Controlling static

The control of static is a major issue within cleanroom environments, particularly within the electronics industry. Voltages as low as 5 volts, which have no real meaning in other industries, can cause catastrophic failure of electronic components, or worse, latent damage that results in field failure, which is by far the most costly in terms of repair and manufacturers' reputation. The problem is predominantly addressed by the use of conductive or static dissipative materials in conjunction with ionisation. Ionisation inhibits the build up of static charges by delivering balanced ionised air to the target surfaces. This in turn prevents electrostatic discharges (ESD) and the electrostatic attraction of airborne particles. If static is not controlled it results in damage to components, which may include semiconductors, PCBs, medical devices and thin film products directly as well as interfering with the operation of vital production equipment.

Demands get tougher As cost competitiveness and quality become even more of an issue within production, manufacturers of MR heads, chips and readers in particular are demanding ionisers that can provide an ion balance of +5 volts, which is achievable throughout the marketplace. UK manufacturer and supplier of electrostatic solutions Meech has been working closely with one company, which has raised a number of issues about the effectiveness of many of the existing ionising blowers on the market. An extensive R&D programme at Meech examined the shortcomings of conventional cleanroom ionisers and consequently prompted the development of a new generation of ionising blowers for use in the cleanroom environment. The company focused on two vital areas of ioniser performance: ion balance and decay speeds. Now companies operating cleanrooms will see improvements in many areas – the manufacturing process, increased yield-rates of components providing greater profitability, and ultimately delivering a better quality of product to their customers.

Recognising the issue After discussions within the high-specification cleanroom industry, it became clear that there were shortcomings with the existing systems. It was recognised that the airflow characteristics needed to be improved in order to overcome problems that included ion imbalance across the work surface, operator discomfort. In addition, the industry was looking for ways to improve decay performance and offset voltages.

The solution With many years experience in dealing with electrostatic solutions, the design engineers tackled each of these problems and came up with the unique design of the Model 220ss cleanroom ionising blower (patent pending). Now launched, this new product is providing operators with the solution to the static problems created in the cleanroom. Fundamental to the high quality performance of the Model 220ss is its construction. Manufactured in stainless steel, a material particularly conducive to cleanroom operation, its 'tear drop' profile is the ideal shape for minimising disturbances to laminar air flows (see Fig. 1). Uniquely the motor has been sealed within the unit to ensure that, except for the emitters, there are no wear parts within the air flow, therefore reducing particulate contamination. For the user, savings are made in general maintenance and set up, due to the presence of only one control circuit, so that time savings can be made during set-up, calibration and maintenance. The 220ss meets the requirements for up to and including Class 10 cleanrooms.

Three become one The major advance and most beneficial design change seen in the 220ss is the incorporation of cross-flow fan technology. Rather than using the conventional multi axial fan arrangement, a single fan has been incorporated that has considerable benefits for the user. With one full-length cross-flow fan, consistent air velocity across the length is virtually guaranteed. This technology also provides effective coverage across the surface of the work area (see Fig. 2), as opposed to the axial fan design that can lead to ion imbalance (see Fig. 3). Axial fans create airflows that are turbulent rather than laminar, resulting in increased contamination, discomfort to operators and uncomfortable noise levels. Another main disadvantage with the use of the axial fan arrangement is the probability of operator presence in the airflow. This affects both ion balance and reduces decay performance. With the use of a cross-flow fan , the operator is kept out of the air flow. Health and safety of operators is of paramount importance and the cross-flow fan improves comfort as well as providing 'whisper-quiet' operation.

Voltage offsets and decay In typical applications, the sensor grill accurately controls the ion balance of the 220ss to +1 volt. However, for extremely sensitive applications where it is crucial that there is an accurate ion balance, a remote ion sensor can be connected, providing an ion balance of +0.25-0.5 volts. A further advantage when using a remote ion sensor is that the sensor itself is placed directly at the critical target area, providing the highest level of ESD security. Fig. 4 illustrates how conventional axial fan technology can result in airflows becoming mixed, resulting in ion imbalance (C) and areas where there is no ionisation (D). In contrast Fig. 5 illustrates the consistent laminar airflow with no ion imbalance or areas without ionisation achieved by the 220ss. In both figures, points (A) and (B) indicate the edge of the airflow from the ionising blower. In Fig. 4 the airflow is far wider above the work area, which can be uncomfortable for the operator and result in poor ionisation due to turbulence created by the operator entering the airflow. Fig. 5 highlights a far narrower airflow thus removing these issues.

The future This new generation of ionising blower is a big step forward for the cleanroom industry and is as a direct response to cleanroom managers demanding higher specifications and less ESD product wastage.