Conformity Magazine - March 2009 - (Page 18) ESD Open Forum Compliance Verification for Common Hand Tools Provided by the ESD Association I understand that hand tools like pliers, tweezers, screwdrivers, nutdrivers, and wire cutters can do damage to ESD sensitive product. How do I determine which ones are safe and which are not? Hand tools can be a significant source of damage to ESD sensitive hardware. The decrease in the width, and distance between the current carrying paths built into integrated circuits (ICs) makes it more likely for smaller charges to jump the gaps, and/or just damage the path. As a result, more and more new IC designs will be classified as HBM Class I, and will be more sensitive to electrostatic discharge and overstress during handling. Everyone involved in handling ESD sensitive products and components must make sure that their ESD control programs keep pace with the increasing sensitivities. In this column, we’ll outline methods for classifying nonelectric hand tools in two dimensions, handle static charging characteristics, and handle resistance. The damage threat from hand tools is Charged Device Model (CDM) charging of the hand tool and Machine Model (MM) discharge to the ESDS item. Volt for volt, MM discharge is more powerful than HBM discharge because the resistance of human body has been removed from the equation. CDM charging can produce two separate discharge events. If you ground a conductor (e.g., the conductive metal blade of a screwdriver) while it is in the presence of any item carrying electrostatic field (a charged piece of plastic, or clothing), the conductor acquires an electrostatic charge that may be sufficient to cause damage when discharged. If the hand tool handle is sufficiently insulative, and acquires a charge, it will remain on the conductive portion of the tool until it is brought in contact with a ground path such as an ESD sensitive component (even if the user is wearing a wrist strap). In this case damage may occur from initial contact with the charged hand, tool, and when the charged component is grounded. Q: A: Handle Static Charging Properties Test (hand tools with non-metallic handles) Test Equipment 1. 2. 3. Hand held static field meter Wrist strap Ground source (static-safe work station, or other) Test Procedure 1. Rub the handle of the hand tool several times against the skin of your forearm, a piece of fabric, or any dissimilar material. Hold the tool by the tip or by any means that leaves as much of the handle as possible exposed to the open air. With the other hand, bring the field meter to the calibrated measuring distance, typically 1” (2.54cm). Note both the reading and the rate of decay, if any. (See Figures 1 and 2) Compare the static charge reading with the damage threshold of the product on which the tool is intended to be used. Tool handles that generate excessive levels of static charge and that do not decay in a matter of seconds are likely to be insulative, and should not be used on ESD sensitive products. 2. 3. 4. 5. Tool In Hand “System” Resistance Properties Test This is the more important of the two tests. In the example above, even though a field strength of 1,440 volts was displayed, the actual amount energy available to cause ESD/EOS damage is not significant by virtue of the small size of the handle. This is not the case for the conductive potion of the tool. Figure 1: Screwdriver handle with a field strength of 1,440 volts Figure 2: Grips on needle nose pliers with no measurable field strength 18 Conformity marCh 2009
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