by Ryne C. Allen and
Gene Felder, Desco Industries
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.
ANSI/ESD S20.20
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
Administrative Support
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.
Withstand Voltages
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 RequirementsThe 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 6.1.1.1 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.
Personnel Safety
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.
Technical Requirements
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.
Tailoring
A companys 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
Conclusion
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
companys 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 advantagea strategic tool
focused on quality, productivity, and customer satisfaction improvement.
References
- ANSI/ESD S20.20-1999 Protection of
Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically
Initiated Explosive Devices), ESD Association, 1999.
- Allen, R., ESD Control and
ROI, EE-Evaluation Engineering, November 1999, pp. S2-5.
- VZAP-95 Electrostatic Discharge
Susceptibility Data Book, Reliability Analysis Center.
- Allen, R., How to Set Up an ESD
Control Program, EE-Evaluation Engineering, February 1999, pp. 112-123.
- Allen, R., Audits Essential to
Successful ESD Control Programs, EE-Evaluation Engineering, December 1999,
pp. 92-97.
About the Authors
Ryne C. Allen is the technical manager
at ESD Systems.com, a division of Desco Industries. Previously, he was chief engineer and
lab manager at the Plasma Science and Microelectronics Research Laboratory at Northeastern
University. The NARTE-certified ESD control engineer is an active member of the ESD
Association on several standards working groups and secretary and webmaster of the local
Northeast Chapter. He graduated from Northeastern University with B.S.E.E, M.S.E.E., and
M.B.A. degrees. ESD Systems, 19 Brigham St., Unit 9, Marlboro, MA 01752-3170,
508-485-7390.
Gene Felder is the corporate product manager at Desco Industries. Before joining Desco, he
was general manager of BW/IP International SR Engineering. Mr. Felder graduated from
California State University with a B.A. in business administration and earned an M.B.A.
from the Anderson School of Management at the University of California at Los Angeles. He
is a member of the ESD Association and APICS-certified in Integrated Resource Management.
Desco Industries, 3651 Walnut Ave., Chino, CA 91710, 909-627-8178, e-mail: Gene.Felder@Desco.com
*Reproduced with Permission,
EE-Evaluation Engineering