ESD Systems - Questions And Answers

As you'd expect, when Operators test ESD wrist straps whilewearing latex finger cots or gloves, the tester does not indicate a passcondition, due to the electrical insulating characteristics of the latexfinger cots or gloves.Is there a potential for static buildup and subsequent discharge on theoutside surfaces of the latex finger cots or gloves? Should we test the ESDwrist straps before wearing the latex finger cots or gloves, or should weperform a different type test while the latex finger cots or gloves areworn by operators? Anonymous, Andover, MA

The Surface Resistivity of ourFinger Cot, Antistatic, Dissipative, Premium: 10^6 – 10^9 ohms, average of 10^8 ohms (ASTM-D257) and will still work (balance charges with ground) with a wrist strap, just won’t “pass” when tested with a tester having a high fail above 1x10^7 ohms. Wrist straps should be tested without finger cots or gloves. When using finger cots or gloves with ESD susceptible devices, they should be ESD safe (conductive or dissipative) and antistatic. If non-ESD latex gloves or finger cots are being used with ESD Susceptible devices, then even a wrist strap won’t prevent possible damage done to the sensitive devices.

ESD Control Programs which include measures to prevent Charge Device Model (CDM) ESD damage will require the use of Static Dissipative Finger Cots or Static Dissipative Gloves. Wrist Strap use is the first line of ESD defense and effectively deals with the Human Body Model (HBM) ESD threats. However, a charged ESD sensitive device can experience a damaging discharge if touched by a person even if that person is properly grounded via a Wrist Strap or Foot Grounder. Increasing the electrical path’s contact resistance is one way to control the speed of the discharge. A good way to accomplish this is by wearing Static Dissipative Gloves or Finger Cots. A device can become CDM charged from motion such as sliding down a defective integrated circuit shipping tube. Another example of CDM charging is by static induction. If an insulator is charged, it will remain charged even if located on grounded mat in an ESD protected workstation. If an ESD sensitive device is placed on that grounded mat, static induction can occur inducing a potential on the ESD sensitive device. A worker properly grounded with a Wrist Strap can touch the charged ESD sensitive device and an ESD event can occur. The recommended solution to this CDM ESD damage is requiring wearing of Static Dissipative Gloves or Finger Cots.

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