Hi everyone,
Why do dissipative worksurface mat define E+6 as lower limit value in ESD standard(S4.1)?
Is there any document indicated that it will reduce static discharge hazard if using E+6 as lower limit?
Happy new year!
Bruce
Thank you for your question. The lower limit for the “dissipative” range of electrical resistance is a “committee chosen value”. Electrical resistance at 10E6 ohm has reasonably fast decay time for objects with a charge that make contact with that surface, so works fine for manual operations. Higher speed operations where contact of charged objects occurs might require lower electrical resistance for adequate decay of charge. The 10E6 ohm level also reduces the current peak from a contact discharge which might be useful to protect sensitive items from being damaged during the decay process. The high end of the so called “dissipative range” for worksurfaces and many other static control items and materials is <1 x 10E9 ohm. Again, this 3 order of magnitude range is “committee chosen” as the range for acceptable dissipation of charge but mostly for manual operations. At the high side of the range, the dissipation rate for static decay might be higher than desirable for high(er) speed operations. For packaging materials, the high side of the "dissipative range is <1 x 10E11 ohm and the decay time approaches multiple seconds.
Be aware that in some industries such as where flammable or explosive atmospheres or materials are involved, the 1 x 10E6 ohm level is specified as the high side of the resistance range acceptable for static control purposes.
Hi Brennanp,
Thank you for detailed reply! And I wonder that:
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How much is the reasonable decay time for objects with a charge which contact with 10E6 ohm electrical resistance? Lower than 1s? Have any experiment document on this?
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Could you please explain more detail on “Higher speed operations where contact of charged objects occurs might require lower electrical resistance for adequate decay of charge.”?
Kindly advise, thank you!
Bruce
Thanks for your follow-up questions. Decay time depends on several factors, the most important are the resistance and capacitance of the interacting bodies. The chart below shows the “calculated” decay time for an object (here a person represented by a resistance of 1500 ohm and a capacitance of 150pF), discharging from 1000 volts to 100 volts. At 1 megohm, the decay time is well under 1 second and at 1 x 10E9 ohm the decay time is 1.72 seconds. This type of information is discussed in numerous ESDA tutorials. The measurements are done very often by static control practitioners.
For high speed operations, we were generically referring to things like pick-and place equipment in circuit card assembly. As the speed of the operation goes up it is necessary to reduce the electrical resistance between contacting elements in the process to assure adequate decay time. But in general, around 1 megohm is satisfactory for most processes. Dropping down to 5 x 10E5 (500,000 ohm) should be fine for many very high speed operations.
As in many electrostatic related issues, it is always best to reduce the actual charge on sensitive items before any possible contact with ground. This is done by selection of contacting materials to reduce triboelectric charging in the process in the first place and by the judicious use of well balanced ionized air when necessary.
Bruce,
To answer your first question, why a lower limit, first lets look at the current standard. The updated version of ANSI/ESD STM4.1 no longer has a lower limit or any limits anymore. This standard test method is now just that, how to conduct a test. For the requiements you need to refer to ANSI/ESD S20.20. In the new release of ANSI/ESD S20.20 the requirement for worksurfaces point to point and point to groundable point for qualification is less than 1x10*9 ohms when tested by ANSI/ESD STM4.1.
Brennan has already answered why the upper limit was chosen. In ANSI/ESD S20.20 there is no required lower limit for worksurfaces but there is a footnote to indicated if Charge Device Model (CDM) type failures are a concern or it cannot be assured that the ESD sensitive parts do not charge, then a lower limit of 1.0x10*6 should be considered. This lower limit is selected to help protect the devices from discharging to fast.
While conductive worksurfaces can be used, evaluation of the process and devices become more critical when when devices are very sensitive to CDM.
Hi John, Brennanp:
Thank you for detailed reply! Appreciate!
Bruce