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First Published in EOS/ESD
Technology April/May 1990
Auditing
for Static Safety
Despite
good intentions and formalized static-control policies, audits can
still uncover numerous offenses against static safety.
Al Boucher
vice President
C.C. Steven & Associated Inc.
Ventura, CA
[Editors Note: No static-control
program is "bulletproof." The best will in the world,
the most costly static-control products, and the most detailed control
plan can be confounded if workers, supervisors, and managers get
the operational details wrong--static control products only work
if they are used properly.
Assuring proper use involves two strategies: first, management follow-up,
supervision, and training; and second, frequent static-control audits
designed to spot unsuspected sources of charge buildup and ESD.
In the article that follows, Al Boucher shares the findings of such
an audit. Boucher has been doing independent static-control audits
longer than there has been a recognized static-control industry,
first with Simco, and now with C.C. Steven.
His material is drawn form the report of an audit conducted at a
major aerospace firm--the name has been omitted to protect the innocent.
And though one might think that a military contractor would by now
have gotten the bugs out of its static-control program, Boucher's
findings below show how vulnerable we all are to lapses.]
On the surface, this company had
a very comprehensive ESD program in place. Its static-control program
addressed all the key elements of a sound control plan:
 A
comprehensive document had been developed that addressed all areas
of concern for implementing an ESD-control program.
Responsible
individuals has been assigned to oversee the implementation of the
program. They were dedicated, intelligent, and sincere about implementing
ESD control.
A
process had been developed that used flow charts and check-off lists
to verify the implementation of the control program.
ESD-control
products such as wrist straps, worksurfaces, floor mats, metallized
bags, conductive containers, antistatic garments, and an assortment
of ESD products were on hand and in use.
ESD problems remained however, and a review of key static-control
tactics and products during a plant survey gave some disquieting
results.
Hard-Laminate
Table Tops
MIL-W-87893 of
November 10, 1987, page 7, paragraph 3.5,states, "The static-dissipative
workstation shall operate for the purpose of removing electrical
charges from its workstation surface or from noninsulative objects
placed on the workstation surface before static discharge occurs."
However, most of this company's work benches were covered with a
laminate material that didn't meet the requirements of the spec.
Therefore, charged conductive objects placed on the surface didn't
lose charge.
Wrist Straps
The wrist strap is the most important
component of an ESD workstation. Its sole function is to eliminate
charge from the body. It is important to note that the wrist strap
will not eliminate charges form the person's clothing.
At this site, testing of wrist straps was performed daily by each
operator to ensure that their wrist strap was performing properly.
Wrist straps were tested on a touch-type tester with settings of
2, 5, and 10 MW.
In Spite of this daily testing, we found a considerable number of
defective coil cords and wrist bands. Many of the defects were intermittent
and could only be detected by wriggling the cord.
With this in mind, employees needed to be trained in proper procedures
for testing and, in addition, periodic testing was needed by supervisory
personnel throughout the day. This could be easily accomplished
by equipping the supervisors with pocket testers.
Another reason for wrist-strap problems was improper tester settings.
Though each tester could be set at 2, 5, and 10 MW, the company
had designated 5 MW to ground as the upper limit. However, test
units throughout the facility were set at different values, many
of them at the undesirable 10 MW level.
The solution to the latter problem was to set the testers at the
designated 5-MW maximum, and then eliminate or secure the selector
switch.
It was also found that placing workstation grounding receptacles
under the table eliminated the problem of inadequate strain relief
on wrist-strap coil cords.
Other observations concerning the use of wrist straps included these:
Some
wrist straps were worn too loosely.
Some
workers did not rest because the importance of testing was not understood.
Some
areas did not maintain testing logs.
Some
wrist straps were hard-mounted, and testing could only be accomplished
with the use of a multimeter. Such inconvenience led to infrequent
testing.
Some
areas did not have conveniently located test equipment. Thus, testing
and logging required a considerable walk.
Grounding
All conductive items must be grounded
to ensure that they do not become inductively charged. These items
include rolling racks, bins, shelves, cabinets, equipment, and machines.
There had been a conscious effort to ground most of these items
throughout the facility. However, we were concerned during the audit
with the rolling metal racks used to transport products and equipment.
These units comprised two or three metal shelves and four supporting
posts. The shelves were attached to the post by plastic wedges,
effectively isolating each shelf from the post and from one another.
The entire assembly was then supported by four rubber wheels. There
are several problems with this configuration.
The
isolation of the various components could allow a potential difference
to exist on the cart.
The
rubber wheels allow the entire assembly to become charged as it
rolls across the floor.
The
entire assembly or components can become charged through induction
if the unit is momentarily grounded in the presence of a field.
Regarding the carts, we recommended the following:
All
parts of these units (shelves, posts, etc.) should be maintained
at the same potential. This could be accomplished by replacing one
plastic shelf wedge (on each shelf) with a metal wedge.
Before
loading or unloading, each cart should be grounded by attaching
a temporary ground.
And with respect to grounding practice in general, we recommended
these items:
Metal
shelves located in the middle of a room were wired together but
there was no connection to earth ground. We suggested a separate
wire to earth ground.
Special
care needed to be taken to ensure that equipment placed on grounded
table mats was properly isolated so that a parallel ground path
did not exist, bypassing the 1-MOhm current-limiting resistor.
Broken ground leads were noted throughout the facility. In some
cases, tape. Some leads were disconnected when tables were relocated
and the wires were left hanging. Such conditions should not exist,
even if the table is grounded at another point. These cables are
safety hazards and also invite unfavorable ESD audit reports. Any
unnecessary wires should be removed.
Work Stands
Work stands used to support work-in-progress
and were composed of a flat or angled metal plate supported by four
metal legs; the worksurface was covered with a static-dissipative
material. At this facility, the assembly sat on the hard-laminated
table top, and it was expected that the contact of the stand's metal
legs with the grounded laminate would effectively ground this assembly.
However, due to the problem previously discussed concerning hard-laminate
table tops, these work stands become floating conductors easily
charged by induction. Until the problem was resolved with the worksurfaces,
we suggested that each stand be grounded independently via 1-MW
resistor.
Garments
The main purpose for wearing conductive
smocks is to suppress static fields on employee clothing. The conductive
fibers woven into the material provide a Faraday cage that prevents
dangerous fields from extending to and damaging sensitive products.
Note that an antistatic garment is not replacement for a wrist strap--an
ungrounded operator wearing a smock can become highly charged under
certain conditions. In many cases a garment should itself be grounded.
We found a number of garment-related problems during the audit and
recommended that supervisors assure that:
Smocks
are completely buttoned to ensure that no part of the employee's
clothing is exposed.
Sleeves
are not rolled up to expose clothing.
At
least some portion of the smock is in contact with the grounded
employee's skin. Otherwise the smock could become inductively charged.
Alternately the garment should itself be grounded.
Insulators
Unlike conductors, insulators can't
be grounded. For this reason, it is extremely important that all
unnecessary insulators be removed from the work area.
This process should begin with suppliers; a company should impose
strict packaging requirements on items shipped into their facility.
The receiving department must ensure that all insulating packaging
materials are removed and discarded before the supplies are forwarded
to the various departments.
It is important to eliminate insulators from the outset, since the
control of insulators in a manufacturing environment is a very difficult
task. Insulators were found throughout the facility in a multitude
of configurations. Here's a partial list of insulators observed
during our audit.
| Vinyl
binders |
Printed
circuit boards |
Plastic
railings on metal racks |
| Hand
tools |
Desolder
wicker spools |
Plastic
covers on test equipment |
| Post-it
notes |
Static-awaremess
labels (!) |
Test-equipment
components |
| Light
fixtures |
Microscope
covers |
Plastic
document holders mounted on metal rack |
| Plastic
boxes |
Equipment
covers |
Key
pads and plastic housing on display terminals |
| Vinyl
stools |
Paper/fiber
wipes |
Plastic
standoffs used to assemble printed circuit boards |
| Plastic
carts |
Vinyl
vacuum hose |
Plastic
shields on equipment |
| Bubble
wrap |
Continuity
tester |
Plastic
parts used on PCB assemblies |
| Wire |
Safety
glasses |
Plastic
calendar holders |
| Masking
tape |
Power
supply housing |
Vinyl
flip charts for manufacturing instructions |
| Curtains |
Data
diskettes |
Plastic
work-instruction folders |
| Plastic
pens |
Solder
spools |
Styrofoam
packaging material |
|
|
Heat
guns with plastic body |
Each of these items constituted
a potential ESD hazard for four reasons:
1. Insulators can charge to thousands of volts with the touch
of a finger.
2. Insulators can charge to different potentials on the same surface.
3. Insulators can store charges of different polarities on the same
surface.
4. The resulting fields can induce a voltage on ungrounded conductors.
As previously stated, all unnecessary
insulators must be removed from the work area. The audited company's
document, "Protection and Control Procedure For Electrostatic
Discharge," tells supervisors to "ensure that no static
generating material such as Styrofoam or plastics is present in
the protected area."
This requirement was not adhered to for various reasons. In some
cases the insulative material was part of a test fixture or equipment,
for example. The solution we suggested was to eliminate all unnecessary
insulative materials and to use ionizers where insulators can't
be eliminated or are part of the work process.
Packaging Materials
Properly used, metallized bags can
offer a great deal of protection from static charge and fields as
a product travels through the facility. As long as the product is
fully enclosed and properly sealed in a metallized bag, it is fully
protected; little further ESD control is needed until the contents
are removed from the bag at a static-safe workstation by a properly
grounded operator.
However, we found numerous bagging problems.
Some
bags weren't sealed.
Some
bags were too small for the product.
We
found products lying unattended on tabletops next to or on top of
the bag.
A
worker was observed carrying a printed circuit assembly to a spray
booth with bare hands.
We
found printed circuit assemblies sitting on a metal rack out of
packaging. Pink poly work travelers were placed on top of the boards.
Each of these practices had to be rectified.
Conductive
Tote Boxes
Conductive totes were used to transport
sensitive assemblies throughout the facility. As with bags, these
must also be covered to provide optimal protection. And also like
bags, we found problems with the way totes were used.
Many
of the tote boxes used masking tape (which tribocharges) to label
the contents. This is not a good practice.
We
also found a variety of insulating tote boxes used throughout the
facility. These boxes had to be removed from the facility.
Vinyl Stools
Vinyl stools
were found throughout the plant. There were two areas of concern,
the vinyl back and the seat. The resulting charge from contact and
separation caused when a person sits and stands can create a hazardous
charge condition. In addition, the stool back can swivel and be
in close proximity to work-in-process on the table. An ungrounded
employee in the presence of these fields could be charged by induction,
and then transfer that charge to the product. These stools were
in violation of the company's own ESD guidelines that required a
minimal distance of 1 meter from protected areas.
The interim solution for the stools was to cover the vinyl parts
with conductive cloth or dissipative vinyl covers. The long-term
solution was appropriately designed static-safe seating.
CRT
Terminals
Computer display terminals used widely
in the factory had strong fields emanating from both screen fields
emanating from both screen and the chassis. Any ungrounded conductor
entering this filed and then momentarily grounded could be charged
by induction.
At one station, PCB assemblies were positioned next to the CRT chassis,
within such a field. This was a very dangerous condition, and should
not have been allowed.
We recommended several attacks on this problem.
The
side of the chassis could be covered with metallized film to suppress
the fields.
The
screen of the CRT could be retrofitted with a commercial shielding
grid designed to suppress these fields.
Remove
CRT's from critical work areas. Avoid contact with static-sensitive
components or assemblies in the presence of CRT fields.
Summary
This audit reflected conditions commonly
found in other electronic facilities. A great deal of money had
been spent on a conglomeration of static-control products, but little
time and resources had been directed toward ensuring that the products
and programs were properly implemented.
The implementation of a successful ESD-control program requires:
A
comprehensive description of the program, outlining policies and
guidelines, both of which must be supported by top management.
Purchase
of ESD-control products that function properly for their intended
use.
Well-trained
supervisors and personnel who understand their role in the ESD program.
Competent
personnel with specifically dedicated resources to enforce and oversee
the successful implementation of the program.
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