Check Your ESD-Protective Products

Will De La Isla

Adapting ANSI/ESD S20.20 for factory auditing leads to an effective in-house ESD control plan.

Say a person buys a luxury car, spends tens of thousands of dollars on it initially, and then never takes it in for an oil change. Most of us would say that this is a poor way to treat an expensive piece of equipment. Likewise, some manufacturers purchase expensive ESD-protective products and then are content to simply use them, feeling confident that they are working correctly. This is a very foolish practice. If ESD-protective products are not properly installed and periodically audited to verify performance, they can fail. And failures can be expensive.

A well-designed ESD control program can help ensure a return on the investment in it by providing performance and maintenance schedules for ESD products. The ESD Association guides the user in designing an ESD control program via ANSI/ESD S20.20.¹ The document titled “Development of an Electrostatic Discharge Control Program” covers the requirements necessary to design, establish, implement, and maintain an ESD control program. When properly employed, the program “protects electrical or electronic parts, assemblies, and equipment susceptible to ESD damage from human body model (HBM) discharges greater than or equal to 100 V.”

S20.20 is very versatile. However, it requires a written ESD control plan. The plan must incorporate several components that are key to its success. These include having a training plan, a compliance verification plan, and a specified technical requirement range. And while the ESD Association program is very good, it lacks practical advice on particular tasks. Importing S20.20 guidelines into a factory setting can preserve ESD product usefulness and safety. This article presents advice for daily implementation of a control plan into a factory.

Compliance Verification

The S20.20 plan requires formal audits and certifications with certain requirements as well as the need to select test equipment. It also states that periodic verifications should be performed. The procedures are set out in section 6.1.3.
Section 6.1.3.² recommends that external and internal audits be performed by both the user and the supplier of the ESD-sensitive products. It advises that routine checks should be performed “based on the control item usage, its durability, and associated risk of failure.”

Table I. Typical testing frequencies required by end-users and recommended procedures and tests (continued below - click to enlarge).

However, these sections do not offer concrete timetables for periodic checks. A successful plan requires users to set verification checks at sufficient intervals. Most users will accept the recommended range set forth by S20.20 Table 1 as their ESD protective product technical requirements. Any deviation from ESD’s Table 1 requires justification. As the document states, “tailoring decisions, including rationale, shall be documented in the ESD Control Program Plan.” Table I in this article is similar to S20.20 but is designed for the factory.

Latent Defects

ESD-protective products are sometimes installed incorrectly, and sometimes they fail to meet the technical requirement range. There are two types of ESD damage: catastrophic failure and latent defects. By definition, normal quality control inspections can catch catastrophic failures. However, these inspections are considerably less effective when detecting latent defects.

Latent defects include a variety of complications such as partial degradation, as explained in ESD TR20.20 section 2.7.2. Partial degradation means that the product may continue to perform its intended function but experiences severely reduced operating life. Latent defects that cause premature failure can lead to costly repairs and, in some cases, pose a serious hazard to employees and other users. That is what makes shipping products with latent defects a far scarier, not to mention more expensive, situation.

The detection technology used now offers little protection against latent defects, especially once the products are assembled. Some studies have indicated that products shipped with latent defects exceed the number of products shipped with catastrophic failure.

Experts estimate the cost of ESD damage at billions of dollars annually. The ESD Association’s Web site, www.esda.org, explains “the cost of damaged devices themselves ranges from only a few cents for a simple diode to several hundred dollars for complex hybrids. When associated costs of repair and rework, shipping, labor, and overhead are included, clearly the opportunities exist for significant improvements.” In lieu of these, following a control plan can reduce problems created by latent effects.

Practical In-House Audit Advice

Users should employ an integrated tester at least daily to verify personnel grounding devices path-to-ground. Typically the integrated tester is located at the entrance of the ESD-protected area. Operators should test their wrist straps and ESD footwear while wearing them, and the results should be logged. While paper logs are acceptable, data acquisition models are available, and they often provide more-effective analysis and management of the data acquired. Computerized records can be particularly important tools for auditing.

The frequency of periodic testing depends on the value of the company’s ESD-sensitive product, the probability of failure, and the potential cost of an ESD-induced failure. Table I provides typical testing frequencies required by end-users, as well as advice on system-type tests to obtain overall information and to make the periodic testing more productive.

For example, a systems-type ESD-worksurface test would incorporate resistance-to-ground (RTG) to equipment ground measurements. It would not just measure the resistance of the mat material or the RTG to the mat’s own ground point.

It’s not always necessary to measure each component; in fact, doing so could waste valuable time. If the RTG measurement to equipment ground is good, the conclusion is that all components are connected and within the technical requirement range. The results of testing should incorporate the following:

• Resistance point-to-point of the worksurface material.
• Snap or other conductor connections to surface material.
• Ground cord or other path-to-ground.

Garments: Place two 5-lb electrodes on each sleeve. Measure the surface resistance.

If a measurement is high, however, troubleshooting includes measuring each component separately, to determine which item is out of spec. Often the failure mode is an indication of improper maintenance, and the product requires only proper cleaning or reinstallation to obtain the desired reading.

It is important to avoid stating in documents that the ESD-protective product is being tested per the ESD Association standard. Rather, use a phrase such as “using modified test methods in accordance with concepts outlined in ESD Association standards.” ESD standards primarily detail test procedures for use in a test laboratory, not for the shop floor.

For auditing a sample quantity, use clear language and provide specific guidelines to direct technicians to look for problems, so that they do not simply check a random sample. When a variety of ESD-protective products are being used, measure at least one representative of each type. Nonrandom sampling enables technicians to measure items that appear to be the most likely to fail. The TR20.20 section 2.⁴ advises that any object, item, material, or person could be a possible source of static electricity.⁸ It advocates the removal of unnecessary nonconductors. It also suggests that manufacturers replace nonconductive materials with dissipative or conductive materials and ground all conductors to control static electricity in the workplace.

Verifying Ionizers

In general, all ionizers should be checked and should not rely on random sample testing.⁷ Periodic testing of the ionizers should provide continual verification of performance. The more critical the application, the more important it is that the ionizer is working correctly. Often, testing is performed during actual use conditions.

All ionizers require maintenance, including emitter point cleaning. Some ionizers have ion-out-of-balance alarms. These feedback controls are designed to sense performance defects, especially if the offset voltage (balance) has degraded because of dirty or corroded points. Some ionizers also have an automatic balancing-feedback mechanism that adjusts the production of different polarity ions to maintain present offset voltage. Sensors detect the net charge and supply this information to analog or digital control circuits. These circuits adjust positive or negative voltage to the emitters.

Conclusion

ANSI/ESD S20.20 is a process document providing guidance for an ongoing ESD-control program. Auditing and periodic checks ensure that the ESD-protective products meet the technical requirement range. If used effectively, companies can turn ESD control programs into a competitive advantage—a strategic tool focused on quality, productivity, and customer satisfaction improvement.

References

1. ANSI/ESD S20.20: “Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices),” ESD Association (ESDA), 1999.
2. ANSI/ESD S4.1: “Worksurfaces—Resistance Measurements,” ESDA, 1997.
3. ANSI/ESD STM12.1: “Seating—Resistive Measurement,” ESDA, 1997.
4. ANSI/ESD STM97.1: “Floor Materials and Footwear—Resistance Measurement in Combination with a Person,” ESD Association, 1999.
5. ANSI/ESD STM97.2: “Floor Materials and Footwear—Voltage Measurement in Combination with a Person,” ESDA, 1999.
6. ANSI/ESD STM2.1: “Garments,” ESDA, 1997.
7. ESD SP3.3: “Periodic Verification of Air Ionizers,” ESDA, 2000.
8. ESD TR20.20: “Handbook for the Development of an Electrostatic Discharge Control Program for the Protection of Electronic Parts, Assemblies and Equipment,” ESDA, 2000.

Will De La Isla is the electronics product manager for Desco Industries Inc. (Marlboro, MA). He oversees the design, production, and sales of ionizers, continuous monitors, and testers.