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HBM Device Failure: The Scourge of Static Sensitive Operations
This close examination of S20.20 reveals the key ingredients to a successful ESD control program.
Multinational manufacturing corporations are racing to pursue the cost benefits of economic geography. In fact, by 2005 as much as 70% of all manufacturing operations will be located in Asia. As part of this dramatic evolution, the impact of ESD is estimated to become the third largest problem in electronic manufacture and device handling.
Enlightened managers know that ESD affects primary assembly and their product warranty as well as the electronics industry in general. Those understanding how ESD influences their process will be the beneficiaries of stable productivity, enhanced product reliability, and an enviable bottom line.
With devices having evolved to their smallest configurations and fastest, most sensitive structures in 30 years, onboard circuit protection currently is very limited. To achieve ESD protection, controls must be built into the process and handling environment more so than ever before.
While it is true that automation is a large part of the process, discharges described by the Human Body Model (HBM) remain a major source of ESD failure. HBM, along with its higher speed and power cousins, Charge Device Models (CDM) and Machine Model (MM), accounts for international ESD losses that now exceed an estimated $90 billion. Yet, the industry spends far less than $2 billion trying to resolve this insidious problem. Two years ago, the Electrostatic Discharge Association (ESDA) announced the availability of third-party certification to the ANSI/ESD S20.20 Standard for the Development of an Electrostatic Discharge Control Program. This milestone document includes specific guidelines for developing an efficient ESD control program for effective device protection. To date, more than 70 corporations have received independent certification, and as many as 1,000 facilities will be certified by 2007.
Close examination of this document reveals the key ingredients to a successful program. One of them is proper personnel grounding. In addition to wrist straps, the document offers options for grounding via footwear and flooring and controlling body-voltage generation.
Personnel Resistance To Ground
In the S20.20 ESD Program Manager Certification Course, ESDA Standards Chair Ron Gibson emphasized that body voltages always are being generated and that wrist straps minimize the effect of these transients based on the body's actual resistance to ground. While quality wrist straps offering 1 Meg Ohms to ground limit body voltage to approximately 10 V or less, degraded wrist straps with 35 Meg Ohms to ground will allow body voltages of up to 100 V. S20.20 requires personnel resistance to ground be less than 35 Meg Ohms. For those handling components with greater sensitivity to ESD, it may be prudent to reduce the voltage and resistance limits.
Resistance Measurement of Personnel to Ground
As part of its tailoring approach, S20.20 allows a means for grounding via footwear and flooring to provide personnel mobility during transport and while standing at a machine or workstation. S20.20 also requires the use of wrist straps when personnel are seated and handling ESD-sensitive devices.
The reference for measuring the personnel system resistance to ground is defined in ESD STM 97.1-1999 Floor Materials and Footwear—Resistance Measurement in Combination With a Person. This basic measurement requires a wide-range ohmmeter and appropriate leads and grounding accessories. The ground point for the measurement should be tested before use in accordance with local codes and ANSI/ESD STM 6.1-1999 Grounding—Recommended Practice. The ground point may be a previously tested ESD common-point ground or an equipment ground reference.
With the test subject wearing ESD control footwear and standing on an ESD control floor, measure the resistance from the hand to the selected ground point using 10 V for the test voltage (Figure 1). This measurement takes into account the individual's body resistance, footwear, and floor resistances.
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*by Stephen A. Halperin, Stephen Halperin & Associates
