ESD Monitoring Triggers Changes in Grounding Techniques

The quest for quality is never ending. At Motorola, this translates into analyzing every aspect of the process to find better and cost-efficient ways to produce the product we sell.

The search for quality has led groups concerned about ESD to find a more economical and reliable way to collect data for wrist-strap monitoring. After evaluating ESD logs for single-line capacitive wrist straps, a trend became apparent.

Twice each shift the operators checked the wrist strap and logged the results. All the results for several months indicated zero failures. This seemed to be exactly what we wanted.

However, after observing how the operators checked their wrist straps, we noticed that they tried several ingenious ways to make the wrist strap pass. Obviously, the protection we were trying to provide our product was questionable.

As a result, the search for a better way to guarantee personnel grounding began by examining capacitive and resistive wrist-strap monitors. We learned that, in the right conditions, a capacitive monitoring system allowed a substantial charge while giving a false “good” result.

Next, we compared the advantages of single-line vs dual-line wrist straps. The dual-line wrist strap gave an extra safety line in the event that the other line broke. We decided to use a resistive monitoring system with a dual wrist strap. No longer did operators try to manipulate the testing process to make wrist straps pass.

Then a new problem developed–dry skin. To address this situation, operators are required to apply ESD lotion throughout their shifts. The lotion reduces the number of failures significantly.

The next problem surfaced while we observed operators using heel straps. Some operations required operators to reach over their heads and some of them stood on their toes to accomplish this.

Since standing on a stool with heel straps negated the grounding connection, we needed a different solution. Step stools were added to the work areas and the heel straps were replaced by wrist straps with cords long enough to allow operators to perform their job function. Since the wrist strap is always connected to the wrist and monitored, now there is little chance of separation from ground.

About the Authors

Michael R. Hoogstra is an Engineering Assistant for the Advanced Power Group in the Analog Integrated Circuits Division at Motorola. Mr. Hoogstra has a degree in electrical and electronic technology. Motorola, Inc., Advanced Power and Computer Peripherals Operation, Analog IC Division, 2100 E. Elliot Rd., Tempe, AZ 85284, (602) 413-3957.

Steve Nosek is a Test Engineer with Motorola ASIC. He has been in electronics for 35 years, and has been addressing ESD problems since 1972. Motorola, Inc., ASIC Final Manufacturing Test, Gate Array Products, Application Specific Integrated Circuits Division, 1300 N. Alma School Rd., Chandler, AZ 85224, (602) 814-5439.

Copyright 1995 Nelson Publishing Inc.

November 1995

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