Environmental and technical characteristics of conductive adhesives versus soldering

6. Occupational health aspects of adhesive and solder technology

6.1 Comparison
6.2  Recommendations
6.2.1 Principles for designing a production line with minimal impact on occupational health
6.3 Handling conductive adhesives safely

The objectives of the occupational health analysis have been to:

1. Compare occupational risk associated with the mounting processes applied when joining electronics with electrically conductive adhesives and tin/lead solders, respectively.
2. Provide recommendations on optimum design of production lines and manual handling when applying adhesives

6.1 Comparison

A detailed analysis for the Danfoss test case can be found in [4]. The conclusion states that it is difficult to judge between the alternatives as exposure data are lacking. However, if proper measures are taken, it is believed that the risk of handling the materials is low for both adhesive and soldering technology.

Another occupational health assessment was carried out at Mekoprint A/S. A comparison with soldering was not possible here as only adhesives are applied. Air measurements had been made here in order to detect possible inhalation exposure connected with adhesive application. Further, an evaluation on skin contact exposure was made. The conclusion – also here – was that proper handling of the adhesives results in a very low occupational risk.

Information on inherent toxic properties of a wide number of adhesives can be found in [5].

6.2 Recommendations

Based on experience from the two occupational health cases, recommendation can be given in relation to design of a production line and to manual handling of conductive adhesives.

6.2.1 Principles for designing a production line with minimal impact on occupational health.

When designing a new production line, a number of occupational health and safety issues have to be taken into consideration.

First of all, the choice of machinery and the design of the work process should support the establishing of a workplace that is feasible from an ergonomic and a psychological point of view. The machinery should be chosen to avoid noise- or vibration related problems.

These issues will not be discussed any further in this paper. The paper focuses on the possible impact on health and safety caused by the use of adhesives.

The conductive adhesives are dispersions of an appropriate conductor (e.g. silver) in a polymeric binder. The typical one-component binders, which are applied in the conductive adhesives, are epoxy, silicone, polyester and polyimide resins. Silicone and epoxy resins are used in two-component binders as well.

For some applications the use of adhesives will require cleaning of the printed wiring board (PWB) before the adhesive is applied. The recommended cleaning agent will most frequently be an organic solvent such as butanone or ethanol.

The units containing and applying the adhesives must be cleaned regularly using a proper solvent. This solvent will often be identical to the solvent used as a thinner for the adhesive.

For a general hazard and risk assessment of the chemicals, please refer to [5].

Recommendations:

	One-component adhesives should be used whenever possible from a technical point of view. This will render the mixing of two components superfluous and reduce the risk of exposure (skin contact and inhalation). In case a two-component adhesive is to be used, the manual mixture of the adhesives should be avoided by establishing a mechanical mixture unit in the assembly line.
	The adhesives should be bought from the supplier in containers ready for installation in the assembly line if convenient. This will eliminate the risk of exposure (skin contact and inhalation) while decanting the adhesive into other containers. Another possibility would be to decant the adhesive to containers for use in the assembly line in one step and the maybe freeze the containers until they should be used.
	Manual handling of the chemicals should be avoided to the greatest possible extent, e.g. when applying the adhesive, mixing or cleaning the accessories.

	Cleaning of the PWB should be carried out in a closed automatic cleaning machine. In different industries the problems related to cleaning technology have been solved in ways leaving only few health- and safety related problems. It is recommended to benefit from these experiences.
	Cleaning of the application part (e.g. a needle) should be done automatically. The cleaning process might be a periodically occurring and integrated part of the production flow.
	If cleaning is planned as a manual or semi-manual process, proper process ventilation will be required if any organic solvents are used. In addition measures have to be taken to avoid skin contact.

Most adhesives will cure only or most efficiently by heating. In case the curing process cannot be brought to an end on the automatically line, the PWB’s should be collected in a well ventilated area in order to ensure fresh air in the breathing zone of the people working there.

It is recommended to establish the possibility for ventilation of the assembly line already when designing since it is difficult to envisage which type of adhesive will be chosen from the technical point of view in due time. Even though a process has been automatized, the possibility for liberation of vapours to the working environment should be avoided. Whether the ventilation has to be process ventilation or a general ventilation of the room depends on the amount and the hazard of chemicals in use.

	The curing of the adhesive should take place in a ventilated area. Some adhesives will start curing as soon as applied requiring proper ventilation of the assembly line. Special notice has to be taken when silicone based adhesives are chosen, as these may liberate irritating vapours during curing.
	Any use of organic solvents requires ventilation as well.

The recommendations given adhere to the design- and planning process. Recommendations on how to handle the chemicals safely are given in the following paragraph.

6.3 Handling conductive adhesives safely

Recommendations on how to design a production line with minimal impact on the occupational health and safety was given above. By following these recommendations, the risk connected with working with conductive adhesives will be reduced.

However, a well considered working practice, the proper use of available technical preventive measures and personal protective equipment as well as good hygiene at work are also necessary in order to minimise the actual risk.

As a basic principle, the preventive measures should be prioritised as follows:

1. Designing the assembly line in a way that reduces risk as much as technical possible
2. Establishing technical measures (e.g. ventilation) where needed.
3. Prescribing the use of personal protection equipment.

Personal instruction at the workplace about the actual health hazards arising from the adhesives and other chemicals is a must in order to enable the employees to handle the hazards and to make the right use of the preventive measures offered. Knowledge about the chemicals and their inherent hazards will provide the necessary background for sound handling of the chemicals.

The instruction must include the following:

	Knowledge about the inherent hazards of the chemicals, e.g. whether the chemicals contain sensitising substances, must be available in order to decide how to work with the chemical and to motivate the employees for using the proper preventive measures.
	Skin contact with chemicals must be avoided. It is of utmost importance that gloves are used properly and that the gloves are resistant to the chemicals. Routines on how often to change the gloves and how to handle the gloves must be implemented.
	Inhalation of vapours and dust must be avoided. It is important for the employees to know how to control the efficiency of the ventilation.
	The hygiene at the workplace must be proper in order to avoid the risk for oral uptake of chemicals (e.g. the metals) due to contaminated cloths and contaminated workplaces.

Whenever required by legislation, the employees must receive the mandatory training as well as a general training is recommended.