| I know that static concerns are very important in your work. Are static concerns eliminated by the antic-static devices you wear or which are in your environment, or do the hand tools you use, such as picks, tweezers and brushes, need to be anti-static? Thanks, Tom | Tom: You are correct, discharge of static energy can damage electronic components. This becomes a greater problem as ICs increase in complexity and decrease in size. I clipped this from our ESD training course.
Electrostatic energy is caused by friction. It�s what causes lightning and the "spark" sound you hear when touching a door knob after walking across a carpet in the winter. It is used to hold the characters on the page during "Xerox" copying and LASER printing. It is also used in residential and commercial air cleaners to make dust stick to the filter.
TRIBOCHARGING. When you rub two different materials together, one becomes positively charged and the other becomes negative. Scientists have ranked materials in order of their ability to hold or give up electrons. This ranking is called the triboelectric series. A list of some common materials is shown here. Under ideal conditions, if two materials are rubbed together, the one higher on the list should give up electrons and become positively charged. You can experiment with things on this list for yourself.
TRIBOELECTRIC SERIES your hand glass your hair nylon wool fur silk paper cotton hard rubber polyester polyvinylchloride plastic
ELECTROSTATIC MACHINES. So, everyday activities generate substantial charges.
Electrostatic voltage (v) Static generation process at 10 to 20% RH at 65 to 95% RH Walking on carpet 35,000 1,500 Walking on vinyl flooring 12,000 250 Worker sitting at a work bench 6,000 700 Handling a vinyl envelope used for work instructions 7,000 600 Picking-up a sandwich plastic bag from a bench 20,000 1,200 Sitting at polyurethane cushioned bench 18,000 1,500
WHO�S TO KNOW? We can not easily see, smell, or feel ESD.
Perception Body�s electrostatic voltage (volts) Not felt when touched at the back of the finger Below 500 Slight sensation when touched at back of finger Above 1,000 Not felt on index finger when touching Below 2,500 Slight sensation on index finger when touching Below 3,000 Audible discharge Above 5,000 Visible spark Above 10,000
Y�OW NOT ANOTHER ONE. Components can be easily damaged by voltages that we can not sense. The breakdown voltage is the voltage level that causes damage to the component.
Device typeBreakdown voltage (v)Device typeBreakdown voltage (v) VMOS 30 to 1,800 Bipolar transistor380 to 7,000 GaAsFET 100 to 300 Junction FET 140 to 10,000 EPROM 100 OP AMP 190 to 2,500 CMOS RA 250 to 2,000 ECL 500 to 1,500 Schottky diode 300 to 2,500 SCR 680 to 1,000 Schottky TTL 100 to 2,500 Film resistor 300 to 3,000
Recognize that at 1/4 of the breakdown voltage shown, these parts can still be damaged even though they may pass the manufacturers� specifications when tested, but the damage may cause them to fail earlier than normal in the field.
ESD CONTROLS. Solving the ESD problem in a factory is complicated and expensive. Employees keep charge generators off the production floor. Factories are equipped with conductive floors that will not generate a charge. Employees wear special coats and/or grounding straps on their shoes. When they are at a work bench, employees wear a grounding cord from their wrist to ground their bodies. Products are packaged in shielded containers during transit. Various types of ionizers are used to produce a gentle flow of ionized air to neutralize electrostatic charges on all kinds of materials. Millions of dollars are spent each year to battle ESD in manufacturing and other professional environments, but the potential for loss due to ESD caused damage is in the billions.
Methods for controlling ESD are:
� Charge prevention � Grounding � Shielding � Neutralization � Education
Each time you start a new task, you need to understand that the charge generated and potentially passed to the product is within the voltage threshold for you plant. You can set that threshold at any level. EIA 625 suggests �200 volts. A 400 volt swing seems high, but you get to pick any level that will prevent damage to your product. Now, finally getting to the point of your question Tom, every time you touch something it generates a charge. The questions about the charge are:
1 Can I keep the charge generated at a low potential? 2 Can I keep it far enough away from the product to prevent damage?
Here�s what I do in selecting:
1 Hand tools: 1a If I could, all tools would be metal with no plastic handles, but I know that unpadded handles are tough on people using hand tools all day long. 1b All things the same, I�d buy a tool with ESD dissipative handles 1c Then I�d buy tools with soft plastic on the handles and then cut the plastic off. 2 Brushes: Need to be antistat
TTYL
Dave F
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I know that static concerns are very important in your wor...
- Jan 11, 1999
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