ESD Control/Prevention

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ESD Controls

The ESD Association suggests focusing on just six basic principles for the development and implementation of an effective ESD control program, namely, 1) ESD Immunity Design-in; 2) definition of the desired level of ESD control; 3) identification of electrostatic protected areas (EPA's); 4) reduction, if not elimination, of static generation; 5) static dissipation and neutralization; and 6) protection of products from ESD.

ESD Immunity Design-in

Prevention of ESD-related problems starts with the ability to produce robust devices that can withstand ESD events. This requires the proper determination of the ESD sensitivity levels of new semiconductor devices prior to their production release. This is best achieved by subjecting representative samples of these devices to industry-standard ESD sensitivity tests using an ESD sensitivity testing system (see example in Fig. 1).

There are several ESD sensitivity testing procedures available today, each one depending on the ESD model being tested for. It is good practice to test a device in terms of different ESD models. Devices that exhibit inadequate immunity to ESD must be redesigned, if possible. This is known as ESD immunity design-in.

Fig. 1. An ESD/Latch-up Tester from KeyTek

Identification of EPA's

Aside from designing ESD immunity into its products, a company must have a sound ESD control program. To begin with, a company must identify all electrostatic protected areas (EPA's) in its factory. An EPA is an area where ESD-sensitive devices will be handled. Every EPA must be adequately protected by ESD controls, the major ones of which are discussed below.

Static Reduction/Dissipation/Neutralization

Over-all Grounding

The backbone of static generation reduction and static dissipation is proper grounding of everything a device touches. The primary means of grounding ESD susceptible (ESDS) items (personnel, equipment, workstations, carts, shelves, etc.) is to provide electrically conductive paths between these items and a common ground. Thus, every factory must have a common grounding point. Detailed information on ESD grounding can be found in ESD Association standard ESD-S6.1, Grounding-Recommended Practices.

Connecting everything to a common ground point essentially puts everything at the same potential (the potential of the common grounding system). As long as everything is in equipotential balance, charging/discharging events will be prevented. It is important to note though that insulators in an Electrostatic Protected Area (EPA) cannot be grounded, so insulative materials must be avoided in EPA's as much as possible.

ESD Association Standard ANSI EOS/ESD 6.1-Grounding recommends a two-step procedure for grounding equipment. The first step is to ground all components of the work area (worksurfaces, people, equipment, etc.) to the common ground, which is also referred to as the 'ESD common point ground'. This ESD common point ground should be properly identified. ESD Association standard EOS/ESD S8.1-1993 recommends its own symbol to identify the common point ground.

The second step is to connect the common point ground to the equipment ground or the third wire (green) electrical ground connection. This is the preferred ground connection because all electrical equipment at the workstation is already connected to this ground. Connecting the ESD control materials or equipment to the equipment ground brings all components of the workstation to the same electrical potential.

If a soldering iron used to repair an ESDS item were connected to the electrical ground and the surface containing the ESDS item were connected to an auxiliary ground, a difference in electrical potential could exist between the iron and the ESDS item. This difference in potential could cause damage to the item. Thus, any auxiliary grounds (water pipe, building frame, ground stake) present and used at the workstation must be bonded to the equipment ground to minimize differences in potential between the two grounds.

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