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Question We are contract manufacturing in printed circuit board assembly so esd sensitive control to raw material such as IC's, BGA and ect is very crucial. If a nonconformance is found where product /material integrity is at stake, what is the best advice to contain or disposition should be taken on the product or material affected?
Answer

The ESD Association produced the ESD TR 12-01 technical report which is entitled “Survey of Constant (Continuous) Monitors for Wrist Straps”. It contains useful information

“Since people are one of the greatest sources of static electricity and ESD, proper grounding is paramount. One of the most common ways to ground people is with a wrist strap. Ensuring that wrist straps are functional and are connected to people and ground is a continuous task.”

“While effective at the time of testing, wrist strap checker use is periodic. The failure of a wrist strap between checks may expose products to damage from electrostatic charge. If the wrist strap system is checked at the beginning of a shift and subsequently fails, then an entire shift’s work could be suspect.” 

What to do with ESD sensitive product handled by ungrounded operators or on an ungrounded worksurface really should be answered by the Quality Manager. Here’s some information that may be helpful.

Most companies would inspect or test ESD sensitive items that were handled by ungrounded operators or stored on an ungrounded surface. This could determine if any catastrophic failures occurred. However, there could be latent defect ESD damage that cannot be detected by normal inspection or testing.

“Electrostatic damage to electronic devices can occur at any point, from the manufacture of the device to field service of systems. Damage results from handling the devices in uncontrolled surroundings or when poor ESD control practices are used. Generally damage can manifest itself as a catastrophic failure, parametric change or undetected parametric change (latent defect).” [ESD Handbook ESD TR20.20 section2.7 Device Damage – Types and Causes]

“When an electronic device is exposed to an ESD event it may no longer function. The ESD event may have caused a metal melt, junction breakdown, or oxide failure. The device's circuitry is permanently damaged, resulting in a catastrophic failure.” [ESD Handbook ESD TR20.20 section 2.7.1 Catastrophic Failures]

“A device that is exposed to an ESD event may be partially degraded, yet continue to perform its intended function. However, the operating life of the device may be reduced dramatically. A product or system incorporating devices with latent defects may experience a premature failure after the user places them in service. Such failures are usually costly to repair and in some applications may create personnel hazards. It is easy with the proper equipment to confirm that a device has experienced catastrophic failure or that a part is degraded or fails test parameters. Basic performance tests will substantiate device damage. However, latent defects are virtually impossible to prove or detect using current technology, especially after the device is assembled into a finished product. Some studies claim that the number of devices shipped to users with latent defects exceeds the number that fail catastrophically due to ESD in manufacturing. Other studies exist that claim that no failure mechanism exists to prove the existence of latent failures. To date, most studies contradict one another as to what magnitude of ESD damage can be attributed to latency. ESD damage is usually caused by one of three events:

• Electrostatic Discharge to the device.

• Electrostatic Discharge from the device.

• Field Induced discharges.”

[ESD Handbook ESD TR20.20 section 2.7.2 Latent Defects]

 
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