Contaminant Clouds Part 3 – Solder Fume

Contaminant Clouds Part 3 – Solder Fume

Soldering is used in a wide variety of industries including electronics, plumbing, and jewellery making. Soldering is the process of joining two metals via a third ‘filler’ metal with a lower melting point than the two base metals and which flows into the joint in between these. The filler metal varies per the specific soldering process but is generally made up of metals (such as tin or lead) and a wetting agent called flux which aids the solder flow. Soft soldering typically uses a tin-lead alloy (although lead free options are now widely applied) as the filter metal. Whereas silver soldering uses a silver alloy at a higher temperature. The base metals are not distorted in this process, as opposed to welding which involves melting the work pieces. And as with most any thermal process, soldering produces fume.

The filler metal alloys melt at different temperatures depending on which specific metal is in use e.g. lead, tin, zinc, silver, cadmium, bismuth. For example, soft solder fillers (such as lead) melt at 350 ˚C. Melt being the key word, not ‘volatilise’. Lead boils and volatilises at 1750 ˚C which turns this into a gas. Likewise, tin melts at 230 ˚C but volatilises at 2600 ˚C. Therefore in a typical tin-lead soft solder process, it is not the tin or lead filler metals themselves which give cause for concern because at solder temperatures (below 450 ˚C) these metals are unlikely to volatilise (the operating temperature is below their boiling point).

However, soldering is a visibly ‘fumey’ process. The main health concern when soldering is the breakdown of the flux. In soft soldering, the flux is commonly rosin based which is a solid pine tree resin. The flux is essential to the process in that it allows the flow of the solder metal and prevents oxidisation. Rosin-based solder flux fume is a potent respiratory sensitiser as well as an irritant to the eyes and upper respiratory tract. The COSHH regulations require that exposure to rosin-based solder flux fume (colophony) should be kept to as low a level as is reasonably practicable. Exposure controls will be needed and may include local exhaust ventilation (LEV) e.g. back-drawn partial enclosure, a positional captor hood, or on-tool extraction.

Solder fume is typically comprised of 99.5% particles and 0.5% gases. When rosin-based solder flux is heated above 200 ˚C this forms resin acid particulates and other gases. The higher the temperature, the more particulate fume is produced i.e. silver soldering will typically produce more fumes compared to soft soldering. In the case of higher temperature soldering, the filler metals may volatilise and create a higher risk of metal fume inhalation, although the risk of flux breakdown and subsequent flux fume inhalation is still present. When soldering, the solder wire will release the rosin flux core when heated forming resin acid particles which comprises the visible cloud of fume. Other vapours and gases formed may also include aliphatic aldehydes (formaldehyde), hydrochloric acid, and various volatile organic compounds. The workplace exposure limit (WEL)  for rosin-based solder flux fume (colophony) is 0.05 mg/m3 8-hour TWA. And individual WELs are also established for some of these other substances such as formaldehyde, acids and metals.

A Tydnall beam (dust lamp) is an easy way to aid in visualising the solder fume cloud and effectiveness of the fume extraction system. From experience, we have seen this reveal fume which was escaping the capture of the on-tool extraction device in place. Showing that a more suitable control measure was needed e.g. a back-draught partial enclosure. The sampling undertaken during this same visit resulted in notable colophony exposure levels, which correlated to the fume escape observed with the dust lamp. Although your first guess may be that a soldering process would create a cloud of metal fume (the solder wire is metal after all), this is not the main composition of the contaminant cloud. What you see when looking at the cloud is actually mainly the resin acid particulates from the flux. With the metal particulates and gases produced from soldering likely to be at trace levels. If personal exposure sampling is deemed necessary, then it is important to focus on the colophony. Although metal particulates and gases could also be sampled in cases where reassurance is needed or on suspicion that levels may be notable in certain cases.