Electrostatic Discharge

Thread Starter

kasko_111

Joined Feb 5, 2023
6
Until now i was doing ESD Tests only with powered device (AC Mains on) but is it also recommended to do ESD without powering on the device
If yes is there any purpose to do ESD Test without powering on the AC Mains.


Looking forward for your Inputs
 
Last edited:

Tonyr1084

Joined Sep 24, 2015
7,852
ESD (Electro Static Discharge) is a phenomena also known as Static Discharge, that annoying sensation when you get out of your car in the winter time when the air is super dry and you turn to push the door closed and you get that dang big snap of electrical discharge. It doesn't matter if a device is powered or not, grounded or not. A discharge into electronic components occurs when the potential energy level of a device is different from another device by several hundred to several thousand volts. When you reach for the doorknob in your house and you get that snap it's because the doorknob is at some voltage potential, likely ultra low voltage and you've gained static charge from simply walking across the carpet. That charge accumulates in your body and you become a potential source of micro-lightning that can destroy electronic components.

The best way to prevent such damage to sensitive electronic components is to keep them and yourself at the same potential (static charge). Wearing wrist straps connected to a bonded earth ground as well as the work surface where the components are being worked with will drain any excess charge away from your body and from the components or PCB's keeping them safe from static discharge.

Static charge can cause two types of failures: First is a catastrophic failure where the device ceases to function at all. The second is worse, the component is damaged and thereby weakened but not in full failure. During testing the device may pass the test but under real world situations and stresses the damage could be weak enough to cause total failure. Devices that are considered "Mission Critical" or "Life Critical" are of the highest order for static protection. Failure to follow proper ESD protocol can put mission or life in danger. Defense, Medical and Automotive are the three most common areas where ESD protocol is critical. Obviously in defense or medical situations - failure can mean someone dies. In automotive, stepping on a pedal and getting no response or a "runaway" response can cause injury or death. Since you're working in medical electronics field, ESD should be something that has been well thought out and defined by an employer, whether they're in the servicing business or production of medical equipment. ISO regulations will have very clear instructions regarding ESD. If you're a home body working on something that someone in the house depends on for life sustaining support then by all means, refer servicing to a qualified service facility. This is not something I'd do at home. Sure, working on my computer, if I blow it up - I'm out some bucks. But on an oxygen generator - blow that up and someone could be at risk. As skilled as I might be I would not undertake servicing medical devices at home.

As for your question about ESD, you need a wrist strap connected to a proper ground. The device you're working on also needs to be grounded. And it's highly advised you have a static mat also grounded for working on such sensitive electronics. In my years of experience I've witnessed a satellite fail because a cart was pushed past it while under test. Though the cart never made contact with the satellite, the chain that drug across the conductive flooring had developed some rust, preventing safe discharging of static buildup to ground. When the static field came close to the satellite it had an immediate and catastrophic failure. You don't even have to touch a device to destroy it. Satellites work on ultra low power, so the components they use can be sensitive to static charges as low as 50Vs (Vs = Volts Static). An ungrounded person can blow out a chip on the workbench just by placing their hand near to the chip. The human body can develop and hold charges in excess of 50KVs. Take an older TV or CRT monitor and place your arm near the screen. Your hair stands on end and is pulled toward the glass. That's because of the static field. Even though you're not touching the screen you ARE exchanging electrons. Exchange a high enough charge and you'll get that annoying, sometimes painful, snap.
 
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