Building a static-safe workstation

Independent research published in 1999 (1) revealed that one major technology company alone estimated its cost for static-related damage to run into millions of dollars but after embarking on a programme of ESD control $5 million was saved in the first year – representing a 3000% return on investment. Other research has substantiated that 70% of failures in electronic devices can be attributed to static damage from people who are incorrectly grounded.

Part of a control programme to drastically lower these losses and failure rates will include static-safe work areas. A static-safe workstation or electrostatic protected area (EPA) is defined as a work area with materials and equipment that limit electrostatic voltages and electrostatic discharge (ESD). This prevents damage to sensitive electronic components and assemblies that could be damaged or destroyed from electrostatic discharges of just a few volts; but in general systems will protect to a level in excess of +200V. In Europe equipment used to create an EPA should comply with the requirements of BS IEC 61340-5-1.

An ESD co-ordinator should be nominated to determine the EPA boundaries, construction and selection of equipment, general design and then audit a new or modified area to ensure that compliance is achieved. The co-ordinator can then issue a certificate of conformance before the EPA is used. Furthermore any departures from the guidelines of BS IEC 61340-5-1 must be recorded on the certificate, together with details of any additional equipment or procedures that have been deemed necessary.

If several EPAs are being created they could be combined into one large area to include an ESD compliant floor. If this is not done, then each workstation forms an individual EPA and in this case devices/ assemblies must be put in ESD protective packaging when they to be moved from one workstation to another.

Specific EPA components

Eight main components are required to construct an EPA workstation, assuming the existence of a suitable workbench.

1. Tablemat. This provides a work surface that dissipates static from conductive items place on it and is usually made of vinyl or rubber. Rubber provides good resistance to solvents, hot solder and soldering irons.
2. Common ground cord. This connects the tablemat and one or two wrist straps to ground and normally has a push connector that snaps to the mat stud.
3. Wrist strap and coil cord. These can be either fabric or metal straps that connect to the wearer's skin to ground via the coiled cord. For comfort they can be supplied in a hypoallergenic material the cord has a built-in resistor for operator safety.
4. Floor mat. This provides a walking surface that dissipates static charges from conductive items placed on it. Usually these mats are made of dissipative rubber, solid vinyl or foamed vinyl for its anti fatigue properties.
5. Floor mat ground cord. This connects the floor mat to ground through the common ground socket.
6. Heel strap or sole grounder. This connects the walking or standing person to ground by using the moisture in the shoes as a body connection. A conductive rubber tread provides connection to a grounded mat or floor. It is mandatory to wear these on both shoes.
7. Constant monitor. This constantly checks that the path to ground is connected and working properly. Most give an audio-visual warning of any malfunction.
8. ESD awareness sign. Simple but important, this is a warning sign that alerts personnel to adhere to ESD control procedures as set up by the ESD co-ordinator.

An EPA ground point should provide a low resistance path (<2 ohms) to ground, usually to mains protective earth. Furthermore ground-bonding points must be provided next to each workstation for wrist strap connection and should be clearly marked; there should also be sufficient points to provide for operators and visitors. The points are connected to ground through a resistance of up to 10⁶ ohms.

Moving components/ assemblies

Moving components or assemblies from one individual EPA to another demands utmost caution. One option is to use clean process bags that are double packed to ensure minimum contamination or ESD damage. Depending on requirements they can be transparent metalised shielding bags or moisture barrier bags. Alternatives are ESD safe boxes or tubes and in all cases the packaging should be clearly labelled to indicate its purpose. Furthermore items that regularly move across an EPA boundary should be marked with an ESD caution label.

Good practice

Having set up the EPA the user must also implement a series of maintenance and operational checks.

1. The wrist strap/ operator combination should be checked daily.
2. Mats should be tested quarterly.
3. Mats should also be cleaned with a purpose made static cleaner.
4. All non-essential plastic items (cups, food containers etc.) should be removed from the workstation because they can generate large amounts of static charge.
5. All personnel must wear wrist straps and foot grounders when seated at the workbench; most people lift their feet off the floor at some point while seated.
6. Parallel ground paths must be avoided. The wrist strap should be the only connection to ground for the operator.
7. Mats must not be "daisy chained"; each mat should be individually connected to ground.
8. Do not ground the mat by one snap and the wrist strap by another; this will add the resistance of the mat to the ground path.

Summary

Good static control practice can quickly more than recover its return on investment through reduced failures; but it will provide even higher returns when saved reputation is taken into consideration. If a product fails because of ESD damage during production testing it is a nuisance to say the least; if it fails when it reaches the customer it is a problem; but if it fails in service it is a potential catastrophe and the cost of this would be at least one thousand times that on an in-house failure – customer dissatisfaction, lost business, even lost jobs.

Perhaps the following is appropriate "for want of a nail, the shoe was lost; for want of a shoe the horse was lost; and for the want of a horse the rider was lost", Benjamin Franklin (1706-90), American Statesman, pioneer in electricity and inventor of the lightning conductor.