Over the next few years, already sensitive Schottky and CMOS (Complementary Metal Oxide Semiconductor) logic forms hardware will be joined by even more sensitive technologies. Therefore, ESD is an increasingly critical factor in the handling and manufacturing of electronic devices.
The larger the electrical charge, the more potential for causing damage to sensitive devices. A part may be damaged and fail immediately or it may take hours or years before the damaged part becomes inoperative (latent failure).
An immediate (or catastrophic) failure occurs when ESD damages the part and renders the part useless immediately after the ESD event. This type of failure would require replacement of the damaged part before any further testing could be accomplished.
A latent failure occurs when the ESD damage is not severe enough to require replacement of the part. This type of failure may be noticed as a glitch in the signal, which only occurs for a microsecond. Further testing may not indicate any damage to the part and operators assume the part to be acceptable. The ESD damage caused the part to fail intermittently whereby a defect was not detected; however, over time and use, the part may degrade until it causes a permanent failure. The latent failures can create more problems as the part is assembled into the final product. The cost to replace parts and equipment after testing and manufacturing is much higher than replacing a single part before assembly
Other failures caused by ESD are often categorized as random, unknown, infant mortality, or manufacturing defect due to the lack or depth of failure analysis performed. A major reason for the above is that few laboratories have equipment or funding required to trace failures to ESD
In many instances, ESD as a cause of failure is not always recognized and, hence, ESD control measures are not adequately implemented
The following precautions must be taken to assure that all electrostatic charges are kept to a minimum:
- All personnel must be trained to be aware of ESD hazards
- Personnel should not wear clothing that can cause the buildup of static charges on the person
- Areas where ESD parts are handled should be limited to only trained personnel to prevent inadvertent damage to parts and equipment
- The level of protection in the work area must be below the damage level of the most sensitive ESD parts that are being handled
- Written procedures outlining detailed steps are required to ensure that personnel follow the proper procedures to avoid high electrostatic potentials
The following are ESD control requirements to consider:
- Verify conformance of ESD protected areas/grounded workbench.
- Proper use of ESD protective equipment.
- Avoid use of charge generating materials in ESD protected areas.
- ESD audits and inspections.
- Inclusion of ESD requirements on documentation.
- Control the relative humidity.
- Use Faraday Cage containers to store and transport ESDS items.
- Air ionizer.
The impact of ignoring ESD is:
- Higher cost for manufacturer and user.
- Lower product quality.
- Unhappy customers, especially if part or assembly fails at a critical time.
- Potential mission failure.
The best method to control the damage caused by ESD is to eliminate any electrostatic charges in the work place. This is easier said than done. A basic guideline to prevent electrostatic charges is to eliminate any unnecessary materials in the work place. Smoking, eating and drinking are some of the most common activities that should be prohibited in the work area. The plastic packaging that most foods are wrapped in carries a high probability for electrostatic charges.
All work surfaces in an ESD-protected area shall be static dissipative (104 to 1011 ohms/square) and electrically connected to the common point ground. Each work area must be checked periodically to determine whether the conductive, static dissipative or insulating properties of the material are still intact.
When required, monitoring equipment such as wrist strap monitors and testers, surface resistivity meters, and ionizers shall be properly calibrated and used.
ESD Protected Areas
Typical ESD Grounded Workbench: A dissipative work surface should go to hard ground while a conductive surface should go to a soft ground.
Static Free Workstation:
- Static Dissipative chair and bench mat
- Conductive smock
- Static awareness labels
- Static safe notebooks
- Static awareness signs
- Shielding bag
- ESD qualified gloves and finger cots
- Air ionizer
- Wrist strap
- Heel and toe grounders
ESD Wrist Strap:
Items to not use in an ESD Protected Area:
Protective packaging is important to protect items while they are being shipped or are removed from an ESD safe environment. Packaging will include conductive materials and anti-static materials. Some applications may use a static dissipative material in place of the conductive.
Most packaging bags and containers provide a layer of anti-static and conductive materials. These materials protect against static fields and the generation of static. Since the protective bags are instrumental in providing shielding the bags may not be punctured or damaged. A puncture in the bag may provide a path for static fields to damage parts.
These bags shall not be sealed with adhesive tapes that can generate static charges.
All packaging should be labeled with a warning sign indicating the items packaged are ESD sensitive.