Controlling solder fume

In a previous post I discussed the health risks associated with exposure to the fume generated during soldering with rosin cored solder. It’s a respiratory sensitiser, and is one of the main causes of occupational asthma in Great Britain.

The fume is generated due to thermal degradation of 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.   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.

In an ideal world we would try to eliminate the risk by using an alternative, rosin free flux. However, this is difficult in practice and most alternatives I’ve come across still generate harmful fume. The amount of fume can be reduced by controlling the soldering temperature. However some fume will still be evolved. So local exhaust ventilation is likely to be needed to minimise the risk to health whenever soldering is being undertaken.

One of the most common type of of extraction system used in workplaces where soldering takes place is the flexible arm captor hood. As I’ve described in a previous post, the problem with external captor hoods like this is that airflow drops off very rapidly with distance from the face of the hood. Positioning is crucial, particularly with solder fume which is hot and rises in a narrow plume before dispersing. Operators tend to position the hoods too far away from the source as moving them close enough would obstruct and interfere with their work. Also, any ambient air movement in the workplace can disrupt the airflow from the extraction, further reducing their ability to capture the fume. Consequently, this type of system is usually ineffective at controlling exposure.

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Another common approach is to install a low volume high velocity (LVHV) system. Here, a small metal tube is attached to the soldering iron, the idea being to capture the fume close to source. The metal tube is then connected to extract ducting via flexible plastic tubing. In principal this should be much more effective then using the flexible arm hoods.

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However, there are a couple of problems which often render these ineffective. First of all operators rarely position the tube close enough to the tip of the soldering iron where the fume is generated, pleading that the tubes “gets in the way of the work”. Secondly, fume that is captured condenses out inside the metal tube and the associated plastic tubing. Unless these are cleaned out very regularly (and, in my experience, this rarely happens) airflow is seriously restricted, significantly reducing the capture velocity and the degree of control.

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Whenever local exhaust ventilation is being applied to attempt to control exposure to airborne contaminants, the best type of hood will usually be a partial enclosure which contains the source. In that case the contaminant doesn’t have to be captured – it is generated within the hood.  If sufficient airflow is provided to draw the contaminant away, and prevent it escaping from inside the hood, there is a good chance that effective control will be achieved. In the past, this approach has not typically been applied to soldering. However one company, working in conjunction with Health and safety Executive Inspectors, has developed such a solution.

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Here, the booth is large enough to contain the work, without interfering with the task. It is constructed of transparent material, so the operator can clearly see what she is doing, while presenting a barrier between the fume and her “breathing zone”. The enclosure also minimises local air turbulence and draughts so that the solder fume rises within the enclosure, relatively undisturbed, and is then captured by the extraction, preventing the operator being exposed to the fume.

This type of hood is by far the most effective way of controlling exposure to solder fume and really needs to become the standard approach for the electronics industry. It is recommended by the HSE in their COSHH Essentials Control Sheet for soldering. Unfortunately employers tend to be quite conservative. Systems with captor hoods have been widely used for many years and it is not easy to convince employers that they’re not the most appropriate approach – particularly when they’ve spent a lot of money purchasing and installing them.

(Note: HSE have produced guidance on the hazards and legal requirements and on the control of rosin cored solder fume.)

Partial enclosures – keeping contaminants out of the user’s breathing zone

In a previous post I discussed why captor hoods are ineffective at controlling contaminants generated by most common industrial processes. Partial enclosures, or booths, are another common type of hood which, in principle, should be more effective  for many situations. This is because they don’t have to actively capture the contaminant as it’s generated inside the hood. The airflow provided by the system doesn’t have to pull the contaminant into the hood, but is intended to

  • prevent the contaminant escaping into the workplace and
  • remove it from the hood.

One of the main problems that is commonly experienced with partial enclosures is that the presence of the user in front of the hood disturbs the airflow and can lead to contaminants being drawn into the user’s “breathing zone”.  This is illustrated in the following diagram.  The blue lines represent the airflow while the red arrows show how contaminants generated in front of the operator just inside the booth are likely to behave.

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There are a number of ways that this problem can be overcome, which are discussed in the Health and Safety Executive publication HSG258Controlling airborne contaminants at work: A guide to local exhaust ventilation

1. Move the source of the contaminants further into the booth so that they are not entrained by the turbulent airflow near to the user. The designer needs to make sure that the booth is deep enough for this arrangement

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2. Insert a transparent barrier at the front of the booth – i.e. increase the degree of containment.

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3. Use a side flow booth where the operator stands at right angles to the horizontal airflow

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4. Use a downdraught booth. Here he worker stands inside the booth where the airflow is vertical (from top to bottom) so the contaminants are pulled down away from the breathing zone.

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Care needs to be taken with the design and application of these downdraught booths to ensure that the presence of the operator, work-piece and work equipment does not disturb the flow too much so that eddies are created that  draw the contaminants into the breathing zone, thereby negating the reason for using this type of booth. It can also be difficult to control highly energetic contaminants (e.g. the spray from a paint spraying operation) with this type of set up.

What’s wrong with captor hoods?

Captor hoods are one of the most common types of inlet that you’ll find on extraction systems. With these hoods, the contaminant is generated and disperses outside the hood. It has to be drawn into the system by the airflow.

Unfortunately, in most cases, they are largely ineffective at controlling contaminants. The following diagram illustrates how airflow drops off very rapidly with distance from the face of the hood. This happens because air is drawn in from every direction – not just from the region where the contaminant is generated.

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As a rough rule of thumb, the velocity a distance equivalent to the diameter of the hood opening will be around 10% of that at the hood face (although the diagram suggests that a captor hood’s performance is even worse than that) .

Here is a common process – a stonemason carving a block of stone with a chisel. This process generates fine dust which will include some respirable crystalline silica – a highly hazardous dust. We can see a captor hood being used to control the dust. The nature of this process means that the dust is created at different locations as the mason works along the block of stone. A dust lamp has been used to visualise the dust created.

The capture zone for the hood (the area where the air velocity is high enough to capture the contaminant) is marked with the dashed yellow circle.

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As you can see when the mason is working close to the hood the dust is created in the capture zone and there is a good chance that it is being controlled. However as the working position changes, dust is created further from the hood and outside the capture zone.

Remember that the dust is invisible to the naked eye, so the mason doesn’t realise that this is happening. As far as he is concerned the dust is being controlled by the LEV. We can see that this is only true when he is working close to the hood.

The mason in the picture is wearing a respirator, so, hopefully, he is protected. However, this will not always be the case and many workers can be exposed to significant concentrations of hazardous contaminants when using captor hoods.

Dusts are particularly difficult to control as they are relatively heavy and often have momentum. A relatively high velocity is needed to draw the particles into the hood. As the velocity drops off very quickly, any dust more than a few centimetres away from the hood is unlikely to be captured effectively.

With gases and vapours lower capture velocities are needed. But as the contaminant tends to disperse over a wide area, they can still be difficult to achieve control with a captor hood.

Captor hoods are often installed as an “off the shelf” solution, and because the purchasers have a mistaken belief that they will control the contaminants. Unfortunately, this is rarely the case. The first step in effective control is to understand the problem –

  • how the contaminant is created,
  • where it is generated (making sure that ALL sources are identified), and
  • how it behaves once released

If LEV is the best, or most practicable solution to the problem (it isn’t always) then the hood should be designed around the process using this information. In many cases it will be possible to design a full or partial enclosure which will almost always be more effective than a captor hood.

Where captor hoods have to be used:

  • they need to be positioned as close as possible to the source of the contaminant
  • the capture velocity needs to be adequate to pull the contaminant into the hood from the furthest point where it is likely to be present
  • the capture zone needs to be clearly defined, bearing in mind that it is easily disrupted, particularly by draughts. It can shrink and expand depending on conditions.
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The capture zone for a flexible arm captor hood

For further details on LEV design, see the HSE LEV website and HSG258Controlling airborne contaminants at work: A guide to local exhaust ventilation – (which can be downloaded free of charge)

Jargon

Paint spraying

When you’ve been working in a particular profession for a while its easy to forget how confusing terminology can be. I find that although we take for granted what is meant by “local” and “general” ventilation, the meaning is not necessarily obvious to someone new to occupational hygiene or to non-specialists, such as managers and workers in industry.

“Local exhaust ventilation” is used to describe extraction systems that extract contaminants close to the source, thereby preventing dispersion into the workplace. Yet although most people would probably interpret “local” as meaning “close to”, I don’t think that “local ventilation” is necessarily understood to mean that capture occurs at source. I’ve seen lots of poorly designed systems, including fans located in walls a fair distance from the source classed as “local extraction”. Similarly the term “general ventilation” is rather vague and I’m not convinced that most people understand what we mean by this – i.e. the use of extraction to dilute contaminants in the ambient workplace air.

There are also some situations where the terminology breaks down. For example, in a walk in spray booth contaminated air is extracted from the whole room, not from near the source. The worker inside the booth is not fully protected as he/she is still located within the contaminant cloud. From his / her perspective this is not local extraction. Yet, at the same time, it is something more than what we would normally class as “general ventilation”.

To overcome these problems, perhaps the use of alternative terms such as source ventilation, room ventilation and dilution ventilation would be better.

Stack Heights

When designing and testing local exhaust ventilation systems we need to pay particular attention to the design of the extraction hoods – where the contaminant enters the system. If this isn’t right then the system is unlikely to be effective at controlling contaminants. However, this doesn’t mean that we shouldn’t ensure that other aspects of the system are properly designed.

In many cases the system will exhaust outdoors and its then important to ensure that any contaminants remaining in the airstream are dispersed effectively so that they do not re-enter the building. This means that they shouldn’t be located too close to any air intakes, vents or windows. It is also particularly important that the stack is high enough. A good “rule of thumb” to follow is that the stack should be at leas one third the height of the building (i.e. it should release at a height 1.33 times the building height).  Its common sense that outlets should also be located away from any windows, doors and inlets. The stacks on the laboratory building shown in the picture above meet these criteria. There are plenty of others out there that don’t! Here’s a few.