Lead in solders

The RoHS Directive was a major catalyst for research and adoption of lead-free solutions in electronic equipment. Alternatives to lead in soldering range from tin (Sn), silver (Ag) and copper (Cu) to bismuth (Bi) and zinc (Zn). These heavy metals do not have the same toxicity and bioaccumulation potential of lead (Pb). ... 

Bismuth is the most diamagnetic of all metals and has low thermal conductivity. Since bismuth expands upon solidification, it is used to make castings for objects subjected to high temperatures. It is used as a replacement for lead in solders, shot for hunting, fishing sinkers, ceramic glazes, and brasses for plumbing applications. It is also used as a carrier for (an isotope of uranium) fuel in atomic reactors. Ionic compounds of bismuth are used in cosmetics and medicine. 

The recommended lead-free solder formulation is Sn-Ag-Cu for board assembly but there are other formulations such as Nickel-Palladium (NiPd), or Nickel-Palladium with Gold flash (NiPdAu). Passive components, to be compatible with a lower temperature Lead process (which is 215°C for 50/50 Tin/Lead formulations and 230°C for 40/60 formulations) and the higher lead-free process of up to 260°C, use pure matte Tin for their contacts. The use of lead in solder is partially based on several potential reliability issues. Pure Tin component leads have been shown to result in inter-metaUic migration in the termination of the electronic component and the growth of tin whiskers which could cause short circuits (which is why there is a exemption for military use (only) components). 

Few technical issues have hit the electronics industry as hard in recent years as the switch from Sn-Pb solder to lead-free attachment. Both by mandate, e.g., the European Commission requirement to eliminate lead in consumer products, as well as voluntarily, electronic component manufacturers and assemblers must come to grips with issues surrounding the transition to lead-free attachment of components. While various industry, academic, and governmental agencies that have studied alternatives to lead in solder now seem to agree on SnAgCu as the preferred alternative, as discussed by Bradley, Handwerker, and Sohn, there is still much to be done to create a minimal-problem transition. 

Lead in solders consisting of more than two elements for the connection between the pins and the package of microprocessors with a lead content of more than 80 percent and less than 85 percent by weight Lead in solders to complete a viable electrical connection between semiconductor die and carrier within integrated circuit Flip Chip packages... 

The choice of the final surface finish for the copper has an effect upon how the solder mask performs. And, with the elimination of lead in solder, the compatibility of new final finishes needs to be confirmed before their implementation. Most of these finishes are applied after the solder mask so the process for mask application can have a significant impact upon final quality of the PCB ... 

The implementation by the European Parliament of Directive 2002/95/EC on the restriction of the use of certain hazardous substances (Restriction of Hazardous Substances [RoHS]) in electrical and electronic equipment, including lead in solder, has had significant... 

AS OF JULY 1, 2006, THE WEEE DIRECTIVE WILL ELIMINATE THE USE OF LEAD IN SOLDER PASTE. THE INDUSTRY IS INTENSIVELY DEVELOPING SOLDER-PASTE SYSTEMS THAT SERVE AS AN ALTERNATIVE TO THE TIN/LEAD (SNPB) SOLDER PASTE USED. BUT SWITCHING TO LEAD-FREE PASTE WILL INCREASE THE NEED FOR CHEMICALLY SUPPORTED CLEANING. By Umut Tosun... 

Many metallic elements are contained in electronic assemblies either as terminations or coatings for component devices or as the electrical circuit, terminations, or coatings on PWBs. The elimination of lead in electronic products requires substitution by other metals that can provide the performance and reliable properties characteristic of traditionally used lead-bearing alloys. There are a number of metal resources that can be selected as substitutes for lead in solders and as coatings for lead-free electronic assemblies. These metals include tin (Sn), silver (Ag), copper (Cu), bismuth (Bi), antimony (Sb), gold (Au), indium (In), nickel (Ni), palladium (Pd), platinum. 

There are several hundred thousand abandoned metal-mining sites in the U.S. that have not been assessed for acid runolf, erosion by weathering, and metal dust emission potential. Any increased demand for metals-such as tin, silver, bismuth, and copper-that might be used as alternatives for lead in solder, will require significantly increased use of water and energy to extract the metals from ore that already is becoming depleted. 

The ROHS Directive would impose substitution on the use of lead, mercury, cadmium, hexavalent chromium, and brominated substances polybrominated diphenyl ethers (PBDE), and polybrominated biphenyls (PBB) in electrical and electronic equipment. The Directive would allow the Commission to establish maximum concentration values, where the presence of specific materials or components is tolerated. Industry welcomes this option because heavy metals covered by the Directive are present in most materials as naturally occurring substances. As for the economic implications, the Commission recognized that substantial costs would be incurred for replacing lead in solders and estimated the additional operating costs of using tin-based solders or other alternatives would be about 150 million Euros per year. 

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