Most ESD Protected Areas (EPAs) will contain a bench or a series of benches. It is important that each bench, or worksurface, is covered with the correct ElectroStatic Discharge (ESD) protective material. They also have to be properly connected to earth using a system of cords and common point grounds. Today’s post will explain in more detail how these ESD protective worksurface work and what you need to look out for.
The purpose of an ESD protective worksurface is to aid in the prevention of damage to ESD sensitive items (ESDS) and assemblies from electrostatic discharge.
ESD worksurfaces, such as mats, are typically an integral part of the ESD workstation, particularly in areas where hand assembly occurs. An ESD protective worksurface provides protection in two ways:
- Providing a low charging (antistatic) worksurface area that will limit static electricity to be generated below potentially damaging levels.
- Removing the electrostatic charge from conductive objects placed on the worksurface.
Types of ESD protective worksurfaces
When deciding to invest in ESD protective worksurfaces, you have the choice of ESD matting (laid-out on a standard non-ESD bench) or ESD benches. Performance-wise there is no difference.
Generally speaking, ESD matting offers a lower initial investment and is easier to replace. On the other hand, some people prefer the robust and consistent approach of ESD benches.
An ESD protective worksurface is usually dissipative. Although conductive materials are the quickest to ground a charge, they can also cause damage by discharging too rapidly. Dissipative worksurfaces have a surface resistance of at least 1 x 104, but less than 1 x 109 ohms. Dissipative materials will dissipate a charge slower and are recommended for handling electronic components. Dissipative materials are usually the preferred choice for bench top worksurfaces.
Grounding of ESD protective worksurfaces
ESD protective worksurfaces need to be grounded. A ground wire from the surface should connect to the common point ground which is connected to ground, preferably equipment ground. For electronics manufacturing a worksurface resistance to ground (Rg) of 1 x 104 to less than 1 x 109 ohms is recommended. Best practice is that ground connections use firm fitting connecting devices such as metallic crimps, snaps and banana plugs to connect to designated ground points. The use of alligator clips is not recommended.
Using a current limiting resistor in the ground cord is the user’s choice. However, the resistor is not for ESD control purposes. The ESD Association standard for grounding is ANSI/ESD S6.1 which recommends a hard ground (no resistor) but allows the use of a current limiting resistor in the mat’s ground cord. “The grounding conductors (wires) from wrist straps, working surfaces, flooring or floor mats, tools, fixtures, storage units, carts, chairs, garments and other ESD technical elements may or may not contain added resistance. Where added resistance is not present, a direct connection from the ESD technical element to the common point ground or common connection point is acceptable and recommended.
Note: Manufacturers may add resistance to the grounding conductors for purposes other than ESD (e.g. current limiting). Added resistance is acceptable for the purposes of controlling ESD provided electrostatic accumulation does not exceed specific EPA requirements. The typical added resistance in grounding conductors is 1 megohm, although other values may be specified.” [ANSI/ESD S6.1 section 5.3.3 ESD Technical Element Conductors]
Using ESD protective worksurfaces
Operators need to ensure that the ESD workstation is organized to perform work and that all unnecessary insulators and personal items are removed. Regular plastics, polystyrene foam drink cups and packaging materials etc. are typically high charging and have no place at an ESD protective workstation.
When working at an ESD workstation, users have to be grounded, too. A wrist strap is arguably the best way to provide a safe ground connection to the operator. While it does not prevent the generation of charges, its purpose is to dissipate these charges to ground as quickly as possible.
When working on high-end sensitive components, the use of Continuous Monitors is recommended. Operators connect their wrist strap to the unit to allow for real-time continuous monitoring. If the wrist strap fails, the unit will alarm.
An option available with most Continuous Monitors is the ability to monitor worksurface ground connections. “Some continuous monitors can monitor worksurface ground connections. A test signal is passed through the worksurface and ground connections. Discontinuity or over limit resistance changes cause the monitor to alarm. Worksurface monitors test the electrical connection between the monitor, the worksurface, and the ground point. The monitor however, will not detect insulative contamination on the worksurface.” [ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps]
When the monitor is connected to an ESD worksurface mat, the amount of current that flows is a function of the total resistance between the monitor and through the working surface to ground. When the resistance of the worksurface is below a pre-set threshold, the monitor will indicate good. Conversely, if the resistance level is high when compared to the monitor’s reference, the unit will alarm. This is an integrating resistance measuring circuit, therefore it is relatively insensitive to externally induced electromagnetic fields.
Maintaining your ESD protective worksurface
An ESD worksurface must be maintained and should be cleaned with an ESD cleaner. Regular cleaners typically contain silicone and should never be used on an ESD working surface.
Operators need to be on guard every day and check visually that ground wires are attached correctly. The company’s compliance verification plan should also include periodic checks of worksurfaces measuring:
- Resistance Point-to-Point (Rp-p) and
- Resistance-to-ground (Rg).
Surface resistance testers can be used to perform these tests in accordance with S20.20 and its test method ESD TR53. If these measurements are within acceptable ranges, the worksurface and its connections are good.
Verifying Surface Resistance using the SCS 701 Analog Surface Resistance Megohmmeter
Most people in the industry consider worksurfaces to be the second most important part of an ESD Control Program, with personnel grounding being the most important.
It is therefore important to install, use and maintain ESD protective worksurfaces correctly. Following all steps outlined above will ensure your ESD sensitive components are protected.
Not sure which ESD worksurface is right for you? Request a free ESD/EOS Assessment for your facility by one of our knowledgeable local representatives to evaluate your ESD Program and answer any ESD questions!