For any worksurface where unprotected ESDS items are worked on, the requirement from ANSI/ESD S20.20 is a resistance to ground of less than 1x10E9 ohms. The reason for a multilayer mat is for a consistant measurement from any place on the worksurface to ground. The path is typically through the disspitive layer to the more conductive layer to ground. Since the conductive layer the limiting resistance is from the top surface to the conductive layer underneath.
For a single layer dissipitive mat, the resistnace can be dependent on the distance to the ground point. Making a measurement from the furtherest point from the ground connection will be the highest resistance of the worksurface. In the case of these materials, if it is a large worksurface, multiple ground connections may be required.
Making multiple measurements during the installation will determine if multiple ground connections are required.
In either case, if the resistance to ground measurment is less than 1x10E9 ohms, the worksurface is compliant.
Something else to consider, is these surfaces are usually hard and slick. Creating potential risk for physical damage or marks and or product sliding unintentionally. ESD mat may be used as an added protection against these factors as well as maintaining the static protection required. You would want to sample a material to insure it meets your physical expectations as some mats are quite different in texture and hardness.
In addition to Chris’ comment above - over time the hard surface may degrade and get scuffed up (already shown in you photograph). More compliance verification measurement may need to be taken on a more frequent basis to ensure that resistances are staying consistent.
Also with degradation of the surface, you may also have a potential to have small debris particles that could contaminate your product.
Greetings of the day to all, from India
As a Freelancing ESD Control CV Auditor I would like to share my experience and opinion too, as under.
A our ESDA’s Senior Expert Mr JK has mentioned in the last para of his comments “ If the resistance to ground measurement is less than 1x10E9 ohms, the worksurface is compliant per ANSI/ESD S 20.20. / IEC 61340-5-1”. And no problem at all
If this compliance verification at less than 1x10E9 ohms remains consistent throughout its life time irrespective any ambient condition then it shall be excellent. and yet again no problem.
But a hard dissipative plastic you have mentioned is as good as an ESD hard laminated surface which in most of the cases at Rh% 40 and above do well with less than 1x10E9 ohms Rg but in dry conditions or even at 30-35% I have seen that both Rpp & Rg shooting up to greater than 10E10/11 ohms many a times. Thus, if your surface has been qualified to be well with in 1 x 10E9 ohms at a lower Rh% per the relevant standard test method in relation to any of the above two standards then perhaps there should be no ambiguity in using the above said material. However, as Mr.JK has explained very clearly subject to the area of the surface it is better to have multiple ground connections to enable for a speedy discharge from the nearest ground path.
In Two- or Three-layer mat surfaces, either the sand witched bottom layer or the middle layer Shall always be conductive and the grounding buttons put on the mats penetrate through the conductive layer enabling for a speedy discharge as in most of the cases the top dissipative layers in mats are more Hygroscopic in nature ( unlike the hard plastic / hard laminate ) and would be with Rpp /Rg well within 10E7-8 ohms and at bit higher Rh% up to 50 it would be even 10E6-7 ohms…
In most of the cases hard plastic (Acrylic sheets) is offered as Antistatic (low static) in the range of Rpp 10E10-11 and a perfect or fast grounding cannot be guaranteed.
However, if your surface is a homogeneous (volumetric) ESC (Electrostatic conductive) with Rpp / Rg /Vg all less than 1 x 10E6 ohms then it shall be very good and fully compliant per both the standards. But, in case you are working on high voltage operations you need to be cautious to avoid any live wire touching the work surface. The ESD control aids are basically for the protection of your ESD sensitive devices that you would-be handling but with some amount of protection to the operators by way of the built-in resistance but not for operator’s protection against high voltage or live electrical contacts.
Look forward to your comments and, thank you all for the opportunity extended to me to share my views in this forum.
Regards,
Vijay
