Mechanical Overstress Induced By High Temperature
Mechanical overstress induced by high temperature is the most common cause of failure in LEDs. If the temperature of the package reaches its 'Glass Transition Temperature'(Tg), whether induced internally by electrical overdrive or externally by high ambient temperature, the epoxy starts to expand rapidly. This stresses many of the internal components of the LED. At extremely cold temperatures, the lens epoxy can crack.
The two figures below illustrate the typical assembly components used in packaging thru-hole and surface-mount LEDs. Wire bond breakage, wire bond detachment, die attach loss all stem from overheated epoxy or cause a delamination between the chip and the epoxy.
Mechanical Stress exerted on either lead wire is another failure mechanism. If the lead wire is soldered while being subjected to stress, or tensile, torsional, or compression stress is applied between lead wires while hot immediately after soldering, open circuits may be generated inside the device. Therefore, be sure to correct the position and direction as necessary, after sufficiently cooling. Also, bending the leads too close to the body of the LED induces similar mechanical stress. Follow manufactures guideline for lead forming (typically recommend forming lead at least = 2mm from the body of a 5mm thru-hole LED).
Finally, Electrostatic Discharge (ESD) is another failure mechanism for InGaN & GaN devices. When handling individual devices or assemblies, be sure the environment is protected against electrostatic electricity. Operators should wear anti-static clothing. Containers and other objects that come into direct contact with devices should be made of anti-static materials and should be grounded to earth via a 0.5- to 1.0-W protective resistor.
