Defining Your Program
The starting point of a sound program is to classify the sensitivity to ESD damage of the devices you need to protect. Classification of these devices should include all simulation models (HBM, MM, and CDM, refer to the ESD STM5 series in Table I) that will properly characterize the devices' sensitivity when handled at various locations within the facility.
- Realize that there may be different sensitivities at different locations within the facility.
- The ESDA standards that aid the sensitivity testing process are ESD STM5.1-1998, ANSI/ESD S5.2-1994, ESD DS5.2-1996, and ESD DS5.3.1-1996. These documents are the most recent in the industry.
- The Military standards that can be used to determine device ESD sensitivity MIL-HDBK-263B, MIL-STD-883D Method 3015.7, MIL-STD-750C/4 Method 1020, MIL-STD-785
- The IEC standards to help classify device sensitivity are CISPR 24 (1997-09) and IEC-61000-4-2 (1995).
- ANSI has a document, ANSI C63.16, that can aid in device sensitivity classification.
If you do not classify the devices then you can assume the worst case for all 3 models, (Classes 0, M0, C0 - refer to the ESD STM5 series standards in Table I), making the program design critical and expensive.
Once the ESD device sensitivities for the various areas in the facility have been determined then this information can be mapped over the complete facility and will act as a guide to designing the ESD control program.
ANSI/ESD S20.20, for the Development of an Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices) is an excellent standard to guide your control program plan development and is available by clicking here.
Now, the location/sensitivity map of the facility needs to be expanded upon by determining what standards you will use to evaluate the success and monitor the program's progress. This map should also consider the transportation systems and traffic flow of the sensitive devices between various working areas. Additional design criteria to ensure device protection that needs to be broadened are listed as follows.
- Minimize voltage or field exposure (remove non-essential charge generators)
- Minimize voltage or field exposure (use of protective packaging during transportation or storage)
- Minimize voltage or field exposure (from machine to device contact, e.g., automated equipment)
- Exposed surfaces and their resistance (controlled discharge times and use of dissipative work surface materials)
- Grounding (power ground distribution) for common point grounds/work surface
- Grounding (floors - traffic areas)
- Grounding (personnel - wrist straps/foot grounders/smocks/gloves)
- Use of air ionization for essential non-grounded or insulative materials/equipment/tools
- Environmental controls (temperature and humidity)
- Training of employees within various affected areas (by far one of the most important factors)
Selecting General Product Criteria for each Area
Which standard(s) should you reference when building your ESD Control program from scratch, updating or evaluating your current program? When you look to build an ESD-safe workstation, you need to know what industry-wide acceptability criteria to comply with.
Table IV will help aid the design and development of an ESD control program for each area using either the ESD ADV-2.0-1994 or the MIL-STD-1686. The former is more recent at this time. Table III lists various ESD Control products and the associated ESDA Standards that can be used to qualify them.
As an example, an ESD Sensitive (ESDS) workstation that is designed for worst case criteria may have the following ESD Control products: an ESD floor; grounded floor mats with use of ESD footwear (such as foot grounders); grounded and monitored table mats covering all exposed surfaces; a common point ground with monitored wrist strap connections; and air ionizers covering all areas on the work surface to which the devices would be exposed. In addition, all exposed insulators and metal surfaces would be replaced with grounded dissipative materials; all non-essential items, especially insulators, would be removed the ESDS area; and most importantly, the ESDS workstation would have ESD Control Trained operator(s) at the helm.
Starting from the ground up, your floor would be the first place to start. One of the most important characteristics of an ESD floor is its ability to conduct charges to ground. The second most important aspect is its anti-static property. One of the main mechanisms of charge generation is triboelectric generation or tribocharging. Some examples of tribocharging are people walking along a floor and carts carrying sensitive devices rolling across a floor. Depending on where the materials in contact with the floor are in the triboseries, voltages of over 30,000 Volts can be attained. If a floor has the property of being anti-static, tribocharging becomes a much smaller concern. The standards documents to help choose a floor are ANSI/ESD S7.1-1994, AATCC Step Test - Method 134-1979, ANSI/EIA-625-1994, MIL-STD-1686, MIL-HDBK-263B, and the AT&T Electrostatic discharge Control Handbook.
Typically protection on an ESDS device should start at receiving, continue to inventory storage, and then travel through its production flow usually from one workstation to the next before ending up in shipping. All throughout its handling, the device should be handled by grounded personnel. The easiest way to ground people who travel from one station to the next, delivering or picking up sensitive materials, is through mobile grounding. Wearing foot grounders (one on each foot) in conjunction with a conductive floor is one way to ensure that the operator is grounded and protected from delivering or receiving an ESD event. There are several ESDA standards to help in the testing and verification of foot grounders and shoes: ESD DSTM54.1-1997, ESD DSTM54.2-1997, and ESD S9.1-1995.
The surfaces where ESDS devices are handled should be both conductive (in the dissipative range) and properly grounded to the equipment grounding conductor to be an effective ESD control element. There are several materials to choose from such as rubber mats, vinyl mats, both single and multi-layered and FRP and Micastat® for rigid or permanent bench surfaces. Conductive metal work surfaces should be discontinued or covered with a dissipative material because it is highly susceptible to causing an ESD event from a metal-metal contact. It is very important to control your discharge time by minimizing the energy transfer by employing resistive materials to ground . The ESDA standards to help characterize a work surface are ESD STM4.2-1998 and ESD ADV53.1-1995.
The human being can be the most dynamic part of a working environment and consequently should be considered one of the most important objects to ground. Wrist straps, a conductive wristband with a connecting ground cord, is the most popular and effective way to ground a person. Wrist straps should always be properly employed when working with ESDS devices. The ESDS standard EOS/ESD S1-1987 can aid in qualifying your wrist straps before implementation.
Materials that must stay with the ESDS work area but are neither conductive nor groundable should be treated with air ionization. Ionizers come in several types, the most popular is the corona discharge air ionizer. Corona discharge air ionizers can have emitters that are powered by AC, DC or pulsing DC high voltage. Air ionizers can be qualified by applying the ESDA standards ANSI-EOS/ESD, S3.1-1991ADV3.2-1995, and ESD SP3.3-1998.
Transportation & Packaging
ESDS devices should always be stored in an enclosed antistatic shielding bag or conductive closed tote or bin when not being handled. This includes inventory storage, transportation, and WIP. Further precautions during transportation include using dissipative carts with conductive wheels or drag chains in conjunction with a conductive floor when transporting ESDS devices in their shielded containers. The standards to help characterize and qualify packaging materials are ANSI/ESD S11.31-1994 for shielding bags, ANSI/EOS/ESD S8.1-1993 for proper use of package markings, ANSI/EIA-541-88 and ANSI/EIA-583-91 for packaging materials.
Determining the product sensitivities within the facility and then mapping this information helps in choosing the right materials to keep each work area under control. Using the ESDA or other related Standards will help your ESD Control program comply with industry-accepted requirements and procedures that govern the materials, products, systems or processes. Acceptability, repeatability, and dependability can be expected from an ESD Control program that employs a good design using the appropriate standards along with proper training and monitoring.
Referring White Paper: http://www.esdsystems.com/whitepapers/wp_standards.asp