It is essential that NASA engineers use proper precautions when working with Electrostatic Discharge (ESD) sensitive hardware, as poor ESD control practices could be the difference between success and failure for a mission. ESD control is a fundamental component of NASA’s workmanship policy, which is why NASA requires operators to verify ESD personnel grounding systems (e.g. wrist or heel straps that are continuously connected to common point ground) are properly functioning before they enter an ESD Protected Area (EPA) or come within one meter of an ESD sensitive item.
Over the past several years wireless or cordless wrist straps have been gaining attention throughout the ESD community. It is advertised that these wrist straps eliminate static buildup without the need of a grounding cord that directly connects to a ground circuit, however, NASA engineers have proven that these products do not function as advertised and do not meet product qualification requirements.
Shoes shuffling along the carpet or clothes brushing skin can create an electrostatic charge. The use of properly grounded ESD wrist or heel straps provides a means to drain the accumulated charge allowing operators to remain at the same electrical potential as the ESD sensitive hardware being handled, thus alleviating one of the conditions that allow for a damaging ESD event to take place.
According to results from testing performed by the NASA Interagency Working Group on Electrostatic Discharge (IAWG-ESD), it was confirmed that wireless wrist straps failed to prevent charge build up or to drain accumulated charge in order to prevent potential discharges. Wireless wrist straps do not meet American National Standards Institute (ANSI)/ESD S20.20 Wrist Strap System performance requirements and therefore are not acceptable for use in ESD controlled areas used to process critical NASA mission hardware.
“ESD wrist straps are the first line of defense against an ESD event,” said Gene Monroe, agency ESD Point of Contact at Langley Research Center. Monroe spearheaded the round-robin laboratory testing, which ultimately led to a Government-Industry Data Exchange Program (GIDEP) alert.
“These wireless wrist straps do not connect you to the ground,” said Monroe. “They do not eliminate that potential difference between the operator and the electronics.”
It typically takes an ESD discharge of more than 2,500 to 3,500 volts for a person to feel a shock, however, a small discharge of 25 volts can damage electronic components without the engineer feeling a thing. In the aerospace industry, many electronics components can be extremely small and ultrasensitive, making them very susceptible to static electricity. According to Bob Vermillion (RMV Technology Group), ESD Subject Matter Expert at Ames Research Center, some specialized circuit cards can cost as much as $1 million, making the consequences for not wearing proper personnel grounding devices severe.
“With these cordless wrist straps, it’s like walking around not even wearing a wrist strap,” said Vermillion. “You can generate hundreds to thousands of volts wearing these.”
Vermillion also performed a series of tests comparing conventional and wireless wrist straps. He, along with NASA engineers from Langley Research Center and Armstrong Flight Research Center, all came to the same conclusion: without proper ESD control practices in place, which includes periodic verification, these devices could present a significant risk to ESD sensitive mission hardware.
Damage from an ESD event can be parametric, which results in reduced functionality of the device, or destructive, resulting in device loss. Troubleshooting and subsequent repair of ESD damaged hardware typically results in additional cost and schedule delays. In the case of a latent failure, the ESD event can damage a device and it will show no signs of defect and continue to function normally until it is too late to repair or replace. If this damage goes undetected, the lifespan of the device can be shortened resulting in premature failure. This latent failure can degrade or end the mission.
“A latent failure is our biggest fear,” said Monroe of NASA flight projects.
Vermillion and Monroe both agree that it’s important to verify with NASA subcontractors that they are using required ESD controls when working on NASA projects. All NASA employees, including subcontractors and suppliers, are required to be compliant with NASA-STD-8739.6 and ANSI/ESD S20.20. This includes the verification of ESD control materials before use; a fundamental and critical quality control used in NASA’s ESD control policy.
Quoting Vermillion’s adage for ensuring Quality Assurance, “trust, but verify.”
Questions about wireless wrist straps may be addressed to Monroe or Vermillion.