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    The six substances are:

     

    • Lead
    • Cadmium
    • Mercury
    • Hexavalent chromium
    • Polybrominated biphenyls or PBB (these are flame retardants)
    • Polybrominated diphenyl ethers or PBDE (this is a family of flame retardants)

     

     

    Since the directive was introduced many exemptions have been requested and over 30 have been granted. These are permitted only under certain specific circumstances, which are:

    If there is no practical alternative material or design or If the substitutes that could be used would have a more negative impact on human health or the environment than the RoHS substance that they would replace.

     

     

     

     

    Hazards of the six RoHS substances

     

    Lead

     

    Lead has been used for thousands of years and its toxicity has been known for most of this time. Lead was used by the Romans to sweeten wine and for water pipes both of which cause lead poisoning with the now well-understood symptoms of blindness, insanity and eventually death. Lead is toxic, a suspect carcinogen and is a mutagen. When ingested or inhaled in small quantities, lead accumulates in the body having a variety of harmful effects. At fairly low levels it detrimentally affects the learning ability of children and causes a reduction in IQ. This was noticed in the USA and Europe in children that lived or attended school near busy roads and who were exposed to lead which was at the time used as an additive in petrol.

     

    Lead poisoning has many symptoms as it affects many essential bodily functions and every organ in the human body. Poisoning has been caused by ingestion of flakes of paint that use lead as a pigment, inhalation of fumes from industrial processes such as metal refining and recycling and from lead dissolved into drinking water from plumbing. Lead water pipes and lead solders have been used for plumbing for many years and are quite safe with hard waters, which deposit a protective layer of lime-scale, but this does not form in soft waters (as in ancient Rome) and so lead dissolves and is ingested.

     

    Industrial processes carried out in Europe and in the USA are strictly controlled by legislation so that workers and local populations are not harmed by lead. Very effective fume extraction and filtration is used and dusts are prevented from escaping by water sprays, washing the wheels of trucks, etc. Workers in US and European factories now rarely have raised blood lead levels because of these measures but this is not the situation in some parts of China, India and Africa where lead refining and recycling is frequently carried out without these types of control measures.

     

    Some examples illustrate these issues.

     

    In Senegal, villagers have been recycling car batteries for years so that large quantities of dust (lead sand) cover everything in their villages. Children are most at risk from inhalation of lead dust and with many falling seriously and in some cases, dying.

     

    There is evidence that uncontrolled car battery recycling is also carried out in India causing illness to workers. Blood lead levels are raised, especially in children in areas where uncontrolled recycling of electrical equipment is carried out in India, China and other countries in Asia and also in Africa. Levels of up to 35 micrograms / dl have been measured whereas 10 micrograms / dl is enough to cause learning disorders.

     

    Blood lead levels are raised from a variety of causes. Ingestion of flakes of lead paint can be a significant cause as well as from industrial pollution of air and from drinking water polluted by wastes from uncontrolled refining and recycling processes.

     

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    Image courtesy of the Basel Action Network (BAN)

     

     

    Cadmium

     

    Cadmium is very toxic and has caused the death of workers and serious illness from lower level ingestion and inhalation. The main effect of cadmium poisoning is kidney damage and renal failure. At higher doses it causes damage to the respiratory system and bone disease. Cadmium is also a carcinogen and mutagen.

     

    Cadmium is normally present at low concentrations in soil and is taken up by plants, so is present in food. Normally the amount present is well below the World Health Organisation (WHO) safe daily ingestion limit but there have been instances where soil has been contamination by cadmium from industrial pollution and fertilisers and has caused serious cadmium poisoning.

     

    Cadmium poisoning resulting in deaths has occurred in Europe and the USA with the more common causes being brazing of cadmium alloys without fume extraction, metal refining and from dusty nickel cadmium battery production (in Sweden). Hygiene in Europe and the USA is now strictly controlled and so cadmium poisoning is now extremely rare. Deaths and illness however have occurred more recently in Asia caused by dusty nickel cadmium battery production (in China) and from electrical equipment recycling although the effects of cadmium from recycling may be less apparent due to the effects of lead poisoning.

     

     

    Mercury

     

    Mercury is also very toxic, a suspect carcinogen and a mutagen. Only very small amounts are used in electrical equipment and so toxic effects are far less likely than lead and cadmium. However, mercury is sometimes used to recover gold from ores and from scrap electrical equipment. Gold dissolves in mercury and is recovered by boiling away the mercury. Mercury vapour is inhaled and collects in soil and water and affects not only workers but also surrounding populations. This has been a serious problem in some parts of South America where illegal gold mining operations were carried out causing illness and deaths from mercury poisoning, although more recent research has suggested that deforestation is also to blame.

     

     

    Hexavalent chromium

     

    Hexavalent chromium is toxic and a known carcinogen when inhaled. It may also be carcinogenic by ingestion but this is less certain. The main risk from hexavalent chromium is from processes that use solutions containing these compounds (chromates) that are used for hard chromium plating and metal passivation.

     

    Hexavalent chromium is also used to treat boiler water to stop corrosion. Workers exposed to chromates are clearly at risk as are local populations if wastes are not treated properly and reach water supplies. Drinking water with even small concentrations causes serious illness and death; the film “Erin Brockovitch” is a true story of an incident in USA where untreated boiler water was dumped and contaminated the water supply. This danger exists anywhere that chromates are used and waste is not correctly treated.

     

     

    Polybrominated biphenyls (PBB)

     

    Polybrominated biphenyls are toxic but there are few reported cases of serious illness. The most well-known incident occurred in Michigan USA where a sack of PBB was used in error instead of a cattle feed additive. Many cattle became ill and some died as well as other animals, but people who consumed contaminated milk and other products also became ill. This type of incident is fortunately extremely unlikely to occur as PBB is now made only in one factory, in China. PBB has been used as a flame redardant, mainly in high voltage power cables.

     

    If the copper is recovered by burning away the plastics on fires, the PBB decomposes to form dioxins and furans which are both very toxic and carcinogenic. However burning most types of plastics on fires is dangerous and all types of plastics can produce dangerous by-products.

     

     

    Polybrominated diphenyl ethers (PBDE)

     

    PBDE is a family of flame retardants some of which are harmful substances although no illness or environmental harm has been directly attributed to these chemicals. Some, such as pentabromodiphenyl ether are clearly harmful substances whereas extensive testing of Decabromodiphenyl ether has shown that this is not harmful to humans and may not have caused harm to the environment. As with PBB and many other plastics and additives, uncontrolled burning of PBDEs can emit dangerous furans and dioxins.

     

     

    Substitutes

     

    Clearly most of the RoHS substances are hazardous but the choice of substitutes needs to be carefully considered as some will be no better and some may be worse. Lead-free solders, based on tin and silver, are clearly less toxic to humans than lead but silver has a considerable environmental impact, for example, silver is toxic to some aquatic organisms, and cyanide is used in silver refining and has caused serious environmental harm as well as deaths of workers from cyanide poisoning.

     

    In the EU mercury is permitted in lamps because fluorescent lamps use far less energy than incandescent lamps.

     

    Replacement of PBB and PBDE needs to be carefully considered. Clearly flame-retardants save many thousands of lives and so should be used but the uncontrolled burning of all types of plastics, with or without brominated flame retardants, emit toxic and carcinogenic substances and so is inadvisable.

     

     

    Uses of the six RoHS substances

     

    The tables below list the main uses of the six RoHS substances in electrical and electronic equipment and show where they are likely to be found:

     

     

     

    SubstanceMain uses
    LeadSolders, termination coatings, paints, pigments, driers, PVC, stabilizers
    CadmiumCoatings, galvanized steel, solders (low melting point), semiconductors, PVC stabilizers, pigments, photocells
    MercuryFluorescent lamps, sensors, relays
    Hexavalent ChromiumCoatings to prevent corrosion (on zinc or aluminium or in paints)
    PBBFlame retardant in certain plastics
    PBDEFlame retardant in many types of plastics


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    Implications of RoHS substance restrictions

     

    There are many implications from restricting the six RoHS substances, some are clearly beneficial but others will cause technical difficulties.

     

     

    Possible Benefits

     

    The main advantage is that workers will not be exposed to these substances. Of course RoHS only restricts substances present in products, not those used to make equipment and so hexavalent chromium may continue to be used in production processes, as long as it is not present in products.

     

    One of the original aims of RoHS was to eliminate these hazardous substances from electrical “scrap” making recycling easier and safer. However, older equipment containing these substances is still reaching end of life and will continue to do so for years to come.

     

    Also, many exemptions are necessary where no substitutes are available and so newer scrap RoHS compliant equipment may also contain some of these substances. People in Asia and Africa are often affected by some of the RoHS materials, especially lead from other sources such as car battery recycling and from lead-based paints. There clearly could be benefits from RoHS but these will be limited if other sources of toxic substances are not also effectively controlled.

     

     

    Substitution of the six RoHS substances is in many cases not straightforward, especially lead. Many uses of lead have no alternatives such as in some types of glass and ceramics and so exemptions are permitted. Lead based solders have however been replaced by lead-free solder alloys but these are not drop- in replacements.

     

     

    Technical Issues

     

    Production: There is no single drop-in replacement alloy for tin/ lead solder. All of the lead-free alloys developed as replacements are “different” in various technically important ways that can affect the ability to manufacture and the reliability of products.

     

    The main issues with lead-free solders are outlined below:

     

    Lead-free solders melt at 30 - 40°C higher than tin/lead.

     

    This has many implications such as:

     

    • It may be necessary to buy new production equipment
    • Some components will be damaged by the higher temperature and so cannot be used (e.g. larger surface mount electrolytic capacitors)
    • The higher temperature can damage Printed Circuit Boards causing delamination, barrel cracking and other board defects. The higher temperature also causes more warping during reflow which creates a variety of problems.

    Lead-free solders wet surface less well and so can affect reliability if this is not resolved (smaller bond areas & weaker bonds).

     

    Lead-free solders are more corrosive when molten and so cannot be used in older wave soldering machines that were designed for tin/lead.

     

    Rework is possible, but more difficult, due to the inferior wetting characteristics but also because component removal in solder pots can completely dissolve copper tracks and pads creating open-circuits

     

     

    Corrosion

     

    Lead-free solder is resistant to corrosion but it wets surfaces less well and uncoated metals may corrode. Corrosion of PCBs and component terminations can be a significant problem in industrial atmospheres, especially if sulphide and chloride are present. This is particularly a problem with electroless nickel – immersion gold (ENIG) coated PCBs and gold plated components because the gold is much nobler than copper and creates a galvanic cell so that the copper pads, tracks and terminals can be “eaten” though completely by corrosion.

     

    Thermal fatigue

     

    Tin/lead solder is susceptible to thermal fatigue but this is well-understood from decades of experience. Thermal fatigue occurs as a result of repeated temperature changes that cause cyclic strain on solder joints, which eventually crack and fail. The thermal fatigue behaviour of lead-free solders is not yet fully understood as there is no long-term field data due to the fact that the alloys are still relatively new.

     

    Accelerated testing has indicated that under low strain conditions, lead-free alloys may be superior to tin/lead but when the strain is high, lead- free solders give inferior test results. This is however a complex subject which is still being researched and although early indications are that lead-free solders will probably be reliable, this is still not yet certain and the differences in their behaviour need to be fully understood when designing equipment.

     

    Tin Whiskers

     

    The last potential problem with “lead-free” is a failure mode referred to as tin whiskers. This does not affect the solder itself but affects the lead-free component termination coatings, most commonly tin.

     

    Tin whiskers form spontaneously as very thin filaments that grow from tin plated coatings that are subjected to compressive stress. Most tin whiskers are quite short but they can be several millimetres long and cause short circuits. This was a problem many years ago that was resolved by adding lead to the tin plated coatings but RoHS prevents this from being used.

     

    Considerable research has been carried out to understand why whiskers form and to develop strategies that minimise the risk.

     

    New tin electroplating formulations have been developed that are “whisker resistant” but must be used under strictly controlled guidelines. The most useful guidance has been published by iNEMI which explains which measures should be taken. This includes using only certain types of coatings and not others. For example, 2 micron matte tin over a non-porous nickel barrier coating on copper lead-frames has a very low risk whereas tin-copper alloy coatings and tin coatings on low TCE (thermal coefficient of expansion) alloy lead-frames have a higher risk of whisker formation. High humidity can also promote whisker growth.

     

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    SEM image of tin whiskers taken by ERA Technology  Ltd

     

     

    Conclusions

     

    Many countries around the world are considering whether to adopt RoHS legislation. India has just agreed on the final text for their version of RoHS legislation and this will come into effect as of 1st May 2012. However,as a result of EU-RoHS, manufacturers in India were already producing lead-free RoHS compliant products. Any equipment made in India for the European market is RoHS compliant, irrespective of where it is sold. Australia has found that a large proportion of equipment sold in its borders is already RoHS compliant, allowing it to be shipped freely to the EU, and so legislation may not be necessary. India is however a very large market for electrical equipment with many products being produced that are not sold in Europe and so RoHS legislation will be vital in encouraging changes to eliminate the six hazardous substances.

     

    The majority of the six substances are harmful to human health and the environment and so, in theory, their elimination should be beneficial. However, it will be necessary to ensure that there are safer substitutes (considering the whole life cycle of the alternative materials) and that other sources of these materials, particularly lead, that are causing poisoning are also controlled effectively. In Europe and the USA, lead in solders does not cause harm because its use is strictly controlled and this would be an alternative option to restrictions.

     

    Design and manufacture of RoHS compliant equipment is far from straightforward. Except for the most basic products, it will not be possible simply to switch solder alloys. Where RoHS compliant alternative components are not available, re-design may be necessary. Production with lead-free solders is more difficult than with tin/lead requiring skill and research to determine optimum conditions, and new equipment may also be required. There are also several long-term reliability issues that are not yet fully understood. The main concerns are thermal fatigue and tin whiskers which potentially could affect equipment that is in use for periods longer than ten years. It is possible to mitigate against these risks but the causes need to be understood by design engineers as well as by following the industry recommended mitigation measures.

     

     

     

     

     

     

     

     

    Please note:

    The information contained in this guide is of a general nature and is not intended to address the circumstances of any particular individual or entity. Although we endeavour to provide accurate and timely information, there can be no guarantee that such information is accurate as of the date it is received or that it will continue to be accurate in the future. No-one should act on such information without appropriate professional advice after a thorough examination of the particular situation.

     

    ©2011 Premier Farnell plc. Permission is granted for reproduction in whole or in part providing Premier Farnell plc is credited. Written in collaboration with ERA Technology Ltd (www.era.co.uk)
    June 2011

     

     

     

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