The European Union Directive on the Restriction of Hazardous Substances (RoHS) - Alternative Materials
P D Goodman, ERA Technology Ltd., Cleeve Road, Leatherhead, Surrey, KT22 7SA, UK, +44 (0)1372 367000, paul.goodman@era.co.uk
ABSTRACT
The European Union intends to introduce the RoHS directive which will require manufacturers of electrical and electronic equipment, sold within the EU area, to stop using six hazardous substances in their products. Manufacturers will need to identify suitable alternative materials to these, except where exemptions apply. The six materials are currently used for a wide variety of applications and in many different products. Lead in solders is well known and much has been published on solders containing lead and lead-free alternatives. However lead is used for many other applications. Materials suppliers have known for some time that the use of these six materials will be restricted and have been working on developing alternatives with mixed success. For many applications, alternatives will be available but there are some products where at present there are no suitable drop-in replacements.
1. Introduction
The European Union originally proposed to introduce a directive that would force manufacturers (or importers to the EU) to recycle their electrical and electronic equipment at the end of its life and also to ban six hazardous substances. This was the WEEE (waste electrical and electronic equipment) directive. However, there are now two separate directives, WEEE will enforce recycling but the substances ban is a separate directive. This directive will ban six hazardous materials from a wide range of electrical and electronic equipment and is the RoHS directive (Restriction of the use of certain Hazardous Substances) and if it is accepted by the European Parliament, it will come into force on the 1st January 2006.1,2 The banned substances are:
- Lead
- Cadmium
- Mercury
- Hexavalent chromium
- Polybrominated biphenyls (PBB)
- Polybrominated diphenyl ethers (PBDE)
The directive applies to equipment sold within the European Union countries irrespective of where they are manufactured. Therefore equipment manufactured in the USA and other countries outside of Europe will have to comply in order to be sold within the EU. Many US and Far East companies have been working on alternative products based on alternatives to these six substances for several years so that by the time the directive comes into force, alternative products will be available in most cases.
The directive includes most consumer electronics and office equipment such as telecommunications and IT equipment, TV sets, videos, and refrigerators. Things like toasters, cameras, lamps, drills, toys and electric toothbrushes are all included and certain medical equipment, tools and automated drink dispensers are also on the list. Equipment used in aeroplanes or for military applications and industrial test equipment are not included.
There are exemptions for components and materials, usually where no suitable alternative exists such as high melting point lead based solders. However Cadmium Mercury Telluride infra-red detectors, which have unique characteristics, are not exempt.
Alternatives for the hazardous materials are increasingly becoming available as materials suppliers develop alternative products but in many cases, there is no single drop-in replacement and the performance of the alternatives are not identical. The processes and facilities that are used to manufacture equipment may need to change quite significantly, which could involve significant investment in new equipment. Examples of issues involved for the more common applications of the six hazardous materials are summarised here.
2. Lead (Pb)
Most discussion and development work has focussed on replacements for lead in solders as this has been perceived as the most difficult, and costly material to replace. Solders based on tin and lead have been used for centuries. Conventional solder alloys are ideally suited for making electrical connections as they have a suitably low melting point, wet metal surfaces well to form strong bonds and are ductile metals. The main use of lead in electronics is in solders which are not only used for making electrical connections to components but also have a variety of other applications. These include:
- Circuit board coatings to maintain solderability – there are several good alternatives available
- Coatings for component leads and terminations – several alternatives such as tin and palladium/nickel
- Etch resist during PCB manufacture - the main alternative is tin
- Die attach solder – high m.pt solders are exempt
- Filament light bulbs – lead in glass and in solder
- Fusible links – low melting temperature alloys
Lead is also found in some piezoelectric materials, dielectrics and glasses however these are likely to be exempt from the RoHS directive. It is interesting to note that some lamps made with lead-free glass have recently been introduced. Matsushita manufactures a new fluorescent lamp made with lead free glass but it contains mercury. Some other lamps made with lead-free glass use lead based solders for electrical connections.
The situation is complicated however by the WEEE directive as lead will have to be separated from end of life equipment during recycling and any lead containing waste will be classified as hazardous. Some manufacturers may wish to avoid lead completely to promote their products as lead free but there are few alternatives for some of these materials.
Lead is also used in lead acid batteries and some older type PVC stabilisers.
Lead-free solders
A great deal of work has been carried out and published on lead-free solder alloys3 but many users are still unclear as to which alloy is the most suitable alternative, how it should be used and what impact it will have on their products. Unfortunately, there is no single drop-in replacement and all of the alternatives have differences in their behaviour to the standard tin lead solder. Table 1 gives some of the more commonly used alternatives with their melting temperature. 63Sn37Pb is also given for comparison.
Replacing lead is not straightforward. The reason becomes clear if one looks at the periodic table of the elements. Once the non-metals, toxic, radioactive, very reactive, rare and high melting point elements are ruled out, there is very little left. Tin is the obvious choice. Zinc is a possibility but is a little too reactive. Indium and bismuth have low melting points but are available in limited quantities, particularly indium. Bismuth can also cause difficulties when WEEE is recycled. This leaves metals with somewhat higher melting points than would be ideal and the main choices are copper and silver which melt at 1083°C and 960°C, respectively.
The most widely used lead-free solders (which contain copper, silver or both) have melting temperatures that are 30 – 40°C higher than tin lead. This has implications with heat sensitive components such as chip capacitors, LEDs as well as some ICs.4 Also, lead-free solders wet metal surfaces less well than tin lead; this combined with the different physical properties results in the possibility of different long term behaviour for example the thermal fatigue performance although several lead free alloys are reported to be superior to tin lead.5 The assembly equipment, reflow ovens and wave solder machines, may need to be changed to accommodate the higher temperatures needed and improved temperature control and a nitrogen environment might also be required to minimise the reflow temperature to protect heat sensitive components.
These changes will increase manufacturing costs. Lead free solders are more expensive, for example Sn3.5Ag0.7Cu alloy is about 2.5 times more expensive than 63Sn37Pb for equal volumes of metal. As well as the cost of new reflow ovens, it is estimated that up to 20% more energy will be required for the higher melting point alloys and there are indications that production rates may be reduced although this is still uncertain and further research needs to be done.
There is a problem with high melting point solders based on lead. Almost all lead free solders are based on tin; it has not been possible to identify low cost ductile alloys that are lead-free and do not contain a major proportion of reactive metals like zinc or aluminium. The highest melting temperature alloy that can be obtained with tin as the major component is about 230°C and this is too low for some applications such as light bulbs that can operate at over well 150°C and die attach solders for power ICs. Therefore these alloys are exempt from the RoHS directive. The only viable lead-free solder alternative is to use gold alloys such as 80Au20Sn, which melts at 280°C. This is used as a semiconductor die attach material although in a typical IC, only 2mg of gold may be used.
Alternative PCB Coatings
Immediately after a printed circuit board has been manufactured, the copper pads are clean and oxide free but during storage, the surface rapidly oxidises and it becomes impossible for solder to wet the surface without the use of acid etchants or corrosive fluxes. Traditionally, PCBs were dipped into a bath of liquid solder which coats the exposed copper with a layer of solder. This is called hot air solder level (HASL) but is not viable with lead-free solders as the higher temperature causes too much damage to the PCB. A variety of board coatings have been developed as alternatives to HASL coatings to maintain the solder wetting properties of the boards during storage. These include organic solderability preservatives (OSP), nickel/gold, silver and tin. The main differences between these are cost, degree of protection and complexity of the processes involved. OSP is the lowest cost but coatings are very thin and easily damaged. Nickel/gold is regarded as the best but is also the most expensive however it does have several technical advantages over the alternatives and is widely used, for example on mobile phone PCBs.
Component lead coatings
A variety of alternatives have been developed including tin which is used by Philips Semiconductors, tin alloys and palladiumbased coatings. There are issues with tin whiskers from the electroplated tin coatings although some manufacturers claim that tin can be used without difficulties as long as precautions are taken. Electroplating tin lead is a well established process but electroplating other tin alloys such as tin copper or tin silver is more difficult as there is a much larger difference between the electrode potentials of these elements than tin and lead and so control of the alloy composition is difficult.
Palladium/nickel, sometimes with a gold flash is used on some semiconductor leadframes because solder wetting properties are good and wire bonds from the semiconductor chip can be made onto the palladium/nickel surface. Texas Instruments uses this for some components but the high price of palladium will limit its use.
3. Cadmium (Cd)
Cadmium’s toxicity is well known but it has many uses in electronics, some of which are difficult to replace.
Coatings
Cadmium electroplated coatings are used to provide corrosion resistance. Cadmium is particularly effective because as a coating, it acts as a sacrificial anode to the underlying metal. Where scratches or gaps in the cadmium coating occur, a galvanic cell is set up and as the cadmium corrodes it protects the base metal which is usually steel. Other metals such as zinc (as in galvanised steel) also protect in this way but cadmium produces a corrosion product that has only one tenth the volume of that from zinc and so removal of old corroded cadmium plated bolts is much easier than galvanised parts. Cadmium is used where long term reliability is required, particularly in aerospace, marine and military applications. There are many possible alternatives to cadmium coatings but performance and characteristics are very different and careful research is needed to make the right choice. Alternatives to be considered include zinc, nickel, silver, tin or alloys such as Zn/Ni, Zn/Sn or Ni/Fe all of which can be deposited by electroplating.6 Cadmium and some of its alternatives are chromate treated to obtain good corrosion resistance but chromate will also be banned. In most cases, cadmium plating is used for applications that are not covered by the RoHS directive and so the directive may have only a small effect on the quantity of cadmium plating used for electrical equipment.
Batteries
It is unclear at present whether batteries as consumables will be included in the RoHS directive but manufacturers may wish to find alternatives for a “green” image. Nickel cadmium batteries are widely used and have technical advantages over lead acid batteries although they are 2 – 3 times more expensive. There are many different types of batteries available today and the choice of an alternative to lead acid or nickel cadmium will depend on the application.7 Some characteristics of the main types of rechargeable batteries are listed in Table 2.
Considerations when choosing a battery type are the operating temperature range, life expectancy, discharge requirements, recharge frequency and depth of discharge between charges. Cost is also a consideration. At present, lithium ion batteries are more expensive than nickel cadmium and require special chargers. However the materials used in these cells are not costly and so the price will inevitably come down. They are already used in some mobile phones and portable computers.
Other uses
Switch contacts where arcing occurs are usually made from silver with cadmium oxide. Contacts made from silver with tin oxide are now available but this material has some limitations in its characteristics and contacts containing cadmium oxide are still widely used.
Other uses of cadmium that may be encountered include low melting point solders, some red/yellow pigments, CdTe solar cells are commercially available from Japan, HgCdTe infra red detectors, additions to some brazing alloys and in photocells. Cadmium was used in thick film inks but the major manufacturers’ products are now cadmium-free. Some plastics imported from the Far East have been found with small amounts of cadmium.
4. Mercury (Hg)
One of the most important uses of mercury is in certain types of lamps. Recently a few manufacturers have introduced a few types of new mercury free lamps. However, fluorescent tubes do contain small amounts of mercury and this is essential for reliable operation and as no alternative currently exists, these are exempt. The maximum amount of mercury permitted under the directive is regarded by Orgalime8 as being too low for some applications. If too little mercury is used in fluorescent lamps, the working lifetime will be reduced.
Mercury has also been used in a wide variety of types of switches including tilt switches and thermostats. Mercury is a unique material as it is the only metal that is liquid at room temperature that does not wet glass, ceramic or plastic surfaces. It also does not oxidise at room temperature. Gallium alloys and sodium/potassium alloys are liquid at room temperature but the gallium alloys wet all surfaces and sodium/potassium is very reactive, and can explode on contact with water. When two switch contacts meet, the actual contact area is extremely small as the surfaces are never mirror flat and perfectly parallel. As a result, the contact resistance can be fairly high and this causes locallised heating that can cause oxidation of the surface leading to an increase in contact resistance. In mercury switches, liquid mercury makes the contact so that the contact area is relatively large.
Suitable alternatives to all types of mercury switches and sensors are now available but these will have differences in their switching performance that may be important with some applications. Mercury tilt switches give bounce free switching with a quick break even when tilted slowly. The contact resistance of mercury switches can be lower than with other types.
Batteries used to be a major user of mercury. Mercury oxide batteries were widely used before 1980 but have now been discontinued. Until more recently small quantities of mercury were used in alkaline batteries but most manufacturers now sell only mercury free products except for a few special types.
5. Hexavalent Chromium (Cr (VI))
Chromium occurs in three stable forms. As the metal, as trivalent chromium (Cr III) which is not toxic and is an essential trace element in the diet and as hexavalent Cr (Cr VI) which is carcinogenic if inhaled and probably also if ingested. Only Cr (VI) is to be banned by the RoHS directive.
Electroplated Chromium
Some parts of electrical equipment may be chromium electroplated. Chromium is used as it has a bright decorative appearance and also because it has good wear properties. Chromium metal itself is acceptable in electrical equipment but it is usually made by electroplating parts in a solution containing hexavalent chromium (CrVI), usually chromic acid. One alternative is to electroplate chromium from trivalent chromium plating baths. The chromium from these, however, has a different appearance and the wear properties are inferior and so is not widely used. Alternative metals can be electroplated to resemble chromium such as nickel and nickel and cobalt alloys such as Co - W.
Conversion Coatings
Cr (VI) based solutions are also used to produce corrosion resistant coatings on metals and to aid adhesion of paint coatings to metals. These are known as conversion coatings because they convert a metal surface into something different. These are widely used on aluminium, steel and galvanised products but may occasionally be used on copper and other metals. Cr (VI) is used because of its unique properties as an excellent corrosion inhibitor. Metal parts in electrical equipment such as steel chassis and aluminium heat sinks may be treated with Cr (VI). The treatment process leaves a very thin layer of a water-insoluble coating which contains hexavalent chromium. The exact chemical form is usually not known but is generally a mixed oxide of the base metal and Cr (VI). Hexavalent chromium is used because it is one of the best corrosion inhibitors known and it is effective for long periods even as a very thin transparent film.
There is no single alternative conversion coating that can replace chromate.9 Each application will be different. In general it is possible to find a chromate-free conversion coating for most metals where the metal is subsequently painted, however, for protecting bare metal, in most cases the alternatives will have inferior performance.
Metallised Plastic
ABS and some other plastics can be coated with metals (usually copper or nickel) using electroless plating. Subsequently the metallised surface can be electroplated with a wide range of metals. One of the process steps involves etching the plastic with chromic acid (contains Cr (VI)) to give good metal adhesion. Until recently no alternatives to chromate etchants for electroless plating of plastics were available but recently a few plating chemical suppliers have introduced chromate-free processes for some plastics.
6. PBB and PBDE flame retardants
The situation with PBB and PBDE is far from clear. PBB is polybrominated biphenyl. Manufacture of this flame retardant ceased in 2000 but some older components, resistors and tantalum capacitors, may contain PBB.
PBDEs are polybrominated diphenyl ethers and cover several products that differ in the number of bromine atoms in the compound. Originally, four products were made with four (tetra-), five (penta-), eight (octa-) and ten (deca-) bromine atoms. Manufacture of the tetra compound has ceased and the penta- compound is not normally found in electrical or electronic equipment (it is used in polyurethane foam) but the octa- and particularly the deca- compounds are widely used. It is clear that PBBs, tetra diphenyl ether and pentadiphenyl ether are harmful substances but there is uncertainty whether Deca and Octa- BDEs are harmful at all.10 Data obtained some years ago which suggested that they may be harmful was carried out with less pure materials which contained tetra- and penta-DBET. The brominated flame retardant manufacturers have argued that the Octa- and Deca- compounds should not be banned as there is no evidence for any harmful effects or that they degrade to harmful compounds. The EU stated that a risk assessment would be carried out but this is long overdue and may never be carried out. At the present time all PBDEs will be banned.
The WEEE directive complicates these issues further. Although brominated flame retardants other than PBB and PBDE are permitted, the WEEE directive states that any plastics with brominated flame retardants must be separated and disposed of separately. This will add to the cost of recycling and as a result discourage the use of these compounds.
Main Uses
Octa and Deca-DBEs use in electrical equipment has declined over the last decade as plastic producers have switched to alternative flame retardants but they are still widely used as these are low cost and very effective flame retardants. The major uses of Octa and Deca-DBEs in electrical equipment are as flame retardants in HIPS (High Impact Polystyrene), ABS, wire and cable insulation and connectors. Components where PBDEs could occur include, although this is by no means an exhaustive list: ABS moulded parts, some types of epoxy and phenolic PCBs, acrylic and PBT connectors, polypropylene parts, HIPS TV cabinets and computer covers, polyethylene and PVC wire and cable insulation.
Alternatives
Finding an alternative flame retardant is a rather complex subject. There are many hundreds of different flame retardants each of which is suitable for certain applications. Flame retardants function in several ways. Brominated flame retardants are particularly effective as they act as free radical scavengers and so can extinguish flames by interfereing with the chemical reactions that occur. Other types such as alumina trihydrate adsorb heat and produce steam both of which cool a fire however, high loadings, normally >50% are needed to be effective. Flame retardancy obtained by each material varies considerably and so changing from PBDE to another type may require the use of considerably more of the chosen alternative in order to achieve the same level of fire safety. However, higher loadings of some fire retardants can be deleterious to the physical and electrical properties of the plastics as well as increasing the cost. New bromine-free (and some also phosphorous-free) flame retarded plastics including HIPS and epoxy laminate are now commercially available but bromine free epoxy laminates are more expensive. Fire safety and physical properties of any alternatives need to be assessed before making a decision to change.
The health hazards from Octa-DBE and Deca-DBE are at present uncertain although EBFRIP (European Brominated Flame Retardant Industry Panel) believe that there is currently no unambiguous evidence that they are harmful. Also, there is no evidence that they decompose after disposal to harmful compounds. Many of the alternative flame retardants have not been as thoroughly studied and some of the phosphorous-based alternatives are classified as harmful.
7. Conclusions
Six “hazardous” substances will be banned from use in electronics and electrical equipment by the RoHS directive. However, there are exemptions and still some argument over the final form of the legislation.
Alternatives for the majority of applications now exist although these nearly always are different in some way. In most instances, no drop-in replacements exist for the hazardous materials and so careful examination of all of the alternatives is needed and often a compromise in performance or cost will have to be accepted. The most appropriate alternatives are listed in Table 3.
The RoHS directive will create a great deal of work for manufacturers of electronic and electrical equipment to locate where these hazardous materials are used, identify suitable alternatives and optimise production processes for these new materials. Some manufacturers may decide to employing consultants such as ERA Technology to help with these changes. It is anticipated that the major effort will be to replace lead containing solders. Alternative solder alloys will require manufacturers to evaluate a large number of solders and solder pastes to identify the best products for their PCBs and to optimise the reflow profiles in order to maintain their product reliability. Chemical suppliers are already working hard to develop alternatives to their products based on hexavalent chromium but will undoubtedly pass on these costs wherever possible. Cadmium free contacts, and mercury-free switches are already available although with differences in their characteristics. Plastics with alternative flame retardants are also available but will not be identical to those made with PBDE and may be more expensive.
There will be other cost implications. Lead-free solders are more expensive than solders containing lead. Also, new reflow ovens may be needed and the cost of components that can withstand higher temperatures and have lead-free solderable terminations will probably be higher. The best alternatives for some applications are at present inferior to products based on the six hazardous substances. This is the case for example, with hexavalent chromium conversion coatings for unpainted metal.
Although not dicussed in detail in this paper, the WEEE directive will also impact on what materials are used in the manufacture of electrical equipment. For example, even though lead is exempt in RoHS for certain applications, any lead arising from recycling will need to be disposed of. Bismuth in scrap causes difficulties to some recyclers if it is used as an alternative to lead and so will add to the end of life cost.
References
- Proposal for a Directive of the European Parliament and of the Council on Waste Electrical and Electronic Equipment. Proposal for a Directive of the European Parliament and of the Council on the Restriction of the use of certain Hazardous Substances in Electrical and Electronic Equipment. Commission of the European Communities, Brussels, 13.6.2000, 2000/0158 (COD) and 2000/0159(COD)
- ENDS Report issue 1183, 21st March 2002
- M. Abtew and G. Selvaduray, Materials Science and Engineering, 27 95 – 141, 2000
- A. Hawes and T Adams, Electronic Production, p 14 – 15, April 2001
- J. S. Hwang, Global SMT and Packaging, 1(3) 10 – 13, 2001
- E. W. Brooman, D. A. Schario, and M. L.Klingenberg, Proceedings of the Symposium on Environmental Aspects of Electrochemical Technology: Applications in Electronics. Pennington, NJ, USA: Electrochem. Soc, 1997. p.219-35 of vii+296 pp. 68 refs. Conference: San Antonio, TX, USA, 7-8 Oct 1996 ISBN: 1-56677-171-4
- R. M. Dell, Solid State Ionics 134 139 - 158, 2000
- Detailed Position of ORGALIME’S Electrical and Electronic Liaison Committee (EELC) in Cooperation with European Sector Committees, 5th Sept. 2000. Orgalime Liaison Group, Brussels
- E. Eichinger, Metal Finishing, 95 (3) 36, 1997
- Comments on the European Commissions Proposals on Waste Electrical and Electronic Equipment, October 2000, European Brominated Flame Retardant Industry Panel
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