Friday, January 21, 2005

ESD garments - do we need them?

In EN 61340-5-1:2001 the user guide suggests that parts of the standard need not be adopted if they are not relevant or needed for the processes carried out. In practice this is open to interpretation. In an electronics facility, is the use of ESD garments really necessary?

I interpret the standard as clearly saying that if you use ESD garments then they must conform to the standard. The more difficult question is, are they necessary and appropriate in your particular case? This is a very hard one to argue and in the end it often boils down to a personal judgement based on your view of your product and processes.

At one extreme if you were making a cheap "throw away" consumer product with cheap components that were not particularly ESD susceptible then I think you might not worry about using ESD garments. At the other extreme if you have a high reliability product with high cost of failure (e.g. satellite) using high susceptibility components then you would clearly take more care and use ESD garments. Most facilities operate somewhere in between.

As another consideration, if your operators all wear cotton tee shirts in a tropical climate there is likely to be little ESD risk. If they may wear long sleeved fleeces or woolly jumpers there may well be some risk from charged clothing and use of ESD garments may be advisable. Another consideration is that use of ESD garments helps to enforce ESD discipline and keep people in the right frame of mind that they should be ESD aware while wearing the coat.

5 comments:

Steve Fowler said...

Many say smocks do not help. Many say smocks are necessary. Most say smocks if ungrounded actually
hurt the ESD program. Some large companies mandate smocks with a wide range of resistance. Many
companies use smocks and don't enforce that they be connected to the person or to ground through wrist straps or footwear. The whole issue is in turmoil.

Cotton clothing especially in humid environments is very good. They can be better than smocks because of the sweat layer connection to the person. Smocks are typically polyester and if not connected to the person they become isolated conductors which is worse for electronics. Cotton clothing with non-ESD protective polyester chair uphostery is bad. Many companies do not have a complete system of ESD protection so the use or non-use of one component is always a problem. If a smock is mis-used, it is a problem not a benefit for ESD programs.

Static Doctor said...

Recieved from Michel Duban and posted by Jeremy Smallwood.....


I nearly completely agree with you for this problem but I would like giving you some comments.

The use of dissipative garment, as all protection in ESD protected area, must taking into account the susceptibility and the sensitivity of components or equipments to be protected.

If you have components no susceptible to parasitic electrostatic field,
dissipative garments are not necessary.
If think CMOS with protection by diodes and resistances network components, bipolar transistors, are not very sensitive to external electrostatic fields.

If you have components susceptible to parasitic electrostatic field,
dissipative garments are very important to limit the influence of parasitic electrostatics field.
I think MOS FET without ESD protection network are very sensitive to electrostatic field; I think level translator CMOS (4050) which are not completely protected against positive level can be more sensitive than the other components of the CMOS family.

More sensitive are components, more important protection must be applicated.

Parasitic electrostatics field must eliminated by all means possible :

1 - Connected to ground all conductors and dissipative materials.
2 - Limit all metal contact ( metal contact :dissipation by Resistance <
10 K ohm).
3 - If metal contact can't be eliminated use ionisation with very low offset,
some materials are guaranty to an off-set < 5 V.
4 Use dissipative garments connected to ground to eliminate parasitic field on usual cloths. Cotton with relative humidity of ~50 % can be considered as dissipative materials.

Anonymous said...

Dear Sir,

We are in the manufacturing unit of telecom products,where we require the ESD Protection.
One thing i want to know that,
we are going to make the conductive flooring in our production unit, with proper grounding .Even after the conductive flooring do we require to wear the ESD cloths and Wrist band.How much it is necessory?
reply soon.

Regards

Sandip Thakor
sandip.thakore@matrixtelesol.com
s_thakor@yahoo.com
India.

Static Doctor said...

Possibly the most important ESD measure in manual handling is to ground all personnel to achieve less than 35 M ohm resistance from their body to ground. You can do this for standing personnel, by using ESD flooring with ESD footwear. In this case the resistance of the flooring may have to be around 35Mohm or less as well as the footwear.

When personnel are seated, you cannot reliably ground them by footwear and flooring as they will probably take their feet off the floor. In this case they need to be grounded via a wrist strap.

After these important ESD precautions, ESD coats may be used to reduce ESD risk due to fields from charged clothing.

Anonymous said...

Morning,

I confirm my first comments on cloths use, and I agree with Jeremy for the necessity to be conform to ESD normalisation documents (S 2020 and or 61 340 -...) .
But If you have very sensitive or ESD susceptible components you must improve the protection.

There are many informations on Internet on class 0 components protection.
But the best document is on JPL Intenet site for components with only 20 V of ESD sensitivity.
Document reference is D 1348.

Best regards.

Michel DUBAN