Engineers learn why not to use field meters in production equipment. Use ESD detectors for troubleshooting.
Lexington, Mass. — "Field meters aren't used correctly," said electrostatic discharge (ESD) consultant Ted Dangelmayer at the Boston Chapter of the IEEE Reliability Society's monthly meeting held at MIT Lincoln Labs. ESD events can damage sensitive ICs at the factory. That's why ESD detection and control is essential to maintaining product yield and profitability.
At the January 10, 2018 meeting, Dangelmayer explained how charged devices discharge when touching metal. The current from a discharge, and the current from coupled radiated fields from an ESD, is known as a "touchdown event." Despite the use of static-dissipative materials in the factory, ICs can still pick up charges from movement. When a charged device touches metal, an ESD event can occur.
Field meters are often used in assembly equipment and around workstations to detect fields that can charge objects. But Dangelmayer warned that these meters change the fields and thus are often ineffective. An alternative is to analyze resistances in production equipment materials and look for potential discharge paths. That's rather time-consuming. Indeed, a resistance as low as 10 kΩ is considered metal. A 10-kV discharge through 10 kΩ is enough to generate 1 A of current. That's certainly enough to cause damage to many sensitive ICs, especially given the fast rise times of ESD-induced current. Dangelmayer noted that some ICs are specified to be sensitive to discharges as low as 50 V. But this spec is misleading. "It's current that damages ICs," he said. "Not voltage."
Dangelmayer recommends using an ESD event detector to identify discharges while production equipment is running. He noted that there are several manufacturers of ESD detectors, with each having strong and weak points. You can even build your own.
Instead of trying to analyze equipment, Dangelmayer recommended that you use an ESD detector to see if discharges occur. If you don't detect discharges, then ESD is likely not the source of IC failures. But it's possible to get false positives: discharges that don't damage ICs. For example, mechanical relays and stepper motors create ESD, but in Dangelmayer's experience, they aren't the cause of device failures. Dangelmayer suggested that IC manufacturing and handling equipment not use relays that cause ESD.
ESD detectors can be quite sensitive, letting them detect low-voltage discharges. In the video below, Dangelmayer shows how an ESD detector is sensitive enough to detect an ESD event from across the room.
Dangelmayer stressed the importance of using ESD detectors during an actual manufacturing run, what he called a "wet run" as opposed to a simulated "dry run." Why? Because only during a wet run do the conditions represent reality.
If you do detect ESD, you then must isolate the source and eliminate false positives. Dangelmayer cited some causes of ESD. In one factory, he discovered fake nozzles used for holding parts to handlers. The material's resistance was too low, causing damaging current to reach the IC. "Try to solve ESD problems with materials, not by using ionizers," he said. He's found instances in which ionizers simply don't work.
Although using an ESD detector in a production machine can change its field characteristics, Dangelmayer did note that some ESD detectors have remote antennas. That lets you keep the unit some distance away, which can alleviate its influence.
— Martin Rowe covers test and measurement for EE Times and EDN. Contact him at email@example.com
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