My factory are using the 2 layers table mat (top green, bottom black) as working surface, the resistance to ground is 1x10e7 ohms (approximately values, including 1 mega ohm grounding cord). And the lifetime is 1 year approximate (the resistance to ground >1x10e9 ohms).
After that I founded the 1 layer table mat (black color) that the resistance to ground is 1x10e7 ohms.
I have tried to evaluate for 2 years by use this as working surface of workstation and measure the resistance every 6 month, found the resistance to ground is 1x10e7 ohms after 2 years passed, that can say the resistance to ground still maintain.
Could you please advise for may questions below?
Can I use the 1 layer table mat instead 2 layers, because the 1 layer evaluate result is better.
What is the effect with ESDS items If use the 1 layer table mat?
The 1 layer meet the S20.20 requirement?
Note: The original workstation’s surface is the wood.
The ANSI/ESD S20.20 requirement from Table 3 for worksurfaces is that they have product qualification resistance per ANSI/ESD STM4.1 of <10^9 ohms both point to point and point to groundable point and that your compliance verification is also <10^9 ohms point to ground tested to ESD TR53.
If your one layer mat meets these requirements then yes, you can use a one layer mat per the requirements of ANSI/ESD S20.20.
I am not completely sure of all the reasons why most in the industry use a two layer mat over a one layer mat but I believe one of the reasons is that the black layer is more porous and more likely would absorb any spills whereas the other layer (typically green or blue) is less porous and can more easily wipe away any spills. If you are able to keep a one layer black mat clean and it still passes the compliance verification tests, I don’t see why you couldn’t continue to use a one layer mat.
Another reason risk with using a one layer mat is that this black mat is likely conductive. You could do a point to point measurement to confirm. This could pose a personal safety risk if the mat were ever energized. The other green or blue layer is typically dissipative and would help to prevent such an issue.
The only other thing I could think of why to use a two layered mat is aesthetics. Some people prefer the cleaner look of the colored mat rather than black.
If anyone else has other reasons for a two layer mat over a one layer mat, please reply
Especially you need to view the key points mentioned by him in the last but one para which is a fact I normally high light to my customers.
Single layer at the bottom is conductive with resistance range normally less than 10^5 ohms which could pose personnel safety risk if any live wire comes in contact with it while at work.
It is advisable for explosive sectors or ordinance units( defense units) where a faster dissipation is preferred due to sparking at times.
In electronics applications , added to the above points it would be aesthetics issue as the conductive layer will be black in color due to conductive carbon blended with the base material.
Just because it is conductive layer at the bottom many suppliers advice the mats can be used without copper foil at the bottom. But, I would strongly advise to use a copper foil below irrespective of whether the table mat is of 2 layer or 3 layer or single conductive layer . My observation at many places is that carbon being brittle material, due to dry weather as well as workload on the surface at times it may develop some cracks at the bottom leading to discontinuity for an effective dissipation and thus a copper foil network below would take care of it . Also, if it is a large area or a running table as the area increases it is better to have the copper foil network below just like it is spread below an ESD floor mats /tiles/epoxy
If your ESD mat is being used as an ESD control item, than it MUST meet the qualification requirements as specified within S20.20/STM4.1. You have not supplied supplied sufficient details for anyone to state the mat in question does or does not meet this. Typically, your mat supplier would state if the product meets/exceeds S20.20 which is a good indicator. However, to meet the qualification requirement, all test data and methods must be provided.
A 2 layer mat would typically have a bottom layer that was conductive which helps insure a good ground connection. With a dissipative top layer which controls the rate of the discharge being transferred to ground and potentially causing damage.
Use of any ESD control item really comes down to the following
what are the requirements of the product (internal and/or customer specified)
does the item meet all qualification/verification requirements
can you provide supporting data of qualification/verification