Phil Storrs PC Hardware book

ElectoStatic Discharge (ESD) precautions

Causes of Electrostatic Discharge in everyday life

Static Electricity is a common occurrence in everyday life. As we go about our lives, we all experience mild electric shocks from objects around us. Friction between, and separation of, dissimilar materials, causes electrons to be transferred from one object to another. This in balance of electrons between objects causes the objects to be at different electrical potentials, relative to one and other. When two bodies of opposite charge, or a charged and an uncharged body come into contact with each other, electrons flow from one body to the other very rapidly, and can generate very high current flows and heat for a very short period of time.

Synthetic materials like those clothes, carpets and furniture are made of are bad offenders, particularly in very dry weather conditions. They are very good insulators and when a charge is built up on them, it will not flow away due to leakage, it must be discharged by coming into contact with another body. These materials generate high voltage Static Electricity charges when rubbed together. All of us have had a mild electric shock from a door handle, a metal cabinet or a similar object, after we have walked some distance over a carpet or vinyl floor, or moved around on a plastic chair. The motion of our body clad in clothing made of synthetic cloth causes us to be charged to a high potential relative to surrounding objects. These mild electric shocks cause us some discomfort when they occur but this is insignificant when we look at the damage they do to modern electronic components.

The following table illustrates the typical magnitude of the voltage developed is some common situations.

Source of ESD voltage Typical Electro Static Voltage in KV
at a relative humidity of 20%
Walking across Vinyl floor
12 KV
Walking across synthetic carpet
35 KV
Opening a plastic bag
20 KV
Arising from a foam cushion
18 KV
Sliding plastic box on a carpet
18 KV
Removing plastic tape for a PC board
12 KV
Removing shrink film from PC board
12 KV
Triggering a vacuum solder remover
8 KV
Spraying aerosol freezer spray
15 KV

An example of how much humidity effects these figures can be shown by looking at the figure for two of the above examples at a relative humidity of 70 to 90 %. The figure for Walking across a vinyl floor falls from 12 KV to 0.25 KV and the opening a plastic bag figure falls from 20 KV to only 0.6 KV.

What minimum discharge voltage can we see and hear ?

Under most conditions a static discharge needs to be about 3500 Volt for us to feel it, 4000 Volt to see a spark in a dark room, and 10,000 Volt in room lighting. We need a at least a 5000 Volt discharge to hear a crackle. Electronic components and assemblies can sustain ESD damage at far below these figures. A potential of 250 Volt can be generated by waving an arm in the air and this can destroy sensitive electronic components. Processors like the Pentium chip can be stressed by voltages as low as 10 to 50 volt and will eventually fail in use.

Modern electronic integrated circuits are getting more and more devices on the piece of silicon. The internal devices are getting smaller and smaller with device sizes now as low as .3 microns. This means the voltage and amount of current from ESD that will damage devices is now very low. The need to make these circuits operate at faster and faster clock speeds has meant the internal protection devices that were used on the input and output leads are now not very often used, as they extra Capacitance they add to the circuit limits the operating speed of the device.

To add the the problem is the fact most of the damage done by ESD does not result in "instant death" of the device. The damage is said to be Latent Damage and often only shows up as degradation of performance and usually eventual failure after some period of functioning apparently normally. Devices damaged in this way are often referred to as the "walking wounded".

Instant death, usually called "Catastrophic Failure", only occurs in about 10% to 20% of devices damaged by ESD. This sort of damage is the best because it will not cause devices to get built into equipment and fail when they are most needed. Imagine a medical life support machine that failed due to an ESD damaged device, when you were on the operating table !.

It is not only the actual contact with devices that can destroy or damage them, they can also be damaged by the electric field radiating out from an electrostaticaly charged body. MOS technology devices, the most common high technology chips in use today, are most susceptible to voltage or electrostatic field damage. Bipolar and TTL devices are more susceptible to current damage, the current generated by the flow of current due to the passage of an Electro Static Discharge through the device.

Typical figures for susceptibility of devices to ESD damage

Device type ESD susceptibility
Bipolar transistors 380 to 7000 Volt
CMOS logic devices 250 to 3000 Volt
EPROM devices 100 volt
Film resistors 300 to 3000 Volt
TTL logic devices 1000 to 2500 Volt
Microprocessor chips as low as 10 Volt

ESD Protection materials

Three types of materials are used to protect Components from ESD.

Conductive protective materials Conductive materials provide the highest level of protection
. Materials such as metals, conductive plastics, and conductive laminates are common, and metal wire impregnated bags are also used. The most common element used to make plastic materials conductive is carbon but this has the disadvantage of shedding material that may contaminate components or assemblies. Another problem with conductive protection is the possibility of static discharge through a conductive bag, the the devices inside.

Static dissipative protective materials
These materials provide a lower level of protection and are made out of the same materials as the Conductive Protection materials. They are thinner than the conductive materials.

Anti-static protective materials
Anti-static materials provide the lowest level of protection. Materials include some melamine laminates, high resistance conductive plastics, virgin cotton, wood and paper products, and static dissipative or conductive materials of very small thickness. The problem with these materials is that they provide protection against ESD induced electrical currents but provide no protection for ESD voltage fields.

Methods of protection

ESD sensitive devices may be protected by one of the following methods:

Grounding is achieved by connecting the work area, the operator and the items being worked on, to a known good grounding point with grounding straps. This can be achieved in the field if the power points have a good earth, by equipping a three pin power plug with a ground wire only, and clip that can be attached to a grounding mat. It is important to test the power point for an effective earth before relying on this method.

Isolation and neutralization is achieved by using th conductive protection materials described above.

ESD safe work areas and protective equipment

Work stations where static sensitive equipment is handled must be equipped with the following equipment:

When repairing equipment out in the field the minimum equipment to ensure a measure of ESD protection is:

Field Service Technicians must also wear suitable clothing and avoid handling objects that can generate dangerous electrostatic potentials. These objects include non protective bags and containers, beverage and food containers, and furniture.

Personal ESD protection equipment

Personal grounding
Personal grounding straps must be worn in close contact with the skin and are usually fitted around the wrist. The function of this strap is to make sure the operator is at the same potential as the work, and the work surface. A 10 Meg ohm resistor is included in the ground wire from the wrist strap to reduce the possibility of a fatal shock occurring if accidental contact with mains voltage, or other high voltage source, should occur. Conductive shoe straps are worn inside shoes and go around the shoe and make contact with the floor. This grounds the operator to the conductive floor mat without the need for a foot strap that would reduce personal mobility and may pose a danger when moving from work station to work station.

Protective clothing
Long sleeved smocks are available that should be worn over static generating clothing. They are made from ESD protective material and have fine conductive threads woven into the cloth. Conductive hair nets are available as hair is a major source of static electricity.

The work station
The conductive bench top surface must be hard wearing and be able to withstand heat and the commonly used chemicals. A light neutral colour is important as it makes small objects placed on the surface, easy to find. Pale blue is the most common colour used. The bench top or mat must be earthed to a ground point through a safety resistor of 10 Meg ohm.

An ESD safe chair will have conductive wheels and drag chains to ensure it makes good contact with the conductive floor mat. The seat cushions should be made from conductive material so no static electricity charges are built up with movement of the operator, on the chair.

Suitable conductive floor coverings are available as rubber matting, tiles or carpet material. Like the other grounding items, the conductive floor covering should be grounded through a 10 Meg ohm safety resistor.

If it is important to have the highest level of protection against ESD then an Ionised Air Blower will neutralise static charges built up on non conductive items, by supplying a constant stream of both positive and negative ions to the work surface.

ESD safe containers

The wide range of static safe containers include:

The most common static-safe bags are the dark metalised transport bags PC cards are usually delivered in. They offer good al round protection and are semi transparent so the items inside can be viewed without the need to remove them from the bag. The pink anti-static bags and the bubble wrap will prevent the generation of electrostatic charges and provide physical protection, but they do not offer electrostatic shielding protection.

Conductive tote boxes with lids should be used to transport materials to and from the work place. Open storage trays and boxes made out of conductive materials can be used for storage on the work station.

Conductive foam is useful for short term storage and for ensuring all the leads of particularly sensitive devices are shorted together during storage.

Metal foil can be used to protect component devices when no suitable specialised containers are available. Care must be taken not to bend the pins on devices when wrapping them in this material.

A wide range of specialised device storage tubes are available and new components are usually supplied in this type of packaging. The most easily recognised one of these is the tubes used to transport and store DIL chips. They protect against ESD and physical damage.

ESD protection and common sense

The most important element in protection against ESD damage is common sense. Never open ESD protective bags and boxes in unprotected areas. Never let anyone not wearing the correct protective gear, handle sensitive equipment. All to often equipment is damaged in the shop, the store area or in the field, by someone opening containers to "look" at the goods.

Ten points for the elimination of ESD damage to PC hardware

  1. Make sure you have a reliable ground point available near the work site
  2. Connect your body to the ground point with a wrist strap.
  3. Ground all equipment you are working on with ground straps
  4. Handle PC components only on a grounded anti-static work surface.
  5. Do not wear clothing which generates static electric charges every time you move.
  6. Do not handle static generating objects while working on PC hardware.
  7. Store all chips and other components in appropriate anti-static containers.
  8. Keep all PC cards in anti-static envelopes until required.
  9. Be sure to turn off the power and remove the power plug from all equipment before working on it.
  10. Do not plug in or remove devices such as printers and modems while the power is on.

Mechanical and Electrical damage

ESD is not the only hazard to PC Computer hardware. Unused boards and drives should be stored in rigid cardboard boxes, so as to eliminate mechanical damage to them from inappropriate storage and handling. Boards and drives have many fragile surface mounted devices exposed on the outside, and these can be knocked off or damages easily, destroying the board or drive.

Under no circumstances should a Peripheral card of any sort be plugged into the Buses on a System Board with the power on. It is impossible to align the pins on the connectors exactly and this will mean adjacent Bus fingers will be shorted together. The chances of destroying both the card and the system board are very great. The various digital signals on the Bus (data address and control) exist along side power rails and earth pins and all you have to do is to short a power rail to a signal pin and the hardware will be destroyed.

The following two images explain another hazard. A technician had just finished assembling a new Pentium 150 Computer and found he had left a screw out of one of the back plane covers. Without switching the computer off, he attempted to screw the missing screw into the back plane but dropped it onto the system board. These images tell the rest of the story. The System Board, the Sound Card and the Video Interface card were destroyed in this incident.

Measuring around PC Computer hardware with Instruments

When we look at the close spacing of the lead wires on the surface mounted devices on PC Computer hardware we can see how it is almost impossible to use Meter and CRO probes to look at the voltages and signals on the devices, without shorting two lead wires together. Shorting two lead wires together often results in destruction of the board. As circuit diagrams are not available (and would be of little use if they were), there is little point in attempting to analyse PC hardware in this way.

About all we can do is look at the voltages on the power supply connector pins and feel the Processor chip to see if it is warming up. If the processor is operating (getting clock signals) it will warm up after being on for a few minutes.

There is only one tool that can be safely used to analyse a PC Bus and that is a POST card.

Copyright © Phil. Storr, last updated 7th December 1998