What is anti static protection

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 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% RH 65-90%
Walking across a carpet

 

 35,000 V

 

 1,500 V

 
Walking on a vinyl tile floor

 

 12,000 V

 

 250 V

 
Vinyl envelopes for work instructions

 

 7,000 V

 

 600 V

 
Worker at bench

 

 6,000 V

 

 100 V

 

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

  • Human Body Model (HBM)
  • Machine Model (MM)
  • Charged Device Model (CDM)

 

What damage does ESD cause in an electronic device?

There are basically two categories of damage from ESD:

(a) 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.

(b) 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.

 

Protection for Electrostatic Discharge Sensitive (ESDS) devices.

(A) Work area:

  1. 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).
  2. 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.
  3. 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.
  4. Avoid bringing sources of static electricity (as shown in page 1) within 1 meter of a static-safe work bench.
  5. Where it is necessary to use air-guns, use special models that do not generate static charges in the air stream.

(B) Personnel:

  1. 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.
  2. The use of an antistatic smock for each worker is highly recommended.
  3. Education and training on ESD preventive measures is invaluable.
  4. A regular audit is also helpful in supporting an ESD program.

(C) Packaging and Transportation:

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

 

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

Write By:
Marek Adamczyk
TOOLSTATIC
Product Engineer – ESD Protection

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