The best programs detail the "who, what, where, when and how" of testing ESD technical elements.

This column deals with an important aspect of ESD control - the process of verifying that everything is working correctly. Before you can reasonably verify that your ESD control measures are working, first prepare an ESD control program plan. A good guide to assist developing the plan is ANSI/ESD S20.20 .

An S20.20 compliant plan includes:

• Training plan. Defines how and when personnel are trained to handle ESD-susceptible items.

• ESD control technical elements. Describes all electrostatic control items used in each process where ESD-susceptible parts are handled (see "Setting Up the ESD Area," Circuits Assembly, December 2005, p. 45).

• Packaging plan. This section guides the selection and use of packaging materials to store, transport and ship ESD-susceptible parts.

• Compliance verification plan. A means for measuring the effectiveness of all ESD controls and process elements.

Each process within a facility that involves ESD-susceptible items requires specific ESD technical elements. This list could include: conductive or dissipative flooring, protective work surfaces, wrist straps, monitors, protective shoes or shoe grounders, garments, chairs and stools, tools and fixtures, protective packaging materials and containers, ionizers and a myriad of other ESD-protective items and test equipment. Each item needs a periodic evaluation to ensure it is working correctly. The compliance verification plan is where you specify the "who, what, where, when and how" of testing the ESD technical elements. "Why" is established in the ESD control program plan.

A compliance verification plan generally lists "who" is responsible for testing the ESD technical elements and monitoring relevant procedures. It could be quality assurance or production personnel, but generally the task falls under the responsibility of the ESD control program manager or ESD coordinator. In large facilities, the daunting task may be spread among department or process area supervisors or designated engineers. This requires a well-defined set of test procedures and specific training for the participants to achieve consistent testing.

"What" is tested and "when" it is tested needs to be specified in the plan. Develop an understanding of each ESD technical element to determine the testing frequency. Establishing the risk of failure is one methodology that can assist in setting the frequency of testing ESD technical elements. For instance, if you rely on wrist straps for grounding personnel, what is the required test frequency to ensure the risk of failure is tolerable? For critical operations the testing may be continuous, thus requiring some sort of wrist strap constant monitor system. For less stringent applications, perhaps testing once per day or shift is enough. You have to decide the criticality of your applications. Make sure that each ESD technical element is evaluated appropriately in the compliance verification plan, using industry-recognized test procedures, and that testing is conducted often enough to ensure that the risk of working without the technical element is acceptable. In other words, what are you going to do with the parts that were handled today if your EPA has technical element failures identified in tomorrow's testing?

Durable items such as flooring and work surfaces generally will not need daily monitoring, except after floor maintenance or bench moving. Routine checks should be run on a weekly basis but a full audit may only be needed quarterly or semiannually after a history is established. Other items require testing depending on how likely they are to fail. Shoe straps require more frequent testing than ESD-protective shoes. Garments may need more frequent evaluation than chairs.

Packaging specifications usually are set ahead of time based on the sensitivity of the product that has to be protected and contractual requirements. Verification of packaging performance may require a third-party evaluation to satisfy an external auditor. Most packaging providers will supply certificates of conformance with each shipment, but it is best to get an outside test house to verify the values periodically.

Table 1 shows an example of a compliance verification plan test frequency. The test frequency for each item is only an example. Choosing the appropriate test method for each ESD technical element is another important component of compliance verification. The industry standard test method will often need modification to suit your applications. Document any changes to the test method in your control program plan. For example, the work surface test method, ANSI/ESD S4.1, measures resistance from the surface to a groundable point. Your application may require a test from the surface to the common point ground. Modification of the test method is acceptable as long as it makes sense and is documented correctly.

Understand the risk involved if an ESD technical element should fail between testing cycles. This will help to determine how frequently a technical element should be evaluated.

 

David E. Swenson retired from 3M after 35 years in ESD control material development and application. He is cofounder and president of Affinity Static Control Consulting, and a longtime member of the ESDA standards committee, ANSI/ESD S20.20 Task Force and board of directors; deswenson@affinity-esd.com.