I have some questions about the ESD S541 and ESD STM 11.13 standard and looking forward your kind reply.
In S541, one of main updates compared to previous version (EIA-541-1988) is adoption of surface resistance (ohms) in place of surface resistivity (ohms/square), and test method changes from ASTM D257 to STM 11.11 ~ 11.13. However it is also mentioned that resistance value is affected by the size and spacing between electrodes （i.e. depends on what kind of probe you used）. It is known that surface resistivity (Ohms/sq) is a property of a material. Theoretically it should remain constant regardless of the method and configuration of the electrodes. So I don’t know why make these changes? because there are many types of electrodes and SR meters in market and will get difference resistance value for a given material.
In STM 11.11 or 11.13, in sample preparation, the test condition is 23C and 12% humidity. 12% is very low and is not feasible or even practical to implement this conditioning test in production mode. We know that temperature and humidity will affect the material resistance, but why set humidity at this low level?
Let me try to answer each question. First, you are correct that ohms/sq is a property material, however, it is only a property material if the material is homogenous. Many of the control products today consist of multi-layer materials. The has been an issue use D257. The reason for reporting ohms instead of ohms/sq is due to the non-homogenous materials that are found today. The other reason that ohms is used in STM 11.11 is the probe is fixed. This allows for a comparison between materials. As you stated, there are many types of electrodes in the market that will give you different measurements. One additional item that changed is the measurement voltage. In STM 11. 11 the voltages are fixed at 10 and 100 volts. D257 allows for higher voltages that are appropriate for insulating materials but can lead to false results in the static dissipative range.
The second issue with temperature and humidity. This is fixed at 23C and 12% as this is the worst case for materials. The 11.xx series of ANSI/ESD standards was mainly for packaging materials. Packaging materials will be in uncontrolled environments. Qualification is done at general worst case condition to ensure they always work. Qualification of materials only needs to be done before implementation and could be done by the supplier or outside lab. Once qualified, compliance verification is done at ambient conditions. See ANSI/ESD S20.20 - 2021 for the requirements for qualification and compliance verifications.