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Environmental Impact of Lead-free Solders
December 31, 1969 |Estimated reading time: 9 minutes
By Ed Smith and Kristine Swanger
Although the movement to regulate and eliminate lead-bearing solders has picked up speed worldwide, there has been little published research on the environmental effects of the available lead-free formulations. Most detrimental environmental effects from soldered products occur during final disposal. Soldered products are disposed of in municipal landfills not the highly regulated hazardous waste landfills. Waste products such as dross, scrap boards and assemblies, and raw metal are disposed of during various manufacturing processes. This article shows the results of the study of seven lead-free solder alloys. These alloys were tested in the various physical forms most likely to occur from printed circuit board (PCB) fabrication, assembly and finished-product disposal to determine the environmental impact from each alloy.
Waste Regulation WorldwideWaste management in industrialized countries ranges from highly regulated, government-controlled operations to programs that are practically nonexistent. Most industrialized countries that significantly regulate industrial/commercial hazardous waste, with or without also regulating household hazardous waste disposal, fall within this range. Hazardous waste is defined as any waste that may "pose a substantial present or potential threat to human health and the environment when improperly treated, stored, transported or otherwise managed." When waste is disposed, it is weathered by rainfall and reacts with other wastes, which allows metal elements and their salts to be leached from the metal surfaces of the waste. If the metal-bearing leachate is allowed to contact stormwater or to migrate into groundwater, local drinking-water supplies are threatened with contamination. Laboratory leaching methods simulate this natural phenomenon.
Deionized Water Leach MethodsMany European countries and Japan utilize deionized (demineralized) water-leaching tests. A draft European test method also specifies deionized water leaching. Texas publishes a seven-day deionized water-leach method, as well. These methods are used to demonstrate the contamination potential to drinking water and groundwater from waste that comes into contact with drinkable water. A portion of the material under study is mixed with some multiple of its weight in deionized water, shaken or tumbled for a specified time, then allowed to leach while being shaken, tumbled or undisturbed. Table 1 highlights each leach method. After the appropriate leach time, the liquid leachate is filtered and analyzed for the constituents of interest. If the leachate shows contaminants higher than the local drinking water standards or other regulatory limits, the waste material is considered to have failed the test.
More Aggressive Leach MethodsAside from the deionized water leaching methods, there are methods that utilize leaching fluids containing various acids. These methods are used to simulate acid rain and to simulate improper waste disposal mixing household waste with industrial waste in the same disposal unit. The synthetic-precipitation leaching procedure (SPLP) developed by the U.S. Environmental Protection Agency (EPA) is one such test. The leaching fluid in this test contains nitric and sulfuric acids, diluted to a pH of 5.00. Some European environmental authorities use a similar leaching fluid. These fluids contain either sulfuric acid or sodium nitrate. The toxicity-characteristic leaching procedure (TCLP) was developed by the U.S. EPA to determine whether a waste was hazardous by virtue of its toxicity. TCLP fluid contains dilute (pH 5.0) acetic acid, which mimics the organic acids typically found in household waste landfill leachate. Consumers typically dispose of their used electronic products by throwing them in the trash; the scenario of electronics being disposed with food and other household waste is highly plausible. California promulgates a soluble-threshold leaching concentration (STLC) test, which utilizes citric acid to mimic the landfill disposal scenario and its effects on waste leaching.
Metal Toxicity1Extensive research has been conducted on the toxicological effects of lead, and there are toxicity concerns with available lead replacement metals.
Silver and silver compounds can cause biological effects such as digestive tract irritation and agryria. Ecotoxicity, reproductive effects and mutagenicity have been observed in laboratory studies; however, toxicological data has not been fully investigated.
Antimony and antimony compounds can cause biological effects such as severe digestive tract irritation with abdominal pain, nausea, vomiting and diarrhea. Toxicological data has not been fully investigated.
Copper and copper compounds can cause biological effects such as severe digestive tract irritation with abdominal pain, nausea, vomiting and diarrhea. Ecotoxicity has been observed in laboratory studies, but toxicological data has not been fully investigated.
Indium and indium compounds have shown developmental toxicity in rats and mice. Particular symptoms of this developmental toxicity include fetal mortality, fetal malformation, reduced fetal weight, and malformations in the tail, ribs, digits and kidneys. Ecotoxicity and mutagenicity have been observed in laboratory studies; more toxicological studies are needed.
Bismuth or bismuth compounds have been suggested to be a carcinogen or a cocarcinogen in rats. Some studies have shown that bismuth can cause chromosomal aberrations in rats. More epidemiological studies are required for a more complete determination.
Based on the toxicity and environmental impact data that are available for lead-replacement solder alloys, it appears that the alloys are also harmful to human health and the environment.
Regulatory ImpactsThere are regulatory concerns for the lead-replacement metals regarding environmental impact and use in the workplace. PCB manufacturers and assemblers moving to lead-free solder materials will need to evaluate these new materials in the workplace for environmental permitting, management and industrial hygiene issues.
Silver and silver compounds. Regulated under Superfund, SARA 313, RCRA, Clean Water Act Toxic Pollutant, California State Superfund Hazardous Substances, CAL-OSHA Director's List of Hazardous Substances and California HWCL Hazardous Wastes.
Antimony and antimony compounds. Regulated under Superfund, SARA 313, Clean Air Act Hazardous Air Pollutant, Clean Water Act Toxic Pollutant, California State Superfund Hazardous Substances, CAL-OSHA Director's List of Hazardous Substances and California HWCL Hazardous Wastes.
Copper and copper compounds. Regulated under Superfund, SARA 313, Clean Water Act Toxic Pollutant, California State Superfund Hazardous Substances, CAL-OSHA Director's List of Hazardous Substances and California HWCL Hazardous Wastes.
Except for the bismuth and indium radionuclides, bismuth, indium and their compounds are not heavily regulated by federal and state authorities. If bismuth and indium alloys are selected by the industry, their use will dramatically increase. Regulation may follow if environmental agencies deem them to have an adverse impact on the environment.
Lead-free replacement solder alloys may not provide the electronics manufacturer any less regulatory burden than that imposed by conventional tin-lead solders.
Experimental MethodsExperiments with eight lead-free alloys were undertaken to show their toxicity relative to each other and to conventional tin-lead solders. Wire solder; solder solids; -325, +500 solder paste (with flux); and solder dross were the physical forms of solder tested. These physical forms of solder mimic the waste streams from PCB fabrication and assembly operations. The alloys chosen were:
- 96.3 tin, 3.2 silver, 0.5 copper
- 96.5 tin, 3.5 silver
- 98 tin, 2 silver
- 99.3 tin, 0.7 copper
- 95 tin, 5 antimony
- 80 tin, 20 indium
- 90 tin, 5 bismuth, 5 silver
- 43 tin, 57 bismuth.
Each of the chosen alloys is commercially available today and several are already in use. Several Japanese manufacturers are utilizing tin-bismuth-silver and tin-copper alloys.2 The National Center for Manufacturing Sciences has studied lead-free solder alloys and narrowed the field of useable alloys to tin-bismuth, tin-bismuth-silver and tin-silver.
Sample Preparation, Leaching and AnalysisEach metal was procured in an elemental state and alloyed under oxygen-free conditions. Solder wire was 0.032" diameter. Solder solids were bar stock, milled to pieces no larger than 0.375 x 0.375". The U.S. EPA mandates this maximum particle size. Solder dross was produced by heating the alloyed solder solids in an ambient atmosphere while occasionally removing the dross from the surface of the solder melt using a titanium bar. An analysis of the oxide content of the dross showed that it contained approximately 90 percent entrapped metal and 10 percent metal oxide.
Samples of solder paste were prepared by alloying the appropriate elements, then blowing them into spheres in an inert atmosphere. The spheres were then sieved to give a -325 to +500 sieve size powder, which is suitable for fine-pitch solder paste printing. A flux paste consisting of reagent-grade rosin gum (20 percent), glycerol (10 percent) and ethanol (70 percent) was prepared. The solder spheres and flux paste were mixed to give a 90 percent solids paste. Typical viscosity of the pastes was in the 350 to 400 kilo-centi-poise (KCPS) range. The ingredients were selected to provide a uniform paste chemistry that would eliminate all variables except the metal constituents. Synthetic activators were not used as they might cause metallic-leaching reactions. The powder sphere size was selected to give a worst-case scenario (maximum leachability). Leaching is a surface phenomenon. Smaller spheres give a higher surface-area-to-volume ratio than larger spheres, presenting a greater opportunity for metal leaching.
After preparation of the "waste" samples, each was leached according to U.S. EPA, Japanese or European protocols, and the leachate analyzed using U.S. EPA metals analysis methods.
Results and ConclusionsThe results show that lead-free solders display several elements leaching at levels above U.S. EPA allowable limits in different leaching media (Tables 2 through 6).
Antimony. Most striking in its apparent toxicity is the 95 tin-5 antimony alloy. The leachable levels found are approximately 10,000 times the maximum allowable in drinking water. The 95 tin-5 antimony alloy studied leached above regulatory limits in every physical form and in all leach methods.
Silver. All lead-free alloys containing silver leached above regulatory limits for the TCLP leach, except for the tin-bismuth-silver, which did show some silver leaching. Silver bearing lead-free alloys were close to the U.S. EPA limit of 0.1 mg/L in drinking water for leach tests using deionized water. When groundwater was used as the leaching media, the silver levels went above the regulatory limit.
Copper. Copper was leached above the STLC (California) regulatory limit in the 99.3 tin-0.7 copper alloy.
Bismuth. Bismuth showed little leachability. It was leachable using the STLC (California) test.
Indium. This leached at 0.1 to 1.0 mg/L in all tests except for the SPLP (synthetic precipitation). Indium did not leach in the SPLP test.
Tin. Tin did not leach significantly. Salts of tin tend to be insoluble in water at room temperature.
The data may also be reviewed by leaching method rather than by metal-alloy element. The SPLP test was ineffective at leaching all elements except for antimony. This demonstrates that acid rain poses limited potential in releasing lead-free solder metals into the environment. Deionized water test methods, such as those proposed or used in both Japan (JST-13) and Europe (preliminary), also tend not to leach lead-free solder alloys, except for antimony. The more aggressive TCLP test, which simulates disposal in a municipal landfill, leaches measurable amounts of tin, silver, copper, antimony, indium and bismuth. This demonstrates that codisposal of electronic wastes with municipal wastes is undesirable. The STLC test used for regulatory purposes in California leaches measurable amounts of tin, silver, copper, antimony, indium and bismuth, with much higher levels of both copper and bismuth than the TCLP. The STLC, like the TCLP, simulates codisposal of wastes.
Lead-free solders are not a panacea for solving the potentially toxic effects from tin-lead solder alloys. These experiments show that most lead-free solders leach at levels that would cause them to be classified as a hazardous waste, failing both silver and antimony levels. If lead-free solders containing silver or antimony are improperly disposed of and contact groundwater, the solders could render that groundwater unsafe to drink per U.S. EPA standards. Solder dross from these alloys carry the same risks, as the dross behaved similarly to the parent alloys in these experiments. Bismuth and indium are not currently regulated and their toxicity has not been widely studied, posing unknown challenges for adopters of lead-free solders.
ACKNOWLEDGMENTSSpecial thanks to Dr. Smith and the staff at EFEH & Associates laboratories for their assistance with the analytical work.
REFERENCES1 "Registry of Toxic Effects of Chemical Substances," National Institutes of Safety and Health, material safety data sheets for the metal elements: 29 and 40 CFR; "Waste Classification Regulation Guidance Manual," California Environmental Protection Agency, August 1994.
2 IPC PC Expo '99 Proceedings, Long Beach, Calif., March 1999.
ED SMITH, senior director of manufacturing and technology, and KRISTINE SWANGER, health, safety and environmental coordinator, may be contacted at K*TEC Electronics, 1111 Gillingham Lane, Sugar Land, TX 77478; (281) 243-5993 (Smith); (281) 243-5605 (Swanger).