LED Internal Construction

Interruptions in the current path respective the increase of electrical resistance have turned out to be frequent failure causes leading to reduced light emission, temporally instable function or even the entire failure. Delamination in the junctions between the chip and bond wire respective the chip and the substrate have also been detected. Since the interface area of the chip to substrate bond essentially influences heat dissipation, disruptions of the interface layer cause an increase of thermal resistance and chip temperature. Consequently, this leads to accelerated ageing of the LED¹³.

In the LED production process are many failure sources to be considered. The soldering process has turned out to be especially critical because thermal overload can cause cracks, detachments or delamination along the border areas due to the various expansion rates of the materials. Extensive qualification tests and stable quality control at the LED manufacturer as well a very good control of the solder process during PC assembly are basics for a long lifetime. Transparent encapsulation materials like silicone or epoxy are not hermetically sealing and therefore do not protect against humidity or other damaging materials¹³.

Furthermore, mechanical stress, for instance while bending the pins of 2-pin-LEDs, or thermal stress like the soldering process, can cause cracks or detachments of the LED joint compound and allow penetration of contaminants down to the chip or metal contact. Under adverse conditions this will result in modifications of the epitaxy layers or in corrosions in the interfaces. Another effect would be the already described consequences due to detachment of the electrical joints or the performance degradation due to damaged reflecting surfaces. Other typical mechanisms that lower the useful lifetime are the ageing of encapsulation materials influenced by UV-radiation, or the degradation of the phosphor of white LEDs, which is also responsible for a color change of white LEDs (known as yellowing). This effect is more intense than the regular ageing of a chip¹³.

Figure 3 X-ray microscopy image of a broken bond wire
Source: LED professional, 2010