27 July 2022Manufacturing / Production Technology, Hardware & Services
Electrostatic discharge (ESD) can cause damage, especially to sensitive electronic components. Static charge can accumulate on surfaces of objects when there is an exchange of charge between the surfaces of two objects that touch, rub against, or are detached from each other, e.g., as is the case when foil is peeled off. One object is charged positively and the other negatively. People can carry a charge of up to 35 000 volts, depending on the environmental conditions.
Figure 1.
A shell of an electrical conductor (e.g., wire mesh or sheet metal) closed on all sides acts as an electrical shield against electrostatic discharges insofar as their contents do not come into contact with the shell itself (Faraday cage). If an electric field builds up between the positive pole and the negative pole, the negatively charged electrons of the shell migrate to the side of the positive pole. As a result, a charge shift occurs within the shell, which generates an electric field in the shell that acts in exactly the opposite direction to the external electric field (Figure 1).
As a result, the effects of the fields in the envelope cancel each other out exactly, and the interior is shielded (Figure 2).
Permanent grounding of working personnel in ESD areas by means of grounding straps, ESD clothing, ESD gloves and antistatic safety shoes as well as further measures such as antistatic work surfaces, floor coverings or conductive covers of office furniture are basic requirements for ESD-protected production.
Figure 2.
But even in an electronics cleaning system with a closed metal chamber, static charge can still occur, especially during the drying process. Humidity control is known to be an active measure against electrostatic discharge. Once an optimum humidity level is reached – typically between 40 and 60% relative humidity – this is sufficient to discharge electrostatic build-up into the air without danger. However, as soon as the humidity drops below 40% during the drying phase while micro-particles can be introduced into the chamber by blowers, the possibility of static charging increases significantly.
Figure 3.
To shield against electrostatic fields within the cleaning chambers of kolb systems, ESD-sensitive electronic cleaning goods are therefore accommodated in special kolb metal washing frames (Figure 3) or kolb metal washing baskets (Figure 4). These fixtures also make use of Faraday’s principle by shielding the cleaning products from electrostatic fields as a closed unit. In addition, the fixtures are grounded via suitable materials with leakage resistances of 103 to 107 Ω. These include, for example, the black ESD casters of the washing baskets and the mounting brackets of the washing frames.
This means that in kolb cleaning systems the products are continuously ESD-protected through all process steps.
For further handling, such as manual loading and unloading of the cleaning goods, all relevant kolb systems have an ESD grounding connection box as standard.
In the standard version of the systems, the double-wall airlock door, the control panel including switching elements, lid, side panels and collecting tray are not ESD-compliant, as these do not come into contact with the cleaning goods at any time. Static charging or contact with ESD-sensitive electronic components can thus be ruled out. The airlock door (used only for heat decoupling of the process chamber against contact) is located in such a way that during loading and unloading and during the process, contact of the cleaning good with the door is not possible.
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