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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.