Wednesday, December 7, 2022

Lead-Free Electronics to Help Lead Good Health

There is a growing concern over use of lead in electronics as exposure to lead even at low levels can affect our health. To come up with lead-free electronics, manufacturers have no option but to abandon lead-tin solder—one of their long-term staples -- Dr S.S. Verma

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Though no study demonstrates environmental or human health risk posed by electronic products in landfills, electronic waste disposal through recycling process poses a great threat to its workers and the environment. Lead is not absorbed through the skin and exposure is through inhalation or ingestion.

The main impetus for the industry to leave lead behind is a ban on lead in electronics imposed in many developed countries. Accordingly, lead must be replaced with other substances in electronic equipment.

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Lead-free alternatives for solder
Solder is the linchpin of electronics manufacturing without which it’s difficult to achieve a proper electronic connection that is durable and reliable. Lead has been ideal for solder. The electronics industry is learning to do without lead, for which it has to abandon lead-tin solder—one of its long-time staples.

For decades lead-tin solder has been used to attach electronic components to printed wiring boards. However, with the body of evidence pointing to adverse health effects of lead, the search for a replacement has spawned intense efforts in the electronics industry and in universities.

Now scientists think they have found some promising leads: solders made of alternative alloys and polymer formulations known as electrically conductive adhesives. Even as efforts to replace lead in solder move ahead, there are still concerns about the impact that newly implemented metals will have on human and environmental health. The alternatives to lead have not been researched extensively in terms of potential health and environmental impacts. Getting rid of lead doesn’t mean to replace it with some-thing that is obviously safer, but to keep looking for lead alternatives that are environmentally benign. Indeed, no metallic alternative to lead is free from environmental concerns.

Solders made of alternative alloys
Because of the considerable toxicity of lead, efforts are being made to replace the traditional soldering alloys with new compositions. Research aims to improve the reliability of lead-free soldering alloys that are used to make electronic devices. However, the reliability of new lead-free materials requires further investigation.

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 biggest benefit for the industry would be to pick one solder, concentrating the development and research efforts on one alloy and making it work. The industry eventually selected a tin-silver-copper combination as the most reliable and easy-to-work-with replacement. The formulation—95.5 per cent tin, 3.9 per cent silver, 0.6 per cent copper—is also known as SAC solder, for the first letters of the chemical symbols of each of the elements (Sn, Ag, Cu).

Tin-silver-copper appears to have reliability at least as good as, if not higher than, tin-lead. With a melting point of 217°C, SAC solder is also the closest in melting point to the conventional lead-tin solder. This does mean, however, an 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. These also mean increase in the time taken to make products, because more time is required to heat and cool the products during the course of their manufacture.

Lead-free electronics
Lead-free electronics

Today, SAC solder is widely used in the industry. However, many of the components can’t withstand the high temperature required by it.

Polymer adhesives
Electrically conductive adhesives (ECAs) are a more experimental alternative to lead-tin solder. These are polymers (silicone or polyamide) containing tiny flakes of metals like silver The polymers adhere to the printed circuit boards and the metal flakes conduct electricity.

ECAs offer a range of advantages. Silver’s electrical conductivity is very high, while electrical resistance is very low. If the current-carrying capability can be boosted, ECAs can replace solder. The temperature required to apply ECAs to circuit boards is far lower than for lead-based solder—150°C compared to 183°C—which offers many advantages. Preliminary studies comparing parts using ECAs instead of solder suggest that ECAs have a much tighter bond than solders—perhaps an order of magnitude better.

ECAs are already available for a small number of applications requiring low power—for instance, liquid-crystal displays. However, these are not ready for the marketplace in general where greater amounts of current are needed. More work is needed on these, as they begin to fail structurally if the component heats up above 150°C. There are other concerns about ECAs as well. With time, the ability of ECAs to conduct electricity drops and resistance to electricity increases. Also, moisture can result in their corrosion.

Lead-free piezoelectric materials of the future
Piezoelectric materials have fantastic properties: squeeze them and they generate an electrical field and vice-versa. They contract or expand when jolted with an electrical pulse. These really took off after the 1950s, with the development of a superior man-made piezoelectric ceramic crystal: lead zirconate titanate, or PZT.

For the last 60 years, PZT has been essential for a myriad of high-tech applications from inkjet printers to digital camera shutters, ultrasonic imagers, fuel injector actuators and igniters for gas barbecue grills. It is the best material for the job at the moment, because it has the greatest piezoelectric effect, good physical durability and can be radically tailored to suit particular applications.

Despite this success many scientists now want to replace PZT with some lead-free material that would be more environmentally benign and enable new piezoelectric applications towards lead-free electronics. To date, however, no suitable successor has been found. Candidates are typically too feeble in their piezoelectric effect and/or physical durability. The lead-free ceramic that is studied is lightweight and can be used at room temperature that could make it an ideal choice for many applications.

Road to eco-friendly electronics
Making lead-free electronics has proved problematic so far. Researchers are trying to develop a method that enables the industrial production of a substance that can be used to replace lead in many electronic applications. Over the past ten years, there has been tremendous growth in research on lead-free alternatives. Till now, a type of material called ‘alkali niobate,’ also known as KNN, is considered to be a likely successor to lead in the electronics industry. However, KNN poses two main problems that have been difficult to resolve: find a KNN variant that ha the properties needed for electronics and develop a method for industrial production of the material.

However, with such experiences, efforts are going in the right direction, and soon we may have very user- and eco-friendly electronics.

The author is an associate professor in Department of Physics at S.L.I.E.T., Longowal, District Sangrur (Punjab)



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