HBM – Ground Zero Electrostatics Blog

Demonstrating CDM Discharge using Common Hand Tools

What is the significance of the time to the charge generation in tribocharging?

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).

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.

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

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).

Insulators or materials with high resistance restricts or prevents flow of electrons across (surface) or through (volume) it’s material.

Conductors or materials with low resistance easily allows the flow of electrons across it (surface) or through (volume) it’s material.

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).

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.

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.

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.”

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.

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.”

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

We don't need no stinking wrist straps, do we?

Q: I have read the White Paper 1: A Case for Lowering Component Level HBM/MM ESD Specifications and Requirements and found the ESD Control Programs and Resulting Data (Chapter 1, Page 20-23) particularly interesting.

Assuming a production environment with ESD flooring, footwear (and clothing), by the time a person walks to a workstation and sits down, the voltage of this persons should not exceed 500V (or even 100V as seen in Figure 3). That would mean even a seated operator in this case would not need to wear wrist strap, that theory would be correct right? After sitting down and this person sits on a stool (feet off the floor) with resistance to floor < 1.0x10exp9ohms, any HBM risk would be further reduced wouldn’t it?

A: Hello ****. Nice try. Even if you have an ESD flooring system and even if you have ESD footwear and even if you have an ESD task chair with ESD casters or an ordinary task chair with an ESD chair cover (very effective as well), ESD smock on… you STILL have to wear the wrist strap when seated at an ESD workstation.

The only time, per ANSI/ESD S20.20-2007 page 4, 8.2 Personnel Grounding, that personnel in the EPA (ESD Protected Area) should be without a wrist strap is when doing standing or walking about operations, and then two conditions must be met;
· “When the total resistance of the system (from the person, through the footwear and flooring to the grounding / Equipotential bonding system) is less than 3.5E7 Ω…”
· “When the total resistance of the system (from the person, through the footwear and flooring to the grounding / Equipotential bonding system) is greater than 3.5E7 Ω and less than 1.0E9 Ω and the BVG is less than 100 v per 97.2…”

This is what is said about seated personnel:

“When personnel are seated at ESD protective workstations, they shall be connected to the grounding / Equipotential bonding system via a wrist strap system.”

Hope this helps. I guess you could say redundancy is good in the realm of ESD. It’s the weak link in the chain that will cause an ESD event. If someone lifts their ESD footwear from the ESD flooring system while seated, they can tribocharge to above 100 volts. It takes only 0.3 seconds of charge time to exceed 20.20 requirements. If personnel is seated and getting up to go to break, it seems best to stand up, remove the wrist strap from the wrist, carefully set it down and walk away from the ESD workstation. Worst case is to take the wrist strap off while still seated, set it down, put your hand on the ESD workstation and near ESDS devices, then stand up out of the task chair before leaving the work station. Under proper conditions and with good bench mats, clean ESD floors, ESD task chairs, etc. in place, no ESD event. The problem with ESD events is that we cannot see, hear, feel them.

The only alternative to not wearing a wrist strap while seated may be the used of a smock with a grounding coil cord attached to it. You can see the footnotes on the 20.20 document at the bottom of page 4 for further details.

We adhere to and meet or exceed requirements put forth in ANSI/ESD S20.20-2007 or IEC 61340-5-1, which assumes a target HBM of 100 volts and less.