It is well known that static electricity can be a component killer. Electrostatic discharge or ESD can damage such things as CPUs, motherboards, memory devices and expansion cards. Fortunately, there are safeguards you can put in place to prevent the harm of ESD, the most effective of which is the anti-static mat.
Basically, an anti-static mat used on bench tops has a surface layer that is static dissipative (which is defined as having a resistance of at least 1 x 10E6 ohms and not more than 10E12 ohms) backed by a conductive layer. This surface layer will dissipate an ESD in a more controlled way than a conductive working surface (which has a resistance of less than 1 x 10E6 ohms), the speed of which can cause damage by dissipating the charge too quickly.
For the floor, anti-static mats are usually simple conductive mats, impregnated with carbon and black in color.
How to Ground Anti-Static Mats
In order for the mats to work efficiently, they need to be grounded. Naturally, as the undersurface of the mat is conductive, if the table and floor are also conductive, then the static will travel to ground via these.
However, it is more common to connect the mat directly to an electrical outlet using a grounding cable. To achieve a connection, many attach a connective snap at one corner of the anti-static mat to which the grounding cable can be attached. Snaps can be:
- Push and clinch, where prongs are bent to secure the snap to the mat.
- Screw on, and
- Riveted, where a punch and anvil are used to provide a secure fitting.
Types of Grounding Cables
There are two options for grounding cables:
- A mat ground simply attaches the mat to ground via a cable going from the snap to an electrical outlet using an eyelet, or #10 ring terminal that screws directly to the outlet.
- A common point ground is often a better option as the wrist strap that grounds the technician is attached to the same point. Grounded personnel are the most important factor in any facility, and this is the best way of ensuring that they are properly grounded.
Grounding cables should have a measured resistance of 1 megohm (1 x 10E6 ohms) ± 20%, which will ensure the proper rate of dissipation.
Do not daisy chain your mats as this will exceed the desirable overall resistance. Each mat must be connected to ground with its own grounding cable if they are to be effective, and you should measure the resistance to make certain it is within the correct parameters. You should also periodically clean both straps and mats if they are to perform optimally. Make sure you follow the manufacturer’s recommendations.
Monitoring and Testing
The grounding of every wrist strap and mat is hugely important and should be tested at least once every day. If the products that are being assembled are valuable, then you should consider installing real-time monitors that will automatically warn you both visually and with an alarm, should the ground of either wrist strap or mat fail. This will significantly reduce the chances of component damage.
It has been found that vector impedance, where a continuous voltage is applied and any distortions monitored, is the most dependable method.
Both layers of benchtop anti-static mats should be regularly tested, both the dissipation layer and the conductive layer, ensuring that any ESD is first dissipated and then conducted to the ground at the correct rate. An ESD Resistance Meter is designed to perform this function.
Anti-static mats are important for hi-tech manufacture, and their correct grounding is vital. If you follow these simple guidelines, your mats and wrist straps will function as they are meant to, and the delicate components will be protected.