Concorde Electronics

Some investigators in the disk drive area have found that under certain test conditions that GMR heads can be altered or damaged if near an activated cell phone. The testing was contrived a bit in that a large coaxial antenna was attached across the magnetic stripe of the GMR head and the cell phone operated within inches of the antenna. Naturally, this sort of condition should not occur in actual practice but it does show that the sensitivity of the particular item was quite high (easy to damage). Of course a charged piece of plastic waved in front of the antenna would have resulted in the same problem or worse. The signal strength of a cell phone should not cause problems to anything of a practical nature. Have a look at papers presented in the EOS/ESD Symposia from 1998-2000 regarding testing of sensitive items like GMR heads for disk drives.

Transforming Technologies

The terms conductive and static dissipative typically refer to resistance or resistivity ranges used in the evaluation of ESD control materials and products. By definition, a conductive material has a surface resistivity of less than 1 x 10E5 ohms per square or a volume resistivity less than 1 x 10E4 ohm-cm. A static dissipative material has a surface resistivity of 1 x 10E5 to 1 x 10E12 ohms per square or a volume resistivity of 1 x 10E4 to 1 x 10E11 ohm-cm. These definitions appear in the ESD Association Glossary as well as in various other static control standards documents. For some materials, surface resistance rather than surface resistivity is often used to define these terms. In this case, a simple conversion factor is applied, dividing the resistivity ranges by 10. Thus conductive becomes less than 1 x 10E4 ohms and static dissipative becomes 1 x 10E4 to 1 x 10E11 ohms, provided that the appropriate electrodes with the correct geometric conversions are used. ANSI/ESD S11.11 provides additional information on this issue. The term antistatic, however, does not refer to resistance or resistivity. By definition, the term refers to a material that resists tribocharging. At one time, the term referenced a resistance value, but it was severely misused and today no longer represents any resistance range.

Source: Transforming Technologies

 We are setting up work stations for handling static sensitive devices. What are some guidelines we should follow to be sure we are adequately protecting our components?

An ESD protective workstation refers to the work area of a single individual that is constructed and equipped with materials and equipment to limit damage to ESD sensitive items. It may be a stand-alone station in a stockroom, warehouse, or assembly area, or in a field location such as a computer bay in commercial aircraft. A workstation also may be located in a controlled area such as a clean room. The basic concept of the ESD protective workstation is to keep all materials and personnel at the same electrical potential. Electrostatic discharge occurs when two objects at different potentials come into contact with or in the proximity of each other. If the potentials are equal, no discharge occurs. The key ESD control elements comprising most workstations are a static dissipative work surface, a means of grounding personnel (usually a wrist strap), a common grounding connection, and appropriate signage and labeling.

Source: Transforming Technologies

Are wrist straps necessary if all other ESD precautions are taken (i.e., two ground [foot] straps, jacket, dissipative flooring, grounded mats, etc.)? If so, why?

Wrist straps are not necessary if an operator is wearing two foot grounders on a conductive grounded floor and doesn’t lift both heels/toes at the same time, like some people do when sitting down. If an operator is also wearing a esd jacket, but is not electrically connecting the jacket to either their body’s skin or ground, then the jacket is providing only partial protection. Charges on the jacket may have no where to go or discharge to if the jacket is not grounded. A popular way to ground the jacket is with a coil cord either attached to a snap on the waist area of the jacket or via a wrist strap snapped to the inside cuff of an ESD jacket.

Source: Transforming Technologies