Airborne pollutants are a composition of particles of different sizes and compositions with again different chemical and physical properties. Solder fume is a good example to illustrate their influence on their environment (Fig. 1). It consists mainly of decomposition products of fluxes, soldering materials and residues of cleaning agents, which can combine to form sticky aerosols. These not only have a negative impact on the health of employees, but can also form firmly adhering layers of dirt and thus contaminate production equipment, which has a lasting negative effect on production quality. If, for example, manufactured electronic assemblies are contaminated with sticky dusts, this can lead to corrosion of the conductor paths, which can result in partial or even complete loss of functionality.
Airborne pollutants are classified according to particle size. This classification (Fig. 2) primarily focuses on the influence of the emissions on the human organism. They are differentiated not only according to whether they are brain, nerve or respiratory tract damaging, but also whether they are inhalable (E fraction) or alveolar (A fraction). For this purpose, there are legal limits according to DIN EN 481, which are 10 mg/m³ for the E-fraction and 1.25 mg/m³ for the A-fraction according to TRGS (Technical Rules for Hazardous Substances) 900. The legal regulations of TA Luft (Technical Instructions on Air Quality Control) allow a total dust mass concentration including fine dust of 20 mg/m³. However, this only applies to dusts that are not harmful to health and does not include CMR substances (carcinogenic, mutagenic, and toxic for reproduction). Airborne pollutants occur at various points in the electronics production.
In the manufacturing process, soft soldering methods are used for the most part, including wave, reflow, vapor phase, laser or hand soldering. This process produces a variety of hazardous substances, e.g. organic tin compounds, hydrogen chloride, formaldehyde, acetaldehyde, acrylaldehyde or butyraldehyde. The following soldering fume concentrations were determined during hand soldering:
- Concentration with leaded solder: approx. 1mg/m³
- Concentration of lead-free solder: approx. 1.3mg/m³.
However, the proportion and quantities of these compounds depend on the composition of the solder and the soldering temperature.
Due to the combination of pollutants in the soldering fumes, combination filters are used for optimal separation (Fig. 3). These consist of resublimation filters, particle filters in several stages and adsorption filters. For larger wave soldering systems, a separate filtration of fluxer and solder wave is used. When using fluxes containing solvents, fans with Ex-equipment are used to avoid explosions or fires.
Gases and vapors
These are created during printing, potting, painting, gluing, cleaning, etc. Typical pollutants are isopropanol, toluene, acids, butanol or resins. The respective safety data sheet is used to determine the relevant substance data. Based on this, the type of filtration is determined, whereby in this case sorption technologies are used, i.e. adsorption by means of activated carbon or chemisorption by means of chemically modified adsorbent. The safety data sheet also provides information about the explosion limits. When using fluxes containing solvents, fans with Ex equipment are used to avoid explosions or fires. If the explosion limit is not safely under the lower explosion limit, Ex-fans must again be used in the extraction system.
They are created in electronics production, for example, during cutting, milling, grinding, polishing or decanting. Typical pollutants are coarse and fine dust and odors. Dry dust is usually separated by means of cartridge filter systems. The filter cartridges can be cleaned and thus used over a relatively long period of time. All dusts have to be checked for flammability before they are extracted and filtered, since in this case Ex-installations have to be used in order to comply with occupational safety and explosion protection.
Laser smoke is mainly generated during labeling/marking, separating, decoating or structuring. Typical pollutants are partly nanoscale particles, but also aerosols and gases.
During laser processing, material transformation takes place through pyrolysis, oxidation, reduction or polymerization. This requires special filter combinations. Furthermore, the composition of the laser smoke is determined by the type of laser source and therefore by the energy input.
Due to the combination of pollutants in the laser smoke, combination filters are mostly used. These consist - as in soldering fume extraction - of resublimation filters, particle filters in several stages as well as adsorption filters.
Due to the diversity of laser fumes, filter geometries and their steps are adapted to the application. Cleanable filter systems are also used. In these systems, the separation quality and thus the filtration performance is increased by adding filter aids.
Recording of air pollutants
An important aspect of air purification is the detection of air pollutants. This is usually done by means of extraction arms and a mounted detection element. Size and type are determined by the pollutants themselves, their thermal and other air movement influences. The level of capture and separation efficiency is decisive for the efficiency or filtration performance of the respective extraction system (Fig. 4).
The proximity to the emission source is of decisive importance - the closer, the better. Not only to capture the majority of all particles before they can spread, but also to minimize the economic effort.
"The rule of thumb that twice the distance between the emission source and the detection element requires at least four times the suction power of the extraction and filter system has proven itself in practice."
The specification of the optimum detection element, extraction arm or extraction hose is determined by the application itself. In production lines, extraction and filter systems are often integrated directly into processing equipment, e.g. laser markers or soldering systems. The detection of contaminants is therefore carried out in an encapsulated system and serves primarily to protect the system itself and the product.
For manual workplaces, different variants are available. For example, a filter unit can be used as a central extraction unit. The respective workstations are equipped with detection elements that are connected to the system via pipes or hoses. The exhaust air flow is activated or deactivated via throttle valves. Depending on the amount of pollutants produced, each manual workplace can also be equipped with an extraction system. Here, primarily mobile systems are used, which are particularly suitable for changing workplaces.
Although many processes in electronics production are standardized, the variety of materials used and processed often requires an individual analysis of the pollutant situation. The use of different organic, inorganic or toxic substances, right up to the use of precious metals, whose recycling must be taken into account when selecting filters, are just a few examples of the "normal abnormal". Standard solutions are no help here. The respective manufacturing situation must be individually evaluated and a corresponding special solution must be designed. In addition to the extraction and separation performance of a filter system, other parameters such as volume, robustness or space requirements also come into play. After all, besides breathing clean air, employees do not want to be disturbed or hindered in their daily tasks.
23 October 2020