Sometimes, there may be breakdowns in the products of LED armature producers. And so the producers may face with serious costs while changing the products on the field. To avoid this type of risks, producers make breakdown analyses and want to minimize these risks in the new designs. Being encountered recently, Electrical Overstress (EOS) and Electrostatic Discharge (ESD) are the principal problems that are difficult to understand while making breakdown analyse. These problems are mostly
confused. In this paper, we will examine the difference between EOS and ESD, the reasons underlying these problems and considerations in designs and production to minimize these breakdowns.
The Difference Between Electrical Overstress (EOS) and Electrostatic Discharge (ESD)
ESD comes first to the mind when we mention EOS. These two terms may be used interchangeably. However, there are small differences between them. ESD occurs when there are momentary high voltage jumps up to kV in a very short time (like nanosecond), while EOS takes longer time (milisecond or second). We can assume ESD as a sub-category of EOS.
- ESD generally occurs when the system is off. It creates small holes/cracks on epitaxial layer (layers that make EPI and LED).
- EOS happens generally in power mode when the system is on. Overheating/burning is the damage (melting in EPI layer/binding wire).
Electrostatic Discharge (ESD)
Electrostatic discharge (ESD) is described as the release of static electric when two objects contact, while static electric is defined as the electrical charge arising out of the imbalance on the surface of a material/electron transfer.
Simply put, ESD means the transfer of static charge from one body to the other (generally through contact). This problem mostly occurs during assembly, production, mounting, transportation and shipping when the product is off. ESD occurs when the static electric is transferred from one object to the other (LED). An ESD event is mostly characterized with “voltage spike.”
The Effects of ESD on LED and Its Symptoms:
The effects of ESD on LED:
ESD can be defined as a sudden burst of energy that is transmitted to ESD device and may harm LED. This sudden burst of energy may create micro-cracks and structural defects in LED, which may lead to following breakdowns:
Latent breakdown: The most problematic of all breakdowns is the latent one. LED keeps working
but partial breakdown leads to low performance or decreases the lifetime of the product. It cannot be detected at the first moment, may emerge after hundreds/thousands of hours when the product is on the field and paves the way for EOS damages.
Disruptive breakdown: LED stops working and becomes off.
• Symptoms:
When LED is damaged because of ESD, some (or all) of the following symtomps may surface:
- It will not be on during nominal current.
- Inverse current leakage will increase due to micro-cracks or structural defects in LED chip ports.
- There will be no anomaly or light output in low current.
• Human Body Model for ESD (HBM):
As stated previously, the reasons behind EDS are various. Human contact, machine, packaging are some of them. The most common reason behind EDS damage is human contact. Human contact model is the most commonly-used model to characterize the sensitivity of an electronic device to the damage arising out of electrostatic discharge. Sensitivity or resistance of LED to ESD is categorized in the following HBM classes.
• Minimizing ESD Risk During Mounting
To minimize ESD risk while mounting LED components and modules, following tools are recommended:
- Y Antistatic Wrist Straps
- Proper Shoes for ESD
- Antistatic Suit / Clothes
Electrical Overstess (EOS)
There are many factors which may cause EOS problem. Spike is defined as fast and short-term sudden electrical transition in voltage, current or transmitted energy in an electrical circuit. These spikes which
may come from various resources such as sudden high current/voltage when the energy is given/cut create EOS problems
• Possible Reasons Behind EOS
• Nondisruptive damages stemming from ESD trigger current/voltage spikes and certainly cause EOS.
• Occuring while power is on and off, these spikes may come from the drive of a LED circuit or system. If there is no extra protection in LED module and a spike more than breakdown value occurs, EOS arises.
• In DOB designs, a spike may occur due to the cable groups around drive circuit and even drive and LED connection themselves may be the source of spike.
• During testing phase of the production, additions to and droppings from LED modules or repairing/intervening the module may cause sudden current spike while the power is on. If this threshold is higher than inrush current specified in datasheet of LED, EOS will emerge and result in serious damage.
• If the driving current of LED is close to maximum output current specified in datasheet of LED, it may exceed the maximum current due to an underdesigned drive that cannot regulate output current or output current tolerance of the drive and may cause breakdown in LED.
• If there is no external protection against power surges and thunderbolt in the system of LED armature, the resultant sudden current/voltage spikes may damage LED modules.
• Any kind of loose contact, crack or skewness in LED assemblies, connectors, cable solderings on LED module may lead to EOS.
• Even if not much damage occurs after one EOS, this may trigger other EOS forms and cause LED to break down.
Symptoms Of EOS on LED
• LED may wholly become non-operating.
• A serious decrase in effectivity, life-time of LED and thermal performance may occur and it may be non-operating after a short while.
• Depending on the extent of the damage, a melted/burnt image, short circuit, ruptured wire, non-uniform light output may be seen on LED. Some damages that are invisible to the eye but visible through microscope may also occur.
Minimizing The Risks Of EOS and ESD During The Design
While choosing LED for our design, we must consider ESD JEDEC Standard classes we mention in technical specifications to minimize ESD risk. LED must absolutely comply with this standard. If protection classes of the future LED differ from each other, the class must be chosen depending on the
class. This detail will minimize ESD risk of LED, yet it is not adequate on its own to protect the whole sytstem from EOS defects and additional measures must be taken. TVS is recommended in the
designs for protection against EOS. It can be used for protecting LEDs against over-chargers and TVS diodes may offer protection up to 30KV. TVS diodes have different covers (smd/dip) and different
product types including unidirectional and bidirectional (two TVS as series). Adding these products, which are compatible with the design, to your LED module will protect your product against possible EOS problems. TVS is always off in the system. When an EOS occurs, it turns on and begins protecting LED if spike voltage is higher than breakdown voltage. While choosing TVS diode, reaction time
and breakdown voltage values should be handled carefully.
