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ESD events are the cause of maddening, difficult-to-duplicate, and intermittent product malfunctions. They consume a great deal of time, annoy all involved, and are often never resolved.
Combating the invisible enemy with an effective ESD control program can produce financial benefits. But the greatest savings come from decreasing latent defects, which are extremely difficult to detect after the component is assembled into a finished product.
Any relative contact and physical separation of materials (or flow of solids, liquids, or particle-laden gases) can generate electrostatic charges. Common sources include personnel, items made from common polymeric materials, and processing equipment. ESD can damage parts by direct contact with a charged source or by electric fields emanating from charged objects that induce a charge on ungrounded sensitive items.
Although having a duration of less than a nanosecond but with peak current reaching several amperes, a discharge of static electricity literally is a miniature lightning bolt, producing heat that can easily burn through microelectronic structures. A human being cannot feel the ESD voltage until it reaches approximately 3,000 V, while the ESD susceptibility or withstand-voltage rating of many components can even be lower than 50 V.
As the drive for miniaturization has reduced the width of electronic device structures to as small as 0.10 µm, electronic components are being manufactured with increased susceptibility to ESD. For product manufacturers using these electronic components, implementing an ESD control program has become crucial. A program of this type must provide continuous ESD protection through all manufacturing steps including inspection, test, storage, shipment, installation, use, maintenance, replacement, and repair.
Where to Begin
An effective ESD control program is only as strong as its weakest link. Focusing on these fundamental ESD control principles is a good place to start:
Clear identification of ESD susceptible components and products.
Grounding of all conductors, including personnel, in an ESD protected area.
Removal of nonessential insulators from ESD protected areas and neutralization of charges on essential insulators via ionization.
Enclosure of ESD-susceptible items in static-shielding packaging while being transported or stored outside an ESD protected area.
An excellent foundation for an ESD control program is ANSI/ESD S20.20-1999.1 According to the ESD Association, "This standard covers the requirements necessary to design, establish, implement, and maintain an ESD control program for activities that manufacture, process, assemble, install, package, label, service, test, inspect, or otherwise handle electrical or electronic parts, assemblies, and equipment susceptible to damage by ESD greater than or equal to 100-V Human Body Model (HBM). When handling devices susceptible to less than 100-V HBM, more stringent ESD control program technical requirements may be required, including adjustment of program Technical Element Recommended Ranges.
In conjunction with the standard, the ESD Association has begun an S20.20 certification program in which it appoints ISO 9000 registrars to be approved to conduct audits according to S20.20. If a company handles ESD-susceptible products, within some period of time, it is likely that a prerequisite for doing business will be to become certified to S20.20.
In addition to staying in business, the benefits of optimizing an ESD control program can be huge. With improved quality, productivity, and customer satisfaction, companies also can convert ESD control from a prerequisite to a competitive advantage. For example, a properly designed and successfully deployed ESD control program can prompt a return on investment of up to 1,000% per year.2
An effective ESD control program requires the support of top management and all other departments within a company. ESD control knowledge and appreciation and a functioning ESD committee with representation from all departments also are imperative.
An ESD control plan should be based on the withstand voltage of the most susceptible components used in the facility or work cell. Examples of ESD-susceptible parts are microcircuits, discrete semiconductors, thick and thin film resistors, hybrid devices, and piezoelectric crystals. Typically, the easiest way to establish the ESD susceptibility of items in your facility is to refer to the Reliability Analysis Center V-ZAP data book that contains ESD susceptibility data for 22,000 devices.3
Writing the Program
Write an ESD control program plan per S20.20 paragraph 6.0.1: "ESD Control Program Requirements-The Program shall include both Administrative and Technical Requirements. The Organization shall establish, document, implement, maintain, and verify the compliance of the Program."4
The plan must include guidelines for personnel training, verification that the plan is being followed, and a list of the technical requirements and ESD protective products approved for use in the program. S20.20 paragraph 220.127.116.11 states:
"The Plan is the principal document for implementing and verifying the Program. The goal is a fully implemented and integrated Program that conforms to internal quality system requirements..Efforts to identify and eliminate defects, and prevent their introduction, shall be a component of the Plan to reduce the cost and risk associated with ESD damage."
Before we get to the key issues of ESD control, it is important to note that personnel safety is paramount. In no way should an ESD control program replace or supersede any requirements for personnel safety.
In the factory, grounding personnel around the AC power line is a possible hazard. Personal grounding should not be used when working around voltages greater than 250 VAC. Although personal grounding items must include a 1-MW resistor to limit current to less than 0.25 mA, ground fault circuit interrupters should be used.
Typical Control Program
Written plans for an ESD control program generally include the following:
Clearly identify ESD protected areas; personnel should never enter an ESD protected area without taking the proper precautions.
Open or remove ESD-susceptible items from protective containers only within an ESD protective workstation or area.
Personnel should be at ground potential when handling ESD-susceptible items. When seated, they must wear a wrist strap fitting snugly on the skin and have it plugged into a common-point ground. If ESD-protected flooring is used as a primary grounding method, then grounders must be worn on each foot in an ESD-protected area.
Remove unnecessary, high-charging materials from ESD-protected worksurfaces, particularly common plastics or any other electrostatic-generating items.
Clean ESD mats and conductive/dissipative surfaces regularly and only with cleaners that do not leave an insulative residue.
Cover work and storage surfaces in ESD-protected areas with static-dissipative material, such as matting or high-pressure laminate. Ground work and storage surfaces; daisy-chaining is not permitted.
Identify all ESD-susceptible items with ESD susceptibility symbols and enclose them within ESD shielding bags or other sealed conductive or shielding container during storage or transportation outside an ESD protected area.
Neutralize insulative material or other static-generating items necessary in the production process with an ionizer. Test ionizers periodically to ensure balance and charge decay.
Test personnel and worksurface grounding devices periodically and maintain records of test results.
Train and test personnel to verify that they understand the principles of ESD control and how to effectively use ESD protective equipment.
Conduct routine auditing and inspection of ESD-protected work areas at regular intervals.
Specify test equipment.
Specify ESD protective products such as wrist straps, mats, flooring, footwear, packaging, garments, and ionizers.
An ESD control program typically will adopt the recommended range listed in Table 1 of S20.20:
ESD Protective Work Surface: <1.0 × 109 W tested per ESD S4.1, with the lower limit of 1.0 × 106 as recommended in ESD-ADV 53.1.
ESD Protective Flooring or Floor Mats: <1.0 × 109 W tested per ESD S7.1. When the flooring/footwear system is the primary grounding method, an S20.20 recommended range is <3.5 × 107 W tested per ESD STM 97.1 where the resistance measurement is in combination with the person or the charge generation is less than 100 V tested per ESD STM 97.2.
Wrist-Strap System: <3.5 ×107 W per ESD S1.1, which will provide a continuous electrical path from the user directly to ground. Alternately, a continuous monitoring system can be used.
ESD Protective Footwear: heel straps, toe straps, or conductive shoes that provide a continuous electrical path from the user directly to the ESD protective flooring or mat <1.0 × 109 W; shoes tested per ESD S9.1.
Electrostatic Generating Sources: Nonessential and personal items shall not be placed on ESD protective worksurfaces. Essential materials under normal use shall not cause or generate static voltages of greater than ±2,000 V within 12 in. of unprotected ESD-susceptible devices.
ESD Protective Garments: 1 × 105 to 1 × 1011 W tested per ESD STM 2.1. ESD smocks may be used to shield ESD-susceptible items from charges on insulative clothing. When worn, they should cover all personal garments above the wrist except at the neck area and make intimate contact with the skin. The ESD smock must be grounded, otherwise it becomes an isolated ungrounded conductor.
Ionizers: ±50 V voltage offset tested per ESD S3.1.
A company's written ESD control plan should be cost-effective. S20.20 may not apply to all situations. Additional precautions may be necessary if the withstand voltage of the components is less than 100-V HBM. In addition to the HBM, the Charge Device Model (CDM) should be considered when tailoring your plan.
Requirements can be relaxed when appropriate. Per S20.20 paragraph 6.0.3, "Tailoring is accomplished by evaluating the applicability of each requirement for the specific application. Upon completion of the evaluation, requirements may be added, modified, or deleted. Tailoring decisions, including rationale, shall be documented in the ESD Control Program Plan."1
To be effective, an ESD control program must be comprehensive and documented into a protection plan that encompasses the processes, personnel, and ESD-susceptible devices. This plan must be adjusted to suit each company's specific needs, but should follow basic guidelines as set forth by S20.20. In addition, management support, personnel training, and consequential follow-up audits are part of an effective ESD control program.5 By following this advice, companies can turn ESD control programs into a competitive advantage-a strategic tool focused on quality, productivity, and customer satisfaction improvement.
1. ANSI/ESD S20.20-1999 Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices), ESD Association, 1999.
2. Allen, R., "ESD Control and ROI," EE-Evaluation Engineering, November 1999, pp. S2-5.
3. VZAP-95 Electrostatic Discharge Susceptibility Data Book, Reliability Analysis Center.
4. Allen, R., "How to Set Up an ESD Control Program," EE-Evaluation Engineering, February 1999, pp. 112-123.
5. Allen, R., "Audits Essential to Successful ESD Control Programs," EE-Evaluation Engineering, December 1999, pp. 92-97.
Referring White Paper: http://esdtraining.esdsystems.com/whtpaper/standards.asp