Towards LEAD (Pb) Free Electronics





Technology, 17 Aug - 2017 ,

Towards LEAD (Pb) Free Electronics
Credit: pixabay.com

Lead is not a problem when contained in electronic equipment, however, when electronic components are deposited in landfills, people may scavenge for equipment and break it open, or the lead may leach out of landfills and into drinking

Indispensible lead

Lead is not a problem when contained in electronic equipment, however, when electronic components are deposited in landfills, people may scavenge for equipment and break it open, or the lead may leach out of landfills and into drinking water. The risk is compounded in countries that receive massive imports of electronic waste.  Use of electrical and electronic products in the society all over the world is growing alarmingly fast, which for a large part end up as Waste Electrical and Electronic Equipment (WEEE) when new and cooler gadgets become available in due course of time. This means that the amount of WEEE is constantly increasing worldwide. However, some countries have developed expertise to recycle and re-use WEEE in the production of new equipment. Most smartphones and other electrical or electronic products contain small amounts of lead, which doesn't sound like a big problem on its own. But when there are many billions of such products, either in daily use or gone astray, the total sums up to very large amounts of lead -- which is a toxic heavy metal. Moreover, lead is often used in materials that are piezoelectric, which means that they consist of crystals that produce an electrical voltage when pressure is applied. Therefore, these materials are widely used as pressure sensors. The reverse function is also important: if we apply an electrical voltage to such materials, they will expand and for instance give a tiny engine that can move very small things around. The most extensive use of piezoelectric and lead-based material occurs in transmitters and receivers, where the ceramic compound lead zirconate titanate (also called PZT) has been difficult to replace. PZT contains approximately 60 per cent lead by weight.  Therefore, the environmental authorities in several countries have agreed to limit the use of lead in electrical and electronic equipment. Products must contain no more than 0.1 per cent by weight of lead in order to be approved for environmental friendly electronics marking.  But there are always exceptions, especially when there are no alternative materials to be found.

Lead poisoning

Lead poisoning can cause a variety of symptoms and it has even been claimed that lead poisoning caused the fall of the ancient Roman Empire. The Romans used lead compounds in their aqueducts, and lead acetate -- also called Sugar of lead -- was even used as a sweetener in wine. As we have become completely dependent on the production of the electronic products,  the problems with lead pollution are much greater. The problem with lead is that the element is absorbed into the body and displaces other chemical substances in such a way that important biological functions are disturbed. Children and pregnant women are especially vulnerable, because lead is easily taken up in bones when growing. Lead exposure, even at low levels, is well known for its harmful effects in general and on children in particular.

The linchpin of electronics

Solder is the linchpin of electronics manufacturing and lead has been ideal for solder, without which, it’s difficult to achieve a proper electronic connection that is durable and reliable. The whole electronics infrastructure was designed around the melting point and physical properties of lead. Lead is malleable and thus easy to work with, and it doesn’t fracture. When lead is combined with tin in the correct proportion (63% tin to 37% lead), the resulting alloy has a low melting point of 183°C, which is another advantage. If not operating at really high temperatures, we have more control over processes, so that the processes aren’t sensitive to slight temperature variations, which are costly to control. Low temperatures also mean less strain on the equipment and materials (such as printed circuit board and components) that must be heated as part of the assembly process. The electronics industry is learning to do without: it has to abandon one of its long-time staples, lead–tin solder. For decades lead–tin solder has been used to attach electronic components to printed wiring boards. However, with the body of evidence pointing to serious adverse health effects of lead, the search for a replacement has spawned intense effort in the electronics industry and in universities. Now scientists think they may have found some promising leads: solders made of alternative alloys and polymer formulations known as electrically conductive adhesives (ECAs).

Alternative alloys

The main approach to replacing lead in solder has been to look for other metals as substitutes. Electronics manufacturers began to look for alternative metals in the 1990s. A search by industry experts for possible replacements for lead–tin solder winnowed down 75 metal alloy alternatives to about half a dozen. The industry eventually selected a tin–silver–copper combination as offering the most reliability and ease to work with as a replacement. The formulation—95.5% tin, 3.9% silver, 0.6% copper—is also known as SAC solder, for the first letters of the chemical symbols of each of the elements (Sn, Ag, Cu).  Experts say that Tin–silver–copper appears to have at least as good reliability if not higher reliability than tin–lead. With a melting point of 217°C, SAC solder also is closest in melting point to the conventional lead–tin solder. This does mean, however, a yet-unquantified increase in energy use. Furthermore, the higher temperature may pose problems for the electronics industry. Higher temperatures mean more stress on components and the entire manufacturing process. Higher temperatures also mean increases in the time it takes to make products, because more time is required to heat and cool the products during the course of their manufacture. SAC solder is used widely in the industry today. However, many of the components being made could not withstand the higher temperatures. That required re-engineering and getting new materials, not only for newer products but for older products. All the older products that had been in production for ten or fifteen years had to be converted over to high temperatures.  There are also short-term consequences of using the new solder. Anytime there is a change in materials, there is a learning curve in using the new materials.

A stickier approach

A more experimental alternative to lead–tin solder is the use of ECAs. These are polymers, such as silicone or polyamide, containing tiny flakes of metals such as silver. The polymers adhere to the printed circuit boards, and the metal flakes conduct electricity. ECAs offer a range of advantages. “If the current-carrying capability can be boosted, ECAs can replace solder. There is another benefit, the temperature required to apply ECAs to circuit boards is far lower than that required for lead-based solder—150°C compared to 183°C.  Moreover, subjecting of the components to lower temperatures and thus less thermomechanical stress enhances their reliability. At present, ECAs are available for a small number of applications requiring low power—for instance, liquid crystal displays—though they are not ready for the marketplace in general, where greater amounts of current are needed.

Solder replacement

Lead-free solder technology is not new. For years, many manufacturers have used lead-free alloys in niche applications to provide a higher melting point or to satisfy particular material requirements. However, the goal of today's lead-free solder research is to determine which alloys should be used to replace the estimated 50,000 metric tons of tin-lead solder currently used each year. Eliminating lead, which is abundant and inexpensive, and replacing it with another element(s) may well increase the cost of raw materials.

·    The materials chosen to replace lead must meet a variety of requirements:

·   They must be available worldwide in quantities sufficient to supply global needs.

·    The replacement alloy must also be considered non-toxic.

·     The substitute alloys must be able to take all forms used by the electronics industry

·      Substitute alloys should also be recyclable

·         These mandates will force the electronics industry to convert from solders that have traditionally contained small amounts of lead (Pb) to alternatives that are Pb-free. While military electronics are currently exempt from these Pb-free imperatives, the consumer market drives the electronics industry, not the military, and what is permitted in consumer electronics will dictate the availability of parts and materials. Electronics for high reliability applications, such as in military equipment, rely heavily on commercial piece parts, circuit boards, and assemblies, the majority of which have transitioned or will transition to Pb-free.

Even as efforts to replace lead in solder move ahead, there still appear to be concerns about the impact that newly implemented metals will have on human and environmental health. “The alternatives to lead have not been researched as well as lead in terms of potential health and environmental impacts. When industries are directed to get rid of lead, they are not given alternatives to replace lead with something that is obviously safer.  It is important to keep looking for lead alternatives that are environmentally benign. Reports indicate that no metallic alternative to lead is free from environmental concerns. For instance, whereas lead may pose a greater public health problem than SAC solder, the latter uses noticeably more energy than lead–tin solder.But the presence of today’s substitutes is good enough as there are alternatives  to chose not to have lead in the electronics products anymore.

Step in the non-toxic direction

The main impetus for the industry to leave lead behind is a ban on lead in electronics already imposed by the many countries. Under these restrictions lead must be replaced by other substances in electronic equipment. In practice, it is not possible to limit or stop the use of lead in such products if we don't have other materials that can deliver the same benefits without being significantly more expensive. Therefore, scientists at the Department of Chemistry at the University of Oslo (UiO) have tried to develop new materials that can replace the lead-containing materials. They have taken a long step in a right and non-toxic direction.  The work is related to making thin films of a material that has the potential to replace lead in electrical and electronic products. The material contains the common elements sodium, potassium and oxygen in addition to the metal niobium, and has no known harmful environmental effects. Strictly speaking, the material isn't completely new, but it has been difficult to produce it on a form that can be used in applications. But now, this problem has been solved by using the technique called Atomic Layer Deposition (ALD) and researchers are now able to make thin films with potassium and sodium as important ingredients, which is something nobody else has been able to do earlier. The innovative thin film consists of crystals, which is created by depositing one atomic layer at a time on a substrate made of silicon. The researchers are far ahead internationally in the use of ALD technology, and research colleagues both far and near have noted that they now are able to make these films with sodium and potassium. Researchers add that there are still some issues that need to be solved before the new thin films can be used in products. They are able to produce the films in the laboratory, so the next step is to find a way to produce quantities that are large and cheap enough to be used industrially. We must also find a way to produce crystals where all the piezoelectric properties points in the same direction, before the materials can be used as sensors or small motors.

Conclusion

Discarding of millions of mobile phones, televisions, computer monitors, and household appliances into landfills worldwide poses a potential environmental hazard if the lead seeps into local groundwater. Lead has already been removed from paint and from gasoline. It was only a matter of time before the electronics manufacturing process went lead-free. These mandates will force the electronics industry to convert from solders that have traditionally contained small amounts of lead (Pb) to alternatives that are Pb-free. While military electronics are currently exempt from these Pb-free imperatives, the consumer market drives the electronics industry, not the military, and what is permitted in consumer electronics will dictate the availability of parts and materials. Electronics for high reliability applications, such as in military equipment, rely heavily on commercial piece parts, circuit boards, and assemblies, the majority of which have transitioned or will transition to Pb-free. Lead-free alternatives that satisfy wave soldering, SMT and hand assembly requirements are available on the market today, although more research is required in the areas of component lead-free alloys, board finish compatibility, flux system development and processing issues. Lead-free solders can be purchased today in all forms – from bar to paste to preforms. Work continues in the development of new flux chemistries that will enable lead-free solders to deliver the same performance as leaded solder materials. Conductive adhesives also are being improved to provide excellent mechanical properties in lower cost formulations that incorporate alternative metal fillers or conductive polymers. In the future, electronic device manufacturers will continue to use both conductive adhesives and solders. Once lead-free regulations go into effect, both adhesives and solders will deliver environmentally safer assembly options, individually and, at times, symbiotically. 


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