Electrostatic Solutions Ltd
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A primary objective of EPA design is that all personnel handling sensitive devices are grounded. The preferred means of grounding personnel is by using wrist straps. If this is not practical, use compliant footwear or heel and toe straps with compliant flooring. Seated personnel should be grounded via wrist straps - footwear grounding is not reliable in this case, and grounding via the seat should not be considered a reliable option.
Any surfaces on which ESDS are placed must be connected to EPA ground, and have resistance-to-ground Rg < 109 Ω. It is good practice in many cases to have Rg>104Ω to help avoid charged device model ESD risks.
An EPA (Electrostatic Protected Area) is a region within which no item or activity must be able to cause damage to a sensitive device. In the simplest case - a field work station - it may consist of a dissipative mat, a wrist strap and common grounding facility for both. At the other extreme it may consist a large manufacturing area where all the measures of 61340-5-1 are implemented. In some cases the EPA may consist of a single bench work area, which may not have an EPA floor.
The ESD Coordinator is responsible for determining EPA boundaries, construction, selection of equipment and general appropriate design of EPA for their particular application. Equipment used within the EPA must comply with the requirements of 61340-5-1.
If possible, a group of EPA work stations should be combined into one large EPA, including an EPA compliant floor. If this is not done, and each work station forms an individual EPA (which may not include an EPA floor). Sensitive devices must then be packaged in ESD protective packaging for transport between work stations.
Marking the EPA boundaries
Personnel must be able to see signs complying with 61340-5-1 before they enter the EPA. Where appropriate (i.e. in a multistation EPA), compliant signs must also be visible within the EPA, and exit signs should mark the boundary of the EPA.
EPA ground system design
The EPA ground provides a low resistance path (<2 Ω) to ground, usually to mains protective earth. A single EPA ground must be used within one EPA facility.
EPA ground bonding points (EBP) must be provided next to each working area or surface. These are points to which wrist straps are connected, and must be clearly marked in compliance with 61340-5-1. There must be sufficient EBPs to provide for operators and visitors, and they must be accessible. EBP are connected to EPA ground through a resistance of up to 106Ω .
Surface to ground resistance values may be achieved by including discrete resistors in the ground path, or by the resistance of the material which is directly grounded.
EPA ground cords are used to connect between groundable points and the EPA ground facility. One or more resistor may be included in the ground cord to give the required resistance to ground value. If a single resistor is used, it must be at the end of the cord at the groundable point. If more than one resistor is included, the resistor at the end connected to the groundable point must be at least half the total resistance in the cord. The ground cord connections, if accessible, must be shrouded with insulating material.
The EPA ground connection system must not be compatible with any connecting system used for any other purpose. The conducting parts of the connectors must be shrouded by insulating material when a cord is connected to the bonding point.
A wrist strap normally has a snugly fitting band worn around the wrist, and cord fitted with a quick release connection. The resistance between the band inner surface and its groundable point must be less than 105Ω. The conducting parts of the connecting system should be shrouded when connected. The strap ground cord usually incorporates a resistor (7.5x105Ω to 5x106Ω ) (see EPA ground cord B). The total resistance from hand to ground must be < 3.5x107Ω.
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Last modified: March 22, 2009
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