“Soft soldering” is a widely used technique in the electronics industry for joining electronic components to printed circuit boards. Traditionally the solder was an alloy of lead and tin, typically containing about 40% lead. It is well known that lead is a highly toxic metal, potentially causing a wide range of harmful effects. Children are particularly susceptible.
The use of lead solders has been effectively banned in Europe for most purposes since 1 July 2006 by the EU Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment (2002/95/EC), commonly referred to as the Restriction of Hazardous Substances Directive or RoHS. The reason for this change was to reduce the discharge of lead into the environment, particularly during the disposal of electronic components. However there are still some applications where lead based solders are permitted.
Where lead containing solders are used, the risk from lead is usually very low. This may seem strange given the high percentage of the metal in the solder. However, soldering is usually carried out at a temperature of around 380 C and significant lead fume is only evolved at temperatures above 450 C. So exposure by inhalation is normally insignificant. This is recognised in the Approved Code of Practice (ACoP) supporting the Control of Lead at Work Regulations 2002 (CLAW). Table 2 in the ACoP (reproduced below) lists processes which are not liable to result in significant exposure to lead. This list includes “Low-temperature melting of lead (below 500°C)” during soldering.
Soldering can produce “dross” – fine particles of solder. Inhalation is likely to occur occur if the dust is disturbed and it may be accidentally ingested if the fingers and hands become contaminated. So its important that appropriate precautions are taken to minimise these risks. However, with well managed soldering processes lead exposure should be minimal.
Despite this a client of ours who carry out soldering with lead based solders (they undertake a process which is exempt from the restrictions imposed by the Restriction of Hazardous Substances Directive) had a visit from a Factory Inspector who insisted that they carry out air sampling and blood lead measurements. The management, who are very conscientious about their responsibilities regarding health and safety, pointed out the guidance in the ACoP, but despite this the Inspector was insistent.
The company asked us to undertake the sampling. I’m glad to say that, as expected, exposures were very low. In fact no lead was detected on any of the samples, meaning that the time weighted average concentrations were less than 2% of the lead exposure limit.
It’s disappointing that the Factory Inspector asked the company to pursue this issue, which involved a significant cost, especially as the HSE’s own guidance is quite clear. Perhaps this case suggests that general Inspectors need to be given more training on occupational hygiene.
The real issue with soldering is the fume produced by the flux – usually containing colophony (also known as rosin), which is manufactured from pine resin, and is usually contained within the soldering wire (rosin cored solder), although liquid fluxes are also used in some cases. The flux is needed to prevent oxidation of components, remove contaminants from the surface of the components, and reduce the surface tension of the molten solder. When heated during soldering it vapourises and condenses into fine particles, which form the fume which is usually clearly visible as a white smoke. Thermal degradation of the colophony also generates irritant gases.
Rosin cored solder fume is a well established respiratory sensitiser, and is one of the main causes of occupational asthma in Great Britain. Colophony fume is generated at temperatures above about 180 C, well below the temperatures associated with soft soldering. So significant concentrations can be evolved. The higher the temperature, the more fume is generated. The lead free solders introduced since the Restriction of Hazardous Substances Directive was implemented in 2006 tend to have higher melting points than the traditional lead:tin types. So, ironically, the elimination of lead for environmental reasons has led to a potential for increased exposures to a potent asthmagen in the workplace.