UBM Tech
UBM Tech

ESD wrist-strap insulation decomposes

-August 14, 2014

Some years ago while replacing a hard drive, I had a motherboard fail from ESD (electrostatic discharge). Ever since then, I've used ESD wrist straps whenever I work on a computer and have never had an ESD problem, even in dry winter conditions. Today, I needed to open a desktop computer so I reached for one of my four ESD wrist straps. To my surprise, all of them had exposed ground wires. The insulation was gone in a few places. What happened?

Figure 1 shows the worst one. Note the difference in insulation color, the dark areas are shiny and when I touched them, they were sticky. The material was a kind of goo that came off on my hands.

Figure 1. The insulation of this ESD wrist strap is decomposing,
exposing portions of the ground wire.

The other three straps—blue, not pink, wristbands—also have exposed wires in some places (Figure 2), but they're not sticky. Because I needed to open the computer right away, I covered the exposed wire on one of them with electrical tape before connecting the clip to the PC chassis.

Figure 2. The ground lead on another wrist strap also shows exposed wire.

I also removed the ground wire from the strap in Figure 1 and measured the resistance from the clip to the snap. It measured 1 MΩ, as expected. Wrist straps have 1-MΩ resistors so any static charge will bleed off slowly and not damage circuits.

The good news is that I was able to open the PC and take care of the problem. It was making an unusually loud noise that sounded like it was coming from one of the three fans. Assuming the PC was near death, I made multiple backups of important files before powering down and opening the PC. The PC was running extremely slow at the time. Hoping for the best, I powered down, donned one of the ESD wrist straps, attached it to the chassis, then opened the computer. The noise was coming from the CPU fan. I cleaned the entire inside of the PC, particularly around the CPU fan and its heat sink. The PC is now back to normal noise level and speed.

Have you ever heard of wrist-strap wire, or any insulation, decomposing into a sticky goo? All four of the straps are about 18 years old and have been stored in a plastic bag. I used one a few months ago while cleaning the PC and didn't see any decomposition at the time.

I'll buy some new ESD wrist straps.

Also see
Bandwidth can limit ESD measurements
Link EMI to ESD events
Tracking down ESD
Check ESD simulators first
ESD measurements enhance simulations
Zapping Things With ESD – Just One More Service I Offer (EE Times)

Martin Rowe, Senior Technical Editor Circle me on Google+ Follow me on TwitterVisit my LinkedIn page

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