History and applications of lead-based alloys
Lead never occurs free in nature but in a compound known as lead sulphide (Galena). The ancient Egyptians used Galena in eye paintings because of its metallic look that served to attract the attention of ancient metal workers. Extraction of lead from its ore was considered an easy exercise as it involved reduction of Galena using lit fires. At 327 degree Celsius melting point, this enables for its flow from the highest point to the lowest point where it is smelted and subsequent collection. Initially, led had minimal use due to its ductility. Its first usage was registered in 3500 B.C. It served well as a material for making containers, conduits and pipes (Humpston and Jacobson, 2004 p.35). Lead pipes used to be very characteristic with the embedded mages of Roman Emperors. Lead is ductile, corrosion proof and can be beaten into any shape hence one of the. Lead is able to flow and collect itself at the base of the fire that is used in its smelting. This is advantageous in reduction reactions as this make it possible for phase separation of lead and gangue as well as ensuring the metal is molded into the recommended shape after its concentration. Ductility and softness of lead has necessitated its use in alloy form. Alloying elements that have jointly been used with lead include Antimony, tin, arsenic and calcium. Antimony ensures that the required hardness and strength is attained and is normally used in making storage battery grids whose grid pates are made from wrought calcium lead or antimonial lead; piping system; battery grids and castings (Shangguan, 2005 p.100). Content of antimony in Lead – antimony alloys are normally in the ratio of 1: 50. But mostly the ratio is 2: 5. Type metals used in printing industry are made up of tin alloys or antimonial lead. Copper is often added to guarantee hardness. The lead sheath that is contained in cables serves two purposes. One, the sheath provides protection against mechanical damage. Secondly, it safeguards the cables against corrosion. Here, chemical lead, arsenic lead and antimonial lead are often used. Lead sheet that is majorly used in chemical industries, building and construction sites is made from pure lead and alloy comprising calcium, lead and tin elements. Solders used in joining materials in industrial and construction sites are made from lead and its component alloys. Tin-lead solders alloy are made at melting temperatures of 182 degree Celsius. Tin –lead alloys melt at varying temperature ranges. This occasioned by the composition of alloys. Lead base bearing alloys. Generous amounts of lead are used in making ammunition used for sporting activities and military training and combat. Lead foil is made from lead and tin (Subramanian, 2007 p.50). Anodes are also made from alloys of lead. They are used in plating of manganese, copper and nickel because they are not easily corroded by acids and sea waters.
Solders, applications, fluxes and metal alloys composition
Because lead is becoming unpopular due to the fact that it contributes to the pollution of the environment, effort has been made towards coming up with solders that are free from lead. In the past, tin antimony solder were used. The amount of tin in the compound was 95 per cent by weight, where as antimony was 5 per cent by weight. Other elements account for 0.05 per cent of the composition of the solder. The composition has a very high melting point than tin lead solder composition. The main application of tin antimony soldier is in the field of refrigeration as well as in joining copper to cast iron. This solder can also be used in soldering electrical and electric connections depending on the temperatures used. It can be used in sweating copper tubing in the solar heating panels, drinking water appliances and air conditioners. This solder is preferable because it’s easier to use, it has good penetration and flow ability, has pleasant appearance as well as higher shear, creep and tensile strength. It comes in the form of wires which can be solid, acid core, rosin core, and organic core that have varying standards and custom diameters. It can also be manufactured in performs which are custom manufactured rings or in bulk alloyed bars.
Other conventional heating methods can also make use of tin antimony alloys. Soldering irons are normally made from these likewise to torch heating. It is often used with the wolverine general purpose soldering flux otherwise known as SILVERBRITE 100 whish is a water soluble flux. The bulk room temperature tensile strength of the soldering flux and the tin antimony alloy solder (95/5 tin antimony) are 6 900 and 6400 psi respectively compared to the tin lead alloy solders which has bulk room temperature tensile strength of 6 000 psi. joints that have been soldered using 95/5 tin antimony alloy solder with L type tube at room temperatures should be able to withstand much pressure to an extent that in case of any breakage, it should be the copper to break but not the soldered joint. Corrosive tests that have been conducted on the tin antimony alloy solder using standard tafel electrochemical techniques and the ASTM-Corrosive water D1384 show that SILVABRITE 100 and 95/5 tin antimony show the following corrosive test data for the flux and the 95/5 tin antimony alloy solder respectively (0.31 and 2.2 mils/year).
Solder that have tin, silver and copper are 95.5 per cent by weight tin, 4 per cent copper, and 0.5 per cent by weight silver (Judd and Brindley, 1992, p.1120). Solder that is made up of tin, antimony, zinc, and silver have 95 per cent copper in their composition, 3 per cent antimony, 1.5 per cent zinc, and 0.5 per cent silver. They are designated as Sn96.5/Ag3.0/Cu0.5, Sn95.5/Ag3.8/Cu0.7 and Sn95.5/Ag4.0/Cu0.5. Sn/96.2/Ag2.5/Cu0.8/Sb0.5 is at times used as low silver content alternative. Solder made up of bismuth, tin, antimony and silver have 1 per cent to 4.5 per cent of bismuth in composition. Presently a solder has been developed that has 90-95 per cent tin, 3-5 per cent antimony, 1-4.5 per cent bismuth and 0.1 to 0.5 per cent silver. Together with the flux tin and silver filler metals can be used to solder ferrous and non ferrous materials whose soldering require low temperatures and have good wetness properties. They are often used in soldering vibration applications. Tin silver solders are used in soldering in food production industries. HAg-4S has 3.4 to 4.5 per cent silver. Its melting point in solid state is 221 degree Celsius and 230 degree Celsius in liquid state. Hag-4S is eutectic, has good flow properties, it usage require the use of low temperatures. It is innocuous. Hag-6S has 5.5 to 6.5 silver. When in solid state it melts at 221 degree Celsius and 27o degree Celsius in liquid state. Because of its wider melting range it is favored for brazing materials with big gap. HAg-100s composition is presented as Sn-Ag-Cu227. In solid state it melts at 260 degree Celsius and 660 degree Celsius in liquid state. Hag-100s is free of lead and can be a perfect substitute of 50/50 brazing filler materials, braze copper and stainless steel. It possesses a very high shearing strength. It is suited for welding and soldering joints of the copper tube.
Initially efforts to mix unclean flux that was made from tin-lead alloy with lead free solder alloys did not give the best of results. Chemical reaction that takes place between the flux and the solder alloy paste interferes with the features of the solder paste a requisite for printing performance. Variability in density between lead free and tin lead solder alloy imply that metal loading of the solder paste has to be different. Lead free solders require higher soldering temperatures therefore the flux has to be very stable at these higher temperatures.
Flux prevents oxidation of the base and the filler materials at such higher temperatures (Suganuma, 2004 p.1). Flux is inert at room temperatures, but acts as reducing agent at very high temperatures like those used when soldering with lead free solder. There are water soluble fluxes and no clean fluxes. Their performance need careful evaluation as it is probable that mild flux may be preferred but may not give the adequate performance. Rosin fluxes come in a variety of the non activated, mildly activated and activated forms. Activated and mildly activated rosin flux posses rosin and other activating agents, usually an acid that enhances the wetness of the joints the flux is applied on. It removes the oxides that are there at the joints. Activated rosin fluxes produce residues that are corrosive that have to be removed from the piece to be joined by soldering. Mildly activated rosin flux produce residues that are not as corrosive as those of activated flux (Bath, 2007 p.200). Cleaning of surfaces they have been applied may be done but not a compulsory exercise.
Development of lead free solders
Because of growing environmental concerns linking lead to environmental pollution, efforts are now being made to develop lead free solders that have different elements. The elements are expected to pose no negative environmental impacts presently or in days to come. The elements are supposed to posses sufficient quantities of base materials that must be available presently and in future. The elements used in development of lead free solders must have melting temperatures similar to those tin and lead alloys used for making solder wires. The temperature should be slightly below 200 degree Celsius. Other issues that were extensively looked at included the color of the material to be used in making the alloy, soldering behavior, the viscosity, stability, and compatibility with other solvents. The characteristics of the solders that have to be looked at are their printing behavior against the established parameters, storage stability of the pastes, reproducibility of the viscosity, formation of voids and distribution of size of particles of the solder.
Developers of lead free solders expect that the elements used in making of such products should be having thermal and electrical conductivities similar to those of tin lead alloys. Elements used in developing lead free alloys should be having recommended joint strength likewise to thermal fatigue resistance, easy to repair, cheap and compatible with the processes in place. Silvers properties qualify it as the best element to be used in the development of lead free alloy solders but issues related to cost slightly make less preferable. Bismuth, another substitute is so brittle apart from having limited supply because it is a by-product of lead mining. Moreover, bismuth is poor conductor electricity and also a poor conductor of heat. Because of toxicity concerns, cadmium is not so favored an element in development of lead free alloys of solder. Silver produced globally is 13,500 tones and its capacity is 15, 000.bismuth produced globally is 4000 tones while its capacity worldwide is 8000. Cadmium’s is 8,000,000 while the global capacity is 10,200,000. Potential problem with potassium, rubidium, sodium, magnesium, caesium and lithium have potential problem because they are alkali metals. Mercury, lead, cadmium are highly toxic where as gallium is short in supply. These elements have low melting points but other properties they have like the toxicity, being alkali metals, and being low in supply disqualify their usage as elements to be used in development of lead free solders.
Copper has generous supply apart from being soluble in tin. This qualifies copper to be used, but it has to be used in low percentages. Indium has low melting point, but is costly. It is not resistant to corrosion and has limited supply. Solders made of alloys of tin and indium are only safe when kept in a place with low humidity or when they are coated to avoid dangers poised on them by corrosion. Tin has adequate supplies and is not so toxic. Zinc boasts of generous supply but has limitations bordering on oxidation. It makes the solder so brittle. The oxidation problems interfere with the use of existing automatic soldering bit. After a series of reviews that was done to come up with the acceptable elements and testing an alloy of CASTIN was adopted. CASTIN contain copper, tin, silver and antimony. Tin accounts for 96.2 per cent of the compound, silver 2.5 per cent, 0.8 per cent copper and 0.5 antimony (Nimmo, 1999 p.1). CASTINs grain structure is coarser hence a misconception that the alloy is brittle. CASTIN has a melting point of 216 degree Celsius.
CASTIN is superior to eutectic 63/37. CASTIN does substitute 63/37 in many soldering applications. When it is used in myriad soldering processes temperature ranges of between 250 to 260 degrees Celsius are administered. When such temperatures are applied on solder, reflow profiles have to be hotter. Soldering bit has to have a temperature of 235 degrees Celsius. At temperatures above 216 degrees Celsius, the span of time has to be between 30-45 seconds. It is proper that soldering done on high density board employ use of nitrogen. This will help in retarding thermal problems with flux and board materials. Hand soldering calla for setting iron at 750 degree Fahrenheit. Circuit boards coated with CASTIN are wonderful. CASTIN is endowed with flatter pads and also has a uniform coating advantages. However, because CASTIN has coarser grain than tin and lead it appears not to come up with joints that are shiny and bright. It creates white and frosty joints that arise due to the choice of cooling rates. When cool down takes place faster, the joint becomes shinier. CASTIN absolutely satisfy all the pre conditions that are required for substances to be used as led free alloys in there use in electrical appliances and making soldering materials. In terms of temperature requisites, physical properties and ease with which CASTIN can be used in industrial set up this alloy serves as a suitable replacement of lead alloys. Its components comprising tin, silver, copper and antimony can be easily processed. It can help in tinning, bear board coating, wave soldering as well as in surface mounting.
Toxicity and Regulation
There are several environmental and toxicological concerns that surround both lead and non-lead solders regarding their extensive use. As a result there are some regulatory measures that have been put in place and they include the following:
Silver and its compounds usually cause biological problems like irritation of the GIT and agryria which presents symptoms of permanent coloration of skin in blue-grey pigment and also the mucus membranes (Smith III, 1998, p.40). These compounds can also lead to reproductive defects and mutations.
Antimony and its compounds also cause similar problems like silver to the digestives system with signs of emesis, diarrhea, nausea, and abdominal pain. There is no documented toxicological information (Smith III, 1998, p.40).
Copper and its compounds also cause problems to the digestive system, with symptoms of nooses, emesis and diarrhea. It also indicates ecotoxicity though not toxicological data is documented. Indium and its compounds have been indicated in developmental problems in mice especially fetal deaths, malformation of various organs including kidneys, tail and ribs. It causes mutagenic. Bismuth and its compounds have been found to be carcinogenic and have the potential to cause chromosomal aberrations (Smith III, 1998, p.40).
These potential negative impacts of these metals have lead to regulatory concern. Silver, antimony, and copper is controlled under SARA 313, Superfund and clean water act among others. Bismuth and indium compounds are more potent are regulated by federal authorities also some state laws (Smith III, 1998, p.40).
Because of the ability of lead to pollute the environment, there uses in making alloys used in various applications like soldering have been phased out with alloys that serve the same functions that lead alloys used to serve but do not have lead. These lead free alloys used to make solder include tin and antimony; tin, silver, copper alloys of solder; and tin, antimony, zinc and silver alloys. These varieties of lead free alloys blend well with flux during the soldering process. Unfortunately for soldering to take place successfully, higher amounts of temperatures have to be used. A major problem with the use of silver has been costs related. Silver also because of its inherent properties does not fully cover the joints soldered a major limitation.
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Shangguan, D. (2005). Interconnect Reliability. California: ASM.
Smith III, E. (1998). Environmental Impacts and Toxicity of lead Free Solders. Including Japanese and European Union Regulations, Surface Mount Technology, Vol. 12, No. 12, p. 40
Subramanian, K.N. (2007). Lead free electronic solders: A special issue of the journal Materials: New York: Springer.
Suganuma, K. (2004). Lead free soldering in electronics. New York: Marcel Dekkar Inc.