Desco - Questions And Answers

We are contract manufacturer, where at our production floor there are some components such as sockets, connectors packaging trays made of insulative materials which can generate very high level of static charge. During assembly process the boards are placed on a grounded workstation and the person who handles the boards are also grounded via a wrist strap. The trays are placed on the workstation with a distance of 10 - 15 " away from the assembled boards. Question: Can the static charge from the insulative trays can damage or weaken the components on the boards? How does induced charge from insulative materials can damaged components on a grounded workstations? What are the solutions? - Anonymous

Static charge from the insulative trays have emits an electric field. It is this field that can influence other materials, especially isolated conductors (circuit traces) causing them to respond to the field and align their surface charges to terminate the fields lines (charging through induction). The induced charge is dependent on the strength of the electric field which is also dependent on the proximity of the electric field (the closer the stronger the field strength). Necessary charge generators, such as the trays you mentioned (if you can’t replace them with static dissipative trays) can be controlled by various methods. The first method would be to keep these trays a minimum of 12 inches from your sensitive devices. This has been determined by empirical studies from the ESD Association to be safe for most materials harvesting electric fields over 2000 V/inch. Section 6.2.3.2. from the ANSI/ESD-S20.20 standard states that:6.2.3.2. Protected Areas Guidance A Protected Area may be a single workstation (fixed or portable), laboratory, room, building or any other area with pre-designated boundaries that contains materials and equipment designed to limit electrostatic potentials. Humidity control may be a key element in an ESD control program. Propensity for charge generation and accumulation increases with a reduction in humidity. All process essential insulators that have electrostatic fields that exceed 2,000 volts should be kept at a minimum distance of 12 inches from ESDS items. 2,000 volts is a measure of the electrostatic field at the point of measurement and is not necessarily directly related to the electrical potential of the item. The accurate measurement of electrostatic fields requires that the person making the measurement is familiar with the operation of the measuring equipment. Most hand held meters require that the reading be taken at a fixed distance from the object. Equipment manufacturers typically specify that the object being measured needs to have certain minimum dimensions. Objects smaller than the minimum dimensions may not provide an accurate reading. Additional guidance related to Protected Areas may be obtained in ESD ADV 2.0. You can download this standard free of charge (assuming you’re wearing a grounded wrist strap). A second method of control would be to ionize these trays with a quality ESD ionizer. Depending on the sensitivity of the components you’re trying to protect, you may need to incorporate both methods of control.

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