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Survey on Lead Free Solder Systems

4 Health and environmental issues

International projects have mainly focused on the technical properties of lead-free solders. The aim of most projects has been to find and investigate ways of lead substitution and not to assess whether the alternatives are recommendable from a health and environmental point of view.

However, some projects have included toxic aspects, see e.g. proceedings from IPCworks’99 (IPC, 1999). Most of these investigations have focused on the inherent toxicity of lead and alternative metals. The investigations have typically been superfluous and often based on non-validated databases as e.g. RTECS. In a recent Danish project (SPM, 1999), the Danish Toxicology Centre made a more in depth assessment of the inherent toxicity of most of the metals used in lead-free and lead containing alloys. It was seen (not surprisingly!) that lead is to be viewed as very toxic but at the same time that several of the other metals have suspicious toxicological and environmental effects as well. However, for most metals relatively few toxicological data are available, whereas a substance as silver is several orders of magnitude more eco-toxic than lead.

The second discussion in relation to actual risk is ‘what is the exposure level when using lead solders?’ A main topic at the IPCWorks’99 conference was that so far no studies have shown that workers in electronics mounting facilities have a higher blood lead level as a result of their occupation and that the likeliness that leaching of lead after end-of-life disposal directly to landfills is very limited as lead will form stable compounds and bind tightly in the soil environment. The latter statement was partly supported as part of the Danish Kamille-project /18/. The leaching from landfills of residues from waste incineration is different, as incineration makes lead very mobile. Again very few data exist for the leaching of alternative metals.

Toxicological profiles for silver, copper, antimony, bismuth, indium, zinc, tin and lead can be obtained from the SPM Report SPM-151. The SPM report generally concluded that lead poses high human toxicity and moderate toxicity towards aquatic organisms. Thus, substitution to lead free solder seems justified with regard to human toxicity.

However, the future general-purpose lead-free solder implies the use of silver, which has low human toxicity but is much more toxic to aquatic organisms than lead. The other metals are low to moderate toxic to humans and of moderate aquatic toxicity. One has to bear in mind that a toxic hazard, e.g. as described in the toxicological profiles not necessarily implies a toxicological risk. The toxic substance, metal or formed compound, e.g. oxide, must be present in (bio)available forms and in sufficiently high concentrations to result in toxic doses in humans or other organisms.

The idea in the current project is to avoid that substitution of lead will cause new foreseeable problems. The selection made is therefore based on existing knowledge from the SPM project (SPM, 1999) and Kamille-project /18/ by also including amounts in the different solders and application of a ranking system. Furthermore, the project has focused on the toxicity of new auxilliary materials (e.g. cleaning agents and fluxes), which have to be introduced in connection with the application of the new solders at different processing conditions as compared to traditional solders.

The use of modern technology, where the solder process takes place in full containment will reduce or eliminate the exposure of workers to solder vapors and fumes to negligible, regardless of the solders used (lead-containing or lead-free). In spite of this there will always be a risk of occupational exposure in connection with maintenance, repair, cleaning, etc., which still makes it justifiable from a human toxicological point of view to avoid lead in solders.

Due to higher processing temperatures, the tendency of chemicals and materials to become airborne increases during lead-free processes compared to lead-containing processes. The uses of the lead-free solders will possible result in an increased evaporation from the solder metals and fluxes as compared to the use of the traditional lead-containing solders.

Furthermore, the concentration of activators (in general irritating and some even allergenic) in the fluxes, seems to be higher (at least the double) in fluxes used in connection with lead-free solders as compared to the fluxes in lead-containing solders. Experience from the practical testing program in this project indicates that the flux of the lead-free solders is more corrosive to the materials. Combined with the fact that the lead free soldering temperature is higher, developing more fumes and vapors this may indicate that the lead-free solder processes may develop more acrid, irritating and in some cases allergenic fumes and vapors than the traditional lead-containing solder processes. From a human working environment point of view it still seems justified to avoid lead in solders, but the risk of exposure is low.

From a wiev of risks to humans from substances spread in the environment this risk is lowered by substitution.

From an ecotoxicological point of view substitution to lead-free solders is more uncertain as emission and environmental fate are uncertain; exposure is therefore uncertain. In a life cycle perspective, the main environmental load will not be during the production/solder process, provided the production is optimized (e.g. with minimal losses during manufacturing), but after end of use.