The Grey Zone
Wrist Strap Testing
In a previous blog post we discovered that ESD is the hidden enemy, unfortunately the ESD event itself is not the only invisible element we have to consider. Not only can we not see or feel electrostatic discharges, or see with the naked eye the damage it causes, but it is also difficult to determine at which point in the process that ESD damage occurred.
Whilst there have long been methods in place to reduce the chances and the strength of ESD Events, these typical methods still leave room for what we call the “Grey Zone”, the period where we cannot be certain our product was protected. Even with a flawlessly managed ESD Control Programme in place, it still leaves us in a reactive, firefighting position.
One of the more widely known examples is found in wrist strap testing. Whilst many companies will be taking the correct precaution of testing their Wrist Straps at least once daily, unfortunately this approach still leave us with a “Grey Zone”. To explain: if each morning the wrist strap has been passing, until Thursday morning you get a fail, then all the product handled between your last pass and your fail becomes suspect, as we don’t know when the wrist strap stopped working. The wrist strap can also have intermittent failures throughout the day and still pass at the tester the next morning. This means that when it does fail at the tester, the grey zone could in fact be larger than just since the last Pass. These intermittent failures can be caused by intermittent flex fatigue, how the wrist strap is worn at the workbench compared to at the tester, or even down to the operators’ dry skin.
Benefits of Continuous Monitors
1. Instant Feedback
Continuous monitors provide operators with instant feedback on the status and functionality of their wrist strap. The instant an operator’s wrist strap or cord fails, whether it is caused by dry skin, wearing the band incorrectly, failing solder connection or something else, the monitor will issue audible and visual (LEDs) alarms alerting the user and supervisor of the problem.
The SCS 724 Workstation Monitor in Use
2. Monitoring of Operator AND Workstation
As well as monitoring your wrist strap, work station monitors also monitor the path to ground of a working surface, ensuring that not only is the operator continuously grounded but so is the work surface.
““For units that also monitor the connection of a worksurface to protective earth, it is also possible to reduce or eliminate the checking of the worksurface as part of the periodic audit of the process.” [IEC TR 61340-5-2:2018 User guide Annex B.2.3 Constant monitors].
Installing the SCS 724 Workstation Monitor to ground the worksurface
3. Detection of Initial Flex Fatigue
Unlike wrist strap testers, continuous monitors detect split-second failures when the wrist strap is still in the “intermittent” stage. This is prior to a permanent “open” but can still result in damage to ESD sensitive components.
“During operation, wrist straps might be stressed and flexed to their limits at a workstation. While a wrist strap is being checked it is typically not stressed, as it would be under working conditions. Openings in the wire at the coiled cord’s strain relief are sometimes only detected under stress. Even if the wrist strap is working properly, a bad or intermittent ground connection will render the wrist strap system less than 100% effective.” [ESD TR20.20 Continuous Monitors Clause 18.2 Wrist Strap Checkers]
The SCS WS Aware Monitor with Big Brother Remotes and Ethernet Output
4. Elimination of Periodic Testing
Many customers are eliminating periodic touch testing of wrist straps and are utilizing continuous monitoring to better ensure that their products were manufactured in an ESD protected environment. Continuous monitors also eliminate the need for users to test wrist straps and log the results.
When using continuous monitoring, operators:
- Don’t have to waste time queuing at a wrist strap test station before each shift.
- Don’t have to remember to complete their daily test logs.
No more paper logs!
Conclusion
If your company manufactures products containing ESD sensitive items, you need to ask yourself “how important is the reliability of our products?” Sooner or later a wrist strap is going to fail. If your products are of such high value that you need to be 100% sure your operators are grounded at all times, then you should consider a continuous monitoring system.
Savings comes from:
- Eliminating time/labour required in verifying a wrist strap before handling ESDs
- Reducing damage to ESDs from broken wrist straps that may go unnoticed with standard wrist strap testers.
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