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ESD....Limiting Resistor

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#59147

ESD....Limiting Resistor | 29 June, 2009

I need some help in setting straight QA which owns our ESD Procedures. 1. Is the limmiting resistor for personel safety or to slow down ESD discharge? Everything I have read indicates it is for personnel safety. 2. Is it an ESD violation to have electrical equipment on the ESD mat even though the equipment is grounded via the power plug? QA's stand is that the electrical equipment on the ESD mat provides an alternate path to ground and thus bypassing the limiting resistor.

I have read ESD TR20.20 and all it mentions that we keep everything at the potential and use a common ground point, preferably the AC ground. I need to make a case with supporting material.

Thanks,

Geo

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#59148

ESD....Limiting Resistor | 29 June, 2009

Current limiting resistor: Personel safety

Electrical equipment on the ESD matt: We need more information. Is the equipment ground of the equipment in contact with the matt? Or are the insulating pads on the bottom of the equipment in contact with the matt? And we have equipment on the matt for what reason?

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#59159

ESD....Limiting Resistor | 30 June, 2009

Dave, We have both situations where the grounded electrical equipment on the mat has insulative pads or does not have insulative pads. The workstations we have this problem with are test stations where electrical equipment is used on the test bench as well as functional test fixtures. Our test fixtures are either made from plastic or sheet metal. Our PCBs are generaly powered by 5-24V DC power supplies when tested on the mat. Seldomly we do test 110v or 220v.

Hope this is enough information.

Geo

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#59161

ESD....Limiting Resistor | 1 July, 2009

You are correct. The resistors are for personal safety. However, I don't think that bench top or floor mats require a 1MO current-limiting resistor. This would be necessary if a wrist strap was being used. The mats we use do have a 1MO resistor on them, but it's just in case a wrist strap is plugged into it. http://www.tecratools.com/pages/datacom/esd.html http://www.intel.com/Assets/PDF/pkginfo/Ch_06.pdf

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#59166

ESD....Limiting Resistor | 1 July, 2009

So, would putting insulating pads on the test fixtures without pads make this issue go away?

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#59168

ESD....Limiting Resistor | 1 July, 2009

It would, but is it correct? This is what QA came back with. "Think of a chassis, connected to earth ground, sitting on top of a mat. In this situation, we are bypassing the meg-ohm resistor to ground." If the mat, the operator and the test equipment are on the same electrical potential (AC ground), does it matter if the test equipment is insulated from the mat?

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#59176

ESD....Limiting Resistor | 2 July, 2009

Nuez,

I don't really see how it's a problem. If your not using a wrist strap, there's no grounding issue. It will work fine as is, equipment insulated or not. The "conductive" mats have many mega-ohms of resistance as it is. Simply "bypassing" the 1MO resistor shouldn't hurt anything or anyone. But, if you are using a wrist strap, it may already have a 1MO built into it. Does it? If so, there's no issue. If not, you would still be fine as long as you don't connect directly to the chassis ground of the equipment. Simply use the banana plug provided on the mat.

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#59233

ESD....Limiting Resistor | 8 July, 2009

Advertisements are not always 100% accurate on technical issues. You don't want instand static discharge. A wrist strap with no resistor could cause that. Therefore put in a current limiting resistor to make the charge dissipate slowly.

Does anyone have any non-advertising sources calling it a safety issue?

And to the OP. Tell your QC department to stop the guesswork and get the proper measuring equipment. How many ohms are there from the mat to the ground. What you say they say, sounds like speculation to me. If they want accurate answers, properly measure the resistances to get the accurate answers.

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#59236

ESD....Limiting Resistor | 8 July, 2009

Stephen,

This link was also in my earlier post.

"The wrist strap cord has a current-limiting resistor for personnel safety."

http://www.intel.com/Assets/PDF/pkginfo/Ch_06.pdf

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#59237

ESD....Limiting Resistor | 8 July, 2009

Actually that reminds me I remember one place I visited that wore two heel straps for saftey purposes.

The reasoning was that if you touched a live wire with one hand and only had the heel strap on the other foot the current would go through your heart.

Even if the current limiting resistor did not have safety advantages you would still need it for ESD reasons.

The OP was talking about mats. JESD625-A Paragraph 6.1 says, "1 G1 (equipment ground) or G2 (earth ground) is acceptable for ESD ground. Where both grounds are used, they shall be connected (bonded) together. 2 R1 is mandatory for all wrist straps. Its value shall be within the limits specified in table 2, 1.a. 3 R2 (for static dissipative worksurfaces) and R3 (for ESD protective floor mats) are optional. When R2 or R3 are used, the resistor values shall be as recommended by the ESD protective equipment manufacturer. ESD protective flooring shall be connected directly to the ESD ground without R3."

Hopefully everyone will be able to figure out the R's without needing the drawing that goes with it. But the point is that the resistors for for static dissipative worksurfaces and for ESD protective floor mats are optional.

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#59323

ESD....Limiting Resistor | 18 July, 2009

hi guys,

can anybody share any info about the maximum acceptable surface resistivity on ESD floor & table mats? Is 10G ohms still ok?

thanks

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