This information HAS errors and is made available WITHOUT ANY WARRANTY OF ANY KIND and without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. It is not permissible to be read by anyone who has ever met a lawyer or attorney. Use is confined to Engineers with more than 370 course hours of engineering.
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.
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.
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.
Two things are required to allow ESD:
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.
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.
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.
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!
Sometimes, I've had a hard time convincing people that volume conductive plastic is quite different than antistatic plastics. To prove the point I came up with the ESD detector below that indicates an ESD episode. Other devices, such as flash bulbs or a radio receiver can also be used. High explosives can detect ESD as well, but should only be used for the most stubborn of critics. The device below is quite sensitive and can be used repeatedly.
The key part is a neon bulb to detect ESD. Neon bulbs typically break down and glow at about 60V (this is over the 20V or so that damage begins). If you place a neon bulb in series with your body, rub your feet on a carpet, and touch the other lead to a doorknob, you will see the bulb light when the spark flies. The problem with this is that the flash can be so dim and quick that it can only be seen in a darkened room. To solve this problem I've come up with a simple circuit that detects even very dim and fast flashes and stores the fact that they have occurred.
Mount the Neon bulb and phototransistor inside an opaque plastic can that 35-MM film comes in (I suppose you will have to find something else these days). The neon bulb lights turning on Q1, a high-gain phototransistor. The transistor pulls down Pin 1 of the 4096. Section A and B of this 4096 form a SET - RESET flip flop. The momentary push button switch activates the reset. The Q output is buffered by section C of the 4096 which in turn drives the LED. No resistor is necessary because the output FET will 'pinch off' at an appropriate current to light an LED. A 9-volt battery with an ON/OFF switch provides power to the circuit.
The first test is to walk on some carpet and spark to a doorknob of course. If it is at all humid in the room you will need to be fast.
Use wood or an anti static work mat (not a conductive mat! - they can KILL you - (These mats can conduct lethal mains voltage!)) for a work surface. Store parts in anti-static plastic bags (paper envelopes or cardboard boxes will work well enough for most applications just don't use plastic containers). Cardboard and paper are naturally anti-static as long as they don't have any plastic coatings. Cardboard containers are fine as long as they are not coated with black conductive materials that provide a destructive path for a static discharge. The only drawback to paper and cardboard containers is they don't provide the vapor barrier needed to preserve the solderability of surface mount parts.
There are anti-static sprays that work to treat plastics that you can't live without (treat your plastic squeeze solder suckers). The sprays need to be repeated and wear off. Carpets can be treated with this spray but be prepared to use a lot and repeat often.
(Hot tip: use an anti-static floor-wax on your plastics and you shouldn't have to ever treat them again! Just dip your plastic bins and trays in. Then let them drip drain and dry. Think of the antistatic floor wax as an almost permanent spray. see http://www.rdmoney.com/floor_finish.htm).
First, paint with enamel (or powered coat) (as long as it is a relatively non conducting paint) any conductive surfaces. This includes the rails on assembly racks! Work surfaces can be covered with an anti static work pad.
I save ground straps for last because they aren't quite as important as the above items. First of all removing static generating materials reduces the need for ground straps (especially if you learn good touch habits.) The biggest reason I don't recommend relying just on ground straps, is because most engineers, hobbyist and technicians don't and won't wear them (yes, you really should wear one). When working in the field there isn't always a ground-strap handy, so understanding what a ground strap does and doesn't do becomes even more important than wearing one.
Ground straps work by connecting your body to ground and bleeding off any charge as fast as it builds up. It supposes that every thing else you touch will also be at ground so there will be no potential difference. But, ground straps won't stop all static damage from occurring in the real world.
Suppose you are sitting at a bench with a ground strap on and someone walks up and hands you a chip. If they have a charge on them from shuffling across the floor and your first contact is with a lead of the chip, you can damage an IC even while wearing a ground strap! The ground strap will even increase the potential for damage in this case because you have increased your total capacitance by connecting yourself to ground!
To avoid this kind of damage requires learning the touch rule. Whenever someone tries to hand you a chip make sure you contact their skin with your skin first, then grab the chip. Think of their skin conductor that will equalize your potentials. (Some people don't want to be touched, so have them place the IC or circuit board on a pink-poly-bag by touching the bag first then placing the part there - when you pick up the item touch the bag first then pick up the part.) If you are working on a computer, touch the bare metal of the case before you touch any PC board then maintain contact with the case by resting your arm or other hand on the case at all times. This is better than being hooked to a ground strap in that it is possible that the frame of the computer case may not be at ground. If you pick up a PC-board from the bench or out of a static bag, always touch the bag first then touch the ground plane of the board first as you pick it up.
Making these touch-rules a habit is the next best thing to having a ground strap on everyone at every single minute. (If you are working with high voltage a whole different set of rules apply - lets assume that there are no voltages above 24 volts and I'll leave learning the one-hand rule for a different article except for one quick point. Never work with high voltages at a bench with a conductive (black) mat; it can kill you!)
The touch rule works even better if every one wears ground straps. If you want to work in electronics as a profession, it is a good idea to start off wearing a ground strap as a habit. Production factories know that wearing ground straps makes an incremental difference in quality. They should always have a built in current limiting resistor in the system or you can expose yourself to shock hazards. That means a watchband tied to ground can kill you if you are working around AC mains voltage so get the real thing.
Dan Anderson is the guy that invented 'anti-static' plastic for Richmond Chemical (the pink in anti-static plastic was his idea too!). The best anti-static videos I've ever seen were by him. He spent a lot of effort trying to help people understand that conductive plastics don't prevent sparks (Remember it is the 'D' in ESD that causes the problem.) Sadly, he was no match for some Minnesota company's marketing arm that was selling conductive (carbon filled, black) plastic for ESD prevention.
He told a story about an ESD episode at NASA that killed an engineer. There was a solid rocket they were working on complete with ignition squib. To make things 'safe', an engineer tied the two leads to the squib together (should have stopped there) and then tied the leads to ground. When they lifted the plastic shroud off the rocket to work on it they generated enough charge to cause a spark to jump from the squib casing to the leads - igniting the squib which started the rocket motor at a bad time. The point of this story is that grounding everything with conductors is not the way to approach ESD control; you must stop the generation of charges if possible and SLOWLY bleed off any charges you can't prevent.
If you have ever walked across a carpet on a dry winter day and produced a spark on the door-knob, you might have noticed that you had to touch something metal - a conductor - to produce the spark. If you paint the door-knob with enamel, you can prevent such sparks - and if you are working with static sensitive materials (explosives anyone?) you might want to paint any exposed metals.
If you have slid across the plastic seat of a car to kiss you partner and instead gotten shocked, you might think the answer is anti-static spray (Woolite is cheaper). Sprays have a limited life time and no one remembers to repeat applications. We are better off dipping plastics in anti-static floor wax - which provides a rather permanent solution. Carpets can be treated by really soaking them with anti-static solution, but again it is not a very permanent solution.
If you were holding hands while you slid across the seat, you would not have gotten shocked. While not as romantic as holding hands, a
|Top Page||wiki Index|
(C) Copyright 1994-2019
All trademarks are the property of their respective owners.