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THE PREVENTION AND CONTROL OF ELECTROSTATIC DISCHARGE (ESD)

An important checklist to ensure that all sources of potential ESD are identified and nullified.

We experience occur­rences of static electric­ity everyday. For exam­ple, walking along a carpeted floor in a heated room during winter generates sufficient static electricity to give us a rather shocking experience when we touch the door knob. While this sudden discharge of static electricity does not result in any harm to the human body, it can be very damaging to electronic devices which are sensitive to electrostatic dis­charge (ESD). It is possible for electronic devices to be dam­aged by ESD that is imperceptible to the human body. This document is intended to shed some light on the sources of ESD and provides guidelines on the prevention and control of ESD.

What are the common sources of static electricity?

The following table shows a sample list of sources of static electricity.

 

Object or Process

Material or Activity

Work Surfaces

Waxed, painted, or plastic surfaces

Floors

Waxed, common vinyl tiles, sealed concrete

Clothes

Common smocks, non- conductive shoes, synthetic materials [e.g. nylon)

Chairs

Vinyl, fiber-glass, finished wood

Packaging

Common plastic bags, foam, trays, tote boxes

Assembly Area

Spray cleaners, heat guns, blowers, plastic fools (e.g. solder suckers, brushes), cathode rays

What are typical examples of static charge inducing situations?

Does humidity have any effect on the induced static charge? The following table shows some typical situations. Please note that humidity has a significant effect on the induced charge. It is not recommended to have relative humidity (RH) that is too low, say, below 30%. ESD control becomes espe­cially challenging at low RH levels. A relative humidity between 40% to 60% is recommended for the typical assembly area.

 

Means of Static Generation

RH 10-20%

RM 65-90%

Walking across a carpet

35,000V

1,500 V

Walking on a vinyl tile floor

12,000 V

250V

Vinyl envelopes for work instructions

7,000 V

600V

Worker at bench

6,000 V

100 V

How does damage from ESD happen?

When a statically-charged person or object touches an electrostatic discharge sensitive (ESDS) device, there is a possi­bility that the electrostatic charge could be drained through sensitive circuitry in the device. If the electrostatic discharge possesses sufficient energy, damage could occur in the device due to localized overheating. Generally, devices with finer geometries are more susceptible to damage from ESD.

The modes in which ESD damage occurs are:                  

·          Discharge to the device

·          Discharge from the device

·          Field – induced discharge

 

What are the classifications of ESD sensitivity?

Electrostatic discharge sensitive (ESDS) parts arc commonly
characterized to three defined models:

·         Human Body Model (HBM)

·         Machine Model (MM)

·         Charged Device Model (CDM)

Based on the models used, the ESDS parts can be classified in accordance with the following table (per MIL-STD-1686C, with HBM subgroups per ESD STM5.1-2001). It should be noted that the HBM, MM, and CDM voltage levels do not correlate with each other.

 

ESD Model

ESD Classification

Voltage Range

Human Body Model (HBM)

0

0V-249V

 

1A

250V-499V

 

1B

500V- 1999V

 

1C

1000V- 1999V

 

2

2000V-3999V

 

3A

4000V-7999V

 

3B

>8000V

Machine Model (MM)

Ml

OV-249V

 

M2

101V-200V

 

M3

201V-400V

 

M4

401V-800V

 

MS

>800 V

Charged Device Model

CI

0V-124V

 

C2

I25V-249V

 

C3

250V-499V

 

C4

SOOV-999V

 

C5

1000V-1499V

 

C6

1500V-2999V

 

C7

>3000V

 

What damage does ESD cause in an elec­tronic device?

There are basically two categories of damage from ESD:

·         Catastrophic damage: The electronic device is rendered inoperable immediately after the ESD event. A semiconductor junction or a connecting metallization could have been damaged by the electrostatic discharge.

Latent damage:

The electronic device appears to be working fine following the ESD event. However, the sensitive circuitry has been damaged and could fail to operate properly at some time in the future.

 Detection of static charges in the work area: A commonly used tool for the detection of static charges is the electrostatic field meter. This tool, when used in conjunction with regular audits on the production floor, is very effective in detecting the presence and magnitude of static charges. Care should be exercised to ensure adherence to the measurement distance as the meter is calibrated to specific distances from the measuring plane. Please consult the user's guide on the correct measuring distance for the specific electrosta­tic field meter model that is being used.

Figure 1, Static safe workbench

Figure 2.   Details of a static-safe work bench.

Figure 3. Static control test station

 

Protection for Electrostatic Discharge Sensitive (ESDS) devices.

WORK AREA:

  It is essential to handle ESDS devices at static-safe workstations. This will prevent yield loss (through catastrophic damage) or, worse, potential reliability failures in the field (through latent damage).

  Where it is impractical or impossible to use antistatic wrist-straps or remove items that are composed of insulative materials at a static-safe workstation, use an air ionizer designed to neutralize electrosta­tic charges or apply topical antistats to control gen­eration and accumulation of static charges.

  When an air ionizer is utilized, it is vital that maintenance procedures and schedules are adhered to in order to ensure that ions generated by the ionizer are sufficiently balanced,

   Avoid bringing sources of static electricity within 1 meter of a static-safe work bench.

   Where it is necessary to use air-guns, use special models that do not generate static charges in the air s

PERSONNEL;

  Any accumulated charge on the body of the human operator should be discharged first before opening the protective container with. ESDS devices inside. The discharge can be accomplished by putting a hand on a grounded surface or, ideally, by wearing a grounded antistatic wrist-strap.

  The use of an antistatic smock for each worker is highly recommended.

  Education and training on ESD preventive mea­sures is invaluable.

  A regular audit is also helpful in supporting an ESD program.

PACKAGING AND TRANSPORTATION:

ESDS devices should be contained in a static protective bag or container at all times during storage or transportation.

Static-safe work bench

Figure .1 shows a typical static-safe work bench. The table top is covered by a static dissipative mat which is grounded through a 1 Meg-ohm resistor. This resistor is required in order to protect the users of the static-safe work bench-in the event that the ground becomes elec­trically live, the resistor will prevent electrical shock at the work bench. The same safety requirement holds true for the antistatic wrist-strap as well.

 Figure 4. Antistatic footwear.

Figure   5.   Labels  to   identify  electrostatic  discharge sensitive (ESDS) devices.

What materials are suitable for dissipating static electricity?

It is recommended that static dissipative materials are used as the medium (e.g. mats, containers) for discharging static charge to ground. These materials have the following properties:

·         Surface resistivity: 1 x 105 to 1. x 1012 ohms/sq

·         Volume resistivity: 1 x 104 to 1 x 1011 ohm-cm

 

Cautionary note: Materials which are conductive (e.g. stainless steel surfaces) are not recommended for use as a static-safe work surface; the low electrical resistance could result in a transient-like (surge) discharge of static electric­ity. A rapid discharge is far more damaging to the electron­ic device than a gradually paced discharge through a static dissipative material.

Figure 2 shows the details of a static-safe work bench. It is vitally important that the wrist-strap and the table mat arc securely grounded (through the 1 Meg-ohm safety resis­tor). In addition, all other materials with which the products come into contact must also be static-safe. The use of an antistatic floor further enhances the protective capabilities of a static-safe work environment. The worker should also wear an antistatic smock.

An example of a Static Control Test Station

Figures 3 shows an example of a test station used to deter­mine whether antistatic wrist-straps or antistatic shoes are working properly,

A green test indicator light means the wrist-strap is worn properly and is working as intended. This picture shows the configuration for testing wrist-straps. The test station can also be configured to test antistatic footwear.

Antistatic footwear

Where a wrist-strap is impractical, e.g. the job requires the worker to walk from one location to another, it is recom­mended that antistatic footwear such as antistatic shoes or heel straps are worn. Figure 4 shows an example of an anti­static heel-strap with the grounding cord running into the socks to make contact with the skin. It is also necessary to use an antistatic floor (e.g. conductive floor tiles) to work together with the antistatic footwear.

Figure 5 shows labels commonly used on containers and packaging to alert anyone who handles the ESDS devices on the need to use static-safe procedures before handling the devices. The label on the left is preferred. The follow­ing verbiage should be placed beside the labels:

Contains parts and assemblies susceptible to damage by

Electrostatic Discharge (ESD)

References:

1.        ANSI/ESD S8.1 -1993: "ESD Awareness Symbols"

2.        ANSI/ESD S20.20 : "ESD Association Standard for the               

         Development of an Electrostatic Discharge Control Program."

    3.     ESD STM5.1 -2001: "Electrostatic Discharge Sensitivity

               Testing-Human Body Model (HBM) Component Level."

4.        MIL-HDBK-263: "Electrostatic Discharge Control       

         Handbook for Protection of Electrical and Electronic Parts,       

         Assemblies and Equipment (excluding electrically-initiated      

         explosive devices)."

    5.     MIL-STD-1686C: "Electrostatic Discharge Control    

              Program for Protection of Electrical and Electronic Parts,      

              Assemblies and Equipment (excluding electrically-initiated

              explosive devices)."

    6.  JESD625-A: "Requirements for Handling Electrostatic-Discharge-Sensitive (ESDS) Devices."

Mini-Circuits Technical Staff can he reached at, Engineering Dept., Mini-Circuits, P.O. Box 350166, Brooklyn, New York 11235-0003, 718-934-4500, www.minicircuits.com

Κώστας Ντόβας

Διπλ. Μηχανολόγος Μηχανικός                      Αθήνα 10-06-2004

 

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