We’re talking about Electrostatic Discharge (ESD) on this blog all the time. But what exactly does it mean and why is it so dangerous?
All matter is constructed from atoms. These atoms have negatively charged electrons circling the atom’s nucleus which includes positively charged protons. As the atom has an equal number of electrons and protons, it balances out having no charge. The problem is that all materials can tribocharge or generate ElectroStatic charges. Most commonly, this happens through contact and separation – examples are:
Unwinding a roll of tape
Gas or liquid moving through a hose or pipe
A person walking across a floor and soles contacting & separating from the floor.
So before we go any deeper let me bring in some interesting fact about tribocharge and how it can be generated literally everywhere:
4 Major Factors that result in Tribocharging process:
1. Surface contact effect
2. Work function
3. Charge back flow
4. Gas Breakdown (Plasma)
Its a long list you can just read the first sentence of each paragraph for the definition of it :D
Surface Contact effect is interaction of two dissimilar material. It can be through mean of friction. There is a possibility that two different surface will stick to each other due to chemical bond. Upon separation some bonds will rupture which leave behind imbalanced charge that generate static shock. This depend on the work function of both the material. [1]
Work function is the property of a material’s ability to hold onto its free electrons (the electrons orbiting the outer most shell of the material). The greater the materials work function, the less likely it is to give up its free electrons during contact (triboelectric generation). The weaker the work function is, the more likely the material will acquire a more positive charge by giving up or loosing some of its free electrons. In general, materials with higher work functions tend to appropriate electrons from materials with lower work functions. [1]
Charge backflow occurs when two materials have been charged possibly from the above mechanisms and are then separated from intimate contact. The backflow of some of this charge imbalance may flow back to the original material reducing to some degree the net charge (charge imbalance) on either surface from tribocharging. [1]
Gas breakdown can occur between two surfaces during separation. The microscopic surface topology of a surface has many peaks and valleys. It is one of these peaks that may have substantial charge that yields a large electric field in a very small area causing corona discharge or the breaking down of the air molecules which were acting as a dielectric (insulator between the two separating surfaces). During this breakdown, charge can be transferred from one surface to the other via the path of the electrified air (plasma). The amount of charge transferred is dependent on the distance of separation and the gas(ses) pressure(s). [1]
Let's take a short break after reading so much detail. Here's a short video
In the short clip above, as the boy jumps up and down, rubbing his feet on the trampoline, he picks up extra electrons. That's why his hair stands straight up in the air: all of the negative charges building up in his body want to repel each other.
But when the boy reaches out and touches fingers with his dad, all of those extra electrons that were building up in his body leap from his finger to his dad's finger, giving him a painful zap.
One observation that I can made is cold and dry area is one of a key parameter that can result in electrostatic. This is very important for manufacturing company since everything is always on the move tribocharging will occur regardless if nothing is done or no precaution is taken in place.
Electrostatic charge is most likely to occur in cold and drier climate and even insulator material still hold charges and the likelihood for an ESD event increase since there we know different polarity attract and same polarity repel then wearing rubber like material (Insulator) will significant increase the chance of getting shock since charges is built up.
Charge Generation Unwinding a Roll of Tape
Unwinding a roll of tap can generate an electrostatic charge
The simple separation of two surfaces can cause the transfer of electrons between surfaces resulting in one surface being positively and the other one negatively charged. With that we’ve just generated an ElectroStatic charge! The amount generated varies and is affected by materials, friction, area of contact and the relative humidity of the environment. At lower relative humidity, charge generation will increase as the environment is drier. Common plastics generally create the greatest static charges.
ELECTROSTATIC DISCHARGE (ESD)
If two items are at the same electrostatic charge or equipotential, no discharge will occur. However, if two items are at different levels of ElectroStatic charge (i.e. one is positively and the other one negatively charged), they will want to come into balance. If they are in close enough proximity, there can be a rapid, spontaneous transfer of electrostatic charge. This is called discharge or ElectroStatic Discharge (ESD). Examples in daily life:
Lightning, creating lots of heat and light
The occasional zap felt when reaching for a door knob
The occasional zap felt when sliding out of a car and touching the door handle
Feeling the Zap when touching a doorknob
Have you felt the zap before?
In a normal environment like your home, there are innumerable ESD events occurring, most of which you do not see or feel. It takes a discharge of about 2,000 volts for a person to feel the “zap”. It requires a much larger ESD event to arc and be seen (e.g. lightning). While a discharge may be a nuisance in the home, ESD is the hidden enemy in a high tech manufacturing environment. Modern electronic circuitry can be literally burned or melted from these miniature lightning bolts. ESD control is therefore necessary to reduce and limit these ESD events.
Type of ESD Device Damages
ESD damage to electronic components can lead to:
Catastrophic Failures
Latent Defects
Catastrophic Failure
Catastrophic failure causes a failure in an ESD sensitive item that is permanent. The ESD event may have caused a metal melt, junction breakdown or oxide failure. Normal inspection is able to detect a catastrophic failure.
Latent Failure
A latent defect can occur when an ESD sensitive item is exposed to an ESD event and is partially degraded. It may continue to perform its intended function, so may not be detected by normal inspection. However, intermittent or permanent failures may occur at a later time.
COSTLY EFFECTS OF ESD
A catastrophic failure of an electronic component can be the least costly type of ESD damage as it may be detected and repaired at an early manufacturing stage. Latent damage caused by ESD is potentially costlier since damage occurs that cannot be felt, seen or detected through normal inspection procedures. Latent defects can be very expensive as the product passes all inspection steps and the product is completed and shipped. Latent defects can severely impact the reputation of a company’s product. Intermittent failures after shipping a product can be frustrating, particularly when the customer returns a product, reporting a problem which the factory again fails to detect. It consequently passes inspection and the product is returned to the customer with the problem unresolved.
The worst event is when the product is installed in a customer’s system, and performs for a while and then performs abnormally. It can be very expensive to troubleshoot and provide repairs in this situation.
An Example from Everfeed Technology Pte Ltd Case Study:
US$7 Device
US$7 Device in board – US$700
US$7 Device in board and in system – US$7,000
US$7 Device and system fails – US$70,000
Industry experts have estimated average electronics product losses due to static discharge to range from 8 to 33%. Others estimate the actual cost of ESD damage to the electronics industry as running into the billions of dollars annually.
Solution
When designing an ESD control program there are two simple rules relative to dealing with charging problems. One is to ground all conductors, the other is to either remove or control all insulative charge generators. Grounding can easily be accomplished with various ESD control products [2]. In lieu of abstaining from the use of non-conductive charge generators, controlling them may be essential to the program. Controlling process necessary insulators can be more involved and may require the use of ionization or surface treatment with a topical antistat solution or spray. Neutralizing of charged objects is accomplished by using a balanced output air ionizer. The target is flooded with a multitude of positive and negative air ions which result in a near zero voltage level relative to ground on the charged surface after just a few seconds of exposure (decay time). The decay time is depending on several factors such as surface proximity to the ionization source, surface area, surface capacitance, level of charge imbalance, etc.
CONCLUSION
Environment in cold condition should always keep a humidity level of 40-50% to avoid ESD event from happening.
It is critical to be aware of the most sensitive items being handled in your factory. As electronic technology advances, electronic circuitry gets progressively smaller. As the size of components is reduced, so is the microscopic spacing of insulators and circuits within them, increasing their sensitivity to ESD. As you can predict, the need for proper ESD protection increases every day.
References [1] R. C., "Triboelectric Generation: Getting Charged", 2000. [Online]. Available: http://www.esdjournal.com/techpapr/ryne/ryntribo.doc. [Accessed: 17- Jan- 2019]
[2]“ESD Control Standards: Setting up an ESD control program”, Ryne C. Allen, EE-Evaluation Engineering, February, 1999