Environmental and technical characteristics of conductive adhesives versus soldering 2.Toxicological aspects of electrically conductive adhesives as compared to tin-lead soldering systems2.1 Toxicological
aspects in connection with joining technology The objectives of the toxicological evaluations were to establish a basis from a health and environmental point of view:
2.1 Toxicological aspects in connection with joining technology In the conventional joining technology of electronic industries tin/lead solder is used. The printed circuit board (PCB) must be cleaned to secure good adhesion (joining). Normally a rosin-containing flux is used. When solder paste is used, the cleaning agents (flux) are integrated in the paste. In some situations, some conductive adhesives require the PCB to be cleaned to ensure good joining of the electronic components to the PCB. Furthermore, stencils, mixing and dispensing equipment must be cleaned after use in order to remove uncured adhesive. Normally a solvent, e.g. ethanol, isopropanol or butanone is used for these purposes. The same goes for solder joining technology, where e.g. stencils must be cleaned for solder paste, usually with alcohols. 2.2 Conductive adhesives Thirteen adhesives were included in the study. A fourteenth adhesive was omitted as it contains nickel. In principle, nickel is prohibited on Danish workplaces. Furthermore, nickel is listed on the List of Undesirable Substances of the Danish Environmental Protection Agency. The toxicology of conductive adhesives is described in [5]. The thirteen adhesives are based on four types of polymers, i.e. 8 epoxy based products, two of which are two-component; two silicone products, one of which is two-component; two polyester products and one acrylate product. Primarily silver is used as the conductive material. In nine products, silver particles are used, and in 2 products other metal particles, i.e. aluminium or copper, plated with silver are used. In two products, carbon is used. In addition to the adhesive polymers and conductive materials, the adhesives also contain low molecular weight molecules, e.g. epoxides, silanes, amines, solvents, fillers, which act as hardeners, cross-linkers, viscosity regulators etc. 2.2.1 Polymers In general, epoxy (one- and two-component) and acrylate products contain reactive low molecular weight substances. Epoxies and acrylates may cause development of skin allergy. Many epoxy systems use amines, which may be corrosive and may also cause development of skin allergy. In addition to causing allergy, many epoxies are genotoxic and mutagenic. The cured polymer materials are non-toxic Silicone products contain low molecular weight silanes, which are irritating or even corrosive, and may have a genotoxic potential. Cross-linking by these small molecules cures silicones. During the curing process, the silanes will release alcohols, normally methanol or ethanol. The polymer materials are non-toxic Polyester products are thermosetting or thermoplastic. The thermosetting products may contain cross-linkers. Thermosetting polyesters are cured at elevated temperatures by reaction with low molecular weight cross-linkers, e.g. silanes. These are irritating or even corrosive, and may have a genotoxic potential. During the curing process, the silanes will release alcohols, normally methanol or ethanol. The polymer materials are non-toxic. 2.2.2 Solvents All the four types of adhesives contain organic solvents, which in general are regarded as irritants and neurotoxic. Most of the solvents will evaporate in the production phase of the electronic equipment. It is evaluated that solvents may cause irritation to eyes and respiratory tract, whereas neurotoxic effects are unlikely to occur due to fairly low exposure concentrations. Exposure in the working environment is relatively easy to control with enclosures and/or exhaust. Minor amounts of high boiling solvents may be left in the final product. This may cause slight nuisance to the consumers during use. 2.2.3 Conductive materials Silver is the most predominant conductive material either per se or as plating on other metals, e.g. copper or aluminium. Carbon is also used as conductive material. Toxicological profiles for the conductive materials can be found in [5]. The conductive material in the adhesives is not important for choice of adhesives from a working environmental point of view. This is merely a matter of environmental toxicity, cf. the later discussion on metals in the environment. 2.2.3.1 Silver Silver is highly inert and generally considered to be of low human toxicity. Silver accumulates in the body. Long-term exposure to silver may cause argyria, which is a cosmetic illness. Soluble silver may affect the liver. Mild allergic skin reactions may be seen in rare occasions. Silver is toxic for the environment. Environmental aspects of silver are discussed below. 2.2.3.2 Copper Copper is an essential element in most organisms and is crucial for the function of many important enzymes in the body. Consequently, the body can regulate uptake and excretion of copper to some extent. Long-term exposure to relatively high doses may affect liver and kidneys slightly, and possibly also the central nervous system. Copper ions are toxic to aquatic organisms, algae and bacteria. The fish toxicity is comparable to that of lead. 2.2.3.3 Aluminium Aluminium is neurotoxic. However, permanent disturbances of the central nervous system are only seen in persons with chronic kidney deficiencies. Long-term inhalation of relatively high doses of aluminium may cause development of lung fibrosis. Long-term exposure to relatively high doses may affect the central nervous system and cause skeletal disorders. Aluminium is toxic to aquatic organisms. The fish toxicity is comparable to that of lead. 2.2.3.4 Carbon Carbon is relatively inert and considered to be of low human toxicity. Dust may irritate skin and mucosae in the eyes and the upper respiratory tract. Long-term inhalation of relatively high concentrations may cause development of lung fibrosis. Carbon may adsorb carcinogenic compounds (PAH), but cancer has not been reported in humans or in experimental animals after inhalation or skin exposure. No data on environmental toxicity were found, but it is estimated that the use of carbon in conductive adhesives will have no environmental impact. 2.2.4 Cleaning agents In connection with conductive adhesive technology, cleaning agents are needed to ensure good conductive joints and to clean stencils, and mixing and dispensing equipment. The common practice is rubbing with alcohols, which is normally sufficient. However, often butanone or the like is used to eliminate rubbing. High concentrations of alcohols may cause irritation of eyes and respiratory tract. Often, the commonly used ethanol is denatured with mineral spirit. On rare occasions, skin contact with mineral spirit may cause development of skin allergy. Butanone irritates skin and the respiratory tract. It has low toxicity, but enhances the toxic effect of other solvents, e.g. the neurotoxic effect caused by n-hexane, which may be present in aliphatic hydrocarbon solvents, and the hepatotoxic effect of chlorinated solvents. Butanone is taken up through the skin 2.2.5 Evaluation of conductive adhesives 2.2.5.1 Working environment Polyester and silicone adhesives are based on non-toxic polymers and contain only a few more ingredients, including solvent and conductive material. During curing, silicone products will release minor amounts of alcohol e.g. methanol or ethanol, which are irritating to the mucosae of eyes and respiratory tract. Inhalation of methanol in very high concentrations may cause damage to the optic nerve and blindness, but these effects will not occur at the concentrations resulting from the emission of methanol from the conductive adhesives. Both types of products may release small amounts of other organic solvents included in the formulation. Uncured silicone adhesives contain low molecular weight silanes, which may irritate the skin on direct contact. Epoxy-based and acrylate-based adhesives contain low molecular weight reactive molecules, which pose risks for development of skin allergy. This requires additional worker protection. Acrylate-based adhesives contain only a few more ingredients, including solvents and conductive material. Epoxy-based adhesives also contain amines or amides as hardeners, which may cause development of skin allergy. The conductive adhesives joining technology requires use of solvents as cleaning agents, cf. above, which are irritating to skin and mucosae of eyes and respiratory tract, and may interfere with the toxic action of other substances. The occupational health risks from the use of electrically conductive adhesives in the electronics production are assessed and discussed in [4]. 2.2.5.2 Consumer From a consumer point of view, the choice of polymer type is not important. Some adhesives may result in minor amounts of high boiling solvents being left in the final product. When they evaporate during use they may cause nuisance to the consumers, i.e. affect the indoor climate especially together with other volatile organic compounds (VOC’s), e.g. from electronics cabinets etc. The emitted amounts of high boiling solvents from conductive adhesives are evaluated to result in very low concentrations. 2.2.5.3 Environment The polymer part of cured adhesives will have no impact on the environment. Neither will the residual amounts of organic solvents and other ingredients have impact on the environment. It is evaluated that only the metals, i.e. silver, copper and aluminium, in the conductive adhesives may have potential impact on the environment, when electronic equipment is disposed of. In conductive adhesives, copper and aluminium are plated with silver. Consequently, silver will have the highest potential impact on the environment of the three. Furthermore, the additional environmental load of copper and aluminium from the use of conductive adhesives is minimal as compared to the amount of copper and aluminium used elsewhere in electronic equipment, e.g. leads and heat sinks. Silver is discussed later in the section on metals in the environment. 2.3 Tin/lead solder Traditional tin/lead solder bars contain 63% tin and 37% lead (Sn63Pb37). Flux must be used in order to secure good joining capabilities. A typical flux contains organic solvents (alcohol and/or mineral spirit), rosin and activator for the rosin. Solder paste is a formulation of metals (tin, lead and often small amounts of silver, e.g. Sn62Pb36Ag2), rosin, activator, and organic solvents. The toxicology of materials used in traditional solder is described in [5] 2.3.1 Lead Lead is a known toxicant to humans and accumulates in the body. Lead is neurotoxic, affects the haemoglobin production, and causes sexual dysfunction and malformation in the foetuses. Furthermore, lead is possibly genotoxic and carcinogenic. Lead is toxic in the environment. Environmental aspects of lead are discussed below. 2.3.2 Tin Tin in high doses is toxic to humans and accumulates in the body. It may cause symptoms from the nervous system and the gastrointestinal tract. Long-term inhalation of tin (or oxides) may cause stannosis (“tin-lungs”). Tin (salts) is a rare skin sensitiser. Tin is toxic for the environment. Environmental aspects of tin are discussed below. 2.3.3 Silver Silver is highly inert and generally considered to be of low human toxicity. Silver accumulates in the body. Long-term exposure to silver may cause argyria, which is a cosmetic illness. Soluble silver may affect the liver. Mild allergic skin reactions may be seen in rare occasions. Silver is toxic in the environment. Environmental aspects of silver are discussed below. 2.3.4 Flux When the flux is heated by applying the hot solder, the rosin may decompose and emit formaldehyde, which is a suspected human carcinogen. Fumes from flux are severely irritating and may cause development of skin and respiratory allergy. Furthermore the solvents evaporate during the process. The solvents are irritating and neurotoxic. Skin contact with flux cause irritation and possibly development of skin allergy. 2.3.5 Solder paste Heating of solder paste will emit fumes of metals and metal oxides, as well as solvents and decomposing products, like formaldehyde. The hazards will be as for the metals and the flux described above. 2.3.6 Cleaning agents In connection with solder technology alcohols like ethanol and isopropanol are used to clean e.g. stencils. High concentrations of alcohols may cause irritation of eyes and respiratory tract. Often, the commonly used ethanol is denatured with mineral spirit. On rare occasions, skin contact with mineral spirit may cause development of skin allergy. 2.3.7 Evaluation of tin/lead solders 2.3.7.1 Working environment During the solder processes fumes of metal and metal oxides may be formed. Vapours and gases may also be emitted to the air in the working environment. These emissions may be inhaled. Especially lead may cause systemic effects, and vapours and gases may irritate the mucosae of eyes and respiratory tract, and possibly cause respiratory diseases with asthma-like symptoms. Skin contact during handling flux and solder paste may irritate the skin and possibly cause development of skin allergy. Repair work implies the risk for inhalation of smoke and vapours from the solder on the PCB’s and the solder thread used when mounting the new component on the PCB. The occupational health risks from the use of traditional tin/lead solder in the electronics production are assessed and discussed in [4]. 2.3.7.2 Consumer As to the use of traditional solder in electronic equipment, it is evaluated that this will cause no nuisance to the consumers. 2.3.7.3 Environment It is evaluated that only the metals, i.e. tin, lead and silver in the traditional solders may have impact on the environment, when electronic equipment is disposed of. The aspects of the metals in the environmental are discussed below. 2.4 Metals in the environment Conductive adhesives predominantly contain silver as the conductive material, whereas the traditional solder contains tin and lead. The environmental fate and toxicity of these three metals have been evaluated in [5] and in order to compare conductive adhesives and traditional solder with respect to environmental effects of their metal contents. 2.4.1 Silver Silver will occur in the environment in the oxidation states 0 and +1. In oxidation state +1, silver will form complexes with oxygen, chloride and sulphate. Even though some of these complexes are mobile, they will form stable complexes with humic components in soil. Thus, silver is estimated to be highly immobile in soil. Lowering pH will increase the mobility of silver. Bioaccumulation of silver is low in the aquatic organisms, but may possibly lead to toxic body burdens. The acute toxicity of silver to freshwater organisms (LC50) is 0.002-0.02 mg/l; the chronic toxicity (NOEC, no observed effect concentration) of silver(+1) is 0.002-0.01 mg/l. Apparently, the acute toxicity to marine organisms is 1-2 orders of magnitude lower than for freshwater organisms; presumably due to the binding of silver ions with chloride ions. A water quality standard for silver ions and soluble silver chloride has been proposed to be 0.0009 mg/l (OECD, 2000). Silver is evaluated to be very toxic for the aquatic environment. 2.4.2 Lead Lead will occur in the environment in the oxidation state +2. Lead precipitates as sulphides, sulphates, carbonates, hydroxides and chloropyromorphite. Sorption of lead in soil is high compared to other metals, and humic substances immobilise lead in most circumstances. Increasing pH will increase the mobility of lead. Lead is significantly accumulated in aquatic organisms. The acute toxicity of lead to aquatic organisms (LC50) is 1.5-40 mg/l; the chronic toxicity (NOEC) of lead is less than 0.3 mg/l (EC50, the concentration that inhibits growth in 50% of the organisms). A water quality standard for lead and lead oxide has been proposed to be 0.0034 mg/l (OECD, 2000). Lead is evaluated to be very toxic for the aquatic environment. 2.4.3 Tin Tin will occur in the environment in the oxidation state +4. Tin of oxidation state +2 is rapidly oxidised to +4. Oxidation increases with pH above 6. Tin may form compounds like oxides, halides, sulphates, phosphates and carbonates in both oxidation states. The mobility of tin is very low. Tin binds to humic substances, and is immobilised. The mobility of tin is very low and is strongly pH dependent No data were available about bioaccumulation of inorganic tin in aquatic organisms. The acute toxicity of tin to aquatic organisms (LC50) is 0.29-50 mg/l; the chronic toxicity (NOEC) of tin(+4) is 0.09-7.8 mg/l. These evaluations are based on the few data that are available. A water quality standard for tin(+4) has been proposed to be 0.02 mg/l (VKI, 2000). Tin is evaluated to be very toxic for the aquatic environment. 2.4.4 Evaluation Silver, lead and tin are immobile at neutral pH in soil containing natural organic matter, and will be located in such layers. The mobility may increase significantly by changes in pH, microbial activity and presence of other contaminants. All three metals are evaluated as very toxic to aquatic organisms. However, the data indicate that silver is much more acute aquatoxic than lead and tin, which are approximately equally toxic. But the question is, if there is any difference when bioavailability is taken into consideration 2.5 Evaluation 2.5.1 Working environment considerations From a working environmental point of view, polyester-based and silicone-based adhesives are to be preferred, because the cured polymers are non-toxic and they contain only a few additional ingredients, including solvent and conductive material. During curing, silicone products will release e.g. methanol or ethanol, which are irritating. Both types of products may release small amounts of other organic solvents included in the formulation. Uncured silicone adhesives contain low molecular weight silanes, which may irritate the skin on direct contact. Epoxy-based and acrylate-based adhesives contain low molecular weight reactive molecules, which pose risks for development of skin allergy. This requires additional worker protection. Acrylate-based adhesives contain only a few more ingredients, including solvents and conductive material. Epoxy-based adhesives also contain amines or amides as hardeners, which may cause development of skin allergy Consequently, from a working environmental point of view, the choice of adhesive should be made according to the following preference:
During the solder processes fumes of metal and metal oxides may be formed. Vapours and gases may also be emitted to the air in the working environment. These emissions may be inhaled. Especially lead may cause systemic effects, and vapours and gases may irritate the mucosae of eyes and respiratory tract, and possibly cause respiratory diseases with asthma-like symptoms. Skin contact during handling flux and solder paste may irritate the skin and possibly cause development of skin allergy. Altogether, the intrinsic occupational hazard for the traditional tin/lead solder technology is evaluated to be higher than for the conductive adhesive technology in most cases. The consequence of this is that metal exposure poses a higher hazard solvent exposure. However, the risk is low for both technologies when adequate preventive measures are taken in the working environment, e.g. enclosures, local exhaust ventilation and personal protection, cf. the report on the occupational health aspects [4]. Amongst the conductive adhesives, epoxy- or acrylate based adhesives pose the largest hazards due to the risk for development of skin allergy. 2.5.2 Consumer considerations Some conductive adhesives may result in minor amounts of high boiling solvents being left in the final product. When they evaporate during use they may cause nuisance to the consumers in the form of indoor climate problems. The risk is estimated to be low because the emitted amounts are small. The use of traditional solder technology in electronic equipment will cause no nuisance to the consumers. 2.5.3 Environment considerations From an environmental point of view, the metals used in joining materials, component terminations and printed circuit boards (PCB’s) are the critical substances. Cured adhesives will have no impact on the environment other than that of the metals. Uncured waste of acrylate- and epoxy-based adhesives may have some environmental impact until cured. Silver, lead and tin are evaluated as very toxic to aquatic organisms. However, the data show that silver is much more toxic than lead and tin, which are approximately equally aquatoxic. All three metals are immobile at neutral pH in soil containing natural organic matter, and will be located in such layers. Furthermore, ions of the three metals may precipitate as insoluble salts with inorganic anions. Thus the question is, if there is any difference when bioavailability is taken into consideration. This question is not easily answered, because the mobility of the metals may increase significantly by changes in pH, microbial activity and presence of other contaminants. Human exposure to the metals through environmental pollution is estimated to be low, because apparently silver, lead and tin are bound firmly to soil components under most conditions. Bioavailable silver may accumulate in aquatic organisms to a low degree and the bioaccumulation of tin is estimated to be low too. Bioavailable lead may accumulate in aquatic organisms in appreciable amounts. Human exposure to the three metals through the food chain is estimated to be low, because of low bioavailability. However, more knowledge about the environmental fate of metals is needed in order to properly assess human exposure via the environment. The life cycle aspects of conductive adhesives and traditional tin/lead solder was also subject for the Kamille II project (c.f. chapter 7). One interesting result from the life cycle study is that the silver layer on component terminals is thicker for components used together with conductive adhesives. It was estimated that the extra amount of silver on the component terminals represents approximately two times the silver coming from a silver-containing adhesive. Changing from traditional tin/lead solder technology to conductive adhesive technology will reduce the lead from the environmental load, but only to some extent because lead is used in components too, e.g. as screening material in cathode ray tubes. The environmental tin load will also be reduced. On the other hand, the silver load will increase. This load may be reduced to some extent, if particles of other metals are used, plated with silver, or even omitted if carbon is used as the conductive material instead. But it must be emphasised that the adhesive technology requires additional silver on the component terminals to secure good joining. 2.6 Conclusions and recommendations From a working environmental point of view, the choice of technology is equivocal. The two technologies impose different hazards, but the occupational risks are minimal when adequate preventive measures are implemented. For the users of electronic equipment, there might be a little emission of VOC’s to the indoor air from electronic equipment manufactured with conductive adhesives. The risk to the consumer is estimated to be very low. From an environmental point of view, it is a question of release of lead and tin, or silver to the environment, unless carbon is used as the conductive material in the adhesives. All the three metals impose environmental hazards, but the actual risks are not easily evaluated. Both for solder and adhesive, recirculation of electronic equipment may reduce environmental release. If the choice falls on conductive adhesive technology, one must choose between the alternatives of polymeric and conductive materials. For the thirteen conductive adhesives included in this project the following orders are preferred based on the evaluations of human toxicity, ecotoxicity and environmental fate. Polymers Conductive materials Consequently, the preferred conductive adhesive would be polyester-based with carbon as conductive material. The technical properties of the participating electrically conductive adhesives have been tested by Delta [6]. The technical aspects of the adhesives must also be considered when choosing. 2.6.1 Other alternatives New technologies are under development, i.e. lead-free solder and conductive polymers as well as other polymers in the conductive adhesives. Conductive polymers make up another interesting possibility. Conductive adhesives may also be based on imide-polymers. However, no commercially available adhesives were identified for the project. It is estimated that this type of adhesive will range along with polyester-based adhesives.
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