ESD and Static Control Questions

Why is it that in tribocharging, there is a big charge produce in short period of time while small charge will be generated at long time? ( at the same force)

(I took the liberty here to respond to the question and go a bit further and look at CDM Testing as described in a recent issue of Conformity.)

A:  First a little background about charge as it relates to ESD (ElectroStatic Discharge).
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 Triboelectric charge is merely the contact and separation of materials.  “It involves the transfer of electrons between materials.”  Which materials lose electrons and which gain them depends on the materials.
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Static electricity can be measured in coulombs, and related to voltage potential via the equation: q=CV.  q = charge in coulombs, C = Capacitance, V = Voltage
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The industry typically uses electrostatic potential and thus uses voltage to look at this energy form.  Voltage is merely charge potential with respect to a ground point or reference and measured in volts (v).�
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Insulators or materials with high resistance restricts or prevents flow of electrons across (surface) or through (volume) it’s material.
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Conductors or materials with low resistance easily allows the flow of electrons across it (surface) or through (volume)  it’s material.
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Insulators and isolated conductors can tribocharge to high voltages and will remain for a long time… so long as energy is not transferred via induction (isolated conductors) by bringing other objects into it’s vicinity and grounding the other object, by grounding the isolated conductor, or by balanced ionization (isolated conductors or insulators).
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When isolated conductors are grounded, they (becoming grounded conductors) will enable electrons to flow easily to ground and the charge upon it will become neutralized and reduced to near zero.�
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Insulators cannot be grounded.  They can induce charge to isolated conductors and can cause electrical overstress/ESD events to isolated conductors at the time they are grounded via the charge field and do not need to contact the isolated conductors in order to do so.
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Here’s another way to say that; “CDM (Charged Device Model) charging can produce two separate discharge events.  Here’s how it works.  If you ground a conductor (the conductive blade of a screwdriver for example) while it is in the presence of any item carrying an electrostatic field ( a charged piece of plastic or clothing), the conductor will acquire an electrostatic charge that may be sufficient to cause damage when discharged.”
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Human Body Model, as is described in ANSI/ESD S20.20-2007… and the ESD control thereof, is concerned with limiting the voltage in the EPA for the protection of ESDS devices (ESD sensitive devices) to 100 volts and a discharge to within that level in less than 0.3 seconds for ESD Technical Elements (some quicker) at minimum.
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I need to know what specifically are you interested in; the HBM, MM (Machine Model), or CDM (Charged Device Model)?  Keep in mind, that “volt per volt, MM discharge is an order magnitude more powerful than HBM discharge because the resistance of human body has been removed from the equation.”

In the article in Conformity, “Demonstrating CDM Discharge Using Common Hand Tools” provided by the ESDA, they state; “The damage threat from hand tools is CDM charging of the hand tool, accompanied by MM discharge to the component or device.”
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Source: Conformity : ESD Open Forum April 2009 pg 20.

The following pics depict the testing I did in my lab in accordance with what I’d learned from a recent Conformity article from the ESD Open Forum entitled Demonstrating CDM Discharge using Common Hand Tools.  It involves charge, not by contact, but by induction;

1-non-esd-screwdriver2

 

2-shockstop-treated-screw-driver2

 

3-gz-shock-stop-sample1

 

4-charging-dp-with-silk1

5-zeroing-field-meter1

6-donning-wrist-strap1

7-confirm-blade-is-at-zero-on-non-esd-sd4

8-touching-blade-for-cdm-charge2

9-cdm-charge-potential-measurement1

10-zeroing-shock-stop-treated-sd-blade4

11-confirm-zero-volts-on-shock-stop-treated-sd1

12-recharging-dp-with-silk1

13-touching-blade-on-esd-treated-sd-for-cdm-charge1

14-cdm-measurement-for-esd-treated-sd1

15-rechecking-cdm-on-non-esd-sd1

16-remeasuring-cdm-on-non-esd-sd1

Q: What is the difference between the symbols used in ESD warning signs / labels: yellow hand w/ black background; black hand w/ yellow background; attention; triange/hand with arch; warning; etc. and how do you decide which one to use?

A: As per ANSI/ESD S8.1-1993, there are two types of ESD awareness symbols identified.

The ESD Susceptibility Symbol:

• 

  ESD Susceptibility symbol

• Consists of the Triangle, the reaching hand with slash through it.
• This means ESD sensitive devices or assemblies are present.
• DO NOT TOUCH or Handle Properly.
• If the device sensitivity is known, it can be added to the label.

ESD Protective Symbol

ESD Protective symbol

• Consists of the Triangle, the reaching hand and an Arc around the Triangle.
• This indicates ESD Protective Material such as chairs, mats, and wrist straps.

Another common ESD symbol for safety would be;

Common Point Ground

• 

  Common Point Ground

• This symbol looks like a bull’s eye and may be black and yellow.
• This represents connecting ESD control materials or equipment to electrical or 3rd wire ground.
• You can consult the standard S6.1 from the ESD Association for more on this.

On our website we have various ESD Labels, ESD Warning Signs and ESD Control Area Signs and Posters for your EPA (ESD Protected Area). The labels can be used on bags, boxes, and totes. The Caution Label symbol is still popular and is represented by a circle with three arrows entering into it.

The Attention Labels utilize the ESD Susceptibility Symbol and are represented by four different types:

1. Small ESD attention labels- Destructible type or tamper-evident and is good for closing small bags containing ESDS components
2. MIL 129 Label - Also destructible type or tamper-evident label
3. ESD Attention Label - a label that also allows writing space
4. Dry Packaging & ESD Susceptible Label

ESD Control Area Signs and Posters.

ESD Control Area Poster

Black on Yellow or Yellow on Black print is just used for ease of reading and the MIL 129 label is the only one that I know that uses a black hand in a yellow triangle instead of a yellow hand in a black triangle.