|Application Note AP8500|
|Protecting your Polar Controlled Impedance Test System from damage from static electricity|
Protecting the CITS from electrostatic discharge
Sensitive electronic test equipment like the Polar Controlled Impedance Test Systems can prove vulnerable to damage from static electricity if proper anti-static procedures are not followed. In this note we look at the best ways to prevent the build up of static charge in the work area — and avoid expensive repairs to your CITS.
Testing the antistatic workstation and surrounding area
The condition of the anti-static workstation – along with anti-static equipment – should be tested regularly. Measure the voltages on the items at your workstation with a suitable ESD meter* (chair, desktop, footrests, floor mats, storage units, packaging, etc.) – don't guess them! ESD-safe work surfaces should not accumulate charge – when, for example, rubbed with a cloth.
Five best anti-static practices!
What is static electricity?
In very simple terms, when insulating materials rub against each other some electrons will be knocked off the surface of one material and will accumulate on the other. This results in a difference in potential of the two materials which can easily rise to several thousand volts.
Just as during a thunderstorm when the clouds become highly charged and eventually short out to ground (lightning) so charged insulating material in the workplace will cause sparks. A common domestic situation where this occurs is removing a synthetic sweater – causing crackling and sparks – or getting out of a car on a very warm dry day and experiencing electric shocks when the car body is touched. In these two cases clothes and seats made of synthetic materials are the insulators involved in creating the static charge.
Electrostatic charge builds up on insulating materials such as clothing, upholstery, floor coverings and packaging in the workplace. If this discharges into the input or outputs of electronic test equipment then serious damage will result. Although Polar's test equipment is designed to minimise this problem it cannot be completely protected against all circumstances. The best policy is to reduce the occurrence of static in the testing environment. This will reduce the risk to Polar equipment, other vendors' test equipment and the associated IT equipment.
Generally, static results from movement and so the prime sources are people and the movement of work. Moving air can also be a problem. In order to prevent the build up of static it is necessary to avoid good insulators such as synthetic clothing and plastic packaging (unless specifically designated as anti-static.)
Many natural materials are inherently static reducing. Cotton, cardboard and wood are better than plastics though their antistatic properties deteriorate as the humidity falls. All problems of static generation get worse when humidity is low.
Extra care is needed on days when the air is very dry (low humidity). This can occur in hot weather and in very cold conditions when the air is heated (and dried in the process) in buildings. Damp air leaks away static charge. Static problems are much reduced in parts of the world where relative humidity is over 75% and are very troublesome below 25% RH. As this changes with the weather and time of year this cannot be relied on as a solution to the problem and should be monitored to ensure protection measures are increased at vulnerable times of year. Probably the most significant environmental factor in electrostatic discharge (ESD) control is relative humidity (RH); relative humidity should be kept within the 50 and 75% range.
Circulating warm air from heaters or air conditioning can cause static to build up on insulating surfaces. Trolley and chair wheels running on synthetic carpet can also create static build up on the mobile device or its operator. The best work environment is one where the surfaces and materials plus the operators clothing are all antistatic (i.e. slightly conductive) so electrical charge leaks away rather than builds up. Probably one of the worst signs of a static problem is where the operator complains of frequently receiving shocks when touching grounded objects such as metal bench frames or test equipment enclosures.
The ideal antistatic workplace.
The workbench should be covered with an antistatic mat that is connected to ground (earthed) as recommended by its manufacturer — see graphic below. All work should be in antistatic containers.
The operator should wear a cotton or a specifically antistatic overall or work coat and be connected to ground by an antistatic wrist strap — below. If the wrist strap is too restrictive a very good alternative is to use a conductive floor mat or better to have the test room floor covered with special conductive plastic flooring.
The operator can then wear special conductive antistatic shoes which provide a discharge path for any static which may build up on the body during work. Another option is to use heel grounding tabs that connect the body to the grounded floor by means of a conductive tape wrapped around and inside a normal shoe. These are normally replaced daily.
Note that clean-room overshoes are not normally antistatic and will completely prevent the effect of heel grounders or conductive footwear. Anyone using overshoes MUST use wrist straps at all times when using static sensitive equipment.
Warning signs should be in place to ensure all other employees are aware this is a special area. Supervision is required to ensure no materials that could cause a hazard are brought into the area . Operator practices also need to be monitored; for example, brushing hair at the workplace could create very high levels of static.
As note above, the most significant environmental factor in electrostatic discharge control is relative humidity (RH); the relative humidity of the ESD-protected area should be monitored frequently and kept within the 50 and 75% range.
As the equipment is subject to wear it is necessary to put in place a program of regular testing to ensure the grounding effect is maintained. Testing of heel grounders and antistatic footwear needs to be performed whenever entering the area if this is the primary means of grounding operators, while the condition of mats, wrist straps and other equipment needs monthly checking. Special testing devices are available which give a “beep” to confirm safe operation and only take a few seconds to use. In more complex form these devices can include data logging to prove compliance with company antistatic programs. For example, the Sentry from Whingate Test Services verifies the integrity of personnel grounding devices such as wrist and foot straps and provides built-in data logging to allow compliance with ISO9000.
The use of materials which can generate high levels of static should be avoided in the designated area. Typical examples are tape dispensers, polystyrene and non-conductive polythene bags.
* Portable, compact ESD meters such as the Simco 91-FMX-004 will be found suitable for most manufacturing antistatic work stations.