INTRODUCTION
This application note describes electrical overstress (EOS) events, their effect on Cree XLamp LEDs and some simple methods of protecting XLamp LEDs against EOS. Electrical overstress is simply exposing an LED to any current greater than the maximum cur*rent specified in that LED's data sheet. The number or length of EOS events is irrelevant because any single EOS event can cause damage to the LED. This dam*age can be exhibited either in an immediate failure or a failure many hours after the EOS event.
2. Transient over-current events Transient over-current events are events that subject the LED to current that is higher than the maximum rated cur*rent on the LED data sheet, either directly through high current or indirectly through high voltage. These events are transient, meaning they happen for a short period of time – typically less than one second. These events are sometimes referred to as "spikes," as in "current spike" or "voltage spike." If the over-current event occurs immediately when the LED is turned on or plugged into a energized power supply (also called "hot plugging"), this over-current event is called "in-rush current."
EFFECTS OF ELECTRICAL OVERSTRESS ON XLAMP LEDS
It is impossible to predict the failure mode of every LED exposed to electrical overstress, but Cree has seen two common symptoms of XLamp LEDs that have had an EOS event cause a catastrophic LED failure. Damage to Bond Wires One common failure mode from EOS is damage to the bond wires inside the LED package, as illustrated in Figure 1 below. This damage usually occurs as a burned wire or a broken wire. In addition, the EOS event can cause damage to other materials in close proximity to the bond wires, such as the encapsulant or phosphor. Damage Near Bond Pads Another common failure mode from EOS is damage to the LED chip itself near the bond pads, as shown in Figure 2 below.
As described in the Causes of Electrical Overstress section, one common form of over-current event occurs when LEDs are connected to an energized power supply or when the power supply is first turned on. This event is called in-rush current.
Cree strongly recommends adding some level of protection to LED modules that do not include an on-board power supply to minimize the risk of an in-rush current event from the separate power supply.
This is only from one LED manufacturer and is only a part of their application note. I have seen another that showed photographs of the actual sputtering of the bond wires.
Curt