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Saturday, July 30, 2011

About ESD - Electro Static Discharge - Transwiki


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[edit] How to keep ESD out of electronics

[edit] In memory of Don Anderson who taught me much about anti-static plastics

[edit] ESD Electro Static Discharge

Everyone who works with electronics has heard about static damage. Unfortunately, there is often as much miss-information as there is hard fact. First of all static electricity does no harm; it is not until there is a discharge (ESD) that we have to worry. ( It is the 'D' in ESD that blows holes in electronic gates!) Sadly, a lot of snake-oil products are sold to make you feel like you have taken care of 'The static problem'. All that you need to keep the marketing types from preying on you whether you are a hobbyist, technician or engineer, is an understanding based in science. Once you realize that the laws of physics are not suspended for ESD (electrostatic Discharge) you will find that you already know most of the basics.

[edit] What is static electricity?

Static electricity is electricity that is static; that is electricity that doesn't move. When you walk across a carpet on a dry winter day, you build up a static charge. As your shoes rub on the carpet there is a net wiping off of electrons that gives your body a potential that is different to ground. If the air were more humid, the charge would bleed off rapidly but on a dry winter day the charge stays on the surface of your skin for a few seconds. Static electricity is a surface phenomenon. The more surface area the greater the charge stored.

[edit] What is electrostatic discharge?

Static electricity doesn't do any damage until it is discharged - and is then called electrostatic discharge or ESD. Quite simply, electrostatic discharge is a spark.

When you walk across the carpet on a cold winter day, the wiping motion of your feet produces a charge separation that is collected and distributed evenly about the surface of your moist and salty (thus conductive) skin conductive skin. When you reach for the doorknob a spark jumps from your conductive skin to the conductive doorknob. If you felt the spark, you had a potential of at least 3500 volts. Every day, when you touch conductive items, small sparks are produced that can't be felt yet have enough energy to damage electronics.

[edit] Requirements for electrostatic discharge

Two things are required to allow ESD:

  • A charge separation.
  • Two associated conductors that collect charge and are brought together initiating the discharge.

Knowing these two requirements puts us in position to rationally tackle the job of preventing ESD. The first requirement to have ESD is that there is a static charge separation; the second requirement is that there is a conductor (for the purposes of this discussion the only meaningful ones are metal, metallic carbon and our skin's sweat layer). ESD always occurs between two conductive surfaces - If you touch the enamel paint (which is non-conductive) on your computer cabinet, no spark will form. You might have noticed that over the years electronic products have gone from having spark producing exposed conductive metal cabinets, to having non conductive materials on the outside that prevents ESD. Designers have figured out that it takes a spark to have ESD, so modern electronic products now have their conductive shielding layers buried in insulating plastic.

[edit] What to keep out of your work area

[edit] Static generators

One way to prevent ESD is to prevent static build up from occurring. Plastics when moved are the worst and most common static generators. Keep all plastics (with the exception of anti-static plastics) out of your work area. This means plastic bags, vinyl report covers, white foam packing material, clear plastic wrappers, clear bubble wrap, Formica table tops, carpeting, plastic bins and packing tape have to go or be treated with an anti-static dip or spray. Keeping static producers out of your work area is the most important step to reduce ESD damage!

There are also anti-static plastics. These plastics won't form a static charge on them because of special additives in the plastic. Pink polyethylene bags are a common example. The gray metallic looking bags should be avoided unless the metalized layer is buried in plastic to keep it from generating a spark. The anti-static plastics have a surface that dissipates static by bleeding the charge off without sparking. A chip or circuit board in a pink-poly bag is safe as long as you touch the bag before you touch the part.

[edit] Conductors

The above requirements to produce ESD also include conductors. If we eliminate unnecessary conductors, we further reduce the possibility of ESD. Just like a doorknob, a metalized bag it is a conductor and can make a spark. (If you can see through a metalized bag, it isn't an effective RFI shield either. You can test this by placing an AM transistor radio inside the bag - if it keeps playing it isn't shielded - the reason these are so popular is because they look hi tech).

Keeping out all non-essential conductors includes metallic carbon. Black lacquered cardboard is conductive - why not use plain cardboard? This means no black-conductive mats, black plastic packaging, besides exposed metabolized bags.

[edit] What is wrong with volume conductive (usually black) plastics?

Antistatic plastics are carefully formulated to not generate charge separations. Black plastic, filled with metallic carbon, is volume conductive. You can put an ohmmeter across these different plastics and see that the antistatics are insulating while the black filled plastics are conductive.

The volume conductive plastic, instead of protecting, can actually produce ESD. The black volume conductive acts as one of the conductors from our list of requirements to produce ESD! The parts leads can act as the other conductor, Thus, if a charge separation exists you can damage a part just by placing it in a conductive bag!

[edit] Lets get scientific -- ESD detectors


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