soldering is still applied in modern times, with due improvements and advancements, in certain special niches of importance in industrial applications. There was a time, and older people may still remember it, before the ubiquitous introduction of plastic containers and implements, when many household items were mostly handmade by a myriad of individual artisans either working for themselves or grouped in factories. They used to cut by scissors thin foils of tin or zinc plated steel, or of lead or zinc, form them by simple tools in useful shapes and then solder them at the overlapping edges with a solder alloy using a low temperature manual torch or heated copper tools. In more modern times virtually all electrical connections in appliances were manually performed by this process with a hand held heated copper tool (usually called a soldering iron), until the appearance of PCB (Printed Circuit Boards) that promoted the development of automatic processes, mostly including dipping of boards into molten solder for performing many joints at once. An essential element of the process is the use of flux required to clean the surfaces from objectionable oxide or other disturbing layers, to allow wetting and metallurgical bond between solder and base material. Bond The nature of the junction is more than adhesion or mechanical attachment, although these contribute to strength: it is rather a metallurgical bond produced at the filler-metal/substrate interface. This property is essential when the purpose of soldering is to provide electrical conductance and continuity of circuit functionality. Advantages The flow and wetting properties of correctly applied solder, permit the joint to be performed in place by itself, without precisely directing the heat source to the connection place. The low temperature means that no damages are generated in other elements, giving a large selection of design items and dimensioning available. Joints can easily be disassembled permitting modifications or repair. The equipment is relatively simple and the process can be automated for mass production. Joint Design Quite usually, the actual joint shape to be selected depends mostly on application, on base metal and on service requirements. As solders are intrinsically weak, strength is seldom a condition. Joints are designed so that the strength requirements are fulfilled by interlocking elements, while the function of the solder is limited to sealing or stiffening, to avoid physical separation or to ensure electrical continuity. Integral with design is the compatible heating process to be employed and the method of introduction of flux and of solder in the joint. This means that both heating method and solder form and application must be specified on drawings or production sheets. For sheet metal, overlapping edges are preferred, possibly interlocked. Wires are usually interwoven before soldering. If possible, solder should be applied from one side on the joint, as its appearance at the other side after soldering is a proof of spread along all the area. This is particularly useful for fitting on tube soldered joints. Precleaning A most important operation to be performed before soldering, precleaning is an essential element to the success of this joining process. All foreign materials like oil, grease, paint, dirt, auxiliary production lubricants, marker traces etc. will interfere with proper surface wetting and solder flowing impairing the quality or making soldering utterly impossible. Therefore, depending on the conditions of the surfaces, degreasing, scrubbing, mechanical abrasion and even acid or alkaline cleaning for removal of most adherent rust or oxides, must be planned into the work method, to ensure the best preliminary conditions. Even rinsing water should be treated if necessary because residues from hard water may interfere with soldering. Precoating In certain cases, for difficult to braze metals, one should consider precoating with a more easily soldered alloy. By this method the use of strong acids during soldering could be avoided, as all residual traces can be rinsed away after precoating. Fluxes The fluxing action consists in the removal of thin oxide layers or other interfering impurities from the surfaces undergoing soldering, to permit intimate contact and wetting of the base metal by the molten solder alloy, through reduction of its surface tension. Fluxes are displaced by the molten solder, and any residues should either be non corrosive, or should be thoroughly eliminated. Fluxes may be applied in form of liquids, pastes or even gaseous materials. In automatic soldering machines, liquid flux is applied by spraying, just before immersion of boards in the molten solder. Fluxes may be noncorrosive, mild, intermediate or highly corrosive depending on base materials and their condition. Fluxes are sometimes in the form of natural occurring resins: these perform well for the soldering process except that they tend to produce fumes, called colophony, which can be irritant or produce allergic reactions. New resin free fluxes are being proposed and available. Mass production Automatic soldering by wave, drag, or dip machines is a successful method for making high-quality, reliable joints. Soldering, if correctly controlled, is one of the least expensive methods for mass fabricating electrical connections. Machines performing soldering under a blanket of inert nitrogen gas are claimed to produce less oxidation (dross) in the molten material, with the advantage of requiring less maintenance, cleaning and solder usage. Solder alloys Selection of a certain solder is based on its melting properties, which depend on its composition, expressed as melting range (Solidus - Liquidus interval) (except for pure metals and eutectic alloys which present a single melting point) and the temperature conditions in service. Tin-lead solders have been most popular in the past and are still in wide use except where applications require the use of materials friendly to the environment. Lead is considered poisonous and many efforts were exerted to remove the chance that lead based residuals find their way to the environment because of negligent production procedures. Due to the wide range of properties deriving from compositions, these alloys are used in many industrial applications, from plumbing, radiators, electrical and electronic connections. The most popular composition for electronic applications is the eutectic alloy of 63% tin (Sn) and 37% lead (Pb), which melts at 183 0C. Specifications set the maximum of impurities in solders, as it was determined that they can impair performance characteristics, like wetting, flow, temperature, strength and oxidation resistance. Elements like Aluminum, Antimony, Arsenic, Bismuth, Cadmium, Copper, Iron, Nickel, Phosphorus, Sulfur, and Zinc, even if within limit in the original filler, tend to increase in the mass of solder during normal use. When used as main constituents, not as unwanted impurities, Antimony, Silver, Zinc, Indium, Bismuth, and Cadmium alloyed with Tin provide classes of solders of special characteristics, like low temperature. Environment friendly solder materials
There has been a growing concern over the continued use of lead in currently used solders, particularly for the electronic industry.
Sample pages of a thorough study on the influence of lead containing solders and on other dangers to the environment can be found in An Article on a New Lead-free Solder Alloy was published in Issue 42 of Practical Welding Letter for February 2007. Click on PWL#042 to read it. To receive every issue of Practical Welding Letter as it is issued, please Subscribe. Some authors contest this being a real problem, or call it a nonproblem, because in their view it is not proven that lead containing solders pose a threat to the environment. While the scientific proof for need of lead-free soldering is questionable, consumer and legislative demand for its production is not. The fact is that a mounting drive pushes towards a new class of solders containing no lead. The switch is not without problems. For one thing, the new solders melt at a temperature higher by 30 to 40 0C to that of current solders. This change alone requires a complete redesign of the applications, involving reevaluation of flux, cleaner, substrate, finish and packaging processes. Specifications Specifications and standards have established requirements for certain solder alloys in terms of composition limits, impurity levels, and a nomenclature for referencing particular compositions. In the United States, the specifications of solder alloy compositions are listed in ASTM B 32. In Europe the International Organization for Standardization (ISO) issued the specification ISO/DIS 9453. In conclusion soldering is a mature process for joining a definite class of items, especially for electrical circuits containing different elements. Automatic machines and procedures are available which permit low cost production with high reliability of performance. The following Documents are relevant to the subject treated. ASTM B32-04 ISO/DIS 9453 AWS SHB
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