Friday, May 20, 2005

>> I have one query, regarding the compliance of acceptable electro-static field
>> strengths to 61340-5-1. I am unsure as what the standard states is an acceptable
>> field potential for items such as vdu screens, printers, fans, walkmans
>> etc. I have a figure of < 10Kv/m but am unsure where this came from.

< 10kV/m is the recommendation of 61340-5-2.

I interpret this as 10kV/m electrostatic field measured at the site of the ESDS parts. Another way of looking at it is that an object that has a surface voltage of 10kV must not come within 1 m of an ESDS, or 1 kV within 10 cm, or 100 V within 1 cm ......This rule of thumb applies to charged insulators.

Older CRT VDUs are certainly an issue, especially direct after switch-on or off. Newer VDUs - especially LCD types - may not be a problem. Laser printers are an issue - papers come out hot (and insulating) and highly charged. I prefer not to have them in the EPA especially as it usually means documentation is around and uncontrolled.

Other items such as radios, walkmans, computer keyboards etc - I don't believe they are generally an issue - but if in doubt measure the fields associated with them. If necessary keep them well away from ESDS. These items are often so contaminated with salts, grease and moisture from people's skin that you would be hard pushed to charge them up. Radios and walkmans can often be kept on a top shelf away from ESDS for example.

Charged conductors are a different matter - an isolated conductor of any significant size charged to 100V within 1 cm of an ESDS could be a risk because of machine model ESD if it touched the part. Conductors should be grounded (and therefore have no significant voltage).

ESD garments and compliance with 61340-5-1

An enquirer asked today:

>> A supplier has said that they are not going to use ESD coats but use a
>> standard poly cotton coats and this was acceptable. I believe this is
>> not correct please can you confirm that they should be using esd type
>> coats

This is an interesting question and really subject to personal judgement of the facility ESD Coordinator.

61340-5-1 includes garments under "ESD Protective items" and says "Specific ESD protective items when used within an EPA shall have the characteristics described......measured in accordance with the test methods.... at the highest and lowest expected or rated humidity values".

In other words you don't have to use ESD protective garments, but if you do use them they must comply with the specifications given. The specifications include:
"Coats, jackets, smocks and overalls shall completely cover all clothing in the area of the arms and torso"
"There shall be electrical continuity between all parts of the garment."
Garments shall characteristics on the outward facing surface in accordance with table 1"
"Garments complying with ESD requirements shall be clearly marked" (with the appropriate ESD symbol recommended)

Table 1 merely says that the point-to-point resistance (Rp) must be less than 10^12 ohms. In addition, a charge decay test is mandatory where Rp > 10^10 ohms or the "material is of non-homogenous woven or other construction containing insulating areas"

So, providing any coat supplied for use in the EPA has Rp < 10^10 ohms over there range of expected or rated operating humidities, it is compliant with the standard. If it has 10^10 ohm < Rp < 10^12 ohm, it should probably also have to pass the charge decay test. In practice many perfectly good ESD coats will fail the charge decay test, and it is planned to be omitted from the next version of the standard! Pragmatically I normally advise users if necessary to waive this last requirement, making note of the technical reasons for doing so.

So in short, if your suppliers coats meet the Rp < 10^12 ohm criterion (especially at low humidities) I would probably accept this as compliance with 61340-5-1.