ESD Damage and Prevention
ESD Damage and Prevention
Over the next few years, sensitive Schottky and CMOS (Complementary Metal Oxide Semiconductor) electronics will be joined by even more sensitive technologies in space system. 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 work could be accomplished.
A latent failure occurs when the ESD damage is not severe enough to detect immediately. This type of failure may be noticed as a glitch in the signal, which only occurs for a microsecond and may categorized in as a “cannot replicate” error. Further testing may not indicate any damage, and the part will be considered to be acceptable; however, over time and use, the damage worsens causing a permanent failure. Latent failures are not just a risk against cost and schedule, where extra time and money is needed to repair late in the development lifecycle, but pose a technical risk to overall mission success after launch.
The opaque nature of latent defects, and the timing of failure well after the ESD event occurred, causes ESD damage to be misdiagnosed as a random failure, unknown cause, infant mortality, or some other manufacturing defect. Failure analysis techniques for discovering ESD damage are specialized and experienced analysts are generally required to provide sufficient investigative depth. Few laboratories have the equipment or experienced personnel for tracing failures to ESD, and few developers have the time, late in the lifecycle, to wait for the depth of investigation required. A lack of awareness of ESD-driven damage can reduce the priority and resources given to ESD control methods.
The potential impacts of ignoring ESD include:
- Lost schedule;
- Higher cost for manufacturer and user;
- Lower product quality;
- Unhappy customers, especially if part or assembly fails at a critical time;
- Potential mission failure.
Eliminating electrostatic charges in the work area is a critical part of ESD control. This can first be done by eliminating unnecessary materials. Unnecessary materials are called nonessential items and can be anything from personal items such as food, drinking cups, family pictures, and tissue boxes to administrative items such as calendars, plastic pens, and cardboard boxes, to technical items that are not necessary for the work being done such as phones, tool boxes, and computer monitors. Electrically insulative items pose a particular risk in an ESD-controlled work area because they do not relinquish their static charge to a bleed-off circuit. When an insulative item is critical for the work being performed, an air ionizer can be used to bath the item with positive and negative charges which serve to neutralize the static charge on the insulative item.
Providing a low-current discharge path for static charge to drain through is foundational in the ESD-controlled work area. Static dissipative is defined as in the 10^4 to 10^11 ohms/square range. To complete the drain circuit, the static dissipative surface must have a conducting contact point that can be electrically connected to a single ground point. See ANSI/ESD S20.20 for explanation about how to arrange a common ground point and equipotential bonding where multiple ground connections are needed. Each work area must be checked electrically periodically to confirm that the conductive, static dissipative or insulating properties of ground points, connections, surfaces, furniture, and work items are still intact. The frequency of verifications should be based on the likelihood of the items and connections being degraded or disturbed over time and with use of the work area. Verification of wrist strap functionality, either using a stand-alone checker or a continuous monitoring system, is recommended before or during each use, respectively. Verification test equipment such as field voltage meters, humidity monitors and surface resistivity meters shall be properly calibrated before use.
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-sensitive 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 reduce static charges to below the damage level of the most ESD-sensitive parts that are being handled (See electronic part data sheets for examples of their ESD sensitivity ratings. The ratings are defined in terms of the lowest charge, in volts, that when discharged through the part, can damage the part. The lower the voltage value, the more sensitive the part is.);
- Use written procedures outlining detailed steps that personel must take that avoid high electrostatic potentials on or near sensitive parts and mitigate against providing a low resistance discharge path between charged items and sensitive items.
The following are ESD control requirements to consider:
- Provide work area surfaces, tools, furniture, and containers whose surface is in the dissipative range of resistance to bleed off static charge without hosting a high energy current;
- Ground the dissipative work surfaces to a single ground point or one of a set of equipotential ground points to complete the ground circuit and to ensure unequal potentials in the work area cause inductive charging or current flow when they come in contact;
- Continuously drain static charge from personnel who directly interact with ESD-sensitive items to the single ground via wrist strap and wire that provides a resistance in the dissipative resistance range;
- Training operators who handle ESDS items to properly recognize and use the ESD controls available in their workspace;
- Maintain area humidity above 30% RH;
- Store ESD-sensitive items in containers with dissipative surfaces. Check the containers electrically to verify their resistance before using them;
- Verify through tests that the ground connections and resistance values for the dissipative surfaces and connections conform to the defined electrical specifications before removing items from ESD-safe containers and using the work area;
- Other ESD protective items and equipment are available but must be verified to meet specifications before use and must be used correctly;
- Avoid use of charge generating materials in ESD protected areas;
- Repeat verification activities regularly (e.g., procedures being followed, measure resistances, electrically confirm equipotential grounding);
- The inclusion of ESD sensitivity levels on documentation and drawings to that personnel can know when they must use ESD control processes;
- Control the relative humidity to above 30%;
- Use Faraday Cage containers to store and transport ESDS items;
- Use an air ionizer to bathe objects for which conduction through dissipative surfaces is not an option for draining static charge (e.g., interacting with hardware when away from an ESD work area, insulators that must be used close to the sensitive hardware and do not readily drain charge through resistive connections).
Figure 1: Wrist Strap checker
ESD Protected Areas
Typical ESD Grounded Workbench: In order to provide a low-current charge drain circuit for statically charged items, the resistance of the connection to ground depends on the resistance of the item being connected. Items such as dissipative work surfaces, with 10^4 to 10^11 ohms/square resistance, should be connected to ground with a very low (<1 ohm, “hard ground”) connection resistance. Conductive items should be grounded by putting them in contact with the grounded dissipative work surface or connected to the ground with a connection resistance in the dissipative range (10^4 to 10^11 ohms; “soft ground”).
Figure 2: ESD Grounded Workbench
Protective packaging is important to protect items while they are being stored outside of an ESD protected area or shipped. Packaging will consist of conductive materials and anti-static materials. Some packaging 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 charge. 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.
Sealing ESD bags with adhesive tapes can generate static charges and should not be an allowed practice. All packaging should be labeled with a warning sign indicating the items packaged inside are ESD sensitive.